In 1981 Florschuetz [4] remarked that the use of air as an active refrigeration system is not a viable alternative because of its low thermal capacity and diffusivity. He found that water is a fluid whose properties allow for a better thermal interchange and consequentially the achievement of higher concentrations without the negative effects of the temperature over the PV cell efficiency.

After this study, a group of authors developed a series of active cooling systems using water (Edenburn, O’Leary and Clements, Chenlo and Cid, Russell). Although each system used the water cooling device to optimise conditions for electricity production, none of them analyse the possibility of taking advantage of the thermal energy produced by the warming up the water.

At present, there are two principal systems which optimise both the electricity production and the thermal energy production:

• CHAPS (Combined Heat And Power Solar), developed at the Australian National University. It consists of a parabolic concentrator with a ratio of 37X which focuses radiation onto a PVT module. The module converts the radiation into thermal and electrical energy with efficiencies of 57% and 11% respectively. The prototype was initially designed as a photovoltaic system with active cooling, the idea later evolved to use the water to capture the thermal energy. Reference data of the thermal gain achieved by the collector is not mentioned in any of the reference publications for the system [5].

• BIFRES, developed at the University of Lleida, is a system which concentrates radiation by Fresnel reflection to a concentration factor of 22X. The hybrid module operates with a nominal thermal efficiency of 59%, permitting the c-Si photovoltaic cells to operate at an optimum efficiency of 11.9% [2].

Both systems positively satisfy the requirements of actively cooling the cells whilst acting as a thermal collector with acceptable efficiencies, above 50%. However, in both cases the PVT module design is not straightforward. Both groups have opted for a tube of circular cross-section appended to an absorber on which the photovoltaic cells are placed. The two systems have significant differences: the heat sink is made of aluminium in the CHAPS system and of copper in the BIFRES system, also the tube developed at the ANU is furrowed with the goal of improving convection into the fluid.

After analyzing these two systems, some improvements may arise: It is well known that rectangular sections have higher Nusselt numbers than circular or square sections. A section with a higher aspect ratio (a), permits a greater thermal interchange into the fluid, where (a) is defined as the quotient between the long and short side of the rectangle [6]. Besides, an attractive concept such as the architectural integration is not well solved in the majority of PVT systems. As a consequence of their dimensions, PVT systems are only suitable for installation on flat roofs.

In this research is proposed, with the same concentration ratio than in the other systems explained before, to reduce the dimensions of the concentrator and the absorber to facilitate the integration in buildings.