Need for Polyhouse
Polyhouse is a way of climate controlled cultivation. It is also some time identified with a greenhouse? In polyhouse all types of activity be it producing seeds, plant nursery, genetic engineering, and of course high quality produce of all types for marketing and getting money for them all season is possible.
Human have been carrying out agricultural practices producing many crops in open fields. These fields have served humanity very well; in fact for many crops open field agriculture is still the only option. However we are now a closed society with boundaries between nations blurring – we encourage free trade and production based on native skills, creativity and infrastructure labor capacity etc. factored in so that specific products of individual nation is desired globally.
The population of world is increasing. This increase requires food production levels to increase also. And paradoxically arable land is slowly reducing. The focus is on produce more with less land.
Cultivation in Polyhouse removes almost all barriers of varied climatic conditions. Farmer gets Full control on climate, fertilizers, water requirement and gives protection from many onslaughts of pests and diseases with automation and less labor. This control in turn allows higher yields with better quality of produce per unit area with no restriction on seasonal variations. With polyhouses requiring lesser field area, individuals get a self employment as area required is less and labor involved is also less.
Types of polyhouse:
Polyhouses construction is limited only by human ingenuity and availability of readily available local material for structure. Also the tenure of polyhouses is also a factor. One thing common to all types is that its roof would be clad with polythene sheets for normal type of polyhouses; whereas poly sheet roofs are used in polyhouses made to last.
Based on structure there are Bamboo structure, Wood structure, Iron structure and Steel structure. Design wise there are curved roof, and the Quonset type.
All modern polyhouse are either Normally Ventilated Polyhouses or the Fan-and-Pad Polyhouses.
Naturally Ventilated Polyhouse is best suited at places where it is neither too hot nor too cold. Here there are opening in the sides which can be closed or opened to let in cold air and the warm air goes out of top ventilator slits.
In Fan&Pad types pads with sump arrangement are placed on the leeward side wall of polyhouse so that the pads are constantly kept wet. On the opposite side fans are put which blow out the air from the inside of the polyhouse. The polyhouses needs to be fully closed except for the pad and fans so that cooler air from the pad circulates around all over inside of polyhouse.
Steps in Polyhouse Construction
While constructing the polyhouse, the following need to be observed.
- Site : the site may be a leveled and preferably an elevated ground from surrounding. If not then as many leveled grounds as smaller polyhouse are to be made.
- Availability of good quality water (Ph 6 to 7 and EC around 0.3 mS/cm or less), and soil pH 5.5 to 6.5 and EC less than 1 ms/cm.
- Site should be approachable; clear from all sides and no tall trees.
- Orientation: orientation should take advantage of available sun shine.
- Size and height : depends on need and land availability; height for smaller polyhouse would be 3.5-4.5 m and for larger polyhouse (greater than 500 sq m. ) would be between 4.5 to 5.5 m.
- Electricity and road connection so that produce is easily transportable to market.
the figures below shows a cross section and main parts of a polyhouse.
Roofing or Cladding Polythene material types:
cladding is based on the type of crop to be raised inside the polyhouses. Some crops require more sunlight and some lesser. Some crops require IR protection and some do not. Normally 200 microns thickness is used in plastic films. There are two types of films – one is monolayer clear transparent UV stabilized films and second type is diffused, anti drip, anti sulphus layering etc.
Climate consideration inside the polyhouse
|Sl||Climate factor||Desirable level||remarks|
|1||Air/wind movement||Inflow:outflow is 1:1||Affects temperature, Relative Humidity and CO2 concentration|
|2||Carbon dioxide CO2||350 – 1000 ppm||Required for good photosynthesis|
|3||Radiation/light||50000 lux||Photosynthesis, photo morphogenesis, photoperiodism|
|4||Temperature||18 – 25 degree C||Cell division, elongation, respiration, photosynthesis, water uptake, transpiration|
|5||Relative humidity RH||60 – 80%||Plant stability|
Structural Design of Polyhouse
The Polyhouses are to be fabricated out of Galvanized Iron Pipes. The foundation can be 60cmx60cmx60cm or 30 cm diameter and one meter depth in RCC of 1:4:8 ratio. The vertical poles should also be covered to the height of 60 cm by concrete cement mix with a thickness of 5cm. This avoids the rusting of the poles. Some other aspects are:
- Tubular structures are preferred. Avoid channel structures as it may twist due to wind pressure.
- Column should run up to top of the Polyhouse to ensure maximum structural strength.
- Structure should withstand dead load, live load and crop load also.
- Foundation should be of Telescopic method not traditional.
- Hot dip galvanizing with min 2mm thickness is desirable.
- Poly-grip mechanism anchored with zig – zag springs and overflow film should be rammed well below the soil.
- Roll up sides and side vents should be covered with insect proof nets.
If the Polyhouse is naturally ventilated, the advantage of natural wind direction has to be taken to the maximum possible.
Size of the Polyhouse
The dimension of naturally ventilated polyhouse should not be more than 50m x 50m. Bigger the polyhouse, more will be the temperature build up due to poor ventilation. The length of evaporative cooled Polyhouse should not be more than 60m.
Spacing between Polyhouse
The spacing between naturally ventilated Polyhouse should be 10 to 15 m. Exhaust from one Polyhouse should not enter the adjacent Polyhouses.
Height of Polyhouse
The maximum height can be up to 5m for 50m x 50m for a big polyhouse. This can be reduced as per the reduced size of the Polyhouse. Higher is the Polyhouse 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.
Roof: transparent cover of a Polyhouse.
Gable: transparent wall of a Polyhouse
Cladding material: transparent material mounted on the walls and roof of a green house.
Rigid cladding material: cladding material with such a degree of rigidity that any deformation of the structure may result in damage to it. Ex. Glass
Flexible cladding material: cladding material with such a degree of flexibility that any deformation of the structure will not result in damage to it. Ex. Plastic film
Gutter: collects and drains rain water and snow which is place at an elevated level between two spans.
Column: vertical structure member carrying the Polyhouse structure
Purlin: a member who connects cladding supporting bars to the columns
Ridge: highest horizontal section in top of the roof
Girder: horizontal structure member, connecting columns on gutter height
Bracings: To support the structure against wind
Arches: Member supporting covering materials
Foundation pipe: Connection between the structure and ground
Span width: Center to center distance of the gutters in multispan houses
Polyhouse length: dimension of the Polyhouse in the direction of gable
Polyhouse width: dimension of the Polyhouses in the direction of the gutter
For greater specifications of the polyhouse technical specification of naturally ventilated greenhouse and fan-and-pad type green house: