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A comprehensive review on latent heat and thermal conductivity of nanoparticle dispersed phase change material for low-temperature applications
Accepted Manuscript A comprehensive review on latent heat and thermal conductivity of nanoparticle dispersed phase change material for low-temperature applications Eanest Jebasingh.B, Valan Arasu.A PII:
S2405-8297(19)30891-8
DOI:
https://doi.org/10.1016/j.ensm.2019.07.031
Reference:
ENSM 856
To appear in:
Energy Storage Materials
Received Date: 27 May 2019 Revised Date:
8 July 2019
Accepted Date: 20 July 2019
Please cite this article as: E. Jebasingh.B,, V. Arasu.A, A comprehensive review on latent heat and thermal conductivity of nanoparticle dispersed phase change material for low-temperature applications, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2019.07.031. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Title
A comprehensive review on latent heat and thermal conductivity of nanoparticle dispersed phase change material for lowtemperature applications Eanest Jebasingh.B
Sequence of Author
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Valan Arasu.A First author Email
[email protected].
First author Address
Department of Mechanical Engineering Thiagarjar College of Engineering
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Madurai- 625015, India Valan Arasu.A
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Corresponding Author
[email protected]
Corresponding Email Corresponding Address
Department of Mechanical Engineering Thiagarjar College of Engineering
Acknowledgment
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Madurai- 625015, India.
The author Eanest Jebasingh B wishes to acknowledge the financial support provided by the Council of Scientific and Industrial Research, India under CSIR
EMR-I)
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SENIOR RESEARCH FELLOWSHIP (Sanctioned Letter No: 08/237/0013/(2018)
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Conflict of interest
We have no conflict of interest to declare.
ACCEPTED MANUSCRIPT A comprehensive review on latent heat and thermal conductivity of nanoparticle dispersed phase change material for low-temperature applications
Abstract:
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Today’s power infrastructure involves unpredictability in both demand and supply. Power management using energy storage is becoming a promising method to have sustainable energy utilization. In recent times, energy storage using latent heat thermal energy storage (LHTES) technology is receiving more considerable attention to
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reducing grid energy demands. LHTES technology have been utilized by using phase change material (PCM)for the last two decades. This review focuses on the change in
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latent heat and thermal conductivity of nanoparticle dispersed phase change material (NDPCM) between the operating temperature range of 20˚C and 37˚C as required in low-temperature applications. The critical feature of this review is that it analyses both the scientific reasons behind the increase or decrease in latent heat and thermal conductivity of base PCM. Dispersion of nanoparticles as well as supporting materials
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into the PCM matrix and the impact of influencing parameters like size, shape and the material of the nanoparticles on the thermal properties of PCM. Dispersion of nanoparticle increases, the thermal conductivity gradually increases while the latent heat decreases. This indicates that the improvement in NDPCM thermal conductivity
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using nanoparticle will be accompanied by reduced latent heat in the NDPCM. However, Thermal conductivity enhancement in NDPCM was higher for carbon-
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based nanomaterial than for metal or metal oxide nanomaterial. Thus, the review will be helpful for new researchers in understanding the underlying science behind the change in critical thermal properties of the base PCM and to further improve the performance of the LHTES system. Keywords: LHTES, phase change material, nanoparticle dispersed PCM, thermal conductivity, latent heat.
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ACCEPTED MANUSCRIPT Nomenclature K Thermal conductivity (W/mK) ∆H Latent heat (kJ/kg)
MWNT
Multi-Wall Carbon Nanotube
NDG NG NP NDPCM
Al2O3 -T CA CNT CTAB
NMP OD OS PA
Nitrogen-Doped Graphene Nano Graphite Nano Particle Nanoparticle Dispersed Phase Change Material N-Methyl-2 Pyrrolidone Octadecane Oleoyl Sarcosine Palmitic Acid
GIC GNP GNS GO GP LA MA
Copper Oxide Nanoparticle Expanded Graphite
PAN PCM
Poly (Acrylamide-Co-Acrylic Acid) Poly acrylo nitrile Phase Change Material
Expanded Graphite Oxide Treated Expanded Graphite Eutectic Hydrated Salt Eutectic Hydrated Salt Expanded Perlite Erythritol Tetra Palmitate Erythritol Tetra Stearate Expanded Perlite Graphene Aerogel Graphene Aerogel (Rgo/Gnp) Graphite Integrated Compound Graphene Nanoparticle Graphite nanosheet Graphene Oxide Graphite Particle Lauric Acid Myristic Acid
PEG PU PVA PW rGO SA SDS SG SiO2 SWNT
Polyethylene Glycol Polyurethane Polyvinyl alcohol Paraffin Wax Reduced Graphene Oxide Steric Acid Sodium Dodecyl Sulfate Spongy Graphene Silicon Dioxide Single Wall Carbon Nanotube
TEA
Tri ethylamine
TEMED
Tetramethyl ethylene diamine
TiO2 xGnP
Titanium Dioxide Exfoliated Graphite Nanoplatelets
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PAAAM
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CuO EG, WPEG EGO EG-T EHS EHS EP ETP ETS Exp GA GA
Treated Aluminium Oxide Capric Acid Carbon Nano Tube Cetyl Trimethyl Ammonium Bromide Copper Nanoparticle
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Cu
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Acronyms 3D-GA Three Dimensional Graphene Aerogel AC Activated Carbon ACCOOH Glacial Acetic Acid Ag Nano Silver Nanoparticle Al2O3 Aluminium Oxide/ Alumina
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Introduction: Most developing countries like India are exposed to warm-to-hot climates due to their geographic latitudinal locations, and favourable environment condition will
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account for the majority of the world’s population increase by 2050 [1]. Surface temperatures are also gradually increasing in developing countries as a result of habitual electricity generations from fossil fuel, thermal power plant, and also
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unpredictable weather events such as heatwaves [2].
For adaptation to changing the climate, one must consider the significance of air
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conditioners (A/Cs) in mitigation of human vulnerability due to unpredictable weather events such as heatwaves. The percentage of households that have air-conditioning units will increase to nearly 99.9% by 2100, from its current 2% [3]. According to the International Energy Agency (IEA), countries like India has seen a rapid increase in household electricity consumption, via increased purchases of appliances like air conditioner. The purchase of air conditioners around the world has increased from
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around 1 million units in 2003-2004 to more than 3 million units in 2010-2011. IEA has also strongly suggested that if this continues, the usage of air conditioning equipment will reach up to 50 million by 2050 [4].
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The international energy agency (IEA) also indicates that developing countries like India has been responsible for almost 10% of the increase in global energy
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demand from 4.4% in 2000 to 5.7 % in 2013 which relates the GDP rating to lower rate [4]. Scientists have predicted that burning of fossil fuels and power generation from thermal power plants will generate a tremendous amount of carbon dioxide (CO2), which would be responsible for over half of GHG (Greenhouse gas) emission induced global warming [5] leading to climate change [6,7]. The growth in air conditioning indicates billions of tons of increased carbon dioxide emissions. Increased saturation of air conditioners has important implications, not only for total electricity generation and carbon dioxide emissions, but also for load management [8].
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ACCEPTED MANUSCRIPT Environmental scientists have predicted that carbon dioxide (CO2) emission will lead to an increase in the global temperature by 1- 3.50 C and the sea level will rise by 15-95 cm by 2050. Increase in global temperature leads to climate change. Climate change will play a significant role in the increased demand for the purchase of air conditioners [9]. Impact of climate change on global emissions have to be considered
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due to energy consumption for cooling needs by 2100 [10]. Researchers also predicted that the efficiency of air conditioners would also decrease when the atmospheric temperature rises from 20°C to 46°C; it is determined that the power consumption for
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air-conditioning units will increase up to 47.1% by 2050 [11].
According to the International Energy Agency (IEA), space cooling is one of
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the prior technologies, which has the highest long-term potential for reducing CO2 emissions [12]. The utilization of renewable or low-grade waste energy with cold energy storage is considered a promising solution to global warming and energy crisis [13]. In 1994, Dorgan and Ellison [14] put forth detailed design procedures for cold thermal energy storage system using ice-cooling. Performance improvement of cool thermal energy storage systems about the building energy efficiency has been
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established through the extensive modeling and simulation techniques. Some of these systems were also field-tested and implemented in buildings located in hot and arid climatic conditions. As a result, these systems have contributed to a reduction in on-
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peak power and energy demand in such buildings by approximately 31% [15,16]. The cold thermal energy storage (TES) may be divided into two main groups:
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sensible heat and latent heat. Due to a large amount of thermal energy absorbed as latent heat (heat of fusion) at nearly constant temperatures during melting and solidification, solid-liquid phase change materials (PCMs) have been adopted as TES media in applications like thermal management. The utilization of cold thermal energy storage using latent heat technology using phase change material (PCM) in A/Cs, is an advanced energy-saving technique for indoor air conditioning application [17]. PCM as a storage medium, the available cold thermal energy or excess cold energy can accumulate into the PCM during the charging process. Then, the stored cold thermal energy can be retrieved and supplied 4
ACCEPTED MANUSCRIPT to the end-user during the discharging process. In virtue of energy storage as a change in internal energy or phase transformation of the storage medium, it offers to alleviate the peak load on electricity grids and utilize power in the off-peak period. The application of thermal energy storage in air-conditioning systems can benefit the customer by lowering the energy costs, improved space temperature control,
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technologically advanced load factor, and slower capital investment in new generation equipment [18].
The energy storage during phase transformation depends on the properties of
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PCM. According to the material properties, the PCM is classified into organic and inorganic. However, the low thermal conductivity of PCMs limits the use of the full
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potential of these materials since it slows down the heat transfer process associated with the charging and discharging processes. To overcome the problem mentioned above, some heat transfer enhancement techniques have been proposed such as the use of fins and heat pipes [19,20], insertion and dispersion of high thermally conductive materials [21], microencapsulation [22], nanoencapsulation [23], etc., Among these techniques, the dispersion of high thermal conductive nanometer-sized materials, such
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as nanoparticles, nanofibres, nanotubes, and nanosheets, in the PCM significantly increases its thermal conductivity [21,24]. This emerging mixture of base PCM and nanoparticle is referred to as nanoparticle dispersed phase change material (NDPCM)
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in this review article. However, the application of NDPCM in the range of 20˚C and 37˚C is discussed in Table 1
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Table 1: Application of NDPCM Reference Application [25] Building Air conditioner
[26]
Electronic cooling
Inference Phase change material embedded with nanoparticles exhibited improved heat transfer mechanisms in charging and discharging processes. Experimental results suggested that the proposed air conditioning system with NDPCM achieved an on-peak and per day average energy savings potential of 36–58% and 24–51%, respectively, for year-round operation while compared to the conventional air conditioning system Electronic applications (to control and avoid 5
ACCEPTED MANUSCRIPT temperature peaks) and plastic injection molds (to reduce temperature oscillations).
Building heating/cooling Indoor temperature controlling,
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[27–30]
NDPCM increases thermal transfer due to the presence of nanoparticle. NDPCM is a preferential potential thermal energy material for improving the energy efficiency in building applications due to its proper phase change temperature range, relatively high latent heat and thermal conductivity, good thermal reliability and chemical stability.
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Highly porous nanosheets, act as nucleators to enhance thermo-physical properties of NDPCM.
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There are many open ‘‘pore structure space’’ based on the pore structure of the nanoparticle. Moreover, there is some adsorption force between nanoparticle and liquid fatty acid phase change material
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Results of numerical analyses revealed that significant gains in the energy efficiency of buildings could be realized by introducing NDPCM into building envelope products. The results indicated a reduction of 79% in energy demand to maintain the interior within the thermal comfort range by application of the NDPCM NDPCM can be considered as energy-saving building materials for residential buildings using radiant floor heating systems A thermal switch is an electromechanical device which opens and closes contacts to control the flow of electrical current in response to temperature change. The thermal switch cuts off the current to critical machinery when a temperature limit is exceeded, preventing potential burn out or failure.
Radiant floor heating
[32]
Thermal switches
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[31]
NDPCM makes it a promising candidate for controlling the heat transport path simply via temperature regulation. NDPCM inevitably requires the increase of the thermal conductivity while
minimizing the variation of the latent heat capacity of the nanocomposite and ideally increases it. In other words, materials should be able to store or dissipate a large
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amount of energy in a short period. Several reviews have been published regarding the characterization and applications of PCM
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composites for thermal energy storage [33]. Despite the contributions given in Table 2, a comprehensive review focusing on the enhancement of thermal conductivity and latent heat capacity of PCMs as thermal energy storage materials, due to the addition of the nanomaterials in the operating temperature range of 20˚C to 37˚C for low temperature applications like indoor cooling to meet the
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human comfort requirements has not yet been carried out. Further, a common thought that the interactions of NP with PCM are influenced by changes in particle shape and size of nanoparticles, which has been analyzed and discussed in this article.
Sharma. R.K., et al. Chenzhen Liu., et al.
Zhenjun Ma., et al. Soares.N., et al.
Lin.Y., et al.
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Muthuvelan., et al.
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Guiyin Fang., et al.
Contents of the Review Reviews previous work on phase change cold storage for air-conditioning systems focusing on two aspects, including phase change materials (PCMs) and applications. The review concentrates on the preparation, thermal properties and applications of shape-stabilized (shape-stabilized PCMs that can be prepared by integrating the PCMs into the supporting material and microencapsulating the PCMs into the shell) thermal energy storage materials Reviews the concept of free cooling, PCMs suitable for free cooling applications in buildings, commercial PCMs that are available, and mapping of free cooling technology along with the promotional policies needed toward energy/environmental sustainability. The review focuses on three aspects: the materials, encapsulation, and applications of organic PCMs, and provides an insight into the recent developments in applications of these materials. The review focuses on preparation and characterization of NanoPCM (nano encapsulated phase change materials), application of NanoPCM in latent functional thermal fluid, dynamics simulation study of NanoPCM and heat transfer enhancement of NanoPCM Reviews the generic framework for the appropriate use of nano-enhanced phase change materials (PCMs) in buildings Reviews the control of domestic energy resources behaviors in the dynamics of energy demand, the advantages of improving thermal storage by using phase change materials, the importance of reducing heating and cooling energy demand (maintaining indoor thermal comfort). Reviews the methods for enhancing the thermal conductivity of PCMs, which include adding additives with high thermal conductivity and encapsulating phase change materials.
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Author Zhai.X.Q., et al.
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Table 2: Summary of a review article on PCM
Reference
[34] [18] [35] [36] [23] [33] [37] [38]
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ACCEPTED MANUSCRIPT The review investigates the experimental research works that report concerning the reason behind improvement or decrement of thermal conductivity and energy storage capacity of NDPCM in the operating temperature range of 20˚C to 37˚C. This review article also includes a critical discussion regarding the influence of size, shape,
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and materials of nanoparticles on latent heat and thermal conductivity of NDPCM. The review is organized as follows: 1) Different preparation methods of NDPCM. 2) Various characterization techniques of NDPCM.
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3) Discussion on latent heat and thermal conductivity of NDPCM for different types of PCMs, nanoparticles, and supporting materials.
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A detailed description of the most relevant experimental studies regarding innovative results, and identification of the critical reasons for the observed increase or decrease in latent heat and thermal conductivity of NDPCM, according to the type of nanoparticles is also included in this review article.
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Nanoparticle dispersed PCM
Organic PCM and inorganic PCM are the two types of commonly used PCMs. Organic PCM has drawn an excellent attraction for its desirable characteristics like
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good heat storage density, melting or solidification with little or no sub-cooling, nonreactiveness with most common chemical reagents and low cost, despite having the
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main drawback of low thermal conductivity [36]. Inorganic PCM has desirable characteristics such as high latent heat storage capacity, low cost, and nonflammability. However, inorganic PCM has some main drawbacks like high supercooling and non-proper phase transition, in addition to low thermal conductivity [39]. Dispersing high conductive nanoparticles in PCM could be a solution to enhance the thermal conductivity [40] and to sustain the thermal performance as a result of enhanced thermal conductivity (Table 3).
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ACCEPTED MANUSCRIPT Table 3: Thermal Conductivities of some commonly used PCM and nanoparticle Thermal conductivity (W/mK)
Material Phase Change Material RT27 Octadecane RT25 Eicosane Tetradecanol Capric acid Nanoparticle Aluminium (Al) Sliver Nanoparticle (Ag) Copper Nanoparticle (Cu) Titanium dioxide (TiO2) Expanded graphite (EG) Exfoliated graphite nanoplatelets (xGnP) Carbon Nanotube (CNT) Graphene
Min Li. Sadegh Motahar., et al. Patrik Sobolciak., et al. Mahdi Nabil., et al. Ju-LanZeng., et al. Dandan Mei., et al.
0.13 0.17 0.26 0.36 0.42 0.48
[41] [42] [43] [44] [45] [46]
237 429 400 8.4 200 1500
[47] [48] [49] [50] [51] [52]
Lingkun Liu., et al. Sung Seek., et al.
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Hao Peng., et al. Rabih M., et al. Sciacovelli A., et al. Arjumand Adil., et al. Teppei Oya., et al. Jinglei Xiang., et al.
Preparation of NDPCM
Reference
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Author
3000 3000
[53] [54]
The usage of PCM is limited because it has low thermal conductivity and leakage
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during phase transition. The low thermal conductivity will decrease the heat transfer rate, and the leakage of PCM causes certain harm to the energy storage system and environment, which limits the further application of PCM. Incorporating the nanomaterial into PCM, called NDPCM, for better improvement in thermal
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conductivity of the PCM is the main focus during the preparation of NDPCM. Table
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4 presents the different preparation methods of NDPCM adopted in experimental studies by different researchers. Table 4: Method of preparation of NDPCM Synthesis process
PCM
Vacuum impregnation method Varnish layer Kneader mixing technique Sonication & Ultrasonication Stirring and Sonication Autoclave method
CA Octadecane Octadecane RT22 Octadecane Tetradecanol
Melting Nanoparticle Temperature (oC) 28.99 30 28.7 25.37 28.91 35.87
EG xGnP EG Graphene xGnP EG
Reference
Ahmet., et al. [55] Seunghwan., et al. [56] Dowan Ki., et al. [57] Nandy Putra., et al. [58] JisooJeon., et al. [31] Yushi Liu., et al. [59] 10
ACCEPTED MANUSCRIPT In general, two main techniques used for dispersion of nanoparticles into the base PCM material: a) One-step method and b) Two-step method. The one-step technique combines the production of nanoparticles and dispersion of nanoparticles in the base PCM in a single step. In the two-step method, the nanomaterial first synthesized or purchased from the market and then dispersed in the base PCM.
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However, the present literature review reveals that most researchers have followed the two-step method only, but some researchers functionalized nanoparticle for better dispersion into the matrix of PCM, for a temperature range of 20˚C to 37˚C.
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Sari et al. [55] from Gaziosmanpasa University employed vacuum impregnation; vacuum flask is used for the dispersion process (Figure 1). The porous
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expanded graphite (EG) 10 g, was placed inside a flask, that in turn was connected to water tromp apparatus to evacuate air from the porous EG. The evacuation process continued for about 30 min inside the vacuum. Then the valve between the flask and the container of PCM was opened to allow the PCM liquid to flow into the flask to cover all the porous material. Finally, the vacuum process ended, and the air was allowed to enter the flask once again in order to force the fatty acid liquid PCM to
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penetrate the pore space of the expanded graphite. In order to test the fatty acid exudation from the porous spaces, the composite PCM was simultaneously heated during the dispersion process at a constant temperature above the melting temperature
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of the fatty acid. The fatty acid absorption amount by EG was evaluated by observing
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fatty acid exudation from the composite.
Figure 1: Vacuum impregnation method of preparation of NDPCM [55]
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ACCEPTED MANUSCRIPT Wi et al. [56] from Soongsil University tried with varnish layer method instead of encapsulation (Figure 2). Organic PCM octadecane was melted and directly mixed with
exfoliated
graphite
nanoplatelets
(xGnP).
The
obtained
homogenous
(Octadecane-xGnP) mixture was poured into a cylindrical shaped hollow to form a cylindrical shape or layer, and the cylindrical layer was covered with varnish for
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preventing the leakage. Then, the composite was dried for 48 h at 25 °C and 50% relative humidity. The cylindrical hollow was made by mixing plaster and water in a ratio of 1:0.45, forming a cuboid having a size of 100 mm (width) × 100 mm (length)
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× 15 mm (height) using a cylindrical mold.
Figure 2: Schematic of the varnish layer method [56]
Kim et al. [57] from Yonsei University used Kneader mixing technique.
Organic PCM octadecane was heated above its melting temperature, and the melted octadecane was directly impregnated into a vacuum treated EG using a stirrer, stirred at 30 rpm and 80˚C for 1 hour and allowed to cool. EG was synthesized from graphite powder using microwave irradiation. The obtained EG powder was dried inside a vacuum oven at 105˚C for 24 h.
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ACCEPTED MANUSCRIPT Putra et al. [58] from Universitas of Indonesia synthesized NDPCM with the help of sonication and ultra-sonication. Figure 3 shows the synthesis process of nano-PCM. Organic PCM RT 22 was melted and directly mixed with graphene by sonication under the frequency of 40 Hz. The obtained NDPCM was kept inside an ultrasonicator bath for 3h under 45˚C until the graphene was fully dispersed into the
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RT 22.
Figure 3: Synthesis process of NDPCM using sonication method [58]
Jeon et al. [31] from Soongsil University followed a simplest common method like two-step method (Figure 4). Octadecane PCM was melted by heating the PCM
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above its melting temperature. Then xGnP was mixed into the liquid PCMs in different mass fractions using a stirrer, stirring at 1000 rpm for 20 minutes. After
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stirring, NDPCM was sonicated for 20 min and allowed to cool.
Figure 4: Schematic of preparation of composite PCMs loaded with xGnP using twostep method [31] 13
ACCEPTED MANUSCRIPT Liu et al. [45] from Changsha University of Science and Technology used autoclave method. Organic PCM Tetradecanol (TD) was mixed with ethanol to improve the absorptivity of the PCM molecule, and then the mixture was melted at room temperature and directly impregnated with expanded graphite (EG) (ethanol was used to improve the uniform dispersion of nanoparticle into the PCM molecule). Then,
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TD- ethanol- EG, was heated above 70˚C in a hotplate to vaporize the ethanol. Characterization of NDPCM
Various techniques have been carried out to characterize PCM with the
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immersion of nanomaterials. Different studies focus on different parameters of NDPCM. NDPCM can be characterized by using a wide range of techniques, and
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some commonly used techniques [30,40,60] are summarized in (Table 5) Table 5: Major technique used to characterize NDPCM Technique Differential Scanning Calorimeter (DSC)
Purpose Determine the phase change temperature and heat of fusion
2
Dynamic Light Scattering (DLS)
Measure the average size and size distribution of nanoparticle in NDPCM
3
Scanning Electron Microscope (SEM)
4
Transmission Electron Microscope (TEM)
5
Fourier Transform Infrared Spectrometer (FTIR)
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S.No 1
X-ray Diffraction (XRD)
Morphology of NDPCM
Inspect the dispersion of nanometer-sized materials and characterize their superficial nanostructures and interfaces Obtain the infrared spectra and determine the chemical group
Measure the crystallite size and structure of nanometer-sized materials
The purpose for dispersion of high thermal conductive nanometer-sized
materials, such as nanoparticles, nanofibers, nanotubes, and nanosheets in the PCM matrix is to increase its thermal conductivity significantly. The thermal conductivity of NDPCMs can be measured using either steady-state methods or transient-state methods [61]. Guarded hot plate method [58] is a typical example of the steady-state
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ACCEPTED MANUSCRIPT method while transient hot-wire method [32], laser flash method [62] and transient plane source method [63] are transient-state methods. Latent heat- An inside view Latent heat is particularly attractive due to its ability to provide high-energy
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storage density in the quasi-isothermal process. A phase change transition from solid to liquid is endothermic, e.g., heat absorption process, the absorbed heat energy is utilized to overcome or break the weak intermolecular attractions in the PCM matrix. In the endothermic process, the energy required to change a gram of a substance from
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solid to liquid state without changing its temperature is commonly called its “heat of fusion.” The heat energy absorbed is used to break down the solid bonds but leaves a
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significant amount of energy associated with the intermolecular forces of the liquid state.
