Greenhouses

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, that is, 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-pads 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).

7.5.1 Greenhouse materials

Traditionally, the first material used for greenhouse cover was glass. As an alternative cover material to glass, oiled paper was tried in the Netherlands during the late eighteenth century and was in common use in Japan well into the twentieth century (Norton, 1992). After the Second World War, plastic materials became more readily available. From the time clear plastic materials were first produced on a commercial scale, their potential for replacing glass in agricultural facilities has been recognized. Nowadays, PVC and polyethylene films are attached internally to the greenhouse framework, thus creating an insulating air gap between the outer cover and the protected artificial environment. Polyethylene is very popular for agricultural applications because it is available in wider sections than most other films and is low cost, although it has a short lifetime of about a year when exposed to typical weather conditions. Additionally, because polyethylene is the most common plastic film used, data for light transmission through this material are readily available.

Generally, plastic materials have inferior light transmission properties compared with glass. Additionally, since they degrade when exposed to heat and ultraviolet light, their useful life is much shorter, typically a few years compared with decades for glass. Condensation on the inner surface of the cover, which under some conditions could persist during the day, reduces the light transmission. This reduction is more pronounced with plastics than with glass because of the higher angle of conduct between the water bubbles and the plastic, leading to a higher proportion of reflected light. The advantages of plastic materials, however, are their low specific mass and high strength, requiring a lightweight structure and lower cost, resulting in lower initial investment.

Although polyethylene is the most widely used plastic film in agriculture, other materials are available, such as polymers containing fluorine compounds, whose radiation transmission properties and resistance to aging are superior to those of polyethylene films. These, however, are more expensive than polyethylene.