Abstract Latent heat storage is one of the most efficient ways of storing thermal energy

Abstract
Latent heat storage is one of the most efficient ways of storing thermal energy. Unlike the sensible heat storage method, the latent heat storage method provides much higher storage density, with a smaller temperature difference between storing and releasing heat. There are large numbers of phase change materials that melt and solidify at a wide range of temperatures, making them attractive in a number of applications. Many different groups of materials have been investigated during the technical evolution of PCMs, including inorganic systems (salt and salt hydrates), organic compounds such as paraffin’s or fatty acids and polymeric materials. The relationships between the structure and the energy storage properties of a material have been studied to provide an understanding of the heat accumulation/emission mechanism governing the material’s imparted energy storage characteristics. This paper reviews previous work on latent heat storage and provides an insight to recent efforts to develop new classes of phase change materials (PCMs) for use in energy storage. Four aspects have been the focus of this review: PCM materials, encapsulation, effect of capsules geometry, and methods of performance enhancement.

Table of Content
Abstract 1
1 Introduction 1
2 Phase changing materials 1
2.1 Classification of PCMs 2
2.1.1 Organic PMCs 2
2.1.2 Inorganic PCMS 3
3 Encapsulation of PCMs 4
3.1 Encapsulation Techniques 4
3.1.1 Coacervation 4
3.1.2 Suspension polymerization 5
3.1.3 Emulsion polymerization 5
3.1.4 Polycondensation 5
3.1.5 Poly additions 6
4 Effect of capsules geometry 6
5 Methods of performance enhancement 6
5.1 Enhancement using high conductive materials 6
5.2 Enhancement using extended heat transfer surface 7
5.3 Enhancement using intermediate heat transfer medium 7
5.4 Enhancement using heat pipe 7
5.5 Enhancement using multiple PCMs 8
6 Conclusion 9
Reference 10

Introduction
Using phase change materials (PCMs) for thermal energy storage (TES) has become an important aspect for energy management. Now days the limited reserves of fossil fuels and concern over greenhouse gas emissions make the effective utilization of energy a key issue. Using PCM for TES provides an elegant and realistic solution to increase the efficiency of the storage and use of energy in many domestic and industrial sector 1 2 3.
The application of PCM for TES plays an important role when energy demand and supply are not equal. Excess energy available in the off-peak time can be stored in TES for later use. Example solar energy is available only in the sunshine hours, thus the excesses heat can may be stored in the daytime and used later in the night hours. Energy storage helps in saving of expensive fuels and reduces wastage of energy and capital cost which leads to cost effective system 2.
TES devices categorized majorly as sensible heat, latent heat and chemical heat storage devices 4. Although the most commonly used devices in industrial application for thermal energy storage , is he sensible heat storage but the latent heat thermal energy storage devices have attracted a wide range of industrial and domestic application . Latent heat thermal energy storage provide large energy storage density with a smaller temperature change 2 4. However, practical difficulties usually arise in applying the latent heat method due to the low thermal conductivity, density change, stability of properties under extended cycling and sometimes phase segregation and subcooling of the phase change materials 5.
Phase changing materials
Phase changing energy storage materials store heat in their latent heat during constant temperature usually solid-liquid phase change is used. Solid-solid phase change are also used. Although for solid-solid phase change, specific latent is less but it have advantage like no leakage and no need of encapsulation. Liquid-gas phase change has highest latent heat of phase change but enormous change in the volume of storage materials is a problem and hence is not used in general4.
The thermal energy stored by latent heat can be expressed as;
Q=m*L
Where m is mass (Kg), Lis the specific latent heat (Kj?Kg).
Classification of PCMs
Phase changing materials broadly classified as Organic, Inorganic and Eutectics. Even though, their melting/freezing temperature lies in an operating range, many of PCMs do not satisfy the criteria required for adequate thermal energy storage devices because no single material can have all the properties required for TES. Therefore, the available materials are to be used and their thermos physical properties are to be improvised by making suitable changes in system design or by using external agents 2 6.
