SOLAR ENERGY

Solar energy—power from the sun—is free and inexhaustible. This vast, clean energy resource represents a viable alternative to the fossil fuels that currently pollute our air and water, threaten our public health, and contribute to global warming. Failing to take advantage of such a widely available and low-impact resource would be a grave injustice to our children and all future generations.In the broadest sense, solar energy supports all life on Earth and is the basis for almost every form of energy we use. The sun makes plants grow, which can be burned as "biomass" fuel or, if left to rot in swamps and compressed underground for millions of years, in the form of coal and oil. Heat from the sun causes temperature differences between areas, producing wind that can power turbines. Water evaporates because of the sun, falls on high elevations, and rushes down to the sea, spinning hydroelectric turbines as it passes. But solar energy usually refers to ways the sun's energy can be used to directly generate heat, lighting, and electricity.The energy transmitted from the Sun. The upper atmosphere of Earth receives about 1.5 × 1021 watt-hours (thermal) of solar radiation annually. This vast amount of energy is more than 23,000 times that used by the human population of this planet, but it is only about one two-billionth of the Sun's massive outpouring—about 3.9 × 1020 .The power density of solar radiation measured just outside Earth's atmosphere and over the entire solar spectrum is called the solar constant.The Sun is about 900,000 miles across and is at least 10 million degrees at its center. The surface of the sun is roughly 6,000°C and its hot gases emit light that has a spectrum ranging from the ultraviolet, through the visible, into the infrared. Photovoltaic or solar cells convert solar power directly into electrical power. Light consists of discrete particle-like packets of energy called photons. Sunlight contains photons with energies that reflect the sun’s surface temperature; in energy units of electron volts. The energy density packed into the photons vary, but the visible region of the light spectrum tends to contain among the highest concentrations of energy that hits the planet.More energy from sunlight strikes the Earth in one hour than all the energy consumed on the planet in a year. At high noon on a cloudless day, the surface of the Earth receives 1,000 watts of solar power per square meter. Sunlight provides by far the largest of all carbon-neutral energy sources. Heat travels in all directions from the Sun and is the ultimate source of all energy on Earth. This energy is responsible for all sorts of weather events, not only scorching heat waves. For instance, wind occurs when sunlight heats the ground, which heats the air above it, which rises, so that cool air whisks in to take its place.In the past decade, solar energy has attracted significant attention from investors, policymakers and the public generally because it is widely available, geopolitically secure and environmentally sustainable. Indeed, solar energy does not create greenhouse gases as a byproduct of generating electricity. Not surprisingly, it is widely considered among the most compelling solutions available for the world's need for clean, abundant sources of energy. Skeptics need only consider the $7.5 billion solar-energy industry - still growing at a rate of more than 30% every year — to appreciate the growing popularity of solar energy in mainstream electricity markets. Still, in 2001, solar electricity provided less than 0.1% of the world's electricity. Solar Heat Collectors According to the World Meteorological Organization, the most reliable (1981) value for the solar constant is 1370 ± 6 W/m2.Solar radiation is attenuated before reaching Earth's surface by an atmosphere that removes or alters part of the incident energy by reflection, scattering, and absorption. In particular, nearly all ultraviolet radiation and certain wavelengths in the infrared region are removed. However, the solar radiation striking Earth's surface each year is still more than 10,000 times the world's energy use. Radiation scattered by striking gas molecules, water vapor, or dust particles is known as diffuse radiation. Clouds are a particularly important scattering and reflecting agent, capable of reducing direct radiation by as much as 80 to 90%. The radiation arriving at the ground directly from the Sun is called direct or beam radiation. Global radiation is all solar radiation incident on the surface, including direct and diffuse.Solar research and technology development aim at finding the most efficient ways of capturing low-density solar energy and developing systems to convert captured energy to useful purposes. Also of significant potential as power sources are the indirect forms of solar energy: wind, biomass, hydropower, and the tropical ocean surfaces. With the exception of hydropower, these energy resources remain largely untapped.Five major technologies using solar energy are being developed.
1, The heat content of solar radiation is used to provide moderate-temperature heat for space comfort conditioning of buildings, moderate- and high-temperature heat for industrial processes, and high-temperature heat for generating electricity.
2, Photovoltaics convert solar energy directly into electricity.
3, Biomass technologies exploit the chemical energy produced through photosynthesis (a reaction energized by solar radiation) to produce energy-rich fuels and chemicals and to provide direct heat for many uses. .