Overview of PCMAphase-change material(PCM) is a substance with a highheat of fusionwhich, while melting and solidifying at a certain temperature, is capable of storing and releasing large amounts of energy. PCMs latent heat storage can be achieved through solid solid, solidliquid, solidgas and liquidgas phase change. However, the only phase change used for PCMs is the solidliquid change. Thermal Energy Storage through Phase Change material has been used for wide applications in the field of air conditioning and refrigeration especially at industrial scale.

Even though the thermal conductivity of phase change materials (PCM) is usually not high, it is sufficient to enhance the global heat transfer conditions of an evaporator with air as external fluid and natural convection as heat transfer mechanism.

Heat is absorbed or released when the material changes from solid to liquid and vice versa; thus, PCMs are classified as latent heatstorage (LHS) units. Initially, the solidliquid PCMs behave likesensible heatstorage (SHS) materials; their temperature rises as they absorb heat. Unlike conventional SHS, however, when PCMs reach the temperature at which they change phase (their melting temperature) they absorb large amounts of heat at an almost constant temperature. The PCM continues to absorb heat without a significant rise in temperature until all the material is transformed to the liquid phase. When the ambient temperature around a liquid material falls, the PCM solidifies, releasing its stored latent heat. A large number of PCMs are available in any required temperature range from 5 up to 190C.Within the human comfort range between 2030C, some PCMs are very effective e.g. Water, masonry or rock.

3.1 Types of phase change materialsA. Organic PCMsParaffin (CnH2n+2) andfatty acids(CH3 (CH2)2nCOOH) Advantages1. Freeze without much super cooling2. Ability to melt congruently3. Self-nucleating properties4. Compatibility with conventional material of construction5. No segregation6. Chemically stable7. High heat of fusion8. Safe and non-reactive9. Recyclable Disadvantages1. Low thermal conductivity in their solid state. High heat transfer rates are required during the freezing cycle2. Volumetric latent heat storage capacity is low3. Flammable. This can be partially alleviated by specialist containment4. To obtain reliable phase change points, most manufacturers use technical grade paraffins which are essentially paraffin mixture(s) and are completely refined of oil, resulting in high costs

B. Inorganic PCMsSalt hydrates (MnH2O) Advantages1. High volumetric latent heat storage capacity2. Availability and low cost3. Sharp melting point4. High thermal conductivity5. High heat of fusion6. Non-flammable Disadvantages1. Change of volume is very high2. Super cooling is major problem in solidliquid transition3. Nucleating agents are needed and they often become inoperative after repeated cycling

C. Eutectic PCMsOrganic-organic, organic-inorganic, inorganic-inorganic compounds Advantages1. Eutectics have sharp melting point similar to pure substance2. Volumetric storage density is slightly above organic compounds Disadvantages1. Only limited data is available on thermo-physical properties as the use of these materials are relatively new to thermal storage application

D. Hygroscopic materialsMany natural building materials are hygroscopic, that is they can absorb (water condenses) and release water (water evaporates). The process is thus: Condensation (gas to liquid) H0; enthalpy increases (endothermic process) absorbs heat (or cools).Whilst this process liberates a small quantity of energy, large surfaces area allows significant (12C) heating or cooling in buildings. The corresponding materials are wool insulation, earth/clay render finishes, etc.

E. Thermal Composites:Thermal-composite is a term given to combinations of phase change materials (PCMs) and other (usually solid) structures. A simple example is a copper-mesh immersed in a paraffin-wax. The copper-mesh within parraffin-wax can be considered a composite material, dubbed a thermal-composite. Such hybrid materials are created to achieve specific overall or bulk properties.Thermal conductivity is a common property which is targeted for maximisation by creating thermal composites. In this case the basic idea is to increase thermal conductivity by adding a highly conducting solid (such as the copper-mesh) into the relatively low conducting PCM thus increasing overall or bulk (thermal) conductivity. If the PCM is required to flow, the solid must be porous, such as a mesh.Solid composites such as fibre-glass or Kevlar-reinforced for the aerospace industry usually refer to a fibre (the Kevlar or the glass) and a matrix (the glue which solidifies to hold fibres and provide compressive strength). A thermal composite is not so clearly defined, but could similarly refer to a matrix (solid) and the PCM which is of course usually liquid and/or solid depending on conditions. They are also meant to discover minor elements in the earth.

