Solar Energy

Solar energy comes from the sun in the form of electromagnetic radiation. This energy generated by the sun exceeds any known form of energy generated by man. The amount of incoming radiation, known as the solar constant (because it is almost invariable), measured by satellite at Earth orbit, is 1370 watts per square meter.

The sun radiates like all other hot bodies. When the sun's radiation reaches the Earth, it contains wavelengths ranging from invisible ultraviolet, which can cause skin sunburn and eye damage (0.2 to 0.38 micrometers), through the visible rainbow colors (0.38 to 0.75 micrometers) and up to invisible infrared, which we sense as heat (0.75 to 2.5 micrometers). Only low levels of ultraviolet and infrared rays reach the Earth's surface since they are strongly absorbed by the Earth's atmosphere.

It is believed that the sun generates energy by the nuclear process called proton-proton fusion in which four hydrogen protons are fused into a single helium alpha particle under conditions of extremely high temperature and pressure that exist within the sun's core. This process releases energy that we know as solar energy, along with two positive electrons and two neutrinos. The sun, as the main source of energy on Earth, delivers light and heat essential to the existence of life. Sunlight drives photosynthesis, an important biochemical process, in which plants, algae, and some bacteria convert solar energy into chemical energy. During photosynthesis, plants produce oxygen and nutrients while absorbing carbon dioxide, thus making Earth a vital, living, and breathing planet. Fossil fuels in the form of coal, oil, and natural gas also come from such plants. Warmth from the sun is essential to keep the water on Earth in its liquid state—hence, without the sun, the Earth would be an icy planet.

Solar energy is the driving force for most environmental processes on the Earth's surface and in the atmosphere. The Earth is heated by sunlight, and to maintain steady temperature it gives energy away to space, thus the Earth begins to radiate. The receiving and giving of heat is made possible by the flow of air, which influences spatial variation of temperature and pressure, thereby creating wind and moisture that determine weather and climate zones. The power of hurricanes is driven by the heating of water and subsequent heat release during condensation. In desert environments, wind power heaps sand into dunes. A portion of incoming solar radiation is reflected by clouds, scattered (making the sky appear blue and the clouds white or gray) and absorbed, e.g., ultraviolet radiation is almost completely blocked by the ozone (three-atom oxygen) layer.

According to Vien's displacement law of physics, since the Earth is cooler than the sun, it must re-emit energy with longer wavelengths than the sun emits, with the peak at far infrared (10 micrometers). Earth's re-emitted heat is absorbed by minor components of the atmosphere called greenhouse gases: carbon dioxide, water vapor, methane, nitrous oxide, and ozone, thus creating a blanketlike protection and keeping the ground warmer by about 33 degrees C than the Earth would otherwise be. Most scientists believe that this natural greenhouse effect has been enhanced by anthropogenic (human-caused) activities that contribute to global warming (the increase of average temperature on Earth). Carbon dioxide, a major greenhouse gas in the atmosphere, has increased significantly due to the burning of coal for electricity generation and heating, as a result of deforestation, and also in the [Page 1638]form of emission of exhaust gases from vehicular engine combustion. Emission of gases from industrial processes, like perfluorocarbons, chlorofluorocarbons, halons, and agriculturally produced methane and nitrous oxide also contribute to global warming. Efforts are being made to limit their release into the atmosphere, but global warming is expected to continue since many of the greenhouse gases have a long lifetime in the atmosphere.

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