Organic PMCs
Organic PCMs possess unique quality of having their solid-liquid phase change temperature within or close to human thermal comfort range between 18 oc and 30 oc. Most organic PCMs used for thermal comfort in building, textiles, etc. However, they decomposed at higher temperatures and their thermal conduction is poor 4.
Organic phase change materials like Paraffin and Non-paraffin’s (like Fatty acids, Easters, Alcohol, and Glycols) mainly discussed in Pielichowska and Pielichowski 1, Alva et al. 4, Farid 5, Sharma et al. 6, Sharma R.K. 2.
Paraffin
Paraffin waxes are saturated hydrocarbons mixture and normally consists of mixture of mostly straight chain n- alkynes, CH3-(CH2)n-CH3. Both the melting points and heat of fusion increase with increase in chain length 9. He W. and Wall S. 9 use the commercial product, Rubitherm, based on a cut form refinery production, was investigated as a PCM for storage. The phase change temperature and the heat of fusion were obtained by thermosensor and DSC analysis. The freezing point of the material is 7 oc and the heat of fusion is 158.3Kj?Kg and the result shows Rubitherm RTS qualifies as a PCM for cooling because of its law temperature and reasonably high heat of fusion.
Non-Paraffin
Fatty acid contain a carboxylic (COOH) functional group on the aliphatic chain. They have a general formula R-COOH where R represents alkyl group. They have low cost, super cooling, chemical stability and not undergoing segregation etc. They also have a few drawbacks like odor, low-density low thermal conductivity, large volume change (approximately 10%) during phase change. Saturated fatty acids from Caprylic (n=8, melting point 16 oc) to stearic acid (n=18, melting point 69 oc) are considered for the TES application 4. Unsaturated fatty acids normally have low phase change temperature. Further extended range of phase change temperature can be obtained through fatty acid eutectics 4.
Among organic PCMs, sugar alcohols have highest melting point and highest latent heat 4. Their phase change temperature makes the suitable heat storage media for medium temperature (90 oc – 250 oc) application like solar heater or waste heat recovery 4. Care should be taken during container design to avoid exposure of sugar alcohols PCM to atmosphere oxygen. Therefore, if sugar alcohols are to be used as PCM cycling and chemical stabilities are the major factors to be tested 4. Sole A. et al. 10 studied cycling and chemical stability of Myo-inositol, Galactitol and D- mannitol by DSC and FTIR.
Among glycols, polyethylene glycol (PEG) used for TES. The difference between melting and freezing point can be as high as 30 oc -40 oc, which is not desirable. One major problem with PEG is that it has the highest supercooling among all organic PCMS 4.
Inorganic PCMS
Appropriate inorganic PCMs chosen for a given operating temperature of the system based phase change temperature. In organic PCMs usually operates in high temperature where organic materials would have thermally decomposed 4.
Inorganic phase changing materials like salt, salt eutectics, salt hydrates, metals and alloys mostly have better thermal conductivity and low volume change but they cool down rapidly and are corrosive12347.
Wang et al. 3 develops anew Si and Al based PCMs with higher temperature suitable for the TES system by varying the ratio of Al, Si and the new agent element added.
A. Mohammed 8, Liu et al. 7 comprehensively reviewed some of inorganic PCMs such as salts, salt eutectics, salt hydrates and some of their thermos physical properties are included such as melting temperature, latent heat of fusion, density, specific heat and thermal conductivity.

Encapsulation of PCMs
Encapsulation is a technique used to hold the material in a sealed container of certain volume. The main goal of capsulation to avoid direct contact between the PCM and the environment to prevent the leakage of the PCM when it is in liquid state and to increase the heat transfer area 2.
Encapsulation Techniques
Sharma et al. 2 states that encapsulation can be made in two possible ways, Macro encapsulation and Micro encapsulation.