3.2 Applicationsof phase change materialsThe applications of PCMs range over a wide variety, but are not limited to: Thermal energy storage Conditioning of buildings, such as 'ice-storage' Cooling of heat and electrical engines Cooling: food, beverages, coffee, wine, milk products, green houses Medical applications: transportation of blood, operating tables, hot-cold therapies Human body cooling under bulky clothing or costumes. Waste heatrecovery Off-peakpower utilization: Heating hot water and Cooling Heat pumpsystems Passive storage inbioclimatic building/architecture (HDPE, paraffin) Smoothingexothermictemperature peaks in chemical reactions Solar powerplants Spacecraftthermal systems Thermal comfortin vehicles Thermal protectionof electronic devices Thermal protection of food: transport, hotel trade, ice-cream, etc. Textiles used in clothing Computer cooling Turbine Inlet Chilling with thermal energy storage

3.3 PCM Selection Criteria:PCM should possess following properties: Thermodynamic properties: 1. Melting temperature in the desiredoperating temperaturerange2. High latent heat of fusion per unit volume3. High specific heat, high density and high thermal conductivity4. Small volume changes on phase transformation and small vapour pressure at operating temperatures to reduce the containment problem5. Congruent melting Kinetic properties1. High nucleation rate to avoid super cooling of the liquid phase2. High rate of crystal growth, so that the system can meet demands of heat recovery from the storage system Chemical properties1. Chemical stability2. Complete reversible freeze/melt cycle3. No degradation after a large number of freeze/melt cycle4. Non-corrosiveness, non-toxic, non-flammable and non-explosive materials Economic properties1. Low cost2. AvailabilityPertaining to the above mentioned properties and the ease of availability, ethylene glycol was chosen as the Phase change material. The specifications of ethylene glycol are as mentioned:

Antifreeze:

Due to its low freezing point ethylene glycol resists freezing. Ethylene glycol disrupts hydrogen bonding when dissolved in water. Pure ethylene glycol freezes at about 12 C (10.4F), but when mixed with water molecules forms a solid crystal structure, and therefore the freezing point of the mixture is depressed significantly. The minimum freezing point is observed when the ethylene glycol present in water is about 70%, as shown below.

Table 3.1: Ethylene Glycol Freezing Point Vs Concentration in WaterSr. No.Water Wt %Freezing Point(C)Sr. No.Water Wt. %Freezing Point(C)

100760-48

210-4870-51

320-7980-45

430-151090-29

540-2311100-12

650-34

However, the boiling point for aqueous ethylene glycol increases monotonically with increasing ethylene glycol percentage. Thus, the use of ethylene glycol not only depresses the freezing point, but also elevates the boiling point such that the operating range for the heat transfer fluid is broadened on both ends of the temperature scale. The increase in boiling temperature is due to pure ethylene glycol having a much higher boiling point and lower vapour pressure than pure water.

Physical PropertiesEthylene glycol is a clear, colourless, odourless liquid with a sweet taste. It is hygroscopic and completely miscible with many polar solvents such as water, alcohols, glycol ethers, and acetone. Its solubility is low however, in non-polar solvents, such as benzene, toluene, dichloromethane, and chloroform. Following are some of the physical properties of ethylene glycol as shown in table.

Table 3. 2: Ethylene Glycol Boiling Point Vs Concentration of WaterSr. No.Water Wt. %Boiling Point(C)Sr. No.Water Wt. %Boiling Point(C)

10100760110

210102870116

320102980124

4301041090140

54010411100197

650107

Ethylene glycol is difficult to crystallize; when cooled, it forms a highly viscous, super cooled mass that finally solidifies to produce a glass like substance. The widespread use of ethylene glycol as antifreeze is based on its ability to lower the freezing point when mixed in water. The physical properties of ethylene glycol- water mixtures are therefore, extremely important in this regard.3.4 Effect of PCM thickness on the melting and solidificationFour PCM thicknesses were considered for the model. The PCM total storage capacity varied between 138 kJ for a 2 mm slab and 345 kJ for a 5 mm slab. The heat load and cooling capacity used to predict the PCM melting and freezing times with different thicknesses correspond to an ambient temperature of 25C and the model results are shown in Figs. 3.1 and 3.2 respectively.

Fig. 3.1 - PCM Thk (mm) v/s Solidification Time (min)1. Maximum 45min required to convert the PCM 5mm thickness material from liquid state to solid state.2. 30min required to convert PCM 4mm thickness material from liquid state to solid state.3. 21min required to convert PCM 3mm thickness material from liquid state to solid state.4. 12min required to convert PCM 2mm thickness material from liquid state to solid state.5. 7 min required to convert PCM 1mm thickness material from liquid state to solid state.

Fig. 3.2 - PCM Thk (mm) v/s Melting Time (mm)

1. Maximum 390min required to convert the PCM 5mm thickness material from liquid state to solid state.2. 280min required to convert PCM 4mm thickness material from liquid state to solid state.3. 180min required to convert PCM 3mm thickness material from liquid state to solid state.4. 85min required to convert PCM 2mm thickness material from liquid state to solid state.5. 40min required to convert PCM 1mm thickness material from liquid state to solid state.The ratio of melting time and solidification time is 8.67 which significantly show the melting process is much slower than solidification i.e. almost 9 times slower than solidification directly contribute it for getting cooling effect more than 7 hours.814