A variable frequency control method and experiments of a direct-expansion solar-assisted heat pump system

摘要

A direct-expansion solar-assisted heat pump hot water system using variable frequency control was studied experimentally. The system mainly consisted of a frequency conversion compressor, a condenser using microchannel heat exchanger, a bare plate solar collector/evaporator, and an electronic expansion valve as the throttling device. Based on experimental data and the opening control method of the electronic expansion valve presented previously, a variable frequency control method was developed, including the initial setting and real-time regulation of the compressor speed. Then the control method was tested under the year-round environmental conditions, which confirmed that the system could obtain a higher coefficient of performance (COP) and control the heating time effectively. It was found that the compressor speed was negatively correlation affected by the solar radiation intensity, and the electronic expansion valve opening was negatively correlation influenced by the compressor speed. Experiments showed that most of the average system COP was above 3.0 throughout the year. Even in the spring for solar radiation intensity of 124 W m(-2) with environmental temperature of 8.5 degrees C and in the winter for solar radiation intensity of 323 W m(-2) with environmental temperature of - 3.0 degrees C, the average system COP still reached 2.81 and 2.58, respectively. Meanwhile, the heating time expectations were found to agree well with experimental data within 20 min accuracy.