Macro encapsulation used very commonly because of its availability in various shape and size. It is mainly used to hold the liquid PCMs and to prevent changes in the composition due to contact with environment, it also add mechanical stability to system if the container is sufficiently rigid 2.
Micro encapsulation is a technique in which a large number of PCM particles of 1-1000µm diameter are enclosed in a solid shell and then arranged in continuous matrix 2. Micro encapsulated phase change materials (MEPCMs) are in the form of pouches, tubes, spheres, or other receptacles and used as heat exchanger 11.
For PCM encapsulation MEPCM need an appropriate properties such as desired morphology, proper diameter distribution, thermal stability, shell mechanical strength and penetration abilities 2. A literature survey on MEPCMs indicates that Urea-formaldehyde (UF) resin, Melamine-formaldehyde (MF) resin and Polyurethanes (PU) usually selected as the microcapsule shell materials for the protection of PCM 2.
There are numerous techniques adopted for Micro encapsulations such as coacervation, suspension polymerization, emulsion polymerization poly condensation polymerization and poly addition polymerizations.
Coacervation
Coacervation is an encapsulation technique that involves the use of more than two colloids 1. Hawlader et al. 12 investigated the influence of different parameters on the characterization and performance of MEPCM in terms of the encapsulation efficiency, energy storage and release capacity. They revealed the complex coacervation and spraying drying method could both be used to prepare micro encapsulation of paraffin wax.
Microcapsules of natural coco fatty acid mixture were prepared using coacervation method 1. The microscopic results shows that microcapsules produced by coacervation process attain a spherical shape geometry and FTIR spectra revealed that the chemical stability of the mixture was not effected by micro encapsulation 1.
Suspension polymerization
Suspension polymerization is encapsulation technique in which the PCM as core filled in a polymer shell 2. Pielichowska K. and Pielichowski K. 1 reviewed that the micro encapsulation of various PCM with a polymer shell of polystyrene by employing a suspension free radical polymer process; it was possible to obtain particles where the PCMs comprised almost 50% of the microcapsules mass. A series of MEPCMs by suspension like polymerization with n-octadcane as the core and styrene-1,4-butylne glycol, diacrylate copolymer (PSB), styrene-divinylbenzane copolymer (PSD), styrene-divinylbenzane 1,4- butylene glycol diacrylate copolymer (PSDB) or polydivinylbenzane (PDVB) as the shell.
Emulsion polymerization
PMMA microcapsules containing docosane, n-octacosane and n-eicosane were prepared by emulsion polymerization and thermal properties evaluated and in all three studies their thermal cycle test showed a good chemical stability 2. SEM analyses revealed that the MEPCMS had compact surface and the average capsules diameter of 160µm.
Polycondensation
An in situ Polycondensation process was applied to prepare a series of melamine –formaldehyde (MF) microcapsules with a PCM core 1. The study demonstrate that the shell structure of MEPCMs can be controlled by the formation process using an optimum dropping rate of shell material of 0.5 ml/min and a temperature increase of 2oc/min. DSV revealed that the melting point of the PCM in shell remained virtually unchanged to that of an uncapsulated PCM 1.
Pielichowska K. and Pielichowski K. 1 reviewed in subsequent development of double- shell structured microcapsules with melamine-formaldehyde resin as the coating material of PCM. In addition, comparison test on the single and double shell structured MEPCMs, polycondensation of melamine and formaldehyde for the shell with hexadecane or octadecane as the core, the effect of the polarity of the PCMs and the types and amount of emulsifiers on the properties of micro encapsulated PCMS, etc.
Poly additions
Chen et al. 13 prepared poly urea microcapsules containing PCM using toluene-2, 4 diisocyante (TDI) and ethylenediamene (EDA) as monomer and butyl stearate as the core material.
