Modeling Thermal Stratification in Fan-Ventilated Greenhouses

Abstract

A two-dimensional thermal model was developed to investigate the thermal stratification in fan-ventilated greenhouses. Model inputs include outside weather (air temperature, humidity, and solar radiation), geometric parameters of crop rows and leaf area index, greenhouse ground and roof temperatures, ventilation rate, and operation of evaporative cooling pads. Comparing predictions with observed data indicated that the air temperature and relative humidity were modeled at acceptable accuracies, with air temperature underpredicted by 1.3C and relative humidity overpredicted by 9%, on average for a planted greenhouse. For an unplanted greenhouse, the air temperature was predicted with an absolute error of 0.7C, while for relative humidity the absolute error was 3%. Vertical temperature variation, defined as maximum temperature minus minimum temperature at approximately the central location of greenhouse, was predicted with an absolute error of 0.1C and a relative absolute error of 10% for the planted greenhouse, while for an unplanted greenhouse it was 0.6C for the absolute error and 12% for the relative absolute error. Simulations with the model suggest that increasing ventilation rate reduced the vertical temperature gradient. Increased ventilation reduced air temperature more at the top than the bottom of the greenhouse. Greater air temperature variation was produced when using evaporative pad cooling than not. Air temperature was reduced more at the bottom than at the top with evaporative pad cooling. The presence of a canopy altered the vertical air temperature distribution and reduced the temperature variation. A sample simulation showed that on a typical summer day at Raleigh, North Carolina, the presence of a canopy row with a height of 1.75 m occupying 69% of the ground area reduced the air temperature variation from 11.5C to 1.8C in a fan-ventilated greenhouse operating with a ventilation rate of 0.087 m3 m-2 s-1 and using evaporative pad cooling. The peak air temperature generally occurred at the top of canopy or somewhat below the canopy top. This finding may have some significance in establishing the location of temperature control sensors in future control systems.