Solar energy refers to the energy that is created and radiated by Sol—the star at the center of the solar system—or, more significantly, the amount of that energy incident on the planet Earth. Solar energy not only is important for human technological efforts but is also the foundation for virtually all biological life on earth. Solar energy also refers to the processes by which light or heat from the sun is collected and converted to some usable form. Aside from geothermal energy, virtually every energy source commonly used by humans is directly or indirectly derived from solar energy. Historically, humans have relied on solar power for a wide number of applications. There are three general categories of solar energy systems in common use today: solar electric, solar thermal, and passive solar.
The source of solar energy is the sun itself or, more specifically, the thermonuclear reactions that take place at the sun's core. Solar energy is the by-product of the nuclear fusion of hydrogen atoms into helium. Nuclear fusion is a highly exothermic reaction, producing prodigious amounts of electromagnetic radiation. This energy radiates out from the sun's core to approximately seven-tenths of the distance from the center to its surface, where it is transferred by convection to the surface of the sun. The energy is then radiated though space.
The sun emits radiation across a wide wavelength spectrum spanning from ultraviolet (approximately 200-400 nanometers) to radio waves (up to 100 meters). The sun does not emit light evenly across this spectrum, however. The highest amount of solar radiation, not coincidentally, is the portion of the spectrum visible to humans (wavelengths between 400 and 700 nanometers). A graph of the solar radiation peaks at the center of the visible light spectrum (yellow and green), falling steeply to the ultraviolet end (shorter wavelengths) and tapering more gradually on the infrared side (longer wavelengths). In short, most of the sun's radiation exists in the form of visible light and heat. Light from the sun propagates in the photosphere, a layer of space at the sun's surface where the majority of its energy is emitted to space. The amount of radiation emitted by the sun is relatively constant, varying less than 1 percent. This measure amounts to an average of 63.5 × 106 watts/meter2 (W/m2).
A relatively small amount of this energy is radiated across space to the surface of the Earth. Incidental variations aside, the amount of energy incident on a surface, termed the radiant flux density, is inversely proportional to the square of the distance from the radiating object. Thus, the radiant flux density of solar energy incident to Earth is significantly reduced as a result of its distance (on average 1.5 × 1011 m). The average radiant flux density measured at the outer edge of Earth's atmosphere is approximately 1,370 W/m2. This mean value is termed the solar constant.
Even though the sun's output is relatively consistent, the amount of sunlight incident on Earth is variable as a result of both astronomical and terrestrial conditions. The Earth's rotation creates the diurnal cycle of light and darkness. Seasonal variations occur because of the tilt of the Earth's axis relative to the plane of its orbit around the sun. During June, July, and August, for example, the northern hemisphere is positioned such that its surface is more perpendicular to solar radiation, creating a cyclically higher solar constant, while at the same time the southern hemisphere is more obliquely situated. This essentially reduces the density of solar radiation hitting that part of the globe, resulting in a lower solar constant for that part of the world. The geometry of the Earth—a sphere—also factors into the local variability of the solar constant. The area around the equator is most perpendicular to the sun, meaning it receives the most solar radiation. The solar equator is the line around the globe that is always perpendicular to the sun. This line is parallel to the plane of the Earth's orbit around the sun, and so is offset from the rotational equator, which at zero degrees latitude is equidistant from the Earth's poles. Generally, the higher the latitude of a geographic area, the less solar energy is available. At its poles, the surface of the Earth is close to parallel to solar radiation, meaning that very little heat is absorbed in these regions. Finally, variability is caused by the imperfectly elliptical course of the Earth's orbit around the sun, coupled with the movement of the sun in its own track around the center of the Milky Way galaxy. One position along the Earth's orbit brings it closest to the sun. This date, January 21, is termed the perihelion, which is winter in the northern hemisphere but summer in the southern hemisphere. On this date, the solar constant is roughly 1,420 W/m2. Conversely, the lowest solar constant, 1,330 W/m2, occurs on June 2—the aphelion, or the position at which the Earth is farthest from the sun. This inconstancy is thought to be chiefly, but not wholly, responsible for the variation in biomes when comparing areas of equal latitude in the two hemispheres.
Another local variable in regard to solar irradiation is weather and regional climate. Geographical, meteorological, and biological features may increase or decrease available solar radiation. The highest concentration of solar irradiation in the United States is in the Sonoran Desert of southeastern California and southwest Utah. This area, situated between two mountain ranges, is extremely arid, with very little precipitation or cloud cover. As a result, this area is viewed as a prime location for large-scale solar energy-harvesting installations of various types. Tracing the same latitude east across the country, there is a steady drop-off of solar radiation incident on the Earth's surface. Southern Virginia only has a third the incident radiation as southern Utah, due mainly to a marked increase in humidity, cloud cover, and precipitation.
