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An effective control logic for a high-pressure fog cooling system for a naturally ventilated greenhouse has been developed. A suitable duration of the fog cooling system operating time is estimated using weather data and the target relative humidity inside the greenhouse with a computational model based on mass and thermal balances of the greenhouse air. The program has three main tasks: (1) to read the dry bulb temperature set point inside the greenhouse for fogging system operation and the target relative humidity defined by the user; (2) to compute a suitable duration of the fog cooling system operation; and (3) to display relevant output information to the user. The duration of fog cooling system operation decreased as the water vapor pressure deficit inside the greenhouse decreased. The control logic of the fog cooling system would be used to lower dry bulb temperature by increasing the relative humidity to the target value. Our results prove that implementing this control logic for greenhouse fog cooling system is technically feasible.

A simple CO 2 control system was developed for a greenhouse with a high natural or forced ventilation rate. This CO 2 control system maintains the difference in CO 2 concentration between inside and outside the greenhouse, ∆C, during daytime being small (ca. 5 µmol mol-1), by supplying a variable amount of CO 2 gas with time. Results showed that this system worked satisfactorily, while the CO 2 concentration outside the greenhouse varied with time in a range between 350 and 450 µmol mol-1 with an average of 380 µmol mol-1. In this system, the CO 2 supply rate was determined based on the time course of ∆C only, and does not consider any physical or physiological mechanisms of CO 2 balance in the greenhouse. Theoretically, the CO 2 supply rate is equal to the net photosynthetic rate of greenhouse crops (Pn), if both rates of CO 2 emitted from the substrate/floor and CO 2 exchange between inside and outside the greenhouse are negligibly small. Hence, this CO 2 control system will be useful not only for monitoring and increasing the Pn in a greenhouse with a high ventilation rate, but also as a diagnosis tool for monitoring the physiological status of greenhouse crops.