It was revealed that the Microcapsules synthesized by using polyetheramine had a smoother and more surface and large mean particle size with a narrow size distribution than those using EDA or diethylene triamine. Further, the microcapsules synthesized with a core/shell weight ratio of 70/30 possessed to optimum properties for TES application 2.
Effect of capsules geometry
Capsules geometry can be a significant parameter to improve the thermal performance of the PCM. Regular geometries like square, cylinder, spherical, have been extensively tested. However, irregular shape geometries such as triangular and trapezoidal are scarce 2. Sharma R.K. reviewed investigation on the solidification of binary mixture of various concentration of ammonia –water filled in a trapezoidal cavity. However, this study does not explicitly investigated the effect of trapezoidal cavity on the solidification rate and is not based on solidification /melting of NEPCM and numerically investigated the effect of trapezoidal cavity on solidification of copper- water non fluid.
Methods of performance enhancement
During the discharging process, the energy released by solidification of the PCM transported from the solid-liquid interface through the growing solid layer to the heat exchanger surface. Hence, the heat transfer coefficient dominated by the thermal conductivity of the solid PCM. However, most PCMs usually provide low thermal conductivity around 0.5 WK/m, which results in poor heat transfer between the HTF and the storage material. Therefore, the design of a cost effective phase change thermal storage system requires the development of proper thermal performance enhancement technique.
Enhancement using high conductive materials
The heat transfer within a PCM storage system can be enhanced by composing high thermal conducting material (sensible heat phase) into the PCM (latent heat phase). In the PCM/ceramic compound, the molten PCM retained and immobilized within the micro-porosity defined by the ceramic network by capillary forces and surface tension, which offers the potential of using direct contact heat exchange 7. Petri et al. 14 tested a packed- bed laboratory scale storage unit containing 1.22 kg composite Na2CO3-BaCO3 (melting point of 700°C)/MgO. The composite has been pressed into cylindrical pellets with a diameter of 2 cm and a height of 1.5 cm. The 200 cycling tests demonstrated the good stability of Na2CO3-BaCO3/MgO composite. Gokon et al. 47 experimentally examined the feasibility of using carbonate (Na2CO3–K2CO3–Li2CO3)/MgO composite material as a thermal storage medium to prolong the cooling time in a double-walled reactor.
Enhancement using extended heat transfer surface
Extension of heat transfer surface using either capsules or finned tubes will reduce the distance for heat transport within the PCM thus improving the heat transfer. Flexible capsules, usually plastic, are used for low temperature PCM applications. To encapsulate the PCMs with melting temperature above 200 ?C, the material is generally expensive. If the stiff capsule is used to contain the PCM, the initial PCM volume should not exceed 80% in order to withstand the pressure variation during the melting/solidification cycling 7. The fin material can be graphite foil, aluminum, steel and copper. Aluminum fins are applicable for temperatures up to 330 ?C. It has been proven to have no degradation after 400 h testing with NaNO3 as a PCM 16.
Enhancement using intermediate heat transfer medium
The concept is based on the reflux evaporation–condensation occurring in the intermediate HTF. The whole storage system consists of a PCM storage unit, discharge and charge heat exchangers placed externally of the PCM at the top and the bottom of the storage unit. The charge heat exchanger is immersed in the liquid intermediate HTF. In the charging process, the liquid HTF absorb energy through vaporization and the vapor flows upwards through the transport channels distributed in the PCM. Then the vapor condenses on the channel’s surfaces and the latent heat of vapor is transferred across the walls to the PCM. The liquefied HTF returns to the pool due to gravity. In the discharging process, the hot PCM causes the liquid HTF to evaporate and the vapor transfers energy to the working fluid passing through the top heat exchanger7.
This concept was first successfully demonstrated by Adinberg et al. 17. In the experiment, they used sodium chloride as the PCM and sodium metal as the intermediate heat transfer medium for a storage temperature of 800 ?C. A metal alloy Zinc/Tin (70/30 wt%) and the eutectic mixture of biphenyl and diphenyl oxide was experimentally investigated as the PCM-HTF system for producing high-temperature superheated steam in the temperature range of 350–400oc 18.
