Comparative performance of a greenhouse cooling strategy with natural ventilation and fogging under different outside climates

Natural ventilation (NV) is the most frequently used method worldwide to maintain appropriate greenhouse conditions for plant growth. However, when adequate ventilation rates to evacuate the excessive heat are not possible, other alternatives such as evaporative cooling systems may enhance the cooling capabilities of the NV systems. Although high-pressure fog systems have shown good results for cooling when coupled with NV, the lack of effective and proven control strategies when combining these two systems, has limited their implementation. A greenhouse cooling strategy was tested by computer simulation, which considered the contribution for cooling and humidification from plant transpiration, within a natural ventilated greenhouse with fog added at variable and controlled rates. The strategy used set points of air specific enthalpy (55.8 kJ kg-1) and vapor pressure deficit (VPD=1.0 kPa) of the greenhouse air to control ventilator openings and fog rates, respectively. These set points would maintain an air temperature and relative humidity (RH) of 24.0°C and 66.5%, respectively. The performance of the control model, developed at the University of Arizona Controlled Environment Agriculture Center (UA-CEAC) in Tucson Arizona was tested for conditions at two different locations in Mexico (Texcoco and San Luis Potosi) each with uniquely different climates. The strategy was capable of maintaining the set points when cooling demands were present in the greenhouse, regardless of the location and its outside climate. It maintained the VPD and the specific enthalpy of the air of 1.02 ± 0.10 kPa and 52.61 ± 2.22 kJ kg-1, respectively for Texcoco; and, VPD and the specific enthalpy of the air was maintained at 1.03 ± 0.12 kPa and 54.77 ± 3.26 kJ kg-1, respectively for San Luis Potosi.