More Info

Thursday, November 13, 2008

Solar greenhouse (technical)

A solar greenhouse works by letting in solar radiation to warm the ground, with the structure then trapping the energy to increase and maintain the temperature at a higher level than it otherwise would be. Generally, a greenhouse blocks the heat inside from mixing with the air outside. See the articles on the greenhouse effect or Earth's atmosphere for more details.


The most basic aspects of greenhouse design are: first, to thermodynamically isolate the system to stop convection and conduction from equalizing the inside temperature with the outside temperature; and second, to provide a covering with a controlled difference between the transparency in the solar radiation band (280 nm to 2500 nm wavelengths) and the terrestrial thermal radiation band (5000 nm to 35000 nm), for the purpose of either raising or lowering the temperature inside the greenhouse. A greenhouse covering which is more transparent to the solar radiation band and less transparent to the thermal radiation band will result in a temperature higher than the surrounding environment, and a greenhouse covering which is more reflective of solar radiation and more transparent to thermal radiation will lower the temperature relative to the surrounding environment.

For the traditional case of a warming greenhouse, such as with a glass covering, a covering material is chosen which will absorb some of the outgoing IR and radiate a portion of it back into the greenhouse environment to reduce radiative energy loss to the sky from the amount that the ambient environment experiences. The use of insulation and more infrared-absorbent glazing enhances the effect by reducing heat loss by conduction and IR radiation.

The soil mass at the base of the greenhouse acts to absorb a portion of the available heat during the solar period of the day for later use as a night time radiant heat source. Installations of subterranean air circulation tubing can be designed to enhance the soil mass heat absorption potential.

With proper subterranean design, underground air circulation tubing can absorb most of the daytime solar gain directly into this soil mass to provide air cooling, prevent overheating and serve as an additional heat source at night. Also the addition of heat storage materials with high heat capacity, such as containers of water or bins of sand and rock absorb heat energy during the day to help prevent greenhouse overheating, and release that energy to maintain the internal temperature during cooling periods, such as during the night.

Practical applications

The modern development of new plastic surfaces and glazings for greenhouses has permitted construction of greenhouses which selectively control the transmittance of both incoming solar radiation wavelengths and outgoing thermal IR wavelengths.

The new materials also provide insulation to reduce conductive loss through the glazing in order to better control the growing environment. The research starts with the blocking of convective heat loss as a given in an isolated system and works toward improving IR absorption and insulation to further reduce radiative and conductive energy loss.

Gardeners sometimes use a "greenhouse-in-a-greenhouse" technique, in which they lay additional IR absorbent plastic sheeting inside a greenhouse in order to provide additional warmth in an isolated area to plants or water pipes.

Another practical application of the greenhouse effect is in the creation of solar cookers. The analysis here compares the thermodynamic properties of several solar cooker designs.

Integrated equipment

Greenhouses are sometimes outfitted with integrated irrigation systems, and/or heat sink-systems which can respectively irrigate the plants or help to store energy from the sun and redistribute it at night (when the greenhouses starts to cool down).

No comments: