The effect of water temperature lift on the performance of cascade heat pump system

The mass flow rate of secondary fluid in a water heater heat pump determines not only the discharge water temperature but also the heat pump performances. As water flow rate decreases, water discharge temperature increases; however, the COP (coefficient of performance) deteriorates due to the increase of high side pressure. In this study, cascade cycle which is used for a water heater heat pump was studied to verify the effect of the water temperature lift in terms of pressure and performance change. Also, the optimal intermediate temperature of cascade cycle with respect to temperature lift was studied and this temperature was adopted for a heat pump transient operation. The heat pump was operated to meet the constant heating capacity and target intermediate temperature by PID control of compressor speed. The transient results showed that water temperature lift determines not only the vertical temperature profile of water reservoir but also the transient performance.