An experimental study was conducted to determine the performance of two evaporative cooling systems, as well as the gradients of temperature and humidity along a greenhouse. The first system was the cooling unit, (CU) compared with the traditional fan-pad one, (F-P). The cooling unit was connected with two perforated ducts inside the greenhouse. The air temperatures and relative humidities on the longitudinal axis were measured outside and just after leaving the media and at each one meter from the pad media. They were also, measured in the traverse axis parallel to the pad media at distances 0.5, 1.5, 2.5 and 3.5 m from the edge of the greenhouse. A steady state mathematical model was modified to predict the greenhouse air temperature according to the state of the air just leaving the cooling media.
According to the experimental results, the non-uniform temperature changes were observed along the greenhouses for the two systems. The average temperatures of the CU greenhouse were 27.9, 30.4, 33.2, 29.1 and 27.9 oC for outside ambient temperatures of 28.7, 32.4, 35.4, 32.1 and 29.4 oC, respectively. The ranges of temperature gradient inside the CU greenhouse were 2.1, 2.1, 2.2, 2.2, and 2.1 oC. While in the F-P greenhouse, they were 5.9, 7.4, 8.2, 5.9 and 6.1 oC for the same ambient temperatures. The average air temperature entering to greenhouse was approximately 6.82 and 7.09 ºC lower than the outside air temperature for CU and F-P systems. The greenhouse of CU system was higher in the relative humidity than F-P one. The average relative humidities were 69.0 and 61.8% for the CU and F-P greenhouses, respectively, with increase percent 12.38%. The averages CU efficiencies were 77.55% and 74.79% in the first and second days, respectively, while they were 72.97% and 70.19% for the same days for F-P system. Consequently, the CU system was averagely more efficient than the F-P system by 6.29% and 6.58%. The water flow rate of the two systems to permit optimum wetness and air saturation were 5.6 and 6.5 L/min for the CU and F-P evaporative cooling systems, respectively. The saving percentage in water circulation was 16.1% for the CU system. The total yield per plant was 6.24 and 5.48 kg/plant for the CU and F-P systems, respectively. The CU system increased the yield per plant with 13.82% over the F-P system.