The influence of shading and evapotranspiration on a ventilated greenhouse environment

A comprehensive experimental study was initiated to investigate the effects of shading rates and plant transpiration on the environment of naturally ventilated greenhouses. Natural ventilation was simulated by a forced ventilation system, which permitted air exchange rates and inside air movements to be controlled. The greenhouse was shaded with one of four black shade nets (30%, 50%, 80%, and 90%). A chrysanthemum crop was chosen to establish a high transpiration canopy. A computational fluid dynamics (CFD) model was validated for greenhouse air velocity profiles, heat transfer, and air exchange rates by comparing numerical results with results from the experimental greenhouse at the same boundary conditions.; The results showed the importance of using shades during high solar load period. Using 50% or 80% shading rates as a solar radiation interceptor significantly reduced the risk of plant stress during high solar loads. Air velocities were an important factor influencing ET rates under shaded greenhouse. The means of daily ET rates for the 50% shade were significantly different for 0.1 and 0.6 m/s air velocity levels at 5 cm above the plant canopy. Increasing air velocities from 0.1 to 0.6 m/s increased the daily ET rate by 16%. For the 0% shade, however, the daily ET means were not significantly different between both velocity levels.; Increasing air change rates with all shading rates were useful in decreasing outlet-inlet temperature differences (ΔT) even with inlet air temperatures of 35°C. When solar radiation was decreased by shades, ET rates responded proportionally and evaporatively cooled the air resulting in low ΔT.; Validating the CFD model for air movements and heat transfer indicated that the CFD method could be used to predict greenhouse air movements and temperature patterns with reasonable accuracy. Validating tests showed that the differences between air velocities predicted by CFD models and those measured were from 0.00 to 0.47 m/s. The differences between predicted outlet air temperatures and those measured were approximately 2°C.