Study on a Frost-Free Air-Source Heat Pump Water Heater System

Abstract

This paper proposes a frost-free air source heat pump water heater (ASHPWH) system with integrated solid desiccant, in which frosting can be retarded by dehumidifying air before it enters the ASHPWH evaporator. A numerical simulation is carried out at a dry-bulb temperature of -7-5.5 °C and a relative humidity of 60-80 % depending on the frosting conditions. The results show that the coefficient of performance (COP) is within the range 3.3-3.8, which is 5-30 % higher than that of the ASHPWH system using the hot-gas defrosting method. Experiments have been carried out to measure the adsorption and desorption characteristics of the desiccant-coated heat exchanger (DCHE), the key component of the frost-free system, and confirm whether frost-free can be realized and how long to keep it, whether the DCHE can be effectively regenerated under the condensation temperature of the heat pump. Experimental results show that frost-free operation can be realized and the frost-free operation times are 17, 20 and 42 minutes at air temperatures of 5, 2 and -7 °C. And the DCHE can be regenerated at a relatively low temperature (around 50 °C), so the condensation heat of the heat pump can be used to regenerate the DCHE. Furthermore, design calculation based on those experimental results was conducted to determine the physical characteristics of the DCHE and evaporator, the layout and dimensions of the heat pump unit. Results show that the heat pump unit at the dimensions of 700 (height) x 1000 (width) x 300 (depth) mm, has a water heating capacity of 4.5 kW under ambient air conditions of 2 °C and 80%.