Controlling greenhouse environments

Crop production in greenhouses involves many interacting systems, some physical and some biological. Environments provided to plants comprise one of the more critical systems because the term "environment" is so inclusive. Air temperature is usually viewed as a major environmental factor for plants. However, light intensity, spectrum and daily integral compose an equally critical part of the environment. Humidity and carbon dioxide concentration are other important environmental parameters that can, and often should, be controlled, or at least modulated. Environments influence pests and diseases. Roots are particularly sensitive to root zone conditions. Virtually everything influencing plant growth composes "environment" in an integrated sense. Environment "control", in contrast, involves primarily physical systems of greenhouses. Environment control in greenhouses differs markedly from environment control in traditional industrial and commercial buildings. Rapidly varying solar intensity creates sudden changes of heating and cooling loads, particularly cooling loads. Thermal time constants of greenhouses are measured in minutes, so a rapidly changing solar environment can lead to problems with control, in particular because of slow response times of heating systems. Slow response times may or may not be important for tropical and semi-tropical greenhouse systems where heating, if any, is primarily at night. Fortunately, response times of mechanical ventilation systems are short and mechanical cooling systems are able to adjust suitably rapidly to changing cooling loads. Response times of natural ventilation systems are less swift, particularly when there is little wind, and natural ventilation is primarily by thermal buoyancy. Matching response times of mechanical (or human) systems that control vents to dynamics of changing environments is critical to successful environmental control.