The effect of cloth temperature on the cooling efficiency of shade cloths in greenhouses

Tests were conducted to examine the role of cloth temperature in the cooling efficiency of shade cloths applied to greenhouses. In the first test, shade cloths (with two thermocouples attached to each) were placed over a pair of small wooden frames, supported by a single layer of clear polyethylene attached to each frame. Water was intermittently applied to 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 shade cloths. A second set of tests placed 50% shade, black plastic shade cloths (with six thermocouples attached to each) over two 6.7 X 12.2 m, double-polyethylene covered, Quonset-style greenhouses. Water was intermittently applied to one of the greenhouses on an alternate basis every other day. The greenhouses were kept empty so that energy gain could be determined by 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 wet white cloth were significantly overpredicted. The model and data showed a clear correlation between shade cloth temperature and floor temperature. For the greenhouse tests, the data showed that energy gain was directly correlated with shade cloth temperature, and that shade cloth temperature was directly correlated with the frequency of water application. Neither set of tests showed radiation screening by the water film to be a major factor influencing cooling.