Screenhouse Microclimate and Ventilation: an Experimental Study

Abstract

A field experiment was carried out to study the microclimate and air exchange rate of an insect-exclusion fine-mesh 60 m by 110 m by 3·2 m commercial screenhouse for pepper cultivation in central Israel. Diurnal courses of plant transpiration, and internal temperature and humidity at three heights were measured at two locations (northern and middle sites) across the screenhouse. Two external towers measured conditions outside the screenhouse. The water vapour mass balance, based on measurements of plant transpiration and internal and external humidity, was used to calculate the air exchange rate and resistance to mass transfer of the screenhouse. Plant transpiration was measured by the heat-pulse technique. Results show that the upper region of the screenhouse interacted strongly with the ambient air, but that during most of the day a temperature inversion inside the screenhouse stabilised the air and reduced mixing. Humidity and temperature profiles show that within the screenhouse temperature increased and absolute humidity decreased with increasing height. Air exchange rates were between 7 and 33 h −1 for windspeeds between 1·5 and 3·5 m s −1 at the two locations measured in the screenhouse. The rate of increase of air exchange rate with windspeed was 7·7 and 3·2 h −1 (m s −1) −1 at the north and middle of the screenhouse, respectively. The higher rate at the northern side demonstrated the influence of wind entering through the sidewalls. Ventilation rates were compared to a theoretical calculation for an open pepper field. For external wind speeds between 1·5 and 3·5 m s −1 covering the field with a screenhouse would reduce ventilation rates by 51–71% in the middle site, respectively, and for the north site by 60–64%. The ventilation rates were higher than those reported for similar size greenhouses, which implies lower resistance to transport in screenhouses as compared with greenhouses of the same size.