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WHAT IS MEAN BY GREENHOUSE ?
A greenhouse (also called a glasshouse) is a building in which plants are grown. These structures range in size from small sheds to very large buildings. A miniature greenhouse is known as a cold frame.

A greenhouse is a structure with different types of covering materials, such as a glass or plastic roof and frequently glass or plastic walls; it heats up because incoming visible solar radiation (for which the glass is transparent) from the sun is absorbed by plants, soil, and other things inside the building. Air warmed by the heat from hot interior surfaces is retained in the building by the roof and wall. In addition, the warmed structures and plants inside the greenhouse re-radiate some of their thermal energy in the infrared spectrum, to which glass is partly opaque, so some of this energy is also trapped inside the glasshouse. However, this latter process is a minor player compared with the former (convective) process. Thus, the primary heating mechanism of a greenhouse is convection. This can be demonstrated by opening a small window near the roof of a greenhouse: the temperature drops considerably. This principle is the basis of the autovent automatic cooling system. Thus, the glass used for a greenhouse works as a barrier to air flow, and its effect is to trap energy within the greenhouse. The air that is warmed near the ground is prevented from rising indefinitely and flowing away.

Although heat loss due to thermal conduction through the glass and other building materials occurs, net energy increases (and therefore temperature) inside the greenhouse.
Greenhouses can be divided into glass greenhouses and plastic greenhouses. Plastics mostly used are PEfilm and multiwall sheet in PC or PMMA. Commercial glass greenhouses are often high-tech production facilities for vegetables or flowers. The glass greenhouses are filled with equipment such as screening installations, heating, cooling, lighting, and may be automatically controlled by a computer.

USES

Greenhouses allow for greater control over the growing environment of plants. Depending upon the level of greenhouse, key factors which may be controlled include, temperature, light, water, fertilizer, and atmosphere. Greenhouses may be used to overcome the poor growing qualities of a piece of land, such as poor light levels, seasons, or soil infertility, thereby improving food production in marginal environments.

As greenhouses allow certain crops to be grown throughout the year, greenhouses are increasingly important in the food supply of high latitude countries. One of the largest greenhouse complexes in the world is in Almeria, Spain, where greenhouses cover almost 50,000 acres (200 km2). It is sometimes called the sea of plastics.

Greenhouses are often used for growing flowers, vegetables, fruits, and transplants. Bumblebees are the pollinators of choice for most greenhouse pollination, although other types of bees have been used, as well as artificial pollination. Hydroponics can be used in greenhouses, as well, to make the most use of the interior space.

Besides tobacco, many vegetables and flowers are grown in greenhouses in late winter and early spring, and then transplanted outside as the weather warms. Started plants are usually available for gardeners in farmers' markets at transplanting time. Special greenhouse varieties of certain crops, such as tomatoes, are generally used for commercial production.

The closed environment of a greenhouse has its own unique requirements, compared with outdoor production. Pests and diseases, and extremes of heat and humidity, have to be controlled, and irrigation is necessary to provide water. Most greenhouses use sprinklers or drip lines. Significant inputs of heat and light may be required, particularly with winter production of warm-weather vegetables.

Greenhouses also have applications outside of the agriculture industry. Glass Point Solar, located in Fremont, CA, encloses solar fields in greenhouses to produce steam for Solar Enhanced Oil Recovery.


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