Phase change material based cold thermal energy storage: Materials, techniques and applications – A review

Accepted Manuscript Title: Phase change material based cold thermal energy storage: materials, techniques and applications-a review Author: C. Veerakumar, A. Sreekumar PII: DOI: Reference:

S0140-7007(15)00383-7 http://dx.doi.org/doi: 10.1016/j.ijrefrig.2015.12.005 JIJR 3213

To appear in:

International Journal of Refrigeration

Received date: Revised date: Accepted date:

18-9-2015 3-12-2015 16-12-2015

Please cite this article as: C. Veerakumar, A. Sreekumar, Phase change material based cold thermal energy storage: materials, techniques and applications-a review, International Journal of Refrigeration (2016), http://dx.doi.org/doi: 10.1016/j.ijrefrig.2015.12.005. 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.

Phase Change Material based Cold Thermal Energy Storage: Materials, Techniques and Applications-A review C. Veerakumara, A. Sreekumarb* Department of Green Energy Technology, Madanjeeth School of Green Energy Technologies, Pondicherry University (A Central University), Puducherry-605014, India. *Corresponding author Email: [email protected] Tel: +91 413 265314 Fax: +91 413 2656758 Highlights 

PCM based cold thermal storage is more advantageous than other techniques such as sensible and thermo-chemical storage techniques.

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Heat transfer rate of the PCM can be increased by inclusion of nano structures.

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Use of less corrosive material is important while developing cold thermal storage system.

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Cold Thermal Storage System aids to increase the efficiency of air-conditioning system.

Abstract This paper gives a comprehensive review on recent developments and the previous research studies on cold thermal energy storage using phase change materials (PCM). Such commercially available PCM’s having the potential to be used as material for cold energy storage are categorized and listed with their melting point and latent heat of fusion. Also techniques for improving the thermo-physical properties of PCM such as heat transfer enhancement, encapsulation, inclusion of Nano structures and shape stabilization are reviewed. Effect of stability due to corrosion of construction materials is also reported. Finally different applications where the PCM can be employed for cold energy storage such as free cooling of building, air-conditioning, refrigerated trucks and cold packing are discussed. Keywords: Cold Thermal Energy Storage, Phase Change Material, Nano Structures, Encapsulation, Free Cooling Nomenclature am

fraction melted (kg)

ar

fraction reacted (kg)

Cp

specific heat capacity (kJ kg-1 K-1)

H

latent heat of fusion (kJ kg-1) 1 Page 1 of 29

m

mass of storage medium (kg)

Q

amount of energy stored (J)

Tf

final temperature (oC)

Ti

initial temperature (oC)

Tm

melting temperature (oC)

∆T

temperature difference (oC)

∆hr

endothermic heat of reaction (kJ kg-1)

Abbreviation PCM

Phase Change Material

HyNC

Hybrid Nano Composite

HyNPCM

Hybrid Nano Phase Change Material

LHTS

Latent Heat Thermal Storage

MPCM

Micro-encapsulated Phase Change Material

DSC

Differential Scanning Calorimetry

SEM

Scanning Electron Microscope

XRD

X-Ray Diffraction

1. Introduction Cooling is one of the major energy consuming processes. Cold Thermal Energy Storage is a process which involves adding cold thermal energy to a medium and extracting it whenever it is needed. During the charging process, the available cold thermal energy can be accumulated in to the storage medium. During discharging process the stored cold thermal energy is retrieved and supplied for the end use. The cold thermal energy can be stored by virtue of change in internal energy or phase transformation of the storage medium. It is an energy saving technology which reduces the electricity peak load by storing cold during off peak hours [1,13] and also for seasonal storage [9]. The commonly used thermal storage methods are 1. Sensible storage 2. Latent storage and 3. Thermochemical storage. In cold thermal energy storage, sensible and latent storage methods are widely used. The energy storage capacity of the sensible heat storage system is based on specific heat capacity and the temperature difference. There is no phase change during energy storage and retrieval. Equation (1) gives the amount of energy stored in a sensible storage system. (1) Latent heat energy storage pulls more attraction because of its high energy storage density [2] of 5-14 times higher than sensible storage [3]. In this technique, a phase change occurs during energy storage and retrieval. The amount of energy stored is based on the latent heat of fusion 2 Page 2 of 29

of the material. PCM is also used to increase the energy storage capacity of a system [4]. Equation (2) gives the amount of energy stored in a latent heat storage system. (2) Thermochemical energy storage system depends on the energy absorbed and released during a chemical reaction by breaking and reforming molecular bonds. The chemical reaction should be completely reversible. The amount of energy stored is based on amount of reacting materials. Equation (3) gives the amount of energy stored in a thermochemical energy storage system. (3) 2. Phase Change Materials for Cold Thermal Energy Storage The materials used in latent heat storage are known as phase change materials. There are some desirable thermo-physical, kinetic and chemical properties [5-8] for a material to be used as a PCM which are listed below.            

