Defrost Smarter: Compact TES for HVAC Peak Load Reduction

Peak electricity demand-not average energy use-drives grid size and costs. Buildings, particularly HVAC systems during extreme weather, set much of that peak. Full-scale thermal energy storage (TES) can help - but its multi-hour capacity, high cost, complex controls, and space needs limit adoption. TES for defrost, by contrast, requires just 10 minutes of storage per hour, which demands higher power density but enables a smaller footprint, simpler integration, and lower cost - while directly addressing "cold blow," a major consumer complaint during defrost. This paper analyzes two approaches for TES-enhanced defrost for air-sourced heat pumps. Both approaches were developed as laboratory scale systems and experimentally tested to evaluate their performance. The experimental results were subsequently used to calibrate the equipment model and inform building-level simulations. The first is a small TES housed inside the outdoor unit, interfacing with the refrigerant through a TES-refrigerant heat exchanger, which enables the indoor air circulation to be turned off during defrost. The second is a phase change materials (PCM) module integrated in the supply air duct, allowing heat exchange between PCM and supply air, which enables continuous heating to the building during defrost. The paper will quantify projected regional benefits, showing how TES-based defrost can reduce energy use, shave peak demand, and lower grid capacity requirements - delivering value for both consumers and the electricity system - while offering a practical, cost-effective, near-marketready solution poised for immediate impact.