A phase transformation from liquid to solid is exothermic. In the freezing process, the moving boundary layer between the solid and liquid phase transfers energy to the PCM molecules. Rapid disordering of a crystal structure due to thermal
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disturbances occurring between the neighboring molecules of PCM causes the effective bonding between them to get strengthened [64]. The heat of fusion, melting, and freezing processes could also change due to the
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immersion of nanoparticle in the matrix of PCM. Determination of latent heat in NDPCM
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The variation trend of enthalpy with the mass fraction of additives mainly resulted from two impact factors. The first impact factor is intermolecular forces between nanoparticle and PCM. Another impacting factor is the non-melting enthalpy of additives, which would drop down the melting enthalpy of composites. When the mass fraction was lower than 1%, the melting enthalpy of NDPCM was mainly influenced by the first impact factor. Rising the mass fraction above 1%, the effect of the second impact factor got larger, and then the melting enthalpy decreased [65]. Further, the realignment of molecules of the PCM matrix in the presence of heavily
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ACCEPTED MANUSCRIPT loaded nanoparticle is impossible, which is also a major reason for the decrease in effective latent heat of the composite [66]. Simple mixing theory, theoretical melting enthalpy of NDPCM can be calculated by using Eq.1 [57] ∗ 1 − ϕ ………………….. (1)
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ΔH
= ΔH
= Theoretical Melting enthalpy of composite,
= Melting enthalpy for base PCM
and
ϕ = weight fraction of nanoparticle
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Effect of Metal and Metal oxide nanoparticles:
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where ΔH
ΔH
Metal nanoparticles research has recently become the focus of intense research work due to their unusual properties compared to bulk metal. Metal oxide nanoparticles represent a field of materials chemistry, which attracts considerable
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interest due to the potential technological applications of these compounds. Metal nanoparticle like silver (Ag) nanoparticle with the size of 10–30 nm diameter and length of 5–15 µm appears like spherical shape, which does not have an impact on the latent heat of the base PCM. However, latent heat in NDPCM was
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reduced in the range of 95% for 1 wt% dispersion of Ag in Eicosane PCM [48]. But, as the Ag nanoparticle concentration increases, the latent heat on NDPCM decreases
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and metal oxides like copper oxide nanoparticle (CuO) have reduced the latent heat of NDPCM considerably [67]. Effect of carbon nanostructural material: High thermal conductivity material, carbon-based materials are widely used as dispersant. Hence, the use of novel allotropes of carbon has continued to rise in recent years.
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ACCEPTED MANUSCRIPT Carbon nanotube (CNT) having a diameter of 8 to 15 nm; length 0.5-2 µm and rod shape has reduced the latent heat in the range of 95.4% for 1 wt% of CNT in NDPCM.
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CNT is a lightweight material and has a smaller particle size, in nanometer scale
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(smaller particle size will increase the intermolecular attraction between the atoms in NDPCM). CNT can easily affect the local interactions in the PCM molecule as well as the diffusion and rearrangement of PCM molecules, owing to its larger surface area. It
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indicates that the multi-porous structure of the CNTs limits the crystal behavior of the PCM [68]. Multi wall carbon nanotube (MWCNT) is one of the premium members in nanomaterial. Use of MWCNT in hexadecane showed
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a contrast result in latent heat. The MWCNT with a diameter of 10–20 nm, length 0.5–2 mm and rod shape, because of their high aspect ratios, large specific surface area, and substantial Vander-Waals attractions, tend to self-aggregate into bundles spontaneously
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in water and organic solvent. Surface functionalization of MWCNT increases the possibility of nano-tubes entanglement and close packing. As a result, latent heat in NDPCM increased from 227.13 kJ/kg to 256.28 kJ/kg for 0.5 wt.% dispersion of MWCNT in hexadecane PCM. The large surface area by surface functionalization was the reason behind the greater intermolecular attraction in
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the PCM, which resulted in its enhanced latent energy [30,69].
Table 6 recaps the melting temperature, and latent heat of NDPCM dispersed with metal, metal-oxide nanoparticle, and carbon nanostructure.
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Table 6: Melting temperature and latent heat of NDPCM
Metal Nanoparticle Rabih., et al. [48] Eicosane
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Reference
Base PCM Material Nanoparticle Melting PCM Temp Material wt.% (˚C) 36 36 36
Ag Ag Ag
1 2 3.5
Melting Latent heat PCM kJ/kg
NDPCM kJ/kg
241 241 241
229 208 192
NanoParticle Size
Shape
dia 10–30 nm; length of 5–15 µm
Spherical
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177
0.025
243.5
200.11
*
*
CuO
0.05
243.5
172.94
*
*
22
GNP
0.5
216.52
22
GNP
1
216.52
25.37
Graphene
0.05
25.37 25.37
Graphene Graphene
0.1 0.15
28
GA
6.25
28 28.81 28.81 28.81
GA GNP GNP GNP
9.09 1 4 7
Octadecane
28.91
xGnP
Bio-based
29.4
xGnP
Octadecane
31.45
31.45 31.45 31.45 32.33 32.33
Min Li [41]
Jisoo Jeon., et al. [31] Seulgi Yu., et al. [40] Yongpeng Xia., et al. [71]
Yi Zen., et al. [68]
RT27
1-dodecanol
SC
28
210.96
60 nm
Planar/ Flake shape
21.5 to 65.5 nm
Rolled thin
3 nm
Flake shape
dia 35 nm
Cylindrical
206.54
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Octadecanol
CuO
163.31
161.43
163.31 163.31
160.95 155.47
238.3
207
238.3 209.33 209.33 209.33
202.8 202.58 193.36 183.62
3
241.97
240.92
5
146.7
143.5
GA
3.22
205.4
195.7
GA GA GA CNT CNT
2.43 1.96 1.63 1 2
205.28 205.28 205.28 231.4 231.4
200.53 203.98 205.28 220.8 212.4
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Jing Yang., et al. [62]
RT-22
28
EP
Nandy Putra., et al. [58]
5
AC C
Carbon nano-material Li-Wu Fan., et al. 1-dodecanol [70]
Ag
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241
Metal- Oxide Nanoparticle B. R. Sushobhan., Octadecane et al. [67]
36
diameter 15µm, thick <10nm * *
length: 0.5–2 µm, dia: 8–15 nm
Rod 19
ACCEPTED MANUSCRIPT
Eicosane
35.7
Graphene
1
262
260.69
35.7 35.7
Graphene Graphene
2 5
262 262
257.546 247.85
* Not reported
dia 5−10 µm, thick of 4−20 nm
Cylindrical
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Xin Fang., et al. [66]
SC
Zhang., et al. [30] performed surface modification of the MWCNT particles in a mixture of two strong acids, concentrated H2SO4 (98%) and HNO3 (70%) at 3:1 volume ratio. MWCNT particles were added to H2SO4–HNO3 in a test tube and sonicated in an
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ultrasonic bath for 6 h and then the MWCNT was heated with reflux at 65 ˚C for 4 h. They reported that modified MWCNT particles took longer time (110 min) to disperse than the original MWCNT particles (45 min). The steric effect of the carboxylic acid group, as well as the irregularity of the modified MWCNT particle surface, resulted in a lower effectiveness of packing and a slower rate of aggregation. The aggregation of modified MWCNT particles eventually appeared, probably due to the low solubility of COOH in
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hexadecane, the hindrance provided by the small size of the acid group, and also the hydrogen bonding interaction among acid groups in modified MWCNT particles.
(SDS),
cetyltrimethylammonium
bromide
(CTAB),
polyvinylalcohol
(PVA),
polyethylene
glycol
(PEG),
AC C
sulfate
EP
Also, they tried various surfactants as additives for dispersing the MWCNT particles in hexadecane, including sodium dodecyl
tetramethylethylenediamine (TEMED), Triethylamine (TEA), glacial acetic acid (AcCOOH), 1-decanol, salicylic acid (SA), Tween80 (polysorbate 80), and Triton X-100 (C14H22O(C2H4O)n) to overcome the rapid aggregation and sedimentation of the nanoparticles in the organic liquid. Stable and homogenous dispersion was attained through surface modification of the MWCNT particles.
20
ACCEPTED MANUSCRIPT They claimed that it was difficult to disperse the MWCNT particles in a hexadecane liquid. The surfactant used to disperse MWCNT must be soluble in hexadecane, and the results showed that the functional group in the surfactants that are supposed to react with the acid groups in MWCNT should be as exposed as possible. Otherwise, the branch near the functional group would produce hindrance and lower
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the dispersion effect. Modified MWCNT particles, plus the addition of 1-decanol as a
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SC
surfactant, were successfully dispersed in hexadecane (Figure 5).
EP
Figure 5: Schematic of the interaction between 1-decanol and modified MWCNT[30] The porous nano-sized carbon-based material like exfoliated graphite
AC C
nanoplatelets (xGnP) was 10 nm thick and 15 µm in diameter. Good dispersion of xGnP in bio-based PCM was obtained easily by capillary and surface tension forces, resulting in no leakage of the melted PCM. Further, the latent heat of NDPCM is 99.56% even for 3wt.% of xGnP in octadecane PCM. Three-dimensional net structure confines heat movement in the PCM composites [31,40]. (Figure 6) shows how the xGnP has an impact on latent heat of various type of NDPCM.
21
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ACCEPTED MANUSCRIPT
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Figure 6: DSC curve of xGnP with various PCM [31]
Liu et al. [59] analyzed using graphene oxide (GO) as nanoparticle dispersion in the PCM matrix and found that the latent heat in NDPCM using GO was reduced
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up to 90% for2wt.% GO, when GO was 500nm in size, 1.318nm thick and was flake/hexagonal shaped. The reason behind the reduction of latent heat in NDPCM was that GO naturally presents weak Vander-wall forces due to the presence of oxide in the GO. In latent energy chemical potential energy play a major role, weak van der Wall force attractions in chemical potential tend to repulsive nature [52, 67]. There is
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chemical potential energy in the weak as well as in the strong ionic and covalent bonds. This chemical potential energy is, however, a major factor influencing the latent energy of a material.
EP
Putra et al. [58] experimentally analyzed using graphene in size ranging from 21.5 to 65.5 nm, width 3–5.5 nm and in the form of rolled-thin sheets dispersed in RT-
AC C
22 was purchased and tried without any future purification. The latent heat (Figure 7) decreased due to the reduction of RT 22 HC in RT 22 HC/graphene nano-PCM. Rolled-thin sheet structure of graphene could quickly fill the gaps in RT 22 HC molecules [50,67]. Therefore, the increase of mass fraction would reduce the latent heat of the nano-PCM. However, Graphene particles would attract RT 22 HC particles to create groups of particles that could give rise to lower energy to melt and freeze because of weak molecular bonds.
22
SC
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ACCEPTED MANUSCRIPT
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Figure 7: Latent heat vs. wt.% of nanoparticle [58]
Li et al. [72] tried surface functionalization on graphene as nanodispersion medium in docosane PCM and observed a contrast result that the latent heat in NDPCM increased from 256.1 kJ/kg to 262.2 kJ/kg. The above results suggested that the addition of surface-functionalized graphene can enhance the latent heat of
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docosane, and has no notable effects on its phase-change temperatures. The contrast result was because there is no change in the molecular structure of docosane and the docosane/graphene composite, but Li., et al. had not reported in his paper about shape
EP
and structure of dispersed spongy graphene in PCM. Surface functionalized spongy graphene had been synthesised by low concentration (0.35–0.4 mg/mL) of
AC C
homogeneous graphene oxide (GO) aqueous suspension mixed with 5vol % pyrrole which was hydrothermally treated in a Teflon-lined autoclave at 80˚C for 12 h to form a N-containing gel and then freeze-dried and annealed at 1050˚C for 3 h under Argon (Ar) atmosphere [72].
Yang et al. [62] have tried surface-modified graphene as nano dispersion medium in octadecane PCM; the graphene was 3 nm in size and flake in nature. Surface functionalized GA (graphene aerogel) was obtained by annealing thermally exfoliated graphene sheets at ~2200 ˚C to remove their residual oxygen-containing groups and to repair their structural defects. NDPCM’s latent heat reduced only 13% even for 6.5 23
ACCEPTED MANUSCRIPT wt.% of GA in octadecane PCM due to annealing of graphene which gives lower density, high porosity, and ultrahigh specific surface area. The PCM molecule can easily be absorbed in the porous nature of GA due to the effect of diffusion force and capillary force between PCM molecule and porous GA. These results prove that
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annealing the nanoparticle has declined the latent heat. [62]. Wang et al. [73] tried naturally obtained graphite as nanodispersion medium in OP10E as PCM. The graphite was 20-30 nm in a spherical structure. Latent heat in NDPCM also showed contrast result: latent heat increases up to 5% for 2 wt% of GNP
SC
in PCM. This result also determined that the same structure of PCM and nanoparticle would produce hindrance and lower the dispersion effect.
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While the nano graphite with diameter 35 nm, cylindrical shape dispersed in RT-22 PCM, Latent heat of NDPCM reduced in acceptable range due to more tightly packed GNP molecules with PCM, but Li [41] suggested that the cost of preparation for nano-graphite/paraffin PCM is acceptable while preparing the nano graphite from ordinary graphite.
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PCM with Supporting material:
Base PCM with supporting material was used for obtaining form- stable PCM. A high porosity supporting material has been used to prevent the base PCM from
EP
leakage during phase transition. Porous construction mainly contributes in prevention of leakage of the PCM molecule, but restricts the crystal arrangement and orientation
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of PCM into the pores.
Advantages of using as supporting material: • • • •
Leakproof without reduction in the thermal property Help to increase thermal performance. Help to retain the shape either in a solid or liquid state Eliminates the need for encapsulation
Average pore size of the supporting materials is exceedingly essential for thermal properties of form-stable composite PCMs. Briefly, if the mean pore size is too small, the PCMs molecular motion might be obstructed; conversely, if it is too large, 24
ACCEPTED MANUSCRIPT
capillary force is not sufficient to pack the liquid PCMs. Controllable pore structure is obtained under the synergistic effect of
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appropriate pyrolysis temperature and crosslinking reaction [74]. Macrospore supporting material like expanded perlite (EP) can maintain the integrity of encapsulation on PCM during the repeated heating/cooling process, meanwhile facilitating the quick heat exchange between the form-stable PCM and the environment.
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Nanoparticles are located mainly in void spaces of the form-stable PCM. Table 7 presents the details of PCM with supporting
Table 7: NDPCM with supporting material
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materials used by researchers.
Supporting Material
PCM
Melting Temp (˚C)
Support Material Material
% of Concen tration
Nanoparticle
Material
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Base PCM Material Reference Sughwan Kim., et al. [63]
Octadecane
28
xGnP
Su-Gwang., et al. [75]
Bio-Based
29.38
xGnP
Hui Li., et al. [76]
Octadecane Octadecane LA-MA CA-MA-SA CA-PA-SA
30.32 30.32 33.62 24.04 25.12
EG EG EG PAN PAN
Coconut oil
26.78
*
*
xGnP
RT25
27
LLDPE
50
27
LLDPE
50
Melting Latent heat PCM NDPCM kJ/kg kJ/kg
44.1
-
-
256.5
104.5
33.33
-
-
149.2
110.6
9.09 11.76 * 10 10
Ag Ag
* *
232.49 232.49 164.5 145.9 145.7
207.9 199.4 122.8 130.5 135.5
23.93
110.4
82.34
EG
5
145
56.2
EG
10
145
59
EP
AC C
Yan He., et al. [77] Huizhen ., et al. [78] Huizhen., et al. [78] Seunghwan Wi., et al. [79] Patrik Sobolciak., et al. [43]
wt.% of Concentrat ion
Nanoparticle Size Surface area (m2/g) 20.41 diameter 15µm, thick <10nm
Shape Flake shape *
300µm
Worm-like Porous
130 nm
Cylindrical
Surface area (m2/g) 20.41
Flake
*
*
25
ACCEPTED MANUSCRIPT
Haiting Wei., et al. [82] Ali Karaipekli., et al. [83] *Not reported
NaMMT
*
xGnP
5
247.6
80.16
CA
29.62
HNT
35
GNP
5
139.77
75.4
29.64
PU
13
Ag
*
29.64
*
*
Ag
*
LA-MA-SA
30.3
aEV
30
Al2O3
Eicosane
36.18
EP
40
CNT
LA-MA-PASA LA-MA-PASA
NDPCM with a nanoparticle as supporting material:
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Huizhen., et al. [81]
28
151.9
118.7
151.9
118.7
SC
Huizhen., et al. [81]
Octadecane
Flake shape
* 1 µm
8.4
160.9
113.7
*
*
0.3
275.91
160.38
O.D. × L 69 nm × 5 µm
Rod
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Yushi Liu., et al. [80] Dandan Mei., et al. [46]
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Phase change behavior of PCM is often restricted by the structure of the supporting framework. So it remains a challenge to achieve the best compromise of high latent heat and high thermal conductivity in form-stable PCMs. Mesoporous carbons with high encapsulation capacity as well as high thermal conductivity are needed for composite form-stable PCM with good thermal energy
EP
storage properties.
AC C
Wang et al. [84] reported a facile method to prepare “graphene-like” mesoporous carbons as supporting frameworks of paraffin PCM. 2 g of MgO was dispersed and stirred vigorously in 800 ml water at 94 °C for 2 hours to form nanosheets, and the suspension was cooled down and precipitated. Then the precipitate was transferred into 25 ml deionized water, into which 2.84 g of sucrose was added with continuous stirring. Subsequently, the mixture was dried at 120 °C in an oven overnight. The collected powders were calcined in a tube furnace under a N2 atmosphere for 4 hours. The products were vigorously stirred in excessive 12 wt% hydrochloric acid for 12 hours and washed thoroughly with deionized water to remove Mg(OH)2 templates. After vacuum freeze 26
ACCEPTED MANUSCRIPT drying at -50 °C for 48 hours, about 0.9 g of the final black spongy powder product was obtained. Then, they prepared NDPCM by a blending and impregnating method. Paraffin was dissolved in alcohol in a 25 ml round bottom flask heated at 80 °C. The prepared porous carbon was added and magnetically stirred continuously until the
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solvent evaporated completely, and then the solid black NDPCM were obtained. Carbon-based NDPCM nanostructure can act as a nucleation agent for increasing the interaction between nanoparticles and form-stable PCM with the help of capillary and surface tension forces. This interaction between nanoparticles and
SC
form-stable PCM helps to retain the energy storage capacity of NDPCM [83]. CNT:
[85] tried surface-functionalized CNTs as a nanodispersion
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Meng et al.
medium in fatty acid PCM. Surface functionalization on CNTs was done by the oxidation of the strong concentrated nitric acid. Surface functionalization not only makes the CNTs become shorter in length and increases their porosity but also can effectively remove the impurities absorbed by the CNTs. While surface
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functionalization has increased porosity level on CNT, leading to an increase in absorption and also a major reason for the reduction in the size of CNT. The result suggested that NDPCM as porous CNT-fatty acid has reduced the latent heat, which
PCM matrix.
AC C
xGnP:
EP
might be because of the larger dispersion of nanoparticles in the molecules of the
The porous nature of carbon-based material xGnP; surface area (20.41 m2/g)
has reduced the latent heat of NDPCM up to 40.74%, even for high dispersion of 44 wt% of xGnP in octadecane PCM. The octadecane almost fully filled the pores in xGnP, which has prevented leakage of NDPCM using physical interactions [63] (Figure 8). Figure 8b shows leakage proof in the molten state of Octadecane/xGnP even at 35˚C, which is higher than the melting temperature of octadecane.
27
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ACCEPTED MANUSCRIPT
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Figure 8: Leakage proof a) in solid-state and b) in the molten state of Octadecane/xGnP [63] EG:
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Expanded graphite (EG) has worm-like microstructure with multiple pores at micro-scale. Expandable graphite has an average particle size of 300 µm and an expansion ratio of 200 ml/g. EG has small grain size and large inner porosity, which is capable of absorbing a high amount of LA-MA PCM. LA-MA eutectic mixture melting temperature is about 33.62℃ and latent heat is 164.5J/g, while the melting
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temperature and enthalpy of LA-MA/EG composite PCM are 34.29℃ and 122.8 J/g respectively [77]. X. Tang et al. [86] reviewed that EG with a well-developed pore network structure can reduce the seepage of liquid PCMs through capillary force. J.L. Zeng et al. [45]have experimentally analyzed TD/EG as form-stable NDPCM. They
AC C
EG.
EP
determined PCM was much restrained since the TD was constrained in the pores of
When the pore diameter of porous EG ranges from 2 nm to 100 µm, NDPCM’s
latent heat was reduced up to 26.71% even for 13 wt% dispersion of EG in PCM. The reason may be that the EG consists of overlapped graphite flakes, forming abundant crevice-like and net-like pores. The porosity of EG was 92.78%, and the pore volume was as high as 28.00 ml/g. Owing to the porous structure, the melted PCM can be easily adsorbed and fill the pores or adhered onto the flakes of EG [87]. Though the microstructure of the EG particle exhibits a flattened irregular honeycomb network, under SEM it appears as a worm-like microstructure with 28
ACCEPTED MANUSCRIPT multiple pores constructed from elementary graphite nanosheets, which provides abundant crevice-like and net-like pores for adsorbing organic substances like octadecane PCM. Latent heat of NDPCM was reduced up to 10.58 % even for 9.09% of EG in octadecane PCM. The reduction of latent heat may be due to the irregular
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honeycomb network [76]. Supporting material like expanded perlite (EP) has a mesoporous structure consisting of pores. Porous construction gives it the ability to absorb PCM molecule at a specified amount into the EP. EP has a porous structure having mesopores and
SC
macrospores; Organic compounds can be absorbed easily in these pores. PCM and EP interactions which are physical and not strong because of the weak alkalinity of the
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expanded perlite (EP) [88]. The weak interaction between the PCM and inner surface of the porous material also leads to a depression of the phase change temperatures of PCM in the porous materials. The interference of porous matrices of EP, the restriction of crystal arrangement and orientation of PCM into the pores resulted in the decline of regularities of crystalline regions and the increase of lattice defect. The CNTs dispersed into form-stable PCM can affect the local interactions as well as the
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diffusion and rearrangement of PCM molecules into the composite during melting and freezing processes. The presence of CNT, together with EP produces offensive networks to allow encapsulating PCM with the help of capillary and surface tension
EP
forces. The presence of CNTs together with EP, produces offensive networks to allow eicosane such as encapsulation due to physical interaction. Figure 9 SEM images This
AC C
show how the mesopores of EP and how the CNT interacts with EP.
phenomenon may result from the reduction of void space within the composite PCM and extension of the contact surface area for the composite particles [83]
29
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ACCEPTED MANUSCRIPT
Figure 9: SEM photographs of (a) EP, (b) CNTs and (c) EP/Eicosane/CNTs(1%) composite PCM.[83]
SC
Pores of the Hollow nanotube (HNT) are in the nm range, and surface area is 57.76 m2/g. Therefore, the maximum mass fraction of PCM like capric acid (CA)
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retained in HNT was 60 wt%. CA/HNT can contain capric acid as high as 60 wt% and maintain its original shape perfectly without any leakage of capric acid. When it was heated above the melting point of CA. Due to its large and smooth unhindered pores, GNP was added to the form-stable PCM like CA-HNT as nanodispersion [46]. Silver nanoparticle (Ag) with 130 nm has a porous network structure with
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cylindrical shape; while coated with PAN material turns to 622 nm. Ag-coated PAN nanofibers appeared to have a larger fiber diameter and rougher surface. While fatty acid- Ag coated PAN has reached about 90 % in latent heat enthalpy efficiencies even
EP
for 10 wt% of PAN [78].
Ag-coated PU membranes as supporting materials with different coating times
AC C
(2h, 6h, and 10h) were selected to absorb PCM through the action of capillary absorption, surface tension forces, and nanoconfinement effects. Phase change enthalpies of synthesized NDPCM were slightly reduced from 132.8 kJ/kg to 106.3 kJ/kg, with the increase of Ag coating time, whereas there was no appreciable impact on the phase transition temperatures. This might be due to the increased average fiber diameters, decreased pore diameters, declined specific surface area, and incremental Ag quantities of Ag-coated PU membranes comparing with original PU membranes. This result proved that using micrometer sized metal nanoparticle could decrease the latent heat dramatically [27].
30
ACCEPTED MANUSCRIPT Metal-oxide like aluminum oxide nanoparticle (Al2O3) has a laminar/ rectangular shape, that decreased the latent heat of the NDPCM which may be due to the slight decrease in the eutectic phase change temperatures of composite PCMs because of the weak attractive interaction between LA-MA-SA PCM molecules and inner surface wall of the Al2O3. Latent heat of melting is 113.7 J/g for Form-
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stable/Al2O3, which is 81.3% and 24.1% higher than that of LA-MA-SA/EV and LAMA-SA/aEV, respectively. Similarly, LA-MA-SA/aEV/ Al2O3 has the highest latent heat of freezing. Compared with the LA-MA-SA/EV, the increase of the latent heat of LA-MA-SA/aEV are attributed to acid treatment. The expanded vermiculite with acid
SC
treatment is partially delaminated, corroded, and the Si-OH groups within the expanded vermiculite surface are increasing. Thus, the adsorption capacity of aEV to
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LA-MA-SA is significantly improved compared with EV. Furthermore, the LA-MASA/aEV/Al2O3 has the highest latent heats among composite PCMs. The reason is that the nano-particle size of Al2O3 in aEV/Al2O3 is beneficial to promote LA-MA-SA adsorption due to the high specific surface area. Form-stable PCM maintains the lamellar morphology, suggesting that aEV shows no significant change in morphology
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after loading. [82].
Summary of latent heat of NDPCM:
The reduction in latent heat enthalpies is typical among almost all PCMs,
EP
which, in general, would be attributed to the reduced mass proportion of PCM being embedded with heat transfer enhancement materials. The nanoparticle has no
AC C
contribution to latent heat, as they did not melt within the temperature range of interest.
