The hybrid Photovoltaic/Thermal (PVT) systems (Fig. 3-i) can be combined with linear Fresnel lenses (Fig. 3-ii) and can be used for space heating and cooling of building interior spaces. The Fresnel lenses with the hybrid PV/T absorbers (FRESNEL/PVT system) aim to maximise the energy conversion from Fresnel lens type solar energy systems, which can be used as transparent material of building atria (Fig. 4-i) or sunspaces (Fig. 4-ii). From the performed laboratory experiments most of the transmitted solar radiation from the lens glazed roof can be absorbed, controlling the illumination of the interior space and providing also electricity and heat to cover several building energy needs. The shading/cooling effect by the suggested system (Fig. 4-ii, b) can adapt about 50% of the needs, only from the heat extraction by the absorber operation, which can be higher if we consider fan or AC operation by the provided electricity from the PVs. In case of using Fresnel lens with 42 cm focal length, the collection of 60%-80% (Fig. 5-i) of the transmitted solar radiation through the transparent cover leaves the rest amount of solar radiation to be distributed in the building space for the illumination needs. Test results showed that a considerable lighting and temperature reduction in the interior space is achieved [22].

Fig. 3 The basic designs of PV/T collectors (i) and the Fresnel lens concept with the absorber types (ii)

An alternative system design is the integration of the linear Fresnel lenses on building fa$ade or inclined roof and the absorber to be PV cells of smaller strip width, to receive the peak of the converged to focal line solar rays. In this design, the non-used converged radiation and also the diffuse radiation by the cells can be absorbed by flat or cylindrical elements placed in a small distance from PV strip (Fig. 5-ii). These elements form an air duct with system thermal insulation and the air can circulate through it to achieve building ventilation. In this system the cell material is reduced (lower PV module cost) and all not-used solar radiation by PV cells is absorbed for effective water heating.