NATURAL VENTILATION OF A GREENHOUSE WITH CONTINUOUS ROOF VENTS - MEASUREMENTS AND DATA-ANALYSIS

Although natural ventilation plays an important role in affecting greenhouse climate, as defined by temperature, humidity and CO:! concentration, particularly in Mediterranean countries, little information and data are presently available on full-scale greenhouse ventilation mechanisms. In this paper, we present the main results of a 4-yr study (1988-1992) involving both experiments and modelling of natural ventilation. Various experimental techniques were used to make full-scale measurements of the air exchange rate in a 400 m(2) plastic-greenhouse, equipped with roof ventilators. Theses included two tracer gas techniques, (i) the decay rate method using N2O as tracer gas in the presence of a crop and (ii) the constant enrichment technique using CO2 but without a crop and (iii) the greenhouse water vapour balance based on measurements of inside and outside air humidity and tomato crop transpiration rates. Good agreement was found between these different experimental approaches. Two models based on a non-dimensional analysis of the flux of ventilation were derived and compared with the measurements and with other results already published in the literature. It was found that the wind effect is the main driving force of ventilation. The air exchange rate is linearly dependent on the wind speed and on the effective opening of the vents. No effect of the wind direction on ventilation performance was found.