PREDICTION OF THE EFFECT OF INSECT-PROOF SCREENS ON CLIMATE IN A NATURALLY VENTILATED GREENHOUSE IN HUMID TROPICAL CLIMATES

Abstract

The impact of different types of insect-proof screens on greenhouse climate in humid tropics needs to be quantified in order to develop technically and economically optimal greenhouse constructions. Thus, physical properties of various insect-proof screens were determined in the laboratory at the Institute of Horticultural and Agricultural Engineering, University of Hannover, Germany. Specially adapted experimental greenhouses were built at AIT campus in Bangkok, Thailand. A dynamic energy and mass balance model of the greenhouse system was developed to predict the greenhouse climate from external weather data and properties of insect screens. Scenario simulations were carried out to predict the effect of screen properties on greenhouse climate. External and greenhouse climate measurements were made in Bangkok. The internal climate measurements were carried out concurrently in two similar, naturally-ventilated greenhouses covered with different insect-proof screens on ventilation openings. Tomato (Lycopersicon esculentum ‘King Kong II’) plants were grown in the greenhouse during the experimental period. Model predictions of greenhouse air temperature were compared to measurements from the two greenhouses and good agreement was achieved. The results form a good basis for decisions on screened greenhouse design improvements and climate control interventions in screened greenhouses in the humid tropical climates. INTRODUCTION Although temperatures and global radiation are very suitable for vegetable cultivation throughout the year in the humid tropics, open air cultivation is hindered by heavy rainfall and high humidity which cause crop damage and infestation by diseases. High incidence of pests such as thrips and aphids is also a big problem. Therefore, greenhouse systems which can provide the optimal plant growth environment while offering maximum protection against pests need to be developed. According to von Zabeltitz (1993), air temperature and air humidity should not be higher in tropical greenhouses than in the open air. Cooling-related expense is one of the most important greenhouse operation costs in the humid tropics. But the efficiency of fan and pad cooling systems in greenhouses in the humid tropics is limited due to high humidity of ambient air (H-J. Tantau, pers. commun., 2004). It is also expensive. Naturally-ventilated greenhouses can result in saving of cooling-related costs. Physical exclusion of pests through the use of insect-proof screens on greenhouse ventilation openings is an increasingly popular way of reducing pesticide applications. A consequence of the use of insect-proof screens is that they restrict air-flow and thus affect the climate in the greenhouse. The effect of insect-proof screens on greenhouse climate depends on the physical properties of the screens (i.e. discharge coefficient and spectral transmissivity) and the ratio of the screen area to floor area. Different authors have reported different values of discharge coefficients of greenhouse openings (Sase and Christianson, 1990; Montero et. al. 1997; Munoz et. al. Proc. IC on Greensys Eds.: G. van Straten et al. Acta Hort. 691, ISHS 2005 45