| The environmental issue and energy conservation have got more and more concerns of the public recent years. It is of an important significance for the sustainable development of Chinese economy to develop renewable energy technologies and reduce the ratio of fossil energy in energy structure. Solar energy is considered to be a promising alternative energy of fossil energy because of its wide distribution, easy accessibility, and pollution free. Meanwhile, the utilization of solar energy needs a large area because of its low energy intensity. As the energy consumption in buildings has a large share of the total energy consumption and there are lot of surfaces with sunshine on buildings, it is very meaningful to integrate solar energy with buildings. Without needs of extra area, solar energy is collected and utilized in the same place. This meets the principle of distributed energy systems.The PV/T (photovoltaic/thermal) collector, which can collect electrical and thermal energy simultaneously, is an integration of PV cells and solar thermal collector. When part of solar energy is converted to electricity, the rest part converts to heat and leads a temperature increase of solar cells. The PV cells in PV/T collector are cooled by coolant (liquid or air) to get a higher electrical efficiency. The heat extracted by coolants is collected for water heating in domestic and industrial applications, or space heating and agricultural drying with air as working fluid. With single area and double output, PV/T collectors have a better overall performance than conventional solar collectors. According to the coolants, PV/T collectors are splitted up into PV/T water collectors and PV/T air collectors. However, PV/T air collector may be left unused in non-heating seasons. Especially in summer, the electrical efficiency of PV/T air collector would be limited because of the high temperature of PV cells without ventilation and overheating protection methods. In building integrating applications, the accumulated heat will increase the cooling load of buildings. As for the PV/T water collector, it may be damaged without anti-freeze solutions when freezing temperatures occur in winter, especially at middle and high latitudes. For freeze protection, drain back systems and antifreeze/water systems are developed. But both of these system will increase initial cost and maintain cost. In addition, heated air for space heating is a more economical way to reduce the energy consumption in cold days. In consideration of the time complementarity of PV/T water collector and PV/T air collector, a new design of tri-functional photovoltaic/thermal solar collector is presented in this thesis. The collector is modified from sheet-and-tube solar collectors. This tri-functional PV/T collector is designed to provide electricity and hot air (PV/air-heating mode) in winter, and provide electricity and hot water (PV/water-heating mode) in the rest of year. The working mode of tri-functional PV/T collector is flexible to fulfill the requirements in different seasons, regions and applications.The main achievements of this thesis are outlined as follows:1. A new tri-functional photovoltaic/thermal solar collector is designed and constructed. Test rigs for PV/air-heating mode and PV/water-heating mode are built. The test rigs can be used to evaluate the effect of flow rate, inlet temperature, and initial conditions on thermal and electrical performance of tri-functional PV/T collector.2. Experimental studies are performed for both PV/air-heating mode and PV/water-heating mode. The advances of tri-functional PV/T collector over conventional solar collectors and PV/T collectors are revealed. It shows that the daily thermal efficiency achieved46.0%with the electrical efficiency of10.2%when the air flow rate was0.042kg/s in PV/air-heating mode. As air mass flow rate increases from0.026kg/s to0.039kg/s, thermal efficiency at zero reduced temperature increases from37.4%to46.4%and heat loss coefficient increases from5.21W/(m2-K) to8.00W/(m2·K). In PV/water-heating mode, the thermal efficiencies of collector and system at zero reduced temperature are56.6%and36.4%respectively. In comparison with the performance of different types of solar collectors recently reported, the tri-functional PV/T collector is proved to be energy-efficient in different working modes and has potential for various applications.3. For two working modes, steady-state model and dynamic model of the collector are developed and validated with experimental results. For PV/air-heating mode, the RMSDs of thermal and electrical efficiencies are2.6%and8.2%, respectively. For PV/water-heating mode, the RMSDs of thermal and electrical efficiencies are3.1%and6.7%, respectively. The simulated and experimental temperatures of collector components also show good agreements.4. Based on the validated models, the effects of different operating parameters on thermal and electrical performance are evaluated. The annual performance of tri-functional PV/T collectors are analyzed with different climates. The thermal efficiencies of collector in Hefei, Beijing, and Xining are38.5%,38.9, and40.1%, respectively. The tri-functional PV/T collector is proved to have better annual performance than separated PV/T water collector and PV/T air collector which could be left unused in some seasons. The thermal and electrical performance in PV/air-heating mode of winter and PV/water-heating mode of summer and the effect on heating/cooling load of building is evaluated in building integrated applications. |
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