Another application intended for the agricultural industry is the greenhouse. The basic function of a greenhouse is to provide environmental conditions that accelerate the process of photosynthesis. Photosynthesis is the driving force for plant growth, in which CO2 is transformed into H2O, using solar energy, to carbohydrates and oxygen. Photosynthesis is highly sensitive to environmental factors.
The requirements for the interior microclimate of a greenhouse vary according to the particular plant species and its stage of growth. This is characterized by the temperature, illumination, and the interior atmosphere, i.e., water vapor, carbon dioxide, and pollutants (nitrogen oxides and sulfur).
The particular method required to create a specified environment and its economic viability depends on the prevailing ambient conditions and the value of the crop to be harvested in the particular greenhouse. It should be noted that a greenhouse designed for a particular climate can produce an environment suitable for a specific crop type, yet the same greenhouse in another location or at a different time of the year may be unsuitable for that same type of crop. Therefore, the plant varieties to be grown in a greenhouse should be chosen to suit the artificial environment that can be achieved economically within the greenhouse.
The main objective for the development of covered areas for growing food was the need for frost protection. Heat is usually obtained from solar radiation and auxiliary sources. As we saw in Chapter 2, by the expression greenhouse effect, we mean that the internal environment of a space is heated by the shortwave solar insolation transmitted through the cover and absorbed by its internal surfaces. These surfaces re-emit heat radiation, which is at longer wavelengths that cannot escape through the cover, and in this way, the heat is trapped into the space.
In places where summers are hot, greenhouses frequently need to be cooled. In areas where summers are not severe and the maximum ambient temperature remains less than 33°C, ventilation and shading techniques work well. In higher-temperature environments, however, where ambient temperatures in summer generally exceed 40°C, evaporative cooling is usually applied, which is the most efficient means of greenhouse cooling. Evaporative cooling can lower the inside air temperature significantly below the ambient air, using fan-pad and fog or mist inside a greenhouse and roof cooling systems. Apart from these systems, two composite systems can be used for both heating and cooling greenhouses: the earth-to-air heat exchanger and the aquifer-coupled, cavity flow heat exchanger. A survey of these systems is given by Sethi and Sharma (2007).
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Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.