Study On Direct Expansion Solar Assisted Heat Pump Water Heater Under Frosting Conditions

A direct-expansion solar-assisted heat pump(DX-SAHP)organically integrates the solar collector with the heat pump evaporator to achieve higher evaporation temperature and Coefficient of Performance(COP)through the assistance of solar radiation.Since the DX-SAHP system suffers from the same problem of collector frosting as traditional air source heat pumps in winter,the research on solar collector/evaporator frosting is one of the important topics to improve system performance and energy utilization efficiency.On the basis of experimental test and simulation calculation,this paper studies and analyzes the heating characteristics of DX-SAHP water heater under frosting conditions and various factors affecting frosting.The main contents are as follows:A test prototype of a variable capacity DX-SAHP water heater with R134a as the working medium is set up.The main components include a bare plate solar collector/evaporator,a fully enclosed rolling rotor variable frequency compressor,a micro-channel condenser and an electronic expansion valve.The working principle and data acquisition device of the experimental platform are briefly described.Based on the test platform,the mathematical models of the components of the heat pump are established,including the compressor model,the condenser model,the expansion valve model and the heat collection/evaporator model.In addition,based on the structural characteristics of the bare plate solar collector/evaporator and the heat and mass transfer process of wet air on the cold surface,the frosting model is established to simulate the winter conditions.The accuracy of the heat pump system model including the frosting model is verified by experiments under typical winter conditions.The mean error of the average condensing temperature,average compressor power consumption and average COP is calculated to be within 5.5%,the mean absolute error of average evaporation temperature is lower than 0.15℃,and the calculation of frost layer distribution is consistent with the experimental results,which proves that the model had high accuracy and reliable prediction effect.Based on the established model of heat pump system including the frosting model,the characteristics of DX-SAHP water heater under frosting conditions are analyzed.The analysis includes meteorological parameters such as ambient temperature,solar radiation intensity,ambient relative humidity and wind speed,as well as operating parameters such as initial water temperature and average compressor speed.The analysis shows that the formation of frost can be effectively inhibited by lowering the ambient temperature below a certain value,and the collector efficiency can be effectively improved at the initial stage of frost crystal growth.Higher solar radiation intensity can significantly improve heating performance and reduce the frosting rate.An increase in wind speed,by contrast,would have the same effect,but to a lesser extent.In addition,the increase of the initial and final temperature of the hot water will alleviate the frosting to some extent,but will lead to the decrease of the performance coefficient of the system,and the increase of the compressor speed will make the frosting worse.Under frosting conditions,the system has a COP of 2.75 or above,and has a reliable heating performance.At the same time,compared with the traditional heat pump evaporator,the heat collection/evaporator shows an advantage in delaying frosting.According to the simulation results and the analysis of the experimental data,the operation control strategy of the DX-SAHP water heater prototype in the winter condition is optimized.Through the multiple linear regression fitting,the calculation of the initial speed of the compressor and the initial opening of the electronic expansion valve is reset before the system started in winter,and the accuracy of the fitting is verified by the residual calculation.According to the characteristics of the winter environment,the strategy of compressor speed regulation and electronic expansion valve opening control during system operation is adjusted,and the effects of the new strategy in improving heating performance and relieving frosting are verified by experiment and simulation results.