Predicting the Interior Conditions in a High Tunnel Greenhouse

Abstract

Simple greenhouses constructed of polyethylene glazing over a steel frame with roll-up sides, called high tunnels, are used in horticulture to extend the growing season, improve crop quality, reduce disease, and reduce pest control issues. The high tunnel is effectively a passive solar structure installed over a crop that is grown using conventional, soil-based methods. Unlike large, hydroponic greenhouses, there are no fans or automated ventilation controls, nor any active heating or cooling systems. High-resolution measurements of air and soil temperatures, relative humidity, solar radiation, and wind speeds were recorded inside Quonset-style passive solar high tunnels, and adjacent open fields, for two growing seasons at two high tunnels in Guelph, Ontario. The side openings on one high tunnel were screened, while the other was left open. Lower than expected frost resistance was observed inside the high tunnels. A one-dimensional parametric energy model was developed to predict air and soil temperatures as a function of weather and high tunnel properties. The model was validated by comparing model predictions of air and soil temperatures to the measurements. A sensitivity study was conducted with the model using different combinations of parameter values to observe the effects of parameter choice on predicted high tunnel microclimate. Parametric models of high tunnel or greenhouse environments were found to be sensitive to choices of model parameters, such as soil, glazing, and thermal properties. The model and data collection was part of a longer study intended to benefit Canadian growers. An ability to accurately predict high tunnel microclimate will help growers choose crops best suited for high tunnels at their location. The data and models resulting from this study will also be useful for identifying methods of reducing energy costs in new high tunnel installations through structural changes and by modifying operating methods.