Fang et al. [89] performed comprehensive measurements on the thermal
storage properties of the prepared NDPCM (Eicosane/GNP) considering the factors including particle size as well as loading level. They obtained graphite nanosheets with different sizes, from the treated NG with the suspensions of N-Methyl-2 pyrrolidone (NMP) using ball milling for 30, 60, and 180 min. The NMP played a vital role in the ball milling process by reducing the energetic penalty for mechanical exfoliation and avoiding graphite nanosheets from re-agglomeration. After that, they 31
ACCEPTED MANUSCRIPT ⸰
filtrated graphite nanosheets repeatedly using DI water, followed by drying at 80 C in a vacuum oven for 48 h. Through AFM images, they determined the thickness of graphite nanosheets to be 5.9, 3.6 and 3.2 nm with the ball milling time for 30 (GNS30), 60 (GNS-60) and 180 (GNS-180) min, respectively. The melting enthalpy of pure eicosane is 249.0 J/g. The incorporation of 3.6 nm and 3.2 nm size nanoparticles led to
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high reduction in latent heat relative to 5.9 nm size and the highest decline of enthalpy around 15% at 5 wt.% for 3.2 nm size nanoparticle, whereas these samples still maintain a latent heat capacity larger than 205 J/g, showing a satisfying latent heat
SC
capacity required for TES systems (Figure 10). Because graphite nanosheets do not undergo the solid–liquid transition in the temperature range evaluated, the enthalpy of composite PCMs is subject to nearly linear drop with the particle content growing.
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Despite no evident increase in enthalpy, these interactions by Lennard-Jones to be enhanced with the growing size of graphite nanosheets, causing smaller drifts in both melting and cooling enthalpies. This study clearly shows the possible reduction for latent heat in NDPCM, is due to the decrease of intermolecular forces and non-melting
AC C
EP
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enthalpy of the nanoparticle.
Figure 10: Latent heat enthalpy vs. particle size [89]
However, Shaikh et al. [69] observed a maximum enhancement of latent heat approximately 13% for the wax/SWCNT composite corresponding to 1% loading of SWCNT. The change in latent heat was modeled by using an approximation for the intermolecular attraction based on the Lennard-Jones potential. Thus, the intermolecular attraction between the molecules of nanoparticles and wax was 32
ACCEPTED MANUSCRIPT considered as the possible reason for the enhancement of latent heat values (Figure
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11)
Figure 11: Latent heat of wax/SWCNT [69]
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Recent studies have demonstrated that the enthalpy of nano-composite PCMs can even be improved due to the Lenard-Jones potential developing between atoms in the crystals. This potential consists of bonded interactions including bond stretching, bending, and torsion as well as non-bonded interactions between CH2 and CH3 sites described by the Lenard-Jones (LJ) potential acting between sites in different
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molecules. According to Lenard-Jones [69,90], intermolecular forces may either be attractive or repulsive. Intermolecular forces can be conveniently divided into two classes: short-range forces, which operate when the centers of the molecules are separated by 3 Å or less and long-range forces, which operate at greater distances.
EP
Generally, if molecules do not tend to interact chemically; the short-range forces between them are repulsive. Long-range forces, or VanderWaals forces, are attractive
AC C
and account for a wide range of physical phenomena, such as friction, surface tension, and adhesion and cohesion of liquids and solids. The chemical potential energy in the weak Vander-Waals attractions is due to strong ionic and covalent bonds. The latent energy will increase while strengthening the weak Vander-walls forces. Effect on thermal conductivity Thermal conductivity is primarily related to the matrix in PCM. Thermal conductivity is associated with collective phonon motions over long distances in a perfect crystal structure and molecular alignment in the matrix of PCM. However, the motion of phonon is slower in the matrix of PCM due to lack of interaction between 33
ACCEPTED MANUSCRIPT atoms in a matrix of PCM and improper alignment of molecules during phase transition like liquid-solid. The improper alignment of molecules leads to an abnormal crystal structure of a lattice of PCM and also leads to delay in the accurate transmission of phase change [91]. It can also lead to a decrease in thermal conductivity due to inappropriate jumping of an atom in the energy level in the matrix
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of PCM. The phonon motion in the matrix of PCM in a particular direction over a long distance can increase the thermal conductivity.
During the melting process, solid-state composite was melted into its liquid
SC
state by increasing the temperature [24]. An increase in temperature accelerates molecular vibrations in the matrix of the orderly solid structure of PCM. Density
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gradient in the liquid state of PCM (due to temperature gradients) generates buoyant forces which may cause convection in the bulk liquid PCM. Besides this convection of liquid, viscous forces tend to slow down the motion [42].
As the viscous force decreases, the thermal conductivity increases in the liquid state in a very negligible amount. However, according to buoyant force, an increase in
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temperature can cause phase transmission due to the pressure variation in a fluid caused by the force of gravity, but, in general, buoyant forces act opposite to the direction of the gravity, buoyant forces cause convection in the bulk liquid PCM. Besides this convection of liquid, viscous forces tend to slow down the motion of the
EP
phonon. This limits the thermal conductivity in the liquid state.
AC C
During the solidification process, the reverse of melting occurs, during which the PCM loses energy to the surrounding, and the atomic or molecules renew their chemical association (i.e., chemical bonds) thus reforming the ordered phase resulting in solidification. PCM forms crystal structures, which increase the area of contact between atoms in the matrix of PCM. According to Gibb free energy, the atom can absorb the energy and gets arranged in an improper mode in the matrix of PCM. The improper alignment of molecules during crystal structure formation tends to decrease the thermal conductivity in solid-state. 34
ACCEPTED MANUSCRIPT The addition of foreign particle in nano-meter or micro-meter size in the matrix of PCM has a potentially positive effect on thermal conductivity due to the ordering of the atomic structure of the solid-liquid or liquid-solid interface [91]. Figure 12 depicts the envisaged movement of phonon in PCM and NDPCM. Dispersion of
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SC
dispersing nano-sized particles in the PCM.
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micro-sized particles causes sedimentation, and this problem can be resolved by
(b)
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(a)
Figure 12: a. Movement of phonon before the dispersion of nanoparticle b. Movement of phonon dispersion after the dispersion of nanoparticle
EP
Dispersion of nano-sized particles can enhance the thermal conductivity of base PCM significantly by reducing the void space in the matrix of PCM. The linear
AC C
change in the thermal conductivity depends on the mass of immersion in a matrix of PCM [83].
The reason behind Thermal conductivity enhancement in NDPCM Thermal conductivity enhancement becomes more pronounced, probably due to more intensive diffusion of nanoparticles like Brownian motion [24], Clustering of nanoparticles [92], Interfacial thermal resistance [91], high aspect ratio [93], purity [32], intermolecular interaction [42] and concentration [94] of nanoparticles. In NDPCM, thermal conductivity enhancement was higher in phase transmission temperature of base PCM, because kinetic energy remains constant; however, phase 35
ACCEPTED MANUSCRIPT change can occur [48]. Different NDPCM thermal conductivity models developed or
AC C
EP
TE D
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SC
RI PT
used in the literature are summarized in Table 8.
36
ACCEPTED MANUSCRIPT
Table 8: Various models for thermal conductivity of NDPCM
− /62 : − /62
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Remarks /234 ; = /<=>?@ + A />?BC?@ − /<=>?@ D E ;
0, ; < ;J − K; ;J − K; ≤ ; < ;J − K; E ; = F ; − ;J + K; ⁄ 2K; , 1, ; > ;J + K;
/011
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Eicosane/ Graphite [94] Eicosane/ Graphene [66]
E] ; \ _ ;, 9 234 Q62 ^62
R01, = 5 × 10U VW X9 YQ3Z [
3 + 9[2Vkk 1 − lkk + Vmm 1 − lmm ] = /234 5 : 3 − 9[2Vkk lkk + Vmm lmm ]
EP
Nan et al. [98,99]
Organic PCM/ Graphene [97]
AC C
Maxwell [95]
9OP Q234 9OP Q234 + 1 − 9OP Q62
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9=
SC
Model Model Formulation developed modified by and used by Maxwell Eicosane/ /62 + 2/ − 29 / /011 = /234 5 [95] Ag [48] /62 + 2/ +9 / Eicosane/ CuO [44] Octadecane / Cu [96]
; ;h01 + −3.0669 × 10cd 9 − 3.91123 × 10cg
_ ;, 9 = 2.8217 × 10cd 9 + 3.917 × 10cg
(taken into account for the Brownian motion in molten NDPCM) lkk = d
op op cq
+ d
lmm = 1 − 2lkk Vkk =
Vmm =
o tuv cq w qcop r/p
R1,kk − R234
R234 + lkk AR1,kk − R234 D R1,mm − R234
R234 + lmm AR1,mm − R234 D
37
ACCEPTED MANUSCRIPT
Koo et al. [104]
PCM/ Al2O3 Numerical Simulation
[105,106]
/011 =
5R< + ˆ ‰Š
qc‹
R< + 5 Š U g
qc‹ gŒ
−
gŒ
VW = •
U
0, 1,
†•€•
‡
• AR1 − R< D: ŽR< +
1 − • + ˆŠ
/62 + 2/ /011 = /234 5 /62 + 2/ +
5 × 10
gŒ
qc‹
…
qc‹ gŒ
gŒ
AR1 − R< D •
For nanoparticle with high porosity and high thermal conductivity.
− 1 − • : AR1 − R< D
− 29 / +9 /
‘ “ VW X9 YQ3Z [ Š ’ _ 234 ”~• @~•
; < ;–h00m?y— ; > ;40>P?y—
qc‹
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Organic PCM/ Foam [103]
|o }~• ⁄}•€• g o‚ pƒ„ × †~•
;, 9
− /62 : − /62
SC
Mesalhy et al. [102]
/011 = R234 {1 +
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Eicosane/ xGnP [101]
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Yu et al. [100]
^62 2x011 + ^62 //1,?ycz>oy0 wW = xW R234 R1,kk =
Numerical simulation of solidification of NDPCM for latent heat thermal energy storage at 0oC. [107–109] {Solidification of water/CuO} W˜™™
Wšƒ›˜
=1−3
†~• ‚q•| †šƒ›˜ †~• † •|‚œ ~• ‚d• œqc †šƒ›˜ †šƒ›˜
¡ A^ , ;, 9 D 6z
=
œc
+ 5 × 10U 9Qžo<0 Ÿz _Š
Wšƒ›˜ “
”~• @~Z
¡ A^ , ;, 9 D 6z
[110]
d
ŽYwq + wd lnA^6z D + wg ln 9 + wU ln 9 lnA^6z D + w¢ lnA^6z D [ ln T + d
Yw£ + w¤ lnA^6z D + w¥ ln 9 + w¦ ln 9 lnA^6z D + wq] lnA^6z D [• {Ÿu¨^©ª©u¨ 9 ≤ 0.04, 300R ≤ ; ≤ 325/} { wq … … wq] = Ÿu − ®__©t©®¨ª ¯w°±® u_ ²wª®³/Ÿ±´ } [108,110]
AC C
EP
Laser flash apparatus used to measure the thermal conductivity Parker et Octadecane R = µ × Ÿ2 × Q al. [111] / GA [71] /234 = Thermal conductivity of PCM ² ⁄¹R ; /62 = Thermal conductivity of nanoparticle ² ⁄¹R ; /žo<0 = Thermal conductivity of base Matrix ² ⁄¹R ; 9 = Volume fraction of the additives; 9OP = weight fraction of the additives; Q = density of the discrete / ⁄¹g ; /011 = ½__®tª©¯® ªhermal conductivity of NDPCM ² ⁄¹R ; ÀŸÁ = Àℎwv® Ÿℎw¨ ® Áwª®³©w° ; ÃÀ = Ãw¨uÄw³ª©t°® ; /<=>?@ & />?BC?@ = ;ℎ®³¹w° tu¨^±tª©¯©ªÆ u_ ÀŸÁ ©¨ Çu°©^ & l©È±©^ Àℎwv® ² ⁄¹R ; ;J & K; = Melting temperature and transition range of PCM / ; VW = ½¹Ä©³©tw° _±¨tª©u¨ u_ E³uɨ©w¨ ¹uª©u¨; X = correction factor for brownian motion; ^ = Ê©w¹®ª®³; Ÿ2 = Specific heat capacity /Ì⁄/ R ; E] = Eu°ªÍ¹w¨¨ tu¨vªw¨ª 1.381 × 10cdg Ì⁄R ; ;h01 = x®_®³®¨t® ª®¹Ä®³w±³® / ; ; = ;®¹Ä®³w±³® / ; w = Aspect ratio of NP; R1,kk = Modified in − plane thermal conductivity of NP ² ⁄¹R ; R1,mm = Out − of − plane thermal conductivity of NP ² ⁄¹R ; x011 = Effective interfacial thermal resistance; R1,?ycz>oy0 = In − plane thermal conductivity of NP ² ⁄¹R ; R< = ;ℎ®³¹w° tu¨^±tª©¯©ªÆ u_ ÀŸÁ ©¨ ªℎ® Äu³u±v ¹®^©w ¹wª³©Ò ² ⁄¹R ; • = The metal foam porosity; R1 = Thermal conductivity of the metal foam ² ⁄¹R ; µ = thermal diffusivity ¹d ⁄v .
38
ACCEPTED MANUSCRIPT a) Concentration of nanoparticles: In NDPCM, thermal conductivity enhancement is generally based on the dispersion of mass fraction of nanoparticles in PCM and operating temperature. The rise in thermal conductivity is observed when the mass fraction of the nanoparticles
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increases in all measured temperatures up to the 2 wt% loading for all kind of nanoparticles in the matrix of PCM [48]. b) Brownian motion:
SC
The random movement of the particles is the stochastic Brownian motion, caused by the impact of the base fluid molecules on the surface of the particles. This
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was discovered by Robert Brown in 1828 and physically described in 1956, by Einstein. According to the kinetic theory of matter, the random motion of each particle is independent and eternal, being composed of translation and rotation movement. Brownian motion is more active in liquids with lower viscosity, for higher temperatures, and the smaller dispersed particle [112–114]
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Brownian motion increases the thermal conductivity in nanofluids due to the following two effects: (1) collisions between Brownian particles, and (2) Nanoscale convection induced by the Brownian particles [112].
EP
Despite the interactions, unless there is some attraction between the particles, the Brownian interparticle collisions could never occur and lead to aggregation.
AC C
Derjaguin [115], Verway [116], Landau and Overbeek (DVLO) developed a theory suggesting that aggregation is determined by the sum of the van der Waals attractive and repulsive forces between the approaching Brownian particles. According to DLVO theory, if the attractive forces are larger than the repulsive forces, the particles will collide and aggregate. On the other hand, if the attractive forces are lower than the repulsive forces, the particles may collide (or not), but without a permanently fixed contact [112].
39
ACCEPTED MANUSCRIPT However, according to Xuan et al. [117], The aggregation of the nanoparticles may also produce a negative impact on the enhancement of the thermal conductivity since the Brownian motion will be slower. According to Choi et al. [118], when the temperature is increased, the viscosity
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of base fluids is decreased, the Brownian motion of nanoparticles is increased, and consequently, convection like effects are remarkably increased, resulting in increased conductivities. These temperature-dependent conductivities are not predicted by Maxwell theory with motionless nanoparticles.
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The effective thermal conductivity of the NDPCM includes the effects of particle size, particle volume fraction, and temperature dependence as well as
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properties of the base PCM and the effects due to Brownian motion. Vajjha et al. [119] has proposed a model for the effective thermal conductivity of nanofluid as /011 = R
while the second term corresponds to Brownian motion with a correction factor. The correction factor takes into account for the Brownian motion in molten PCM and not
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in the solidified PCM. Valan Arasu et al. [105,106] used the same model in the numerical studies of nanoparticle dispersed paraffin wax PCM. Abdollahzadeh et al. [120] have numerically analysed the effects of Brownian
EP
motion on freezing of PCM containing nanoparticles; they made the following assumptions for generating the governing equation: (a) the NDPCM is an
AC C
incompressible Newtonian fluid. (b) thermos physical properties of the NDPCM are constant; (c) buoyancy force is modeled with Boussinesq approximation; (d) base PCM and the solid nanoparticles are in thermal equilibrium.
They determined,
Brownian velocity as a function of the temperature of the liquid phase only, and it becomes slower near the solidification front whose the temperature is low. The averaged motion of the particles is an interesting consequence of the Brownian movement of particles in fluids with an externally sustained and constant temperature gradient: It becomes apparent that particle dispersion is higher and the Brownian force is stronger when the local fluid temperature is higher. When there is a 40
ACCEPTED MANUSCRIPT temperature gradient in the flow domain of the suspension, small particles disperse faster in hotter regions and slower in colder regions. It must be noted that the particles in suspension will not fully accumulate in the colder region. Interparticle collisions in the colder regions, where the particle concentration becomes higher, would disperse the particles stronger than in the hotter regions, where the particle concentration is
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lower. Thus, in the absence of other dispersion mechanisms – such as turbulence, velocity fluctuations, shear forces, lift forces, etc. A dynamic equilibrium for the particle concentration will be established, with lower particle concentrations in the
SC
hotter regions and higher concentrations in the colder regions [121,122].
Lamas [112] suggested, based on the review of numerical study results, that the
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Brownian motion intensity has a probably negligible impact on the interparticle collisions since the average collisions and standard deviation seems to be independent of the base fluid viscosity and temperature. In addition, the results suggested that the interparticle interaction and spatial distribution depends on the aspect ratio of the nanoparticles. For a given concentration, the average distance between particles is reduced for elongated particles, increasing the influence produced by the displacement
TE D
of the fluid. Interparticle interactions and spatial distribution increases with increasing nanoparticles aspect ratios [123].
Increase in temperature, increases the interparticle collision by Brownian diffusion
EP
of the particle (When two-particle collide, the solid-solid heat transfer mode could increase the effective thermal conductivity of the Nanofluid). The high surface-to-
AC C
volume ratio of the nanoparticles induces strong van der Waals interparticle interactions. These interactions, associated with the Brownian movement, cause the aggregation of the nanoparticles, which begin to behave as micrometer particles. This will result in the settlement and clogging of the channels, but also in the decreasing of the thermal conductivity of the Nanofluid [124]. c) Clustering of nanoparticles: In NDPCM’s solid phase, a cluster of nanoparticles has enhanced thermal conductivity due to the aggregation of nanoparticles by physical interaction like 41
ACCEPTED MANUSCRIPT adhesion forces between the neighbouring nanoparticles in the matrix of NDPCM. When the clusters are formed, thermal conductivity can be enhanced even in the solidstate due to the aggregation. In NDPCM, clustering of nanoparticles is the main factor for larger enhancement of thermal conductivity in solid-state than in the liquid state. Then clustering holds the key to the thermal conductivity enhancement of NDPCM
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[24].
However, clusters are formed by a collection of atoms up to about 50 atoms. The cluster of nanoparticles has a high possibility of sedimentation of nanoparticles in
concentration of nanomaterial in NDPCM [92].
SC
NDPCM. But, desirable stability can be achieved for a relatively low mass
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In NDPCM, thermal conductivity enhancement was of monotonic order for the mass between 1 wt% and 2 wt% of nanomaterial. (Monotonic is a standard deviation relation between the mass of nanomaterial and PCM molecule in the matrix of NDPCM; the relation was non-linear with either entirely never increases (or) never decreases). However, when the mass of nanomaterial is less than 1 wt% in NDPCM,
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the thermal conductivity enhancement is dependent on operating temperature. The source of the sharp rise in the measurements near the melting temperature is attributed to the non-equilibrium state of the samples near the phase transition. With nanoparticle loadings of 1 and 2 wt%, the thermal conductivity was monotonically
EP
raised as the concentration of the additives was increased for all the measured
AC C
temperatures [44].
In NDPCM, thermal conductivity enhancement was of a non-monotonic order for the mass-less than 1 wt% of nanomaterial immersion in NDPCM and also nonmonotonic for the mass greater than 2 wt% of nanomaterial immersion in the matrix of PCM. (Non-monotonic is a standard deviation relation between the mass of nanomaterial and PCM molecule in the matrix of PCM, which linearly increases or decreases). Non-monotonic relation between the thermal conductivity and the mass fraction that was independent of the temperature range studied, was exhibited when the mass fraction was greater than 2%. For a given particle loading, the measured thermal conductivity values were generally insensitive to the measurement 42
ACCEPTED MANUSCRIPT temperature. [44]. In NDPCM, for the highest loading of the nanoparticles being 10 wt%, the measured thermal conductivity values were the greatest quantities recorded
AC C
EP
TE D
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SC
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at each measurement temperature [44].
43
ACCEPTED MANUSCRIPT
Mass greater than 10 wt% of nanomaterial immersion in PCM has a high possibility for formation of sedimentation of nanoparticles and also the highest loading of nanomaterial in PCM exhibits non-proper crystal structural formation of solidification
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process [92]. Table 9 condenses the thermal conductivity of NDPCM in the dispersion of metal, metal- oxide nanoparticle and carbon nanostructure
Reference PCM
Melting Temp (˚C)
Material
Octadecane
28
TiO2
0.5
0.17
0.79
Eicosane
28 28 36.18 36.18 36.18
TiO2 TiO2 CuO CuO CuO
1 2 1 2 3.5
0.17 0.17 0.58 0.58 0.58
0.56 0.83 0.61 0.64 0.65
CNT CNT CNT xGnP xGnP GA GA GNP
0.05 0.1 0.15 0.1 0.45 6.25 9.09 1
0.15 0.15 0.15 0.15 0.15 0.23 0.23 0.13
0.15 0.15 0.16 0.16 0.19 4.61 5.92 0.37
Octadecane
Sivasankaran., et al.[32]
Octadecane
Jing Yang., et al. [62]
Octadecane
Min Li., et al. [41]
RT27
27 27 27 27 27 28 28 28.81
EP
Carbon nano-material Sivasankaran., et al.[32]
AC C
Mahdi Nabil., et al. [44]
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Metal-Oxide nano-particle
SadeghMotahar., et al. [42]
Thermal conductivity (W/mK) wt.% Pure NDPCM of Con PCM
Nano- Particle
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Base PCM
SC
Table 9: Thermal conductivity of NDPCM
Analysis Method
Nano-particle
Size
Shape
25nm Spherical
HD Dia 5-15 nm
dia 0.7−2 nm
Rod
think 6−10 nm; dia 25 µm
*
LFA
3nm
Flake
HD
dia 35 nm
*
HW
44
ACCEPTED MANUSCRIPT
Seulgi Yu., et al. [40]
Biobased
Seulgi Yu., et al. [40]
Biobased
Yongpeng Xia., et al. [71]
Octadecane
28.91 28.91 29.4 29.4 29.4 29.4
xGnP xGnP CNT CNT xGnP xGnP
3 5 1 3 1 3
0.50 0.50 0.15 0.15 0.15 0.15
0.87 1.00 0.41 0.49 0.27 0.61
31.45
GA
1.63
0.15
0.26
31.45 31.45 36
GA GA CNT
1.96 2.43 1
0.15 0.15 0.41
TCi
RI PT
Octadecane
SC
Jisoo Jeon., et al. [31]
0.39 0.43 0.57
THB
think 10 nm; dia 15 µm
*
100 nm
Hallow tube
think 10 nm; dia 15 µm
Hexagonal
*
Flake
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d) Aspect ratio:
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Nitesh Das., et al. [93] Eicosane dia 1.35nm * * Mikali Temirel., et al. think 15 nm; dia Eicosane 36.4 xGnP 1.57 0.41 1.05 * HW [101] 25 µm * Not reported HW= Hot-Wire., LFA= Laser flash apparatus., HD= Hot Disk., TCi= TCi Thermal Conductivity analyser., THB= Transient Hot Bridge
EP
In NDPCM, thermal conductivity enhancement depends on aspect ratio, because the aspect ratio is the ratio of
AC C
Length/Diameter (or) Length/Thickness, which mainly depends on particle size. Thermal conductivity enhancement of PCM is higher for smaller diameter of nanoparticle than the larger diameter of nanoparticle since small diameter nanoparticle can form a chain-like structure due to higher interaction forces [125].
45
ACCEPTED MANUSCRIPT Effect of Metal nanoparticle and Metal oxide nanoparticle: Metal-based Nanoparticles are synthesized from metals to nanometric sizes either by destructive or constructive methods. The nanoparticles have distinctive properties such as sizes as low as 10 to 100 nm and shapes like spherical and cylindrical. They have characteristics like the high surface area to volume ratio and
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surface charge density.
Metal nanoparticle like silver (Ag) nanoparticle with the size of 10–30 nm diameter and length of 5–15 µm appears to be spherical shaped and does not have a
SC
higher impact on thermal conductivity on NDPCM. The thermal conductivity on NDPCM was increased only by 33.33 % for 2 wt.% of Ag nanoparticle in organic
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PCM but has shown contrast result on thermal conductivity of NDPCM for 10 wt.% of Ag nanoparticle which shows that there is no strong interaction between Ag particles and Eicosane [37].
Metal oxide-based nanoparticles are synthesized to modify the properties of their respective metal-based nanoparticles. These nanoparticles have possessed excellent
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property when compared to their metal counterpart.
In NDPCM, metal oxide nanoparticle like titanium dioxide nanoparticles (TiO2) of size 25 nm with spherical shape was dispersed in the matrix of PCM; the thermal
EP
conductivity of eicosane enhanced from 0.17 W/mK - 0.56 W/mK when analyzed with the hot disk. Generally, TiO2 has the property of sensational dispersion, chemical
AC C
stability, and non-toxicity. However, the spherical shape and property like sensational dispersion have great attraction between nanoparticles and the PCM molecule in NDPCM [42].
