Thermal-photovoltaic solar hybrid system for efficient solar energy conversion Yu. Vorobiev a, * , J. Gonza ´lez-Herna ´ndez b,1 , P. Vorobiev c , L. Bulat d a CINVESTAV-Quere ´taro, Libramiento Norponiente 2000, Quere ´taro 76230, Queretaro, Mexico b CIMAV, Miguel de Cervantes 120, Chihuahua 31109, Mexico c Facultad de Ingenierı ´a, Universidad Auto ´ noma de Quere ´taro, Queretaro, Mexico d St. Petersburg State University of Refrigeration and Food Engineering, St. Petersburg 191002, Russia Received 1 November 2004; received in revised form 13 April 2005; accepted 18 April 2005 Available online 2 August 2005 Communicated by: Associate Editor Xavier Matthew Abstract A hybrid solar system with high temperature stage is described. The system contains a radiation concentrator, a pho- tovoltaic solar cell and a heat engine or thermoelectric generator. Two options are discussed, one with a special PV cell construction, which uses the heat energy from the part of solar spectrum not absorbed in the semiconductor material of the cell; the other with concentration of the whole solar radiation on the PV cell working at high temperature and cou- pled to the high temperature stage. The possibilities of using semiconductor materials with different band gap values are analyzed, as well as of the different thermoelectric materials. The calculations made show that the proposed hybrid sys- tem could be practical and efficient. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Photovoltaic panel; Energy flux concentrator; Heat engine; Thermoelectric generator 1. Introduction A key problem in solar cell science and engineering is the limited efficiency of solar energy conversion by pho- tovoltaic cells, which results from the impossibility of an efficient utilization of the wide solar spectrum with only one semiconductor material. To overcome this problem, the combinations of different materials have been consid- ered (multi-junction cells, see Henry, 1980; Sze, 1981; Yamaguchi, 2003), as well as the concept of a special material with an intermediate band of electronic levels within the band gap (Luque and Martı ´, 1997, 2001). All these devices (both existing and hypothetical ones) are very sophisticated and expensive, while their effi- ciency is still not satisfactory. Theoretical limits of photovoltaic conversion efficiency (Landsberg and Bade- scu, 1998; Landsberg, 2002) for a multi-junction cell pre- dicts an efficiency of about 90%, but in practice not even half of that value has been obtained. On the other hand, the solar energy converters using a high temperature 0038-092X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2005.04.022 * Corresponding author. Tel.: +52 442 4414 916; fax: +52 442 4414 916/938. E-mail address: vorobiev@qro.cinvestav.mx (Yu. Voro- biev). 1 On sabbatical leave from CINVESTAV. Solar Energy 80 (2006) 170–176 www.elsevier.com/locate/solener