The XR nonimaging photovoltaic concentrator Aleksandra Cvetković a , Maikel Hernández b , Pablo Benítez a , Juan C. Miñano a , Ruben Mohedano b , Asunción Santamaría a a Universidad Politécnica de Madrid (UPM). ETSI Telecomunicación, C. Universitaria, 28040 Madrid SPAIN b Light Prescriptions Innovators Europe, Marques de Urquijo 14, 28008, Madrid, Spain ABSTRACT A novel XR nonimaging concentrator is presented. The concentrator is formed by one reflective (X) and one refractive (R) free-form surface, calculated by the SMS method. The application of SMS method to this configuration consists of the simultaneous design of both optical surfaces. As it is common in the SMS devices, the XR concentrator achieves high efficiency with a low aspect ratio (thickness to entry aperture diameter ratio), and can perform close to the thermodynamic limit of concentration (maximum acceptance angle for a given geometrical concentration). The photovoltaic system is based on this new concentrator design and 1 cm 2 multijunction tandem cells (developed by Spectrolab with predicted efficiency of 40% at 25ºC). For the geometrical concentration of 800x the preliminary results show the acceptance angle of ±2 deg. Two different strategies for the improvement of irradiance distribution on the solar cell, for two different designs will be presented: (1) The XR with an ultra-short kaleidoscopic prism, (2) the non-rotational XR design with 3D Kohler integrating lenticulations; this last design is based on the latest advances in the Nonimaging Optics design techniques. INTRODUCTION Concentrating the sunlight onto the smaller solar cells has always been a promising technique for reducing the cost of photovoltaic solar energy. The additional optical elements add complexity to the system, which has to deal with the problems as cooling of the cells, high local irradiances and optical precision, so for getting the high efficiency and low cost, only a few optical surfaces can be used. One of the keys for the adequate performance and commercial success of a photovoltaic concentration system (CPV) is the acceptance angle. This becomes more critical when high concentration is needed; this is the case when using multi-junction solar cells. The high acceptance angle system allows, from the manufacturing point of view, high mass production tolerances of all its components. Thus the module assembling, system installation and the cost of the structural elements constrains can be relaxed and the global cost of the concentration system could potentially decrease. Today a very few devices adaptable for photovoltaic application work close to the thermodynamic limit of concentration. Here we present one of these designs, that is called XR. It was developed for wireless optical communications, and its adaptation and optimization for photovoltaics is now under progress in a consortium led by The Boeing Company. The general objective is to co- develop a cost effective high-concentration wide- acceptance PV system. The XR concentrator has been designed with the Simultaneous Multiple Surface optical design method in two dimensions (SMS2D) [1]. The SMS is the most advanced design method in nonimaging optics (see Chapter 8 in ref. [2]), which is the branch of optics dealing with maximum efficiency light transfer problems. The XR is a nonimaging lens-mirror combination composed by a primary rotational symmetric aspheric (non-parabolic) mirror and by rotational symmetric aspheric secondary lens which encapsulates the solar cell (see Figure 1). The nomenclature XR stands for the reflection (X) and refraction (R) that the sunlight undergoes from the input aperture to cells. Contrary to the classical parabola plus nonimaging secondary configuration, the simultaneous design of the two optical surfaces of the XR allows the concentrator to be very compact (around 0.25, to be compared to 1-1.5 for the classical systems), without sacrificing performance. The XR can even reach isotropic illumination of the receiver (β = 90º), if needed. Figure 1. Cross section of an XR concentrator