Simulating the effect of condensation type on greenhouse shortwave radiation transmission and utilization

To investigate the effect of condensation type (filmwise or dropwise) on the shortwave radiation transmission and utilization in greenhouses, the Gembloux Dynamic Greenhouse Climate Model (G.D.G.C.M.) was modified by introducing two series of solar radiation transmittance values: one that is used when the cover is dry, the other when the cover is wetted by condensation. Simulations were carried out for the whole Belgian typical reference year, assuming that a tomato crop was grown from 1 December to 30 November in an infinitely large polyethylene covered greenhouse at 51 degrees North latitude. The solar radiation transmission was simulated for some idealized cases: condensation which does not affect the transmittance (reference state), filmwise condensation, and condensation as cap shaped drops with contact angles of 30, 60, and 90 degrees. The assumption of complete covering of the wet part of the roof by condensation implies that the simulation results should be considered limit values between which the real results are situated. The simulations pointed out that the solar radiation flux density inside the greenhouse with film condensation was 1% (summer) to 3% (winter) higher than for the reference state. For condensation as hemispherical drops, however, a decrease of 10% (summer) to 20% (winter) with respect to the reference state was found. The lower incoming shortwave radiation flux density in the greenhouse covered with hemispherical drops resulted in a decrease of the yearly vegetation transpiration flux density of about 9%, while the greenhouse heating requirements increased by about 3%. For the greenhouse covered with a condensation film, it was found that the slightly higher amount of solar radiation entering the greenhouse had hardly any influence on the yearly heating requirements and on the vegetation temperature and transpiration