Influence of nanomaterials on properties of latent heat solar thermal energy storage materials – A review

Energy Conversion and Management 83 (2014) 133–148

Contents lists available at ScienceDirect

Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Review

Influence of nanomaterials on properties of latent heat solar thermal energy storage materials – A review G. Raam Dheep, A. Sreekumar ⇑ Solar Thermal Energy Laboratory, Centre for Green Energy Technology, Pondicherry University, Puducherry 605014, India

a r t i c l e

i n f o

Article history: Received 19 December 2013 Accepted 21 March 2014

Keywords: Nanomaterials Phase change materials Thermo-physical properties Thermal energy storage

a b s t r a c t Thermal energy storage system plays a critical role in developing an efficient solar energy device. As far as solar thermal devices are concerned, there is always a mismatch between supply and demand due to intermittent and unpredictable nature of solar radiation. A well designed thermal energy storage system is capable to alleviate this demerit by providing a constant energy delivery to the load. Many research works is being carried out to determine the suitability of thermal energy storage system to integrate with solar thermal gadgets. This review paper summarizes the numerous investigations on latent heat thermal energy storage using phase change materials (PCM) and its classification, properties, selection criteria, potential research areas and studies involved to analyze the thermal–physical properties of PCM. Ó 2014 Elsevier Ltd. All rights reserved.

Contents 1. 2.

3. 4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Classification of thermal energy storage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Sensible Heat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Chemical reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Latent heat storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1. Organic PCM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2. Inorganic PCM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3. Eutectics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Research areas in solar latent thermal energy storage system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Literature review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction The world is hurtling towards two major crises: serious energy shortages and accelerating climate change. Together, they threaten to destroy the achievements of human civilization. Solutions to both the crises are interlinked; the diversification of the fuel base and adoption of emerging clean and green alternative for energy production. Many scientists are involved in search of new and renewable sources of energy and have proved that solar energy has the maximum potential to solve the problem of energy crisis. ⇑ Corresponding author. Tel.: +91 413 2654314; fax: +91 413 2656758. E-mail address: [email protected] (A. Sreekumar). http://dx.doi.org/10.1016/j.enconman.2014.03.058 0196-8904/Ó 2014 Elsevier Ltd. All rights reserved.

133 134 134 134 134 135 135 135 135 136 147 147

The use of solar energy for thermal applications such as water heating, space heating, cooking, and drying sometimes becomes unreliable as the solar energy is variable and unpredictable; hence there exists a mismatch between the availability and utilization time. Therefore it necessitates an effective thermal energy storage system to store the energy whenever it is available, which can be utilized during non-solar hours. Thermal energy storage system will reduce the imbalance between the demand and the supply of energy, thereby improving the performance of the system and reduce the cost incurred due to loss of energy [1–3]. Thermal energy can be stored as a change in internal energy of a material as sensible heat, latent heat and thermo-chemical or combination of all these. Thermal energy storage system requires some

134

G. Raam Dheep, A. Sreekumar / Energy Conversion and Management 83 (2014) 133–148

Nomenclature am ar Cp Csp Clp dt m

fraction melted fraction reacted specific heat (kJ/kg K) average specific heat between Ti and Tm (kJ/kg K) average specific heat between Tm and Tf (kJ/kg K) change in time mass of heat storage medium (kg)

Q Tf Ti Tm Dhm Dhr

quantity of heat stored (J) final temperature (°C) initial temperature (°C) melting temperature (°C) heat of fusion per unit mass (kJ/kg) endothermic heat of reaction

distinctive features such as high heat storage efficiency, large storage capacity per unit mass and volume, very small heat losses, should be non-corrosive and long life with less expensive [4–8]. This paper gives review on classification, studies of thermo-physical properties, thermal stability and thermal reliability of phase change materials. Apart from that the effect of encapsulation, shape stabilization and influence of nanomaterials on the properties of phase change materials are also discussed which will be an aid to the scientific community.

heat storage techniques, such as high energy density, ambient temperature storage and possibility of heat pumping and long distance transport. Technical compatibility of this method is yet to be proven. There are three modes of storage using chemical reactions: reversible reactions, thermo-chemical pipeline energy transport and chemical heat pump storage. Thermal energy is stored and retrieved by breaking and reforming of molecular bonds through reversible chemical reactions [4,9,11,12]. Thermal storage is relied on the amount of storage material, endothermic heat of reaction and the extent of conversion given by the Eq. (2)

2. Classification of thermal energy storage

Q ¼ ar mDhr

Thermal energy can be stored as either sensible heat or latent heat or as chemical storage based on the specific heat capacity, latent heat and chemical reactions of the material [9]. Basic classification of thermal energy storage is given in Fig. 1. 2.1. Sensible Heat In sensible heat storage, thermal energy is stored based on the specific heat capacity of the material. Here the temperature of the material varies and does not undergo any phase transformation during charging or discharging cycles [4,9,11,12]. The amount of energy stored is given by Eq. (1). The amount of heat stored depends upon the amount of the storage material, specific heat of the medium and difference between the change in temperature at initial and final stage. Some examples of the solid and liquid sensible heat storage medium are given in Table 1.

Z

Tf

ð2Þ

2.3. Latent heat storage In latent heat storage system, the process of storing and retrieving the thermal energy is based on the latent heat of fusion, where storage medium undergoes a phase transformation. The heat stored during the phase change process of the material is called latent heat. As the source temperature increases the chemical bonds of the PCM material breaks up which leads to the transformation from one phase to other [9,11–14]. Here the temperature is almost constant with less temperature swing. Energy stored in latent heat storage medium is given by Eq. (3)

Q¼

Z

Tm

mC p dt þ mam Dhm þ

Z

Ti

Tf

mC p dt Tm

¼ m½C sp ðT m  T i Þ þ am Dhm þ C lp ðT f  T m Þ

ð3Þ

2.2. Chemical reactions

Phase transformation of the material can be solid–solid, solid– liquid, or liquid–gas. Transformation of crystalline nature from one to other will be observed in solid–solid latent heat storage material whereas phase change of the material will be used to store thermal energy in other two methods of the latent heat storage materials. Solid–solid has an advantage of small change

Thermal energy can also be stored using the reversible chemical reactions. It is more advantageous compared to sensible and latent

Table 1 Examples of sensible heat storage materials.

Q¼

mC p dt

ð1Þ

Ti

¼ mC p ðT f  T i Þ

Thermal Energy storage

Sensible heat storage

Latent Heat storage

Solid

Solid-Liquid

Liquid

Liquid-Gas

Chemical Reaction heat storage Thermochemical reactions Thermochemical pipeline energy transport

Solid-Solid Chemical heat pump storage Fig. 1. Classification of thermal energy storage.

Materials

Operating temperature (°C)

Heat capacity (J/kg K)

Density (kg/m3)

Water Therminol Engine oil Lithium Sodium Ethanol Butanol Octane Aluminium Brick Concrete Cast iron Copper Sodium carbonate Limestone Magnesium oxide

0–100 9 to 343 Upto 160 180–1300 100–760 Upto 78 Upto 118 Upto 126 Upto 660 1000 1000 Upto 1100 Upto 1000 Upto 850 Upto 825 Upto 2800

4.190 2.100 1.880 4.190 1.300 2.400 2.400 2.400 0.896 0.840 1.130 0.837 0.383 1.090 0.900 0.960

1000 750 888 510 960 790 809 704 2707 1698 2240 7900 8954 2510 2500 3570

135

G. Raam Dheep, A. Sreekumar / Energy Conversion and Management 83 (2014) 133–148

Fig. 2. Classification of latent heat storage materials for thermal energy storage.

in volume and greater design flexibility but small latent heat compared to solid–liquid and liquid–gas which has larger latent heat and larger volume changes. Latent heat storage materials can be classified based on temperature, phase transition and compounds used as shown in Fig. 2. 2.3.1. Organic PCM A phase change material which contains carbon atom is known as organic PCM. It is classified into paraffin and non-paraffin. PCM materials with the general chemical formula CnH2n+2 are categorized under paraffin, where the heat of fusion and melting point increases with the increasing value of carbon atom number. Nonparaffin PCM is the compounds which contain functional groups such as alcohols, glycols, esters and fatty acids. Organic PCM’s are available for a wide range of temperatures which are stable till 300 °C. Examples of organic PCM are shown in Table 2. Few of the advantages of using organic PCM are no tendency to segregate, chemically stable, high heat of fusion, no tendency of supercooling and compatible with all containers except plastic at high temperature. Some of the demerits are low thermal conductivity, high cost, sometimes flammable and mildly corrosive [4,9,11–14]. 2.3.2. Inorganic PCM Inorganic PCMs are materials which consist of salt hydrates, nitrates and metallic’s. Inorganic PCM can also be used for higher