Enhancement using heat pipe
It is well known that heat pipes have high effective thermal conductivity. They can be incorporated into phase change thermal storage systems to serve as thermal conduits between the HTF and the PCM. The heat pipes can transfer heat between the HTF and the PCM with evaporation and condensation of the heat pipe working fluid occurring at the ends of the heat pipes 7. Shabgard et al. 19 investigated the impact of the number of heat pipes as well as their orientation relative to the HTF flow direction and the gravity vector in two distinct system configurations.
Enhancement using multiple PCMs
Thermal storage systems employing multiple PCMs of different melting temperatures is another attractive heat transfer enhancement technology. In this type of system, a few modules containing different PCMs and different melting temperatures are connected to each other in series. When employing multiple PCMs to enhance the thermal performance of latent storage systems, it is important to select the appropriate PCMs and relative proportions of the PCMs. The multiple PCMs in shell and tube units should be in the flow direction that the melting temperature decreases in the charging process and increases in the discharging process 7. In the case that the encapsulated PCM is immersed in a large storage tank so that the temperature variation of the HTF can be ignored along the flow direction, in order to extract maximum benefit, the multiple PCMs should be arranged in the radial direction than in the axial direction (flow direction) 19. Wang et al. 20 experimentally investigated the melting process of a cylindrical heat capsule filled with three PCMs with the lowest melting point PCM at the center and other PCMs arranged in the of increasing melting point from the center to the outer. The thermal performance of a same capsule filled with only PCM2 was also evaluated for the purpose of comparison. The results showed that the melting time was 37–42% shorter in the three PCMs capsule than that in the single PCM capsule.

Conclusion
Organic and inorganic compounds are the two most common groups of PCMs. Most organic PCMs are non-corrosive and chemically stable, exhibit little or no subcooling, are compatible with most building materials and have a high latent heat per unit weight and low vapor pressure. Their disadvantages are low thermal conductivity, high changes in volume on phase change and flammability. Inorganic compounds have a high latent heat per unit volume and high thermal conductivity and are non-flammable and low in cost in comparison to organic compounds. However, they are corrosive to most metals and suffer from decomposition and subcooling, which can affect their phase change properties. The applications of inorganic PCMs require the use of nucleating and thickening agents to minimize subcooling and phase segregation. Significant efforts are continuing to discover those agents by commercial companies. Since PCM microencapsulation is a potential solution for the corrosion issue, a standard testing procedure must be established for leakage testing, since most of the currently studied encapsulated PCMs are not tested. In addition, microencapsulation can be used for heat transfer enhancement and to prevent phase separation during the melting process, as a result stability testing is crucial to the final evaluation of the encapsulation process effects. The other basic parameter used to improve the heat transfer is proper selection/design of capsules geometry and size.
Heat transfer enhancement is another research area that requires additional future studies, especially given the poor thermal conductivities of most of the inorganic PCMs. One potential enhancement is dispersion of highly conductive nanoparticles within PCMs. However, there is a lack of studies for inorganic PCMs enhancement using nanoparticles as compared to organic PCMs. Another open research area in heat transfer enhancement is heat pipes, as it was mentioned earlier only few studies were reported using this technique.
In conclusion, latent heat energy storage is very important for the development and efficiency improvement of energy network systems, in general, and especially in solar energy utilization.

?
Reference
1. Kinga Pielichowska, Krzysztof Pielichowski, Phase change materials for thermal energy storage. Progress in Materials Science 65 (2014) 67–123.
2. R.K. Sharma, P. Ganesan, V.V. Tyagi, H.S.C. Metselaar, S.C. Sandaran, Developments in organic solid–liquid phase change materials and their applications in thermal energy storage. Energy Conversion and Management 95 (2015) 193–228.