The upper part of the Earth's atmosphere reflects 31 percent of all solar radiation, including many wavelengths that would be quite harmful to organic life. Only visible light, infrared, and a small amount of ultraviolet radiation are allowed to pass through. An additional 17 percent of the radiation is absorbed by or diffused into the atmosphere, and roughly 4 percent is reflected from the Earth's surface back into space. In all, only about 50 percent of incident radiation is received by the Earth's surface. Almost all of the solar energy received by the atmosphere and the Earth's surface is converted into longwave infrared radiation, where it eventually is re-radiated into space. Various gases in the atmosphere, however, such as methane and carbon dioxide, act as an insulating blanket, retarding the transmission of this heat.
Solar energy is a renewable source and in terrestrial terms is essentially unlimited. The sun will continue to produce energy at roughly the same output for billions of years. Fossil and nuclear fuels are in far more limited supply. The total amount of solar energy incident on the Earth can be determined by combining the mean solar constant with atmospheric and terrestrial interference and multiplying this by the surface area of the Earth. This averages 89 petawatts. The amount of energy humans use is roughly 17.8 terawatts—0.02% of available solar resources.
Solar energy is clean during generation. There are no waste products associated with solar energy collection, unlike other forms of energy such as combustion or fission. This is because solar energy systems do not produce energy but merely harvest an existing resource.
Solar energy is versatile. Humans have used energy from the sun for a wide variety of applications throughout recorded history. Current uses of the sun's energy can be divided into two types: passive and active. Passive solar strategies use the sun's rays for warmth and light. Active strategies convert solar radiation into another form, usually electricity.
The largest issue with harvesting solar energy is that it is diffused more or less evenly across the Earth's surface. The diffuse nature of solar energy requires large collecting areas to provide enough power density for many applications. This aspect of solar energy, coupled with the relative efficiency of various harvesting strategies, is the main reason for the relatively high cost of solar power in relation to fossil fuels—another barrier to widespread solar energy applications. Currently available photovoltaics—devices that convert visible light into electricity—have nominal efficiencies between 5 and 20 percent. The diurnal cycle causes solar power availability to be periodic rather than constant—a trait common among renewable energy resources. This leaves two options: either implement an alternative energy source at night and during periods of inclement weather or adopt some type of energy storage solution. Economics and sociocultural inertia are also major impediments to the wholesale adoption of solar energy systems, though several countries and other municipalities have begun to incentivize solar energy and other renewable systems on the scale of both utilities and individuals.
Solar energy is the root source of many other forms of energy that we use today. The only types of energy commonly in use not directly derived from solar energy are geothermal energy, which taps into heat from deep in the Earth, and nuclear energy, which splits rare, unstable Earth elements to produce heat. Wind, for example, is generated by heat from the sun warming the Earth's oceans and land masses, causing planetary-scale convective currents. Hydroelectricity, produced mainly by damming rivers, is dependent on precipitation cycles (weather) and the seasonal melting of ice. These phenomena are linked to solar radiation. About 0.1 percent of solar radiation becomes entrained in biomass, particularly plankton and other plants, as it expires. This small amount of trapped energy is the basis for fossil fuels as well as biofuels, such as ethanol, and other forms of biopower. Fossil fuels are derived from prehistoric plant life that thrived under the sun millions of years ago and over time was converted into coal, oil, and natural gas.
By far the largest use of solar energy on Earth is life itself. Solar energy is the foundation on which almost all life on Earth depends. The only exception are deep-sea ecosystems that are based on the heat and nutrients released by underwater volcanoes. Plants use sunlight as the basis for photosynthesis to construct sugars out of carbon dioxide, water, and trace nutrients. This chemical energy is harvested by higher orders of life in a complex web of dependencies. Photosynthesis is also the process that produces oxygen, without which most animal life cannot exist.
Active energy systems convert a power source into some other usable form. Active solar energy applications turn energy from the sun into electricity. Two main types emerge: photic systems use visible light, and thermic systems use solar heat as a power source. Photovoltaics are, as the name implies, photic. Solar thermal electric facilities, also referred to as concentrated solar power, or CSP, are thermic. Photovoltaics are made up of semiconductor material that is exposed to light from the sun. At the quantum level, individual photons from the sun hit electrons within the photovoltaic material, freeing them from their atomic bonds. These free electrons will induce electrical flow along a closed circuit. CSP systems are quite different. Using solar concentrator technology, these facilities gather heat from the sun over large areas of land. This heat is used to produce steam, which then turns a turbine-driven electrical generator—quite similar in operation to conventional centralized power plants.
Both systems produce electricity from solar energy. No other fuel source is required. No harmful environmental byproducts are created by their use. Photovoltaics are, on the whole, quite durable and are very versatile. Photovoltaics can power very small point-of-use applications and can be combined in arrays of many megawatts in size. Though not widespread, CSP systems are somewhat less expensive than large-scale photovoltaic arrays. The electrical generation technology of these facilities is virtually the same as fossil fuel power plants. The electrical output of CSP facilities is similar to that of small fossil fuel plants, in the several hundred megawatt range.
