Simulation Analysis On A Direct Expansion Solar Assisted Heat Pump Water Heater

The total building energy consumption has rapidly increased with the development of economy and society in China, which occupies much higher propotion in total energy consumption. Hot water consumption is the very important part of total building energy consumption. The scheme of combining the heat pump with solar energy application system has been proposed and developed by many researchers around the world. If the collector and evaporator are combined into one unit (collector/evaporator) and the system is used to supply hot water, the system is called the direct expansion solar assisted heat pump water heater (DX-SAHPWH). The DX-SAHPWH has several advantages, such as superior thermodynamic performance and collector efficiency, lower running cost, better integration with buildings. In this study, the focus aims at improving the performance and commercialization of the system. The results could be rather helpful to the study and may engender a great benefit to the large-scale application of the system. The main works are summarized as following.The distributed parameter and homogeneous flow mathematical models of solar collector/evaporator and condenser were established and solved with the tube length for a iterative criterion. The lumped parameter models of compressor and thermostatic expansion valve, the refrigerant charge model and the thermodynamic properties model of refrigerant were also established. The calculating procedures of the refrigerant charge and the thermodynamic properties of refrigerant were put forward. The every component model was combined with the refrigerant conditions and the DX-SAHPWH model was established. Three iterative criterions (refrigerant charge criterion, mass flow rate criterion and superheat criterion) and three iterative parameters (evaporating pressure, condensing pressure and superheat degree) were obtained and the simulation program of DX-SAHPWH was coded according to the flow chart of the system.The calculating and experimental results were compared based on the DX-SAHPWH prototype developed by Institute of Refrigerant and Cryogenics of Shanghai Jiao Tong University. The comparing results showed that average relative errors of the performances of the system were below 1%(except that the average relative error of the average collector efficiency was 4.6%). The calculating results of the instantaneous performances of the system agreed well with the experimental results.Based on the calculating results, the operational parameters and the performances of the system were obtained. The effects of lots of parameters on the coefficient of performance (COP), collector efficiency, the power of compressor, heat power of hot water and collector power of collector were analyzed. The effects of changing the refrigerant charge on heating time, the superheat degree of the collector outlet, evaporating pressure, condensing pressure, the power of compressor, collector efficiency and COP were also analyzed. Some references to judge optimum refrigerant charge were put forward.