Solar energy (Encyclopedia of Environmental Issues, Revised Edition)
The sun was essentially humankind’s only source of energy before the Industrial Revolution. In its broadest definition, solar energy is remarkably diverse and takes many forms. Hydropower, which is derived from the kinetic energy of moving water, is possible because water evaporated from the oceans by solar radiation subsequently falls in mountainous areas and flows to lower elevations. Wind energy results from the uneven solar heating of the planet’s surface. Biomass contains solar energy that has been stored by photosynthesis, as do fossil fuels. Defined more narrowly, viable solar energy technologies include active and passive systems that make use of the sun’s heat energy and photovoltaic cells that convert the sun’s energy into electricity.
According to the National Renewable Energy Laboratory (NREL), each day 46,700 quadrillion British thermal units (quads) of energy fall on the coterminous United States alone—substantially more than the 101.527 quads the nation consumed in peak year 2007, and more than one hundred times the total world consumption for the year 2006. This energy is available to anyone who is able to collect, transform, and store it.
Between 2000 and 2009 solar energy generation in the United States nearly quadrupled, rapidly increasing from 909 million kilowatt-hours at the beginning of the decade to almost 3.6 billion kilowatt-hours by its end. Between 2001 and 2009 U.S. venture capital and private...
(The entire section is 399 words.)
Solar Thermal Energy (Encyclopedia of Environmental Issues, Revised Edition)
The use of solar energy for heating and cooling is based on the simple fact that when an object absorbs sunlight it gets hot. The heat energy may be used in several ways: to provide space heating and cooling, to drive engines, or to heat water or other fluids. If this is accomplished by nothing other than appropriately designed and situated buildings and without moving parts, it is termed passive solar technology. Passive solar architecture was introduced about two thousand years ago by the Greeks and is a common feature in traditional Islamic architecture.
The simplest passive method of solar heating used in the Northern Hemisphere involves situating a building so that its windows face south and its long axis runs east-west (in the Southern Hemisphere, the windows must face north). During the winter the sun is low in the sky and provides heating to the windows. In the summer, when the sun is high in the sky, most of the radiation falls on the roof. The building’s windows and walls are constructed to minimize heat transfer by conduction, convection, and radiation. Sufficient interior heat capacity, generally in the form of concrete walls or floors, keeps the building from overheating during the day while storing excess heat for release during the night. Double-paned windows with a layer of air between the sheets of glass are effective for preventing conduction and convection. Glass is transparent to visible and...
(The entire section is 720 words.)
Concentrating Solar Power (Encyclopedia of Environmental Issues, Revised Edition)
Concentrating solar power (CSP) technology uses direct solar radiation to produce electricity on a commercial scale. It is best suited to locations with direct sunlight, clear skies, and dry air, which is why CSP facilities tend to be located in desert areas. In a CSP system, an array of reflectors or lenses concentrates sunlight onto a collector, producing temperatures as high as 3,000 degrees Celsius (5,432 degrees Fahrenheit). There the heat may be retained in a storage system containing molten salts or some other fluid, or it may be converted into mechanical energy. Sun-tracking systems orient the reflectors or lenses to take best advantage of the available sunlight.
The most common solar concentrator design, and the one that has enjoyed the most commercial success, is the parabolic trough, a long array of curved mirrors that concentrates sunlight on receiver tubes that parallel each mirror along its focal line. Inside the tubes is a liquid medium, which is heated by the sunlight. The liquid is conveyed to a central collector, where it heats water to produce steam that drives an electric turbine. Another design, the linear Fresnel reflector system, involves a receiver tube positioned above several mirrors. This allows the mirrors to track the sun more effectively. Yet another design is the parabolic dish, which concentrates the sun’s energy on a central thermal receiver. Such a system is often combined with a...
(The entire section is 413 words.)
Photovoltaic Solar Power (Encyclopedia of Environmental Issues, Revised Edition)
Photovoltaic solar cells, first developed in 1954 by scientists at the Bell Telephone Laboratories, are solid-state devices that convert sunlight into electricity. The earliest solar cells were made using single-crystal silicon wafers. Silicon is an important type of element known as a semiconductor, which has properties between those of conductors and insulators.
