Transcritical, Field Test, CO2, Heat Pump, Optimal discharge Pressure, COP
Air-source transcritical CO2 heat pump water heater (ATHW) can supply hot water from 60 ? to 90 ? at high efficiency with environment-friendly refrigerant CO2 for commercial, residential and industrial applications. Several optimal discharge pressure correlations for transcritical CO2 heat pump have been proposed in the past few years, most of which are related to the ambient temperature, the evaporation temperature and the gas cooler outlet temperature. In an earlier study, the authors’ research group had presented a study on the dependency of the optimal discharge pressure on the ambient temperature and the hot water outlet temperature. In this study, a revised model for optimal discharge pressure is developed based on experimental results. In order to validate the optimal discharge pressure model developed, field tests are conducted to evaluate the performance of an air-source transcritical CO2 heat pump water heater in practical application. The system is comprised of a semi-hermetic reciprocating compressor, a counter-flow tube-in-tube gas cooler, a counter-flow internal heat exchanger, a fin-and-tube evaporator, and an electronic expansion valve (EEV) driven by electrically operated step motor. A Siemens SIMATIC S7-200 Programmable Logic Controller (PLC) was used to regulate the compressor discharge pressure by adjusting the EEV opening and the water flow rate by changing the frequency of the variable speed water pump. Field tests were conducted under three different operating scenarios: the nominal test condition, high water supply temperature condition and low ambient air temperature condition. The results show that the coefficient of performance (COP) can achieve 3.76 in the nominal test condition with 15? water inlet temperature and 80? hot water supply temperature. Even when the hot water temperature is higher than 90?, the COP remains at 3.21 with 20? dry-bulb temperature and 15? wet-bulb temperature. Under low ambient air temperature condition, the COP was 2.19 with the hot-water supply temperature of 60?. Comparison between the field test results and the model predictions show that the maximum relative error of discharge pressure control was 5.6% in the low temperature condition, while the maximum relative error of system COP was only 4.7%. With the reasonable agreement observed between the field test results and the model prediction. It is reasonable and effective to model the optimal discharge pressure as the function of the ambient temperature and the water outlet temperature.