Research On The Photothermal Performance Of Compound Parabolic Concentrator Based On Solar Vacuum Tube

Solar energy,as a low-cost and inexhaustible energy source with zero-pollution,provides an important technical means for addressing the requirement of household heating,industrial and agricultural drying and cooling,large-scale power transmission and other aspect.Exploring the method to efficiently convert and utilize solar energy resources is of enormous significance for alleviating the gradual shortage of fossil fuels and the increasing of environmental pollution.CPC shows enormous potential and wonderful application prospects in effectively improving the photo-thermal performance of solar evacuated tube.Aimed at the problem of light energy loss caused by the gap between evacuated tubes,a composite solar collector was constructed that the back side of all-glass evacuated tube was coupled with compound parabolic concentrator（CPC）.In addition,a combination of theoretical modeling,numerical solution and experimental verification was used to evaluate and analyze the photo-thermal conversion characteristics of the CPC vacuum tube.The obtained research results could provide some technology references for the efficient utilization of solar energy resources and the optimization design of solar collector system.The research work in detail and its results mainly include the following:1.According to the reflection law of geometric optics and the theory of plane analytical geometry,the mathematical model of CPC reflector was deduced.Based on the concentrating characteristics of CPC reflector on solar light,the physical model of the CPC evacuated tube was established,and its optical characteristics were analyzed and verified by simulation.The inherent law which the optical efficiencies of evacuated tube varied with projected incident angle and their mathematical relationship were revealed.2.A mathematical model of steady heat transfer for the CPC vacuum tube was established according to the energy conservation,and solved by iterative calculation based on the home-built C programming language.The experimental verification system for photo-thermal conversion was also constructed.The obtained numerical solutions fit in well with the experimental results,and the relative errors were no more than 9%.3.From the perspective of thermal resistance,the heat transfer process of evacuated tube was analyzed.The results indicated that the vacuum interlayer between absorber tube and cover tube played a crucial role in hindering heat transfer.The thermal-radiation resistance between the outer wall of cover tube and the sky(R_co-sky,rad)was far greater than the thermal-convection resistance between the outer wall of cover tube and surrounding air(R_co-air,conv),so the convective heat transfe between the outer wall of cover tube and surrounding air dominated the heat loss from cover tube to ambient.4.Under the weather conditions of various proportions of diffuse irradiation(R_dif),the experimental results revealed that the CPC had a significant advantage and potential in improving the performance of vacuum tubes.Compared with ordinary vacuum tube,the CPC enabled the the temperature rise of working water in the vacuum tube to increase by24.2%～29.3%,with the thermal efficiency of vacuum tubes 24.3%～29.2%.5.The influence of the main parameters on the calculation was analyzed and discussed.The results demonstrated the correction factor of CPC reflector（M_f）should was in the range of0.8～0.9.When the temperature on the outer wall of absorber tube(T_ao)did not exceed 360K,it is reasonable and reliable to calculate with an average emissivity of the selective absorption coating as a constant.The deviations of the tilt angle of the vacuum tube（β）had no significant effect on the acquisition of solar beam and diffuse radiation.