Design and experimental test of refractive secondary optics on the electrical performance of a 3-junction cell used in CPV systems
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This work analyzes and compares the effects of the secondary optics on the performance of a triple junction solar cell used in a compact HCPV prototype unit. The HCPV system is composed of triple junction III-V (Ga0.5In0.5P, Ga0.99In0.01As and Ge) solar cells that have a circular shape with an active area of 4.15 mm2. The optics consists of a primary PMMA square Fresnel lens (75 mm – side) with constant pitch and a refractive secondary optic (RTP) made of dielectric material. The overall geometrical concentration ratio is 1300×. The tracking system is a tip-tilt type two-axis mechanism driven by stepper motors. The HCPV secondary optics were firstly designed at Polo Tecnologico Andrea Galvani and then tested on the prototype unit at the Engineering Faculty of Università Politecnica delle Marche in Ancona, Central Italy. The aim of the paper is to present the numerical and experimental performance results of two different secondary optics and to assess the effects of the main construction parameters (i.e.: the geometry of the secondary optics and the distance between the two optics) on the concentration efficiency. Moreover, two different 3 J cell receiver types were tested, the Insulated Metal Substrate technology (IMS) and the Direct Bonded Copper (DBC) technologies. The experimental tests were performed under real outdoor operating conditions, therefore also the Direct Normal Irradiance (DNI) was measured. In general, the free-form optics showed significant improvements in terms of overall irradiance and homogeneity. In the best configuration the electric efficiency achieved 39.55%, neglecting the primary optics losses, thus confirming that the presented system setup is able to reach the highest standards of CPV technology performance.