| With the development of energy shortage problems all over the world,numerous of renewable energy technologies have emerged.Solar energy is clean,cheap and endless,and can be wildly obtained through various manners,which make it a typical clean energy resource.On the other hand,heat pump is another promising energy saving technology which can absorb heat from mid-low temperature resources and allows for temperature upgrading and capacity expansion in the interim.Direct-expansion solar-assisted heat pump(DX-SAHP)system combines solar energy and heat pump,which means that the solar energy can be considered as direct or indirect heat source of the heat pump system.Through this way,the faultiness of low energy density and unstable of the solar radiation is offset.As an intelligent extension,the DX-SAHP system not only takes advantage of heat pump but also obtains a wider heat sources.And it can be considered as a superior solar energy heat utilization technology.In this work,thermal performance of the DX-SAHP system is theoretically analyzed.The main work is as follows:The experimental set-up of DX-SAHP system has been designed,fabricated and tested for domestic hot water supply.The micro-channel solar collector/evaporator,compressor,micro-channel condenser,water tank and electronic expansion valve are employed and the refrigerant of R290(propane)is adopted in the system.Corresponding data collection and control system also has been designed and built for collecting,organizing and analyzing experimental data.The uncertainties of the thermal performance indexes have been calculated,and the results show that the values of uncertainties are below 5%.In addition,the DX-SAHP system has been operated under spring conditions for testing the experimental performance of the system.The experimental results indicate that the heating capacity of the system can be maintained in a range of 905-1460 W,and the COP(coefficient of performance)of the system can reach above 2.71 while the collector efficiency above 0.71.Based on experimental set-up,the models of equipment and thermodynamic properties of the R290 have been established.It mainly contains mathematical model of micro-channel solar collector/evaporator,compressor,micro-channel condenser,water tank and electronic expansion valve,and the model of thermal conductivity,dynamic viscosity,surface tension and other thermodynamic properties of the R290.The corresponding correlations are used with the various state of R290 for improving calculation accuracy.The simulation program is written using Matlab software with convergence criteria of enthalpy and refrigerant charge.The effect of ambient parameters and operating parameters on the system performance is analyzed at corresponding superheat degree by using simulation program.The parameters mainly contain ambient temperature,solar radiation intensity,wind speed,superheat degree,compressor speed,initial water temperature and refrigerant charge.Based on the same operating condition,the simulation result and experimental result are comparatively analyzed.And the result shows that the relative error of the simulation result is 5.29%.Specially,the effect of refrigerant charge on the system performance at fixed superheat degree and various superhet degrees are respectively analyzed.The results show that,when system at fixed superheat degree,the evaporating pressure and refrigerant mass flow rate both decrease with increasing refrigerant charge.While the evaporating pressure and refrigerant mass flow rate both increase with rising refrigerant charge at various superheat degree.In the other hand,the COP,collector efficiency and condensing pressure both increase with the increasing refrigerant charge regardless of superheat degree. |