Copper oxide nanoparticles (CuO) of size 5- 15 nm with spherical shape was dispersed in the matrix of eicosane PCM [44]. The values of the thermal conductivity increases as the nanoparticle loading is raised up to 5 wt%, while the thermal conductivity of the composite decreased when the loading is increased to 6.5 wt.%, probably due to more intensive diffusion of nanoparticles, e.g., Brownian and thermophoretic diffusion, at higher temperatures. Also, the discrepancy could result 46
ACCEPTED MANUSCRIPT from the neglected temperature dependence on the thermal physical properties, effects of diffusion of nanoparticles in the liquid, and volume change upon freezing. The pronounced void formation was also observed in the solid layer upon freezing. Effect of Carbon nanostructural material:
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In NDPCM, Thermal conductivity is enhanced to a higher range due to the presence of CNT. CNT has the thermal conductivity of (3500 W/mK), and CNT is a low-density material with the high surface area (50 - 1315 m2/g). High surface area helps to increase a strong interaction between CNT and PCM molecule in the matrix
SC
of NDPCM. CNT has high aspect ratios, large specific surface area, and substantial Van der Waals attractions and these carbon nanotubes tend to self-aggregate into spontaneously.
Flexibilities
increase
the
possibility of
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bundles
nano-tubes
entanglement and close packing [30].
Babaei et al. [91] experimentally studied CNT of diameter 100 nm and hollow tube shape dispersed in bio-based PCM. The thermal conductivity of NDPCM increased from 0.15W/mK – 0.41 W/mK even for 1 wt.% of CNT in bio-based PCM.
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The hollow tube structure was able to hold the arrangement of PCM molecule in the matrix of NDPCM. Therefore, the CNT provides a template for crystallization leading the PCM molecules to get aligned with the nanotube, which in turn leads to an
EP
increase in thermal conductivity in a greater range. The template helps to improve the phonon motion in a particular direction over a long distance. The phonon motion in
AC C
the matrix of PCM in a particular direction over a long distance can increase the thermal conductivity [91]. In NDPCM, nano graphite (GNS), 60nm and planar shape were dispersed in
organic PCM. Thermal conductivity of NDPCM increased from 0.1725 W/mK 0.2631 W/mK even for one wt. % of GNS. Two-dimensional planar structure leading to reduced geometric contribution of phonon mismatch-induced phonon scattering at the filler-matrix interfaces is responsible for the low thermal interface resistance of GNS [70].
47
ACCEPTED MANUSCRIPT The porous nano-sized carbon-based material like exfoliated graphite nanoplatelets (xGnP), 10 nm thick and 15 µm in diameter having hexagonal nature were dispersed. Thermal conductivity of NDPCM increased from 0.15 W/mK - 0.27 W/mK even for 1 wt. % of sample [31,40]. Exfoliated graphite sheets were in a
the high specific surface area (SSA) of xGnP.
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combined layer structure. Thermal conductivity enhancement of PCM might be due to
Liu et al. [59] used graphene oxide (GO) before it reduces to graphene. The particle size of GO ranges between 500 nm to 2.5 µm, and the thickness of GO is
SC
about 1.318 nm and appeared to be hexagonal shaped. Thermal conductivity of NDPCM increased from 0.70 W/mK – 1.05 W/mK even for 2 wt. % of GO. Due to a
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large number of oxygen-containing functional groups on the surface of GO, which made GO nanosheets compatible well with PCM; the large two-dimensional sheet structure of GO can block migration of phonon transfer resulting in reduced thermal conductivity of the PCM electric charges in NDPCM composite, resulting in a smaller electrostatic interaction.
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Putra et al. [58] experimented with directly obtained graphene of size ranging from 21.5 to 65.5 nm with the width 3–5.5 nm in the form of platelets/rolled-thin sheets dispersed in RT-22. Thermal conductivity enhancement of 306 % even for 0.05
EP
wt.% graphene was obtained.
The thermal conductivity of NDPCM increased dramatically to the higher
AC C
range due to the presence of GNP. GNP had high thermal conductivity (3000–5000 W/mK) and low-density carbon nanostructure. Plate like GNP structure helps to form a proper molecular alignment in NDPCM. Proper molecular alignment leads to proper phonon motion in the matrix of PCM in a particular direction over a long distance, thus increasing thermal conductivity (Figure 13) of NDPCM. In NDPCM, thermal conductivity enhancement by GNP is due to high interfacial thermal resistance [58].
48
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ACCEPTED MANUSCRIPT
Figure 13: Thermal conductivity of RT-22 with graphene nanoparticles [58]
Li et al. [72] synthesized surface-functionalized graphene by annealing process and dispersed it in decosane PCM. The thermal conductivity of NDPCM increased
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from 0.26 to 0.59 W/mK in the presence of spongy graphene of 3 mg/cm3, i.e., more than 2 times in such a low graphene concentration. Dispersion of Supporting material:
EP
Supporting material has high porosity and low thermal conductivity. The pores lead to an increase in the absorption of PCM molecule and thus prevents the leakage
AC C
of PCM. But, the thermal conductivity of PCM with supporting material decreases due to the low thermal conductivity of the supporting material and lack of photon transmission in the matrix of form-stable PCM. However, thermal conductivity can be enhanced in form-stable PCM, by immersion of nanoparticles. Nanoparticles increase photon transmission, hence the thermal conductivity in form- stable NDPCM. Table 10 presents the measured values of the thermal conductivity of PCM with supporting material and nanoparticles.
49
ACCEPTED MANUSCRIPT
Table 10: Thermal conductivity of form-stable NDPCM Supporting material
Reference
wt.% wt.% Material of Con of Con
LA-MA-PA LA-MA-PA LA-MA-PA
CNT CNT CNT
10 20 30
-
Octadecane
28
xGnP
5
-
Octadecane
28
xGnP
44.1
-
Octadecane
28
xGnP
1
-
Octadecane
28
xGnP
3
Tetradecanol
35.87
EG
Tetradecanol
35.87
RT25 Octadecane
0.41 0.63 0.64
-
0.83
2.12
-
0.26
1.36
-
0.28
0.91
-
-
0.28
0.98
10
-
-
0.42
3.22
EG-T
10
-
-
0.42
3.11
27
LDPE
50
EG
5
0.26
0.42
28
Na-MMT
*
xGnP
5
0.83
2.12
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* * *
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PatrikSobolciak., et al. [43] Su-Gwang., et al. [80] Sayanthan Rama., et al. [126] Dandan Mei., et al. [46] Haiting Wei., et al. [82] Ahmet S., et al. [88]
Analysis Method
NDPCM
EP
Ju-Lan Zeng., et al. [45]
-
Pure
SC
Material
Nanoparticle
Size
Shape
HD
dia 8-15nm & length 50µm
Rod
TCi
Surface area (m2/g) 20.41 Flake shape
DRX
300 µm *
DICO
LDPE 200 µm
TCi
Surface area (m2/g) 20.41 dia 1-2 µm; thick 1-5 nm
* Flake shape
RT 27
28.81
EP
49.5
xGnP
1
0.34
0.51
CA
29.62
AC C
Su-Gwang Jeong., et al. [80] Sughwan Kim., et al. [63] Seunghwan Wi., et al. [56]
Thermal conductivity (W/mK)
Nanoparticle
Melting Temp (˚C) 21 21 21
PCM Xin Men., et al. [85]
Support Material
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Base PCM Material
HNT
35
GNP
5
0.48
0.76
HW
*
*
LA-MA-SA
30.3
aEV
30
Al2O3
8.4
0.26
0.67
*
*
Laminar
CA
32.14
EP
55
EG
10
0.09
0.14
HW
*
*
Cylindrical
50
ACCEPTED MANUSCRIPT
Methyl stearate
32.5
App
5
EG
15
0.83
3.14
Ali Karaipekli., et al. [83]
Eicosane
36.18
EP
40
CNT
0.3
0.22
0.19
Eicosane Eicosane
36.18 36.18
EP EP
40 40
CNT CNT
0.5 1
0.22 0.22
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Hassina., et al. [127]
0.24 0.32
KD2
3-4 µm
Flake
O.D. × L 69 nm × 5 µm
Rod
SC
* Not reported HW= Hot-Wire., DRX = DRX-i-RX Hot-Wire., LFA= Laser flash apparatus., HD= Hot Disk., TCi= TCi Thermal Conductivity analyzer
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CNT
Meng et al. [85] analyzed surface-functionalized CNTs as nanodispersion medium in fatty acid PCM. Surface functionalization of CNTs was done by the oxidation of strong concentrated nitric acid. By oxidation, since CNT has a multi-porous
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structure; fatty acids were successfully absorbed into the porous structure of the CNTs. Figure 14 shows SEM images of porosity of CNT on treating and how it reacts with the fatty acid. However, scattering of phonon is possible by acid treatment and thermal
AC C
composite fatty acids/CNTs.
EP
conductive net-work formed between the fatty acids and the CNTs will lead to a greater enhancement in thermal conductivity of the
51
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ACCEPTED MANUSCRIPT
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Figure 14 : SEM image of CNTs a) untreated CNTs, b) treated CNTs and the fatty acids/CNTs composite with various weight percentages of the fatty acids, c) 50 wt% fatty acids and d) 60 wt% fatty acids [85]
xGnP
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High SSA (specific surface area) in xGnP is due to exfoliation of graphite interaction compound (GIC). During the exfoliation of GIC, a large amount of expansion increases its size which is due to breaking of the balloon wall (formed by sliding the carbon layer relative to one another; sliding enables the balloon wall). High
EP
SSA in xGnP leads to high porosity, high porosity with high thermal conductivity help to increase the absorption force between the PCM molecule and xGnP in the NDPCM
AC C
[63,65]. The strong interaction with higher thermal conductivity helps to enhance thermal conductivity in PCM. EG:
EG, have worm-like microstructure, small grain size, and large inner porosity, which is capable of absorbing a high amount of PCM molecule. Higher the EG porosity will result in higher absorption capability. The large surface area due to rapid heating at high temperature in a muffle furnace expanded the graphite. Large surface area and pores in the EG help to increase the absorbance. The pore size of the EG
52
ACCEPTED MANUSCRIPT network is at micro-scale. A large amount of well-developed pore network structure of raw worm-like EG can act as the carrier of phonon in the PCM molecule by physical interaction [86]. J.L. Zeng., et al. [45] have analyzed surface modified EG by concentrated nitric
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acid; the surface volume has increased from 200 ml/g to 0.2 ml/g. Thermal conductivity of NDPCM has increased from 0.42 W/mK to 2.76 W/mK even for 7 wt.% of EG. This result suggests that the increased surface volume has more possibility in increasing the thermal conductivity.
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Supporting material with low thermal conductivity has high porosity. The pores were mesoporous. Thermal conductivity decreases due to low thermal conductivity in
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the supporting material and lack of phonon transmission in the matrix of form-stable PCM. Thermal conductivity can be enhanced in form-stable PCM, by immersion of nanoparticles. Higher thermal conductivity particles like nanoparticle will increase the connectivity of the structure. Nanoparticle like carbon-based nanostructure is added to form-stable PCM molecule in NDPCM. These nanoparticles increased the phonon
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transmission in particular direction in NDPCM during the phase change process [78], and hence, the thermal conductivity of NDPCM is enhanced in form- stable PCM. Karaipekli et al. [83] tried EP as supporting material which has a mesoporous
EP
structure consisting of pores and holes with a diameter below 100 µm. The presence of CNTs together with EP produces networks like encapsulation. The EP was
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encapsulated (C20) in the PCM with the help of capillary and surface tension forces to form-stable PCM. The CNT with dia 6-9 nm and rod shaped, which was used as nanoparticle dispersion in EP/Eicosane form-stable PCM. The uniform dispersion of CNT additives in the composites played a beneficial role in enhancing the heat transfer performance of EP/Eicosane composite. Moreover, the thermal conductivity of EP/C20/CNTs (0.3 wt%) decreased, but the thermal conductivity of EP/C20/CNTs (0.5 wt%) was increased. This phenomenon may be resulted from the reduction of void space within the composite PCM and increased contact surface area for the composite particles.
53
ACCEPTED MANUSCRIPT Jeong et al. [80] have analyzed with octadecane as PCM encapsulated in NaMMT as supporting material and xGnP as a nanoparticle. PCMs molecules were retained easily in the pores of xGnP. PCMs could integrate into the structure of xGnP and Na-MMT due to their physical bonding without changing their chemical properties. xGnP is more porous than Na-MMT powder so that more PCM could be
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incorporated into the porous structure of xGnP. Thermal conductivity of form-stable PCM octadecane/Na-MMT/xGnP increased from 0.83 W/mK to 2.12 even for 5 wt.% of xGnP.
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Summary of thermal conductivity of NDPCM:
In general, the thermal conductivity of suspended nanometer-sized particles of
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PCM is higher than the pure PCM. Moreover, for high aspect ratio materials (CNT and GnP), in the presence of interfacial thermal resistance, the effective thermal conductivity enhancement of the nanocomposite becomes insensitive to the
nanomaterial thermal conductivity when the ratio R6z ⁄R234 > 1000. For low aspect
ratio materials like metal nanoparticle and spherical nanoparticles, the thermal conductivity ratio of nanoparticle and the PCM is an order of lesser magnitude
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(R6z ⁄R234 > 1000) [128].
In NDPCM, smaller sized NP behaved extremely well in improving the
EP
thermal conductivity when they were dispersed into the base PCM. It is noteworthy that the interparticle repulsive forces and the Brownian motion exhibited by NP were
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appreciable, which in turn was evident from the improved thermal conductivity. However, the researchers have experienced contrast result in a case for smaller nanosheets that possess higher specific surface area; phonons are easily scattered at the filler/PCM interfaces. As a consequence, the thermal conductivity of sample containing 5 wt.% of GNS-30 (5.9nm) is observed to be 1.57 W/m K, which is 258% higher than the baseline of eicosane, while for GNS-60 (3.6 nm) and GNS-180 (3.2 nm) at the same concentration, the enhancement ratios are 180% and 109%, respectively (Figure 15). The researcher has suggested that thermal conductivity on
54
ACCEPTED MANUSCRIPT NDPCM does not depend on the smaller size of the nanoparticle, but depends on the
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kind of heterogeneous nucleation [89].
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Figure 15: Thermal conductivity vs. wt.% of GNS [89] Nabil et al. [44] observed effective thermal conductivity using CuO with dia 515 nm and spherical shape as nanodispersion in eicosane PCM and measured experimentally by using the transient plane source technique. The effective thermal conductivity was analyzed by subjecting the NDPCM to one of the three solidification
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procedures, i.e., ambient solidification, ice-water bath solidification, or oven solidification method (Figure 16). In NDPCM, an increase in thermal conductivity depends on the results which suggest that at a constant concentration of particles if the
EP
solidification process takes place over a longer period, the thermal conductivity values increase. The hypothesis is that longer durations of phase change leads to the
[44].
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formation of bigger micron-scale grains in the structure of solid NDPCM composites
.
55
ACCEPTED MANUSCRIPT Figure 16: Thermal conductivity vs temperature [44] Harish et al. [32] Prepared NDPCM by using directly obtained CNTs of size ranging between 0.7-2 nm and rod shape dispersed in octadecane. They suggested that higher thermal conductivity enhancement is observed in the solid-state which is possibly attributed due to the formation of continuous networking structure by
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collective phonon motions over long distances between the phase transitions (Figure 17).
In the liquid phase, while the alignment parameter of NDPCM matrix has a
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relatively low value than solid-state, the molecules at and near the nanoparticle exhibit much stronger alignment even though the interfacial effect was limited to 1 nm. While
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in solid-state, the molecules are at closest distance from the surface of the NP, but
EP
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remains elevated to the bulk value even 2–3 nm away from the NP surface [32].
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Figure 17: Thermal conductivity of NDCPM during the phase change from solid to liquid state [32]. Fang et al. [66] used directly obtained graphene in the thickness range of 4−20
nm and planar shape as nanodispersion in eicosane by using sonication process. Thermal conductivity has enhanced only 152.8 % even for 2 wt.% graphene. The reason behind the lower enhancement in thermal conductivity may be because the diameter of graphene is 5-10 µm and due to intensive sonication for 30 min. However, [58] it was suggested that thermal conductivity enhancement while using graphene as nanodispersion is vigorous due to sonication for 3 hours. Hence, this result suggested
56
ACCEPTED MANUSCRIPT that decreasing the sonication time also had more possibility of decreasing the thermal conductivity of NDPCM [66]. Zhang et al. [129] analyzed octadecane PCM encapsulated with cement as supporting material and EG as a nanoparticle. EG was 500 µm, and microstructure of
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the EG particle exhibits a flattened irregular honeycomb network constructed from elementary graphite nanosheets, which provides abundant crevice-like and net-like pores for adsorbing organic substances. The result shows contrast due to the micro size of high thermal conductivity particle. However, the result suggested that the
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micro size of the particle has more possibility for sedimentation and decreases the thermal conductivity [129].
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Overall summary:
Thermal storage system performance is mainly dependent on the thermal properties of the PCM. Dispersing nanoparticle to improve the thermal conductivity and hence the heat transfer rate of the storage system is the current trend, although it has positive or negative effects on the thermal storage capacity of NDPCM. The
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essential points obtained from the present review are summarised below: • Immersion of carbon-based nanomaterial shows better performance than metal or metal oxide nanomaterial in the temperature range of 20˚C to 37˚C because,
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besides increasing thermal conductivity, they moderately decreased the energy storage capacity.
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• From the graph (Figure 18), it is seen that carbon-based nanomaterial like graphene and xGnP are commonly preferred nano additives.
• It should be noted that the thermal properties of NDPCM depend on the mass of nanoparticles added and primary melting behavior of PCM. The thermal properties of NDPCM may increase or decrease in the melting range of PCM outside the temperature range considered in the present work, i.e. 20˚C to 37˚C. • The summary on Latent heat and thermal conductivity of PCM on the addition of different nanoparticles is listed in Table 11.
57
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Figure 18: Research work on NDPCM in the temperature range of 20˚C to 37˚C
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• From the graph (Figure 18), it is seen that carbon-based nanomaterial like graphene and xGnP are commonly preferred nano additives. • It should be noted that the thermal properties of NDPCM depend on the mass of nanoparticles added and primary melting behavior of PCM. The thermal properties of NDPCM may increase or decrease in the melting range of PCM
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outside the temperature range considered in the present work, i.e. 20˚C to 37˚C. • The summary on Latent heat and thermal conductivity of PCM on the addition of different nanoparticles is listed in Table 11. • Thermal properties like latent heat and thermal conductivity of NDPCM shows
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better performance on smaller sized nanoparticle rather than larger sized nanoparticle. However, 1-10 nm size nanoparticle depends on surface effect
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and quantum size effect.
• It should be noted that the thermal properties of NDPCM depend on the mass of nanoparticles added and primary melting behavior of PCM. The thermal properties of NDPCM may increase or decrease in the melting range of PCM outside the temperature range considered in the present work, i.e. 20˚C to 37˚C. • Addition of the nanoparticle tends to reduce the energy storage capacity of the matrix PCMs. But, a high enhancement of thermal conductivity, giving rise to a high heat transfer rate can compromise the decrease in the rate of energy storage capacity. 58
ACCEPTED MANUSCRIPT
Table 11: Parameters influencing the latent heat and thermal conductivity of NDPCM PCM (Melting temp. In oC)
Concentration (%)
2
Decrease Increase in in Latent Thermal heat Conductivity (%) (%) 86.31 133.3
[48]
-
94.90
-
[78]
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Particle Size
Ref
O.D. × L 10-30 nm × 5–15 µm
Eicosane (36)
Cylindrical
130 nm
LA-MA-SA (32.76)
Rod
Diameter 0.7−2 nm
Octadecane (27)
0.1
-
102.5
[32]
Rod
O.D. × L 8–15 nm × 0.5-2 µm
1-dodecanol (32.33)
1
95.42
-
[68]
Hallow
100 nm
Bio Based (29.4)
1
90.93
266.2
[40]
GA
Flake
3 nm
Octadecanol (28)
6.25
86.87
2004.35
[62]
GNP
Platelets
Width <2 µm, thick <2 nm
Paraffin RT-22 (25.37)
0.1
98.55
613.3
[58]
Cylindrical
Diameter 5−10 µm, thick of 4−20 nm
Eicosane (35.7)
2
98.30
152.8
[66]
Cylindrical
Width 16 nm
Eicosane (36)
1
97.34
171.1
[93]
Eicosane (36.02)
1
98.34
182.4
[89]
Eicosane (36.02)
1
94.75
197.8
[89]
Eicosane (36.02)
1
98.89
230
[89]
Paraffin RT-27 (28.81)
1
96.78
288.8
[41]
1-dodecanol (22)
1
95.39
-
[70]
CNT
GNS
3.2 nm Flake broken
3.6 nm
CuO
EP
5.9 nm Cylindrical
Diameter 35 nm
Flake
60 nm
Cylindrical
Diameter 25 µm, think 6−10 nm
Octadecane (27)
0.1
-
107.9
[32]
Flake
Diameter 15 µm, thick 10 nm
Bio Based (29.4)
3
98.50
397.4
[40]
Rod
Diameter 15µm, thick <10nm
Octadecane (28.91)
3
99.57
-
[31]
Spherical
Diameter 5-15 nm
Eicosane (36.18)
1
-
117.5
[44]
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xGnP
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Spherical
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Ag
Particle Shape (or) Structure
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Particle name
59
ACCEPTED MANUSCRIPT • The surface-functionalized nanoparticle has shown a different effect based on the methods of surface modification. Surface-functionalized nanoparticle shows higher thermal conductivity enhancement. • Latent heat and the thermal conductivity of the NDPCM, irrespective of nanoparticle shape, i.e. spherical or wire or rod, depend only on melting
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temperature and thermo-physical characteristics of base PCM as well as the type of application.
• Supporting material having high porosity are used to prevent the base PCM
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from leakage during phase change. However, the average pore size of the supporting materials is exceedingly essential for thermal properties of formstable composite PCMs. Mesoporous carbons like supporting material with
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high encapsulation capacity as well as high thermal conductivity are needed for composite form-stable PCM to increase the thermal properties of NDPCM Future work
Many researchers have employed different variants of the two-step preparation
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method for the synthesis of NDPCM, but there are still many aspects worth for further investigations. Suggestions for future research works are summarised below: 1. The most commonly used two-step method for preparation of NDPCM has the problem of agglomeration of the nanoparticle. More efficient and
EP
agglomeration free preparation method for NDPCM has to be developed. 2. Determination of optimum level of nanoparticle concentration at which
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NDPCM will have maximum enhancement on thermal conductivity with minimum degradation of latent heat capacity.
3. Nanoparticle synthesis methods can be standardized by changing the reaction parameters like temperature and time, which would change the physical structure of the nanoparticle materials. 4. Modified nanoparticle shows better performance on latent heat of NDPCM by reducing the dispersion effect on NDPCM. More experimental research works involving surface modification of nanoparticles have to be conducted.
60
ACCEPTED MANUSCRIPT 5. Study on the effect of nanoparticle-surface modification on thermal and rheological properties and stability of NDPCMs can be carried out. 6. Variation of size and shape of the same nanoparticle in same phase change materials can be examined to understand their effect on the thermal
Conclusion
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performance of NDPCMs in the low-temperature range.
This review consists of three parts: preparation of nanoparticle dispersed phase change material, characterization techniques of NDPCM and thermal properties like
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latent heat and thermal conductivity of NDPCM.
This review analyses the impact of the dispersion of various nanoparticles like
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metal, metal oxide, and carbon-based materials in the PCM matrix with the scientific reason behind the impact. This review will help the researchers to understand and select the appropriate nanomaterial for enhancing the thermal performance of latent heat thermal energy storage systems working in the temperature range of 20˚C to 37˚C.
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From this review, it is concluded that the dispersion of nanoparticle with PCM molecule would reduce the energy storage capacity of the matrix PCMs because they do not contribute to latent heat storage. Moreover, a remarkable enhancement of
storage.
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thermal conductivity is acceptable for a slight decrease in the rate of latent heat
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Impact on Latent heat of NDPCM does not depend on the size of nanoparticle but depends on the dispersion of mass concentration of nanoparticles in the PCM matrix. However, latent heat of NDPCM can even improve due to the Lenard-Jones potential forming between PCM molecule and nanoparticle. However, the dispersion of nanoparticle increases; the thermal conductivity gradually increases while the latent heat decreases. This indicates that the improvement in NDPCM thermal conductivity using nanoparticle will be accompanied by reduced latent heat in the NDPCM. Therefore, a suitable mass fraction of nanoparticle should be determined based on the application. 61
ACCEPTED MANUSCRIPT Thermal conductivity enhancement in NDPCM was higher for carbon-based nanomaterial than for metal or metal oxide nanomaterial. Carbon-based nanomaterial has more efficient networking structures which are strongly integrated without any micro-cracks or loose interface in PCM molecule than the case of metal and metal
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oxide nanoparticle dispersion that results in superior performance. Use of NDPCM in a wide range of application appears promising, but the oftenpoor performance of NDPCM and lack of theoretical understanding of the mechanisms remains a challenge and hinder the widespread commercial application of
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NDPCM. This review article brings out the impact of material, shape, size, surface modification of nanoparticles and supporting material on the important thermal
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properties like latent heat and thermal conductivity of PCM and brings to light the possible underlying science under the change in the thermal properties of PCM. Future work is required to reveal the Surface-functionalized nanoparticle, to facilitate optimization of the latent heat and thermal conductivity of NDPCM. Also, the Surface-functionalized nanoparticle will be the route for the characterization
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techniques for carrying out a large number of experimental works. Therefore, the
new NDPCM.
