In Fig. 1 the efficiency factor is plotted as a function of the absorber temperature for a collector without and with thermotropic overheating protection at a solar irradiation of 1200 W/m2 and ambient air temperatures of 0 and 30°C. The thermotropic layer exhibits a solar transmittance of 0.90 in clear state and of 0.10 in opaque state. The collector without a thermotropic layer reaches maximum
absorber temperatures of ~160°C and ~175°C at ambient temperatures of 0 and 30°C, respectively. It is observable that the stagnation temperatures can be controlled by the use of thermotropic layers. To achieve maximum absorber temperatures of 90°C, switching temperatures of the thermotropic film between 55 and 60°C are required. A slight effect of the ambient air temperature on the efficient working temperature range is discernible. At elevated ambient air temperatures of 30°C the efficiency drop is shifted to lower temperatures. However, the efficient working temperature exceeds 60°C even at an ambient temperature of 30°C. Thus the overheating protected collector is appropriate for domestic hot water and space heating applications.
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
absorber temperature [°C]
Fig. 1. Collector efficiency versus absorber temperature of a solar collector with twin-wall sheet glazing and
black absorber (a=0.95, є=0.90) at a solar irradiation of 1200W/m2 and ambient air temperatures of 0°C and
30°C; solid lines: collector without overheating protection (no TTL); dashed lines: collectors with thermotropic
glazing (switching temperature: 55-60°C).
In Fig. 2 the effect of the thermotropic layers switching range on maximum absorber temperatures at a solar irradiation of 1000W/m2 and an ambient air temperature of 20°C is shown. The residual solar transmittance of the thermotropic layer in opaque state is varied between 0.20 and 0.60. The transmission of the layer in clear state remains constant at 0.85. Compared to layers exhibiting a solar transmittance of 0.90 in the clear state (Fig. 1), the collector efficiency is shifted to slightly lower values. As to layer design, this indicates that the solar transmittance should exceed 0.85 in the clear state. The collector without thermotropic overheating protection reaches absorber temperatures of about 160°C. It is observable that by applying thermotropic layers exhibiting a residual transmittance of 0.30 to 0.35 the absorber temperatures do not exceed 90°C. Layers exhibiting transmittance values above 0.35 lead to a successive increase of maximum absorber temperatures up to 130°C for a solar transmittance of 0.60 in the scattering state.
20 30 40 50 60 70 80 90 100 110 120 130 140 150
absorber temperature [°C]
Fig. 2. Collector efficiency versus absorber temperature of solar collectors with twin-wall sheet glazing and black absorber (a=0.95, є=0.90), at a solar irradiation of 1000W/m2 and an ambient air temperature of 20°C; variation of switching performance of the thermotropic layers (TTL) (solar transmittance: 0.85 in clear state and 0.20 to
0.60 in opaque state).