Condensation and radiation transmittance of greenhouse cladding materials, part 3: Results for glass plates and plastic films

The effect of condensate on the transmittances of single glass, low-emissivity glass, double glass, ordinary low-density polyethylene, anti-drop-condensation polyethylene and anti-dust polyethylene was investigated experimentally. The directional-hemispherical transmittances of the dry and the wet (i.e. covered with condensate) materials were measured under laboratory conditions using monochromatic radiation. Under dry conditions, the directional-hemispherical transmittances of the three plastic films and the single glass plate coincided within 3% for incidence angles up to 60 degrees. The directional-hemispherical transmittances of dry low-emissivity glass and especially dry double glass were significantly lower than the three plastic films and the single glass plate. Depending on the cladding material and incidence angle, the effect of the presence of condensate on the directional-hemispherical transmittance was insignificant or negative (a reduction of up to 23% on plastic films without anti-drop agents). The presence of condensate caused similar transmittance reductions for single and double glass reaching 13-15% at incidence angles of 50-65 degrees. The transmittance reductions determined for the wet low-emissivity glass were slightly smaller and limited to a maximum of 11% at an incidence angle of 70 degrees. The addition of anti-drop agents to plastic films suppressed the transmittance reduction totally. The different influence of the condensate on the transmittance of different cladding materials was mainly due to shape of the condensate drops, which were much flatter on glass than on non-anti-drop plastics. Assuming uniformly diffuse radiation, condensate was found to lower the hemispherical-hemispherical transmittance by 8% for single and double glass, 5% for low-emissivity glass, 11% for low-density polyethylene and 13% for anti-dust polyethylene. (C) 2000 Silsoe Research Institute.