[1]
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S2405-8297(19)30891-8
DOI:
https://doi.org/10.1016/j.ensm.2019.07.031
Reference:
ENSM 856
To appear in:
Energy Storage Materials
Received Date: 27 May 2019 Revised Date:
8 July 2019
Accepted Date: 20 July 2019
Please cite this article as: E. Jebasingh.B,, V. Arasu.A, A comprehensive review on latent heat and thermal conductivity of nanoparticle dispersed phase change material for low-temperature applications, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2019.07.031. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Title
A comprehensive review on latent heat and thermal conductivity of nanoparticle dispersed phase change material for lowtemperature applications Eanest Jebasingh.B
Sequence of Author
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Valan Arasu.A First author Email
[email protected].
First author Address
Department of Mechanical Engineering Thiagarjar College of Engineering
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Madurai- 625015, India Valan Arasu.A
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Corresponding Author
[email protected]
Corresponding Email Corresponding Address
Department of Mechanical Engineering Thiagarjar College of Engineering
Acknowledgment
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Madurai- 625015, India.
The author Eanest Jebasingh B wishes to acknowledge the financial support provided by the Council of Scientific and Industrial Research, India under CSIR
EMR-I)
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SENIOR RESEARCH FELLOWSHIP (Sanctioned Letter No: 08/237/0013/(2018)
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Conflict of interest
We have no conflict of interest to declare.
ACCEPTED MANUSCRIPT A comprehensive review on latent heat and thermal conductivity of nanoparticle dispersed phase change material for low-temperature applications
Abstract:
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Today’s power infrastructure involves unpredictability in both demand and supply. Power management using energy storage is becoming a promising method to have sustainable energy utilization. In recent times, energy storage using latent heat thermal energy storage (LHTES) technology is receiving more considerable attention to
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reducing grid energy demands. LHTES technology have been utilized by using phase change material (PCM)for the last two decades. This review focuses on the change in
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latent heat and thermal conductivity of nanoparticle dispersed phase change material (NDPCM) between the operating temperature range of 20˚C and 37˚C as required in low-temperature applications. The critical feature of this review is that it analyses both the scientific reasons behind the increase or decrease in latent heat and thermal conductivity of base PCM. Dispersion of nanoparticles as well as supporting materials
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into the PCM matrix and the impact of influencing parameters like size, shape and the material of the nanoparticles on the thermal properties of PCM. Dispersion of nanoparticle increases, the thermal conductivity gradually increases while the latent heat decreases. This indicates that the improvement in NDPCM thermal conductivity
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using nanoparticle will be accompanied by reduced latent heat in the NDPCM. However, Thermal conductivity enhancement in NDPCM was higher for carbon-
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based nanomaterial than for metal or metal oxide nanomaterial. Thus, the review will be helpful for new researchers in understanding the underlying science behind the change in critical thermal properties of the base PCM and to further improve the performance of the LHTES system. Keywords: LHTES, phase change material, nanoparticle dispersed PCM, thermal conductivity, latent heat.
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ACCEPTED MANUSCRIPT Nomenclature K Thermal conductivity (W/mK) ∆H Latent heat (kJ/kg)
MWNT
Multi-Wall Carbon Nanotube
NDG NG NP NDPCM
Al2O3 -T CA CNT CTAB
NMP OD OS PA
Nitrogen-Doped Graphene Nano Graphite Nano Particle Nanoparticle Dispersed Phase Change Material N-Methyl-2 Pyrrolidone Octadecane Oleoyl Sarcosine Palmitic Acid
GIC GNP GNS GO GP LA MA
Copper Oxide Nanoparticle Expanded Graphite
PAN PCM
Poly (Acrylamide-Co-Acrylic Acid) Poly acrylo nitrile Phase Change Material
Expanded Graphite Oxide Treated Expanded Graphite Eutectic Hydrated Salt Eutectic Hydrated Salt Expanded Perlite Erythritol Tetra Palmitate Erythritol Tetra Stearate Expanded Perlite Graphene Aerogel Graphene Aerogel (Rgo/Gnp) Graphite Integrated Compound Graphene Nanoparticle Graphite nanosheet Graphene Oxide Graphite Particle Lauric Acid Myristic Acid
PEG PU PVA PW rGO SA SDS SG SiO2 SWNT
Polyethylene Glycol Polyurethane Polyvinyl alcohol Paraffin Wax Reduced Graphene Oxide Steric Acid Sodium Dodecyl Sulfate Spongy Graphene Silicon Dioxide Single Wall Carbon Nanotube
TEA
Tri ethylamine
TEMED
Tetramethyl ethylene diamine
TiO2 xGnP
Titanium Dioxide Exfoliated Graphite Nanoplatelets
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PAAAM
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CuO EG, WPEG EGO EG-T EHS EHS EP ETP ETS Exp GA GA
Treated Aluminium Oxide Capric Acid Carbon Nano Tube Cetyl Trimethyl Ammonium Bromide Copper Nanoparticle
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Cu
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Acronyms 3D-GA Three Dimensional Graphene Aerogel AC Activated Carbon ACCOOH Glacial Acetic Acid Ag Nano Silver Nanoparticle Al2O3 Aluminium Oxide/ Alumina
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Introduction: Most developing countries like India are exposed to warm-to-hot climates due to their geographic latitudinal locations, and favourable environment condition will
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account for the majority of the world’s population increase by 2050 [1]. Surface temperatures are also gradually increasing in developing countries as a result of habitual electricity generations from fossil fuel, thermal power plant, and also
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unpredictable weather events such as heatwaves [2].
For adaptation to changing the climate, one must consider the significance of air
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conditioners (A/Cs) in mitigation of human vulnerability due to unpredictable weather events such as heatwaves. The percentage of households that have air-conditioning units will increase to nearly 99.9% by 2100, from its current 2% [3]. According to the International Energy Agency (IEA), countries like India has seen a rapid increase in household electricity consumption, via increased purchases of appliances like air conditioner. The purchase of air conditioners around the world has increased from
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around 1 million units in 2003-2004 to more than 3 million units in 2010-2011. IEA has also strongly suggested that if this continues, the usage of air conditioning equipment will reach up to 50 million by 2050 [4].
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The international energy agency (IEA) also indicates that developing countries like India has been responsible for almost 10% of the increase in global energy
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demand from 4.4% in 2000 to 5.7 % in 2013 which relates the GDP rating to lower rate [4]. Scientists have predicted that burning of fossil fuels and power generation from thermal power plants will generate a tremendous amount of carbon dioxide (CO2), which would be responsible for over half of GHG (Greenhouse gas) emission induced global warming [5] leading to climate change [6,7]. The growth in air conditioning indicates billions of tons of increased carbon dioxide emissions. Increased saturation of air conditioners has important implications, not only for total electricity generation and carbon dioxide emissions, but also for load management [8].
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ACCEPTED MANUSCRIPT Environmental scientists have predicted that carbon dioxide (CO2) emission will lead to an increase in the global temperature by 1- 3.50 C and the sea level will rise by 15-95 cm by 2050. Increase in global temperature leads to climate change. Climate change will play a significant role in the increased demand for the purchase of air conditioners [9]. Impact of climate change on global emissions have to be considered
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due to energy consumption for cooling needs by 2100 [10]. Researchers also predicted that the efficiency of air conditioners would also decrease when the atmospheric temperature rises from 20°C to 46°C; it is determined that the power consumption for
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air-conditioning units will increase up to 47.1% by 2050 [11].
According to the International Energy Agency (IEA), space cooling is one of
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the prior technologies, which has the highest long-term potential for reducing CO2 emissions [12]. The utilization of renewable or low-grade waste energy with cold energy storage is considered a promising solution to global warming and energy crisis [13]. In 1994, Dorgan and Ellison [14] put forth detailed design procedures for cold thermal energy storage system using ice-cooling. Performance improvement of cool thermal energy storage systems about the building energy efficiency has been
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established through the extensive modeling and simulation techniques. Some of these systems were also field-tested and implemented in buildings located in hot and arid climatic conditions. As a result, these systems have contributed to a reduction in on-
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peak power and energy demand in such buildings by approximately 31% [15,16]. The cold thermal energy storage (TES) may be divided into two main groups:
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sensible heat and latent heat. Due to a large amount of thermal energy absorbed as latent heat (heat of fusion) at nearly constant temperatures during melting and solidification, solid-liquid phase change materials (PCMs) have been adopted as TES media in applications like thermal management. The utilization of cold thermal energy storage using latent heat technology using phase change material (PCM) in A/Cs, is an advanced energy-saving technique for indoor air conditioning application [17]. PCM as a storage medium, the available cold thermal energy or excess cold energy can accumulate into the PCM during the charging process. Then, the stored cold thermal energy can be retrieved and supplied 4
ACCEPTED MANUSCRIPT to the end-user during the discharging process. In virtue of energy storage as a change in internal energy or phase transformation of the storage medium, it offers to alleviate the peak load on electricity grids and utilize power in the off-peak period. The application of thermal energy storage in air-conditioning systems can benefit the customer by lowering the energy costs, improved space temperature control,
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technologically advanced load factor, and slower capital investment in new generation equipment [18].
The energy storage during phase transformation depends on the properties of
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PCM. According to the material properties, the PCM is classified into organic and inorganic. However, the low thermal conductivity of PCMs limits the use of the full
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potential of these materials since it slows down the heat transfer process associated with the charging and discharging processes. To overcome the problem mentioned above, some heat transfer enhancement techniques have been proposed such as the use of fins and heat pipes [19,20], insertion and dispersion of high thermally conductive materials [21], microencapsulation [22], nanoencapsulation [23], etc., Among these techniques, the dispersion of high thermal conductive nanometer-sized materials, such
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as nanoparticles, nanofibres, nanotubes, and nanosheets, in the PCM significantly increases its thermal conductivity [21,24]. This emerging mixture of base PCM and nanoparticle is referred to as nanoparticle dispersed phase change material (NDPCM)
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in this review article. However, the application of NDPCM in the range of 20˚C and 37˚C is discussed in Table 1
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Table 1: Application of NDPCM Reference Application [25] Building Air conditioner
[26]
Electronic cooling
Inference Phase change material embedded with nanoparticles exhibited improved heat transfer mechanisms in charging and discharging processes. Experimental results suggested that the proposed air conditioning system with NDPCM achieved an on-peak and per day average energy savings potential of 36–58% and 24–51%, respectively, for year-round operation while compared to the conventional air conditioning system Electronic applications (to control and avoid 5
ACCEPTED MANUSCRIPT temperature peaks) and plastic injection molds (to reduce temperature oscillations).
Building heating/cooling Indoor temperature controlling,
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[27–30]
NDPCM increases thermal transfer due to the presence of nanoparticle. NDPCM is a preferential potential thermal energy material for improving the energy efficiency in building applications due to its proper phase change temperature range, relatively high latent heat and thermal conductivity, good thermal reliability and chemical stability.
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Highly porous nanosheets, act as nucleators to enhance thermo-physical properties of NDPCM.
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There are many open ‘‘pore structure space’’ based on the pore structure of the nanoparticle. Moreover, there is some adsorption force between nanoparticle and liquid fatty acid phase change material
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Results of numerical analyses revealed that significant gains in the energy efficiency of buildings could be realized by introducing NDPCM into building envelope products. The results indicated a reduction of 79% in energy demand to maintain the interior within the thermal comfort range by application of the NDPCM NDPCM can be considered as energy-saving building materials for residential buildings using radiant floor heating systems A thermal switch is an electromechanical device which opens and closes contacts to control the flow of electrical current in response to temperature change. The thermal switch cuts off the current to critical machinery when a temperature limit is exceeded, preventing potential burn out or failure.
Radiant floor heating
[32]
Thermal switches
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[31]
NDPCM makes it a promising candidate for controlling the heat transport path simply via temperature regulation. NDPCM inevitably requires the increase of the thermal conductivity while
minimizing the variation of the latent heat capacity of the nanocomposite and ideally increases it. In other words, materials should be able to store or dissipate a large
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amount of energy in a short period. Several reviews have been published regarding the characterization and applications of PCM
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composites for thermal energy storage [33]. Despite the contributions given in Table 2, a comprehensive review focusing on the enhancement of thermal conductivity and latent heat capacity of PCMs as thermal energy storage materials, due to the addition of the nanomaterials in the operating temperature range of 20˚C to 37˚C for low temperature applications like indoor cooling to meet the
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human comfort requirements has not yet been carried out. Further, a common thought that the interactions of NP with PCM are influenced by changes in particle shape and size of nanoparticles, which has been analyzed and discussed in this article.
Sharma. R.K., et al. Chenzhen Liu., et al.
Zhenjun Ma., et al. Soares.N., et al.
Lin.Y., et al.
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Muthuvelan., et al.
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Guiyin Fang., et al.
Contents of the Review Reviews previous work on phase change cold storage for air-conditioning systems focusing on two aspects, including phase change materials (PCMs) and applications. The review concentrates on the preparation, thermal properties and applications of shape-stabilized (shape-stabilized PCMs that can be prepared by integrating the PCMs into the supporting material and microencapsulating the PCMs into the shell) thermal energy storage materials Reviews the concept of free cooling, PCMs suitable for free cooling applications in buildings, commercial PCMs that are available, and mapping of free cooling technology along with the promotional policies needed toward energy/environmental sustainability. The review focuses on three aspects: the materials, encapsulation, and applications of organic PCMs, and provides an insight into the recent developments in applications of these materials. The review focuses on preparation and characterization of NanoPCM (nano encapsulated phase change materials), application of NanoPCM in latent functional thermal fluid, dynamics simulation study of NanoPCM and heat transfer enhancement of NanoPCM Reviews the generic framework for the appropriate use of nano-enhanced phase change materials (PCMs) in buildings Reviews the control of domestic energy resources behaviors in the dynamics of energy demand, the advantages of improving thermal storage by using phase change materials, the importance of reducing heating and cooling energy demand (maintaining indoor thermal comfort). Reviews the methods for enhancing the thermal conductivity of PCMs, which include adding additives with high thermal conductivity and encapsulating phase change materials.
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Author Zhai.X.Q., et al.
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Table 2: Summary of a review article on PCM
Reference
[34] [18] [35] [36] [23] [33] [37] [38]
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ACCEPTED MANUSCRIPT The review investigates the experimental research works that report concerning the reason behind improvement or decrement of thermal conductivity and energy storage capacity of NDPCM in the operating temperature range of 20˚C to 37˚C. This review article also includes a critical discussion regarding the influence of size, shape,
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and materials of nanoparticles on latent heat and thermal conductivity of NDPCM. The review is organized as follows: 1) Different preparation methods of NDPCM. 2) Various characterization techniques of NDPCM.
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3) Discussion on latent heat and thermal conductivity of NDPCM for different types of PCMs, nanoparticles, and supporting materials.
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A detailed description of the most relevant experimental studies regarding innovative results, and identification of the critical reasons for the observed increase or decrease in latent heat and thermal conductivity of NDPCM, according to the type of nanoparticles is also included in this review article.
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Nanoparticle dispersed PCM
Organic PCM and inorganic PCM are the two types of commonly used PCMs. Organic PCM has drawn an excellent attraction for its desirable characteristics like
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good heat storage density, melting or solidification with little or no sub-cooling, nonreactiveness with most common chemical reagents and low cost, despite having the
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main drawback of low thermal conductivity [36]. Inorganic PCM has desirable characteristics such as high latent heat storage capacity, low cost, and nonflammability. However, inorganic PCM has some main drawbacks like high supercooling and non-proper phase transition, in addition to low thermal conductivity [39]. Dispersing high conductive nanoparticles in PCM could be a solution to enhance the thermal conductivity [40] and to sustain the thermal performance as a result of enhanced thermal conductivity (Table 3).
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ACCEPTED MANUSCRIPT Table 3: Thermal Conductivities of some commonly used PCM and nanoparticle Thermal conductivity (W/mK)
Material Phase Change Material RT27 Octadecane RT25 Eicosane Tetradecanol Capric acid Nanoparticle Aluminium (Al) Sliver Nanoparticle (Ag) Copper Nanoparticle (Cu) Titanium dioxide (TiO2) Expanded graphite (EG) Exfoliated graphite nanoplatelets (xGnP) Carbon Nanotube (CNT) Graphene
Min Li. Sadegh Motahar., et al. Patrik Sobolciak., et al. Mahdi Nabil., et al. Ju-LanZeng., et al. Dandan Mei., et al.
0.13 0.17 0.26 0.36 0.42 0.48
[41] [42] [43] [44] [45] [46]
237 429 400 8.4 200 1500
[47] [48] [49] [50] [51] [52]
Lingkun Liu., et al. Sung Seek., et al.
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Hao Peng., et al. Rabih M., et al. Sciacovelli A., et al. Arjumand Adil., et al. Teppei Oya., et al. Jinglei Xiang., et al.
Preparation of NDPCM
Reference
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Author
3000 3000
[53] [54]
The usage of PCM is limited because it has low thermal conductivity and leakage
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during phase transition. The low thermal conductivity will decrease the heat transfer rate, and the leakage of PCM causes certain harm to the energy storage system and environment, which limits the further application of PCM. Incorporating the nanomaterial into PCM, called NDPCM, for better improvement in thermal
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conductivity of the PCM is the main focus during the preparation of NDPCM. Table
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4 presents the different preparation methods of NDPCM adopted in experimental studies by different researchers. Table 4: Method of preparation of NDPCM Synthesis process
PCM
Vacuum impregnation method Varnish layer Kneader mixing technique Sonication & Ultrasonication Stirring and Sonication Autoclave method
CA Octadecane Octadecane RT22 Octadecane Tetradecanol
Melting Nanoparticle Temperature (oC) 28.99 30 28.7 25.37 28.91 35.87
EG xGnP EG Graphene xGnP EG
Reference
Ahmet., et al. [55] Seunghwan., et al. [56] Dowan Ki., et al. [57] Nandy Putra., et al. [58] JisooJeon., et al. [31] Yushi Liu., et al. [59] 10
ACCEPTED MANUSCRIPT In general, two main techniques used for dispersion of nanoparticles into the base PCM material: a) One-step method and b) Two-step method. The one-step technique combines the production of nanoparticles and dispersion of nanoparticles in the base PCM in a single step. In the two-step method, the nanomaterial first synthesized or purchased from the market and then dispersed in the base PCM.
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However, the present literature review reveals that most researchers have followed the two-step method only, but some researchers functionalized nanoparticle for better dispersion into the matrix of PCM, for a temperature range of 20˚C to 37˚C.
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Sari et al. [55] from Gaziosmanpasa University employed vacuum impregnation; vacuum flask is used for the dispersion process (Figure 1). The porous
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expanded graphite (EG) 10 g, was placed inside a flask, that in turn was connected to water tromp apparatus to evacuate air from the porous EG. The evacuation process continued for about 30 min inside the vacuum. Then the valve between the flask and the container of PCM was opened to allow the PCM liquid to flow into the flask to cover all the porous material. Finally, the vacuum process ended, and the air was allowed to enter the flask once again in order to force the fatty acid liquid PCM to
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penetrate the pore space of the expanded graphite. In order to test the fatty acid exudation from the porous spaces, the composite PCM was simultaneously heated during the dispersion process at a constant temperature above the melting temperature
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of the fatty acid. The fatty acid absorption amount by EG was evaluated by observing
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fatty acid exudation from the composite.
Figure 1: Vacuum impregnation method of preparation of NDPCM [55]
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ACCEPTED MANUSCRIPT Wi et al. [56] from Soongsil University tried with varnish layer method instead of encapsulation (Figure 2). Organic PCM octadecane was melted and directly mixed with
exfoliated
graphite
nanoplatelets
(xGnP).
The
obtained
homogenous
(Octadecane-xGnP) mixture was poured into a cylindrical shaped hollow to form a cylindrical shape or layer, and the cylindrical layer was covered with varnish for
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preventing the leakage. Then, the composite was dried for 48 h at 25 °C and 50% relative humidity. The cylindrical hollow was made by mixing plaster and water in a ratio of 1:0.45, forming a cuboid having a size of 100 mm (width) × 100 mm (length)
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× 15 mm (height) using a cylindrical mold.
Figure 2: Schematic of the varnish layer method [56]
Kim et al. [57] from Yonsei University used Kneader mixing technique.
Organic PCM octadecane was heated above its melting temperature, and the melted octadecane was directly impregnated into a vacuum treated EG using a stirrer, stirred at 30 rpm and 80˚C for 1 hour and allowed to cool. EG was synthesized from graphite powder using microwave irradiation. The obtained EG powder was dried inside a vacuum oven at 105˚C for 24 h.
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ACCEPTED MANUSCRIPT Putra et al. [58] from Universitas of Indonesia synthesized NDPCM with the help of sonication and ultra-sonication. Figure 3 shows the synthesis process of nano-PCM. Organic PCM RT 22 was melted and directly mixed with graphene by sonication under the frequency of 40 Hz. The obtained NDPCM was kept inside an ultrasonicator bath for 3h under 45˚C until the graphene was fully dispersed into the
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RT 22.
Figure 3: Synthesis process of NDPCM using sonication method [58]
Jeon et al. [31] from Soongsil University followed a simplest common method like two-step method (Figure 4). Octadecane PCM was melted by heating the PCM
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above its melting temperature. Then xGnP was mixed into the liquid PCMs in different mass fractions using a stirrer, stirring at 1000 rpm for 20 minutes. After
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stirring, NDPCM was sonicated for 20 min and allowed to cool.
Figure 4: Schematic of preparation of composite PCMs loaded with xGnP using twostep method [31] 13
ACCEPTED MANUSCRIPT Liu et al. [45] from Changsha University of Science and Technology used autoclave method. Organic PCM Tetradecanol (TD) was mixed with ethanol to improve the absorptivity of the PCM molecule, and then the mixture was melted at room temperature and directly impregnated with expanded graphite (EG) (ethanol was used to improve the uniform dispersion of nanoparticle into the PCM molecule). Then,
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TD- ethanol- EG, was heated above 70˚C in a hotplate to vaporize the ethanol. Characterization of NDPCM
Various techniques have been carried out to characterize PCM with the
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immersion of nanomaterials. Different studies focus on different parameters of NDPCM. NDPCM can be characterized by using a wide range of techniques, and
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some commonly used techniques [30,40,60] are summarized in (Table 5) Table 5: Major technique used to characterize NDPCM Technique Differential Scanning Calorimeter (DSC)
Purpose Determine the phase change temperature and heat of fusion
2
Dynamic Light Scattering (DLS)
Measure the average size and size distribution of nanoparticle in NDPCM
3
Scanning Electron Microscope (SEM)
4
Transmission Electron Microscope (TEM)
5
Fourier Transform Infrared Spectrometer (FTIR)
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S.No 1
X-ray Diffraction (XRD)
Morphology of NDPCM
Inspect the dispersion of nanometer-sized materials and characterize their superficial nanostructures and interfaces Obtain the infrared spectra and determine the chemical group
Measure the crystallite size and structure of nanometer-sized materials
The purpose for dispersion of high thermal conductive nanometer-sized
materials, such as nanoparticles, nanofibers, nanotubes, and nanosheets in the PCM matrix is to increase its thermal conductivity significantly. The thermal conductivity of NDPCMs can be measured using either steady-state methods or transient-state methods [61]. Guarded hot plate method [58] is a typical example of the steady-state
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ACCEPTED MANUSCRIPT method while transient hot-wire method [32], laser flash method [62] and transient plane source method [63] are transient-state methods. Latent heat- An inside view Latent heat is particularly attractive due to its ability to provide high-energy
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storage density in the quasi-isothermal process. A phase change transition from solid to liquid is endothermic, e.g., heat absorption process, the absorbed heat energy is utilized to overcome or break the weak intermolecular attractions in the PCM matrix. In the endothermic process, the energy required to change a gram of a substance from
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solid to liquid state without changing its temperature is commonly called its “heat of fusion.” The heat energy absorbed is used to break down the solid bonds but leaves a
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significant amount of energy associated with the intermolecular forces of the liquid state.
