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A Review on Augmentation in Thermal Performance of Solar Water Heater using Phase Change Material

The nature of solar energy is intermittent due its variable magnitude in daytime and zero throughout the night or during overcast conditions, which limiting its use in hospitals and in various industrial and domestic applications particularly in water heating. Because of inconsistent nature of solar energy some type of thermal energy storage (TES) system is essential for its effectual utilization and to meet the energy demand at all the times. Phase change material (PCM) is a very good option for latent heat TES because of its high thermal storage density and isothermal behaviour at the time of charging and discharging. The PCM absorb 5 to 14 times more heat as comparison to sensible heat storge material such as water, stone, bricks or rocks etc. The PCM with nano-sized-particle can enhanced the performance of LHTS systems. In this review, ongoing research and development on the performance of solar water heater (SWH) incorporating with PCM’s and nano-embedded PCM is summarized and to provide viewpoint for future research and development.


Introduction
Solar energy is most promptly available, free, environmentally clean and ultimate auspicious alternate source of energy available since ancient times. The importance of solar energy and its magnitude are very well known to everyone. The available value of its magnitude is dependent on time and also dependent on the meteorological states of the location of that place. In the past two decades a wide variety of solar energy-based water heating innovations have been developed by the researchers through  The nature of solar energy is intermittent due its variable magnitude in daytime and zero throughout the night or during overcast conditions, which limiting its use in hospitals and in various industrial and domestic applications particularly in water heating. In solar water heating the energy source and demand commonly mismatch each other especially in domestic applications. The peak demand of heated water is early in the morning and also in late evening while the peak solar radiation come about noon. So, because of inconsistent nature of solar energy some type of TES system is essential for its effectual utilization and to meet energy demand at all the times. TES system provides a thermal energy reservoir to meet the energy demands consistently in day and night and reduces the gap between energy source and demand in domestic applications. The application of solar water heaters is mainly depending on efficient thermal storage unit of the system. The TES systems are sensible heat thermal storage system and latent heat thermal storage system. Because of non-isothermal behaviour in charging and discharging of energy and also small thermal storage capacity per unit volume sensible heat thermal storage system are not too much in use. Latent heat thermal storage system has high thermal storage density with solid-liquid phase change and they can store thermal energy at a constant temperature with respect to the phase change temperature of TES substance. [29][30][31] The materials which can melt and solidify at particular temperature and also capable for storing and releasing huge amount of heat consistently by undergoing a phase transition are called phase change materials. [3,14,16,[23][24][25][26] PCM is a very good option for LHTS because of its high TES density and isothermal behaviour during charging and discharging. The PCM should possess the following properties: 1. High latent heat of fusion and thermal conductivity per unit volume 2. Should have small vapour pressure. 3. Should have marginal change in volume during phase transformation. 4. Should be chemically stable. 5. Should exhibit very small or no-subcooling. 6. Minimum variation in density during phase transformation. 7. They should be non-corrosive, nontoxic, non-inflammable and also non explosive. 8. Should have high heat transfer coefficients.
[32-37] The PCM absorb 5 to 14 times more heat as comparison to sensible heat storge material such as water, stone, bricks or rocks etc. the PCM absorbs and releases energy almost at constant temperature. They can store large amount of energy with a small change in temperature. The latent heat thermal storage (LHTS) tank is shown in fig.4. The latent heat is more or less 100 times higher

Methodology and Experimental Investigations
The thermal performance of a SWH generally depends on absorption and conduction of solar radiation, transmittance and thermal conductivity of working fluid. [1] Experimentally studied the effect of PCM in a LHTS tank. They use a layer of encapsuled PCM at the bottom of a storage tank to store large amount of heat at the time of sunshine. They used paraffin wax as a PCM. The PCM filled metallic capsules form a thick layer of 4 cm parallel to the absorbing plate at the bottom of the tank. The metallic capsules of PCM are used because at the time of phase change from solid to liquid by PCM the melted layer of PCM would not be parallel to the absorbing plate and also necessitated because of inclination of the absorbing system. The PCM store energy in the form of latent heat by changing its phase from solid to liquid. At the time of early morning, evening, night and in overcast period the hot water is drawn out from the tank, which is substituted by cold water. The cold-water gains energy from the PCM. The PCM release energy by changing its phase from liquid to solid. The solar water heaters with PCM at the bottom of the tank meet the requirement of heated water during off-sunshine hours. [2] used paraffin as a PCM in encapsulated spherical capsules with melting temperature 60 ± 1 o C and water as working fluid. The LHTS system remarkably reduces the size of TES tank as compared to ordinary TES system. As the time increases during charging of PCM the efficiency and cumulative heat stored (a) (b) Fig. 6 Variation in Cumulative heat storage and efficiency Vs. Time at ɛ = 0.49 [2] of LHTS system decrease and increases respectively at different mass flow rate of working fluid as shown in fig. 6. As time increases the temperature of the working fluid in the storage tank increases Table1: [2,4,17] Fig. 7. Experimental set-up used by Nallusamy et al. [10] losses from the tank increases, hence efficiency of SWH decreases. The different material for encapsulation of PCM is shown in fig.3. [3] the charging of PCM during initial period of time is high and decreases as time increases because the difference in temperature of water and storage tank also decreases with time. [7] experimentally studied that if a PCM module at the top of the storage tank is added than it would give higher storage density. [8] The thermal storage system consists of two heat absorbing units. From which one is SWH which supplies hot water during sun-shine and other one is PCM thermal storage unit which supplies hot water during early morning, evening and in night time.
They use paraffin as a PCM in small aluminium cylinders for thermal storage. During charging process gradual increase in the temperature of PCM is followed by the isothermal melting. The charging time of PCM can be reduced increasing the area of the absorbing plate. The LHTS system are reliable and viable option for storage of solar energy. [9,10] PCM based SWH come up with 23% more cumulative and life cycle savings as comparison to ordinary SWH. The experimental set-up is shown in fig.7. [12] Experimentally investigate the performance of shell and tube TES system with paraffin wax as PCM.
The energy efficiency increases from 63.88% to 77.41% and exergy efficiency decreases from 9.58% to 6.02% with the increase flow rate of working fluid from 0.033 kg/min to 0.167 kg/min. [13] Experimentally investigate that the cooling rate of water reduces due to PCM and remarkably increase the utilization of solar energy. The efficiency and thermal storage capacity of SWH increases from 31.25% to 44.63% and 3260.4 kJ to 4656.5 kJ respectively. [15] Experimentally studied that the efficiency of SWH increase and cooling rate of water decreases with application of PCM. Due to increase in the thermal storage capacity the size of storage tank reduces. [18]  Experimentally studied the performance of SWH with paraffin wax as a PCM with and without nanofiller. They used aluminium oxide as a nanofiller in the paraffin wax. The hot water temperature is higher with nanofiller as compare to without nanofiller. Aluminium nanofiber also increases the thermal conductivity of the PCM. [19] Studied thermal performance of SWH with PCM and PCM nanocomposite. They used paraffin wax as a PCM and paraffin wax nano composite with Sci and CuO as a nano-sized-particle. The energy efficiency of the system with PCM and PCM nano-composite are 38.3% and 41.7% respectively. The exergy efficiency of the system with PCM and PCM nano composite are 2.18% and 3.23% respectively. The thermal conductivity of paraffin wax nano composite remarkably increased to 22.53% as comparison to base paraffin wax.
[20] Experimentally investigated thermal performance of SWH with a new design of shell and finned tube LHTS system which is shown in fig 8. They used paraffin wax as a PCM. The experiment is carried out by using thermal storage material as water, paraffin wax as a PCM and both paraffin wax and water. They achieved highest Fig. 8. Shell and Finned type thermal storage system [20] daily efficiency as 65% with PCM and water as a thermal storage material. The SWH is capable to provide hot water at all the times ranging from 50 to 60.4 o C. [21] Experimentally studied the effect of nano-embedded PCM on the performance of SWH. They used CeO2 as a nano-sized-particles with mass fraction of 0.5%, 1.0% and 2.0% in paraffin. The thermal conductivity of nano-embedded PCM increased to 108.89% with 2.0% mass fraction of CeO2 as comparison to paraffin. The nano-embedded PCM with mass fraction of CeO2 1.0% shows excellent thermal properties as compare to 0.5% and 2.0% mass fraction. They achieved highest daily efficiency and exergy efficiency as 79.2% and 5.1 % respectively for nano-embedded PCM with 1.0% mass fraction of CeO2.