I. Seginer, Dvora Kantz
Sep 1, 1989
Journal of Agricultural Engineering Research
A steady-state computational model was developed to simulate the night-time energy and vapour balances of a greenhouse. It was then used to determine the heating flux required to maintain a crop canopy at a given temperature and the greenhouse air at a given humidity. The model can simulate the effect of a single- or double-layer cover of arbitrary transmissivity to long-wave radiation. It was assumed that the transmissivity of the ceiling drops to zero whenever it is covered with condensate. One source and three sinks of vapour were considered: the vapour supplied by crop transpiration could be removed from the air by any desirable combination of condensation on the ceiling, ventilation and hygroscopic absorption. Condensation on the ceiling is the least expensive means of removing moisture from the air. Whenever the rate of condensation on the ceiling cannot, by itself, maintain a sufficiently low humidity in the greenhouse, the choice is between ventilation and dehumidification. The loss of energy by ventilation is linearly related to the vapour conductance of the canopy and in mild climates may become a significant fraction of the total loss. Dehumidification may conserve most of the energy which otherwise leaves the greenhouse through ventilation, but adds to the capital costs of the installation. The more ventilation is required over the season to maintain temperature and humidity levels, the greater becomes the relative advantage of dehumidifiers. Generally speaking their relative advantage should show up in mild weather conditions, in well insulated greenhouses, with strongly transpiring plants (at night), and when a low humidity is required.