| Solar energy is a kind of new energy. As to renewable clean resources, it is the most remarkable and widely used resource that attracts many researchers’interest. Our country is speeding up the development and utilization of solar energy, to release the present tension of the energy problems. Solar thermal utilization technology in reducing fossil fuels consumption, promoting energy conservation and emissions reduction has been played and continued to play an very important role.Flat-plate solar collector is a kind of solar collectors which is widely used in all countries in the world. Comparing with other solar collectors, flat-plate solar collector has advantages of energy saving, high efficiency, easy processing and wide application, etc. The fluid channel size in solar collector is commonly large, more than5millimeters, so the volume of water heat capacity is relatively large, it needs a longer time to reach the desired water temperature. Also the temperature difference of inlet and outlet is small, heat loss of the collector system is large and thermal efficiency in high temperature range is low. This kind of collector can only be used in low temperature range. In this paper, we have designed a new flat-plate solar collector, applying the concept of micro channels heat transfer into it, wishing to develop a kind of non focusing type solar collectors which can he used in middle temperature occasions such as solar desalination.At first, this paper has designed and built a microchannel flat-plate solar collector experimental equipment. MicroChannel size was determined to be0.7millimeters, the distance between the transparent plate and absorber plate was determined to be17millimeters. Secondly, we conducted the forced convection experiment by giving an inlet water velocity, from the aspects of energy balance, instantaneous efficiency and system heat loss, to analyse the thermal performance of the collector. We founded that the instantaneous efficiency increased with the increase of flux, system heat loss primarily caused by the top part. And then we analysed the heat transfer process in the microchannel, the slender structure made the water have sufficient time to make heat exchange with the wall of absorber plate. The temperature difference between water and absorber plate was small, less than3degree centigrades, resulting in achieving higher outlet temperature. Natural convection cycle experiment refered to the natural circulation process in a closed collector system which due to the inlet and outlet water temperature difference was also studied. Under similar experimental conditions, the highest temperature we got in natural convection experiment was lower than forced convection. It is concluded that forced convection can enhance the heat transfer coeilicient of water, and it is easier to meet the requirement of reaching the high water temperature.Numerical simulation can help to analyse the heat transfer characteristics in the microchannel from microcosmic aspect. The simulation based on experimental data, the results showed consistency of the experiment and simulation. Taking advantage of numerical simulation can also study the characteristics and differences in niicrochanncls and regular channels. It is founded that heat transfer coefficient, outlet temperature and drag coefficient decreased with the increase of the flow channel size. Therefore, in microchannei, it can further improve the effect of heat transfer if we found ways to reduce the resistance. Numerical simulation can also be used to analyse the velocity and temperature field distribution in flow channel. |
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