The design of the two double fagade sections is based on an earlier simulation study by Charron and Athienitis [4]. The double fagades in the outdoor test-room were fully instrumented with thermocouples attached at several points on the cavity surfaces, hot wire anemometers for velocity measurement, pressure sensors and a weather station. Note that the back panel in each fagade consists of polystyrene enclosed on both sides with 1 cm thick plywood panels; the total RSI value is equal to 1 and it has negligible heat storage. The heat transfer through the panel to the room was negligible compared to other energy flows.

Figures 2 and 3 below compare results from the two fagades for January 26, 2004. This day was cold and clear with negligible wind. The data were collected every minute and averaged for about half hour from 11:20 am to 11:50 am. Quasi-steady state conditions existed with no major (not more than 5%) change of any of the parameters measured.

As can be seen from Figure 3, much higher thermal efficiencies are obtained when we have air flow on both sides of the PV than when the PV panel is exposed. For the case of Figure 2, the electricity generated was 87 W and the thermal energy (heating of air) 337 W without taking into account the significant heating at the inlet; as can be seen from Fig. 2 the air at the inlet is heated about 3 °C and this effect may also be due to some air leakage from the room into the cavity. The resulting electrical efficiency was about 10% and the thermal efficiency 37% for a total efficiency of 47%.

By comparison, in the configuration of Figure 3, the electrical efficiency of the PV was only 6% (but it was not at its maximum power point) and its thermal efficiency was 65% for a total of 71%. One disadvantage of the double cavity configuration of Fig.3 is that the PV operates at a higher temperature — a maximum temperature of 40.7°C when the outside temperature is -17 °C. The temperature of the air exiting the cavities may be controlled by varying the flow rate and mixing with indoor air.

Studies of the temperature and velocity profiles across the horizontal (air gap) were also performed. Figure 4 shows a typical inlet and outlet temperature profile corresponding to the measurements in Fig. 2. The velocity profiles that were used to compute the average velocity (equal to 0.6 m/s) showed a buoyancy-induced peak near the PV followed by flat region in the middle. The flow is complex, definitely a mix of natural and forced (fan — induced) convection, laminar at the inlet and turbulent near the top of the cavity.