Modelling evapotranspiration in off-line simulations of greenhouse climate control

Abstract

The design of greenhouse environmental control involves simulations under various climate and system configuration alternatives. These off-line model-based simulations cannot use online measured greenhouse conditions (temperature, humidity) and must rely on historical weather data and on well-established greenhouse and crop properties. An important element of such simulations is the partitioning of the dissipated energy into sensible and latent heat, which can be achieved in different ways. Specifically, the rate of evapotranspiration (ET) may be obtained from 1) knowledge of the aerodynamic and stomatal canopy resistances, 2) estimating ET from atmospheric conditions and control settings, or 3) estimating the Bowen (sensible to latent heat) ratio in the ventilation stream (VBR) from the atmospheric conditions. The objective of this study is to compare the simulated behaviour of a tomato greenhouse, based on these three quite different approaches, both among themselves and against a full-year data set from a commercial Dutch cherry-tomato greenhouse. Using a quasi-steady-state simulation-optimization program with hourly time steps, trajectories of indoor conditions (temperature, humidity, CO2 concentration), control actions (heating, ventilation, CO2 enrichment, thermal screen), yield and ET were obtained, and their monthly averages presented. At the monthly time scale the simulated results are very similar among themselves and to those observed, with the exception that the simulated summer CO2 concentrations, and hence fruit yield, are higher than those observed, possibly due to the optimization inherent in the simulations. The considerable VBR discrepancy between the three ET formulations during autumn is due to low nighttime ET, but this has a negligible effect on the daily calculated ET. In conclusion, it seems that for off-line design purposes a simple linear dependence of ET on global radiation is sufficient. Adaptation of the model to existing facilities may require a more refined version of the VBR approach.