Investigating the performance of a thermal energy storage unit with paraffin as phase change material, targeting buildings’ cooling needs: an experimental approach

For the reduction of the electricity cost for cooling buildings in hot climate zones, thermal energy storage at low temperature using phase change materials is a potential solution. The storage medium can be cooled during periods of low electricity cost and then absorb the cooling load any time of the day. In this study, an experimental cold thermal energy storage system using organic phase change materials (A9 and A14) with melting temperature of 9 or 14 °C is investigated. The system is designed for real application conditions coupled with a heat pump which cools down the organic material through the circulation of water in the pipes and fan coil units which dissipate their thermal load in the initially cooled storage medium. It comprises of a thermal energy storage tank, inside which is positioned a staggered heat exchanger with finned tubes immersed in phase change material. The results from the experiments yielded shorter process duration (for both melting and solidification) and higher heat transfer rate when the feed rate was higher. Moreover, the temperature of the water entering and exiting the tank is more than 5 °C for A9 organic material for more than half the capacity of the tank thus, using A9, the fan coil units can operate efficiently for a long time. The heat transfer rate exceeds 5 kW for 32 and 24 min using A9 and A14 respectively and the thermal load that the tank can absorb from the fan coils is above 6 kWh per tank. During the heat pump operation, A9 results to a more constant water outlet temperature over the time compared to A14 which in addition is not affected significantly by the water flow rate. Moreover, the staggered heat exchanger transfers less than 10 kW of heat when the water outlet temperature ranges between 5 and 10°C, a power value which can easily be provided by a typical commercial heat pump.