THE EFFECT OF CLOTH TEMPERATURE ON THE COOLING EFFICIENCY OF SHADE CLOTHS IN GREENHOUSES

Abstract

Tests were conducted to examine the role of cloth temperature in the cooling efficiency of shade cloths appliedto greenhouses. In the first test, shade cloths (with two thermocouples attached to each) were placed over a pair of smallwooden frames, supported by a single layer of clear polyethylene attached to each frame. Water was intermittently appliedto one of the shade cloths every other day. A 60% shade, black cloth was used for the first half of testing and a 40% shade,white cloth was used for the second half. A mathematical model was developed to describe the energy balance on the shadecloths. A second set of tests placed 50% shade, black plastic shade cloths (with six thermocouples attached to each) over two6.7 . 12.2 m, doublepolyethylene covered, Quonsetstyle greenhouses. Water was intermittently applied to one of thegreenhouses on an alternate basis every other day. The greenhouses were kept empty so that energy gain could be determinedby air temperature rise alone. Output from the thermal model was compared to data observed during the shade frame tests.Model output agreed well with the observed shade cloth and floor temperatures, except that floor temperatures under the wetwhite cloth were significantly overpredicted. The model and data showed a clear correlation between shade cloth temperatureand floor temperature. For the greenhouse tests, the data showed that energy gain was directly correlated with shade clothtemperature, and that shade cloth temperature was directly correlated with the frequency of water application. Neither setof tests showed radiation screening by the water film to be a major factor influencing cooling.