Implementation of a Greenhouse Cooling Strategy with Natural Ventilation and Variable Fogging Rates

Abstract

A control strategy for greenhouse cooling with natural ventilation and variable high-pressure fog was evaluated via computer simulation and verified experimentally. Two set points were used, one based on air specific enthalpy (56kJ kg-1) for determining the vent openings, and the other based on vapor pressure deficit (VPD) of the air (1.0 kPa) for controlling the fogging rate. These set points would maintain the greenhouse air at a temperature of 24C and a relative humidity of 67%. Achieving the VPD set point was the priority, and excessive air exchange was avoided through adjustments of the vent openings when fogging demands were beyond the operational capacity of the fogging system. Results of simulations and experiments from four different days (18, 19, 29 June and 3 July 2011) were in good agreement. The performance of the control strategy developed was satisfactory to maintain the greenhouse indoor climate close to the set points with 1.1 0.4 kPa and 26C 1.7C for inside air VPD and temperature, respectively (with relative humidity of 67% 8%). The implementation results demonstrated that the control strategy was capable of reducing the air VPD by an average of 4.2 kPa when the average outside air VPD was 5.4 kPa and reducing the air temperature by an average of 10.5C when the average outdoor air temperature was 37C, and the greenhouse relative humidity was increased by an average of 52% compared to outside during the four experiment days. Deviations between the simulated and measured variables inside the greenhouse were attributed to a prevailing high and variable-magnitude outside wind speed during the experiments as well as to the time delay on the retrieval of climate data needed for computations of the strategy. Real-time climate parameters with no delay are preferable for effective computations of ventilation rates and desired fogging rates. The strategy that was developed in this study maintained the VPD close to the selected set point for all the experimental periods evaluated. Finally, the control strategy developed effectively maintained desirable climate conditions inside the greenhouse, and the simulation results were experimentally validated.