Performance investigation of a cascade heat pump water heating system with a quasi-steady state analysis

The cascade refrigeration system has been applied to heat pump water heaters to overcome the problems which arise from the high compression ratio at low ambient temperature. In this study, the performance of a cascade heat pump water heating system is investigated with a quasi-steady state analysis to find out the transient behavior of the system. The water-heating system consists of the cascade heat pump water heater, which uses R134a and R410A as refrigerants, and a water storage tank. The steady-state cascade heat pump model is developed based on experimental results and the dynamic storage tank model is created by thermodynamic equations. The model computes the several major parameters related with the system performance, such as the heating capacity, the power consumption, the COP (coefficient of performance), and the temperature distribution in a storage tank. Moreover, the performance of the system is calculated for various conditions to research the influence of ambient temperature and hot water demand.