Capacity control of a cascade multi-purpose heat pump using variable speed compressor

Cascade multi-purpose heat pumps are designed to concurrently meet the hot water (HW) and cooling needs, and the HW and heating demands of buildings. However, the capacity of heat pumps deteriorates as the building load increases. At such conditions, the capacity of the heat pump needs to be enhanced to ensure that its rated value is maintained. This study examines the adoption of a variable speed compressor in the low-stage (LS) of a water source cascade multi-purpose heat pump for capacity control at different outdoor conditions in many operating modes. The study showed that the variable speed compressor is highly reliable for the capacity control of the cascade multi-purpose heat pump in all operating modes at varying outdoor temperature conditions. However, the variable speed compressor to be adopted for the capacity control of the cascade multi-purpose heat pump should have a higher capacity and wider range of compressor speeds to meet the rated capacity of the cascade multi-purpose heat pump at severe outdoor temperature conditions in HW mode and heating-HW mode. Furthermore, the variable speed compressor should be installed in the low stage cycle and hot water cycle to ensure precise capacity control, improved system efficiency and energy savings.