Numerical And Experimental Study On The Direct-expansion Solar Assisted Heat Pump System

One third of the world's total energy consumption is building energy consumption,including space heating,space cooling and water heating consumption.Solar assisted heat pump is an effective way to reduce building energy consumption and cope with the worldwide energy crisis.Solar assisted heat pumps combine solar energy with traditional heat pump systems,and can utilize clean and renewable solar energy to improve the performance of heat pumps.Direct-expansion solar assisted heat pumps(DX-SAHP)have the advantages of compact structure and low cost.Also,DX-SAHP can prevent the nighttime freezing problem of collectors.Researches have proved that the performance of DX-SAHP systems is better than traditional heat pumps.However,study on DX-SAHP systems is insufficient in some respects.Firstly,DX-SAHP system can be applied in space heating and water heating.Most experimental studies focus on the water heating application of the system,due to the limited experiment facilities.The performance of DX-SAHP system using air source heat exchangers is quite different from that using water cooling heat exchangers.As an important application,the space heating performance of DX-SAHP systems needs further research.Secondly,most experiments were conducted in the ourdoor environment.Affected by the meteorological condition,the test condition is unsteady,which leads to the difficulty to quantitatively analyze the effects of environment parameters on the system.Therefore,experiments under steady conditions are necessary,in order to analyze the system performance quantitatively.Thirdly,frosting problem is one of the most important problems in the application of heat pumps.The heating performance and frosting characteristics of DX-SAHP systems under frosting conditions need to be researched.But the problem has seldomly been discussed.Forthly,besides of heating performance,the DX-SAHP system needs to offer space cooling in real application,which also lacks of study.To research these important problems,in this paper,experiments are conducted in the enthalpy difference lab and test a DX-SAHP system with bare plate collectors.The system is tested under a series of steady conditions.The numerical model is proposed and predicts the performance of the system.Besides,the performance of the system is compared with the performance of a DX-SAHP system using fin-and-tube heat exchangers.The main work and conclusions are as follows.1.Experiments test the heating performance of the DX-SAHP system under different steady conditions.The results show that the increase of ambient temperature can increase the system heating capacity and COP.When solar irradiance is 300 W/m2,relative humidity is 50%and ambient temperatures are 5.0?,10.0? and 15.0?,COP is 2.12,2.18 and 2.26.When the relative humidity increases from 70%to 90%,the evaporating heat exchange rate increases by 35.0%,and COP increases from 1.78 to 2.07.When the ambient temperature is 15.0?,the relative humidity is 50%and the solar irradiances are 0 W/m2,100 W/m2,200 W/m2,300 W/m2 and 500 W/m2,COP is 2.07,2.09,2.14,2.26 and 2.36.The improvement caused by solar irradiation on the heating performance of the system is significant.2.The conditions when the DX-SAHP system frosts are experimentally studied.When there is no solar irradiation,frost occurs when the ambient temperature is between 7.0 ? and 6.0 ?,and the relative humidity is between 50%and 70%.When solar irradiance is 100 W/m2 and ambient temperature is higher than-3.0 0C,frosting can be totally prevented.Lower ambient temperature or higher relative humidity call for higher solar irradiance to prevent frosting.3.The frosting pattern of the DX-SAHP system is analyzed according to the experiments.The experiment is conducted for 360 min under the condition that ambient temperature is 1.0 ?,relative humidity is 70%and no solar irradiation.Compared with traditional air source heat pumps,DX-SAHP system with bare solar collectors frosts slower.The performance of the system is improved instead of obstabled by the frost.4.The effects of ambient parameters on the DX-SAHP system under frosting conditions are analyzed.Solar irradiation can effectively retard or even totally prevent the frosting of the system,and improve the heating performance.The decrease of ambient temperature causes the decrease of the system heating performance.The collectors get frosted more serious when relative humidity is higher.When ambient temperature and relative humidity are low,the air humidity is not large enough to form frost although the evaporating temperature is lower than the freezing point.5.The DX-SAHP system is researched with numerical methods.A dynamical model is presented and solved in MATLAB.The simulation results are compared with experimental results to validate the model.The frosting rate and frost thickness are analyzed in the frosting process.According to the simulation results,the frosting rate and frost thickness are the largest when ambient temperature is-1 ?,with relative humidity of 70%and no solar irradiation.Solar irradiation can retard frosting.When solar irradiance rises from 0 W/m2 to 100 W/m2,frosting is prevented,but COP decreases from 1.97 to 1.89.As solar irradiance keeps rising to 500 W/m2,heating capacity increases from 1082W to 1610W and COP from 1.89 to 2.21.Althouth the decrease of ambient temperature lowers the system heating performance,but the frosting process benefits the heat exchange.When ambient temperature is lower than-5?,no frost forms.The effect of relative humidity on the system performance is mainly realized by affecting the frosting process.6.Experiments on the space cooling performance of the system are conducted.When ambient temperature increases from 25.0 ? to 30.0 ?,the cooling capacity decreases by 14.9%.When wind speed changes from 4 m/s to 5 m/s,COP increases by 3.9%,but further increase of wind speed affects the system performance little.As solar irradiance increases from zero to 800 W/m2,the cooling capacity reduces by 34.9%and COP changes from 1.36 to 0.82.Solar irradiation significantly effects the system cooling performance.7.The cooling performance of the DX-SAHP system is simulated and optimization suggestion is presented.To improve the cooling performance,the collectors should not accept solar irradiation when working as the condenser.The optimal area of the collectors is 8 m2,and optimal wind speed is 5m/s.8.Experiments test the DX-SAHP system with the solar fin-and-tube heat exchanger.The results are compared and analyzed with the simulation results of the DX-SAHP system with bare solar collectors.Under nominal heating condition,when there is no solar irradiation,frosting of the fin-and-tube heat exchanger is more serious than the bare solar collector,and both of them weaker than traditional air souce heat exchanger.The frosting process do not retard the system performance.Moreover,solar irradiance of 100 W/m2 can prevent frosting of both heat exchangers.The fin-and-tube heat exchanger absorbs heat from ambient better than the bare solar collector with no solar irradiation,and gets better system performance.While as solar irradiance increases,bare solar collector performans better.