A design methodology for thermal storages in heat pump systems to reduce partial-load losses

The paper deals with the sizing and the arrangement of thermal energy storages in heat pump systems for building service. Partial-load losses are an increasingly important phenomenon in high-efficiency buildings, even for inverter heat pumps, due to the low average thermal demand compared to the installed capacity. The paper discusses partial-load losses through the data collected during a "hardware-in-the-loop" experimental campaign. Then, it presents a straightforward methodology that can be practically used by HVAC designers to easily determine the volume and the best arrangement of thermal energy storages in heat pump systems. Shortly, thermal storage in a series configuration is an effective solution to make the compressor work at its maximum performance, provided that the thermal efficiency of the storage is higher than the partial-load penalization coefficient of the heat pump. The proposed design methodology is applied to a test case, consisting of a high -efficiency dwelling: depending on the heat pump control characteristics, the electricity consumption can be reduced up to 45 % using a thermal storage. For standard values of the degradation coefficient (0.75-0.9), the seasonal energy saving is in the range 10-20% with a tank loss coefficient lower than 5 W/K.