Electric conduction in silicon results from the movement of negative charges (electrons) and positive charges (holes). One way to cause this movement is to add arsenic or phosphorus atoms—which have five outer-shell electrons—to the pure silicon, creating a semiconductor that has excess negative charge (n type). The addition of boron atoms with three outer-shell electrons creates a p-type semiconductor. Electric conduction occurs when p- and n-type slices are placed in close contact.
The simplest solar cell is a p-n junction sandwiched between two conductors. One of the tricks to fabricating efficient solar cells centers on the design of the top conductor. It should be large enough to capture the electrons but not so large that it blocks sunlight from passing into the center of the sandwich. Sunlight enters the top of the cell and excites electrons in the n-type layer, causing them to jump to the p-type layer, where they are captured and carried away to do work in the external circuit.
The earliest solar cells converted sunlight to...
(The entire section is 749 words.)
Further Reading (Encyclopedia of Environmental Issues, Revised Edition)
Cassedy, Edward S. “Solar Energy Sources.” In Prospects for Sustainable Energy: A Critical Assessment. New York: Cambridge University Press, 2000.
Foster, Robert, Majid Ghassemi, and Alma Cota. Solar Energy: Renewable Energy and the Environment. Boca Raton, Fla.: CRC Press, 2010.
Kalogirou, Soteris. Solar Energy Engineering: Processes and Systems. Burlington, Mass.: Academic Press, 2009.
Naff, Clay Farris, ed. Solar Power. Detroit: Greenhaven Press, 2007.
National Renewable Energy Laboratory. 2009 Renewable Energy Data Book. Golden, Colo.: U.S. Dept. of Energy, Office of Energy Efficiency and Renewable Energy, 2010.
National Research Council. Electricity from Renewable Resources: Status, Prospects, and Impediments. Washington, D.C.: National Academies Press, 2010.
O’Keefe, Philip, et al. The Future of Energy Use. 2d ed. Sterling, Va.: Earthscan, 2010.
Pimentel, David, ed. Biofuels, Solar, and Wind as Renewable Energy Systems: Benefits and Risks. New York: Springer, 2008.
(The entire section is 130 words.)
Background (Encyclopedia of Global Resources)
Solar energy has provided continuously almost all of Earth’s energy, which humans have exploited, directly or indirectly, since prehistoric times. The steady evolution of solar technology, however, has been interrupted sporadically by the discovery of plentiful and inexpensive nonrenewable fuels, such as coal, oil, and uranium. Successive civilizations have shortsightedly overexploited each new resource by treating it as an inexhaustible supply governed only by price and availability.
Designing buildings to optimize the use of the Sun during various seasons began in ancient Greece. During the fourth century b.c.e., a severe shortage of wood for fuel necessitated that homes be designed to take advantage of the abundant sunlight during the moderately cool winters while taking advantage of shade during the summer. Because glass or other transparent materials for doors and windows did not yet exist, houses had to be designed to collect as much sunlight as possible during the short winter days. This was achieved by designing houses with (in the Northern Hemisphere) south-facing covered porches that were closed to the north. During the winter the low-angled sunlight streamed through the porch and warmed the interior rooms. During summer, rooms were shaded from the high Sun by the porch roof.
The ancient Romans used wood at such a prodigious rate that by the first century b.c.e., timber had to be imported from more than 1,000...
(The entire section is 739 words.)
Nature of Solar Energy (Encyclopedia of Global Resources)
Every day, the Earth receives ten thousand times more energy from the Sun than humans derive from all other alternative energies and nonrenewable fuels combined. Above Earth’s atmosphere, 170 billion megawatts of power are available, but the energy’s intensity is diluted to 430 British thermal units per hour per square foot (Btu/hr/ft2); this is attenuated to between 100 and 200 Btu/hr/ft2 at Earth’s surface, thus requiring large collector areas to capture significant amounts of usable energy. Nevertheless, industrialized societies have come to realize that past patterns of energy consumption cannot be sustained. A viable energy future requires not only that solar energy be harnessed but also that technologically advanced societies modify their lifestyles to live in closer harmony with nature before an energy crisis of global proportions decimates humanity.
