Application of simulation models to optimize the control of thermal screens

The heat and mass transport through screens has been described in a model in dependence of material properties of screens, screen- and window openings and ambient conditions. The model was integrated with a simulation model for greenhouse environment and validated with greenhouse experiments. The model was used to study the performance of combined screen and minimum ventilation control strategies with different humidity setpoints, with respect to greenhouse air humidity level, energy consumption and the number of control actions. The humidity control strategies were based on either first proportional control of the screen followed by control of the minimum ventilation or vice versa. Results indicated a slightly better performance of strategies using first proportional control of the screen with respect to energy consumption and the number of hours with humidity exceeding the setpoints. Energy consumption is only influenced in a small way by the settings for maximum screen aperture during the night, but is strongly affected by the setpoints for humidity. Based on the simulation study several control strategies were selected and tested in a greenhouse experiment. At low outside temperatures the humidity can be kept below or around the setpoints by control of the screen aperture. At higher outside temperatures this appeared impossible, for low RH-setpoints, but even opening the screen entirely did not get the humidity lower.