Thermal energy storage based (TES-based) reverse cycle defrosting control strategy optimization for a cascade air source heat pump

Abstract

Thermal energy storage based (TES-based) reverse cycle defrosting method is a feasible way to reduce energy requirements for defrosting of cascade air source heat pumps (CASHPs). The energy stored in the phase change material based heat exchanger (PCM-HE) is the heat source for both the high stage cycle (HSC) and low stage cycle (LSC) during the TES-based reverse cycle defrosting process, thus the heat provided to the HSC and LSC are coupled, affecting the defrosting performance in LSC and heating performance in HSC. To shorten the defrosting time and maintain the indoor thermal environment, the heat distribution between HSC and LSC during TES-based reverse cycle defrosting was explored. It was found that the heat distribution pattern was mainly influenced by the refrigerant flowrate. Therefore, controlling the refrigerant flowrate of HSC and LSC during the TES-based reverse cycle defrosting is a possible way to regulate the heat distribution between HSC and LSC. In this paper, an experimental investigation of the control strategy for TES-based reverse cycle defrosting of CASHPs is presented. The defrosting performance and indoor heating performance during TES-based defrost operation with LSC compressor frequencies of 60 Hz, 75 Hz, 90 Hz, and 105 Hz were tested. Then HSC EEV openings of 80%, 90% and 100% at the optimized LSC compressor frequency of 90 Hz were considered. It was concluded that the optimal TES-based reverse cycle defrosting strategy is to maintain the LSC compressor frequency at 90 Hz and the HSC EEV opening at 80% for the equipment considered in this study. The defrosting time was 440 s and the average indoor heating capacity was 5.23 kW. The heat provided to the HSC and LSC accounted for 38.7% and 61.3% of the total heat released by PCM-HE, respectively.