In hybrid PVT/AIR systems the thermal unit for air heat extraction, the necessary pump and the external pipes for fluid circulation constitute the complete system that extracts the heat from PV modules and transfers it to the final use. Practical considerations in PVT/AIR system design include the evaluation of the thermal and electrical efficiency improvement with respect to the system cost. The hybrid PV/T systems consisting of PV modules freely exposed to ambient temperature without any thermal protection have high top thermal losses and therefore their operating temperature is not high. To increase the system operating temperature, an additional transparent cover is necessary (like the glazing of the typical solar thermal collectors), but this has as result the decrease of the PV module electrical output because of the increasing reflection and absorption of the solar radiation.

In most PVT/AIR systems the air circulates through a channel formed between the rear PV surface and the system thermal insulation, and in some other systems through channels on both PV module sides, in series or in parallel flow. The usual heat extraction mode is the direct air heating from PV module rear surface by natural on forced convection and the thermal efficiency depends on channel depth, air flow mode and air flow rate. Small channel depth and high flow rate increase heat extraction, but increase also pressure drop, which reduces the system net electrical output for the forced air flow, because of the increase of the fan power. In applications with natural air circulation, the small channel depth reduces air flow and this results to an increase of PV module temperature. In these systems large depth of air channel (minimum 0.1 m) is necessary (Bhargava et al, 1991).

The aim of our research is to design efficient PVT/AIR systems based on low cost modifications. The main concepts are concerned with the reduction of PV module temperature, the improved air heat extraction by the circulating air and the reduction of heat losses by using thermal insulation on system back side and edges. A low cost modification was investigated in University of Patras, by which satisfactory air heating, reduced PV module temperature and low increase of the opposite channel wall temperature can be achieved (Tripanagnostopoulos et al, 2000b). This modification consists of a thin flat metallic sheet (TFMS) placed inside the system air channel and along the air flow. This TFMS element doubles the heat exchanging surface area in the air channel, aiming also to the reduction of the heat transfer to the back side of the PVT/AIR system. The experimental models were constructed in the University of Patras and consisted of commercial type mc-Si PV modules in combination with a laboratory made air heat recovery unit (HRU). The experimental study is based on two hybrid PVT/AIR module designs (Tripanagnostopoulos et al, 2002) of same aperture surface area Aa (Aa = 0.4 m2). The first model is that of the simpler form without additional glazing, named PV/T UNGLAZED or PVT/UNGL and the second model is that with the additional glazing, the PV/T GLAZED or PVT/GL. These experimental models have an air duct of 10 cm width and a thermal insulation layer to avoid thermal losses from the non-illuminated system surface sides.

We consider that these systems can be installed on the horizontal roof, on the tilted roof or on the fagade of a building. Horizontal and tilted roof installation are more interesting at low latitudes, while building fagade (and high tilted roof) installation are more effective for medium and high latitude applications because of the lower sun altitude angles. In the case of tilted roof system installation, the PV/T systems are placed one beside the other and on the southern building inclined roof surface. The tilted roof integrated systems are additionally thermally insulated on their rear surface, compared to the ones installed on horizontal roof, as they are attached on the tilted roof. The additional thermal protection

increases the thermal efficiency of the system, but the lower thermal losses keep the PV temperature at a higher level, operating therefore with reduced electrical efficiency.

The systems that are considered for installation on the tilted building roof are the standard PV modules and the usual type PVT/AIR systems. The experimental models that simulate the PV and PV/T systems for the tilted roof installation, are the PV-TILT, PVT/UNGL-TILT and PVT/GL-TILT models. They are consisted of the same basic system units (PV, PVT/UNGL and PVT/GL) and were tested with an additional thermal insulation on their rear surface, considering the thermal protection of the tilted roof to the attached system thermal losses from this surface.

Considering PV/T solar systems installed on horizontal building roof the parallel rows keep a distance from one row to the other, in order to avoid PV module shading. We suggest the use of stationary flat diffuse reflectors (REF) placed properly between the PV modules from the higher part of the one row of them to the lower part of the next row. This installation increases solar input on PV modules almost all year, resulting to an increase of electrical and thermal output of the PV/T systems. The systems that combine the PVT/AIR modules with diffuse reflector are the models PVT/UNGL+REF and PVT/GL+REF. The suggested diffuse reflectors differ to the specular reflectors, as they avoid the illumination differences on module surface and the reduction of the electrical efficiency, because they provide an almost uniform distribution of reflected solar radiation on PV module surface. The systems were tested with slope equal to the latitude of Patras (38.25o). In the experiments with the diffuse reflector, the PVT/AIR systems were tested for variable additional solar radiation (concentration ratio CR=1.0 to CR=1.5) to get data for different angles of incidence between system and sun.

Aiming to the increase of PVT/AIR performance three prototypes with the TFMS modification were constructed and tested by University of Patras. These models were all of unglazed type and were distinguished in pVt/TFMS, the simple type for horizontal roof, the PVT/tFmS+REF, the system with the diffuse reflector and the PVT/TFMS-TILT, the system for tilted roof installation. The systems consisted of the same basic unit (mc-Si PV module and HRU of 10 cm air channel) with an additional thin aluminium sheet in the middle of the air channel and parallel to the air flow.