Utilizing solar energy as a major source of energy for the world has several advantages. First, solar energy is virtually inexhaustible and is constant (at least above the Earth’s atmosphere). Second, it is clean; the only direct environmental impact may be aesthetic—some active collection systems are conspicuously ugly. However, more attractive large-scale devices could be designed, and smaller units could be integrated into residential structures so as to be less obvious. Finally, the collected energy is “free” after the initial cost of purchase and...
(The entire section is 292 words.)
Types of Solar Energy Systems (Encyclopedia of Global Resources)
All solarheating systems share two common elements: a device for collecting energy from the Sun to provide electricity, heat, or air-conditioning and a facility for storing the energy when it is not needed. PV systems convert the Sun’s radiant energy directly into electricity, while thermal solar units provide heat for interior spaces or hot water for domestic uses. Concentrating collectors are used to create steam that can power air-conditioning units or generate electricity.
Thermal solar energy is captured in active or in passive systems. Active systems require electricity to power pumps or fans, while passive systems convert sunlight directly into interior space heating. Active systems may be subdivided into those that use flat-panel stationary collectors and systems that focus incoming solar rays in order to achieve temperatures high enough to create steam. Active solar heating systems transfer a Sun-heated medium (air or water) from an exterior south-facing collector (north-facing if located in the Southern Hemisphere) to the point of use or to a storage facility. For air systems the storage facility is a bed of rocks, while water systems store heat in tanks of water. Active concentrating systems focus sunlight by one of two possible means: power towers consisting of mirror arrays, which reflect sunlight from a large area into a small central region, or troughs of parabolic mirrors, which focus sunlight...
(The entire section is 604 words.)
Active Solar Heating Systems (Encyclopedia of Global Resources)
These types of systems, first developed in the early decades of the twentieth century, were largely forgotten (after some considerable initial interest) when inexpensive fossil fuels became widely available. The oil embargo of 1973 renewed interest in these systems, and government tax credits helped homeowners defray the initial high cost of purchasing and installing active systems.
All active solar systems have the following components: a collector, a pump or fan to circulate the heat transfer fluid, and a means of storing excess energy. Active systems have two basic uses. Either they are used to heat the interior of a building (or at least to preheat the circulating fluid), or they are used for domestic hot water (DHW). The circulating fluid for DHW systems is always water, and the storage unit is a water tank, typically having a 50-gallon capacity. Space heating units may use either circulating air or circulating water, but air systems are more common.
Solar flat-plate collectors consist of an enclosed rectangular box containing a metal plate with a flat black surface covered by nonreflecting tempered glass. Tubes to conduct water across the collector are soldered to the plate for water systems, while in air systems channels direct the airstream. The entire box must be watertight and well insulated. When sunlight strikes the black surface, light is changed into heat, which is transferred to the fluid...
(The entire section is 978 words.)
Passive Solar Space Heating (Encyclopedia of Global Resources)
This type of heating is achieved entirely by natural means—heat circulates by natural convection without pumps or fans. A well-designed passive system should include these elements: south-facing insulated (double-paned) windows to collect the winter sunlight, interior thermal mass to prevent overheating, night insulation to cover the windows, overhangs above the windows to keep out the summer sunlight, and sufficient insulation to minimize heat loss.
Passive systems may be categorized as one of three types, depending on the relative locations of the windows and thermal mass. These are termed direct-gain, indirect-gain, and attached-gain (or greenhouse) systems. In direct-gain systems large south-facing windows admit sunlight, which falls on the thermal mass, usually brick, tile, or concrete. The mass, which may be incorporated into a floor, a wall, or even an earth-filled planter, must be sized to the total area of south-facing glass—the greater the area, the greater the mass required to prevent overheating and to store heat efficiently for evening. Without sufficient thermal mass, indoor temperatures can exceed 32° Celsius during the day while plummeting to uncomfortably low temperatures at night.
Indirect-gain systems have a massive wall, constructed of brick or barrels of water, located close to the south-facing glass. The outside-facing surface of the mass is painted black, which transforms...
(The entire section is 382 words.)
Photovoltaic Systems (Encyclopedia of Global Resources)
Although passive and active solar technologies are well understood, the direct conversion of sunlight into electricity by means of photocells is still being developed. Once too expensive for typical homeowners, new technologies are reducing the cost so that it is becoming a cost-effective and viable energy alternative, particularly in remote areas where the cost of running conventional power lines would be prohibitive.
