Analysis of High Concentration Passively Cooled CPV Module Designs Using Mirror Optics

Abstract

M. Dreger, M. Wiesenfarth, T. Schmid and A. W. Bett Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstrase 2, 79110 Freiburg, Germany, max.dreger@ise.fraunhofer.de, phone: +49 761 4588 5007, fax: +49 761 4588 8555 ABSTRACT: In high concentrating photovoltaic (HCPV) modules the solar radiation is focused on high efficient multi-junction solar cells. One option to distinguish between different system approaches is the applied optics. For example, Frensel lenses or parabolic mirrors can be used as primary optics. Another aspect of the module design is how the thermal energy is distributed. There are actively and passively cooled systems. In any case, the way of thermal management is essential for the high performance of the system. In this paper passively cooled systems with mirrors as primary optics are investigated. Different designs of modules with on-axis, off-axis and Cassegrain optics were chosen and evaluated in terms of thermal management, optical efficiency, acceptance angle and manufacturing efforts. For the analysis of the optical and thermal behavior, ray tracing and FEM simulations were applied. In terms of temperature of the solar cell the Cassegrain design has the lowest operating temperature as the heat spreader does not shade the primary optics and can be generously dimensioned. Comparing the acceptance angle and the optical efficiency a special on-axis design with the heat spreader assembled inside of the module performs best. Keywords: CPV module, module design, mirror optics, passive cooling, thermal management, optical efficiency 1. INTRODUCTION With concentrating photovoltaic (CPV) systems, a cost competitive and robust technology is available to the market. In CPV modules the solar radiation is focused on multi-junction solar cells. The key success factor for the technology is the high efficiency of the multi-junction solar cell. Several system designs have been realized. One approach to classify the different system designs is the primary optics used. For example, parabolic mirrors are applied. This optics is particularly suitable for very high concentrations of 1000x and more. As the solar cell is still costly, increasing the concentration factor is one way to reduce the module costs. In addition, in comparison to lens optics, mirror optics show no chromatic aberration. Another aspect of the module design is how the thermal energy is distributed. There are actively and passively cooled systems. In any case, the way of thermal management is essential for the high performance of the system. Large primary optics can be used if active cooling is used. A well-known example for such a system is the dish system developed by Solar System in Australia [1]. If the passively cooled (so called micro dish) option is chosen, the size of the primary mirror is limited. The advantage is that passively cooled systems need very low maintenance and do not require any failover. An early grid connected passively cooled system installed in 2005 is the MicroDish developed by Spectrolab [2]. However, there are multiple ways to assemble the optics. Hence, there are different system designs as for example developed and demonstrated by SolFocus [3], Beghelli [4, 5], AtemEnergia [6, 7], Soltech Renovables [8] or Brightleafs technology [9]. In this work different concepts for micro dish systems are investigated using theoretical modeling. The advantages and challenges of the approaches are compared. 2 INVESTIGATED CPV MODULE DESIGNS CPV is a particularly integrative technology. Therefore, it is very important to match the design of all components very well. In addition, the manufacturing processes must be planned already in the design phase. These aspects need to be considered when starting a module development. In this paper, different designs with mirror optics and passively cooled receivers are evaluated. All investigated systems were designed for a geometrical concentration of 1000x. The solar cells, electrical contacts and mirror optics are protected against ambient conditions like dust and humidity. This is realized by incasing the components with a frame and cover glass. Thus, high reliability and long-term stability of the module can be guaranteed. The design approaches are different particularly in the arrangement of the optics. On-axis, off-axis and Cassegrain optics were investigated. For each module design, the design of the heat spreader was adapted. In the following the different designs are described. In Design 1, shown in Figure 1, an off-axis optics is used. The light path is indicated in black. The mirror is comprised of a paraboloidal section. The optical axis does not contain the mechanical center of the input aperture. The solar radiation is reflected to the solar cell assembled parallel to the incoming radiation vertically onto the module wall. Hence there is no shading of the mirror by the solar cell.