Table 2 Examples of organic phase change materials. Materials

Melting point (°C)

Latent heat of fusion (kJ/kg)

Density (kg/m3)

Formic acid Acetic acid Glycerin D-lattic acid Polyethylene glycol 600 Cyanamide Methyl eicosanate Camphene Chloroacetic acid Trimyristin Bee wax Bromcamphor Durene Acetamide Succinic anhydride Benzoic acid Stibene Benzamide Alpha glucose Salicylic acid O-mannitol Hydroquinone

7.8 16.7 17.9 26 20–25 44 45 50 56 33–57 61.8 77 79.3 81 119 121.7 124 127.2 141 159 166 172.4

247 187 198.7 184 146 209 230 238 130 201–213 177 174 156 241 204 142.8 167 169.4 174 199 294 258

1226.7 1050 1260 1249 1100 1080 851 842 1580 862 950 1449 838 1159 1104 1266 1164 1341 1544 1443 1489 1358

Table 3 Examples of inorganic phase change materials. Materials

Melting temperature (°C)

Heat of fusion (kJ/kg)

Density (kg/m3)

NaNO3 RbNO3 KNO3 KOH CsNO3 AgBr PbCl2 Ca(NO3)2 LiCl FeCl2 MgBr2 CaI2 NaCl KF BaCl2 PbSO4 MgSO4 MgF2 BaF2 CaF2 BaSO4 SrSO4

306 312 334 380 409 432 501 560 610 677 711 783 802 858 961 1000 1130 1263 1320 1418 1512 1605

182 31 266 149.7 71 48.8 78.7 145 441 337.9 214 142 482 468 76 133 122 938 119 391 188 196

2260 3685 2109 2044 2500 1100 5600 2113 2070 3160 3720 3956 2160 2370 3856 6200 2660 3150 4890 3180 4500 3960

temperatures up to 1500 °C. Few examples of inorganic PCMs are given in Table 3. Inorganic PCMs are superior in terms of low cost, easy availability, sharp melting point, high thermal conductivity, high heat of fusion and lower volume change. It is associated with demerits such as supercooling, segregation, materials degradation, corrosion of heat exchangers, low specific heat and decrease in heat of fusion after few cycles due to incongruent melting [4,9,11,12,15,16]. 2.3.3. Eutectics Eutectic PCMs are mixture of two or more compounds at a particular percentage of composition. The compounds can be of any combination like organic-organic, inorganic–inorganic and organic–inorganic. These types of PCMs melt and freeze congruently without any segregation. They freeze to an intimate mixture of crystals leaving less opportunity for the compounds to separate. Similarly during melting, different compound melts simultaneously which also gives less probability of compound separation [4,9–12]. Few examples of eutectic PCM are given in Table 4. 3. Research areas in solar latent thermal energy storage system Thermal energy collected from the solar receiver by heat transfer fluid (HTF) is allowed to pass through the heat exchanger wherein the PCM absorbs heat from the HTF and undergoes a

136

G. Raam Dheep, A. Sreekumar / Energy Conversion and Management 83 (2014) 133–148

Table 4 Examples of eutectic phase change materials. Material composition (wt%)

Melting temperature (°C)

Heat of fusion (kJ/kg)

Density (kg/m3)

Thermal conductivity (W/mK)

NaF–MgF2 (75 + 25) NaF–MgF2 (67 + 33) LiF–MgF2 (67 + 33) NaF–CaF2–MgF2 (65 + 23 + 12) LiF–NaF2–MgF2 (33.4 + 49.9 + 17.1) LiF–NaF2–MgF2 (46 + 44 + 10) Na2CO3–Li2CO3 (56 + 44) NaCl–MgCl2 (50 + 50) Li2CO3–K2CO3–Na2CO3 (31 + 35 + 34) MgCl2–NaCl–KCl (63 + 22.3 + 14.7) NaCl–Na2CO3–NaOH (7.8 + 6.4 + 85.5) LiCl–LiOH (37 + 63) KCl–NaCl–CaCl2 (5 + 29 + 66) KCl–BaCl2–CaCl2 (24 + 47 + 29)

832 832 746 745 650 632 496 450 397 385 282 262 504 551

650 616 947 574 860 858 368 429 275 461 316 485 279 219

4660 4650 – – 1150 1200 2110 0960 2040 0950 – 1100 1000 950

2.68 2.14 2.63 1.58 2.82 2.24 2.33 2.24 2.31 2.25 2.13 1.55 2.15 2.93

phase transformation. PCM material stores enormous amount of thermal energy during this phase transition, which can be retrieved later and during the retrieval process the PCM again changes its phase. Wide scope of research potential exists in latent heat thermal energy storage system integrated with solar thermal energy devices, which includes primarily identification of suitable PCM material and design of a PCM container with adequate heat transfer facility [9,10]. Fig. 3 depicts the various possible research activities that can be performed in the field of thermal energy storage system. The research activities involved in thermal energy storage is divided into three categories. First step involves selection and optimization of kinetic and thermo-physical properties of PCM and designing of suitable heat exchanger followed as second step. Finally performance evaluation based on technical and economical aspects must be optimized for the commercialization of the system. Therefore it is necessary to perform all these analysis to develop the solar energy based thermal storage system. Latent heat storage system

Identification of suitable PCM based on temperature requirement

4. Literature review Fabrication of heat exchanger

Selection of heat exchanger PCM material

Construction material

Thermo physical property studies

Experimental analysis

Mathematical simulation Comparison and analysis of data

Thermal cycle testing of freezing and melting Kinetic property studies

Life expectancy analysis Material Compatibility

Optimization of properties of identified PCM

Thermo-physical properties of the PCMs are analyzed using (i) differential thermal analysis (DTA), and (ii) differential scanning calorimeter (DSC). Here the sample and reference material is heated at a constant rate. Usually alumina is used as a reference material; the temperature difference between the two is proportional to the heat flow, which is recorded in the DSC curve. From the curve latent heat of fusion is calculated using the area under the peak and melting temperature is estimated by the tangent at the point of greatest slope on the face portion of the peak. Apart from these studies, thermal conductivity, viscosity, corrosion studies and accelerated thermal cycle tests are also performed to analyze the thermal reliability of the PCM. Enhancement in heat transfer properties of the PCM are studied by encapsulation, shape stabilization and incorporation of nanomaterials in PCM. Once the thermo-physical properties of PCMs are optimized, it is integrated with the solar thermal collectors with the help of a suitable PCM container with superior heat transfer facility.

Model development

Testing and analyzing

Prototype development

Testing and analyzing

Integration with solar system Performance evaluation

Economic analysis

Commercial product Fig. 3. Flowchart showing the potential research areas in solar thermal energy storage.

Many researchers presented reviews on low and high temperature phase change materials based on classification, need, selection criteria, long term characteristics, compatibility with heat exchanger materials, numerical models [6–17]. Methods of enhancing thermal and physical properties through incorporation of nanomaterials [18], encapsulation [19], shape stabilization and PCM slurries [20–22] were also reviewed in large numbers. In this section, a detailed review on phase change materials is presented based on thermo-physical properties like melting and cooling temperature, latent heat of melting and crystallization, thermal conductivity, degree of supercooling, viscosity, thermal stability and thermal reliability when PCM is subjected to nanoparticles addition, microencapsulation and shape stabilization. Feldman et al. [23] and Arndt et al. [24,25] reported the measurement of melting point, freezing point and latent heat of few organic PCMs of fatty acid esters, ethoxylated alcohols, alkyl phenol and sulphur compounds with low melting temperature of 10– 90 °C for space heating and cooling applications. Aboul-Enein and Olofa [26] analyzed the thermophysical properties of hexadecane, decanol and caprilic acid PCM for latent cold storage system (0– 20 °C). Calorimetric measurements of phase change temperature, phase transition enthalpy, degree of supercooling and variation of specific heat with temperature (in the solid and liquid phases near the phase transition temperature) for these three PCM is shown in Table 5. Desgrosseilliers et al. [27] studied the phase change properties of lauric acid based on the purity of the material. The author compared the reagent grade [98% pure] with practical grade [<80%

137

G. Raam Dheep, A. Sreekumar / Energy Conversion and Management 83 (2014) 133–148 Table 5 Thermo-physical properties of organic PCMs. Materials

Phase transition temperature (K)

Phase transition enthalpy (kJ/kg)

Specific heat capacity (kJ/kg K)

