Reducing the Electricity Use and Peak Power Impacts of Defrost Cycles Using Thermal Energy Storage

Heat pumps extract heat from the outdoors using an evaporator temperature below that of the ambient air. If this temperature is also below the air dewpoint and 0 degrees C, frost forms on the evaporator. This frost is periodically removed-often by reversing the refrigeration cycle to draw heat from the indoor heat exchanger and deliver it to the outdoor coil, which are typically 5-10 minutes long. While it effectively melts frost, this process cools the building and often triggers inefficient electric resistance heat to prevent cold air from reaching occupants. This paper investigates the use of thermal energy storage (TES) to mitigate the defrost cycle's negative impacts on occupant comfort, energy use, and peak electric demand. The TES is an air-to-PCM heat exchanger, added to the supply duct of a heat pump. The heat pump charges the TES (melts the PCM) during normal heating and discharges the TES (freezes the PCM) during defrost, heating the air that is cooled during the reverse cycle. We show how well a small TES can meet the energy and power requirements for the short, high demand defrost period. Modeling results indicate that peak electric demand could be reduced by more than 30% and annual energy savings of 3-4%. Experimental results validate the TES models and highlight the challenge of delivering high power over a short duration-particularly for configurations that maintain indoor heating during defrost. This study demonstrates the feasibility of using TES in small-commercial air-source heat pumps to improve defrost performance. Next steps include scaling up the TES prototype and evaluating its performance when integrated with an air-to-air heat pump.