The melting temperature of the PCM should be in the range of operating temperature High Latent heat of fusion High thermal conductivity High density Low volume change during phase change Low degree of super cooling Less corrosive to the construction materials Low degradation Chemically stable Non-toxic and non-flammable Easily available Cost effective

The Fig.1 shows the classification of phase change materials used for cold thermal energy storage. 2.1. Organic PCM Organic PCMs are carbon based compounds. These are generally classified as paraffin and non-paraffin. The latent heat of fusion of the PCM increases with increase in molecular weight or number of carbon atoms. The advantages of organic PCMs are its chemical stability and high latent heat of fusion but the main disadvantage is its low thermal conductivity. Some of the organic PCMs from the literature are listed in Table 1. 2.2. Inorganic PCM Inorganic PCMs are generally metallic and hydrated salts. The main advantages of inorganic PCMs are low cost, good thermal conductivity but it is corrosive in nature with the construction materials. Some of the inorganic PCMs from the literature are listed in Table 2. 2.3. Eutectic PCM

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Eutectic PCM is a mixture of two or more PCMs in order to achieve the desirable melting point. The melting point of the PCM is one of the major criteria for selecting PCM for cold storage applications. The Eutectic PCM can be further divided into Eutectic PCMs for low temperature cooling systems and Eutectic PCMs for high temperature cooling systems. Also it can be categorised as Organic Eutectic PCMs and Inorganic Eutectic PCMs which are listed in Table 3 and Table 4 respectively. 2.4. Commercial PCMs PCMs which are commercially available in market also can be used for cold thermal energy storage applications. Some of the commercially available PCMs are listed in Table 5 [40-57]. 3. Selection of PCM for Cold Thermal Storage application Selecting a right PCM for a particular cold storage application is an important criterion in which much attention to be paid. The exact phase transition temperature should be known to design a cold storage system. Differential Scanning Calorimetric analysis at varying heating and cooling rate provides the important properties of PCMs like latent heat of fusion and exact phase transition temperature. Also its thermal stability, reactivity with the construction materials are some of the other properties to be noted. 4. Heat transfer enhancement A most common problem of using latent heat cold storage is low heat transfer rate especially while using organic phase change materials. This will lead to practical problems such as incomplete melting/freezing, loss in extraction of stored energy, etc. In order to increase the heat transfer rate several techniques such as inclusion of nano-structures, encapsulation and shape stabilization of PCM are experimented by many researchers.

4.1. Inclusion of Nano structures Nano structures are embedded into the pure PCM to increase the rate of heat transfer during the cyclic phase change process. The shape and size of the nano structures are important in a way that the surface to volume ratio of nano structure alters the thermo-physical properties of the PCM [58]. Metal and metal oxide nano structures are commonly used which enables micro convection between nano structures and base PCM and thereby the thermal conductivity also increased. The effect on thermo-physical properties of phase change materials due to the dispersion of nano structures are reviewed by M.A. Kibria et al. [59] and G. Raam Dheep et al. [60]. R. Parameshwaran et al. [61] experimentally investigated on the variation of thermal properties of organic ester PCM when silver-titania nano particles are embedded in it with different proportions. Fig.2. gives the scheme for preparation of hybrid nano composite phase change material. DSC graph of pure PCM and PCM with different mass proportions of HyNC is shown in Fig.3. in which the thermal conductivity was significantly altered thereby the melting and freezing time were also decreased. The heat storage and releasing characteristics of the pure PCM and the HyNPCMs were also studied and presented as in Table 6. It is obvious that the variation in melting temperature and latent heat of fusion of the 4 Page 4 of 29

PCM is very less even after 1000 thermal cycles. These improved results facilitated the Hybrid Nano Phase Change Material (HyNPCM) as a viable material for cold storage application.

