Operating characteristics of an air-source heat pump under frosting/defrosting conditions

Abstract

This paper presents an experimental investigation on the behavior of an air-source heat pump operating under frosting/defrosting conditions. Dynamic data of parameters, such as the frost layer thickness, compressor suction/discharge pressures, fin temperature, heating capacity, power consumption, and fan electric current, were acquired during the test. Based on these experimental data, the operating characteristics of the heat pump under frosting/defrosting conditions were analysed, and the influence of frosting/defrosting on the performance and reliability of the heat pump was investigated. It was shown that a critical frost layer thickness existed, below which the heat pump presented no noticeable parameter changes. When the frost layer grew above the critical value, the compressor discharge pressure increased rapidly with a slight decrease in the suction pressure, but the shaft power remained almost unchanged. However, due to the increase in airflow resistance, the airflow rate dropped and the fan electric current increased abruptly, leading to fan performance deterioration and a reduction in the system heating capacity. At 50 per cent nominal airflow rate, the frost layer formed later and grew slower than at the nominal airflow rate. Under defrosting conditions, the suction pressure experienced a large drop, which led to frequent on-off switching of the compressor due to pressures falling beneath the low-level limit. This could be avoided by recirculating a portion of the high pressure gas in the compressor receiver back to the compressor inlet through a pipe fitted with a solenoid valve controller.