Solar PV cells installed on individual residences are particularly beneficial for providing intermediate load demand because they provide electricity during the sunniest and hottest part of the day, when the demand for air-conditioning is maximum. Such systems are also cost-effective for utility companies because if less electricity needs to be provided, the cost of upgrading transmission lines and associated equipment is reduced. When individual PV systems are tied to the grid, excess energy produced during the day can be fed back into the grid, while nighttime requirements are available from the grid. Home systems connected to the grid eliminate the need for large banks of storage batteries, which are expensive and potentially dangerous. Cost-effective grid-connected systems are becoming common in Japan and Germany.
As the world market for PV systems increases from less than 1 percent of new generating systems to hundreds of times this level by the mid-twenty-first century, these systems will also...
(The entire section is 431 words.)
Impact and Economics of Solar Energy (Encyclopedia of Global Resources)
The rate at which the world consumes energy is a function of its population and the average energy consumed per person. Both of these are increasing; the world population is predicted to reach 10 billion by the middle of the twenty-first century, while the world’s two largest countries by population (China and India) are expected to increase their per capita energy use by three to five times during the same time.
The world is in the process of transitioning to an alternative energy future, with solar power as one of the major components. Estimates indicate that the solar industry will grow by 20 to 30 percent each year until 2050. Solar can provide energy for underdeveloped countries that lack fossil fuels, thus creating resources to develop the infrastructure. When solar resources are abundant, solar can also provide water for agriculture as well as potable water through desalination.
The shift to a solar economy will be driven by two aspects affecting energy supply and demand. First, the most abundant source of clean, renewable energy for both industrialized and developing countries is solar. Second, a shift away from large, centralized power plants to small, locally generated solar energy is inevitable.
Because approximately one-fourth of the annual U.S. energy consumption is for heating and cooling, solar energy could have a significant impact. In most regions of the United States,...
(The entire section is 428 words.)
Further Reading (Encyclopedia of Global Resources)
Berman, Daniel M., and John T. O’Connor. Who Owns the Sun? People, Politics, and the Struggle for a Solar Economy. White River Junction, Vt.: Chelsea Green, 1996.
Bradford, Travis. Solar Revolution: The Economic Transformation of the Global Energy Industry. Cambridge, Mass.: MIT Press, 2006.
Chiras, Daniel D. The Homeowner’s Guide to Renewable Energy: Achieving Energy Independence Through Solar, Wind, Biomass, and Hydropower. Gabriola, B.C.: New Society, 2006.
Goswami, D. Yogi, Frank Kreith, and Jan F. Kreider. Principles of Solar Engineering. 2d ed. Philadelphia: Taylor & Francis, 2000.
Hayden, Howard C. The Solar Fraud: Why Solar Energy Won’t Run the World. 2d ed. Pueblo West, Colo.: Vales Lake, 2004.
Hestnes, Anne Grete, Robert Hastings, and Bjarne Saxhof, eds. Solar Energy Houses: Strategies, Technologies, Examples. 2d ed. London: James & James, 2003.
Kryza, Frank T. The Power of Light: The Epic Story of Man’s Quest to Harness the Sun. New York: McGraw-Hill, 2003.
Naff, Clay Farris, ed. Solar Power. Detroit: Greenhaven Press, 2007.
Nansen, Ralph. Sun Power: The Global Solution for the Coming Energy Crisis. Ocean Shores, Wash.: Ocean Press, 1995.
Ramsey, Dan, and David Hughes. The Complete Idiot’s Guide to Solar Power for Your Home. 2d ed. New York: Penguin...
(The entire section is 242 words.)
Definition (Encyclopedia of Global Warming)
Solar energy includes all human-derived means of collecting and utilizing energy from the Sun, for such applications as heat, air conditioning, or electric power generation. Two primary classes of solar energy arephotovoltaics, converting radiant energy into electricity, and thermal solar, using radiant energy for heat or directly to power air-conditioning units. Thermal solar energy may be subdivided into active systems and passive systems.
Active systems require another source of energy, typically electricity, to power pumps or fans. These systems may be used to provide domestic hot water, to heat the interior of a building, or in some cases to provide steam. Active systems include flat-panel, exterior, stationary collectors and systems that concentrate solar rays in order to produce higher temperatures. Arrays of mirrors have also been used to concentrate diffuse sunlight in a small region, where extremely high temperatures may be achieved. Other focusing systems use troughs of parabolic mirrors to reflect light to a central axis, where a contained moving fluid transports the heat.
