Effects of Ventilation Rate on the Environment of a Fan-Pad Evaporatively Cooled, Shaded Greenhouse in Extreme Arid Climates

A study was conducted to examine the influence of two ventilation rates on the environment of a shaded greenhouse equipped with fan-pad evaporative cooler during typical summer days in extreme arid conditions. Temperature, relative humidity, and amounts of water and electrical energy consumed for cooling were investigated in a 39- x 9-m Quonset shaded greenhouse planted with cucumbers. Cooling was provided via an evaporative pad and two exhaust fans, each had a manufacturer's ventilation rating of 10 m3/s [two fans provided 1 air exchange per minute (ACM) and one fan provided 0.5 ACM]. One fan was used from 0700 to 2000 h every other day, and two fans were used for the same period on the alternate days. Averages of daytime air temperature and relative humidity inside the greenhouse with a ventilation rate of 0.5 ACM were 33.6°C and 33.5%, respectively, as compared to 30.2°C and 37.5%, respectively, with a ventilation rate of 1 ACM, while the averages of outside temperature and relative humidity were 38.7°C and 11%, respectively. Due to the greenhouse length, temperature gradients along the direction of the airflow were observed. The highest temperatures differences occurred between locations 5 and 38 m from the cooling pad and the temperature difference between theses two locations was 7.5°C and 12°C at ventilation rates of 1 and 0.5 ACM, respectively, during extreme hours of the day (from 1230 to 1500 h). The temperature difference increased as the solar radiation increased for both ventilation rates. The solar radiation effect on temperature rise was more severe for the 0.5-ACM ventilation rate. Results showed that the daytime average of cooling water consumption with a ventilation rate of 0.5 ACM was 8.4 L/m2 of floor area. Whereas, with a ventilation rate of 1 ACM, the average was 14 L/m2 of floor area. A simple model for predicting the rate of water consumption by the evaporative cooling system was proposed. The model suggested that the daytime average of cooling water consumption with ventilation rates of 0.5 and 1 ACM were 7.9 and 16.3 L/m2 of floor area, respectively. The average electrical energy consumed from 0700 to 2000 h with ventilation rates of 0.5 and 1 ACM were 48.2 and 99.6 Wh/m2, respectively.