P. Chandrasekaran et al. [62] investigated the thermal properties of nano-fluid phase change material. In the study, multi-walled carbon nano tubes were mixed in deionised water with a nucleating agent. The results showed that the heat transport properties were increased along with charging and discharging time. L. Fan et al. [63] experimentally investigated the thermal properties of CuO nano particle enhanced cyclohexane as phase change material. The thermal conductivity was measured using the transient plane source technique for both solid and liquid phases. Unidirectional freezing of PCM samples were also studied using a specially designed equipment shown in Fig.4. and compared with numerical predictions. The thermal conductivity of the Nano enhanced Phase Change Material samples (NePCM) were measured using an experimental setup shown in Fig. 5. From the result given in Table 7 it was found that the thermal conductivity increases with increase in concentration of nano particles in liquid phase. R. Parameshwaran et al. [64] worked on thermal performance improvement of variable airconditioning system coupled with silver nano particles embedded latent heat thermal storage system. The pictorial view of silver nanoparticles embedded PCM samples at different stages of charging and discharging cycles are shown in the Fig. 6. The experimental results suggested that the proposed air-conditioning system achieved an on-peak energy saving potential of 36-58 % and daily energy saving potential of 24-51 % when compared with the conventional air conditioning system. The combined effect of silver nano particle embedded LHTS with ventilation techniques was also studied by the same researcher and the result showed that the overall thermal performance of the system was improved. Kalaiselvam et al. [65] reported the heat transfer characteristic and thermodynamic behaviour of aluminium and alumina nano particle embedded PCM for building application. The experimental results of six PCMs namely 60% n-tetradecane: 40% n-hexadecane, Capric/Lauric Acid, CaCl2.6H2O, n-Octadecane, n-Hexadecane, and n-Eicosane were analysed. The study proved that the nano particle embedded PCM (60% n-tetradecane:40% n-hexadecane) took less freezing time compared to its pure form. It was noted that selecting specific nano particles for a particular PCM is an important criteria for determining charging and discharging rate. The experimental investigation of CuO-Oleic acid nano fluids for thermal energy storage for cooling systems was reported by Harikrishnan et al. [66]. Thermophysical properties were analysed with different mass fraction of CuO nano particles. Fig.7. and Fig. 8 show the sedimentation photograph and DSC curve of pure oleic acid and oleic acid with different mass fraction of CuO nano particles. The study reported that the nano fluids with 0.5, 1.0, 1.5 and 2 wt% of CuO nanoparticles mass fractions saved complete solidification time by 10.71, 16.07, 19.64, 27.67% and melting time by 7.14, 14.28, 25, 28.57% respectively. Also the thermal conductivity of composite PCM was increased as compared to base fluid. Shuo Zhang et al. used multi-wall carbon nano-tube (MWCNT) in organic liquid nhexadecane to decrease supercooling. Thermal analysis of n-Hexadecane with different concentrations of MWCNT ranging from 0.1 to 1.0 w/w% was done with DSC. Fig. 9. shows 5 Page 5 of 29

the DSC curves of pure n-Hexadecane. Here MWCNT is also used as nucleating agent and the DSC curves of the hexadecane with MWCNT of different fractions as the nucleating agent for heating and cooling process is given in Fig.10. The result of the study summarized in Table.8. reveals that supercooling decreases significantly with 0.1% and 0.5% but only slight variation over 1.0% concentration.

4.2. Encapsulation of PCM Encapsulation of PCM is a practical method to increase the heat transfer and it prevents PCM from mixing with the heat transfer fluid. The PCM containment used for encapsulation should possess qualities such as strength, flexibility, corrosion resistance and thermal stability [70]. In addition, it should provide necessary surface area for heat transfer and structural stability. Different types of encapsulation methods are macro-encapsulation, microencapsulation and nano-encapsulation. The PCM filled in blocks, pouches, spherical capsules etc. in macro scale made of metallic or polymeric film is referred as macro-encapsulation (Fig.11. to Fig.16.). The thermal performance of a macro-encapsulated PCM storage tank was investigated by Sih-Li Chen et al.[68]. Authors used cylindrical capsules with water as PCM and ethylene glycol as HTF. Experimental results showed that low coolant temperature and high flow rate increases the efficiency of the storage tank. Micro-encapsulation and nano-encapsulation refers to the filling of PCM in capsules made of polymer in micro and nano scale respectively. A performance study on micro-encapsulated PCM for air-conditioning application was done by Y. Allouche et al. [71]. The SEM micrograph of microencapsulated PCM is shown in Fig.17. The different thermo-physical properties such as specific heat, thermal conductivity, density, and enthalpy were experimentally determined. Also thermal performance of 100 L horizontal tank with tube bundle heat exchanger was experimentally determined. The result was compared with water based sensible storage system. The study shows that the amount of energy stored by PCM was 53% higher than that of water based sensible storage system of same tank capacity. Microcapsules of n-Octadecane with different n-butyl methacrylate copolymer based encapsulation were prepared and the surface morphologies were studied by X. Qiu et al.[72]. From the result it was found that the MicroPCMs with P(BMA-co-MAA) possess high latent heat of fusion and thermal resistance. The BMA based encapsulation of PCM increases the thermal regulation potential. 4.4. PCM Slurries The thermal capacity and heat transfer rate of heat transfer fluid or storage medium can be increased by using micro/nano encapsulated PCM slurries. Z. Youssef et al. [74] reviewed different properties of Clathrate hydrate slurry, microencapsulated PCM slurry, shapestabilized PCM slurries and phase change material emulsions. The study throws light on choice of suitable PCM slurries for laboratory and industrial applications. L. Huang et al. [75] studied on phase change slurry made up of paraffin blend with a melting point in the range of 2-12 oC and water emulsion. The results indicated that the paraffin/water emulsion with a 6 Page 6 of 29

