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

摘要

The cooling performance of an externally mounted, flat-woven, black-polypropylene shade cloth (manufacturer’s shade rating of 55%) was examined under both dry and wet conditions. Wetting was accomplished by intermittently sprinkling the cloth with water when outside solar levels were greater than 400 W m–2. Compared to an unshaded greenhouse, the dry shade cloth reduced the rate of energy gain by about 26%, less than one-half the amount suggested by the shade rating. At the same time, electrical energy consumption was also reduced by about 8% due to reduced operation of the cooling equipment in the shaded house. Under the wet cloth, the reduction in rate of energy gain improved to about 41%, of which 3.5% was attributable to the increased shading provided by the water film. Air temperature rise along the house was reduced by 18% under the dry cloth and 40% under the wet cloth. Leaf temperature rise was reduced by only about 9% under the dry cloth; however, the value is misleading because leaf temperatures were reduced nearly uniformly along the house whereas air temperatures were reduced primarily at the exhaust end. Under wet shade, leaf temperature rise was reduced nearly 43% and electrical energy consumption by 21%.