A phase transformation from liquid to solid is exothermic. In the freezing process, the moving boundary layer between the solid and liquid phase transfers energy to the PCM molecules. Rapid disordering of a crystal structure due to thermal
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disturbances occurring between the neighboring molecules of PCM causes the effective bonding between them to get strengthened [64]. The heat of fusion, melting, and freezing processes could also change due to the
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immersion of nanoparticle in the matrix of PCM. Determination of latent heat in NDPCM
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The variation trend of enthalpy with the mass fraction of additives mainly resulted from two impact factors. The first impact factor is intermolecular forces between nanoparticle and PCM. Another impacting factor is the non-melting enthalpy of additives, which would drop down the melting enthalpy of composites. When the mass fraction was lower than 1%, the melting enthalpy of NDPCM was mainly influenced by the first impact factor. Rising the mass fraction above 1%, the effect of the second impact factor got larger, and then the melting enthalpy decreased [65]. Further, the realignment of molecules of the PCM matrix in the presence of heavily
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ACCEPTED MANUSCRIPT loaded nanoparticle is impossible, which is also a major reason for the decrease in effective latent heat of the composite [66]. Simple mixing theory, theoretical melting enthalpy of NDPCM can be calculated by using Eq.1 [57] ∗ 1 − ϕ ………………….. (1)
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ΔH
= ΔH
= Theoretical Melting enthalpy of composite,
= Melting enthalpy for base PCM
and
ϕ = weight fraction of nanoparticle
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Effect of Metal and Metal oxide nanoparticles:
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where ΔH
ΔH
Metal nanoparticles research has recently become the focus of intense research work due to their unusual properties compared to bulk metal. Metal oxide nanoparticles represent a field of materials chemistry, which attracts considerable
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interest due to the potential technological applications of these compounds. Metal nanoparticle like silver (Ag) nanoparticle with the size of 10–30 nm diameter and length of 5–15 µm appears like spherical shape, which does not have an impact on the latent heat of the base PCM. However, latent heat in NDPCM was
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reduced in the range of 95% for 1 wt% dispersion of Ag in Eicosane PCM [48]. But, as the Ag nanoparticle concentration increases, the latent heat on NDPCM decreases
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and metal oxides like copper oxide nanoparticle (CuO) have reduced the latent heat of NDPCM considerably [67]. Effect of carbon nanostructural material: High thermal conductivity material, carbon-based materials are widely used as dispersant. Hence, the use of novel allotropes of carbon has continued to rise in recent years.
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ACCEPTED MANUSCRIPT Carbon nanotube (CNT) having a diameter of 8 to 15 nm; length 0.5-2 µm and rod shape has reduced the latent heat in the range of 95.4% for 1 wt% of CNT in NDPCM.
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CNT is a lightweight material and has a smaller particle size, in nanometer scale
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(smaller particle size will increase the intermolecular attraction between the atoms in NDPCM). CNT can easily affect the local interactions in the PCM molecule as well as the diffusion and rearrangement of PCM molecules, owing to its larger surface area. It
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indicates that the multi-porous structure of the CNTs limits the crystal behavior of the PCM [68]. Multi wall carbon nanotube (MWCNT) is one of the premium members in nanomaterial. Use of MWCNT in hexadecane showed
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a contrast result in latent heat. The MWCNT with a diameter of 10–20 nm, length 0.5–2 mm and rod shape, because of their high aspect ratios, large specific surface area, and substantial Vander-Waals attractions, tend to self-aggregate into bundles spontaneously
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in water and organic solvent. Surface functionalization of MWCNT increases the possibility of nano-tubes entanglement and close packing. As a result, latent heat in NDPCM increased from 227.13 kJ/kg to 256.28 kJ/kg for 0.5 wt.% dispersion of MWCNT in hexadecane PCM. The large surface area by surface functionalization was the reason behind the greater intermolecular attraction in
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the PCM, which resulted in its enhanced latent energy [30,69].
Table 6 recaps the melting temperature, and latent heat of NDPCM dispersed with metal, metal-oxide nanoparticle, and carbon nanostructure.
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Table 6: Melting temperature and latent heat of NDPCM
Metal Nanoparticle Rabih., et al. [48] Eicosane
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Reference
Base PCM Material Nanoparticle Melting PCM Temp Material wt.% (˚C) 36 36 36
Ag Ag Ag
1 2 3.5
Melting Latent heat PCM kJ/kg
NDPCM kJ/kg
241 241 241
229 208 192
NanoParticle Size
Shape
dia 10–30 nm; length of 5–15 µm
Spherical
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177
0.025
243.5
200.11
*
*
CuO
0.05
243.5
172.94
*
*
22
GNP
0.5
216.52
22
GNP
1
216.52
25.37
Graphene
0.05
25.37 25.37
Graphene Graphene
0.1 0.15
28
GA
6.25
28 28.81 28.81 28.81
GA GNP GNP GNP
9.09 1 4 7
Octadecane
28.91
xGnP
Bio-based
29.4
xGnP
Octadecane
31.45
31.45 31.45 31.45 32.33 32.33
Min Li [41]
Jisoo Jeon., et al. [31] Seulgi Yu., et al. [40] Yongpeng Xia., et al. [71]
Yi Zen., et al. [68]
RT27
1-dodecanol
SC
28
210.96
60 nm
Planar/ Flake shape
21.5 to 65.5 nm
Rolled thin
3 nm
Flake shape
dia 35 nm
Cylindrical
206.54
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Octadecanol
CuO
163.31
161.43
163.31 163.31
160.95 155.47
238.3
207
238.3 209.33 209.33 209.33
202.8 202.58 193.36 183.62
3
241.97
240.92
5
146.7
143.5
GA
3.22
205.4
195.7
GA GA GA CNT CNT
2.43 1.96 1.63 1 2
205.28 205.28 205.28 231.4 231.4
200.53 203.98 205.28 220.8 212.4
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Jing Yang., et al. [62]
RT-22
28
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Nandy Putra., et al. [58]
5
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Carbon nano-material Li-Wu Fan., et al. 1-dodecanol [70]
Ag
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241
Metal- Oxide Nanoparticle B. R. Sushobhan., Octadecane et al. [67]
36
diameter 15µm, thick <10nm * *
length: 0.5–2 µm, dia: 8–15 nm
Rod 19
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Eicosane
35.7
Graphene
1
262
260.69
35.7 35.7
Graphene Graphene
2 5
262 262
257.546 247.85
* Not reported
dia 5−10 µm, thick of 4−20 nm
Cylindrical
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Xin Fang., et al. [66]
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Zhang., et al. [30] performed surface modification of the MWCNT particles in a mixture of two strong acids, concentrated H2SO4 (98%) and HNO3 (70%) at 3:1 volume ratio. MWCNT particles were added to H2SO4–HNO3 in a test tube and sonicated in an
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ultrasonic bath for 6 h and then the MWCNT was heated with reflux at 65 ˚C for 4 h. They reported that modified MWCNT particles took longer time (110 min) to disperse than the original MWCNT particles (45 min). The steric effect of the carboxylic acid group, as well as the irregularity of the modified MWCNT particle surface, resulted in a lower effectiveness of packing and a slower rate of aggregation. The aggregation of modified MWCNT particles eventually appeared, probably due to the low solubility of COOH in
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hexadecane, the hindrance provided by the small size of the acid group, and also the hydrogen bonding interaction among acid groups in modified MWCNT particles.
(SDS),
cetyltrimethylammonium
bromide
(CTAB),
polyvinylalcohol
(PVA),
polyethylene
glycol
(PEG),
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sulfate
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Also, they tried various surfactants as additives for dispersing the MWCNT particles in hexadecane, including sodium dodecyl
tetramethylethylenediamine (TEMED), Triethylamine (TEA), glacial acetic acid (AcCOOH), 1-decanol, salicylic acid (SA), Tween80 (polysorbate 80), and Triton X-100 (C14H22O(C2H4O)n) to overcome the rapid aggregation and sedimentation of the nanoparticles in the organic liquid. Stable and homogenous dispersion was attained through surface modification of the MWCNT particles.
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ACCEPTED MANUSCRIPT They claimed that it was difficult to disperse the MWCNT particles in a hexadecane liquid. The surfactant used to disperse MWCNT must be soluble in hexadecane, and the results showed that the functional group in the surfactants that are supposed to react with the acid groups in MWCNT should be as exposed as possible. Otherwise, the branch near the functional group would produce hindrance and lower
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the dispersion effect. Modified MWCNT particles, plus the addition of 1-decanol as a
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surfactant, were successfully dispersed in hexadecane (Figure 5).
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Figure 5: Schematic of the interaction between 1-decanol and modified MWCNT[30] The porous nano-sized carbon-based material like exfoliated graphite
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nanoplatelets (xGnP) was 10 nm thick and 15 µm in diameter. Good dispersion of xGnP in bio-based PCM was obtained easily by capillary and surface tension forces, resulting in no leakage of the melted PCM. Further, the latent heat of NDPCM is 99.56% even for 3wt.% of xGnP in octadecane PCM. Three-dimensional net structure confines heat movement in the PCM composites [31,40]. (Figure 6) shows how the xGnP has an impact on latent heat of various type of NDPCM.
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Figure 6: DSC curve of xGnP with various PCM [31]
Liu et al. [59] analyzed using graphene oxide (GO) as nanoparticle dispersion in the PCM matrix and found that the latent heat in NDPCM using GO was reduced
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up to 90% for2wt.% GO, when GO was 500nm in size, 1.318nm thick and was flake/hexagonal shaped. The reason behind the reduction of latent heat in NDPCM was that GO naturally presents weak Vander-wall forces due to the presence of oxide in the GO. In latent energy chemical potential energy play a major role, weak van der Wall force attractions in chemical potential tend to repulsive nature [52, 67]. There is
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chemical potential energy in the weak as well as in the strong ionic and covalent bonds. This chemical potential energy is, however, a major factor influencing the latent energy of a material.
EP
Putra et al. [58] experimentally analyzed using graphene in size ranging from 21.5 to 65.5 nm, width 3–5.5 nm and in the form of rolled-thin sheets dispersed in RT-
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22 was purchased and tried without any future purification. The latent heat (Figure 7) decreased due to the reduction of RT 22 HC in RT 22 HC/graphene nano-PCM. Rolled-thin sheet structure of graphene could quickly fill the gaps in RT 22 HC molecules [50,67]. Therefore, the increase of mass fraction would reduce the latent heat of the nano-PCM. However, Graphene particles would attract RT 22 HC particles to create groups of particles that could give rise to lower energy to melt and freeze because of weak molecular bonds.
22
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ACCEPTED MANUSCRIPT
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Figure 7: Latent heat vs. wt.% of nanoparticle [58]
Li et al. [72] tried surface functionalization on graphene as nanodispersion medium in docosane PCM and observed a contrast result that the latent heat in NDPCM increased from 256.1 kJ/kg to 262.2 kJ/kg. The above results suggested that the addition of surface-functionalized graphene can enhance the latent heat of
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docosane, and has no notable effects on its phase-change temperatures. The contrast result was because there is no change in the molecular structure of docosane and the docosane/graphene composite, but Li., et al. had not reported in his paper about shape
EP
and structure of dispersed spongy graphene in PCM. Surface functionalized spongy graphene had been synthesised by low concentration (0.35–0.4 mg/mL) of
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homogeneous graphene oxide (GO) aqueous suspension mixed with 5vol % pyrrole which was hydrothermally treated in a Teflon-lined autoclave at 80˚C for 12 h to form a N-containing gel and then freeze-dried and annealed at 1050˚C for 3 h under Argon (Ar) atmosphere [72].
Yang et al. [62] have tried surface-modified graphene as nano dispersion medium in octadecane PCM; the graphene was 3 nm in size and flake in nature. Surface functionalized GA (graphene aerogel) was obtained by annealing thermally exfoliated graphene sheets at ~2200 ˚C to remove their residual oxygen-containing groups and to repair their structural defects. NDPCM’s latent heat reduced only 13% even for 6.5 23
ACCEPTED MANUSCRIPT wt.% of GA in octadecane PCM due to annealing of graphene which gives lower density, high porosity, and ultrahigh specific surface area. The PCM molecule can easily be absorbed in the porous nature of GA due to the effect of diffusion force and capillary force between PCM molecule and porous GA. These results prove that
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annealing the nanoparticle has declined the latent heat. [62]. Wang et al. [73] tried naturally obtained graphite as nanodispersion medium in OP10E as PCM. The graphite was 20-30 nm in a spherical structure. Latent heat in NDPCM also showed contrast result: latent heat increases up to 5% for 2 wt% of GNP
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in PCM. This result also determined that the same structure of PCM and nanoparticle would produce hindrance and lower the dispersion effect.
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While the nano graphite with diameter 35 nm, cylindrical shape dispersed in RT-22 PCM, Latent heat of NDPCM reduced in acceptable range due to more tightly packed GNP molecules with PCM, but Li [41] suggested that the cost of preparation for nano-graphite/paraffin PCM is acceptable while preparing the nano graphite from ordinary graphite.
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PCM with Supporting material:
Base PCM with supporting material was used for obtaining form- stable PCM. A high porosity supporting material has been used to prevent the base PCM from
EP
leakage during phase transition. Porous construction mainly contributes in prevention of leakage of the PCM molecule, but restricts the crystal arrangement and orientation
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of PCM into the pores.
Advantages of using as supporting material: • • • •
Leakproof without reduction in the thermal property Help to increase thermal performance. Help to retain the shape either in a solid or liquid state Eliminates the need for encapsulation
Average pore size of the supporting materials is exceedingly essential for thermal properties of form-stable composite PCMs. Briefly, if the mean pore size is too small, the PCMs molecular motion might be obstructed; conversely, if it is too large, 24
ACCEPTED MANUSCRIPT
capillary force is not sufficient to pack the liquid PCMs. Controllable pore structure is obtained under the synergistic effect of
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appropriate pyrolysis temperature and crosslinking reaction [74]. Macrospore supporting material like expanded perlite (EP) can maintain the integrity of encapsulation on PCM during the repeated heating/cooling process, meanwhile facilitating the quick heat exchange between the form-stable PCM and the environment.
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Nanoparticles are located mainly in void spaces of the form-stable PCM. Table 7 presents the details of PCM with supporting
Table 7: NDPCM with supporting material
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materials used by researchers.
Supporting Material
PCM
Melting Temp (˚C)
Support Material Material
% of Concen tration
Nanoparticle
Material
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Base PCM Material Reference Sughwan Kim., et al. [63]
Octadecane
28
xGnP
Su-Gwang., et al. [75]
Bio-Based
29.38
xGnP
Hui Li., et al. [76]
Octadecane Octadecane LA-MA CA-MA-SA CA-PA-SA
30.32 30.32 33.62 24.04 25.12
EG EG EG PAN PAN
Coconut oil
26.78
*
*
xGnP
RT25
27
LLDPE
50
27
LLDPE
50
Melting Latent heat PCM NDPCM kJ/kg kJ/kg
44.1
-
-
256.5
104.5
33.33
-
-
149.2
110.6
9.09 11.76 * 10 10
Ag Ag
* *
232.49 232.49 164.5 145.9 145.7
207.9 199.4 122.8 130.5 135.5
23.93
110.4
82.34
EG
5
145
56.2
EG
10
145
59
EP
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Yan He., et al. [77] Huizhen ., et al. [78] Huizhen., et al. [78] Seunghwan Wi., et al. [79] Patrik Sobolciak., et al. [43]
wt.% of Concentrat ion
Nanoparticle Size Surface area (m2/g) 20.41 diameter 15µm, thick <10nm
Shape Flake shape *
300µm
Worm-like Porous
130 nm
Cylindrical
Surface area (m2/g) 20.41
Flake
*
*
25
ACCEPTED MANUSCRIPT
Haiting Wei., et al. [82] Ali Karaipekli., et al. [83] *Not reported
NaMMT
*
xGnP
5
247.6
80.16
CA
29.62
HNT
35
GNP
5
139.77
75.4
29.64
PU
13
Ag
*
29.64
*
*
Ag
*
LA-MA-SA
30.3
aEV
30
Al2O3
Eicosane
36.18
EP
40
CNT
LA-MA-PASA LA-MA-PASA
NDPCM with a nanoparticle as supporting material:
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Huizhen., et al. [81]
28
151.9
118.7
151.9
118.7
SC
Huizhen., et al. [81]
Octadecane
Flake shape
* 1 µm
8.4
160.9
113.7
*
*
0.3
275.91
160.38
O.D. × L 69 nm × 5 µm
Rod
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Yushi Liu., et al. [80] Dandan Mei., et al. [46]
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Phase change behavior of PCM is often restricted by the structure of the supporting framework. So it remains a challenge to achieve the best compromise of high latent heat and high thermal conductivity in form-stable PCMs. Mesoporous carbons with high encapsulation capacity as well as high thermal conductivity are needed for composite form-stable PCM with good thermal energy
EP
storage properties.
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Wang et al. [84] reported a facile method to prepare “graphene-like” mesoporous carbons as supporting frameworks of paraffin PCM. 2 g of MgO was dispersed and stirred vigorously in 800 ml water at 94 °C for 2 hours to form nanosheets, and the suspension was cooled down and precipitated. Then the precipitate was transferred into 25 ml deionized water, into which 2.84 g of sucrose was added with continuous stirring. Subsequently, the mixture was dried at 120 °C in an oven overnight. The collected powders were calcined in a tube furnace under a N2 atmosphere for 4 hours. The products were vigorously stirred in excessive 12 wt% hydrochloric acid for 12 hours and washed thoroughly with deionized water to remove Mg(OH)2 templates. After vacuum freeze 26
ACCEPTED MANUSCRIPT drying at -50 °C for 48 hours, about 0.9 g of the final black spongy powder product was obtained. Then, they prepared NDPCM by a blending and impregnating method. Paraffin was dissolved in alcohol in a 25 ml round bottom flask heated at 80 °C. The prepared porous carbon was added and magnetically stirred continuously until the
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solvent evaporated completely, and then the solid black NDPCM were obtained. Carbon-based NDPCM nanostructure can act as a nucleation agent for increasing the interaction between nanoparticles and form-stable PCM with the help of capillary and surface tension forces. This interaction between nanoparticles and
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form-stable PCM helps to retain the energy storage capacity of NDPCM [83]. CNT:
[85] tried surface-functionalized CNTs as a nanodispersion
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Meng et al.
medium in fatty acid PCM. Surface functionalization on CNTs was done by the oxidation of the strong concentrated nitric acid. Surface functionalization not only makes the CNTs become shorter in length and increases their porosity but also can effectively remove the impurities absorbed by the CNTs. While surface
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functionalization has increased porosity level on CNT, leading to an increase in absorption and also a major reason for the reduction in the size of CNT. The result suggested that NDPCM as porous CNT-fatty acid has reduced the latent heat, which
PCM matrix.
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xGnP:
EP
might be because of the larger dispersion of nanoparticles in the molecules of the
The porous nature of carbon-based material xGnP; surface area (20.41 m2/g)
has reduced the latent heat of NDPCM up to 40.74%, even for high dispersion of 44 wt% of xGnP in octadecane PCM. The octadecane almost fully filled the pores in xGnP, which has prevented leakage of NDPCM using physical interactions [63] (Figure 8). Figure 8b shows leakage proof in the molten state of Octadecane/xGnP even at 35˚C, which is higher than the melting temperature of octadecane.
27
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ACCEPTED MANUSCRIPT
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Figure 8: Leakage proof a) in solid-state and b) in the molten state of Octadecane/xGnP [63] EG:
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Expanded graphite (EG) has worm-like microstructure with multiple pores at micro-scale. Expandable graphite has an average particle size of 300 µm and an expansion ratio of 200 ml/g. EG has small grain size and large inner porosity, which is capable of absorbing a high amount of LA-MA PCM. LA-MA eutectic mixture melting temperature is about 33.62℃ and latent heat is 164.5J/g, while the melting
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temperature and enthalpy of LA-MA/EG composite PCM are 34.29℃ and 122.8 J/g respectively [77]. X. Tang et al. [86] reviewed that EG with a well-developed pore network structure can reduce the seepage of liquid PCMs through capillary force. J.L. Zeng et al. [45]have experimentally analyzed TD/EG as form-stable NDPCM. They
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EG.
EP
determined PCM was much restrained since the TD was constrained in the pores of
When the pore diameter of porous EG ranges from 2 nm to 100 µm, NDPCM’s
latent heat was reduced up to 26.71% even for 13 wt% dispersion of EG in PCM. The reason may be that the EG consists of overlapped graphite flakes, forming abundant crevice-like and net-like pores. The porosity of EG was 92.78%, and the pore volume was as high as 28.00 ml/g. Owing to the porous structure, the melted PCM can be easily adsorbed and fill the pores or adhered onto the flakes of EG [87]. Though the microstructure of the EG particle exhibits a flattened irregular honeycomb network, under SEM it appears as a worm-like microstructure with 28
ACCEPTED MANUSCRIPT multiple pores constructed from elementary graphite nanosheets, which provides abundant crevice-like and net-like pores for adsorbing organic substances like octadecane PCM. Latent heat of NDPCM was reduced up to 10.58 % even for 9.09% of EG in octadecane PCM. The reduction of latent heat may be due to the irregular
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honeycomb network [76]. Supporting material like expanded perlite (EP) has a mesoporous structure consisting of pores. Porous construction gives it the ability to absorb PCM molecule at a specified amount into the EP. EP has a porous structure having mesopores and
SC
macrospores; Organic compounds can be absorbed easily in these pores. PCM and EP interactions which are physical and not strong because of the weak alkalinity of the
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expanded perlite (EP) [88]. The weak interaction between the PCM and inner surface of the porous material also leads to a depression of the phase change temperatures of PCM in the porous materials. The interference of porous matrices of EP, the restriction of crystal arrangement and orientation of PCM into the pores resulted in the decline of regularities of crystalline regions and the increase of lattice defect. The CNTs dispersed into form-stable PCM can affect the local interactions as well as the
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diffusion and rearrangement of PCM molecules into the composite during melting and freezing processes. The presence of CNT, together with EP produces offensive networks to allow encapsulating PCM with the help of capillary and surface tension
EP
forces. The presence of CNTs together with EP, produces offensive networks to allow eicosane such as encapsulation due to physical interaction. Figure 9 SEM images This
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show how the mesopores of EP and how the CNT interacts with EP.
phenomenon may result from the reduction of void space within the composite PCM and extension of the contact surface area for the composite particles [83]
29
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ACCEPTED MANUSCRIPT
Figure 9: SEM photographs of (a) EP, (b) CNTs and (c) EP/Eicosane/CNTs(1%) composite PCM.[83]
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Pores of the Hollow nanotube (HNT) are in the nm range, and surface area is 57.76 m2/g. Therefore, the maximum mass fraction of PCM like capric acid (CA)
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retained in HNT was 60 wt%. CA/HNT can contain capric acid as high as 60 wt% and maintain its original shape perfectly without any leakage of capric acid. When it was heated above the melting point of CA. Due to its large and smooth unhindered pores, GNP was added to the form-stable PCM like CA-HNT as nanodispersion [46]. Silver nanoparticle (Ag) with 130 nm has a porous network structure with
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cylindrical shape; while coated with PAN material turns to 622 nm. Ag-coated PAN nanofibers appeared to have a larger fiber diameter and rougher surface. While fatty acid- Ag coated PAN has reached about 90 % in latent heat enthalpy efficiencies even
EP
for 10 wt% of PAN [78].
Ag-coated PU membranes as supporting materials with different coating times
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(2h, 6h, and 10h) were selected to absorb PCM through the action of capillary absorption, surface tension forces, and nanoconfinement effects. Phase change enthalpies of synthesized NDPCM were slightly reduced from 132.8 kJ/kg to 106.3 kJ/kg, with the increase of Ag coating time, whereas there was no appreciable impact on the phase transition temperatures. This might be due to the increased average fiber diameters, decreased pore diameters, declined specific surface area, and incremental Ag quantities of Ag-coated PU membranes comparing with original PU membranes. This result proved that using micrometer sized metal nanoparticle could decrease the latent heat dramatically [27].
30
ACCEPTED MANUSCRIPT Metal-oxide like aluminum oxide nanoparticle (Al2O3) has a laminar/ rectangular shape, that decreased the latent heat of the NDPCM which may be due to the slight decrease in the eutectic phase change temperatures of composite PCMs because of the weak attractive interaction between LA-MA-SA PCM molecules and inner surface wall of the Al2O3. Latent heat of melting is 113.7 J/g for Form-
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stable/Al2O3, which is 81.3% and 24.1% higher than that of LA-MA-SA/EV and LAMA-SA/aEV, respectively. Similarly, LA-MA-SA/aEV/ Al2O3 has the highest latent heat of freezing. Compared with the LA-MA-SA/EV, the increase of the latent heat of LA-MA-SA/aEV are attributed to acid treatment. The expanded vermiculite with acid
SC
treatment is partially delaminated, corroded, and the Si-OH groups within the expanded vermiculite surface are increasing. Thus, the adsorption capacity of aEV to
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LA-MA-SA is significantly improved compared with EV. Furthermore, the LA-MASA/aEV/Al2O3 has the highest latent heats among composite PCMs. The reason is that the nano-particle size of Al2O3 in aEV/Al2O3 is beneficial to promote LA-MA-SA adsorption due to the high specific surface area. Form-stable PCM maintains the lamellar morphology, suggesting that aEV shows no significant change in morphology
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after loading. [82].
Summary of latent heat of NDPCM:
The reduction in latent heat enthalpies is typical among almost all PCMs,
EP
which, in general, would be attributed to the reduced mass proportion of PCM being embedded with heat transfer enhancement materials. The nanoparticle has no
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contribution to latent heat, as they did not melt within the temperature range of interest.
