The Penman-Monteith evapotranspiration equation as an element in greenhouse ventilation design

Transpiration is a major cooling mechanism of greenhouse crop canopies. The larger this energy dissipation component, the smaller the required ventilation rate. The evaporation coefficient, namely the fraction of the radiation load dissipated as latent heat, is therefore an important ventilation design parameter, of which designers have currently no more than qualitative knowledge. In an attempt to overcome this difficulty, the present study proposes to incorporate the Penman-Monteith evapotranspiration equation into the standard ventilation design formula. With this modification the design adapts automatically not only to different radiation loads and temperatures, but also to different humidity conditions. The new design formula can be modified for greenhouses with an evaporative cooling system, and either air temperature or crop temperature may be used as the design criterion. The applicability of this approach depends on the availability of reliable bulk Penman-Monteith coefficients, which may be improved by a joint analysis of data from different climatic regions. (C) 2002 Silsoe Research Institute. Published by Elsevier Science Ltd. All rights reserved.