The Planar Construction Of Compound Parabolic Concentrators And The Research On The Condensing Properties

Solar energy,as an emerging renewable energy source,holds the promise to replace fossil fuels without sacrificing our living standard.To utilize this natural resource more efficiently,solar concentrators are widely used in solar thermal collection systems as an effective means to improve system collection temperature and heat collection.Compound parabolic concentrator(CPC)is a typical non-imaging type of concentrator,which has the advantages of good system stability,high optical efficiency and a wide range of acceptable angles.However,the existing CPCs also have some problems that need to be solved.The large-curvature cylindrical glass mirrors used to make the reflective surface of CPCs are relatively costly in terms of manufacturing,transportation and maintenance,and the energy density distribution on the surface of the CPC absorber is extremely non-uniform.In view of the defects and shortcomings of existing CPCs,this paper proposed an optimized design method of multi-sectioned compound parabolic concentrators,aiming to further optimize the energy-gathering characteristics of CPCs and reduce the comprehensive cost,conducts a comparative study of the key energy-gathering indexes of concentrators.The research results obtained in this paper could provide some technical references for the optimal design of concentrators and the efficient utilization of solar energy.The research work and results of this paper include the following aspects:1.An optimization design method of multi-sectioned compound parabolic concentrator(M-CPC)based on the Monte Carlo ray-tracing method is proposed.Further,a multi-objective genetic algorithm is combined with the design method,and a CPC with novel structure is optimized based on it.Five M-CPCs with different total number of flat plate reflective surface are constructed.The combination of multi-objective genetic algorithm and the optimization design of M-CPC can reduce a large number of meaningless calculations and improve the efficiency and accuracy of the calculation.An M-CPC solid model is fabricated by additive manufacturing technology,and a laser verification experimental platform is built.Good agreement is shown between the experimental and calculated results,with a maximum error of 2.20 mm and an average error of 0.74 mm.2.Calculations are carried out for the energy-gathering characteristics of the M-CPCs constructed in the paper before and after optimization,and the results showed that all the optimized M-CPCs could significantly improve the uniformity of energy density distribution on the absorber and reduce the peak energy density while maintaining high optical efficiency.Under the typical irradiation conditions with the solar radiation energy density of 1000 W/m~2,M-CPC5 could reduce the average energy non-uniformity coefficient from 2.23 to 0.92,while its peak energy density can be reduced from 46002.67 W/m~2 to 2851.00 W/m~2.The flat plate reflective surface structure could avoid the generation of the high energy spots and could protect the glass absorber and the selective absorption coating on the surface.3.The numerical solution and analysis of the acceptable angle of M-CPC before and after optimization showed that the acceptable angle range of M-CPC is expanded.The acceptable angle range of M-CPC5 is expanded from-60°-60°to-70°-+70°.compared with the standard CPC.Moreover,the M-CPC could solve the problem of sudden cooling and heating of the absorber caused by the sudden change of energy density,which could improve the stability of the system and reduce the selectivity of the concentrator system to the sky radiation conditions.The analysis results also show that with the increase of the total number of flat plate reflective surface of M-CPC,the concentrator properties would become closer and closer to those of CPC as the base model.4.Based on the regional typical meteorological year data and the theory of direct dispersion separation of solar radiation,the mathematical model of annual collected radiation of the concentrator is constructed,and the numerical solution of annual average collected solar radiation energy of five M-CPCs is obtained by calculation.The results show that the total annual collected radiant energy of M-CPC5 is 3717.86 MJ/m~2under the lighting condition that the optical aperture is placed horizontally along the north-south direction,and the annual collection efficiency is 68.39%.About 43.6%of the total collected radiant energy comes from sky diffuse radiation,which could help the CPC resist weather transients.The horizontal placement avoids the bracket structure at the bottom of the concentrator and maintains the low center of gravity of the concentrator.It also avoids the shading problem between the concentrators and improves the utilization of the site.