Efficient Control of Cascade Heat Pumps Using Variable Speed Compressors

Building load varies continuously throughout the year, causing heat pumps to operate at part load. At such situations, the capacity of the heat pump needs to be controlled to meet the building load. The use of variable speed compressors to control the capacity of heat pumps is found to be the most energy-efficient. However, the efficient control of the capacity of the cascade heat pump using variable speed compressors is lacking in the open literature. This study investigates an energy-efficient capacity control method for a cascade heat pump by adopting variable speed compressors to match the rated capacity of a cascade heat pump at varying indoor and outdoor entering water temperature (EWT) conditions. Three capacity control strategies were employed in this study namely: low temperature (LT) compressor speed control, high temperature (HT) compressor speed control, and simultaneous LT and HT compressor speed control. COP of the cascade heat pump was almost similar in simultaneous control of the LT and HT compressor speeds and HT compressor speed control but significantly higher in simultaneous LT and HT compressor speed control than LT compressor speed control at all outdoor EWT conditions. However, the cascade heat pump had no significant difference in its COP at varying indoor EWT using all three capacity control strategies. It is recommended that water-to-water cascade heat pumps adopt variable speed compressors at the HT side and constant speed compressors at the LT side for efficient and cost effective capacity control at varying building loads.