ALTERNATIVE DESIGN FORMULAS FOR THE VENTILATION RATE OF GREENHOUSES

A fresh look at a standard design formulae for ventilation of greenhou ses is presented. The new approach emphasizes crop-canopy temperature rather than air temperature and covers situations with positive as wel l as negative ventilation Bowen ratios (ratio of sensible to latent he at). 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 e xperimental data for parameter estimation. First, a midday crop-in-gre enhouse model is developed. It relates canopy temperature to environme ntal and operational conditions and can be used, at least in principle , to find the dependence of stomatal and aerodynamic conductances on t he ventilation rate. If canopy-temperature data are not available, but transpiration data make it possible to develop a simple predictive fo rmulae, representative values of the stomatal and aerodynamic conducta nces can be obtained. These can then be used to calculate the required ventilation rate. The procedure is illustrated with data from a tempe rate-maritime climate. Some improvement over the standard approach can be made even with just standard environmental data. With these an emp irical relationship may be found between the Bowen ratio and the tempe rature rise in the greenhouse. The Bowen ratio is then used in the rev ised design formulae. This approach removes some of the guesswork curr ently involved in selecting the evaporation coefficient for the standa rd design formulae. This procedure is illustrated with data sets from mediterranean locations, which also demonstrate the effect of evaporat ive cooling. (C) 1997 Silsoe Research Institute.