At this time, both systems produce electricity at a cost somewhat higher than the lowest fossil fuel-based utility rates. The Department of Energy estimates the average U.S. household uses roughly 27 kilowatt-hours of electricity every day. If all this energy were to be supplied by photovoltaics, the average home would require roughly 325 square feet (30 square meters) of photovoltaic cells rated at 10 percent efficiency, if the array were situated in southern Utah. If the array were situated in the southeast United States, the array would need to be almost three times as large, or 1,000 square feet (100 square meters). Other types of active solar energy collectors have higher efficiency rates.
As fossil fuels become scarcer, it is anticipated that this fossil fuel-based electrical energy will rise in cost, whereas the technology involved in photovoltaics and CSP systems will continue to fall in price. The growth of CSP facilities has been quite slow for decades, though in 2008 and 2009 a number of new facilities were announced.
Certainly the biggest challenge active solar energy systems face is the variability of solar radiation primarily resulting from the diurnal cycle and weather patterns. Both photic and thermic active solar systems work best in arid regions that are dominated by clear days.
CSP systems in particular are ill suited for areas that are largely overcast, as they rely solely on direct solar radiation. Photovoltaics have been successfully used virtually all over the world, regardless of local weather patterns. Photovoltaics can produce power in low-light and overcast conditions, though at a reduced rate.
Solar energy as the sole source of electricity requires the ability to store energy for use at night and over prolonged overcast periods. Small-scale photovoltaic systems often use chemical batteries as a storage medium, though the creation of hydrogen through solar energy-driven electrolysis has been studied and implemented to some degree. The manufactured hydrogen can be stored and used to power a fuel cell. A wide variety of large-scale energy storage systems for both CSP and utility-scale photovoltaic arrays have been proposed and, to some degree, implemented. Some examples are compressed air energy systems—in which air is compressed in underground caverns, thermal energy storage in the form of phase change material, molten salts or solid thermal mass, induced hydropower (where solar energy is used to mechanically pump water to the “upstream” side of a hydroelectric dam), flywheels, or large-scale hydrogen production.
A wide and varied number of solar heating applications have been put to use by humans for millennia. There have been and continue to be a number of agricultural uses of heat from the sun, from extending the growing season through the use of greenhouses to the drying of crops. Solar radiation is used as a source of process heat for a number of industrial enterprises. Solar heat can aid quality-of-life issues, especially in economically disadvantaged regions of the world. Solar cookers have been introduced in areas where fuel for cooking is scarce. Solar distillation provides clean water in places where the local water source is contaminated or otherwise unreliable.
The most frequent use of solar heating today is for domestic purposes, either for the creation of hot water or for space heating. Solar thermal systems for heating water or air comprise, at a minimum, three components: some type of collector that absorbs light and heat from the sun, a fluid that transfers the heat, and some type of thermal storage. Solar thermal systems can be sized for a single home or for campus or district applications. Solar thermal systems of this type have relatively high conversion efficiencies (50-70 percent), though on average only about 25 percent of the absorbed energy is available for its intended use. It is rare for solar thermal systems to supply all the heat energy needed by homes or other domestic users, in large part because of seasonal or climatic variation, though this limitation can be overcome by the use of larger collector arrays, very large and/or well-insulated storage tanks, or usage prescriptions. It is commonly accepted that energy efficiency strategies are more cost-effective than active solar systems.
Solar energy can supplement or replace other energy sources for lighting and thermal comfort. Daylighting can obviate the need for electric lighting during the day. Almost all buildings have some transparent aperture for view or lighting; optimizing these strategies can reduce or eliminate electrical demand in regard to artificial lighting, especially in buildings that have diurnal occupancy patterns such as corporate or governmental offices. Passive heating strategies convert solar radiation into space heating. Direct sunlight is allowed to penetrate the building envelope through glazing; this radiation is absorbed by the interior surfaces of the building that subsequently warm the space. Several of these strategies incorporate the use of massive materials to store the collected heat for night-time use. Solar heat can also be used for cooling: A solar chimney is a tall, glazed space situated above the occupied area of a building. Sunlight penetrates the chimney, heating the air inside, which causes the air to rise through vents at the top. This creates negative air pressure inside the building, inducing air flow.
Passive solar strategies must be incorporated into the building during the design process, often requiring customization to meet the objectives of a particular project. Oversizing apertures for these systems can cause overheating during the day or excessive heat loss at night. Building shape, size, location, and orientation all affect the potential for passive solar strategies. Moreover, these strategies are region and climate specific. Low-energy or zero-energy buildings normally incorporate passive solar strategies. The highest form of passive solar buildings closely replicates natural processes. This quality is termed biomimicry.
Biomass Energy, Daylighting, Energy Storage, Fossil Fuels, Fuel Cell, Green Power, Hydrogen, Photovoltaics (PV), Renewable Energies, Solar Concentrator, Solar Thermal Systems.
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