Model calculations and the few data that are available show that over 100 L water condense yearly on each square meter of a greenhouse cover. It is known that the presence of condensate reduces light transmission. This effect is suppressed to some extent by adding film-forming (anti-drop) additives to plastic film covers and surface structures or coatings on hard cover materials. There is a need, therefore to assess the effect of the surface treatment on the loss of light. On the other hand, condensation releases the energy that was used for evaporation, thereby warming-up the cover and somewhat decreasing the heating requirement of the greenhouse. The amount of condensation energy that is recovered may be expected to depend on the external and internal climate conditions. In this work we analysed the effect of condensation on light transmission and energy budget of a greenhouse, with seven different cover materials. Various internal vs external conditions were created by placing the model greenhouse (about 3¿4 m) in a large climate chamber. Each experiment was repeated for two temperature differences between inside and outside (10 and 20°C) and two air movements in the greenhouse (7.5 and 15 cm s-1). Light transmissivity was reduced by 9% on average, with large differences among materials. Anti-drop coatings did suppress this effect, as did a surface structure meant to increase light diffusivity of the material. As far as energy is concerned, the overall heat transfer coefficient (U-value) of the greenhouse increased by an average of 16% (single layers) or 12% (double layer covers) when wet. Obviously there was an effect of the temperature difference on the U-value, which was found to be consistent with the heat transfer theory, whereas little effect was found of the air movement within the house.
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