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 ca-pacity 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 cas-cade 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) con-ditions. Three capacity control strategies were employed in this study namely: low tem-perature (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 us-ing 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 com-pressors at the LT side for efficient and cost effective capacity control at varying building loads. (c) 2022 The Author(s). Published by Elsevier B.V. on behalf of African Institute of Mathematical Sciences / Next Einstein Initiative. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )

Automated summary

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 the heat pump at varying indoor and outdoor water temperatures. The results show that the COP of the heat pump is higher in the simultaneous control of the LT and HT compressor speeds and HT compressor speed control compared to LT compressor speed control. However, the control strategies have no significant difference in COP at varying indoor water temperatures. The study recommends using variable speed compressors at the high-temperature side and constant speed compressors at the low-temperature side for efficient and cost-effective capacity control of cascade heat pumps.