The concentrating solar systems use reflective (flat or curved mirrors) and refractive (mainly Fresnel lenses) optical devices. These solar energy systems are characterized by their concentration ratio (CR, or simpler C) and can be combined with “linear focus” (2D) or “point focus” (3D) absorbers for low (C<10X), medium (C<100X) or high (C>100X) ratios, respectively. Concentrating systems with C>2.5X must use a system to track the sun, while systems with C<2.5X can operate with stationary concentrating devices. The low concentrating ratio systems (C<10X) are of particular interest for the photovoltaics as they are of linear geometry and thus one tracking axis is enough for their efficient operation. The distribution of solar radiation on PV module and the temperature rise of it affect the electrical output. The uniform distribution of the concentrated solar radiation on PV surface and the application of a suitable cooling mode contribute to an effective system operation, considering the achievement of the maximum electrical output. In low concentration photovoltaics, flat and curved reflectors, Fresnel lenses and dielectric lens type concentrators have been studied. The performed works can be grouped in systems with V-trough reflectors [1-4], achieving concentration ratios up to two with east-west or north-south orientated reflectors, CPC (Compound Parabolic Concentrator) type reflectors [5-10], which are usually static and CR<2.5, refractive concentrators of 3D acrylic lens [11-13] and linear Fresnel lenses [14-16]. Comparison results give an idea about the benefits of concentrating photovoltaics and point-focus concentrating systems with a fixed flat plate PV module [17-19] show that the concentrating systems produces 37% greater electrical energy than the flat PV modules.

In the University of Patras, research works on low concentration photovoltaics have been performed last years [20-25]. PVs can be combined with thermal collectors to form hybrid photovoltaic/thermal systems, which can be used to buildings contributing to the reduction of the required available roof or faqade installation surface area. In PV/T system applications and considering that the electricity is of priority, the operation of PV modules at lower temperatures is necessary in order to keep PV cell electrical efficiency at a sufficiently higher level [23]. This demand limits the effective operation range of PV/T system thermal unit in lower temperatures and the extracted heat can be mainly used for low temperature thermal needs (space heating and natural ventilation of buildings, air or water preheating, etc). To overcome it, the system can be combined with a typical thermal collector system circulating the preheated water of PV/T unit in the lower part of it [24]. The PVs and the PV/T systems can be combined with linear concentrators, as of Fresnel lenses or flat type and Compound Parabolic Concentrating (CPC) reflectors to achieve cost effective solar energy conversion systems [25]. This subject is very interesting for the practical application of the photovoltaics, which would result to a wider application of these solar energy conversion systems.