Passive systems require no input energy beyond that of the Sun itself; they convert sunlight directly into space heat for buildings. Three types of passive systems are direct gain, indirect gain, and attached gain. Direct gain systems utilize large expanses of glass facing toward the equator to admit sunlight, as well as substantial amounts of...
(The entire section is 374 words.)
Significance for Climate Change (Encyclopedia of Global Warming)
Global warming is likely caused in part by the anthropogenic release of carbon dioxide (CO2) from the combustion of fossil fuels. Moreover, fossil fuels, as a nonrenewable resource, are being depleted by energy-intensive nations such as the United States. Enough solar energy reaches Earth to provide virtually all of humanity’s energy needs if it can be collected and used efficiently. Solar technology is not yet capable of exploiting solar energy to such an extent, but research is ongoing. Meanwhile, fossil fuels still provide the vast majority of the energy consumed in the production, delivery, and utilization of consumer goods.
Crude oil is the most important raw material for industrial manufacturing, but it is a nonrenewable resource that may reach peak production in the near future. Ecological destruction is also associated with resource recovery and manufacturing. Thus, it is in the best interests of contemporary industrialized societies to engage in a concerted effort to transition to renewable energy and environmentally sustainable resource use by terminating the dependence on fossil fuels.
Solar energy is a dilute source of energy that lends itself to small-scale applications. In almost any location in the United States, homeowners can reduce their dependence on fossil-fuel-derived energy by passive solar design or retrofitting an existing structure for passive solar gain. Although even a good...
(The entire section is 368 words.)
Further Reading (Encyclopedia of Global Warming)
Anderson, Bruce, and Malcolm Wells. Passive Solar Energy. 2d ed. Amherst, N.H.: Brick House, 1994. Authoritative work containing succinct but inclusive information sufficient to guide both amateurs and professionals in constructing a passively heated solar dwelling.
Chiras, Daniel D. The Solar House: Passive Heating and Cooling. White River Junction, N.H.: Chelsea Green, 2002. Provides all the detailed information and diagnostic aids necessary to design and build a solar home incorporating solar power for both heating and electricity.
Michaels, Tim. Solar Energy Utilization. New York: Van Nostrand Reinhold, 1979. Presents comprehensive information on active systems, including sizing, collectors, and storage.
Ramsey, Dan. Solar Power for Your Home. New York: Alpha Books, 2003. Covers virtually all aspects of solar power, including photovoltaic systems and wind energy. Includes ideas for improving energy efficiency, calculations for sizing systems, and the practical means to achieve an energy-efficient lifestyle.
Scheer, Hermann. The Solar Economy: Renewable Energy for a Sustainable Global Future. London: Earthscan, 2004. Explores how and why humans must transition from fossil fuel economies to an alternative-energy-based supply. Provides the economic analysis to show how this can be achieved with minimal disruption to society.
Solar Energy International....
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Solar Energy (Science Experiments)
Solar Cells: Will sunlight make a motor run?
Sunlight has been recognized as a powerful source of energy since ancient times. "Burning glasses" that dated back to 7 B.C. have been found in the ruins of Nineva (now part of Iraq). These glasses were similar to magnifying lens and could concentrate sunlight into a beam hot enough to start a fire. Each day, Earth receives about 4 quadrillion kilowatt-hours of solar energyAny form of electromagnetic radiation that is emitted by the Sun., generated by nuclear reactions deep inside the Sun's mass. While we receive a lot of solar energy, it is not easy to harness. Environmental concerns and our limited supply of fossil fuels make finding ways to gather and concentrate solar energy efficiently an urgent challenge.
Think of the Sun as a constantly active hydrogen bomb: a swirling, mass with eruptions that give off great amounts of energy. Within the Sun's center, the temperature is about 25,000,000°F (14,000,000°C). About 700 million tons (635 million metric tons) of hydrogen fuse into 695 million tons (630 million metric tons) of helium each second. What happens to the missing five...
(The entire section is 2427 words.)
