Solar energy availability in greenhouses as affected by condensation on cladding materials

To investigate the solar radiation transmission through cladding materials covered with condensation on their inner surface a new experimental set-up was constructed. Transmittance experiments were carried out on vertical glass plates and polyethylene films covered with condensation without and with runoff for several incidence angles in the horizontal plane using perpendicularly polarized radiation. It was observed that condensation on a glass plate slightly decreased the solar radiation transmittance when compared with the dry state. The reduction in transmittance increased from 0.8% at normal incidence to 8.5% at an incidence angle of 60 degrees. Results for polyethylene pointed out that condensation affected the solar radiation transmission and so the available solar energy in the greenhouse much more than in a glasshouse. At normal incidence, a transmittance reduction of 20.8% with respect to the dry state was obtained. For higher incidence angles, the transmittance reduction decreased gradually to 7.2% at an incidence angle of 60 degrees. For both glass and polyethylene, it was observed that the start of runoff gave rise to a transmittance increase of up to 2% when compared with the phase of condensation without runoff. (C) 1998 Elsevier Science Ltd. All rights reserved.