Solar greenhouse (technical)

A solar greenhouse works by letting in solar radiation and trapping the energy from that radiation to increase and maintain the internal temperature above that of the temperature outside.

Incoming solar radiation

A solar greenhouse is basically a structure designed for gardening. The idea is to maintain a higher temperature inside in order to extend the growing season. In a gardening greenhouse, a building is constructed out of glass or transparent plastic glazing which lets in most of the short wavelength radiation (visible light) reaching the Earth's surface from the Sun. This directional radiation strikes the soil and other surfaces inside the greenhouse where part of it is absorbed and warms them. This absorbed solar radiation is the energy source for the greenhouse.

The absorbed energy increases the temperature of those objects. The heated surfaces then transfer this heat energy by the three heat transfer mechanisms: conduction, convection and radiation. The warmed objects radiate energy as infrared radiation in all directions to the surroundings, some of which goes back through the glazing. The glass is partially absorbent to infrared radiation, so this results in only a fraction of the energy escaping that would otherwise escape in an open environment. The warmed objects also conduct energy to adjacent materials, such as through the glass, into the heat storage materials, into the ground, and into the immediately adjacent air. This warmed air transfers heat within the greenhouse by simple convection, but as the air is trapped by the greenhouse covering the convection is limited to the area within the greenhouse.

The role of convection

In any fluid undergoing unequal heating, the major form of heat distribution is by convection. In the free atmosphere the heated air above warmed ground expands and rises creating a convection cell. Within the confined volume of a closed greenhouse this convective flow is limited to the available volume of the greenhouse and thus is much less effective than in the free environment. Of course, if convection is permitted between the outside air and the air within a greenhouse by an open window, the convection will cause the temperature to normalize with the outside environment, as convection will transfer the heat via the window to the outside surrounding environment. Ventilation such as this is sometimes used to keep the greenhouse from over-heating in hot weather. However, external convection such as this can only act to bring the temperatures inside and outside of the greenhouse into balance.

The most basic aspects of greenhouse design are: first, to thermodynamically isolate the system to stop convection and conduction from equalizing the temperature with the ambient temperature; and second, to provide a covering that is transparent to incoming solar radiation to provide the heat source for the greenhouse. After these two basics have been established, 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 greenhouse is able to maintain a higher temperature from its environment because it is thermodynamically isolated due to insulation and the absence of convection with the surrounding environment, and the absorption of a portion of the outgoing radiation by the glazing. The environment loses much more energy to the sky in the form of thermal IR radiation, which for the greenhouse is partially absorbed by the glass, or other glazing, and thus the energy is prevented from escaping the system by blocking convective flow and a part of the outgoing IR.

Thermal infrared radiation absorption

Any object within the greenhouse environment that absorbs the shortwave solar energy will act as a blackbody and radiate infrared radiation in all directions. This omnidirectional infrared radiation is absorbed by all components of the greenhouse environment, including the glazing material. The insulation ability of modern glazing to thermal conduction, combined with its thermal IR absorption, will result in a temperature gradient across the glazing. The inside of the glazing will exchange heat with the inside air through convection and with the inside surfaces through blackbody radiation, while the outside surface will exchange heat through convection with the outside air and through blackbody radiation with the much cooler upper sky. The net effect is a loss of heat energy through the glazing, although the use of double pane or insulating glazing can drastically reduce this loss. Modern research on greenhouse glazing has produced materials that are highly effective at absorbing thermal infrared radiation and returning a significant portion to the internal greenhouse environment.

A greenhouse is designed to be out of thermal equilibrium with its surroundings. The radiative difference between the absorbed shortwave radiation that enters and the longwave radiation that exits is the "heat source" for the greenhouse environment. The rise in temperature is due to the energy difference between the inward net flow of radiative energy minus the outward net flow of energy which makes it through the insulation by conduction and convection. The use of insulation and more infrared-absorbent glazing enhances the effect by reducing heat loss by conduction and IR radiation. 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.[1] 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.

References

• Analytical Spectroscopy Research Group, Spectroscopy Overview, http://www.pharm.uky.edu/ASRG/general_spectroscopy.html Describes the operation of the greenhouse effect both globally and in greenhouses.
• Bowling, Sue Ann, How Do Greenhouses Work?, April 20, 1987, Alaska Science Forum. http://www.gi.alaska.edu/ScienceForum/ASF8/817.html.
• Earth Radiation Budget, http://marine.rutgers.edu/mrs/education/class/yuri/erb.html
• Fairey, Philip; An Analysis of Greenhouse Cookpot Design Considerations For Low-Cost Solar Cookers, Florida Solar Energy Center, http://www.fsec.ucf.edu/bldg/pubs/cookpot/ , accessed 3–30–2005.
• Fleagle, RG and Businger, JA: An introduction to atmospheric physics, 2nd edition, 1980
• Fraser, Alistair B., Bad Greenhouse http://www.ems.psu.edu/~fraser/Bad/BadGreenhouse.html
• Giacomelli, Gene A. and William J. Roberts1, Greenhouse Covering Systems, Rutgers University, downloaded from: http://ag.arizona.edu/ceac/research/archive/HortGlazing.pdf on 3–30–2005.
• Henderson-Sellers, A and McGuffie, K: A climate modelling primer (quote: Greenhouse effect: the effect of the atmosphere in re-readiating longwave radiation back to the surface of the Earth. It has nothing to do with glasshouses, which trap warm air at the surface).
• Idso, SB: Carbon Dioxide: friend or foe, 1982 (quote: ...the phraseology is somewhat in appropriate, since CO2 does not warm the planet in a manner analogous to the way in which a greenhouse keeps its interior warm).
• Joliet O., et al.; Horticern – An Improved Static Model for Predicting the Energy-Consumption of a Greenhouse, Agricultural and Forest Meteorology 55(3–4): 265–294 Jun 1991, A detailed explanation of the gardening greenhouse mechanism.
• Kiehl, J.T., and Trenberth, K. (1997). Earth's annual mean global energy budget, Bulletin of the American Meteorological Society 78 (2), 197–208.
• Lindzen, Richard S., Some Coolness Concerning Global Warming, Bulletin American Meteorological Society, v.71 #3 Mar. 1990. http://eaps.mit.edu/faculty/lindzen/cooglobwrm.pdf Accessed 3–30–2005.
• Meteorological Service of Canada FAQ, http://www.msc.ec.gc.ca/education/scienceofclimatechange/understanding/FAQ/sections/1_e.html – which describes the greenhouse effect and its resemblence to greenhouses.
• Number Watch, September/October 2002, Greenhouse effect, http://www.numberwatch.co.uk/greenhouse_effect.htm – accessed 3–30–2005.
• Piexoto, JP and Oort, AH: Physics of Climate, American Institute of Physics, 1992 (quote: ...the name water vapor-greenhouse effect is actually a misnomer since heating in the usual greenhouse is due to the reduction of convection)
• Stanford University, Planetary Habitability, Chapter 7 A Clement Climate, http://pangea.stanford.edu/courses/gp025/webbook/07_clement.html Earth Science Web Book which discusses greenhouses.
• Wayne, R.P. 3rd edition, 2000), Chemistry of atmospheres, section 2.2.2, p52 (Quote: by supposed analogy with the behaviour of panes of glass, the effect is often called the greenhouse effect (in reality, greenhouses are effective almost entirely because they inhibit convection rather than because they trap radiation))
• Wood, R.W. (1909). Note on the Theory of the Greenhouse, Philosophical Magazine 17, p319–320. For the text of this online, see http://www.wmconnolley.org.uk/sci/wood_rw.1909.html