The horizontal temperature distribution of the ventilated collector is shown in Fig. 3. From the temperature difference of the three sensors respectively at the top and the bottom of the absorber plate can be concluded that the horizontal variations are small.

These temperature variations are correlated with periods of high wind speeds measured. Along the vertical axis the absorber temperature is significantly lower in the bottom than in the top with a maximum difference of 42 K. Almost 35 K of the temperature increase occurs over the lower part of the absorber. This fraction varies to some extent with the tilt angle but generally a larger temperature difference was observed in the lower half than in the upper half [10].

For the reference collector the temperature increase over the lower half of the absorber is typically 25 K while the increase over the upper half is 5 K.

1.2. Sensibility of slit opening and tilt angle (set-up A)

The effect of the size of the slit aperture and the collector tilt angle on the temperature reduction relative to the reference collector was investigated.

The temperature reduction (of the ventilated relative to the reference collector) is displayed in Fig. 4 as a function of the temperature difference Trmc-Ta. The reduction is given for measurements with slit apertures of 10, 15, 20 mm and tilt angles of 45, 60, 90°. In Fig. 4 (a) the tilt angle is constant at 45° and it is clearly shown that increasing slit apertures result in higher temperature reductions.

In Fig. 4 (b) the slit aperture is constant while the tilt angle is varied. Here the temperature reduction is largest when the collector tilt angle is 90°. Although the same maximum temperature reduction is obtained with tilt angles of 90° and 60° it occurs at different temperatures of the reference plate.

Fig. 4. Temperature reduction of the ventilated — relative to the reference collector as a function of the
temperature difference Trmc — Ta for slit apertures of 10, 15, 20 mm and tilt angles of 45, 60, 90°; [set-up A]