For the model collector a specially designed 10mm thick triple-wall sheet with channels for transport of water (Solarnor, NOR) was used as the polymeric absorber [5]. Black absorbers with solar absorptivity a of 0.95 and infrared emissivity s of 0.90 were considered. Heat transfer medium was water with a flow rate of 50 kg/(m2h). The collector was covered by a 10 mm polycarbonate twin-wall sheet with intrinsic air filled channels (width 9.5 mm; spacing layer thickness 0.5 mm) acting as an additional insulation layer. A theoretical thermotropic layer was attached to the back side of the glazing. For the considered thermotropic layer constant solar transmittance and reflectance values in the clear and opaque state were assumed. In the switching temperature range a steep switching with a linear change from the light transmission to reflection was chosen. Solar transmittance of the film in clear state was set to 0.90 or 0.85. In opaque state, the transmittance was assumed to decrease to values between 0.10 and 0.60. Absorption in the thermotropic layer was neglected. The air gap between collector glazing and absorber was 10 mm. The solar collector is confined by a 50 mm and a 30 mm thick insulation on the rear side and the lateral surface, respectively. As to solar collector position south orientation and an angle of 45° to the horizontal was selected. Ambient air temperatures of 0, 20 and 30°C were considered. Solar irradiations of 1000 and 1200 W/m2 were modeled.

3. Results and Discussion