Impact of water drops on the visible radiation transmittance of glazings under outside radiant conditions

Experimentally determined directional-hemispherical transmittances for visible radiation were used to calculate the effect of condensate on the hemispherical–hemispherical transmittance values under diffuse radiant conditions for single glass and polyethylene. The hemispherical–hemispherical transmittances were determined in the horizontal plane for different inclinations of both glazing materials assuming a completely overcast sky and a clear sky. In all cases, condensate was found to reduce the transmittance. Under an overcast sky, the transmittance of single glass was reduced by the presence of condensate from 8 up to 13% (on a relative scale) for increasing inclination angles of the glass plate, while for the polyethylene film, the transmittance reduction due to condensate decreased from 20 down to 13%. Under a clear sky, depending on date, time, and orientation of the glazing material, the transmittances of single glass and polyethylene were reduced by between 6 and 15% and by between 8 and 21%, respectively, due to the presence of condensate. These results revealed that the impact of the presence of condensate on the transmittance of a glazing cannot be omitted from design and performance calculations of solar energy systems.