The Study Of Thermal Performance Of Directly Expanded Solar Heat-pump Water Heater

With compact structure and low cost,the direct expansion solar assisted heat-pump water heater(DX-AHPWH)has been proved to be significantly better than the traditional air-source heat-pump water heater(AHPWH).The available research has not provided satisfied basic theory and design specification for the DX-AHPWH.Although most research drew a conclusion that the DX-AHPWH is superior to the AHPWH,few research provided applicability domains.This research aims to explore the applicability domains and investigate the basic theory and design specification of the DX-AHPWH.The main work and findings are as follows:(1)The DX-AHPWH simulation program was written to calculate thermal performance and analyze the influence of environmental parameters on the thermal performance.The study finds that with the increase of environment temperatures from0℃to 40℃,the heating time decreases by 53.7%,and the average COP increases by130.9%.When the solar radiation intensity is increased from 0W/m~2 to 800W/m~2,the heating time would decrease by 26.6%,and the average COP increases by 40.7%.When the wind speed increases from 0m/s to 6m/s,the heating time decreases by 18.7%,and the average COP increases by 23.1%.The results show that the environmental temperature has the greatest influence on the thermal performance of the DX-AHPWH,next does the solar radiation intensity and the wind speed.(2)For the collector-evaporator is a important component,the sensitivity coefficient was introduced to determine its key design parameters.It is found that the sensitivity coefficient of the collector-evaporator area to the average heating performance coefficient and the average hot water heating power is 0.201 and 0.223,respectively.The effect of collector-evaporator area is the most significant,followed by the collector-evaporator absorptivity.Consequently,the collector-evaporator area and absorptivity are determined as the key design parameters.The research lay the theoretical foundation for the system design and optimization.(3)Based on the same hot water demand,under the four meteorological parameters on summer sunny day,summer cloudy day,winter sunny day and winter cloudy day in Xi’an,the thermal performance of the DX-AHPWH and the AHPWH were compared by their simulation programs.The results show that the thermal performance of the DX-AHPWH is better than that of the AHPWH in summer sunny day,summer cloudy day and winter sunny day.The former is inferior to the latter only in winter cloudy day.In conclusion,the DX-AHPWH has higher thermal performance under the meteorological conditions in Xi’an.The research ideas and methods can be applied to other regions.(4)The thermal performance of the DX-AHPWH and the AHPWH were compared,and the relationship between the comparison results and environmental parameters was obtained.If the heating time is taken as performance index,the DX-AHPWH has higher thermal performance while the ambient temperature(y)and solar radiation intensity(x)are on the right side of the straight line y(28)0.1235x-27.066,while they are on the left side the AHPWH has higher thermal performance.If the COP is taken as performance index,the DX-AHPWH has higher thermal performance while the ambient temperature(y)and solar radiation intensity(x)are on the right side of the straight line y(28)0.1492x-37.893,while they are on the left side the AHPWH has higher thermal performance.The research achieved applicability domains while the DX-AHPWH is superior to the AHPWH.