Experimental Study Of Heating Performance And Frosting Characteristics Of A Solar/Air Source Variable Capacity Finned Heat Pump Water Heater

The advantages of the solar/air source direct expansion variable capacity finned heat pump water heater are:1)Absorbing energy from both solar radiation and air efficiently;2)Less system components and saving cost by applying direct expansion structure;3)Better matching among different components with variable frequency compressor and electronic expansion valve;4)Larger energy absorbing surface by finned evaporator;5)The system can operated in solar,air,dual modes by employing fan and automatic lifting glass cover plate,so the system can work efficiently in different conditions.In this paper,with the stable ambient parameters provided in the enthalpy difference lab,the influence of ambient temperature,solar irradiation,compressor frequency and working modes on the heating performance of the system are investigated.Results show that:1)Ambient temperature,solar irradiation and working modes almost have no effect on energy consumption of compressor.When the ambient temperature and solar irradiation rise,heat gains of the evaporator and COP increase;2)The energy consumption of compressor increases linearly with water temperature,and the heat gain of evaporator and instant COP decrease linearly with water temperature;3)When the compressor frequency increases,the energy consumption of compressor and heat gains of evaporator increase;4)When the ambient temperature is between 20.00? to 30.00?,and the solar irradiation is between 500 W/m~2 to 900 W/m~2,it is recommended to operate the system in solar mode with compressor of 60 Hz;Frost formation is an important problem for practical application of heat pump.The influence of compressor frequency on the frost formation for the system working in solar mode were investigated in this paper.Results show that:1)Because of the small surface of evaporator and maldistribution of refrigeration,some frost forms when the system operates in solar mode under higher ambient temperature,higher solar irradiation and lower humidity,and the frost form unevenly on the evaporator;2)At the beginning,the frost accumulates with the rising of the water temperature.After the water temperature reaches a certain value,the frost gradually dissolves with the rising of the water temperature until totally disappears;3)When the ambient temperature and solar irradiation increase,frosting becomes less severe,and COP increases;4)When the compressor frequency decreases,frosting become less severe,and the average COP increases.The influence of compressor frequency on the frost formation of the heat pump under air mode were studied in this paper.Results show that:1)The frost form more severe on the right side of the inlet,outlet and surface of the evaporator;2)When the compressor frequency increases,energy consumption of the system increases.Before water temperature rises to 43.5?,heat gain of the evaporator is less with higher compressor frequency,and then heat gain of the evaporator is larger with higher compressor frequency.Before the water temperature rises to 47?,instant COP is less with higher compressor frequency,and then instant COP is larger with higher compressor frequency;3)When the compressor frequency decreases,the temperature of inlet,outlet and surface of the evaporator and the evaporating pressure increase,the frost become less severe,heating time increases,frosting time decreases,and COP increases.