Microclimate and Air Exchange Rates in Greenhouses covered with Different Nets in the Humid Tropics

Abstract

The effect of different mesh sizes of nets (40, 52 and 78 mesh) to cover ventilation openings of greenhouses located in the humid tropics on microclimate and air exchange rates was studied. Experiments were carried out in three identical greenhouses (each size of 10 m wide by 20 m long). These greenhouses were covered with a plastic film on the top, and different insect-proof nets on the roof and sidewalls, respectively. Water vapour, transpired from 300 tomato plants cultivated in each greenhouse, was used to determine air exchange rate. In addition, an energy balance method, based on solar radiation and daily microclimate in and outside the greenhouse, was used to predict the air exchange rate. A good agreement was achieved between the measured and predicted air exchange rates. Results show that the use of nets at different mesh sizes had a significant effect on microclimate and air exchange rate. Compared to the 40 mesh greenhouse, the reduction of air exchange rate of about 50% and 35% was obtained for the 78 and 52 mesh greenhouses, respectively. Consequently, the internal air temperature was also increased by 1–3 °C. Regarding air temperature rise, only minor differences have been observed due to very large ventilation openings. However, differences in absolute humidity were much more pronounced and statistically significant. Humidity in 78 mesh greenhouse was consistently approx. two times higher than 40 mesh greenhouse, while 50% increment was observed in the 52 mesh greenhouse. Based on internal microclimate and its ability to exclude some major insects, the 52 mesh net seems to be selected as a compromise size of nets appropriate for tropical greenhouse.