paraffin mass fraction of 30–50 mass% is suitable for cold storage and distribution applications. Fig.18. shows the appearance of the MPCM slurry and SEM microscopic image of the MPCM particles. X. Wang et al. [76] proposed a new design of air-conditioning system with microencapsulated PCM storage tank. A feasibility study was carried out with mathematical model and theoretical calculations. The properties of MPCM slurry with different mass fractions are given in Table 9. The study revealed that the MPCM slurry storage tank is suitable for cooling load shift from daytime to night time and it is economically feasible. A work on raising the evaporative cooling potential by using combined cooled ceiling and MPCM slurry storage was also presented by the same author [77]. The evaporative cooling availability and utilization were assessed for five different climatic cities in china. A design methodology for designing MPCM slurry storage tank was also proposed. 4.5. Shape stabilized PCM Shape stabilized PCMs are special kind of PCMs which consists of working material and a supporting material. The supporting material remains in solid phase even when the working material undergoes phase change. The shape stabilized PCMs are prepared by physical and chemical methods. The physical methods involve adsorbing, blending, impregnation etc. and the chemical methods involve sol-gel method and graft co-polymerization. [78-82]. L. Feng et al.[84] prepared a shape stabilized PCM of Polyethylene glycol and activated carbon by physical impregnation method. The physical properties such as phase change temperature and enthalpy were observed for different weight percentage and molecular weight. Fig.19. shows the SEM image of PEG/SiO2 shape stabilized PCM. From the result it was concluded that the phase change properties are influenced by PEG segments from the porous structure of the activated carbon. T. Qian et al.[85] prepared a novel shape stabilized composite PCM of polyethylene glycol as PCM material and SiO2 as a carrier matrix by temperature assisted sol-gel method. The different characterization techniques such as DSC, FT-IR, SEM and XRD were adopted for the prepared PCM. The study concluded that the super-cooling extend, melting and solidification time of the shape stabilized PCM were 22.3%, 26.5% and 22.6% less than those of normal PEG. A paraffin blend/porous opal composite shape stabilized PCM was prepared by Z. Sun et al. [86]. The new PCM was characterized by SEM, FT-IR, DSC and thermal cycling test. The result showed that the phase transition temperature and latent heat of fusion were well suitable for indoor thermal energy storage applications. A composite shape stabilized PCM was prepared by Li et al. [87]. The main advantage of shape stabilized PCM is it can undergo phase transition without any containers to hold it. Shape-stabilized PCMs are generally used for space cooling of buildings. Capric acid-lauric acid eutectic mixture totally encapsulated into porous graphite with the mass fraction of 80.47% was prepared by M. Feng et al. [88]. The result showed that the melting and freezing time were reduced and the thermal conductivity was increased. A shape stabilized PCM of Capric acid adsorbed into Expanded Perlite (EP) prepared by Sari et al. [89] possess good thermal stability even after 5000 thermal cycles. Fig.20. shows the DSC curves of Capric acid and Capric acid adsorbed into Expanded Perlite. Fig.21. shows the DSC curves of CA/EP composite PCM before and after thermal cycling.

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5. Corrosion While developing a thermal storage system it is necessary to be focussed on corrosion of the construction material when it is used with different PCMs at low temperature. P. Moreno et al. [90] evaluated corrosion on different metals with salt hydrate as PCM. Four commercial PCMs used for HVAC applications were tested and the results were presented. Corrosion test of five different materials namely aluminium, stainless steel 316, stainless steel 304, carbon steel and copper with four different PCMs (one inorganic mixture, one ester and two fatty acid eutectics) was performed and the result were presented by G. Ferrer et al.[91] The result revealed that aluminium specimen is suitable except inorganic PCM. Copper can be used for fatty acid PCMs. Stainless steel 316 and Stainless steel 304 was suitable for all type of PCMs tested. Fig.22(a-d) shows the photograph of metal samples before and after immersed in different PCMs. E. Oró et al. [92] tested four commonly used metals (copper, aluminium, stainless steel 316, and carbon steel) and four polymers (poly-propylene, high density polyethylene, polyethylene terephthalate, and polystyrene) with nine different PCMs used for cold storage. The study reported that stainless steel is most suitable metal for PCM container. 6. Applications of PCM in Cold Thermal Storage Systems In this section, different applications of PCM in cold energy storage are discussed. There are wide ranges of applications such as air-conditioning, free cooling of building, food storage, medical, cold packing etc. A capital cost investment study of paraffin wax as cold thermal energy storage material was done by He et al. [93]. The incorporation of storage system helps to save tremendous amount of conventional energy in addition to reduction in initial capital cost, maintenance and other operational costs. Also the efficiency of the existing system can be improved by implementing cold energy storage technology. 6.1. Free cooling and Air-Conditioning Free cooling is a technique refers to the cooling of building’s interior in day time by using night time cold of the ambient air. Thermal energy storage systems are employed for this inorder to provide long time air-conditioning. The TES system stores the night time cold of the air and supplies in day time. The storage medium for free cooling is in the form of sensible or latent heat storage. The LHTES by using PCM is preferred due to its high energy storage density. The performance of the free cooling system is better if the diurnal temperature range of the day is 15oC. C. Arkar et al. [94] investigated free cooling efficiency results of a mechanical ventilation system with LHTES which consists of encapsulated paraffin RT20 as PCM. From the analysed results it is found that the free cooling technique reduces the size of the mechanical ventilation system and therefore enables better thermal comfort. K.Yanbinga et al. [95] proposed an innovative design of night ventilation system with PCM packed bed storage. An experimental setup of night ventilation system with and without LHTES was compared in this work. Fig.23. gives the schematic of NVP system. A mathematical model was developed and its result was compared and validated with the result of experimental model. The experimental result showed that the NVP system can prominently improve the thermal 8 Page 8 of 29

