Intermittent application of water to an externally mounted, greenhouse shade cloth to modify cooling performance

The cooling performance of an externally mounted, flat-woven, black-polypro pylene shade cloth (manufacturer's shade rating of 55%) was examined under both dry and wet conditions. Wetting was accomplished by intermittently spr inkling the cloth with water when outside solar levels were greater than 40 0 W m(-2). Compared to art unshaded greenhouse, the dry shade cloth reduced the rate of energy gain by about 26%, less than one-half the amount sugges ted by the shade raring. At the same time, electrical energy consumption wa s also reduced by about 8% due to reduced operation of the cooling equipmen t 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 increase d 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 te mperature rise was reduced by only about 9% under the dry cloth; however th e value is misleading because leaf temperatures were reduced nearly uniform ly along the house whereas air temperatures were reduced primarily at the e xhaust end. Under wet shade, leaf temperature rise was reduced nearly 43% a nd electrical energy consumption by 21%.