| In order to cope with the energy crisis and environmental pollution,the transformation of the world energy structure is imminent.Solar energy has great potential as a pollution-free renewable energy source,but its energy density is low and difficult to use,concentrating collectors can help solve this problem.The receiver is the core component of the opticalthermal conversion of solar collectors,and the power generation efficiency is directly affected by the optical-thermal performance of the receiver.The cavity receiver is widely used in the field of solar heat utilization because of its stable structure,simple manufacture,and low maintenance cost.However,their optical-thermal efficiency is not high enough,and the radiation heat loss on the absorption surface leads to the poor thermal performance of the receiver,so its performance needs to be optimized.In view of the relatively high radiation loss of the absorption surface,based on the radiation heat transfer theory of the enclosure,a cavity receiver with secondary reflectors was designed for parabolic trough collectors in this paper.The optical model and the heat transfer model of the cavity receiver were established,and the influence of the length and tilt angle of the secondary reflector on the heat loss of the cavity receiver was studied.The numerical simulation shows that the heat loss of the receiver decreases with the increase of the length of the secondary reflectors,and the optical-thermal performance of the receiver is optimal at a reflector length of 15 mm and a tilt angle of 90°.Through the optimized cavity receiver,the optical-thermal efficiency of the system could be increased by 1.8%.The total heat loss could be reduced by up to 26.3% at an absorber surface temperature of 423 K,compared with the cavity receiver without reflectors.The performance of the receiver can be further improved by reducing the percentage of radiation heat loss of the absorber surface.Therefore,a cavity receiver with the selective transparent film was proposed in this paper.The optical model and the heat loss model of the receiver were established with band approximation.The optimal cut-off wavelength of the ideal selective transparent film and the influence of actual film on the optical-thermal performance of the receiver under various concentration ratios and surface temperature were explored.It was found that the optimal cut-off wavelength of the ideal film was distributed around 1.3μm,1.8μm,and 2.5μm,the optical-thermal efficiency of the absorber could be improved by 5.7-58.2% by the ideal film.In this paper,TCO film was used as the actual selective transparent film.Compared with no film condition,setting TCO film could improve the optical-thermal efficiency of the absorber by at most 25.1% at the absorber surface temperature of 873 K and the concentration ratio of 30.However,the optical-thermal efficiency of the absorber would be reduced by setting TCO film under the condition of high concentration ratio as well as low surface temperature.The range of operating conditions under which the TCO film could improve the optical-thermal efficiency of the absorber was concluded,providing an analytical method for the application of selective transparent films in engineering practice. |
