Planning of Greenhouse Facility
---------------------------------------
A greenhouse, has basically the purpose of providing and maintaining a growing
environment that will result in optimum production at maximum yield. The agriculture in the
controlled environment is possible in all the regions irrespective of climate and weather. As an enclosing structure for growing plants, greenhouse must admit the visible light portion of solar radiation for the plant photosynthesis and, therefore, must be transparent. At the same time, to protect the plants, a greenhouse must be ventilated or cooled during the day because of the heat load from the radiation. The structure must also be heated or insulated during cold nights. A greenhouse acts as a barrier between the plant production areas and the external or the general environment.
1.1 Site selection and orientation
A greenhouse is designed to withstand local wind, snow and crop loads for a specific cropping activity. In this way, the structure becomes location and crop specific. The building site should be as level as possible to reduce the cost of grading, and the site should be well aerated and should receive good solar radiation. Provision of a drainage system is always advisable. It is also advisable to select a site with a natural windbreak. In regions where snow is expected, trees should be 30.5 m away in order to keep drifts back from the greenhouses. To prevent shadows on the crop, trees located on the east, south, or west sides should be at a distance of 2.5 times their height.
1.2 Structural design
The most important function of the greenhouse structure and its covering is the protection of the crop against hostile weather conditions (low and high temperatures, snow, hail, rain and wind), diseases and pests. It is important to develop greenhouses with a maximum intensity of natural light inside. The structural parts that can cast shadows in the greenhouse should be minimized.
The different structural designs of greenhouse based on the types of frames are available. A straight side wall and an arched roof (a- type) is possibly the most common shape for a greenhouse, but the gable roof (b type) is also widely used. Both structures can be free standing or gutter connected with the arch roof greenhouse. The arch roof and hoop style (c -type) greenhouses are most often constructed of galvanized iron pipe. If tall growing crops are to be grown in a greenhouse or when benches are used, it is best to use a straight side wall structure
(d- type) rather than a hoop style house, this ensures the best operational use of the greenhouse. A hoop type greenhouse is suitable for low growing crops, such as lettuce, or for nursery stock which are housed throughout the winter in greenhouses located in extremely cold regions. A Gothic arch frame structure (e- type) can be designed to provide adequate side wall height without loss of strength to the structure.
Loads in designing the greenhouse structures include the weight of the structure itself and, if supported by the structure, loads of the equipment for the heating and ventilation and water lines. The load may also include the weight of crops, if they are trailed to a support system carried by the greenhouse frame; and also loads from wind and snow. Greenhouse structures should be designed to resist a 130 km/h wind velocity. The actual load depends on wind angle, greenhouse shape and size, and the presence or absence of openings and wind breaks.
The ultimate design of a greenhouse depends on the following aspects:
(i) The overall structural design and the properties of the individual structural components.
(ii) The specific mechanical and physical properties which determine the structural behavior of the covering materials.
(iii) The specific sensitivity of the crop to light and temperature to be grown in the greenhouse.
(iv) The specific requirements relevant to the physical properties of the covering material.
(v) The agronomic requirements of the crop.
1.3 Covering materials
The various factors; to be considered while selecting the greenhouse covering material; are light transmission, weight, resistance to impact, and durability to outdoor weathering and thermal stability over wide range of temperatures. Before selecting the covering material, two important points should be taken into consideration: the purpose for which greenhouse facility is intended and service life of material. In temperate regions where high temperatures are required, the covering material with high light transmission and far IR absorption must be selected. Also the loss of heat by conduction should be minimum. Another important aspect in selection of covering material is the service life of the material
Covering material Life span
1. Glass and acrylic sheet -----20 years
2. Polycarbonate and fiberglass-reinforced polyester sheet ------5-12 years
3. Polyethylene----- 2-6 months
4. Polyethylene stabilized for UV rays------- 2-3 years
The ideal greenhouse selective covering material should have the following
properties:
(i) It should transmit the visible light portion of the solar radiation which is utilized by plants for photosynthesis.
(ii) It should absorb the small amount of UV in the radiation and convert a portion of it to fluoresce into visible light, useful for plants.
(iii) It should reflect or absorb IR radiation which are not useful to plants and which causes greenhouse interiors to overheat.
(iv) Should be of minimum cost.
(v) Should have usable life of 10 to 20 years.
Such a covering material will obviously improve CEA performance, reducing solar heat load, increasing light levels and crop yields.