Hexadecane

3.5–2.5

231.9

Decanol

3.5–5

181–181.6

Caprilic acid

5–6

150–151.2

1.45–1.64 at 245–275 K (solid phase) 2.2–2.22 at 305–320 K (liquid phase) 1.477–1.8 at 235–260 K (solid phase) 2.25–2.31 at 290–300 K (liquid phase) 1.77–1.96 at 225–275 K (solid phase) 2.15–2.23 at 300–310 K (liquid phase)

pure] and found that onset of melting temperature for 98% pure and <80% pure were 43 °C and 43.3 °C with heat of fusion 180 kJ/kg and 184 kJ/kg and remains to be stable even after 500 accelerated thermal cycles. Therefore lauric acid irrespective of purity acts as a suitable PCM for thermal energy storage which is less expensive, easily available and nontoxic. Accelerated thermal cycle testing of calcium chloride hexahydrate PCM for latent heat storage was performed by Tyagi and Buddhi [28] to check its reliability after 1000 cycles. It is reported that calcium chloride hexahydrate acts as a good inorganic PCM for low temperature applications which has only small variations in the latent heat of fusion and melting. The heat of fusion and melting temperature of CaCl26H2O based on the number of thermal cycles is given in Table 6. El-Sebaii et al. [29,30] experimentally investigated melting point and latent heat of fusion by fast thermal cycling for 1000 cycle of magnesium chloride hexahydrate (MgCl26H2O). The study was performed using extra water principle to avoid segregation of PCM during solidification. The author concluded that MgCl26H2O solidifies with a slight degree of supercooling in the range of 0.1– 3.5 °C. El-Sebaii also investigated the thermal cycling of acetanilide (C8H9NO) and magnesium chloride hexahydrate (MgCl26H2O) for 500 cycles. The compatibility of the selected PCMs was also analyzed with the container material of aluminum and stainless steel samples through surface investigation using Secondary Ion Mass Spectroscopy (SIMS) technique. Corrosion studies concluded that acetanilide is compatible with aluminium and shows small corrosion in contact with steel but magnesium chloride hexahydrate is not compatible with either aluminum or stainless steel. SEM images showing the compatibility of stainless steel and aluminium immersed in acetanilide and magnesium chloride hexahydrate for 500 cycles is shown in Fig. 4. Shukla et al. [31] performed 1000 thermal cycling tests with some selected organic and inorganic phase change materials (PCMs). Paraffin wax (A), Paraffin wax (B), Paraffin wax (C), Sodium hydroxide (NaOH), Di-Sodium borate decahydrate (Na2B4O710H2O), Ferric nitrate hexahydrate (Fe(NO3)36H2O), Barium hydroxide octahydrate (Ba(OH)28H2O) and erythritol (C4H6OH4) are few of the PCMs selected for the study. The study reported that the Table 6 Variation of melting temperature and latent heat of fusion of CaCl26H2O for 1000 thermal cycle. S. no

No. of cycles

Melting temperature (°C)

Heat of fusion (kJ/kg)

1 2 3 4 5 6 7 8 9 10 11 12

1 10 100 200 300 400 500 600 700 800 900 1000

23.26 26.85 27.14 24.62 24.79 24.34 24.54 24.41 24.26 24.15 23.95 23.26

125.4 138.1 117.9 130.3 130.0 135.3 130.1 127.1 129.5 129.6 122.3 125.4

selected inorganic PCMs are not suitable for latent heat thermal energy storage due to large variation in thermo-physical properties whereas paraffin waxes (A, B, and C) and erythritol has good thermal reliability in terms of latent heat of fusion and melting temperature and tends to be most promising PCM for low temperature applications. Harikrishnan and Kalaiselvam [32,33] experimentally analyzed the thermo-physical properties of CuO incorporated oleic acid PCM for cooling applications. In which it is reported that, heat transfer rates during melting and solidification has been enhanced more than the base PCM depending upon the different mass fraction of CuO. Author also examined TiO2 incorporated stearic acid as a phase change material for solar thermal energy storage applications. It is revealed that the complete melting and solidification time of PCM was reduced by 43% and 41% based on the composition of TiO2. Karunamurthy et al. [34] investigated the variation in thermal conductivity, charging and discharging time by the addition of CuO nanoparticles at various concentrations both experimentally and analytically. The charging and discharging time was decreased around 50% at a volume concentration of 0.16% of CuO nanoparticles along with slight enhancement in thermal conductivity. Valan Arasu et al. [35] investigated the effects of Al2O3 and CuO nanoparticles on both melting and solidification rates of paraffin wax. Results showed that dispersing nanoparticles in smaller volumetric fractions increase the heat transfer rate. Also found that thermal performance of paraffin wax is greater for Al2O3 than that of CuO nanoparticles. Jesumathy et al. [36,37] reported the heat transfer characteristics of paraffin PCM with and without CuO nanoparticles based on few important issues like temperature distribution, heat transfer phenomenon during total melting and solidification, Reynolds number based on inlet of heat transfer fluid and finally the heat transfer characteristics. The results showed that by the addition of CuO nanoparticles to the paraffin wax enhances the heat conduction, natural convection, thermal conductivity and viscosity very effectively. The graph showing the variation in viscosity and thermal conductivity for different weight% of CuO are shown in Fig. 5. The phase change behavior of paraffin wax, microcrystalline wax, and inorganic PCM Na2SO410H2O and CaCl26H2O with and without Al2O3 particles was reported by Liu and Chung [38]. Experiments were performed to determine the melting and solidification temperatures, supercooling, heat of fusion, and thermal cycling stability. The study concluded that inorganic materials with and without additives are not suitable for thermal energy storage applications, due to incongruent melting, large supercooling, and thermal cycling instability. Wang et al. [39] studied the thermal performance of the paraffin wax by varying the mass fraction of c-Al2O3 nanoparticles. It was shown that the addition of c-Al2O3 nanoparticles reduced both melting point and latent heat capacity but increased the thermal conductivity compared to pure paraffin wax due to the interaction of Al2O3 nanoparticles with paraffin wax molecules. Fan and Khodadadi [40] determined the effect of CuO nanoparticles on the thermal conductivity of cyclohexane using transient

138

G. Raam Dheep, A. Sreekumar / Energy Conversion and Management 83 (2014) 133–148

Fig. 4. SEM images of (a) and (b) stainless steel and aluminium at 0th cycle, (c) and (d) stainless steel and aluminium at 500th cycle for acetanilide, (e) and (f) stainless steel and aluminium at 500th cycle for magnesium chloride hexahydrate.

plane source technique both in solid and liquid state at different temperatures. Unidirectional freezing of PCM was investigated experimentally and compared with the numerical values simulated using one-dimensional Stefan model. It is found that the thermal conductivity of cyclohexane was enhanced in liquid state with increasing concentration of CuO nanoparticles whereas in solid state it shows nonlinear changes. Ho and Gao [41] studied the effective thermophysical properties, such as latent heat of fusion, density, dynamic viscosity and thermal conductivity of n-octadecane PCM embedded with Al2O3 nanoparticles with the composition of 5 and 10 wt% prepared by emulsifying alumina by means of non-ionic surfactant. The change in the values of melting temperature, freezing temperature and latent heat of fusion is given in Table 7. Thermal conductivity, density and dynamic viscosity studies shows that there is a relative enhancement which tends to be non-linear with increased mass fraction of nanoparticles compared to that of pure paraffin. Teng and Yu [42] prepared nanocomposite enhanced phase change materials (NEPCMs) by direct synthesis method and experimentally studied the thermal performance of paraffin with the additives concentration varying at 1, 2, 3 wt% of Al2O3, TiO2, SiO2

and ZnO nanoparticles. The study revealed that among the additives, TiO2 incorporated paraffin have significant potential for enhancing the thermal storage characteristics of paraffin. Experimental analysis was performed on phase change performance of sodium acetate trihydrate by Hu et al. [43]. It has high storage density and thermal conductivity but large supercooling and phase segregation, hence nucleating agents should be used to prevent the supercooling. AlN nanoparticles was chosen as a nucleating agent and it is found that incorporation of 5 wt% of AlN nanoparticles in PCM has shown no tendency of supercooling. Effect of copper nanowires with high aspect ratio on the properties of Tetradecanol was investigated by Zeng et al. [44]. Results showed that by the addition of Cu nanowires the phase change enthalpy and thermal stability have been decreased whereas the thermal conductivity and melting speed were increased. Fig. 6 shows the SEM images of Cu nanowires and changes in thermophysical properties of tetradecanol with respect to different weight% of Cu nanowires. Zeng et al. [45] also reported effect of silver (Ag) nanowires at different compositions. Fig. 7 shows the changes in the heat of fusion and thermal conductivity with respect to varied mass fraction of silver nanowires. The study

G. Raam Dheep, A. Sreekumar / Energy Conversion and Management 83 (2014) 133–148

139

Fig. 5. Effect of various weight% of CuO nanoaprticles on (a) dynamic viscosity and (b) thermal conductivity at different temperatures.