comfort level of the indoor environment. The experimental installation of the system is shown in Fig.24. U. Stritih et al. [96] experimentally analysed the cooling of buildings using night-time cold accumulation in a phase change material. Fig.25. describes the principle function of PCM free cooling system. In this paper some of the phase change materials used for free cooling was listed. The paraffin with melting point of 22oC was used and an experiment was conducted. For different air velocities, the air temperatures, heat flux and heat as a function of time was reported as in Fig.26, 27 and 28. K. Nagano et al. [97] proposed a new floor supply air conditioning system using phase change material granules with a melting point temperature of 20oC. A small scale experimental system was constructed and the granular PCM was introduced in it. In this experiment three different sample of PCM Hexadecane, Octadecane and mixture of Hexadecane and Octadecane is absorbed in glass beats and used as granular PCM. This is more suitable for floor supply in air conditioning system and it is better than sensible storage system. The efficiency of a gas turbine is increased by cooling the inlet air. J.P. Bédécarrats et al.[98] employed an encapsulated phase change refrigeration storage for inlet air cooling of a gas turbine and tested for different hot and wet climate at New Delhi, India. A. Lazaro et al. [99] experimentally tested the PCM–air real-scale heat exchangers. The experimental setup for air–PCM heat exchangers at University of Zaragoza is shown in Fig.29. From the result it was concluded that PCM with low thermal conductivity and low total energy storage capacity, but with efficiently designed heat exchanger has higher cooling power and hence an effort should be made to design heat exchangers instead of to enhance PCM thermal conductivity. A concept of increasing the cooling efficiency of a building air-conditioner using phase change material which melts at 20oC was reported by N. Chaiyat et al. [100]. The result of mathematical model and experimental model was compared and concluded that the electrical consumption of the modified model was reduced to 36.27 kWh/day from 39.36 kW h/day at an operating time 15 h/day. B. M. Diaconu [101] performed a quantitative energy analysis of an air-conditioning system for an office building which works on solar ejector cycle with low temperature thermal energy storage and the advantage of its each configuration was reported. A quantitative energy analysis for assessing the parameters which influence the rated system power, ejector energy efficiency etc. were also presented. 6.2. Food storage D. Leducq et al.[102] studied PCM packaging for ice cream storage and transportation and compared the result with polystyrene packing. The result showed that the PCM packing is more efficient when compared to insulation packing. Similar work was presented by E. Oro et al.[103]. A PCM package for commercial ice cream container was designed and tested. Both mathematical model and experimental results were compared and from that it was concluded that PCM package is beneficial when it is outside the freezer. The chilled food refrigeration cabinet operates in the temperature range of 0o to 5oC was tested with PCM storage by W. Lu et al. [104]. The addition of nucleating agents results in reduced supercooling. 9 Page 9 of 29

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thermal conductivity and expedited unidirectional freezing of cyclohexane-based nanoparticle suspensions utilized as nano-enhanced phase change materials (NePCM). International Journal of Thermal Sciences. 62, 120-126. [64] Parameshwaran, R., Kalaiselvam, S., 2014. Energy conservative air conditioning system using silver nano-based PCM thermal storage for modern buildings. Energy and Buildings. 69, 202-212. [65] Kalaiselvam, S., Parameshwaran, R., Harikrishnan, S., 2012. Analytical and experimental investigations of nanoparticles embedded phase change materials for cooling application in modern buildings. Renewable Energy. 39, 375-387. [66] Harikrishnan, S., Kalaiselvam, S., 2012. Preparation and thermal characteristics of CuO–oleic acid nanofluids as a phase change material. Thermochim Acta. 533, 46–55. [67] Zhang, S., Wu, J, Y., Tse, C, T., 2012. Effective dispersion of multiwall carbon nano tubes in n-Hexadecane through physiochemical modification and decrease of supercooling. Solar Energy Materials & Solar Cell. 96, 124-130. [68] Sih-Li, C., Chin-Lung, C., Chun-Chuh, T., Tzong-Shing, L., Ming-Chun, K., 2000. An experimental investigation of cold storage in an encapsulated thermal storage tank. Experimental Thermal and Fluid Science. 23, 133–144. [69] Antony Aroul Raj, V., Velraj, R., 2010. Review on free cooling of buildings using phase change materials, Renewable and Sustainable Energy Reviews. Renewable and Sustainable Energy Reviews. 14, 2819–2829. [70] Regin, A.F., Solanki, S.C., Saini, J.S., 2008. Heat transfer characteristics of thermal energy storage systems using PCM capsules: a review. Renewable and Sustainable Energy Reviews. 12, 2438–2458. [71] Allouche, Y., Varga, S., Boudena, C., Oliveira, C.A., 2015. Experimental determination of the heat transfer and cold storage characteristics of a microencapsulated phase change material in a horizontal tank. Energy Conversion and Management. 94, 275– 285. [72] Qiu, X., Lu, L., Wang, J., Tang, G., Song, G., 2014. Preparation and characterization of microencapsulated n-octadecane as phase change material with different n-butyl 13 Page 13 of 29

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15 Page 15 of 29

Fig.1. Classification of PCM for cold thermal energy storage [60].