Fang et al. [89] performed comprehensive measurements on the thermal
storage properties of the prepared NDPCM (Eicosane/GNP) considering the factors including particle size as well as loading level. They obtained graphite nanosheets with different sizes, from the treated NG with the suspensions of N-Methyl-2 pyrrolidone (NMP) using ball milling for 30, 60, and 180 min. The NMP played a vital role in the ball milling process by reducing the energetic penalty for mechanical exfoliation and avoiding graphite nanosheets from re-agglomeration. After that, they 31
ACCEPTED MANUSCRIPT ⸰
filtrated graphite nanosheets repeatedly using DI water, followed by drying at 80 C in a vacuum oven for 48 h. Through AFM images, they determined the thickness of graphite nanosheets to be 5.9, 3.6 and 3.2 nm with the ball milling time for 30 (GNS30), 60 (GNS-60) and 180 (GNS-180) min, respectively. The melting enthalpy of pure eicosane is 249.0 J/g. The incorporation of 3.6 nm and 3.2 nm size nanoparticles led to
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high reduction in latent heat relative to 5.9 nm size and the highest decline of enthalpy around 15% at 5 wt.% for 3.2 nm size nanoparticle, whereas these samples still maintain a latent heat capacity larger than 205 J/g, showing a satisfying latent heat
SC
capacity required for TES systems (Figure 10). Because graphite nanosheets do not undergo the solid–liquid transition in the temperature range evaluated, the enthalpy of composite PCMs is subject to nearly linear drop with the particle content growing.
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Despite no evident increase in enthalpy, these interactions by Lennard-Jones to be enhanced with the growing size of graphite nanosheets, causing smaller drifts in both melting and cooling enthalpies. This study clearly shows the possible reduction for latent heat in NDPCM, is due to the decrease of intermolecular forces and non-melting
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EP
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enthalpy of the nanoparticle.
Figure 10: Latent heat enthalpy vs. particle size [89]
However, Shaikh et al. [69] observed a maximum enhancement of latent heat approximately 13% for the wax/SWCNT composite corresponding to 1% loading of SWCNT. The change in latent heat was modeled by using an approximation for the intermolecular attraction based on the Lennard-Jones potential. Thus, the intermolecular attraction between the molecules of nanoparticles and wax was 32
ACCEPTED MANUSCRIPT considered as the possible reason for the enhancement of latent heat values (Figure
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11)
Figure 11: Latent heat of wax/SWCNT [69]
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Recent studies have demonstrated that the enthalpy of nano-composite PCMs can even be improved due to the Lenard-Jones potential developing between atoms in the crystals. This potential consists of bonded interactions including bond stretching, bending, and torsion as well as non-bonded interactions between CH2 and CH3 sites described by the Lenard-Jones (LJ) potential acting between sites in different
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molecules. According to Lenard-Jones [69,90], intermolecular forces may either be attractive or repulsive. Intermolecular forces can be conveniently divided into two classes: short-range forces, which operate when the centers of the molecules are separated by 3 Å or less and long-range forces, which operate at greater distances.
EP
Generally, if molecules do not tend to interact chemically; the short-range forces between them are repulsive. Long-range forces, or VanderWaals forces, are attractive
AC C
and account for a wide range of physical phenomena, such as friction, surface tension, and adhesion and cohesion of liquids and solids. The chemical potential energy in the weak Vander-Waals attractions is due to strong ionic and covalent bonds. The latent energy will increase while strengthening the weak Vander-walls forces. Effect on thermal conductivity Thermal conductivity is primarily related to the matrix in PCM. Thermal conductivity is associated with collective phonon motions over long distances in a perfect crystal structure and molecular alignment in the matrix of PCM. However, the motion of phonon is slower in the matrix of PCM due to lack of interaction between 33
ACCEPTED MANUSCRIPT atoms in a matrix of PCM and improper alignment of molecules during phase transition like liquid-solid. The improper alignment of molecules leads to an abnormal crystal structure of a lattice of PCM and also leads to delay in the accurate transmission of phase change [91]. It can also lead to a decrease in thermal conductivity due to inappropriate jumping of an atom in the energy level in the matrix
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of PCM. The phonon motion in the matrix of PCM in a particular direction over a long distance can increase the thermal conductivity.
During the melting process, solid-state composite was melted into its liquid
SC
state by increasing the temperature [24]. An increase in temperature accelerates molecular vibrations in the matrix of the orderly solid structure of PCM. Density
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gradient in the liquid state of PCM (due to temperature gradients) generates buoyant forces which may cause convection in the bulk liquid PCM. Besides this convection of liquid, viscous forces tend to slow down the motion [42].
As the viscous force decreases, the thermal conductivity increases in the liquid state in a very negligible amount. However, according to buoyant force, an increase in
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temperature can cause phase transmission due to the pressure variation in a fluid caused by the force of gravity, but, in general, buoyant forces act opposite to the direction of the gravity, buoyant forces cause convection in the bulk liquid PCM. Besides this convection of liquid, viscous forces tend to slow down the motion of the
EP
phonon. This limits the thermal conductivity in the liquid state.
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During the solidification process, the reverse of melting occurs, during which the PCM loses energy to the surrounding, and the atomic or molecules renew their chemical association (i.e., chemical bonds) thus reforming the ordered phase resulting in solidification. PCM forms crystal structures, which increase the area of contact between atoms in the matrix of PCM. According to Gibb free energy, the atom can absorb the energy and gets arranged in an improper mode in the matrix of PCM. The improper alignment of molecules during crystal structure formation tends to decrease the thermal conductivity in solid-state. 34
ACCEPTED MANUSCRIPT The addition of foreign particle in nano-meter or micro-meter size in the matrix of PCM has a potentially positive effect on thermal conductivity due to the ordering of the atomic structure of the solid-liquid or liquid-solid interface [91]. Figure 12 depicts the envisaged movement of phonon in PCM and NDPCM. Dispersion of
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dispersing nano-sized particles in the PCM.
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micro-sized particles causes sedimentation, and this problem can be resolved by
(b)
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(a)
Figure 12: a. Movement of phonon before the dispersion of nanoparticle b. Movement of phonon dispersion after the dispersion of nanoparticle
EP
Dispersion of nano-sized particles can enhance the thermal conductivity of base PCM significantly by reducing the void space in the matrix of PCM. The linear
AC C
change in the thermal conductivity depends on the mass of immersion in a matrix of PCM [83].
The reason behind Thermal conductivity enhancement in NDPCM Thermal conductivity enhancement becomes more pronounced, probably due to more intensive diffusion of nanoparticles like Brownian motion [24], Clustering of nanoparticles [92], Interfacial thermal resistance [91], high aspect ratio [93], purity [32], intermolecular interaction [42] and concentration [94] of nanoparticles. In NDPCM, thermal conductivity enhancement was higher in phase transmission temperature of base PCM, because kinetic energy remains constant; however, phase 35
ACCEPTED MANUSCRIPT change can occur [48]. Different NDPCM thermal conductivity models developed or
AC C
EP
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SC
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used in the literature are summarized in Table 8.
36
ACCEPTED MANUSCRIPT
Table 8: Various models for thermal conductivity of NDPCM
− /62 : − /62
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Remarks /234 ; = /<=>?@ + A />?BC?@ − /<=>?@ D E ;
0, ; < ;J − K; ;J − K; ≤ ; < ;J − K; E ; = F ; − ;J + K; ⁄ 2K; , 1, ; > ;J + K;
/011
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Eicosane/ Graphite [94] Eicosane/ Graphene [66]
E] ; \ _ ;, 9 234 Q62 ^62
R01, = 5 × 10U VW X9 YQ3Z [
3 + 9[2Vkk 1 − lkk + Vmm 1 − lmm ] = /234 5 : 3 − 9[2Vkk lkk + Vmm lmm ]
EP
Nan et al. [98,99]
Organic PCM/ Graphene [97]
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Maxwell [95]
9OP Q234 9OP Q234 + 1 − 9OP Q62
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9=
SC
Model Model Formulation developed modified by and used by Maxwell Eicosane/ /62 + 2/ − 29 / /011 = /234 5 [95] Ag [48] /62 + 2/ +9 / Eicosane/ CuO [44] Octadecane / Cu [96]
; ;h01 + −3.0669 × 10cd 9 − 3.91123 × 10cg
_ ;, 9 = 2.8217 × 10cd 9 + 3.917 × 10cg
(taken into account for the Brownian motion in molten NDPCM) lkk = d
op op cq
+ d
lmm = 1 − 2lkk Vkk =
Vmm =
o tuv cq w qcop r/p
R1,kk − R234
R234 + lkk AR1,kk − R234 D R1,mm − R234
R234 + lmm AR1,mm − R234 D
37
ACCEPTED MANUSCRIPT
Koo et al. [104]
PCM/ Al2O3 Numerical Simulation
[105,106]
/011 =
5R< + ˆ ‰Š
qc‹
R< + 5 Š U g
qc‹ gŒ
−
gŒ
VW = •
U
0, 1,
†•€•
‡
• AR1 − R< D: ŽR< +
1 − • + ˆŠ
/62 + 2/ /011 = /234 5 /62 + 2/ +
5 × 10
gŒ
qc‹
…
qc‹ gŒ
gŒ
AR1 − R< D •
For nanoparticle with high porosity and high thermal conductivity.
− 1 − • : AR1 − R< D
− 29 / +9 /
‘ “ VW X9 YQ3Z [ Š ’ _ 234 ”~• @~•
; < ;–h00m?y— ; > ;40>P?y—
qc‹
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Organic PCM/ Foam [103]
|o }~• ⁄}•€• g o‚ pƒ„ × †~•
;, 9
− /62 : − /62
SC
Mesalhy et al. [102]
/011 = R234 {1 +
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Eicosane/ xGnP [101]
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Yu et al. [100]
^62 2x011 + ^62 //1,?ycz>oy0 wW = xW R234 R1,kk =
Numerical simulation of solidification of NDPCM for latent heat thermal energy storage at 0oC. [107–109] {Solidification of water/CuO} W˜™™
Wšƒ›˜
=1−3
†~• ‚q•| †šƒ›˜ †~• † •|‚œ ~• ‚d• œqc †šƒ›˜ †šƒ›˜
¡ A^ , ;, 9 D 6z
=
œc
+ 5 × 10U 9Qžo<0 Ÿz _Š
Wšƒ›˜ “
”~• @~Z
¡ A^ , ;, 9 D 6z
[110]
d
ŽYwq + wd lnA^6z D + wg ln 9 + wU ln 9 lnA^6z D + w¢ lnA^6z D [ ln T + d
Yw£ + w¤ lnA^6z D + w¥ ln 9 + w¦ ln 9 lnA^6z D + wq] lnA^6z D [• {Ÿu¨^©ª©u¨ 9 ≤ 0.04, 300R ≤ ; ≤ 325/} { wq … … wq] = Ÿu − ®__©t©®¨ª ¯w°±® u_ ²wª®³/Ÿ±´ } [108,110]
AC C
EP
Laser flash apparatus used to measure the thermal conductivity Parker et Octadecane R = µ × Ÿ2 × Q al. [111] / GA [71] /234 = Thermal conductivity of PCM ² ⁄¹R ; /62 = Thermal conductivity of nanoparticle ² ⁄¹R ; /žo<0 = Thermal conductivity of base Matrix ² ⁄¹R ; 9 = Volume fraction of the additives; 9OP = weight fraction of the additives; Q = density of the discrete / ⁄¹g ; /011 = ½__®tª©¯® ªhermal conductivity of NDPCM ² ⁄¹R ; ÀŸÁ = Àℎwv® Ÿℎw¨ ® Áwª®³©w° ; ÃÀ = Ãw¨uÄw³ª©t°® ; /<=>?@ & />?BC?@ = ;ℎ®³¹w° tu¨^±tª©¯©ªÆ u_ ÀŸÁ ©¨ Çu°©^ & l©È±©^ Àℎwv® ² ⁄¹R ; ;J & K; = Melting temperature and transition range of PCM / ; VW = ½¹Ä©³©tw° _±¨tª©u¨ u_ E³uɨ©w¨ ¹uª©u¨; X = correction factor for brownian motion; ^ = Ê©w¹®ª®³; Ÿ2 = Specific heat capacity /Ì⁄/ R ; E] = Eu°ªÍ¹w¨¨ tu¨vªw¨ª 1.381 × 10cdg Ì⁄R ; ;h01 = x®_®³®¨t® ª®¹Ä®³w±³® / ; ; = ;®¹Ä®³w±³® / ; w = Aspect ratio of NP; R1,kk = Modified in − plane thermal conductivity of NP ² ⁄¹R ; R1,mm = Out − of − plane thermal conductivity of NP ² ⁄¹R ; x011 = Effective interfacial thermal resistance; R1,?ycz>oy0 = In − plane thermal conductivity of NP ² ⁄¹R ; R< = ;ℎ®³¹w° tu¨^±tª©¯©ªÆ u_ ÀŸÁ ©¨ ªℎ® Äu³u±v ¹®^©w ¹wª³©Ò ² ⁄¹R ; • = The metal foam porosity; R1 = Thermal conductivity of the metal foam ² ⁄¹R ; µ = thermal diffusivity ¹d ⁄v .
38
ACCEPTED MANUSCRIPT a) Concentration of nanoparticles: In NDPCM, thermal conductivity enhancement is generally based on the dispersion of mass fraction of nanoparticles in PCM and operating temperature. The rise in thermal conductivity is observed when the mass fraction of the nanoparticles
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increases in all measured temperatures up to the 2 wt% loading for all kind of nanoparticles in the matrix of PCM [48]. b) Brownian motion:
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The random movement of the particles is the stochastic Brownian motion, caused by the impact of the base fluid molecules on the surface of the particles. This
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was discovered by Robert Brown in 1828 and physically described in 1956, by Einstein. According to the kinetic theory of matter, the random motion of each particle is independent and eternal, being composed of translation and rotation movement. Brownian motion is more active in liquids with lower viscosity, for higher temperatures, and the smaller dispersed particle [112–114]
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Brownian motion increases the thermal conductivity in nanofluids due to the following two effects: (1) collisions between Brownian particles, and (2) Nanoscale convection induced by the Brownian particles [112].
EP
Despite the interactions, unless there is some attraction between the particles, the Brownian interparticle collisions could never occur and lead to aggregation.
AC C
Derjaguin [115], Verway [116], Landau and Overbeek (DVLO) developed a theory suggesting that aggregation is determined by the sum of the van der Waals attractive and repulsive forces between the approaching Brownian particles. According to DLVO theory, if the attractive forces are larger than the repulsive forces, the particles will collide and aggregate. On the other hand, if the attractive forces are lower than the repulsive forces, the particles may collide (or not), but without a permanently fixed contact [112].
39
ACCEPTED MANUSCRIPT However, according to Xuan et al. [117], The aggregation of the nanoparticles may also produce a negative impact on the enhancement of the thermal conductivity since the Brownian motion will be slower. According to Choi et al. [118], when the temperature is increased, the viscosity
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of base fluids is decreased, the Brownian motion of nanoparticles is increased, and consequently, convection like effects are remarkably increased, resulting in increased conductivities. These temperature-dependent conductivities are not predicted by Maxwell theory with motionless nanoparticles.
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The effective thermal conductivity of the NDPCM includes the effects of particle size, particle volume fraction, and temperature dependence as well as
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properties of the base PCM and the effects due to Brownian motion. Vajjha et al. [119] has proposed a model for the effective thermal conductivity of nanofluid as /011 = R
while the second term corresponds to Brownian motion with a correction factor. The correction factor takes into account for the Brownian motion in molten PCM and not
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in the solidified PCM. Valan Arasu et al. [105,106] used the same model in the numerical studies of nanoparticle dispersed paraffin wax PCM. Abdollahzadeh et al. [120] have numerically analysed the effects of Brownian
EP
motion on freezing of PCM containing nanoparticles; they made the following assumptions for generating the governing equation: (a) the NDPCM is an
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incompressible Newtonian fluid. (b) thermos physical properties of the NDPCM are constant; (c) buoyancy force is modeled with Boussinesq approximation; (d) base PCM and the solid nanoparticles are in thermal equilibrium.
They determined,
Brownian velocity as a function of the temperature of the liquid phase only, and it becomes slower near the solidification front whose the temperature is low. The averaged motion of the particles is an interesting consequence of the Brownian movement of particles in fluids with an externally sustained and constant temperature gradient: It becomes apparent that particle dispersion is higher and the Brownian force is stronger when the local fluid temperature is higher. When there is a 40
ACCEPTED MANUSCRIPT temperature gradient in the flow domain of the suspension, small particles disperse faster in hotter regions and slower in colder regions. It must be noted that the particles in suspension will not fully accumulate in the colder region. Interparticle collisions in the colder regions, where the particle concentration becomes higher, would disperse the particles stronger than in the hotter regions, where the particle concentration is
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lower. Thus, in the absence of other dispersion mechanisms – such as turbulence, velocity fluctuations, shear forces, lift forces, etc. A dynamic equilibrium for the particle concentration will be established, with lower particle concentrations in the
SC
hotter regions and higher concentrations in the colder regions [121,122].
Lamas [112] suggested, based on the review of numerical study results, that the
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Brownian motion intensity has a probably negligible impact on the interparticle collisions since the average collisions and standard deviation seems to be independent of the base fluid viscosity and temperature. In addition, the results suggested that the interparticle interaction and spatial distribution depends on the aspect ratio of the nanoparticles. For a given concentration, the average distance between particles is reduced for elongated particles, increasing the influence produced by the displacement
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of the fluid. Interparticle interactions and spatial distribution increases with increasing nanoparticles aspect ratios [123].
Increase in temperature, increases the interparticle collision by Brownian diffusion
EP
of the particle (When two-particle collide, the solid-solid heat transfer mode could increase the effective thermal conductivity of the Nanofluid). The high surface-to-
AC C
volume ratio of the nanoparticles induces strong van der Waals interparticle interactions. These interactions, associated with the Brownian movement, cause the aggregation of the nanoparticles, which begin to behave as micrometer particles. This will result in the settlement and clogging of the channels, but also in the decreasing of the thermal conductivity of the Nanofluid [124]. c) Clustering of nanoparticles: In NDPCM’s solid phase, a cluster of nanoparticles has enhanced thermal conductivity due to the aggregation of nanoparticles by physical interaction like 41
ACCEPTED MANUSCRIPT adhesion forces between the neighbouring nanoparticles in the matrix of NDPCM. When the clusters are formed, thermal conductivity can be enhanced even in the solidstate due to the aggregation. In NDPCM, clustering of nanoparticles is the main factor for larger enhancement of thermal conductivity in solid-state than in the liquid state. Then clustering holds the key to the thermal conductivity enhancement of NDPCM
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[24].
However, clusters are formed by a collection of atoms up to about 50 atoms. The cluster of nanoparticles has a high possibility of sedimentation of nanoparticles in
concentration of nanomaterial in NDPCM [92].
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NDPCM. But, desirable stability can be achieved for a relatively low mass
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In NDPCM, thermal conductivity enhancement was of monotonic order for the mass between 1 wt% and 2 wt% of nanomaterial. (Monotonic is a standard deviation relation between the mass of nanomaterial and PCM molecule in the matrix of NDPCM; the relation was non-linear with either entirely never increases (or) never decreases). However, when the mass of nanomaterial is less than 1 wt% in NDPCM,
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the thermal conductivity enhancement is dependent on operating temperature. The source of the sharp rise in the measurements near the melting temperature is attributed to the non-equilibrium state of the samples near the phase transition. With nanoparticle loadings of 1 and 2 wt%, the thermal conductivity was monotonically
EP
raised as the concentration of the additives was increased for all the measured
AC C
temperatures [44].
In NDPCM, thermal conductivity enhancement was of a non-monotonic order for the mass-less than 1 wt% of nanomaterial immersion in NDPCM and also nonmonotonic for the mass greater than 2 wt% of nanomaterial immersion in the matrix of PCM. (Non-monotonic is a standard deviation relation between the mass of nanomaterial and PCM molecule in the matrix of PCM, which linearly increases or decreases). Non-monotonic relation between the thermal conductivity and the mass fraction that was independent of the temperature range studied, was exhibited when the mass fraction was greater than 2%. For a given particle loading, the measured thermal conductivity values were generally insensitive to the measurement 42
ACCEPTED MANUSCRIPT temperature. [44]. In NDPCM, for the highest loading of the nanoparticles being 10 wt%, the measured thermal conductivity values were the greatest quantities recorded
AC C
EP
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SC
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at each measurement temperature [44].
43
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Mass greater than 10 wt% of nanomaterial immersion in PCM has a high possibility for formation of sedimentation of nanoparticles and also the highest loading of nanomaterial in PCM exhibits non-proper crystal structural formation of solidification
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process [92]. Table 9 condenses the thermal conductivity of NDPCM in the dispersion of metal, metal- oxide nanoparticle and carbon nanostructure
Reference PCM
Melting Temp (˚C)
Material
Octadecane
28
TiO2
0.5
0.17
0.79
Eicosane
28 28 36.18 36.18 36.18
TiO2 TiO2 CuO CuO CuO
1 2 1 2 3.5
0.17 0.17 0.58 0.58 0.58
0.56 0.83 0.61 0.64 0.65
CNT CNT CNT xGnP xGnP GA GA GNP
0.05 0.1 0.15 0.1 0.45 6.25 9.09 1
0.15 0.15 0.15 0.15 0.15 0.23 0.23 0.13
0.15 0.15 0.16 0.16 0.19 4.61 5.92 0.37
Octadecane
Sivasankaran., et al.[32]
Octadecane
Jing Yang., et al. [62]
Octadecane
Min Li., et al. [41]
RT27
27 27 27 27 27 28 28 28.81
EP
Carbon nano-material Sivasankaran., et al.[32]
AC C
Mahdi Nabil., et al. [44]
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Metal-Oxide nano-particle
SadeghMotahar., et al. [42]
Thermal conductivity (W/mK) wt.% Pure NDPCM of Con PCM
Nano- Particle
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Base PCM
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Table 9: Thermal conductivity of NDPCM
Analysis Method
Nano-particle
Size
Shape
25nm Spherical
HD Dia 5-15 nm
dia 0.7−2 nm
Rod
think 6−10 nm; dia 25 µm
*
LFA
3nm
Flake
HD
dia 35 nm
*
HW
44
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Seulgi Yu., et al. [40]
Biobased
Seulgi Yu., et al. [40]
Biobased
Yongpeng Xia., et al. [71]
Octadecane
28.91 28.91 29.4 29.4 29.4 29.4
xGnP xGnP CNT CNT xGnP xGnP
3 5 1 3 1 3
0.50 0.50 0.15 0.15 0.15 0.15
0.87 1.00 0.41 0.49 0.27 0.61
31.45
GA
1.63
0.15
0.26
31.45 31.45 36
GA GA CNT
1.96 2.43 1
0.15 0.15 0.41
TCi
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Octadecane
SC
Jisoo Jeon., et al. [31]
0.39 0.43 0.57
THB
think 10 nm; dia 15 µm
*
100 nm
Hallow tube
think 10 nm; dia 15 µm
Hexagonal
*
Flake
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d) Aspect ratio:
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Nitesh Das., et al. [93] Eicosane dia 1.35nm * * Mikali Temirel., et al. think 15 nm; dia Eicosane 36.4 xGnP 1.57 0.41 1.05 * HW [101] 25 µm * Not reported HW= Hot-Wire., LFA= Laser flash apparatus., HD= Hot Disk., TCi= TCi Thermal Conductivity analyser., THB= Transient Hot Bridge
EP
In NDPCM, thermal conductivity enhancement depends on aspect ratio, because the aspect ratio is the ratio of
AC C
Length/Diameter (or) Length/Thickness, which mainly depends on particle size. Thermal conductivity enhancement of PCM is higher for smaller diameter of nanoparticle than the larger diameter of nanoparticle since small diameter nanoparticle can form a chain-like structure due to higher interaction forces [125].
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ACCEPTED MANUSCRIPT Effect of Metal nanoparticle and Metal oxide nanoparticle: Metal-based Nanoparticles are synthesized from metals to nanometric sizes either by destructive or constructive methods. The nanoparticles have distinctive properties such as sizes as low as 10 to 100 nm and shapes like spherical and cylindrical. They have characteristics like the high surface area to volume ratio and
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surface charge density.
Metal nanoparticle like silver (Ag) nanoparticle with the size of 10–30 nm diameter and length of 5–15 µm appears to be spherical shaped and does not have a
SC
higher impact on thermal conductivity on NDPCM. The thermal conductivity on NDPCM was increased only by 33.33 % for 2 wt.% of Ag nanoparticle in organic
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PCM but has shown contrast result on thermal conductivity of NDPCM for 10 wt.% of Ag nanoparticle which shows that there is no strong interaction between Ag particles and Eicosane [37].
Metal oxide-based nanoparticles are synthesized to modify the properties of their respective metal-based nanoparticles. These nanoparticles have possessed excellent
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property when compared to their metal counterpart.
In NDPCM, metal oxide nanoparticle like titanium dioxide nanoparticles (TiO2) of size 25 nm with spherical shape was dispersed in the matrix of PCM; the thermal
EP
conductivity of eicosane enhanced from 0.17 W/mK - 0.56 W/mK when analyzed with the hot disk. Generally, TiO2 has the property of sensational dispersion, chemical
AC C
stability, and non-toxicity. However, the spherical shape and property like sensational dispersion have great attraction between nanoparticles and the PCM molecule in NDPCM [42].
