HEAT AND MASS TRANSFER IN A LARGE EVAPORATIVE COOLED GREENHOUSE EQUIPPED WITH A PROGRESSIVE SHADING

An experimental study was conducted in a large commercial greenhouse equipped with a fan and pad evaporative cooling system and a half-shaded plastic roof. Experimental measurements were carried out at four different locations inside the greenhouse: (i) next to the cooling pads, (ii) 15 m away from the cooling pads (unshaded section), (iii) 45 m away from the cooling pads (shaded section) and (iv) before the extracting fans. The experimental results showed strong climate heterogeneity along the air stream from cooling pads to extracting fans. The overall efficiency of the system was quite high (~ 80%) and in combination with the dry and warm outside climate conditions enabled the air to enter the greenhouse with a temperature even 10ºC lower than that of outside air temperature. However, due to the long air path through the greenhouse, air exits the greenhouse even 8ºC warmer than the inlet air. Air vapour pressure deficit varied from 0.5 kPa (after the cooling pads) to 2 kPa (extracting fans). Transpiration rate was found larger at the second (shaded) part of the greenhouse mainly due the higher vapour pressure deficit found at this section of the greenhouse. Crop-to-air temperature difference was smaller at the second part of the greenhouse (45 m) indicated that the crop in this part was cooler than in the first one as a result of the higher transpiration rate at this section.