Fig.2. Scheme for preparation of HyNC and HyNPCM [61]

16 Page 16 of 29

Fig.3(a). DSC graphs obtained for the pure PCM and at different mass proportions of HyNC dispersed into the PCM in early cycles [61]

Fig.3(b). DSC graphs obtained for the pure PCM and at different mass proportions of HyNC dispersed into the PCM after 1000 cycles [61]

17 Page 17 of 29

Fig.4. Schematic diagrams of (a) the apparatus of the unidirectional freezing experiment and (b) the arrangement of thermocouples [63]

Fig.5. Photograph of the arrangement of the experimental setup for measuring thermal conductivity of NePCM samples [63]

Fig.6. Pictorial view of silver nanoparticles embedded PCM samples at the (a) commencement of charging cycle (b) near completion of charging cycle (c) intermediate stage of discharging cycle (d) near completion of discharging cycle [64]

18 Page 18 of 29

Fig.7. Sedimentation Photograph of pure oleic acid and oleic acid with different mass fraction of CuO nano particles [66]

Fig.8. DSC measurements of CuO-Oleic acid nanofluids [66]

Fig.9. DSC melting and freezing curves of the pure hexadecane at 5 oC/min scanning rate and the characteristic temperatures Tm, Tm.peak, Tf.peak

Fig.10. DSC curves of the hexadecane with MWCNT of different fractions as the nucleating agent (NA): heating process and (b) cooling process.

(a)

Fig.11. Commercial rectangular macro-encapsulated PCM [74]

19 Page 19 of 29

Fig.12. PCM encapsulated in spheres [57]

Fig.13. PCM tube encapsulation [69]

Fig.14. PCM metal ball encapsulation [69]

Fig.15. PCM encapsulated in aluminium panel [69]

Fig.16. PCM encapsulated in aluminium pouches [69]

20 Page 20 of 29

Fig. 17. SEM micrograph of PCM suspension taken with 5000X magnification [71]

Fig.18. Appearance of the MPCM slurry and SEM microscopic image of the MPCM particles [75]

Fig.19. SEM image of PEG/SiO2 [84]

21 Page 21 of 29

Fig.20. DSC curves of CA, and form-stable CA/EP composite PCM [89]

Fig.21. DSC curves of the form-stable CA/EP composite PCM before and after thermal cycling [89]

Fig.22(a). Carbon Steel specimen immersed in SP21E [91]

Fig.22(b). Copper immersed in Capric acid (73.5%)/Myristic acid (23.5%) eutectic [91]

Fig.22(c). Copper immersed in Capric acid (75.2%)/Palmitic acid (24.8%) eutectic [91]

22 Page 22 of 29

Fig.22(d). Aluminium immersed in SP21E [91]

Fig.23. Schematic of NVP system [95]

Fig.24. Experimental installation [95]

Fig.25. Principal function of PCM “free-cooling system”: (left) cooling of PCM at night, (right) cooling of building during the day [96]

23 Page 23 of 29

Fig.26. Outlet air temperatures at different inlet air temperatures and airflows [96]

Fig.27. Heat flux of the buffer at different inlet air temperatures and airflows [96]

Fig.28. Amount of cold [96]

24 Page 24 of 29

Fig.29. Experimental setup for air–PCM heat exchangers at University of Zaragoza [99]

Table 1- Organic PCMs Material Micro encapsulated tetradecane n-Tetradecane Paraffin C14 Formic acid Polyglycol E400 n-Pentadecane Paraffin C15 Tetrabutyl ammonium bromide Isopropyl Palmitate Isopropyl Stearate Propyl Palmitate Caprylic acid Dimethyl Sulfoxide Paraffin C16 Acetic acid Polyethylene Glycol 600 Glycerin n-Hexadecane n-Heptadecane Butyl Stearate Dimethyl Sabacate Octadecyl 3-mencaptopropylate Paraffin C17 Paraffin C16-C18 Paraffin C13-C24 Ethyl Palmitate Lactic Acid

Melting Point o C 5.2 5.5 5.5 7.8 8 10 10 10-12 11 14-18 16-19 16 16.5 16.7 16.7 17-22 17.9 18 19 19 21 21 21.7 20-22 22-24 23 26

Heat of fusion kJ kg-1 215 215 228 247 99.6 193.9 205 193-199 95-100 140-142 186 150 85.7 237.1 184 198.7 210-236 240 140-200 120-135 143 213 152 189 122 184

Ref. [17,40] [17,40] [16] [18,3] [6,9] [9] [3,16] [22] [9] [9] [22,25] [6,9,16,22] [9,22] [3,16] [9] [6] [22] [9,18] [9] [6,9,19,22,25] [2,6,29] [2] [6] [6,9,22,25] [2,6,29] [18] [6] 25 Page 25 of 29

