Climatic effects of two cooling systems in greenhouses in the Mediterranean area: External mobile shading and fog system

Abstract

Mediterranean horticulture under plastic requires the use of cooling systems to avoid situations of thermal stress. Traditionally, greenhouses are cooled by natural ventilation combined with cover whitening. Nevertheless, the use of such a shading technique can cause a reduction in radiation during those times of day which are not thermally stressful, needlessly limiting the photosynthetic capacity of plants. The efficiency of two cooling methods which avoid the disadvantages associated with fixed shading was evaluated: external mobile shading (S) and fogging (F). In the first experiment a fixed shading system (R) was compared with a mobile shading controlled by temperature settings (S1); in the second experiment mobile shading was governed by both radiation and temperature (S2) then compared to F. The different settings for the activation of S applied in each experiment gave rise to differences in the average of transmissivity, yielding 38% in S1 and 56% in S2. Cooling methods F, S1 and S2 were all similarly efficient in reducing high air temperatures when compared to R. The inside to outside air temperature gradient gradually diminished in all cases due to the increased influence of transpiration as crop leaf area increased. It is important to use very precise control settings of mobile shading, especially when growing species of high light saturation. Fogging caused the greatest reduction of VPD, especially during the first stages of growth, although it implies the use of water, which is an important consideration in areas with scarce water resources.