Evaporative cooling of a ventilated greenhouse rose crop

Greenhouse crops may suffer from excessive heat load in warm weather. A procedure is developed to evaluate latent heat cooling by means of crop transpiration and free water evaporation from a wet pad and fan system. The procedure uses concurrent external climatic factors as input data. It treats construction characteristics (dimensions and radiometric properties of the roof cover), plant foliage (leaf area, stomatal conductance) and ventilation rate as parameters to calculate heat transport coefficients. Measurements in a greenhouse rose crop (Rosa indica L cv ‘Mercedes Long’) show that the numerical solution of the energy balance equation predicts accurately crop transpiration, foliage temperature, air temperature and humidity inside the greenhouse. With ventilation rates of 30 volume changes per hour and external air humidity below 50%, transpiration of a plant well supplied with water, cools the foliage and the air in the greenhouse below external temperature even when solar radiation is at its maximum value. Cooling obtained with an evaporative wet pad at the air inlet lowers vapour pressure deficit in the greenhouse and decreases transpiration rate. Still, total latent heat dissipation added to pad evaporation and crop transpiration is higher than that obtained by crop transpiration without the wet pad. The combined solution of the energy balance of the air passing through the evaporative pad and of the crop predicts accurately transpiration of the rose crop and internal temperature and humidity. The evaporative pad cools the air considerably; but the lowering of transpiring leaf temperature is only minor. Evaporation from the pad decreases when external humidity increases. Crop transpiration rate when the wet pad operates is nearly independent of external humidity and ventilation rate.