Control of greenhouse-air energy and vapor pressure deficit with heating, variable fogging rates and variable vent configurations: Simulated effectiveness under varied outside climates.

• Strategy kept setpoints when either cooling or heating was needed in the greenhouse. • Fogging lowered VPD and ventilation lowered enthalpy while heating increased it. • Transpiration cooled the greenhouse air by adding up to 84.5% of the water vapor. • Control not effective when it wasn't psychrometrically possible to reach setpoints. Climate in greenhouses is continuously influenced by the outside environmental conditions. Outside conditions are always variable, especially in locations with extreme climates. Under non-extreme climates, greenhouses equipped with natural ventilation systems coupled with on–off controllers may be sufficient to effectively maintain appropriate climate conditions for plant growth. However, additional and flexible climate control systems, such as variable frequency drive (VFD) controllers, may be needed under extreme environments. In this study, the performance of a greenhouse climate control strategy, which considers plant transpiration, includes heating, operates variable vent configurations and a fogging system on a VFD basis, was tested by computer simulations. The strategy uses a set point of air specific enthalpy (55.8 kJ kg−1) to control the heaters and vents while a set point of Vapor Pressure Deficit (VPD) of 1.0 kPa is used to control the fogging system. Simulations were performed for two locations with different climates to test the strategy's capabilities. Set points were effectively maintained whenever cooling demands existed in the greenhouse. For that, the crop played a key role by adding to the atmosphere on average 55.14 and 84.53% of the water vapor by means of plant transpiration for the first and second locations, respectively. When demands for heating existed, the set points were maintained whenever it was psychrometrically possible. When the outside air is at high humidity levels, the set points cannot be achieved under this approach. Under such conditions, the set points may be continuously and simultaneously kept if a dehumidification action is added to the strategy. [ABSTRACT FROM AUTHOR]