Collector/Evaporator Flow Channel Shape Optimization And Operation Analysis Of The DX-SAHPWH System

Solar water heater and air source heat pump water heater are both high-efficient water heating systems with application of renewable energy source.Which are also the most promising water heating technologies nowadays.Though the operation performance of the solar energy water heater completely depends on the solar irradiation,running stability and reliability is poor;while efficiency of the air source heat pump water heater is restricted by ambient air temperature,and low temperature environment will lead to decrease of output heating capacity as well as energy efficiency coefficient of the system.The direct expansion solar assisted heat pump water heater(DX-SAHPWH)studied in this paper is a combination of solar energy and heat pump technology.DX-SAHPWH system combines the advantages of solar energy and air source heat pump system,and overcomes the shortcomings of both at the same time.The advantages of the DX-SAHPWH system are as following:(1)The performance of the system is high-efficient,and the operation of the system is stable and reliable;(2)Compact structure,low cost of installation & maintenance,and easy integration with the building facade;(3)The system collector/evaporator using refrigerant flow as working medium.Compared with conventional solar collector using water as working medium,since temperature of the working fluid is low,the solar energy collection efficiency of the collector gets improved.On the other hand,there is no problems such as pipeline corrosion,winter antifreeze and short service life.(4)Because of the application of double heat source(solar and air),DX-SAHPWH system suitable for different climate condition has good market development and potential applications.And the widespread application of this system is conducive to promote energy conservation and emissions reduction goal of China.Theoretical analysis,simulation and experimental research methods are adopted in this paper to complete the DX-SAHPWH system design,optimization and operation characteristics analysis.In this paper,the specific work content mainly includes:First,the DX-SAHPWH system is analyzed with first law and second law of thermodynamics.Through the thermodynamic analysis,factors that affect the performance and the performance evaluation parameters of the system are determined.At according to thermodynamics analysis results,the components(mainly compressor with the collector/evaporator)matching of DX-SAHPWH system are studied,and optimization design method are proposed.Second,the collector/evaporator is core technology of the DX-SAHPWH system.And the heat transfer effect of the collector/evaporator is also one of the key factors that affect the operating performance of the system.In this paper,the structure of the collector/evaporator flow channel networks has been optimized to improve the heat transfer performance.The prototype has been built and experiments have been carried out to verify the performance of optimization design.The experimental results show that the optimized flow channel structure enhances the heat transfer performance of the collector/evaporator,and effectively improves the efficiency of the heat pump system.When initial water temperature is 25 ?,the set value of hot water temperature of 55 ?,the ambient air temperature is 26 ?,and the sun radiation intensity is 700 w/m2,DX-SAHPWH using the conventional serpentine flow channel collector/evaporator(Type I)performs an average COP of 4.58 during the process of water heating operation,and the average output heating capacity is 1.37 k W;And with the optimized flow channel structure Type II and Type III,the system average COP are increased to 5.25 and 5.56 respectively,while the average output heating capacity are increased to 1.61 k W and 1.71 k W respectively.Third,through the theoretical and experimental research,the effects of the water temperature,ambient iar temperature,the intensity of solar radiation and sky long wave radiation on the performance of DX-SAHPWH system has been analyzed in this paper.Under various weather conditions,operating performance of the DX-SAHPWH system performance is tested and compared with that of the air source heat pump water heater(ASHPWH)under the same operating condition.Test results show that,compared with the ASHPWH system,the DX-SAHPWH system under clear day condition has significant performance advantages;while under clear night condition,the DX-SAHPWH system shows a poor performance.And with overcast sky conditon,there is no significant difference between operating performances of both systems.As for the exergy loss,the component shows largest exergy loss ratio in the ASHPWH system is the compressor,while for the DX-SAHPWH system,the component shows largest exergy loss ratio in the ASHPWH system is the collector/evaporator.Fourth,the simulation model based on the prototype is established to study on the performance of DX-SAHPWH system in different climate zones.The annual operation simulation results of different area are analized.And according to data regression analysis of the simulation results,weighted average value of the air temperature and solar radiation intensity are advised to be adopted as the rated condition for annual performance evaluation for the DX-SAHPWH system.And results show that there is a binary linear relationship amongh the system average COP,the weighted average air temperature and the weighted average intensity of solar radiation.At the same time,considering both investment and running cost of system,optimized matching of the compressor and collector/evaporator is studied.According to the results of optimization calculation,in various climate conditions,the optimal ratio of compressor displacement volume and collector/evaporator area locates within the scale of [0.28,0.35].Fifth,the performance of the air source heat pump water heater was mainly affected by ambient temperature and water temperature.With small temperature difference between day and night,the performances of the system changes little with hours throughout the day.On contrast,since the intensity of solar radiation have great influence on the performance of the DX-SAHPWH system,and solar irradiation conditions change significantly throughout the day,the DX-SAHPWH system has significant difference in performance at different hours of the day.The simulation calculation analysis shows that,compared with the conventional thermostatic control strategy,time-sharing thermostatic control with consideration of solar irradiation conditions of can significantly improve the daily average performance of the DX-SAHPWH system.Research contents in this paper can provide reference for DX-SAHPWH system design.And the results obtained in this paper have important significance on the commercial promotion of and related performance evaluation standards of the DX-SAHPWH system.