Numerical And Experimental Study On The PV Direct-coupled Solar Water Heating System

Solar water heating systems (SWHS) are widely used. However, the conventional SWHS requires auxiliary electric power and an ON/OFF differiential temperature sensing controller to power and control the AC pump. Since the flow rate of the AC pump is constant, the pump operates between ON and OFF frequently, especially under low irradiation. This operational instability can easily lead to system malfunctions and reduces the pump life. Moreover, the controller must keep working24hours a day all the year round, to make the system work properly. Thus, the previous researcher proposed the use the PV coupled DC pump to circulate the fluid in a SWHS. Since the flow rate requirement of the SWHS has a natural relationship with the availability of solar irradiation, the solar irradiation can be used as the control signal and the power source of the pump. Hence, PV coupled SWHS require no controller, temperature sensors or auxiliary electric power. Among all the PV coupled SWHSs (PV-SWHS), the PV direct-coupled SWHS is more simple and reliable, and promising to be widely used.However, there are limited studies on the comparison of the PV-SWHS and the traditional SWHS, and on the optimization of PV direct-coupled SWHS. Hence, this thesis propose a PV-SWHS with a new design of PV module, and build a comparative test rig to measure the performance of the PV-SWHS and the traditional SWHS under the same condition and at the same time. A system model and a simulation program are built and validated with experiment results. Then, the main factors which influence the thermal performance of the system, are found and investigated. And a design method for PV-SWHS is proposed. Then, the optimized PV-SWHS is compared with the traditional SWHS under different weather and operation conditions. The main content and results are as below.1. A SWHS is built and the input characteristics of the DC brushless pump is measured after installed into the system. According to the input curves, two different PV modules with different designs method are proposed. And two PV-SWHS with these two different PV modules are built.2. A comparative test rig is built to measure the performance of the PV-SWHS and the traditional SWHS under the same condition and at the same time. The results show that on sunny days, the PV-SWHS has nearly the same thermal efficiency as the traditional SWHS, and slightly higher thermal efficiency after improving the design of the PV module. Under low irradiation, the PV-SWHS gains much more heat than the traditional SWHS, which indicates the potential of the PV-coupled SWHS having much higher efficiency than the traditional SWHS on cloudy days. And the modified design of the PV module can reduce the requirement of PV cells and increase the efficiency of the system.3. A system model and a simulation program are built and validated with experiment results. It is found that the simulation program has good accuracy in evaluate the thermal performance, the hydraulic performance, and the electric performance.4. With the simulation program, the different factors which influence the system performance is investigated. And the optimized value of the major factor are gained, according to the results. The research is divided into two parts. In the first part, the influence of the PV module designs on the system flow rate profiles is investigated. In the second part, the influence of the flow rate profiles on the system thermal performance is investigated. The results show that the startup irradiation of the pump is the most important factor, and the optimized value is about250W/m2.5. Three different connection methods (PV-type1, PV-type2, PV-MPPT) are investigated. And the results show that different connection methods lead to significantly different flow rate profiles. And the PV-type2is more reliable and cheaper. It is the first choice of the small and medium scales PV-SWHS.6. A design method for PV-SWHS is proposed and the optimized PV-SWHS is compared with the traditional SWHS under different weather and operation conditions. The results shows, both in common conditions and bad conditions, the PV-SWHS has the same thermal efficiency with the traditional SWHS. Even in bad conditions, PV-SWHS can work well without a controller and the DC pump operates placidly. However, the AC pump in the traditional SWHS operates between ON and OFF frequently all the day.7. Economic assessment of the PV-SWHS is conducted with emphasis on the fluid circulation and control parts. The results show that compared with the conventional SWHS, the PV-SWHS is cheaper and has no operation costs. Moreover, considering the maintenance costs of the system, the PV-SWHS is much more economical.