Effect of the Secondary Optics and the Receiver Design on the Performance of a Triple Junction Solar Cell

In this paper, the prototype of a compact and small scale HCPV system with two-axis tracking mechanism for building integration purposes is presented and the experimental measurements are reported. The system is composed of 4 groups of optics and photovoltaic cells, each containing 8 receivers. The cells are triple junction III-V (Ga0.5In0.5P, Ga0.99In0.01As and Ge) and they have a circular shape with an active area of 4.15 mm2. The optics consists of a primary and a secondary optics: the former is a PMMA square Fresnel lens (75 mm – side) with constant pitch; the latter is a refractive optics (RTP) realized with dielectric material, that i) improves the light homogeneity on the cell area, ii) reduces the negative effects of the sun-tracking misalignment and iii) increases the concentration ratio. The overall geometrical concentration ratio is 1300x. The two-axis tracking system is a tip-tilt type driven by three stepper motors, one for the tip movement and the other two for the tilt movement. The alignment with the sunrays is guaranteed by an algorithm that controls the electrical motors. The HCPV prototype was tested at the Engineering Faculty of Università Politecnica delle Marche in Ancona, Central Italy. The aim of the paper is to present the performance results, both simulated and experimental, of two different secondary optics and to assess the effect of the distance between the primary and the secondary optics on the concentration efficiency. In addition, two different solutions for the 3J cell receiver where tested, the Insulated Metal Substrate technology (IMS) and the Direct Bonded Copper (DBC). The electrical I-V curves are presented in order to assess the performance of the CPV system using the two different solutions. The experimental tests are realized under real outdoor operating conditions, therefore also the direct normal radiation (DNI) was taken into account.