TRANSMISSION PROPERTIES OF DRY AND WET GREENHOUSE CLADDING MATERIALS AND RELATION TO CROP PHOTOSYNTHESIS

Abstract

Solar transmission properties of dry and condensate covered single glass and a common polyethylene film (PE) were measured as a function of incidence angle. The experiments revealed that scattering by dry glass was negligible, in contrast to dry PE which scattered 47% of the transmitted radiation. With the presence of condensate, single glass scattered 81 to 86% of the totally transmitted radiation, depending on incidence angle. At an incidence angle of 30degrees, condensate lowered the total transmittance by 4%. The scattered fraction of the radiation transmitted by the PE film was increased to 78-83% by the condensate drops, but the total amount of transmitted radiation was reduced by 22-23%. To assess the effect of scattering on plant growth, net photosynthesis rates were simulated for a tomato crop. The photosynthesis rate was predicted to be 16% higher under dry polyethylene when compared with dry glass due to the scattering proper-ties of the plastic film. The presence of condensate gave rise to a 13% higher photosynthesis rate under single glass and a reduction of 6% under polyethylene when compared with the respective materials in the dry state.