A simple model for ventilation rate determination in screenhouses

The objective of this work was to study and model the ventilation rate in screenhouses. Thus, microclimate variables and crop transpiration as well as the air velocity were measured in three screenhouses covered by different screens: (i) a clear insect-proof screen, (ii) a white insect proof screen and (iii) a green shade screen, with values of shading factors to solar radiation measured in the lab of about 13%, 34% and 36%, respectively. The porosity of the screens was found 0.46 for the insect proof and 0.63 for the shading screen. The ventilation rate was estimated using the decay rate 'tracer gas' method, using the water vapour as tracer gas. The results showed that the insect proof screens reduced at the same rate the inside screenhouse air velocity, since they had the same geometrical characteristics. The internal air velocity in the insect proof and the shading screenhouses was about 20% and 44%, respectively, of that measured outside. The ventilation rate data obtained were used to calibrate a model that can be used for the prediction of ventilation rate in screenhouses, taking into account the geometrical characteristics of the screens used and of the screenhouse and the outside wind speed.