Optimization of greenhouse ventilation for humid tropics

Abstract

In any agricultural sector, the need of the hour is to maximize the yield per unit area to cater the needs of our population, which is exploding at an alarming rate. Protected cultivation or controlled environment agriculture is one of the methods to increase the crop production from unit area. Of the various forms of protected cultivation, greenhouses are most common. Our state falls under humid tropical climatic condition. Greenhouse cooling is to be done during peak hours of daytime under this climatic condition. Ventilation plays an important role in green house cooling. A study was conducted to determine the optimum greenhouse ventilation for humid tropics and to analyze the effect of different cooling methods viz. natural ventilation, fan and pad system, mist system and roof shading on greenhouse cooling. The study showed that natural ventilation could reduce the greenhouse temperature to a great extent. The different percentages of ventilation used for the experiment were 13.8, 11.5, 9.2, 6.9, 4.6 and 2.3. The study was also conducted without ventilation. As percentage ventilation increases, greenhouse cooling increases and inside relative humidity decreases. 13.8 percent ventilation gave maximum greenhouse cooling and this value is taken as the optimum percentage of natural ventilation. But the natural ventilation alone cannot meet the cooling requirement during peak hours. Fan and pad system is found to be not effective for greenhouse cooling in this particular study. Even 2.3 percentage of natural ventilation condition gave better cooling than fan and pad cooling system. Misting is an effective method to lower down the greenhouse temperature within a short period of time. Misting was done along with fans at different percentages of ventilation and the different climatic parameters were studied. The results showed that while misting, the maximum cooling was obtained at 2.3 percent of ventilation. Effect of roof shading on greenhouse cooling was tested with one shade net over the greenhouse. Roof shade has a significant effect on greenhouse cooling. Th inside temperature of the greenhouse was higher than the outside temperature by more than lOoe during peak hours of the day time, when no ventilation was provided and fan and pad system, mist system and shade cover were

not used. But when the green house was covered with a shade net, an inside to outside temperature difference of one or two degree was observed. Also, while operating fan and pad system and mist system, shaded condition gave better cooling. Fan and pad system operated without shade cover, could not bring down the greenhouse temperature below ambient temperature. But for the shaded condition, greenhouse temperature could be lowered to a value less than the ambient temperature. Misting under shaded condition at 2.3 percent ventilation gave maximum cooling. Misting inside an unshaded greenhouse at 2.3 percent ventilation could bring down the greenhouse temperature to 32°C, but misting inside a shaded greenhouse at 2.3 percent ventilation lowered the greenhouse temperature to 29°C. Use of two shade net layers did not give any additional cooling, and at this condition the light intensity inside the greenhouse was only 10 percent of the outside light intensity. It only increases the cost of the greenhouse and hence it is not recommended to use two shade net layers for greenhouses.