Study On Optical Properties Of Three - Dimensional Asymmetric CPC With Full Internal Reflection Cross Under Horizontal Uniaxial Tracking

Total internal reflection based CPC（ITR-CPC）, sharing the advantages of a wide range of acceptable angle for solar radiation and no optical loss due to reflections, have received much attention in recent years. The symmetric three-dimensional CPC can be produced by either rotating a 2D-CPC 360 o about its optic axis（truncated cone shaped 3D-CPC, T-3D-CPC） or crossing two 2D-CPC（C-3D-CPC）, but the asymmetric three-dirmensional CPC must be produced by crossing one asymmetric 2D-CPC with another symmetric or asymmetric CPC（C-3D-ACPC）. For 3D-CPC used in systems tracking the sun by horizontal south-north axis, the projection incidence angle of solar rays on the plane(θ_p,//), parallel to sun-tracking axis and normal to sun-tracking surface asymmetrically varies in a wide angle in one year, therefore, the asymmetric 2D-CPC（2D-ACPC） perpendicular to sun-tracking axis must be used for efficient radiation concentration. However, with the increase of azimuth angle of sun-tracking axis deviating from the south-north direction, the annual variation of θ_p,// becomes more symmetric but the annual collectible radiation decreases. Therefore, to the first concern in the design of C-3D-ACPC is to determine how the asymmetry of acceptance angle of 2D-ACPC affects the optical performance of C-3D-ACPC for the design of C-3D-ACPC based horizontal axis sun-tracking system.In this work, a comparison of optical performance between TIR based T-3D-CPC and C-3D-CPC with identical acceptance half-angle and geometric concentration was made based on ray-tracing analysis first, and results show that the optical efficiency of T-3D-CPC averaged over its acceptance half-angle is slightly higher that that of C-3D-CPC. Then three C-3D-ACPC, formed by crossing a symmetric 2D-CPC oriented in the sun-tracking axis（S-CPC） and an asymmetric 2D-CPC perpendicular to sun-tacking axis（A-CPC）, were designed and theoretically investigated by ray-tracing analysis in terms of optical efficiency, and annual collectible radiation was estimated when they are applied to systems tracking the sun about south-north horizontal axis. In these C-3D-ACPCs, the acceptance angle of S-CPC is set to 40o（acceptance half angle is 20o）, the acceptance angle of A-CPC is also set to be 40 o but not symmetric for two reflectors. The acceptance of A-CPC in one of these C-3D-ACPCs is determined based on the requirement ensuring the sun with its acceptance angle for at least 10 hours in all day of a year in Kunming. Results show that the optical efficiency increases first then decreases with the increase ofp,/ /q, and with the increase of acceptance half-angle difference between two reflectors of A-CPC, the optical efficiency averaged over its acceptance angle decreases.Finally, the annual radiation collected by C-3D-ACPCs in the system tracking the sun by south-north horizontal axis was estimated based optical efficiency obtained by ray-tracing analysis, solar geometry and monthly horizontal radiation data. Results indicate that C-3D-ACPCs with a small acceptance half-angle difference between two reflectors of A-CPC perform better in sites with lower site latitude, whereas those with high acceptance half-angle difference between two reflectors of A-CPC perform better in the sites with high site latitude in terms of annual collectible radiation and annual average optical concentration. This means that, C-3D-ACPCs used in the system tracking the sun by horizontal south-north axis should be designed based on site latitude where they are used.