Optimal CO2 enrichment of greenhouses in mild climates

The formulation of a simple optimization criterion led to two approaches for the enhancement of CO2 enrichment. Both result in a reduced ventilation rate and hence in less 'waste' of CO2. One approach is to reduce the energy load on the greenhouse and the other, for which some examples are shown, is to increase the humidity of the exhaust air. Optimal control trajectories showed the existence of three daytime operating regimes, which switch from one to the next as the energy load increases: A: Enrichment without ventilation. B: Simultaneous ventilation and enrichment. C: Ventilation without enrichment. Evaporative cooling increases the duration of regime A at the expense of regime C. Reference to canopy temperature, rather than to air temperature, reduces the apparent advantage of evaporative cooling, except when it is applied at or close to the canopy, or when it prevents water stress.