Investigation On The Joint Effect Of Frsoting And Fouling On The Performance Of Air Source Heat Pumps

Driven by the“coal to clean energy”reform,the development of renewable energy substitute for fossil fuels has aroused widespread concern.Air source heat pump(ASHP),as a renewable energy technology,earned widely usage in buildings space heating and water heating for their higher energy efficiency.However,a heating season usually coincides with serious air pollution in China,hence frosting and fouling appears concurrently on ASHPs’outdoor coils.Detailed knowledge of frosting and fouling effects are important for promoting energy efficiency in heating seasons.For this purpose,in this paper,detailed investigations on the frosting and fouling problems of ASHPs were conducted,the following four aspects were covered:Firstly,experiments were carried out to understand the detailed frosting process.The change of the airflow impedance,frost mass,total thermal resistance of the evaporator and the output heating capacity of ASHP during the frosting process were studied.The results demonstrated that after a period of rapid increase,the airflow impedance would reach to an inflection point,where the rapidly increased airflow impedance became fairly constant.This inflection point divided the frosting process into two stages.In the first stage,the frost mass accumulated rapidly,but the thermal resistance increased slowly;in the second stage,the trend shifted between frost mass and thermal resistance,with the former one increased slowly and the latter one increased rapidly.Besides,the reduction in output heating capacity of ASHP in the second stage was about three times faster than that in the first stage.Therefore,a defrosting operation should be initiated in the junction of these two stages at the latest,so as to minimize the output heating capacity reduction of ASHPs.Based on this inflection point,the effects of air temperature and relative humidity on frosting process were studied systematically,and the results were used to identify a defrosting map for the initiation time of defrosting.Secondly,it was noted that the existing frosting maps are specific for unique systems.To evaluate the frosting performance of arbitrary system,a semi-experimental method,to depict frosting evaluation plot,was proposed in the present study.In the frosting evaluation plot,frosting region was clarified,and the frosting driving forces with the effect air temperature and relative humidity(RH)were evaluated,through theoretical calculation of Gibbs free energy change.After that,experiments were conducted to verify the frosting evaluation plot,and the results demonstrated the present method was feasible.As implementation of the semi-experimental method,the frosting evaluation plot with impact of air velocity was drawn.The results suggested that,increased air velocity would help to restrain frosting.Thirdly,the uneven distribution of frosting and fouling was explored,based on visualization experiments.Owing to lower temperature of air leaving the evaporator than air entering the evaporator,the surface temperature at evaporator outlet side is supposed to be lower than that the inlet side,as a result,the evaporator inlet side may escape from frosting while the outlet side frosting.Based on the semi-experimental method proposed in this study,a frosting evaluation plot was depicted to divide the frosting regions and dewing regions,both at the air inlet and outlet side.Based on the frosting evaluation plot,the air temperature and RH ranges leading to uneven frosting were clarified.On the other hand,visual results showed that the particulate fouling mostly deposited on the air inlet side of the evaporator,which opposite to frosting.Therefore,in the uneven frosting region,when frost and fouling coexist on the surface of the evaporator concurrently,both the air inlet and outlet side of the evaporator will be blocked,leading to seriously reduction on the operating performance.The experimental results showed that the joint effect of frosting and fouling on the performance of air source heat pump is significantly greater than the sum of frosting and fouling effects alone.Finally,the effect of fouling on the performance and frosting characteristics of air source heat pump in heating mode was studied.In this study,four levels of fouling,namely none fouling(0 g/4m~2),mild fouling(15 g/4m~2),moderate fouling(40 g/4m~2)and severe fouling(70 g/4m~2),were concretized on a finned tube heat exchanger successively.After that,experiments were conducted to investigate the impact of fouling on the heating performance of an air source heat pump water heater,and results demonstrated that the impact of fouling on the heat transfer performance would be exacerbated with decreased air temperature.In addition,the frosting evaluation plots were depicted at four levels of fouling.The frosting evaluation plots suggested that increased level of fouling would expand the frosting regions,and could hasten the frosting process.As verification,signal factor experiments were carried at different levels of fouling,and the results demonstrated that the increase of fouling level would result in faster frosting rate.