Alternative design formulae for the ventilation rate of greenhouses

A fresh look at a standard design formulae for ventilation of greenhouses is presented. The new approach emphasizes crop-canopy temperature rather than air temperature and covers situations with positive as well as negative ventilation Bowen ratios (ratio of sensible to latent heat). The latter situation is characteristic of arid conditions, and is the result of intense evapotranspirational cooling on hot, dry days. Two design formulae are considered, depending on the availability of experimental data for parameter estimation. First, a midday crop-in-greenhouse model is developed. It relates canopy temperature to environmental and operational conditions and can be used, at least in principle, to find the dependence of stomatal and aerodynamic conductances on the ventilation rate. If canopy-temperature data are not available, but transpiration data make it possible to develop a simple predictive formulae, representative values of the stomatal and aerodynamic conductances can be obtained. These can then be used to calculate the required ventilation rate. The procedure is illustrated with data from a temperate-maritime climate. Some improvement over the standard approach can be made even with just standard environmental data. With these an empirical relationship may be found between the Bowen ratio and the temperature rise in the greenhouse. The Bowen ratio is then used in the revised design formulae. This approach removes some of the guesswork currently involved in selecting the evaporation coefficient for the standard design formulae. This procedure is illustrated with data sets from mediterranean locations, which also demonstrate the effect of evaporative cooling. (C) 1997 Silsoe Research Institute.