Section 1: Introduction
Greenhouse glazing materials allow shorter wavelength radiation (i.e. visible light) to pass through but long wavelength radiation such as infrared (heat) is trapped inside the greenhouse. The temperature inside of a greenhouse may be up to 30° F higher than the ambient temperature outside of the greenhouse (hence, the greenhouse effect). Because of this property, greenhouses may require both summer (or during high temperature seasons generally) and winter (or during cool seasons generally) cooling systems in order to maintain optimal temperatures for the crops being grown.
In the summer, both high temperatures outside the greenhouse (ambient air temperatures) and solar heating of the air inside of the greenhouse necessitates the use of various cooling systems to remove hot air from inside the greenhouse and to bring in air from outside of the greenhouse. In some cases, if the outside air temperature is appropriate, simply replacing the air inside of the greenhouse with outside air might be adequate to maintain acceptable air temperatures inside of the greenhouse. In other cases, the outside air temperature may be above the optimal temperature for the crop and the air brought from the outside of the greenhouse may need to be cooled as it enters the greenhouse (to increase the cooling potential). In the winter, cooling may be required because of the solar input. However, since the air temperature outside of the greenhouse is usually lower than the optimal temperature for the crops being grown, exchanging inside air with the outside air is usually adequate to cool the greenhouse. Therefore, the cooling system (or cooling strategies) used in the summer and those used in the winter are often different.
Cooling systems used in both summer and winter may be generally classified into two categories. The first is passive systems. These cooling systems do not use energy to cool the greenhouse. In contrast, active cooling systems use energy in some way to cool the greenhouse.
