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

• Published in: Solar energy Vol. 176; pp. 572 - 580
• Main Authors: Kong, Xiangqiang, Sun, Penglong, Jiang, Kailin, Dong, Shandong, Li, Ying, Li, Jianbo
• Format: Journal Article
• Language: English
• Published: New York Elsevier Ltd 01.12.2018; Pergamon Press Inc
• Subjects: Environmental conditions; Evaporators; Expansion; Experimental data; Experiments; Frequency control; Gas expanders; Heat exchangers; Heat pump; Heat pumps; Heating; Hot water heating; Microchannels; Radiation; Solar collectors; Solar energy; Solar radiation; Temperature; Throttling; Time compression; Variable frequency control; Heat pump; Variable frequency control; Experiments; Solar energy
• ISSN: 0038-092X; 1471-1257
• DOI: 10.1016/j.solener.2018.10.067

•A direct-expansion solar-assisted heat pump system was studied experimentally.•A variable frequency control method for the system was developed.•The control method was tested under the year-round operating conditions. 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 °C and in the winter for solar radiation intensity of 323 W m−2 with environmental temperature of −3.0 °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.