A combined thermal energy storage system for domestic applications

Abstract

Four storage systems are tested and compared experimentally during charging and discharging cycles using three flow-rates ( 4 mls-1 , 8 mls- 1 and 12 mls- 1). The four storage systems are a packed bed latent heat storage system of adipic acid, a sensible heat packed bed of granite pebbles with an average diameter of 31.9 mm and two combined storage systems. The first combined system is composed of adipic acid PCM capsules at the top of the storage with granite pebbles at the bottom in a 1:1 ratio (i.e., 50 % PCM capsules at the top and 50 % granite pebbles at the bottom). The second combined system is composed of granite pebbles at the top and adipic acid PCM capsules at the bottom in a 1:1 ratio (i .e., 50 % granite pebbles at the top and 50 % PCM capsules at the bottom). The combined storage system with PCM at the top shows generally better energy and exergy charging rates for all flow-rates considered. The sensible heat storage system shows the worst thermal performance in terms of the energy and exergy charging rates. For discharging cycles with different flow-rates, the worst thermal performance is seen with the sensible heat storage system which shows lower discharging energy and exergy rates. The latent heat storage system shows the best thermal performance, however, its performance deteriorates as the discharging flow-rate increases. The thermal performance of the combined system with PCM at the bottom improves with an increase in the flow-rate whereas the combined system with PCM at the top shows a slight deterioration in its performance. The thermal performance during discharging for the combined system with PCM at the top is comparable to the performance of the latent heat storage system. The packed bed latent storage shows the highest energy and exergy efficiencies with flowrates of 4 mls- 1 and 12 mls- 1 . However, the results suggest the best flow-rate of 8 mls- 1 for the combined system with PCM at the top, where the energy and exergy efficiencies of the combined system with PCM at the top is greater than for the other storage systems. The worst performance in terms of the energy and exergy storage efficiencies is seen with with the sensible heat packed bed of granite pebbles. The combined system with PCM at the top shows the best overall thermal performance and its cost will be less than that of a pure latent heat storage sytem, thus it is a viable option for domestic applications.