ABSTRACT: The last decades there has been a lot of improvements in greenhouse farming, as a result of the increasing demand for high-quality products. There is a dominant usage of glass for greenhouses in Europe, but this is very expensive. A cheaper option is a plastic covering, which covers the whole crop and therefore influences the whole environment.
There are two materials used the most in plastic coverings: polyethylene (LDPE) and Ethylene Vinyl Acetate (EVA). LDPE is the cheapest of the two and is available in large sheets. It has some disadvantages: it’s lifetime is short with a maximum of two or three years and it has a poor heat retention (60% - 80% will pass through). The thermal characteristics of EVA are better. They both transmit a lot of light and are therefore very decent for usage in greenhouses. EVA, on the other hand, is liable to stretching at high temperatures and could damage the greenhouse by flapping on its structure.
There are some techniques to get the best of both materials. Co-extrusion is one of them, in which you make multiple layers of EVA and polyethylene combined with each other to exclude the negative characteristics of both materials. Another simple technique is to use multiple layers of the same material. This could save up to 30% of the heat losses.
For building a greenhouse with a plastic covering there are some important considerations to keep in mind:
- East-west orientation gives the most light-transmission during winters and less during summers.
- Asymmetric shapes with a lower slope orientated to the south gives a better transmission of diffuse light.
- An arched roof improves light transmission, if it has a high radius curvature.
- Make sure the covering can’t flap against the structure
- Use simple methods to change the covering.
- Use effective ventilation with a very good efficiency.
- Minimize condensation problems by choosing the correct roof with the correct slope and covering material.
- The lifetime of the covering should be over 1 year.
- Collect and store the rainwater during rainy seasons to use it for dry seasons.
For good ventilation you need holes in the sidewalls, at the ridge and in the top of the greenhouse. The top ventilation is provided by two different slopes of the ceiling so they meet at different levels on the ridge line. The type of ventilation system is dependent on the size of the greenhouse and the crops inside it. A multispan structure will accomplish these requirements the best in the environment of Kenya. These structures are expensive, but have some important advantages: land can be used more efficiently, the environmental conditions are better, due to its larger volume and its efficient fan ventilation.
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http://www.agnet.org/library.php?func=view&id=20110729171201 (accessed at 08-09-2016)