Table 7 Variation in melting temperature, freezing temperature and latent heat of fusion as a function of different weight% of Al2O3 nanoparticles. Mass fraction of nanoparticles (xp) (wt%)

Melting temperature (°C)

Freezing temperature (°C)

Latent heat of fusion (kJ/kg)

0 5 10

26.5 26.0 26.3

25.1 25.0 25.3

243.1 225.6 212.3

clarified that the changes in the properties of 1-TD/Ag nanocomposite PCM is due to the strong ability of Ag nanowires of high aspect ratio and thermal conductivity interfaces. The results proved that Ag nanowire is a strong material to enhance the thermal con-

ductivity of organic PCMs, which can be employed in thermal energy storage, thermal protection, and also in thermal interface materials. Parameshwaran et al. [46] investigated the performance of organic ester by incorporating silver nanoparticles in terms of latent heat capacity, thermal conductivity and heat storage and release capabilities. Report shows, latent heat capacities decreased by 7.88% in freezing and 8.91% in melting whereas thermal conductivity of composite PCMs increased from 0.284 to 0.765 W m1K1, which is 10–67% more compared to base PCM. The span of composite PCM in freezing and melting cycles was enhanced by 41% and 45.6%, whereas freezing and melting time was reduced by 30.8% and 11.3 %. Thermal–physical performance of 1-tetradecanol (TD) with silver nanoparticles PCM nanocomposite was investigated by Zeng

Fig. 6. SEM images of Cu nanowires (a), PCM with 58.9% (b), PCM with 1.32% (c) of Cu nanowires. Effect of different weight% of Cu nanowires on phase change enthalpy (d), thermal stability (e) and thermal conductivity (f).

140

G. Raam Dheep, A. Sreekumar / Energy Conversion and Management 83 (2014) 133–148

Fig. 7. Effect of different mass fraction of Ag nanowires on heat of fusion (a) and Thermal conductivity (b).

et al. [47]. The composition of both Ag nanoparticles and tetradecanol was varied and the corresponding thermal stability, thermal conductivity and phase change enthalpy were analyzed. The results showed that the thermal conductivity of the composite material was enhanced as the percentage of Ag nanoparticles increased; also the composites had relatively large phase change enthalpy but slightly decreased in phase change temperature. The thermal stability of the composite was also close to that of pure TD. Wang et al. [48] studied the thermal characteristics, heat charging and discharging rates of paraffin wax with various weight% (0.1, 0.5, 1, 2) of copper nanoparticles. Surfactant was used to improve the stability of suspension. Studies show that there is only small change in phase change temperature but large variation in latent heat with respect to the addition of nanoparticles. The heat charging and discharging rates were reduced compared to pure paraffin. It is shown that the melting time for composite PCM with 0.5% and 1% copper nanoparticles was reduced to 22.9% and 33.6%. Similarly the freezing time was reduced to 21.5% and 24.8% than pure PCM. The change in thermal properties was due to the enhancement of thermal conductivity of paraffin wax by the addition of copper nanoparticles. Nanofluid based PCM was developed by Wu et al. [49] by dispersing small amounts of few nanoparticles such as Cu, Al, and C/Cu. Five dispersants namely GA, Span-80, cetyl trimethyl ammoniumbromide (CTAB), Hitenol BC-10 and sodium dodecylbenzenesulfonate (SDBS) were used for stable suspension of nanoparticles. It was revealed that Cu/paraffin with Hitenol BC-10 was more stable and the latent heat shifted to lower values compared to pure paraffin, but melting and freezing temperatures were almost same as that of pure paraffin. The latent heat and phase change temperature show the small variation after 100 thermal cycles but the heating and cooling rate of PCMs were increased due to the addition of Cu nanoparticles. A latent heat storage nanocomposite PCM stearic acid with multi-walled carbon nanotube was prepared and investigated its thermophysical properties by Li et al. [50]. Fig. 8 shows the experimental procedures involved in preparing stearic acid/ MWCNT nanocomposite PCM. Experimental results show that the addition of multiwall carbon nanotube (MWCNT) improves the

thermal conductivity and also increases the discharging rate to about 91% and lowers the charging rate to 50% of stearic acid, when the volume fraction of the additive is 5%. It is concluded that the MWCNT is an effective additive for enhancing the heat transfer performance of latent heat thermal energy storage system. Thermal performance of palmitic acid and paraffin with different composition of multiwall carbon nanotubes (MWCNT) was also studied by Wang et al. [51–54]. MWCNTs were treated by acid oxidation, mechanochemical reaction, ball milling, and grafting so that hydroxyl groups, carboxylic groups, and amidocyanogen were introduced onto the surfaces of the MWCNTs. The functionalized MWCNTs were used to prepare the nanocomposite PCM in order to study its thermo-physical properties in both solid and liquid state. Enhancement in thermal conductivity and studies on thermo-physical properties of palmitic acid (PA)/MWNT composite was also experimented by Ji et al. [55]. Multiwalled carbon nanotubes (MWNTs) dispersed in PA was functionalized by oxidation (O-MWNTs) and also by adsorption of pyrogallol (f-MWNTs). Enhancements of thermal properties are explained based on the weight% of O-MWNTs and also on the different oxidation time of MWNTs. The effects of multi-walled carbon nanotubes (MWNTs) with and without surface modification by surfactants such as CTAB and SDBS on phase change enthalpy (DH) and thermal conductivity (j) of palmitic acid (PA) was investigated by Zeng et al. [56]. It was observed that CTAB has capability to disperse MWNT uniformly and separately in PA which tends to enhance the properties of the PCM. Teng and Yu [57] also experimentally analyzed the performance of paraffin with three different concentration (1, 2, 3 wt%) MWCNT. The author described that MWCNT can reduce the melting onset temperature but increases the solidification onset temperature of pure paraffin. Shaikh et al. [58] performed the experimental analysis on the latent heat of wax by adding single wall carbon nanotubes (SWCNTs), multiwall CNTs, and carbon nanofibers with numerical modeling to calculate the change in latent heat based on Lennard–Jones potential. The study concluded that higher molecular density and large surface area were the reasons to have greater intermolecular attraction that results in enhanced latent energy.

Fig. 8. Schematic representation for preparing stearic acid/MWCNT nanocomposite PCM.

G. Raam Dheep, A. Sreekumar / Energy Conversion and Management 83 (2014) 133–148

The addition of various carbon nanofillers at different weight% and its effects on the thermal–physical properties of paraffin-based nanocomposite PCMs was investigated experimentally by Fan et al. [59]. Nanofillers such as short and long multi-walled carbon nanotubes, carbon nanofibers, and graphene nanoplatelets (GNPs) were added to the base PCM. Fig. 9 shows the changes in phase change enthalpies of paraffin wax with different nanofillers during melting and solidification. It is concluded that, presence of nanofillers slightly decreases phase change enthalpies and has negligible influence on phase change temperatures and thermal conductivity of nanocomposite PCMs that strongly depends on the size and shape of the nanofillers. Elgafy and Lafdi [60] experimentally and analytically studied the thermal performance of the paraffin wax filled with carbon nanofibers. The cooling rate was determined using the transient temperature response measured during its solidification process. A comparative study was made on the effect of carbon nanofibers surface characteristics on thermal performance of paraffin wax. Cui et al. [61] experimentally exploited the carbon nanofiber and carbon nanotube additives on the thermal behavior of soy and paraffin wax PCM. From the results author concluded that, carbon nanofiber acts as a better additive than CNT to enhance the thermal properties of PCM due to its better dispersion in the matrix. Zhong et al. [62] has reported on the thermal behavior of octadecanoic acid (OA) PCM consisting of graphene aerogel (GA). Here GA acts as supporting material in which OA was impregnated with the help of capillary forces. The thermal conductivity of GA/OA was about 2.635 W/mK with a GA of 20 vol%, which was about 14 times larger than that of OA (0.184 W/mK). The composite PCM presents a high heat storage capacity of 181.8 kJ/kg, which was slightly decreased compared to the OA (186.1 kJ/kg). Xia and Zhang [63] studied on thermal properties, heat storage and retrieval durations and thermal conductivity of acetamide (AC)/expanded graphite (EG) composite PCM with 10 wt% of EG as the effective heat transfer promoter and compared with that of pure AC. The study revealed that the melting/freezing points shifted from 66.95/ 42.46 °C for pure AC to 65.91/65.52 °C for AC/EG composite, and the latent heat decreased from 194.92 to 163.71 kJ/kg. Thermal conductivity studies showed that there is a fivefold increase compared to the pure PCM. In addition, heat storage and retrieval tests in a latent thermal energy storage unit showed that the heat storage and retrieval durations were reduced by 45% and 78%, for composite PCM. The thermal physical properties of paraffin wax with 1, 2, 3, 5 and 7 wt% of exfoliated graphite nanoplatelets (xGnP) was experi-

Fig. 9. Phase change enthalpies of paraffin wax with different nanofillers during melting (a) and solidification (b).

