Study On Characteristics Of Small Fixed Capacity Direct-Expansion Solar-Assisted Heat Pump Water Heater

The hybrid heat pump system based on air source is the most complete one in the prior art.The direct-expansion solar-assisted heat pump(DX-SAHP)system effectively combines solar energy with an air source heat pump,which receives widespread attention.The performances of the DX-SAHP water heater are studied in this paper.The main contents are as follows:The experimental platform of a small DX-SAHP water heater was built in Qingdao.The test errors of the system are calculated,which shows that the errors of coefficient of performance(COP)and collector efficiency are within the tolerance.Besides,it is verified that even in rainy days,the system COP can still reach 3.42 with the average solar radiation intensity of only16 W·m-2,the average ambient temperature of 24.8℃,the average wind speed of 2.1 m· s-1.In order to study the thermal performance of the DX-SAHP system more comprehensively,a mathematical model based on the above experimental platform was established.It is found that the deviations between the simulated value and the experimental value are mostly within the range of 10%.Through comparative calculations,the climatic conditions of the typical years in Qingdao are obtained.In the actual process,the cold water temperature is difficult to obtain.A simplified calculation method is selected to calculate the monthly average cold water temperature throughout the year.The results show that the cold water temperature in February is the lowest at 12℃ and the cold water temperature in August is the highest at 20.6℃.The energy efficiency of the system was calculated throughout the year.The system COP in the winter is the lowest with an average of 3.61.The average collector efficiency throughout the year is 1.29.The annual energy consumption of the system is 784.6 kW·h.In August,the unit hot water consumption is the lowest with only 0.25 kW·h·L-1 and that in January is the highest,reaching 0.44 kW·h·L-1.The annual energy consumption of the DX-SAHP water heater is only 22.8%of the electric water heater.The static investment recovery period of the DX-SAHP water heater is 1.48 times of the electric water heater.The dynamic annual value is only 48%of the electric water heater.The annual CO2 emission reduction is 16507.8 kg.It is verified that the DX-SAHP water heater is with good energy-saving,economy and environmental protection.The influence of the degree of superheat on system performance us studied.It is found that the optimal range of the degree of superheat of the system is 5-10℃.The effects of solar radiation intensity,initial water temperature and operating frequency of the compressor on system performance are also studied.The results show that the COP increases while the operating time of the system decreases with the increase of solar radiation intensity.The system COP is decreases while the initial water temperature in the water tank increases.The change of compressor pressure ratio is the most sensitive one when the operating frequency changes.As the operating frequency increases,the system COP gradually decreases while the collector efficiency of the collector increases.