Cold energy storage in a packed bed of novel graphite/PCM composite spheres

Abstract

The low thermal conductivity of phase change materials (PCMs) are hindering wider use. To enhance the thermal conductivity of PCM (water), a novel approach of encapsulated water in a high thermal conductivity graphite sphere was investigated. For a single graphite/water composite sphere, the results showed that the effective thermal conductivity of the PCM (water) is increased by 12 times, and hence the freezing and melting times of the PCM is reduced by 53.7% and 44.4% respectively, compared to a non-graphite sphere filled with the same amount of water. In addition, a graphite/water composite sphere exhibits minimum super-cooling, indicating that the graphite matrix material plays a good role as a nucleating agent for water freezing. The thermal behaviour of a packed bed filled with 50 graphite/water composite spheres was tested. The influence of heat transfer fluid (HTF) inlet temperature and volumetric flow rate on the thermal behaviour of the packed bed latent heat storage system are experimentally investigated. The results showed that both had a significant effect on freezing time. The freezing time at HTF inlet temperature of −3.5 °C and volumetric flow rate of 1000 mL/min is 5.3 h. However, it is decreased by 45% and 65% when the HTF inlet temperature is decreased from −3.5 °C to −5.5 and −7.5 °C, respectively. It is observed that by adjusting the HTF inlet temperature and volumetric flow rate during the discharge process, the required output temperature and average melting rate can be achieved in practical applications. The developed ice storage system can be easily adapted to a variety of cooling applications.