| As an important way of solar heat utilization,solar collector has been widely studied in recent years.The non-concentration collector is small in size and easy to be combined with buildings,but its collecting temperature is low.The concentrator has high temperature,but it is large in size and needs a complex tracking system,so the cost is high.Therefore,Compound Parabolic Concentrator(CPC)solar collectors which can achieve a certain multiple concentration without tracking system were proposed.The non-uniformity distribution of the heat flux in the absorber formed by CPC will accelerate the aging of the absorber.Moreover,higher local temperature will reduce the performance of the collector.Due to the size effect of microchannel parallel flow flat tube,the influence of non-uniform heat flux on heat collection will be homogenized.Therefore,a CPC solar collector with parallel flow microchannel as heat absorber is proposed.The main research contents and conclusions are as follows:(1)The effects of environmental factors(solar radiation intensity,ambient temperature),operation factors(inlet temperature,circulating flow)and structural factors(CPC concentration ratio,absorber length)on the operation characteristics of the collector have been investigated by establishing a heat transfer model of a collector unit based on the energy balance equation.It is found that the ambient temperature and inlet temperature has little influence on the fluid temperature rise,but the radiation has the opposite effect.The increase of circulating water flow causes a significant reduction in fluid temperature rise when it is less than 3L/h.When it is higher than 3L/h,the collector efficiency and fluid temperature rise tend to be flat.CPC concentration ratio has little effect on the efficiency of the collector,but has a considerable impact on the fluid temperature.As the height of CPC increases exponentially with the concentration ratio,it is difficult to integrate with the building.The longer the absorber,the higher the fluid temperature can be obtained with little change in efficiency.Therefore,considering the collector efficiency,fluid temperature rise and manufacturing cost,the collector design can choose the concentration ratio of 3,the design length of microchannel absorber can choose 1m,and the collector circulation flow can choose 3L/h less than the recommended flow of flat plate collectors in actual use.(2)Through the analysis of the main heat loss of the collector,the influence of different concentration ratio on the natural convection in the closed cavity formed by CPC,microchannel and glass cover is studied.The result shows that the increase of the concentration ratio and inclination angle will inhibit the natural convection within the cavity.The higher the inlet temperature,the stronger convection will be.(3)An experimental test system was designed and built to measure the dynamic performance of the collector under various flow rate,inlet temperature and incidence angle.As shown by experimental,the collection efficiency and fluid temperature difference more rapid decrease with a rise in inlet temperature and incidence angle.The maximum collection efficiency reaches up to 0.8.(4)The homogenization effect of microchannel on CPC non-uniform concentration was acquired by the calculation of heat flux distribution and measurement of fluid temperature.There was evidence that the variation of non-uniformity is opposite to that of inlet temperature and flow rate.The flow rate affects the temperature distribution nonuniformity at the outlet of microchannel a little.With the increase of the incident angle,the nonuniformity of the non-uniform heat flux on the surface of the microchannel increases from 0.9 to 1.4,but the nonuniformity of the temperature distribution at the outlet section of the microchannel is smaller,below 0.045.The results show that the microchannel has a good homogenization effect on the non-uniform heat flux under different working conditions,and the non-uniformity of the microchannel outlet is less than 0.106.This provides a basis for the feasibility of combining CPC with microchannel,and also provides a reference for reducing the heat loss of the collector and optimizing the design of the collector. |
