INTERMITTENT APPLICATION OF WATER TO AN EXTERNALLYMOUNTED, GREENHOUSE SHADE CLOTH TO MODIFY COOLING PERFORMANCE

Abstract

The cooling performance of an externally mounted, flat-woven, black-polypropylene shade cloth(manufacturers shade rating of 55%) was examined under both dry and wet conditions. Wetting was accomplished byintermittently sprinkling the cloth with water when outside solar levels were greater than 400 W m2. Compared to anunshaded greenhouse, the dry shade cloth reduced the rate of energy gain by about 26%, less than one-half the amountsuggested by the shade rating. At the same time, electrical energy consumption was also reduced by about 8% due toreduced operation of the cooling equipment in the shaded house. Under the wet cloth, the reduction in rate of energy gainimproved to about 41%, of which 3.5% was attributable to the increased shading provided by the water film. Airtemperature rise along the house was reduced by 18% under the dry cloth and 40% under the wet cloth. Leaf temperaturerise was reduced by only about 9% under the dry cloth; however, the value is misleading because leaf temperatures werereduced nearly uniformly along the house whereas air temperatures were reduced primarily at the exhaust end. Under wetshade, leaf temperature rise was reduced nearly 43% and electrical energy consumption by 21%.