141

mentally studied by Kim and Drzal [64]. It was observed that thermal conductivity of paraffin/xGnP composite increased with xGnP loading percentage. DSC curves shows two peaks at 35 °C and 55 °C, where the first peak corresponds to solidphase transition and second peak corresponds to solid to liquid phase transition. It was also found that the latent heat of paraffin/xGnP composite PCMs did not decrease as loading xGnP contents to paraffin. The study elucidated that xGnP can be considered as an effective heat diffusion promoter to improve thermal conductivity of PCMs without reducing its latent heat storage capacity in paraffin wax. The melting and freezing curves and latent heat storage performance of paraffin/xGnP composite PCMs during phase transition is shown in Figs. 10 and 11. Table 8 shows the phase change characteristics of the composite PCM for different weight% xGnP. Zhang et al. [65] prepared novel gelatinous shape-stabilized PCMs with polyol acetal derivatives such as sorbitol, mannitol, xylitol, and pentaerythritol into paraffin at different weight%. Among these gelators, sorbitol based PCM exhibited good thermal stability and no leakage of PCM was found even greater than melting point of PCM material. Additionally 3 wt% expanded and exfoliated graphite (EG) were also dispersed in paraffin which exhibited good thermal conductivity, phase change temperature and heat storage density. Heat storage and retrieval time were reduced with rapid release and absorption of heat compared to pure paraffin wax. Shape-stabilized lauric acid (LA)/activated carbon (AC) composites were synthesized and its thermal properties were presented by Chen et al. [66]. Here AC was used as an adsorbent and LA as PCM. Due to the surface tension forces and capillary action, LA is uniformly adsorbed and dispersed into the pores of activated carbon which ensures no leakage of melted LA. The latent heat increased from 12.22 to 65.14 kJ/kg during melting and 10.69 to 62.96 kJ/kg during freezing based on the decreasing weight% of activated carbon. The thermal conductivity was also improved by 4.67% and 71.1% during solidifying and melting state compared to that of pure LA. It was concluded that, carbonaceous layers create a physical protective barrier on the surface of the composites which helps to improve the thermal stability of the composites. A novel form-stable nanocomposite phase change material (PCM) was prepared by absorbing capric acid (CA) into halloysite nanotube (HNT) by Mei et al. [67] for thermal energy storage applications. The composition and thermal properties of CA/HNT is given in Table 9. Graphite (G) was added into PCM to improve thermal storage performance. Thermal storage and release rates were increased by 1.8 times and 1.7 times as compared with the composite without graphite as shown in Fig. 12. The study explicated that CA:HNT:G nanocomposite PCM is cost-effective latent heat storage material due to its properties like high adsorption capacity of CA, high heat storage capacity, good thermal stability, low cost and simple preparation method. Paraffin wax dispersed in two different exfoliated graphite nanoplatelets with a lateral size of about 15 lm and 1 lm in diameter (xGnP-15 and xGnP-1) were prepared by Xiang and Drzal [68] using direct casting and roll milling methods. Fig. 13 shows the schematic for sample preparation using casting and roll milling. The latent heat capacities of nanocomposite PCMs showed only a very small change also the thermal stability was improved due to the presence of exfoliated graphite nanoplatelets compared to pure paraffin wax. It is concluded that the enhancement of thermal physical properties of the PCM depends on the nanofillers of larger aspect ratio, better orientation and lower interface density. Phase change enthalpy and thermal conductivity of graphene/1octadecanol composite PCM was investigated as a function of graphene content by Yavari et al. [69]. The graphene was dispersed in PCM using solvent evaporation technique. The schematic representation of preparing PCM nanocomposite and the variation of

142

G. Raam Dheep, A. Sreekumar / Energy Conversion and Management 83 (2014) 133–148

Fig. 10. Melting and freezing curves of composite paraffin at different weight% of xGnP.

Table 8 Phase change properties of nanocomposite PCM with varying weight% of xGnP. Nanocomposite PCM

Pure paraffin Paraffin/xGnP 1% Paraffin/xGnP 3% Paraffin/xGnP 5%

Fig. 11. Phase change transition and phase change performance of paraffin/xGnP composite PCM.

latent heat of fusion and thermal conductivity is shown in Fig. 14. The thermal conductivity of the nanocomposite increased nearly 140% at 4wt% of graphene while the drop in the heat of fusion

Phase change temperature (°C) Melting

Cooling

Solid– solid

Solid– liquid

Solid– solid

Liquid– solid

35.3 35.1 34.9 35.4

55.2 55.1 55.1 54.9

32.9 32.9 33 32.9

51.1 51 51.4 50.8

was only 15.4%. It is claimed that the enhancement in thermal properties of 1-octadecanol obtained with the addition of graphene is superior to the effect of other nanofillers such as silver nanowires and carbon nanotubes. Li [70] prepared the paraffin based composite PCM by incorporating nanographite with a particle diameter of 35 nm and studied the effect of incorporation on phase change temperature, latent heat and thermal conductivity. The results indicated a considerable decrease in melting temperature but 7.41% increase in thermal conductivity. Table 10 and Fig. 15 shows the phase change properties based on the effect of incorporating different mass fraction of nanographite in paraffin.

143

G. Raam Dheep, A. Sreekumar / Energy Conversion and Management 83 (2014) 133–148 Table 9 Thermo-physical properties of nanocomposite PCM with varying weight% of capric acid:halloysite nanotube:graphite. CA:HNT:G (wt%)

Melting point (°C)

Melting latent heat (kJ/kg)

Freezing point (°C)

Freezing latent heat (kJ/kg)

50:50:0 55:45:0 60:40:0 60:35:5 65:35:0 100:0:0

28.64 29.23 29.34 29.56 29.58 29.62

56.27 66.76 75.52 75.40 85.89 139.77

25.08 25.13 25.28 25.36 25.31 25.57

56.85 67.43 75.81 75.35 86.24 140.12

Fig. 12. Melting (a) and freezing (b) curves of CA:HNT and CA:HNT:G nanocomposite PCM.

Fig. 13. Schematic representation for preparing paraffin wax/xGnP nanocomposite PCM using casting and roll milling methods.

Mehrali et al. [71] prepared and studied the thermal properties of form-stable composite phase change material by impregnating paraffin at different weight% in graphene oxide sheets using vac-

uum Impregnation method. The composite PCM containing 48.3 wt% of paraffin is found to be more stable that melts at 53.57 °C with a latent heat of 63.76 kJ/kg and solidifies at 44.59 °C with a latent heat of 64.89 kJ/kg. Thermal reliability of the composite PCM was analyzed for 2500 cycles and it was observed that the composite PCM melts and solidifies at 55.42 °C and 44.55 °C with the latent heats of 62.67 kJ/kg and 63.11 kJ/kg. The thermal conductivity of the composite PCM at molten (60 °C) and solid state (25 °C) shows three times increase compared to pure paraffin. The study made clear that graphene oxide sheet acts as a supporting and protective layer which decreases the degree of supercooling and also increases the thermal conductivity of the PCM to make more suitable for solar thermal energy storage applications. Phase change property of sebacic acid with and without expanded graphite was analyzed by Wang et al. [72]. It was shown that composite PCM has a phase change temperature of 128 °C with 187 kJ/kg heat of fusion which is slightly decreased compared to pure sebacic acid. It was also investigated that there is a small loss of PCM after 3000 heating and cooling cycles. Fig. 16 shows the effect of packing density on latent heat and thermal conductivity. Studies on thermal storage properties of polyethyleneglycol (PEG) is widely been studied by many researchers. Wang et al. [73] studied the effect of pore structure of the supporting materials on the phase change properties of the composite PCM. Wang used PEG as a PCM with expanded graphite (EG), active carbon (AC) and ordered mesoporous carbon (CMK-5) as supporting materials with various pore structures to synthesize the shape stabilized PCM. It was found that PEG/EG with pore diameter of 1.3  104 has a higher stability than other two carbon materials and proved that, stability of the PCM depends on the capillary forces of the pores and hydrogen bonding of the functional group. Feng et al. [74] studied the influence of different weight% of PEG with mesoporous active

144

G. Raam Dheep, A. Sreekumar / Energy Conversion and Management 83 (2014) 133–148

Fig. 14. Schematic representation for preparing graphene/1-octadecanol composite PCM (a), latent heat of fusion (b) and thermal conductivity (c) for various graphene variation.

