The impact of the slit aperture size and the flap’s sensitivity setting is shown in Fig. 5 for the measurements with set-up B on September 12 (a) and September 15 (b). On both days, the collector tilt angle was 60°. For (a) the flap was adjusted to open at approximately 80 °C and for (b) slightly over 75 °C. “Flap setting: 8 mm” in Fig. 5 means that the ventilation flap in the top of the collector frame opens when the absorber’s relative, longitudinal expansion is larger than 8 mm.

The opening of the flap can be clearly seen from the comparison of the temperature profiles of the completely closed reference collector (Tr) and the ventilated collector with flap (Ttest). The slit aperture was 15 mm in (a) and 20 mm in (b). The maximum temperature in (a) was for the reference collector 122 °C and for the test collector 110 °C with AT = (12.1 ± 1.0) K.

In (b) the maximum temperatures in the reference — / test collector were 128 °C / 111 °C with AT = (17 ± 1.0) K. In the comparison the variations of the solar irradiance for (a) between 14:30-

15:45 should be considered; still the desired effects on the temperature reduction between reference — and test collector due to increased flap sensitivity and slit aperture is obvious.

Fig. 5. Maximum temperatures in the reference — and test collector with different flap-sensitivity settings and
slit apertures for Sept. 12 (a) and Sept. 14 (b); Ig is the global solar irradiance, Ta the ambient temperature

[set-up B]