The design of greenhouse should be based upon sound scientific principles which facilitates controlled environment for the plant growth. Controlled environment plant production systems are used widely throughout the world to produce plant materials and products at a time or place, or of a quality that can not be obtained outdoors. Controlled environment agriculture requires far more capital investment per unit area than field agriculture and thus must essentially be correspondingly more intensive to justify investment costs.
The greenhouse is a structure covered with a transparent material for admitting natural light for plant growth. The main components of greenhouse like structure, covering/glazing and temperature control systems need proper design for healthy growth of plants.
Under Indian conditions, Quonset type, multispan greenhouse is most suitable, because of its low cost and ease of fabrication. Ultra violet resistant low density polyethylene (UVLDPE) single film cladding of 200 micron thickness is sufficient for Naturally Ventilated ( NV) greenhouse and fan and pad (FP) greenhouses. This should be fully tightened by stretching on the structure to avoid fluter and tearing. It should not be nailed or screwed to the structure as it gives the chance for tearing. The T-Lock of LLock should be used for fastening the sheet at structure, as this does not tear the sheet and sheet replacement is easy.
The structure has to carry the following loads and is to be designed accordingly.
a) Dead load: weight of all permanent construction, cladding, heating and cooling equipment, water pipes and all fixed service equipments to the frame.
b) Live load: weights superimposed by use (include hanging baskets, shelves and persons working on roof). The greenhouse has to be designed for a maximum of 15 kg per square meter live load. Each member of roof should be capable of supporting 45 kg of concentrated load when applied at its centre.
c) Wind load: The structure should be able to withstand winds of 110 kilometer per hour and at least 50 kg per square meter of wind pressure.
d) Snow load: These are to be taken as per the average snowfall of the location
The greenhouse should be able to take dead load plus live load or dead load plus wind load plus half the live load.
The greenhouses are to be fabricated out of Galvanized Iron Pipes. The foundation can be 60cmx60cmx60cm or 30 cm diameter and one meter depth in PCC of 1:4:8 ratio. The vertical poles should also be covered to the height of 60 cm by PCC with a thickness of 5cm. This avoids the rusting of the poles.
Orientation of the greenhouse is a compromise for wind direction, latitude of location and type of temperature control. Single greenhouses with latitude above 40°N should have ridge running east to west to allow low angle light to enter from side rather than ends. Below 40°N the ridge of single greenhouses should be oriented from north to south, since the angle of sun is much higher. This orientation permits the movement of shadow of the gutter across the green house. The location and orientation of the greenhouse should avoid falling of shadow on the adjacent greenhouses. To avoid the shading effect from one green house to another greenhouse these should be oriented East to West. However, the wind direction and latitude are also to be considered.
If the greenhouse is naturally ventilated, the advantage of natural wind direction has to be taken to the maximum possible. The maximum dimension (length) of greenhouse should be perpendicular to the wind direction especially in summer. For fan and pad greenhouse the natural wind direction should be same as the air blown by fan.
Size of the greenhouse
The dimension of NAV GH should not be more than 50m x 50m. Bigger the greenhouse, more will be the temperature build up due to poor ventilation. The length of evaporatively cooled greenhouse should not be more than 60m.
Spacing between greenhouses
The spacing between naturally ventilated green house should be 10 to 15 m so that the exhaust from one greenhouse should not enter the adjacent greenhouse.
Height of greenhouse
The maximum height can be up to 5m for 50m x 50m green house and this can be reduced as per the reduced size of the green house. Higher is the greenhouse more is the wind load for structure and glazing. The side ventilation can be of 2 m width and roof ventilation is 1m in width.
The greenhouses are to be designed for necessary safety, serviceability, general structural integrity and suitability. The structure should be able to take all the necessary dead, live, wind and snow loads. The foundation, columns and trusses are to be designed accordingly. The greenhouse structures are to be designed to take up the loads as per design loads prescribed by the National Greenhouse Manufactures Association (NGMA of USA) standards –1994.