3. Zhengyun Wang, Hui Wan, Xiaobo Li, Dezhi Wang, Qinyong Zhang, Gang Chen, Zhifeng Ren, Aluminum and silicon based phase change materials for high capacity thermal energy storage. Applied Thermal Engineering 89 (2015) 204–208.
4. Guruprasad Alva, Yaxue Lin, Guiyin Fang, An overview of thermal energy storage systems. Energy 144 (2018) 341–378.
5. Mohammed M. Farid, Amar M. Khudhair, Siddique Ali K. Razack, Said Al-Hallaj, A review on phase change energy storage: materials and applications. Energy Conversion and Management 45 (2004) 1597–1615.
6. Atul Sharma, V.V. Tyagi, C.R. Chen, D. Buddhi, Review on thermal energy storage with phase change materials and applications. Renewable and Sustainable Energy Reviews 13 (2009) 318–345.
7. Ming Liu, Wasim Saman, Frank Bruno, Review on storage materials and thermal performance enhancement techniques for high temperature phase change thermal storage systems. Renewable and Sustainable Energy Reviews 16 (2012) 2118–2132.
8. Shamseldin A. Mohamed, Fahad A. Al-Sulaimana, Nasiru I. Ibrahim, Md. Hasan Zahir, Amir Al-Ahmed, R. Saidur, B.S. Y?lba?, A.Z. Sahin, A review on current status and challenges of inorganic phase change materials for thermal energy storage systems. Renewable and Sustainable Energy Reviews 70 (2017) 1072–1089
9. Bo He, Fredrik Setterwall, Technical grade paraffin waxes as phase change materials for cool thermal storage and cool storage systems capital cost estimation. Energy Conversion and Management 43 (2002) 1709–1723.
10. Aran Solé, Hannah Neumann, Sophia Niedermaier, Ingrid Martorell, Peter Schossig, Luisa F. Cabeza, Stability of sugar alcohols as PCM for thermal energy storage. Solar Energy Materials & Solar Cells 126 (2014) 125–134.
11. Cemil Alkan,, Ahmet Sari,, Ali Karaipekli, Orhan Uzun, Preparation, characterization, and thermal properties of microencapsulated phase change material for thermal energy storage. Solar Energy Materials & Solar Cells 93 (2009) 143–147.
12. Hawlader MNA, Uddin MS, Khin MM. Microencapsulated PCM thermal-energy storage system. Applied Energy2003; 74:195–202.
13. Chen L, Xu L, Shang H, Zhang Z. Microencapsulation of butyl stearate as a phase change material by interfacial polycondensation in a polyurea system. Energy Conversation Management 2009; 50:723–9.
14. Petri RJ, Ong ET, Olszewski M. High temperature composite thermal storage systems. In: 19th Intersociety energy conversion engineering conference. 1984.
15. Gokon N, Nakano D, Inuta S, Kodama T. High-temperature carbonate/MgO composite materials asthermal storage media for double-walled solar reformer tubes. Solar Energy 2008; 82:1145–53.
16. Steinmann W-D, Laing D, Tamme R. Development of PCM storage for process heat and power generation. Journal of Solar Energy Engineering 2009; 131:041004–9.
17. Adinberg R, Yogev A, Kaftori D. High temperature thermal energy storage an experimental study. J Physics IV France 1999; 9:PR3
18. Adinberg R, Zvegilsky D, Epstein M. Heat transfer efficient thermal energy storage for steam generation. Energy Conversion and Management 2010; 51:9–15.
19. Shabgard H, Bergman TL, Sharifi N, Faghri A. High temperature latent heat thermal energy storage using heat pipes. International Journal of Heat and Mass Transfer 2010; 53:2979–88.
20. Wang J, Ouyang Y, Chen G. Experimental study on charging processes of a cylindrical heat storage capsule employing multiple-phase-change materials. International Journal of Energy Research 2001; 25:439–47.