Copper oxide nanoparticles (CuO) of size 5- 15 nm with spherical shape was dispersed in the matrix of eicosane PCM [44]. The values of the thermal conductivity increases as the nanoparticle loading is raised up to 5 wt%, while the thermal conductivity of the composite decreased when the loading is increased to 6.5 wt.%, probably due to more intensive diffusion of nanoparticles, e.g., Brownian and thermophoretic diffusion, at higher temperatures. Also, the discrepancy could result 46
ACCEPTED MANUSCRIPT from the neglected temperature dependence on the thermal physical properties, effects of diffusion of nanoparticles in the liquid, and volume change upon freezing. The pronounced void formation was also observed in the solid layer upon freezing. Effect of Carbon nanostructural material:
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In NDPCM, Thermal conductivity is enhanced to a higher range due to the presence of CNT. CNT has the thermal conductivity of (3500 W/mK), and CNT is a low-density material with the high surface area (50 - 1315 m2/g). High surface area helps to increase a strong interaction between CNT and PCM molecule in the matrix
SC
of NDPCM. CNT has high aspect ratios, large specific surface area, and substantial Van der Waals attractions and these carbon nanotubes tend to self-aggregate into spontaneously.
Flexibilities
increase
the
possibility of
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bundles
nano-tubes
entanglement and close packing [30].
Babaei et al. [91] experimentally studied CNT of diameter 100 nm and hollow tube shape dispersed in bio-based PCM. The thermal conductivity of NDPCM increased from 0.15W/mK – 0.41 W/mK even for 1 wt.% of CNT in bio-based PCM.
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The hollow tube structure was able to hold the arrangement of PCM molecule in the matrix of NDPCM. Therefore, the CNT provides a template for crystallization leading the PCM molecules to get aligned with the nanotube, which in turn leads to an
EP
increase in thermal conductivity in a greater range. The template helps to improve the phonon motion in a particular direction over a long distance. The phonon motion in
AC C
the matrix of PCM in a particular direction over a long distance can increase the thermal conductivity [91]. In NDPCM, nano graphite (GNS), 60nm and planar shape were dispersed in
organic PCM. Thermal conductivity of NDPCM increased from 0.1725 W/mK 0.2631 W/mK even for one wt. % of GNS. Two-dimensional planar structure leading to reduced geometric contribution of phonon mismatch-induced phonon scattering at the filler-matrix interfaces is responsible for the low thermal interface resistance of GNS [70].
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ACCEPTED MANUSCRIPT The porous nano-sized carbon-based material like exfoliated graphite nanoplatelets (xGnP), 10 nm thick and 15 µm in diameter having hexagonal nature were dispersed. Thermal conductivity of NDPCM increased from 0.15 W/mK - 0.27 W/mK even for 1 wt. % of sample [31,40]. Exfoliated graphite sheets were in a
the high specific surface area (SSA) of xGnP.
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combined layer structure. Thermal conductivity enhancement of PCM might be due to
Liu et al. [59] used graphene oxide (GO) before it reduces to graphene. The particle size of GO ranges between 500 nm to 2.5 µm, and the thickness of GO is
SC
about 1.318 nm and appeared to be hexagonal shaped. Thermal conductivity of NDPCM increased from 0.70 W/mK – 1.05 W/mK even for 2 wt. % of GO. Due to a
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large number of oxygen-containing functional groups on the surface of GO, which made GO nanosheets compatible well with PCM; the large two-dimensional sheet structure of GO can block migration of phonon transfer resulting in reduced thermal conductivity of the PCM electric charges in NDPCM composite, resulting in a smaller electrostatic interaction.
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Putra et al. [58] experimented with directly obtained graphene of size ranging from 21.5 to 65.5 nm with the width 3–5.5 nm in the form of platelets/rolled-thin sheets dispersed in RT-22. Thermal conductivity enhancement of 306 % even for 0.05
EP
wt.% graphene was obtained.
The thermal conductivity of NDPCM increased dramatically to the higher
AC C
range due to the presence of GNP. GNP had high thermal conductivity (3000–5000 W/mK) and low-density carbon nanostructure. Plate like GNP structure helps to form a proper molecular alignment in NDPCM. Proper molecular alignment leads to proper phonon motion in the matrix of PCM in a particular direction over a long distance, thus increasing thermal conductivity (Figure 13) of NDPCM. In NDPCM, thermal conductivity enhancement by GNP is due to high interfacial thermal resistance [58].
48
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ACCEPTED MANUSCRIPT
Figure 13: Thermal conductivity of RT-22 with graphene nanoparticles [58]
Li et al. [72] synthesized surface-functionalized graphene by annealing process and dispersed it in decosane PCM. The thermal conductivity of NDPCM increased
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from 0.26 to 0.59 W/mK in the presence of spongy graphene of 3 mg/cm3, i.e., more than 2 times in such a low graphene concentration. Dispersion of Supporting material:
EP
Supporting material has high porosity and low thermal conductivity. The pores lead to an increase in the absorption of PCM molecule and thus prevents the leakage
AC C
of PCM. But, the thermal conductivity of PCM with supporting material decreases due to the low thermal conductivity of the supporting material and lack of photon transmission in the matrix of form-stable PCM. However, thermal conductivity can be enhanced in form-stable PCM, by immersion of nanoparticles. Nanoparticles increase photon transmission, hence the thermal conductivity in form- stable NDPCM. Table 10 presents the measured values of the thermal conductivity of PCM with supporting material and nanoparticles.
49
ACCEPTED MANUSCRIPT
Table 10: Thermal conductivity of form-stable NDPCM Supporting material
Reference
wt.% wt.% Material of Con of Con
LA-MA-PA LA-MA-PA LA-MA-PA
CNT CNT CNT
10 20 30
-
Octadecane
28
xGnP
5
-
Octadecane
28
xGnP
44.1
-
Octadecane
28
xGnP
1
-
Octadecane
28
xGnP
3
Tetradecanol
35.87
EG
Tetradecanol
35.87
RT25 Octadecane
0.41 0.63 0.64
-
0.83
2.12
-
0.26
1.36
-
0.28
0.91
-
-
0.28
0.98
10
-
-
0.42
3.22
EG-T
10
-
-
0.42
3.11
27
LDPE
50
EG
5
0.26
0.42
28
Na-MMT
*
xGnP
5
0.83
2.12
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* * *
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PatrikSobolciak., et al. [43] Su-Gwang., et al. [80] Sayanthan Rama., et al. [126] Dandan Mei., et al. [46] Haiting Wei., et al. [82] Ahmet S., et al. [88]
Analysis Method
NDPCM
EP
Ju-Lan Zeng., et al. [45]
-
Pure
SC
Material
Nanoparticle
Size
Shape
HD
dia 8-15nm & length 50µm
Rod
TCi
Surface area (m2/g) 20.41 Flake shape
DRX
300 µm *
DICO
LDPE 200 µm
TCi
Surface area (m2/g) 20.41 dia 1-2 µm; thick 1-5 nm
* Flake shape
RT 27
28.81
EP
49.5
xGnP
1
0.34
0.51
CA
29.62
AC C
Su-Gwang Jeong., et al. [80] Sughwan Kim., et al. [63] Seunghwan Wi., et al. [56]
Thermal conductivity (W/mK)
Nanoparticle
Melting Temp (˚C) 21 21 21
PCM Xin Men., et al. [85]
Support Material
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Base PCM Material
HNT
35
GNP
5
0.48
0.76
HW
*
*
LA-MA-SA
30.3
aEV
30
Al2O3
8.4
0.26
0.67
*
*
Laminar
CA
32.14
EP
55
EG
10
0.09
0.14
HW
*
*
Cylindrical
50
ACCEPTED MANUSCRIPT
Methyl stearate
32.5
App
5
EG
15
0.83
3.14
Ali Karaipekli., et al. [83]
Eicosane
36.18
EP
40
CNT
0.3
0.22
0.19
Eicosane Eicosane
36.18 36.18
EP EP
40 40
CNT CNT
0.5 1
0.22 0.22
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Hassina., et al. [127]
0.24 0.32
KD2
3-4 µm
Flake
O.D. × L 69 nm × 5 µm
Rod
SC
* Not reported HW= Hot-Wire., DRX = DRX-i-RX Hot-Wire., LFA= Laser flash apparatus., HD= Hot Disk., TCi= TCi Thermal Conductivity analyzer
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CNT
Meng et al. [85] analyzed surface-functionalized CNTs as nanodispersion medium in fatty acid PCM. Surface functionalization of CNTs was done by the oxidation of strong concentrated nitric acid. By oxidation, since CNT has a multi-porous
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structure; fatty acids were successfully absorbed into the porous structure of the CNTs. Figure 14 shows SEM images of porosity of CNT on treating and how it reacts with the fatty acid. However, scattering of phonon is possible by acid treatment and thermal
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composite fatty acids/CNTs.
EP
conductive net-work formed between the fatty acids and the CNTs will lead to a greater enhancement in thermal conductivity of the
51
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Figure 14 : SEM image of CNTs a) untreated CNTs, b) treated CNTs and the fatty acids/CNTs composite with various weight percentages of the fatty acids, c) 50 wt% fatty acids and d) 60 wt% fatty acids [85]
xGnP
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High SSA (specific surface area) in xGnP is due to exfoliation of graphite interaction compound (GIC). During the exfoliation of GIC, a large amount of expansion increases its size which is due to breaking of the balloon wall (formed by sliding the carbon layer relative to one another; sliding enables the balloon wall). High
EP
SSA in xGnP leads to high porosity, high porosity with high thermal conductivity help to increase the absorption force between the PCM molecule and xGnP in the NDPCM
AC C
[63,65]. The strong interaction with higher thermal conductivity helps to enhance thermal conductivity in PCM. EG:
EG, have worm-like microstructure, small grain size, and large inner porosity, which is capable of absorbing a high amount of PCM molecule. Higher the EG porosity will result in higher absorption capability. The large surface area due to rapid heating at high temperature in a muffle furnace expanded the graphite. Large surface area and pores in the EG help to increase the absorbance. The pore size of the EG
52
ACCEPTED MANUSCRIPT network is at micro-scale. A large amount of well-developed pore network structure of raw worm-like EG can act as the carrier of phonon in the PCM molecule by physical interaction [86]. J.L. Zeng., et al. [45] have analyzed surface modified EG by concentrated nitric
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acid; the surface volume has increased from 200 ml/g to 0.2 ml/g. Thermal conductivity of NDPCM has increased from 0.42 W/mK to 2.76 W/mK even for 7 wt.% of EG. This result suggests that the increased surface volume has more possibility in increasing the thermal conductivity.
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Supporting material with low thermal conductivity has high porosity. The pores were mesoporous. Thermal conductivity decreases due to low thermal conductivity in
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the supporting material and lack of phonon transmission in the matrix of form-stable PCM. Thermal conductivity can be enhanced in form-stable PCM, by immersion of nanoparticles. Higher thermal conductivity particles like nanoparticle will increase the connectivity of the structure. Nanoparticle like carbon-based nanostructure is added to form-stable PCM molecule in NDPCM. These nanoparticles increased the phonon
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transmission in particular direction in NDPCM during the phase change process [78], and hence, the thermal conductivity of NDPCM is enhanced in form- stable PCM. Karaipekli et al. [83] tried EP as supporting material which has a mesoporous
EP
structure consisting of pores and holes with a diameter below 100 µm. The presence of CNTs together with EP produces networks like encapsulation. The EP was
AC C
encapsulated (C20) in the PCM with the help of capillary and surface tension forces to form-stable PCM. The CNT with dia 6-9 nm and rod shaped, which was used as nanoparticle dispersion in EP/Eicosane form-stable PCM. The uniform dispersion of CNT additives in the composites played a beneficial role in enhancing the heat transfer performance of EP/Eicosane composite. Moreover, the thermal conductivity of EP/C20/CNTs (0.3 wt%) decreased, but the thermal conductivity of EP/C20/CNTs (0.5 wt%) was increased. This phenomenon may be resulted from the reduction of void space within the composite PCM and increased contact surface area for the composite particles.
53
ACCEPTED MANUSCRIPT Jeong et al. [80] have analyzed with octadecane as PCM encapsulated in NaMMT as supporting material and xGnP as a nanoparticle. PCMs molecules were retained easily in the pores of xGnP. PCMs could integrate into the structure of xGnP and Na-MMT due to their physical bonding without changing their chemical properties. xGnP is more porous than Na-MMT powder so that more PCM could be
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incorporated into the porous structure of xGnP. Thermal conductivity of form-stable PCM octadecane/Na-MMT/xGnP increased from 0.83 W/mK to 2.12 even for 5 wt.% of xGnP.
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Summary of thermal conductivity of NDPCM:
In general, the thermal conductivity of suspended nanometer-sized particles of
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PCM is higher than the pure PCM. Moreover, for high aspect ratio materials (CNT and GnP), in the presence of interfacial thermal resistance, the effective thermal conductivity enhancement of the nanocomposite becomes insensitive to the
nanomaterial thermal conductivity when the ratio R6z ⁄R234 > 1000. For low aspect
ratio materials like metal nanoparticle and spherical nanoparticles, the thermal conductivity ratio of nanoparticle and the PCM is an order of lesser magnitude
TE D
(R6z ⁄R234 > 1000) [128].
In NDPCM, smaller sized NP behaved extremely well in improving the
EP
thermal conductivity when they were dispersed into the base PCM. It is noteworthy that the interparticle repulsive forces and the Brownian motion exhibited by NP were
AC C
appreciable, which in turn was evident from the improved thermal conductivity. However, the researchers have experienced contrast result in a case for smaller nanosheets that possess higher specific surface area; phonons are easily scattered at the filler/PCM interfaces. As a consequence, the thermal conductivity of sample containing 5 wt.% of GNS-30 (5.9nm) is observed to be 1.57 W/m K, which is 258% higher than the baseline of eicosane, while for GNS-60 (3.6 nm) and GNS-180 (3.2 nm) at the same concentration, the enhancement ratios are 180% and 109%, respectively (Figure 15). The researcher has suggested that thermal conductivity on
54
ACCEPTED MANUSCRIPT NDPCM does not depend on the smaller size of the nanoparticle, but depends on the
SC
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kind of heterogeneous nucleation [89].
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Figure 15: Thermal conductivity vs. wt.% of GNS [89] Nabil et al. [44] observed effective thermal conductivity using CuO with dia 515 nm and spherical shape as nanodispersion in eicosane PCM and measured experimentally by using the transient plane source technique. The effective thermal conductivity was analyzed by subjecting the NDPCM to one of the three solidification
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procedures, i.e., ambient solidification, ice-water bath solidification, or oven solidification method (Figure 16). In NDPCM, an increase in thermal conductivity depends on the results which suggest that at a constant concentration of particles if the
EP
solidification process takes place over a longer period, the thermal conductivity values increase. The hypothesis is that longer durations of phase change leads to the
[44].
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formation of bigger micron-scale grains in the structure of solid NDPCM composites
.
55
ACCEPTED MANUSCRIPT Figure 16: Thermal conductivity vs temperature [44] Harish et al. [32] Prepared NDPCM by using directly obtained CNTs of size ranging between 0.7-2 nm and rod shape dispersed in octadecane. They suggested that higher thermal conductivity enhancement is observed in the solid-state which is possibly attributed due to the formation of continuous networking structure by
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collective phonon motions over long distances between the phase transitions (Figure 17).
In the liquid phase, while the alignment parameter of NDPCM matrix has a
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relatively low value than solid-state, the molecules at and near the nanoparticle exhibit much stronger alignment even though the interfacial effect was limited to 1 nm. While
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in solid-state, the molecules are at closest distance from the surface of the NP, but
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remains elevated to the bulk value even 2–3 nm away from the NP surface [32].
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Figure 17: Thermal conductivity of NDCPM during the phase change from solid to liquid state [32]. Fang et al. [66] used directly obtained graphene in the thickness range of 4−20
nm and planar shape as nanodispersion in eicosane by using sonication process. Thermal conductivity has enhanced only 152.8 % even for 2 wt.% graphene. The reason behind the lower enhancement in thermal conductivity may be because the diameter of graphene is 5-10 µm and due to intensive sonication for 30 min. However, [58] it was suggested that thermal conductivity enhancement while using graphene as nanodispersion is vigorous due to sonication for 3 hours. Hence, this result suggested
56
ACCEPTED MANUSCRIPT that decreasing the sonication time also had more possibility of decreasing the thermal conductivity of NDPCM [66]. Zhang et al. [129] analyzed octadecane PCM encapsulated with cement as supporting material and EG as a nanoparticle. EG was 500 µm, and microstructure of
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the EG particle exhibits a flattened irregular honeycomb network constructed from elementary graphite nanosheets, which provides abundant crevice-like and net-like pores for adsorbing organic substances. The result shows contrast due to the micro size of high thermal conductivity particle. However, the result suggested that the
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micro size of the particle has more possibility for sedimentation and decreases the thermal conductivity [129].
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Overall summary:
Thermal storage system performance is mainly dependent on the thermal properties of the PCM. Dispersing nanoparticle to improve the thermal conductivity and hence the heat transfer rate of the storage system is the current trend, although it has positive or negative effects on the thermal storage capacity of NDPCM. The
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essential points obtained from the present review are summarised below: • Immersion of carbon-based nanomaterial shows better performance than metal or metal oxide nanomaterial in the temperature range of 20˚C to 37˚C because,
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besides increasing thermal conductivity, they moderately decreased the energy storage capacity.
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• From the graph (Figure 18), it is seen that carbon-based nanomaterial like graphene and xGnP are commonly preferred nano additives.
• It should be noted that the thermal properties of NDPCM depend on the mass of nanoparticles added and primary melting behavior of PCM. The thermal properties of NDPCM may increase or decrease in the melting range of PCM outside the temperature range considered in the present work, i.e. 20˚C to 37˚C. • The summary on Latent heat and thermal conductivity of PCM on the addition of different nanoparticles is listed in Table 11.
57
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Figure 18: Research work on NDPCM in the temperature range of 20˚C to 37˚C
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• From the graph (Figure 18), it is seen that carbon-based nanomaterial like graphene and xGnP are commonly preferred nano additives. • It should be noted that the thermal properties of NDPCM depend on the mass of nanoparticles added and primary melting behavior of PCM. The thermal properties of NDPCM may increase or decrease in the melting range of PCM
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outside the temperature range considered in the present work, i.e. 20˚C to 37˚C. • The summary on Latent heat and thermal conductivity of PCM on the addition of different nanoparticles is listed in Table 11. • Thermal properties like latent heat and thermal conductivity of NDPCM shows
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better performance on smaller sized nanoparticle rather than larger sized nanoparticle. However, 1-10 nm size nanoparticle depends on surface effect
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and quantum size effect.
• It should be noted that the thermal properties of NDPCM depend on the mass of nanoparticles added and primary melting behavior of PCM. The thermal properties of NDPCM may increase or decrease in the melting range of PCM outside the temperature range considered in the present work, i.e. 20˚C to 37˚C. • Addition of the nanoparticle tends to reduce the energy storage capacity of the matrix PCMs. But, a high enhancement of thermal conductivity, giving rise to a high heat transfer rate can compromise the decrease in the rate of energy storage capacity. 58
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Table 11: Parameters influencing the latent heat and thermal conductivity of NDPCM PCM (Melting temp. In oC)
Concentration (%)
2
Decrease Increase in in Latent Thermal heat Conductivity (%) (%) 86.31 133.3
[48]
-
94.90
-
[78]
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Particle Size
Ref
O.D. × L 10-30 nm × 5–15 µm
Eicosane (36)
Cylindrical
130 nm
LA-MA-SA (32.76)
Rod
Diameter 0.7−2 nm
Octadecane (27)
0.1
-
102.5
[32]
Rod
O.D. × L 8–15 nm × 0.5-2 µm
1-dodecanol (32.33)
1
95.42
-
[68]
Hallow
100 nm
Bio Based (29.4)
1
90.93
266.2
[40]
GA
Flake
3 nm
Octadecanol (28)
6.25
86.87
2004.35
[62]
GNP
Platelets
Width <2 µm, thick <2 nm
Paraffin RT-22 (25.37)
0.1
98.55
613.3
[58]
Cylindrical
Diameter 5−10 µm, thick of 4−20 nm
Eicosane (35.7)
2
98.30
152.8
[66]
Cylindrical
Width 16 nm
Eicosane (36)
1
97.34
171.1
[93]
Eicosane (36.02)
1
98.34
182.4
[89]
Eicosane (36.02)
1
94.75
197.8
[89]
Eicosane (36.02)
1
98.89
230
[89]
Paraffin RT-27 (28.81)
1
96.78
288.8
[41]
1-dodecanol (22)
1
95.39
-
[70]
CNT
GNS
3.2 nm Flake broken
3.6 nm
CuO
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5.9 nm Cylindrical
Diameter 35 nm
Flake
60 nm
Cylindrical
Diameter 25 µm, think 6−10 nm
Octadecane (27)
0.1
-
107.9
[32]
Flake
Diameter 15 µm, thick 10 nm
Bio Based (29.4)
3
98.50
397.4
[40]
Rod
Diameter 15µm, thick <10nm
Octadecane (28.91)
3
99.57
-
[31]
Spherical
Diameter 5-15 nm
Eicosane (36.18)
1
-
117.5
[44]
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xGnP
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Spherical
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Ag
Particle Shape (or) Structure
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Particle name
59
ACCEPTED MANUSCRIPT • The surface-functionalized nanoparticle has shown a different effect based on the methods of surface modification. Surface-functionalized nanoparticle shows higher thermal conductivity enhancement. • Latent heat and the thermal conductivity of the NDPCM, irrespective of nanoparticle shape, i.e. spherical or wire or rod, depend only on melting
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temperature and thermo-physical characteristics of base PCM as well as the type of application.
• Supporting material having high porosity are used to prevent the base PCM
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from leakage during phase change. However, the average pore size of the supporting materials is exceedingly essential for thermal properties of formstable composite PCMs. Mesoporous carbons like supporting material with
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high encapsulation capacity as well as high thermal conductivity are needed for composite form-stable PCM to increase the thermal properties of NDPCM Future work
Many researchers have employed different variants of the two-step preparation
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method for the synthesis of NDPCM, but there are still many aspects worth for further investigations. Suggestions for future research works are summarised below: 1. The most commonly used two-step method for preparation of NDPCM has the problem of agglomeration of the nanoparticle. More efficient and
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agglomeration free preparation method for NDPCM has to be developed. 2. Determination of optimum level of nanoparticle concentration at which
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NDPCM will have maximum enhancement on thermal conductivity with minimum degradation of latent heat capacity.
3. Nanoparticle synthesis methods can be standardized by changing the reaction parameters like temperature and time, which would change the physical structure of the nanoparticle materials. 4. Modified nanoparticle shows better performance on latent heat of NDPCM by reducing the dispersion effect on NDPCM. More experimental research works involving surface modification of nanoparticles have to be conducted.
60
ACCEPTED MANUSCRIPT 5. Study on the effect of nanoparticle-surface modification on thermal and rheological properties and stability of NDPCMs can be carried out. 6. Variation of size and shape of the same nanoparticle in same phase change materials can be examined to understand their effect on the thermal
Conclusion
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performance of NDPCMs in the low-temperature range.
This review consists of three parts: preparation of nanoparticle dispersed phase change material, characterization techniques of NDPCM and thermal properties like
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latent heat and thermal conductivity of NDPCM.
This review analyses the impact of the dispersion of various nanoparticles like
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metal, metal oxide, and carbon-based materials in the PCM matrix with the scientific reason behind the impact. This review will help the researchers to understand and select the appropriate nanomaterial for enhancing the thermal performance of latent heat thermal energy storage systems working in the temperature range of 20˚C to 37˚C.
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From this review, it is concluded that the dispersion of nanoparticle with PCM molecule would reduce the energy storage capacity of the matrix PCMs because they do not contribute to latent heat storage. Moreover, a remarkable enhancement of
storage.
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thermal conductivity is acceptable for a slight decrease in the rate of latent heat
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Impact on Latent heat of NDPCM does not depend on the size of nanoparticle but depends on the dispersion of mass concentration of nanoparticles in the PCM matrix. However, latent heat of NDPCM can even improve due to the Lenard-Jones potential forming between PCM molecule and nanoparticle. However, the dispersion of nanoparticle increases; the thermal conductivity gradually increases while the latent heat decreases. This indicates that the improvement in NDPCM thermal conductivity using nanoparticle will be accompanied by reduced latent heat in the NDPCM. Therefore, a suitable mass fraction of nanoparticle should be determined based on the application. 61
ACCEPTED MANUSCRIPT Thermal conductivity enhancement in NDPCM was higher for carbon-based nanomaterial than for metal or metal oxide nanomaterial. Carbon-based nanomaterial has more efficient networking structures which are strongly integrated without any micro-cracks or loose interface in PCM molecule than the case of metal and metal
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oxide nanoparticle dispersion that results in superior performance. Use of NDPCM in a wide range of application appears promising, but the oftenpoor performance of NDPCM and lack of theoretical understanding of the mechanisms remains a challenge and hinder the widespread commercial application of
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NDPCM. This review article brings out the impact of material, shape, size, surface modification of nanoparticles and supporting material on the important thermal
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properties like latent heat and thermal conductivity of PCM and brings to light the possible underlying science under the change in the thermal properties of PCM. Future work is required to reveal the Surface-functionalized nanoparticle, to facilitate optimization of the latent heat and thermal conductivity of NDPCM. Also, the Surface-functionalized nanoparticle will be the route for the characterization
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techniques for carrying out a large number of experimental works. Therefore, the
new NDPCM.
[1]
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