Table 10 Phase change properties of nanocomposite PCM with varying weight% of nanographite. Properties

Phase change temperature (°C) Latent heat (kJ/kg) Thermal conductivity (W/ mK)

Nanographite mass fraction (%) 0

1

4

7

10

28.81

27.73

27.5

27.66

27.8

209.33 0.1264

202.58 0.3650

193.26 0.4971

183.62 0.5685

181.81 0.9362

carbon (AC) to prepare stable PCM by blending and impregnating method. The result obtained clarified that phase change enthalpies of the composite PCM increased with increasing weight% of PEG. Similar studies on thermo-physical properties of PEG were also analyzed by incorporating graphene oxide (GO) [75], sulfonated graphene (SG) [76] and SiO2 [77] at different weight%. Qian et al. [78] incorporated diammonium phosphate (DAP) in addition to the supporting material of Tetraethoxysilane (TEOS). The studies with the addition of DAP showed that the PCM has good flame retarding capacity and good thermal stability. Molecular dynamics (MD) simulation was performed by Rao et al. [79] to study about heat and mass transfer mechanisms of nanoparticle-enhanced PCM and nano-encapsulated PCM at molecular and atomic scale. The nano-encapsulated PCM was stud-

Fig. 15. Effect of nanographite mass fraction (%) on latent heat and thermal conductivity of paraffin.

ied using n-octadecane as core and SiO2 as shell material. Nanoparticle-enhanced PCM was studied by addition of Al nanoparticles on four different PCMs such as n-nonadecane, n-eicosane, n-heneicosane and n-docosane. The study expressed that thickness of the shell and size of the nanoparticles added to the PCM are to be

G. Raam Dheep, A. Sreekumar / Energy Conversion and Management 83 (2014) 133–148

optimized, which plays an important role in enhancing the thermal storage properties of the PCM like self diffusion coefficient, torsion, extension of the core material, mobility, latent heat and thermal conductivity. The author also simulated to understand the melting behavior of the phase change materials slurry using molecular dynamics [80]. The simulations were performed based on constant temperature, constant volume and constant pressure to study the self diffusion coefficient and concluded that molecular dynamic simulation is an effective way to predict the thermal storage behavior of the PCM. Zhang et al. [81] developed a novel microencapsulated PCM to enhance the thermo-physical properties and phase change characteristics using n-octadecane PCM core encapsulated in an inorganic silica shell. These microcapsules varied in the size ranging from 7 to 16 lm. The study revealed that microencapsulated PCM has

Fig. 16. Variation of thermal conductivity and latent heat with respect to packing density of sebacic acid/expanded graphite.

145

good thermal stability and enhanced thermal conductivity due to the presence of highly thermal conductive silica shell. Figs. 17 and 18 shows the steps involved in preparing and the SEM images of silica microencapsulated n-octadecane PCM respectively. The paper explained that by controlling the loading percent of n-octadecane and acidity of reaction, the microencapsulated PCM can achieve good phase-change performance, high encapsulation efficiency, and good anti-osmosis property. Li et al. [82] prepared and studied the thermal properties of paraffin microencapsulated in SiO2 based phase change composite. Here paraffin acts as a core and SiO2 as shell prepared by hydrolysis and polycondensation of tetraethyl orthosilicate. Fig. 19 shows the steps involved in preparing paraffin microencapsulated SiO2 PCM. Table 11 reveals that encapsulation ratio of 31.7% silica microencapsulated paraffin has a melting and freezing temperature of 56.5 °C and 45 °C, whereas for pure paraffin it was 56 °C and 50.5 °C. The degree of supercooling was increased to 11.5 °C from 5.5 °C for microencapsulated paraffin. The study elucidates that due to microencapsulation, the latent heat during melting and freezing were also decreased to 45.5 kJ/kg and 43.8 kJ/kg from 143.5 kJ/kg and 144 kJ/kg. The composite PCM was thermally stable even after 30 melting and freezing cycles. Pure paraffin completely melted when heated at a temperature of 70 °C for 10 min but silica encapsulated paraffin showed no leakage or melting even when heated at 70 °C for 20 min as it gives the mechanical strength and prevents the seepage of melted paraffin. Similar studies based on microencapsulation of PCM were also studied by Huang et al. [83] on disodium hydrogen phosphate heptahydrate (Na2HPO47H2O) as PCM core and polymethylmethacrylate (PMMA) as shell material and by Qiu et al. [84] on noctadecane as PCM core and poly butyl methacrylate (PBMA) and poly butyl acrylate (PBA) as shells. The phase change temperature and latent heat was stable and showed only small changes after 1000 thermal cycles. Stearic acid/polycarbonate (SA/PC) based microencapsulated phase change material with and without iron fillings to improve

Fig. 17. Schematic representation for preparing silica microencapsulated n-octadecane PCM.

Fig. 18. SEM images of silica microencapsulated n-octadecane PCM.

146

G. Raam Dheep, A. Sreekumar / Energy Conversion and Management 83 (2014) 133–148

Fig. 19. Schematic representation of preparing paraffin microencapsulated SiO2 PCM.

Table 11 Phase change properties of paraffin and silica microencapsulated paraffin. S. no

1 2

Phase change material

Pure paraffin Paraffin@SiO2

Phase change temperature (°C) Melting

Freezing

56 56.5

50.5 45

thermal storage and release rates was studied by Zhang et al. [85]. The melting and freezing temperatures are 60 °C and 51.2 °C with the latent heat of melting and crystallization are 91.4 kJ/kg and 96.8 kJ/kg. The incorporation of iron fillings has improved the heat storage and release rates up to 23%. SA/PC is found to have good thermal reliability even after 1000 thermal cycle. Preparation, characterization, thermal properties and thermal reliability of neicosane coated with polymethylmethacrylate (PMMA) shell for thermal energy storage application was reported by Alkan et al. [86]. The microencapsulated PCM has a melting and solidification temperatures of 35.2 °C and 34.9 °C with the latent heat of 84.2 kJ/kg and 87.5 kJ/kg. Accelerated 5000 thermal cycling was performed to study the thermal reliability of the PCM and observed that there are no large variations in thermal properties and tends to be most promising PCM for thermal energy storage. Konuklu et al. [87] studied the thermal energy storage performance of microencapsulated caprylic acid with three different shell materials. Urea–formaldehyde resin, melamine–formaldehyde resin and urea–melamine–formaldehyde resin were used as wall materials prepared using coacervation method. Among all these shell materials urea–formaldehyde resin based microencapsulated PCM proved to be most suitable for thermal energy storage applications. The melting and freezing temperature was found to be 15.4 °C and 9.3 °C for caprylic acid and 13.9 °C and 5.11 °C for microCA. The latent heat of melting and freezing are 158.4 kJ/kg and 162.3 kJ/kg for caprylic acid and 93.9 kJ/kg and 106.1 kJ/kg for microCA respectively. Author also optimized parameters such

Degree of supercooling (°C)

5.5 11.5

Latent heat of fusion (kJ/kg) Melting

Freezing

143.5 45.5

144 43.8

as emulsion temperature and emulsion stirring time which influences the latent heat capacity of the microCA. The results suggested that, the emulsion temperature should be between 50 °C and 70 °C with stirring time of 120 min, values higher or lower than these has decreased the thermal storage capability of the microencapsulated PCM. Immiscibility of PCM and stability of supporting material are the two main characteristics of microencapsulated PCM for thermal energy storage. Trigui et al. [88] prepared wax with low-density polyethylene (LDPE) by melt mixing method and analyzed the phase change properties of the composite PCM for passive solar wall applications. Ehid et al. [89,90] also prepared composite PCM of paraffin/high density polyethylene (HDPE) and paraffin/ graphite nanofibers (GNF)/HDPE to study about the structural and thermal properties, effects of changing polymer loading level, geometric stability to prevent settling of incorporated materials. From the results it is found that the incorporated materials settles at the bottom after 2–5 cycles but by the addition of 10% of HDPE, the settling of incorporated materials is prevented and the shape also remains stabilized. Fig. 20 shows the influence of HDPE on the settling of graphite nanofibres. Three different types of fatty acid eutectics capric acid (CA)–lauric acid (LA), capric acid–palmitic acid (PA), and capric acid–stearic acid (SA) was prepared by Cai et al. [91,92] using melt-blending and ultrasonication. The thermal performance of PCM was analyzed by preparing nanofibers of eutectic PCM using polyamide (PA6) and polyacrylonitrile polymers. Extended graphite and

Fig. 20. (a) Settling of GNF without HDPE and (b) uniform distribution of GNF with addition of HDPE after thermal cycling.