1-Dodeconol Octadecyl Thioglyate Vinyl Sterate Paraffin C18 n-Octadecane Methyl Sterate

26 26 27-29 28 28-28.1 29

200 90 122 244 245 169

[20] [2] [21] [2] [18] [18]

Table 2- Inorganic PCM Material H2O H2O + Polyacrylamide K2HPO4.6H2O LiClO3.3H2O ZnCl23H20 K2HPO4.6H2O NaOH.(3/2 H2O) NaOH Na2CrO4.10H2O KF.4H2O Na2SO4.10H2O FeBr3.6H2O Mn(NO3)2.6H2O CaCl2.6H2O CaCl2.12H2O LiNO3.2H2O LiNO3.3H2O

Melting Point o C 0 0 4 8 10 13 15-15.4 16 18 18.5 21 21 25.8 29 29.8 30 30

Heat of fusion kJ kg-1 333 295 109 155-253 200 231 198 105 125.8 190.8 174 296 189/296

Ref. [22,40] [22,40] [3] [6,9,22,25] [9] [6,9] [6] [2] [6,9,22] [9] [31,32] [26] [2,6,27,28,29] [6] [2,6] [6]

Table 3- Organic Eutectic PCMs Material Composition 91.67% Tetradecane + 8.33% Hexadecane Hexadecane + Tetradecane (2:3-0:1 by volume) Tetradecane + Docosane Tetradecane + Geneicosane Caprylic acid + Lauric acid (9:1 by mol) Microencapsulated 94% Tetradecane + 6% Tetradeconol 96% Tetradecane + 4% Tetradeconol 94% Tetradecane + 6% Tetradeconol Lauryl alcohol + Caprylic acid (2:3 by quality) Pentadecane + heneicosane HS-1, HS-4, HS-8, HS-9 Caprylic acid + Palmitic acid (2:3 by quality) C14,C15,C16,C17,C18 (33.4:47.3:16.3:2.6:0.4) Dodecanol + Caprylic acid (40.6:59.4 by quality) Pentadecane + Docosane Pentadecane + Octadecane

Melting Point o C 1.7 1.7-5.3 1.5-5.6 3.54-5.56 3.77 5.2 5.5 5.5 6.2 6.23-7.21 6.48-8.14 6.54 7 7 7.6-8.99 8.5-9

Heat of fusion kJ kg-1

Ref.

156.2 148.1-211.5 234.33 200.28 151.5 206.4 206.4 202.1 173.2 128.25 143.2-147 116.5 158.3 178.6 214.83 271.93

[33] [10,11] [33] [33] [12] [17] [17] [17] [41] [18] [41] [41] [13] [41] [15] [15] 26 Page 26 of 29

Hexadecane + Tetradecane Capric acid + Lauric acid (65:35 by mol) with 10% Cineole Capric acid + Lauric acid (65:35 by mol) with 10% Methyl Salicilate Capric acid + Lauric acid (65:35 by mol) with n-Pentadecane (9:1/7:3/5:5 by volume) 90% Capric acid + 10% Lauric acid 38.5% Triethylolethane + 31.5% H2O + 30% Urea Capric acid + Lauric acid (65:35 by mol) with 10% Eugenol 45% Capric acid + 55% Lauric acid 48% Butyl Palmitate + 48% Butyl Sterate + 3% Other Capric acid + Lauric acid 65% mol Capric acid + 35% mol Lauric acid 61.5% mol Capric acid + 38.5% mol Lauric acid 65-90% Methyl Palmitate + 35-10% Methyl Sterate Capric acid + Palmitic acid 34% C14H28O2 + 66% C10H20O2 Octadecane + Docosane Octadecane + heneicosane Capric acid + Stearic acid 50% CH3CONH2 + 50% NH2CONH2

10-14.5 12.3 12.5

147.7-182.7 111.6 126.7

[10,11] [14] [14]

13.3/11.3 /10.2 13.3 13.4-14.4 13.9 17-21 17 18 18-19.5 19.1 22-25.5 22.1 24 25.5-27 25.8-26 26.8 27

142.2/149.2/1 57.8 142.2 160 117.8 143 140 120 140.8-148 132 120 153 147 203.8 173.93 152 163

[14] [9] [9,16,18] [14] [9,15,22] [15] [15] [9,15,22] [15] [6,18] [35] [6] [18] [18] [35,36] [6]

Table 4- Inorganic Eutectic PCMs Material Composition 31% Na2SO4 + 13% NaCl + KCl 16% + 40% H2O 40% Tetra n-butyl ammonium bromide + 2% Borax 76% Na2SO4.H2O 45% Tetra n-butyl ammonium bromide 45% CaCl2.6H20 + 55% CaBr2.6H2O Mn(NO3).6H2O + MgCl2.6H2O 45-52% LiNO3.3H2O + 48-55% Zn(NO3)2.6H2O 45% Ca(NO3)2.6H2O + 55% Zn(NO3)2 66.6% CaCl2.6H2O + 33.3% MgCl2.6H2O 50% CaCl2 + 50% MgCl2 + 6H2O 48% CaCl2 + 4.3% NaCl + 0.4% KCl + 47.3% H2O 47% Ca(NO3)2.4H2O + 53% Mg(NO3)2.6H2O 60% Na(CH3COO).3H2O + 40% CO(NH2)2

Melting Point o C 4 9 9.3 12.5 14.7 15-25 17.2 25 25 25 26.8-29 30 30

Heat of fusion kJ kg-1

Ref.

