Effects of ventilation on the energy balance of a greenhouse with bare soil

Abstract

Measurements of the radiation and energy balances in a polyethylene greenhouse covering a bare, dry sandy soil were used to determine the heat dissipation efficiency of four ventilation methods. The 62% of incident solar radiation transmitted by the roof provided the heat load on the system. The four ventilation treatments comprised passive ventilation obtained by rolling up opposite end walls, activating two fans mounted in one gable and opening the opposite wall, the same configuration but with a single fan, and completely closing the greenhouse. Opening opposite walls generated sufficient draught to exchange air at a rate of 44 volumes per hour. Operating one and two fans produced exchanges of 8 and 13 volumes per hour, respectively, much below the specified rating of the equipment, because of pressure losses across insect-proof nets on the vents. Closing the greenhouse limited the air exchange to three volumes per hour. For the climatic conditions of the experiment, external wind speed and internal buoyancy forces affected passive ventilation, but had no significant effects on fan ventilation. Despite the dryness of the top layer of soil, the latent heat flux remained a large term in the energy balance. High ventilation rates diminished soil heat flux, increased sensible heat flux and slightly reduced latent heat flux.