SUBOPTIMAL CO2 ENRICHMENT OF GREENHOUSES

Greenhouse CO2 enrichment in warm climates is restricted by the need to ventilate, leading some growers to intermittent enrichment, where enrichment and ventilation alternate several times an hour. This strategy relies on the heat and CO2 capacity of the system, characterized by a heating time constant of the order of 10 min, during which period ventilation may be suspended. It is shown that, for slowly changing weather, the optimal CO2 enrichment is basically not intermittent (bang-bang control), but rather quasi steady state (smoothly varying singular control). As the disturbance (weather) frequency increases, the quasi steady-state (QSS) solution becomes less and less optimal. Nevertheless, due to the difficulties involved in implementing a truly optimal control (the need for accurate weather forecast and high control fluxes), the sub-optimal QSS control may be a better choice. We chose to try a controller which aims to follow the QSS temperature and CO2 setpoints at all disturbance frequencies. The performance of this controller for high disturbance frequencies is a few per cent lower than the truly optimal solution, but over the whole season this effect may not be significant. On the other hand, the controller is likely to be more robust. Implementation of the QSS solution requires simulaneous ventilation and enrichment, properly balanced.