234 187 114.4 195.5 140 125.9 220 130 127 95 190.8 136 200.5

[6] [41] [41] [41] [16] [18] [22] [2] [2] [2,6,29] [7] [7] [39]

Table 5- Commercial PCMs Name

Type

RT3 RT4 RT5 RT6 MPCM(6)

Paraffin Paraffin Paraffin Paraffin Paraffin

Melting Point o C 3 4 5 6 6

Heat of fusion kJ kg-1 198 182 198 175 157-167 27 Page 27 of 29

ClimSel C7 E17 E19 RT20 Emerest 2325 Emerest 2326 FMC RT21 ClimSel C21 S21 E21 SP22 A17 A22 ClimSel C23 S23 GM ClimSel C24 GR25 A24 SP22A4 ClimSel C24 S25 SP25A8 LatestTM25T RT26 A26 RT27 STL27 S27 LatestTM29T E30 ClimSel C32

Salt solution Salt Hydrate Salt Hydrate Paraffin Fatty acid Fatty acid Paraffin Paraffin Salt Solution Salt Hydrate Salt Hydrate Blend Paraffin Salt Hydrate Salt Hydrate Ceramic + Paraffin Salt Hydrate Granule Paraffin Blend Salt Solution Salt Hydrate Blend Salt Hydrate Paraffin Paraffin Paraffin Salt Hydrate Salt Hydrate Salt Hydrate Salt Hydrate Salt Hydrate

7 17 19 20 20 20 20-23 21 21 21 21 22 22 23 23 23.5-24.9 24 23-25 24 24 24 25 25 24-26 24-28 26 27 27 27 28-29 30 32

130 143 146 140 134 139 130 134 122 170 150 180 145 148 175 41.9 216 145 165 180 180 180 175 131 150 184 213 183 175 201 212

Table 6- Heat storage and release characteristics of the pure PCM and the HyNPCMs [61] Process

Parameter

Heat Commencement storage (min) (freezing) Completion (min) Reduction (%) Heat Commencement release (min) (melting) Completion (min) Reduction (%)

HyNC mass loading in wt (%) 0.1 0.5 0.8 1.0 18.7 17.3 16.7 16.0

1.5 15.0

36.7

34.5

32.9

31.5

29.6

28.0

76.3

5.08 75.0

12.18 73.7

15.23 72.3

18.78 71.7

23.86 69.8

93.3

90.5

88.4

85.3

83.5

81.4

-

1.70

3.41

5.24

6.03

8.52

0 (Pure) 19.7

Table 7- Adopted thermal conductivity (unit: W/mK) from measurements of both liquid and solid cyclohexane-based NePCM samples with various loading of copper oxide nano particles. [63] Mass fraction (wt%) Liquid (10oC) Solid (-5oC) 0 0.1225 0.2689 28 Page 28 of 29

1 2 4

0.1231 0.1238 0.1243

0.2764 0.2865 0.2716

Table 8- Melting and crystallization properties of hexadecane with MWCNT of different factions as the nucleating agent NA (0.0 wt%) NA (0.1 wt%) NA (0.5 wt%) NA (1.0 wt%) NA (2.0 wt%) NA (10.0 wt%)

∆Hm (J g-1) 227.13 196.58 256.28 237.90 244.81 219.60

∆Hf (J g-1) 225.81 196.33 250.59 239.77 244.22 223.03

Tm.peak (oC) 22.67 22.33 22.25 23.83 22.75 22.70

Tf.peak (oC) 14.08 17.42 15.42 13.67 13.83 14.08

∆T (oC) 8.59 4.91 6.83 10.16 8.92 8.67

Table 9- Properties of the MPCM slurry [76] MPCM slurry (mass fraction)

Density (kg m-3)

Specific heat (J kg-1oC-1)

0.1 0.2 0.3 0.4

987 976 933 911

3945 3707 3470 3232

Thermal Conductivity (Wm-1oC-1) 0.575 0.551 0.528 0.505

Latent heat (kJ kg-1) 19.6 39.2 58.8 78.4

Table 10- The changes in thermal properties of the CA/EP composite PCM with respect to thermal cycling number [89] No. of thermal cycling 0 1000 3000 5000

Tm (oC)

∆Hm(J g-1)

Tf (oC)

∆Hf (J g-1)

31.80 31.32 29.16 30.25

98.12 96.98 93.43 95.54

31.61 31.36 31.30 31.52

90.06 94.42 88.39 90.60

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