The effects of the opening of the electronic expansion valve in the high-stage cycle on the performance of a cascade heat pump water heater

Abstract

The opening of the electronic expansion valve (EEV) in the high-stage cycle of a cascade heat pump not only affects the suction and discharge statuses of the compressor, but also regulates the intermediate temperature, thereby, determines the system performance. In this study, we conducted experimental analyses on a cascade heat pump water heater that was equipped with an EEV in the high-stage cycle (high-stage EEV) and an inverter compressor in the low-stage cycle (low-stage compressor) to investigate the effects of the opening of the high-stage EEV on the suction and discharge statuses of the high-stage compressor and the system performance. Our results indicated that the optimum opening of the high-stage EEV maximized the coefficient of performance, and the increase in the frequency of the low-stage compressor from 150 to 225 Hz increased the optimum opening of the EEV from 37.5% to 50%. Moreover, the decrease in the opening of the high-stage EEV and the increase in the frequency of the low-stage compressor increased the heating capacity of the system by 4.88W/% and 30.14W/Hz, respectively. Furthermore, the opening of the high-stage EEV had little influence on the suction superheat of the high-stage compressor, specifically, the suction superheats ranged from 6.0 to 9.5 °C at all of the EEV openings (32.5–75.0%). Nevertheless, low EEV openings increased the discharge pressure of the high-stage compressor approaching the safe operating limit of the compressor. Hence, the control on the opening of the high-stage EEV should be aimed to optimize the system performance and reduce the discharge pressure of the high-stage compressor.