Thermal Energy Storage for Improved Defrosting in a Cascade Heat Pump

Thermal energy storage (TES) decouples the time at which thermal energy is generated from the time at which it is used. TES has been studied for decades and is widely applied in domestic hot-water systems and large heating and cooling plants. More recent work has examined TES coupled with split residential and packaged commercial systems for electric load shifting, but these concepts have not yet seen broad market adoption because of limited financial benefit, high cost, and substantial volume and weight. This paper investigates a different TES application: maintaining continuous space heating during heat-pump defrost cycles without relying on a backup heater. The concept integrates TES into the intermediate heat exchanger of a cascade heat pump. During normal operation, the bottom cycle supplies heat to the evaporating refrigerant in the top cycle while charging, or melting, a phase change material (PCM). When defrost is required, the bottom cycle reverses. Both cycles then draw heat from the PCM as it freezes: one continues delivering heat to the building, while the other delivers heat to the outdoor coil to remove frost. The model predicts nearly 30% lower electric energy use, driven by the more efficient and shorter defrost events. This results in an effective COP of 1.71 vs. 1.21 for the baseline. These results show how a low-cost, low-weight TES system can maintain space heating while offsetting a high-cost heating source: the electric resistance element typically used during defrost. We also describe plans for a future cascade refrigerant-to-refrigerant heat exchanger integrated with a high-conductivity PCM.