Influence of humidity control for decreasing air temperature without shading on light environment and leaf temperature in a greenhouse

The method of control of fog cooling and ventilation windows to decrease air and leaf temperatures in hot summer season without shading was examined. Ventilation rate and evapotranspiration rate were calculated by heat-balance method every two minutes. If these values were accurate (more than zero), the method of fog cooling on base of heat-balance method was conducted. But these values were not accurate in many cases, the main method was adjusting ventilation window angles on base of relative humidity (aimed at 75-90%) and spraying fog for 90 s every 2 min. The data of PPF, air and leaf temperatures, relative humidity on both the fog cooling greenhouse and the shading greenhouse were recorded from 9:00 to 16:00 on 8 September 2008. If the solar radiation outside the greenhouse was higher than 0.65 kW m-2, a shading curtain was spread in the shading greenhouse. Tomatoes were cultivated using rockwool system in both greenhouses from 28 August. The inside air temperature of the fog cooling greenhouse was 3.9°C lower than that outside the greenhouse, and this value was larger than that in the shading greenhouse (1.4°C). Leaf temperatures in both greenhouses were nearly at the same level. Average PPF value (956 μmol m^-2 s^-1) in the fog cooling greenhouse was higher than that in the shading greenhouse (513 μmol m^-2 s^-1). The fog cooling treatment maintained about 80% of relative humidity. Enthalpy in the fog cooling greenhouse was sometimes higher than that outside the greenhouse. The treatment of controlling relative humidity with fogging and adjusting ventilation was effective on decreasing air and leaf temperatures without decreasing PPF in greenhouse. It was thought that monitoring of enthalpy levels inside and outside the greenhouse was necessary in case of adjusting angles of ventilation windows on base of relative humidity.