Solar Energy (World of Earth Science)
Earth's surface receives energy from processes in Earth's interior and from the Sun. Heat from the interior comes from radioactive elements in the mantle and core, tidal kneading by the Moon and Sun, and residual heat from the earth's formation. This interior heat is radiated through the surface at a global rate of 3 1013 watts (W)bout .07 W per square yard (.06 W/m2). The Sun, in contrast, provides 1.73 1017 W, 5,700 times more power than Earth radiates from within and about 30,000 times more than is released by all human activity. Clouds, air, land, and sea absorb 69% of the energy arriving from the Sun and reflect the rest back into space. The ocean, which covers about 70% of the earth's surface, does about 70% of the absorbing of solar energy.
Between its absorption as heat and its final return to space as infrared radiation, solar energy takes many forms, including kinetic energy in flowing air and water or latent heat in evaporated water. Solar energy keeps the oceans and atmosphere from freezing and drives all winds and currents. A small fraction of Earth's solar energy income is intercepted by green plants, providing the flow of food energy that sustains most
earthly life. Only a few organisms, including thermophilic bacteria infiltrating the crust and organisms specialized to live in the vicinity of hydothermal deep-sea vents, derive their energy from Earth's interior rather than from the Sun.
Regional variations in solar input contribute to weather patterns and seasonal changes. On average Earth's surface is more nearly at a right angle to the Sun's rays near the equator, so the tropics absorb more solar energy than the higher latitudes. This creates an energy imbalance between the equator and the poles, an imbalance that the circulation of the atmosphere and oceans redress by transporting energy away from the equator. During each half of the year the daylight side of each hemisphere is tilted at a steeper angle to the sun than during the other half, and so intercepts less solar energy; this results in seasonal climatic changes.
Solar energy is also of technological importance. Utilization of the Sun as an energy source has been routine on spacecraft for decades and is becoming more frequent on the ground. Electromagnetic radiation from the Sun, unlike the major conventional power sources, produces no smokestack emissions, greenhouse gases, or radioactive wastes; and its production cannot be manipulated for profit or political leverage. On the down side, sunlight is a diffuse or spread-out energy source compared to any fuel and is directly available only during the day. Yet, even at high latitudes in Europe and North America, where most of the world's energy is consumed, the ground receives from the Sun a long-term average of 83.6 W per square yard (100 W/m2). This average is inclusive of "dark" hours. Both indirect and direct harvesting of this energy income is possible. Indirect solar schemes, including wind power, wood heat, and the burning of alcohol, methane, or hydrogen, run on energy derived at second hand from sunlight. Direct schemes use sunlight as such to heat buildings or water, generate electricity, or supply high-temperature process heat to industrial systems.
Because conventional electricity generation is expensive and polluting, much effort has been devoted to solar electricity generation. Electricity can be generated from sunlight either thermally or photovoltaically. Thermal methods focus the Sun's rays on looped pipes through which molten salt, hot air, or steam flows. This hot fluid is then used either at first or second hand to run generators, much as heat from coal or nuclear fuel is used in conventional power plants. Photovoltaic electrical generation depends on flat, specially designed transistors (solar cells) that convert incident light to electricity. At 83.6 W/yard2 (100 W/m2) average solar input, 38 square yards (32 m2) of 33% efficient solar cells square 18 feet (5.5 m) on a sideould supply 800 kilowatt-hours of electricity per
month, the approximate usage of the average U.S. household. An efficiency of 32.3% has been demonstrated in the laboratory, but most commercial photovoltaic cells are only about 10% efficient. Unlike the unused heat from a ton of coal or uranium, however, the sunlight not converted to electricity by a solar cell entails neither monetary cost nor pollution, and so cannot be viewed as waste.
Despite its obvious advantages, photovoltaic electricity generation has long been limited to specialized off-grid applications by the high cost of solar cells. However, cell prices have fallen steadily, and several large-scale photovoltaic electricity projects are now under way in the U.S. and elsewhere.
See also Atmospheric circulation; Coronal ejections and magnetic storms; Energy transformations; Global warming; Insolation and total solar irradiation; Meteorology; Ocean circulation and currents; Seasonal winds; Solar illumination: Seasonal and diurnal patterns; Solar sunspot cycles; Sun; Ultraviolet rays and radiation