A. K. Athienitis*, A. Tzempelikos* and Y. Poissant**

*Concordia University, BCEE Dept, Montreal Canada
**CETC Varennes — PV & Hybrid Systems Program, Natural Resources Canada

Introduction

This paper describes an outdoor experimental investigation of double fagades with integrated photovoltaic panels. Results predicted with a simple analytical one-dimensional model are compared with the experimental results and the impact of convective heat transfer coefficient is studied.

Photovoltaic modules may be integrated in buildings to form the exterior envelope layer while generating electricity. Their cost-effectiveness is thus improved in comparison with stand-alone systems that need a separate support structure, particularly when they replace expensive envelope exterior layers such as high quality precast panels, granite or marble. However, their electrical conversion efficiency, which is presently about 15%, is rather low in comparison with thermal systems; typically about 70-80% of the solar radiation incident on the PV panels is lost by convection and infrared radiation to the outdoor environment. By placing the PV modules in the interior of an airflow window (attached to the inner or outer glazing), and passing fresh air through the glazing cavity, we achieve the twin objectives of capturing much of the absorbed solar energy that would otherwise be lost as heat, while cooling the PV panels and thereby raising their electrical conversion efficiency. Thus, the window or double fagade functions as a cogeneration device that generates both electrical and thermal energy. Usually, some daylight should be transmitted through the system, that is, the PV modules should not cover the whole available area, but rather 50-70%. Thus, a significant percentage of incident solar radiation is transmitted as daylight, potentially reducing by a corresponding amount the electricity consumption for lighting.

A PV/thermal workshop [1] organized by the International Energy Agency concluded that there is an urgent need for research on effective integration of photovoltaic and solar thermal systems so that physical systems can be developed that perform both functions in a reliable and optimal manner. While several BIPV projects have been built around the world, such as the Mataro Library [2] in Spain and some progress on modelling has been reported [3, 4], there is no systematic procedure for their overall optimization and prediction of reliability. Part of the reason is the complexity, particularly of PV-airflow windows, which require consideration of thermal, electrical and daylighting performance. Motorized blinds may be employed in an airflow window with PV to control daylight transmission into a room in conjunction with dimming of electric lights and heating/cooling needs.

While many researchers have considered various system configurations and developed thermofluid models to investigate performance, there is a need for systematic optimization of such systems to raise their overall performance and cost effectiveness.