Performance characteristics of a cascade multi-functional heat pump in the winter season

Cascade multi-functional heat pumps are becoming popular due to their ability to simultaneously provide cooling, heating, and hot water. In this study, the performance characteristics of a water-to-water cascade multi-functional heat pump designed to provide heating, hot water, and simultaneous heating and hot water during the winter season are analyzed according to variation in secondary fluid temperature. The simultaneous heating and hot water cycle was optimized to meet the design heating and hot water capacities by increasing the compressor speeds of the low stage and high stage cycles above those in the heating and hot water cycles. An increase in the outdoor heat exchanger entering water temperature increased the performance of the cascade multi-functional heat pump in all operating modes, while an increase in the indoor heat exchanger entering water temperature resulted in a decrease in the performance of the cascade multi-functional heat pump in all operating modes except the capacity of the hot water cycle in heating-hot water mode. Furthermore, an increase in the hot water entering water temperature resulted in a decrease in the performance of the cascade multi-functional heat pump. The cascade multi-functional heat pump in heating-hot water mode showed similar performance trends as individual heat pumps for space heating and hot water generation and provided much stable hot water capacity at varying outdoor entering water temperature conditions. Cascade multi-functional heat pumps have the potential to provide better energy efficiency at higher heating and hot water loads, and can, therefore, be used for simultaneous space heating and hot water generation in winter seasons.