storageandcanop

01/10/2021

A greenhouse

A hoop barn kits, cold stove or greenhouse is an enclosed, walk-in place used for growing plants, both ornamental and horticultural, to protect them from excessive cold at certain times of the year. It is usually equipped with a translucent outer cover of glass or plastic, which allows the control of temperature, humidity and other environmental factors, which is used to promote the development of plants. The greenhouse takes advantage of the effect produced by solar radiation which, when passing through a glass or translucent plastic, heats the environment and the objects inside; these, in turn, emit infrared radiation, with a longer wavelength than solar radiation, so they cannot pass through the glass on their way back, and are trapped and produce the heating of the environment. Emissions from the Sun to the Earth are short wave, while those from the Earth to the exterior are long wave. Visible radiation can pass through glass, while some infrared radiation cannot.
The glass or plastic works as a selective transmission medium for various spectral frequencies, and its effect is to trap energy in the greenhouse, which heats the indoor environment. It also serves to prevent convective heat loss. 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 automatic cooling system.
In the absence of a covering, the absorbed heat would be removed by convective currents and by the emission of infrared radiation (wavelength longer than visible light). The presence of glass or plastics prevents the transport of the accumulated heat to the outside by convection and obstructs the outflow of part of the infrared radiation. The net effect is the accumulation of heat and an increase in the temperature of the enclosure. See solar greenhouse (technical) for a more detailed discussion of a solar greenhouse.
Glasses have very little resistance to the passage of heat by transmission (in fact, for single glazing, the thermal transmission coefficient is considered to be zero and only the sum of the surface resistances is taken into account), so that, contrary to what some believe, having two different temperatures on each side, there are significant transmission losses (the glass has a transmittance of U = 6.4 W/m²-K, even higher if it is in an inclined position with respect to the vertical). The result is that the higher the temperature, the lower the heat retention effect, i.e. the higher the temperature, the higher the losses and the lower the performance of the system.

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