ADVERTISEMENTS:
During winter months when temperature in green house goes down below the optimal range of temperature required to produce quality blooms. Heat loss in green house is through conduction, infiltration and radiation. Metallic frame, polythene, sash bars, etc. are the sources of heat loss through conduction whereas leakages and holes are the sources of infiltration.
In radiation warm objects emits radiant energy which passes through air to colder objects without warming air significantly and objects become warms. Glass, venyl plastic, and water fiber reinforced plastic (FRP) are relatively opaque to radiant every whereas clear polythene is not. Therefore, polyethylene used as covering material for poly houses losses heat rapidly through radiation.
Heat Loss Through Various Green House Covering
ADVERTISEMENTS:
The requirement for heating the green house is adding of heat at the rate it is being lost.
ADVERTISEMENTS:
Source of Energy:
There are three sources of energy viz. fossil fuel, solar, and electricity. Among these, fossil fuel source of energy is mostly used for heating the green houses. Among fossil fuel, coal or gasoline can be used to heat up the water or steam. The hot water or steam is then circulated through pipes inside the green house and desired effect is obtained.
For this the central boiler system is installed. The capacity of boiler depends upon the size of range of green house. It is known that much heat is required to warm the boiler and its associated plumbing material before actual heat is received in the green house. It is therefore, very necessary to match the boiler capacity in view of green-house area and plumbing units.
ADVERTISEMENTS:
For smaller range of green house, hot water is used with a water temperature of 82 to 95°C. Large volume of water is required because 1 Btu (British thermal unit) heat unit is obtained from 1 lb of water when it losses 1°C temperature. For circulating the water circulating pipes are required.
For this the pipe diameter of 5.1 mm is used. Placement of these pipes in green house is very important. If all the pipes are stacked on the side wall or end wall, undesirable pattern of hot air flow will occur and certain cold spots will be created in the center. This can be avoided by placing one third pipes across the green house running from end to end.
The remaining of two third pipes is placed in stacks along the outer wall. To reduce the cost of pipe, fin pipes should be used. Fin pipes are conventional pipes with numerous thin metal plates radiating out ward which increases its surface area which transfer heat more rapidly and evenly.
It is known that 1 foot of fin pipe is equal to 4 feet conventional pipes. However, for better results fin pipes should be alternate with conventional pipes at equal intervals. Pipes in coils can also be used and these coils can be arranged in two styles. For hot water system box style is used whereas for steam, tram borne is used.
Apart from central heating system, localized system is also popular because of low initial price. In this small heaters with self-contained fire box system are installed in each green house. But in long term, this system proves costly due to high cost of maintenance than a central system. Low intensity of infrared radiant heaters can also be used which can save 30% energy over conventional heaters. So far, solar system of heating is found useful only for small or hobby green house.
Heat Conservation:
Efforts should be made to conserve the heat of green-house so that green-house production becomes economical. A sizeable decrease in heat loss can be made possible through green-house design and by taking extra care during peak days of winter e.g. by double covering (it saves 40% energy). Covering of glass by films, wall insulation, by sealing leaks and thermal screen which are popular among green house in cold season.
Thermal screens are curtains of different material such as polyethylene, polyester film; polyesters film strips or polyester cloth. It can be drawn eave to eave or gutter to gutter as well as around inner perimeter of green house. These thermal screens reflect back the heat to plant and soil. Such steps can save the consumption of fuel by 20-60%.
ADVERTISEMENTS:
Automation can also be done in bigger green-house if used on large scale. By planting green wind break in north and south side at a distance of 2.5 times the height of green house, 5-10% fuel saving can be achieved.
