Michele Pellegrino, G. Flaminio, S. Bolognesi and C. Privato

ENEA Centro Ricerche, Localita Granatello. P. O. Box 32,1-80055 Portici (NA), Italy.

tel:+39-81-7723-267, Fax:+39-81-7723-344; mail to Michele. Dellearino@_nortici. enea. it

Though on theory an integrated PV/T should have been proved to be more efficient than the simple addition of a thermal collector and a PV module, in practice this has very rarely happened in spite of the positive testing of components in research laboratories worldwide. Reasons have been very different. Number one the two technologies have never been very well matched each other: if for the PV part the cooling or the ventilation has proved to be of some usefulness in terms of increased efficiency, for the thermal collector the performance is surely worst than the traditional component, since while the low operating temperature is good for the PV part component at the same time gives problems for the thermal collector design and for the thermal output outlet water with very low enthalpy, t< than 50 °C. Number two there is a seasonal mismatch, since it just in the summertime when there is the best insolation that the thermal recovery is less necessary and for the high glassy fagade of buildings there could be not need for thermal contribution even in the winter. Possible solution could be represented by the thin film technology, especially in term of amorphous materials. In fact this material is less sensitive to the increasing of temperature and it is boasted for even a positive coefficient of temperature in some cases, so allowing the PV/T component to work at a little higher temperatures. The other opportunity is the Photovoltaics Building Integration that could bring advantages from an economical point of view, for the waited costs reduction and the less competition for the required area, and from a technical point of view for the transition toward a real integrated PV/T system with optimised materials. In addition each sector can benefit of the other experience, the high tech nature of PV for the thermal collector and the market sectors and the existing standards for the PV. Still it could be possible to work on the demand side by making more efficient and more appropriately designed buildings with heat storage system for the higher insolation periods. The paper intends to give a contribution to the understanding of the obstacles that hinder the developing of this technology. At the same time results on a PV/T module made of a tandem amorphous Si are presented.

Introduction

Usually the performances of PV modules are determined at Standard Conditions, e. g. junction temperature of 25 °C and irradiance of 1000 W/m2 at AM 1.5, corresponding to the irradiance at zenith at the sea level and at the latitude of 45 °. Of course that situation does not happen very often during the normal operation conditions and for instance the temperature is several degrees Celsius higher than the room temperature, since even in the case of very efficient cells the energy converted into electricity is only a small fraction, normally between the 5-15 % of the incoming radiation. The international Standard IEC 1215 for the approval and the qualification of the module type prescribes the determination of the NOCT the Nominal Operating Temperature Condition, that it is nearer to the real operating conditions temperature.

The increase of temperature has an effect on the efficiency since its decreasing with the temperature is about 0.4 % for the crystalline and 0.2 % for the amorphous silicon as it can be seen in figure 1.

To cope with this problem the module ventilation, usually natural or even forced, is provided. Another good solution, even though more complicated, is to draw the heating by means of a fluid inside the device so obtaining the PVT Photovoltaic Thermal module, figure 2. In that way the benefit is double:

a) cooling of the photovoltaic part increasing the peak power,

b) producing a warm fluid for the domestic heat water or space heating application.

But even if this seems reasonable in theory, on practice there have been many obstacles that have hindered the diffusion process. The paper tries to give some possible reasons for the difficulty that technology has experienced and the possible solutions together with some experimental data on a tandem amorphous PVT Si module.