Effects of cover properties, ventilation rate, and crop leaf area on tropical greenhouse climate
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Experimental results and validation of a simple greenhouse climate model are analysed according to data sets from six prototype greenhouses with three different plastics (reference N0, and two levels of near-infrared reflecting pigments N1 and N2); two ratios of ventilation openings to greenhouse covering area (0.223 and 0.427); a wide range of tomato leaf area index (0.01–4.97); and for three crop-growing periods that represent year-long tropical lowland climatic conditions in Purwakarta (107°30′E, 6°30′S, altitude 25 m), Indonesia. The model with a calibration factor for indirect absorbed solar radiation (indicating the part of the radiation absorbed by the greenhouse cover, structural elements, and soil surface released into the air) Λ of 0.1 satisfactorily calculated greenhouse air temperature TAir with less than 2% error and greenhouse air water vapour pressure deficits DAir with less than 10% error. The errors were higher at low values for the leaf area index. The model performance slightly improved by including the effect of leaf area index on Λ as an exponential term. Measurements and calculations demonstrated that TAir was affected more by variations of ventilation and leaf area index than by the applied cover properties. The leaf area index had the highest impact on greenhouse air temperature, implying that a large proportion of the cooling is achieved by the crop itself. The results enable the model to be used in the design of optimum greenhouse systems for tropical lowland Indonesia.