Investigation of thermal aspects of building integrated photovoltaic/thermal solar collectors

Abstract

In this study a novel building integrated photovoltaic/thermal (BIPVT) solar collector was developed, tested, modelled and optimised both experimentally and theoretically. Experimental testing found that glazing the prototype collector improved the maximum thermal efficiency by approximately 25% and decreased heat loss, by a factor of two, relative to an unglazed collector. Additionally, the spectral absorptance of a photovoltaic (PV) cell and several coloured absorber samples were characterised.

The experimental data was subsequently used in the development and validation of an optimisation model for BIPVT style collectors. Numerical optimisation showed that the collector thermal efficiency could be improved by maximising the geometric fin efficiency, reducing the thermal resistance between the PV cells and the absorber, and by increasing the transmittance-absorptance product of the PV cells and/or the absorber. The results showed that low cost materials, such as mild steel, could be used without significantly affecting the BIPVTs thermal efficiency. It was also shown that there was potential to develop coloured BIPVT collectors with acceptable thermal efficiencies. Finally, the model showed that potentially the air space in an attic could be used rather than traditional insulating materials.

Subsequent computational and experimental fluid dynamics studies found that the heat transfer coefficients in a scale-model attic would result in R-values similar to mineral wool type insulation and therefore may provide sufficient insulation of a BIPVT in a cold roof building. In these studies the validity of an existing correlation for natural convection in an attic-shaped enclosure was extended to Grashof numbers in the range 10^7 to 10^9 from its previous range, 2.9 x 10^6 to 9 x 10^6.

The use of a single vertically mounted baffle was also found to reduce the natural convection heat transfer coefficients in attic-shaped enclosures. This led to the development of a new generalised correlation that can be used to determine the Nusselt number in an attic-shaped enclosure with regard to the proportions of the baffle.

This work has shown that it is possible to achieve satisfactory thermal performance from BIPVT style collectors fabricated from low cost materials such as colour coated mild steel. Further it has demonstrated that there is potential to reduce the cost of such systems by integrating them into a building rather than onto a building.