The Sun
From Thermal-FluidsPedia
Give me the splendid silent sun with all his beams full-dazzling. ~ Walt Whitman (1819 - 1892)
Since ancient times, humans have demonstrated a basic understanding of solar energy by designing shelters that protected them from the cold of winter and the heat of summer. Socrates suggested that houses should be built facing south in order to maximize the heat of the winter sun. Anasazi Indians designed their dwellings under cliff overhangs, maximizing the solar gain in winter and providing shade in summer. The sheltering overhangs also reduced radiant heat loss at night and protected against cold winds. Furthermore, some houses were built with thick stone and clay walls and had small windows to retain heat during the day and release it gradually at night.
Today, solar power has been proven to be highly effective not only in providing heat to buildings and swimming pools, but also in cooling applications and for the generation of electricity. As with many other sources of energy, there are limitations on where this energy source can be used. The amount and intensity of solar energy available varies depending on geographical location, the position of the sun in the sky, weather conditions, and the orientation of the collector and surrounding objects. Solar energy can be used worldwide, especially in regions with high insolation. This includes large areas of Africa, Australia, the Middle East, Central and South America, and the southwestern United States.
In this chapter, we start by looking at the sun itself and the amount of radiation that can potentially be intercepted by various collectors on earth. We will discuss passive and active systems, as well as thermal and photovoltaic systems for utilizing solar energy for heating, cooling, and producing electricity.
The sun is a 1.4 million kilometer wide sphere of extremely dense, hot gas that resides approximately 150 million kilometers from the earth (Figure 1). Composed predominately of hydrogen and helium, its central temperature and pressure are estimated to reach 15-25 million kelvins and 300 billion atmospheres – conditions suitable for nuclear fusion to take place. As we will discuss later in nuclear energy, fusion involves combining atoms of hydrogen into a helium nucleus and heat. The sun loses mass at a rate of about 4.3 billion kilograms every second while producing an enormous amount of energy (a). At its outer layer ionized gases both absorb and emit a continuous spectrum of radiation, causing this layer to be essentially opaque. Temperatures drop to about 5,800 K at the surface of the sun.
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References
(1) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005
Additional Comments
(a) This loss of mass is nothing to worry about! By some estimates, the sun will have enough energy to support life on earth for another five billion years.
Further Reading
Markvart, T., and Castanar, L., Solar Cells: Materials, Manufacture and Operation, Elsevier Publishing Company, 2005.
Galloway, T., Solar House, Elsevier Publishing Company, 2004.
Stine, W. B., and Harrington, R. W., Solar Energy Systems Design, John Wiley and Sons, Inc., 1985.
Solar Energy, Direct Science Elsevier Publishing Company, the official journal of the International Solar Energy Society, covers solar, wind and biomass energies.
External Links
National Renewable Energy Laboratory: Solar Research (http:// www.nrel.gov/solar).
Energy Efficiency and Renewable Energy: Solar Energy, US Department of Energy (http://www.eere.energy.gov).
American Solar Energy Society (http://www.ases.org).
Solar Electric Power Association (http://www.solarelectricpower.org).
California Solar Center (http://www.californiasolarcenter.org).