G. Raam Dheep, A. Sreekumar / Energy Conversion and Management 83 (2014) 133–148

carbon were also incorporated in the nanofibers and it was found that the enthalpies of melting and freezing were higher with increased thermal energy storage and retrieval rates, but has only very small changes in phase transition temperature of composite PCM. The author also investigated the properties of the nanofibres of eutectic PCM using polyethylene terephthalate (PET) [93] at different mass ratios and found phase change temperature of composite fibers decreased but heat enthalpies of melting and crystallization was maintained with small variations. The study concluded that amount of composite fibers plays an important role in thermal storage properties of the PCM. Xiao et al. [94] studied the phase change properties of salt nitrates (eutectic KNO3/NaNO3) by incorporating expanded graphite (EG). Rogers and Janz [95], Lopez et al. [96] and Acem et al. [97] also analyzed the thermophysical properties of the individual salt hydrates and eutectic mixtures of KNO3/NaNO3 for high temperature thermal energy storage applications. Thermal conductivity of NaNO3 was found to be 0.244 W/(mK) whereas for KNO3 is 0.156 W/(mK) at room temperature of 25 °C with the phase change temperature of 299.59–336.35 °C. These values increased to 10– 50% and latent heat of melting decreased to 5–20% by incorporating EG at different weight%. 5. Conclusion Thermal energy storage system based on the latent heat capacity of phase change material is an effective method to store solar thermal energy. This has been the topic of exhaustive research for several years and few researchers have come out with some promising results. They are succeeded in addressing the drawbacks associated with use of latent heat storage materials to make the technology to have wider acceptability. This review paper presents a detailed review on thermal energy storage materials and outlines the research areas related to the development of phase change materials, analyzing the thermo-physical properties, thermal reliability and thermal stability. Experimental methods and techniques to overcome certain demerits associated with usage of PCM such as low thermal conductivity, supercooling and incongruent melting through nanomaterial incorporation and encapsulation are also been presented. The conclusions derived out of the literature review are summarized below:  Inorganic PCMs are more suitable for low temperature to high temperature thermal energy storage but has got serious disadvantages of corrosion, incongruent melting and supercooling. Therefore organic PCM are mostly preferred due to wide operating temperature, high latent heat and congruent melting but these PCMs has low thermal conductivity and low heat transfer rates.  Metals (Ag, Al, Cu), metal oxides (Al2O3, CuO, MgO, ZnO and TiO2) and carbon based (expanded graphite sheets, SWCNT, MWCNT, graphene nanosheets, graphene flakes, nanofibers, active carbon) nanomaterials are incorporated with PCM to achieve efficient phase change properties. In general carbon based nanostructure with high aspect ratio tends to be the most promising nano-additives due to enhanced thermo-physical properties as compared to the metal and metal oxide nanomaterials.  Microencapsulation is another method to enhance thermal conductivity, increase heat transfer rate, reduce PCM interaction with the outside environment, and also control the volume change of PCM with increase in operating temperature and this helps to prevent leakage during phase change. Using these techniques together with selection of suitable materials for incorporation and encapsulation, it is possible to develop high efficient solar thermal energy storage system based on phase

147

change materials. Therefore latent heat storage systems by using phase change materials prove to be the most promising technique in the field of solar thermal energy storage. References [1] Duffie John A, Beckman William A. Solar engineering and thermal processes. Wiley; 1980. [2] Dincer Ibrahim, Rosen Marc A. Thermal energy storage systems and applications. Wiley; 2011. [3] Tiwari GN. Solar energy fundamentals, design, modeling and applications. New Delhi: Narosa; 2005. [4] Garg HP, Prakash J. Solar energy fundamentals and applications. New Delhi: Tata McGraw-Hill; 2002. [5] Garg HP, Mullick SC, Bhargava AK. Solar thermal energy storage. Holland: Reidel; 1985. [6] Nkwetta Dan Nchelatebe, Haghighat Fariborz. Sust Cities Soc 2014;10:87–100. [7] Agyenim F, Neil H, Eames P, Smyth M. Renew Sustain Energy Rev 2010;14. [8] Abhat A. Rev Phys Appl 1980;15:477–501. [9] Sharma Atul, Tyagi VV, Chen CR, Buddhi D. Renew Sustain Energy Rev 2009;13:318–45. [10] Zalba Belen, Marın Jose M, Cabeza Luisa F, Mehling Harald. Appl Therm Eng 2003;23:251–83. [11] Sharma SD, Sagara Kazunobu. Int J Green Energy 2005;2:1–56. [12] Sharma Someshower Dutt, Kitano Hiroaki, Sagar Kazunobu. Res Rep Fac Eng Mie Univ 2004;29:31–64. [13] Sarier Nihal, Onder Emel. Thermochim Acta 2012;540:7–60. [14] Farid Mohammed M, Khudhair Amar M, Razack Siddique Ali K, Said Al-Hallaj. Energy Convers Manage 2004;45:1597–615. [15] Kenisarin Murat M. Renew Sustain Energy Rev 2010;14:955–70. [16] Cardenas Bruno, Leon Noel. Renew Sustain Energy Rev 2013;27:724–37. [17] Al-abidi Abduljalil A, Bin Mat Sohif, Sopian K, Sulaiman MY, Th. Mohammed Abdulrahman. Renew Sustain Energy Rev 2013;20. [18] Khodadadi JM, Fan Liwu, Babaei Hasan. Renew Sustain Energy Rev 2013;24: 418–44. [19] Jamekhorshid A, Sadrameli SM, Farid M. Renew Sustain Energy Rev 2014;31:353–63. [20] Youssef Ziad, Delahaye Anthony, Huang Li, Trinquet François, Fournaison Laurence, Pollerberg Clemens, et al. Energy Convers Manage 2013;65:120–32. [21] Egolf PW, Sari O, Kitanovski A. University of applied sciences of western Switzerland international report; 2003. [22] Chen Lin, Wang Ting, Zhao Yan, Zhang Xin-Rong. Energy Convers Manage 2014;79:317–33. [23] Feldman D, Shapiro MM, Banu D. Sol Energy Mater 1986;13:1–10. [24] Arndt PE, Dunn JG, Willix RLS. Thermochim Acta 1984;79:55–68. [25] Arndt PE, Dunn JG, Willix R. Thermochim Acta 1981;48:237–40. [26] Aboul-Enein S, Olofa SA. Renewable Energy 1991;1(5/6):791–7. [27] Desgrosseilliers Louis, Whitman Catherine A, Groulx Dominic, White Mary Anne. Appl Therm Eng 2013;53:37–41. [28] Tyagi VV, Buddhi D. Sol Energy Mater Sol Cells 2008;92:891–9. [29] El-Sebaii AA, Al-Heniti S, Al-Agel F, Al-Ghamdi AA, Al-Marzouki F. Energy Convers Manage 2011;52:2606–14. [30] El-Sebaii AA, Al-Amir S, Al-Marzouki FM, Faidah Adel S, Al-Ghamdi AA, AlHeniti S. Energy Convers Manage 2009;50(3104):1771–7. [31] Shukla Anant, Buddhi D, Sawhney RL. Renewable Energy 2008;33:3104–11. [32] Harikrishnan S, Kalaiselvam S. Thermochim Acta 2012;533:46–55. [33] Harikrishnan S, Magesh S, Kalaiselvam S. Thermochim Acta 2013;565:137–45. [34] Karunamurthy K, Murugumohankumar K, Suresh S. Digest J Nanomater Biostruct 2012;7(4):1833–41. [35] Valan Arasu A, Sasmito Agus P, Mujumdar Arun S. Front Heat Mass Transf 2011;2:043005. [36] Jesumathy Stella P, Udayakumar M, Suresh S. J Mech Sci Technol 2012;26(3): 959–65. [37] Jesumathy Stella, Udayakumar M, Suresh S. Heat Mass Transfer 2012;48: 965–78. [38] Liu Zongrong, Chung DDL. Thermochim Acta 2001;366:135–47. [39] Wang Jifen, Xie Huaqing, Li Yang, Xin Zhong. J Therm Anal Calorim 2010;102:709–13. [40] Fan Liwu, Khodadadi JM. Int J Therm Sci 2012;62:120–6. [41] Ho CJ, Gao JY. Int Commun Heat Mass Transfer 2009;36:467–70. [42] Teng Tun-Ping, Chao-Chieh Yu. Nanoscale Res Lett 2012;7(611):611. [43] Peng Hu, Da-Jie Lu, Fan Xiang-Yu, Zhou Xi, Chen Ze-Shao. Sol Energy Mater Sol Cells 2011;95:2645–9. [44] Zeng Ju-Lan, Zhu Fu-Rong, Sai-Bo Yu, Zhu Ling, Cao Zhong, Sun Li-Xian, et al. Sol Energy Mater Sol Cells 2012;105:174–8. [45] Zeng JL, Cao Z, Yang DW, Sun LX, Zhang L. J Therm Anal Calorim 2010;101:385–9. [46] Parameshwaran R, Jayavel R, Kalaiselvam S. J Therm Anal Calorim 2013;3064–9. [47] Zeng JL, Sun LX, Xu F, Tan ZC, Zhang ZH, Zhang J, et al. J Therm Anal Calorim 2007;87(2):369–73. [48] Nan Wang, Shuo Yang, Dongsheng Zhu, Xiaoping Ju, Preparation and heat transfer behavior of paraffin based composite containing nano-copper particles. In: 7th International conference on multiphase flow. Tampa, FL USA; May 30–June 4, 2010.

148

G. Raam Dheep, A. Sreekumar / Energy Conversion and Management 83 (2014) 133–148

[49] Shuying Wu, Zhu Dongsheng, Zhang Xiurong, Huang Jin. Energy Fuels 2010;24:1894–8. [50] Li Ting Xian, Lee Ju-Hyuk, Wang Ru Zhu, Kang Yong Tae. Energy 2013;55:752–61. [51] Wang Jifen, Xie Huaqing, Xin Zhong, Li Yang. Carbon 2010;48:3979–86. [52] Wang Jifen, Xie Huaqing, Xin Zhong. J. Appl. Phys. 2008;104:113537. [53] Wang Jifen, Xie Huaqing, Xin Zhong, Li Yang, Chen Lifei. Sol Energy 2010;84:339–44. [54] Wang Jifen, Xie Huaqing, Xin Zhong. Thermochim Acta 2009;488:39–42. [55] Ji Peijun, Sun Huanhuan, Zhong Yunxia, Feng Wei. Chem Eng Sci 2012;81:140–5. [56] Zeng JL, Cao Z, Yang DW, Xu F, Sun LX, Zhang XF, et al. J Therm Anal Calorim 2009;95(2):507–12. [57] Teng Tun-Ping, Yu Chao-Chieh. Int J Chem Eng Appl 2012;3(5). [58] Shaikh Shadab, Lafdi Khalid, Hallinan Kevin. J Appl Phys 2008;103:094302. [59] Fan Li-Wu, Fang Xin, Wang Xiao, Zeng Yi, Xiao Yu-Qi, Zi-Tao Yu, et al. Appl Energy 2013;110:163–72. [60] Elgafy Ahmed, Lafdi Khalid. Carbon 2005;43:3067–74. [61] Cui Yanbin, Liu Caihong, Hu Shan, Yu Xun. Sol Energy Mater Sol Cells 2011;95:1208–12. [62] Zhong Yajuan, Zhou Mi, Huang Fuqiang, Lin Tianquan, Wan Dongyun. Sol Energy Mater Sol Cells 2013;113:195–200. [63] Xia L, Zhang P. Sol Energy Mater Sol Cells 2011;95:2246–54. [64] Kim Sumin, Drzal Lawrence T. Sol Energy Mater Sol Cells 2009;93:136–42. [65] Zhang Xiaoxing, Deng Pengfei, Feng Rongxiu, Song Jian. Sol Energy Mater Sol Cells 2011;95:1213–8. [66] Chen Zhi, Shan Feng, Cao Lei, Fang Guiyin. Sol Energy Mater Sol Cells 2012;102:131–6. [67] Mei Dandan, Zhang Bing, Liu Ruichao, Zhang Yatao, Liu Jindun. Sol Energy Mater Sol Cells 2011;95:2772–7. [68] Xiang Jinglei, Drzal Lawrence T. Sol Energy Mater Sol Cells 2011;95:1811–8. [69] Yavari Fazel, Fard Hafez Raeisi, Pashayi Kamyar, Rafiee Mohammad A, Zamiri Amir, Yu Zhongzhen, et al. J Phys Chem C 2011;115:8753–8. [70] Li Min. Appl Energy 2013;106:25–30. [71] Mehrali Mohammad, Latibari Sara Tahan, Mehrali Mehdi, Metselaar Hendrik Simon Cornelis, Silakhori Mahyar. Energy Convers Manage 2013;67:275–82. [72] Wang Shuping, Qin Peng, Fang Xiaoming, Zhang Zhengguo, Wang Shuangfeng, Liu Xiaohong. Sol Energy 2014;99:283–90. [73] Wang Chongyun, Feng Lili, Li Wei, Zheng Jie, Tian Wenhuai, Li Xingguo. Sol Energy Mater Sol Cells 2012;105:21–6. [74] Feng Lili, Zheng Jie, Yang Huazhe, Guo Yanli, Li Wei, Li Xingguo. Sol Energy Mater Sol Cells 2011;95:644–50.

[75] Qi Guo-Qiang, Liang Cheng-Lu, Bao Rui-Ying, Liu Zheng-Ying, Yang Wei, Xie Bang-Hu, et al. Sol Energy Mater Sol Cells 2014;123:171–7. [76] Li Hairong, Jiang Ming, Li Qi, Li Denian, Chen Zongyi, Waping Hu, et al. Energy Convers Manage 2013;75:482–7. [77] Li Jingruo, He Lihong, Liu Tangzhi, Cao Xuejuan, Zhu Hongzhou. Sol Energy Mater Sol Cells 2013;118:48–53. [78] Qian Yong, Wei Ping, Jiang Pingkai, Li Zhi, Yan Yonggang, Ji Kejian, et al. Energy Convers Manage 2013;76:101–8. [79] Rao Zhonghao, Wang Shuangfeng, Peng Feifei. Int J Heat Mass Transf 2013;66:575–84. [80] Rao Zhonghao, Wang Shuangfeng, Maochun Wu, Zhang Yanlai, Li Fuhuo. Energy Convers Manage 2012;64:152–6. [81] Zhang Huanzhi, Wang Xiaodong, Dezhen Wu. J Colloid Interface Sci 2010;343:246–55. [82] Li Benxia, Liu Tongxuan, Luyang Hu, Wang Yanfen, Gao Lina. ACS Sustainable Chem Eng 2013;1:374–80. [83] Huang Jin, Wang Tingyu, Zhu Panpan, Xiao Junbin. Thermochim Acta 2013;557:1–6. [84] Qiua Xiaolin, Song Guolin, Chu Xiaodong, Li Xuezhu, Tang Guoyi. Thermochim Acta 2013;551:136–44. [85] Zhang Ting, Wang Yi, Shi Huan, Yang Wantai. Energy Convers Manage 2012;64:1–7. [86] Alkan Cemil, Sari Ahmet, Karaipekli Ali. Energy Convers Manage 2011;52:687–92. [87] Konuklu Yeliz, Unal Murat, Paksoy Halime O. Sol Energy Mater Sol Cells 2014;120:536–42. [88] Trigui Abdelwaheb, Karkri Mustapha, Krupa Igor. Energy Convers Manage 2014;77:586–96. [89] Ehid Ryan, Fleischer Amy S. Energy Convers Manage 2012;53:84–91. [90] Ehid Ryan, Weinstein Randy D, Fleischer Amy S. Energy Convers Manage 2012;57:60–7. [91] Cai Yibing, Gao Chuntao, Zhang Ting, Zhang Zhen, Wei Qufu, Jinmei Du, et al. Renewable Energy 2013;57:163–70. [92] Cai Yibing, Zong Xue, Zhang Jingjing, Yiyuan Hu, Wei Qufu, He Guangfei, et al. Sol Energy Mater Sol Cells 2013;109:160–8. [93] Cai Yibing, Ke Huizhen, Lin Liang, Fei Xiuzhu, Wei Qufu, Song Lei, et al. Energy Convers Manage 2012;64:245–55. [94] Xiao X, Zhang P, Li M. Energy Convers Manage 2013;73:86–94. [95] Rogers DJ, Janz GJ. J Chem Eng Data 1982;27:424–8. [96] Lopez J, Acem Z, Barrio EPD. Appl Therm Eng 2010;30:1586–93. [97] Acem Z, Lopez J, Barrio EPD. Appl Therm Eng 2010;30:1580–5.