THE SUN AND ITS RADIATED ENERGY
The word "solar" originated from the Latin "solaris," which means "of the sun". The Sun like all stars, continuously emits radiation from thermonuclear reactions.
As the temperature rises, this thermal radiation rapidly goes up. Just for a reference, in any object the electromagnetic emission per unit surface area is proportional to the fourth power of its absolute temperature. As for the Sun, the scientists estimate that the power radiated from its surface is about 63 megawatt per sq. meter. The intensity of this radiation drops with the squared distance from the Sun. Although our planet intercepts only a small fraction of this radiation, its total amount is enormous. The exact number is determined by the Earth cross section area and the Earth-Sun distance. For more information see our analysis and a calculation of the energy that reaches the Earth.
SOLAR ENERGY USES
In a broad sense, sunlight is one of the few fundamental physical sources for most forms of energy, which we can practically utilize.
For example, the sun's heat causes temperature differences between various areas, which produce winds whose power can be used in a home wind generator. Sunlight's heat also causes evaporation of water, which then falls on high elevations and flows down to the sea. Its streams can be used in hydroelectric turbines. However, the term "solar energy uses" usually refers to the applications in which the Sun's radiation is converted directly to usable products. Currently the resulting products of sunlight conversion systems fall into three basic categories: electricity, fuels, and heat. The first major application of photovoltaic technology was launched in US in 1958 with powering orbital satellites. Today this technology is utilized in both large-scale and small-scale applications. Examples of solar applications and uses in the world are central power stations, space installations, private home systems, outdoor lights, and portable consumer electronic devices. In residential homes sunlight can be used today for heating water and swimming pools and for generating electricity.|
HOW DOES IT WORK
Electromagnetic radiation in general interacts with matter in different ways in different parts of its spectrum: it can be reflected, absorbed or transmitted. When electromagnetic waves strike an object they can cause excitation of the molecules, excitation and ejection of electrons, and oscillation of charge carriers in the object. As the result, a portion of the radiated photons can convert to heat, electricity or chemical energy. A system that uses sunlight energy can be active or passive. In active systems, the Sun's radiation is converted or transported by various controlled electrical or mechanical equipment. For examples, photovoltaic (PV) based devices and solar power plants can actively generate electricity from sunlight. In PV systems, the electromagnetic radiation is transferred to electricity in the form of DC voltage by PV cells. This voltage can then be transformed by solid-state SMPS converters to required forms, such as conventional household AC voltage (read more about SMPS). In the power plants, the heat from the solar thermal collectors heats a fluid, which produces steam that turns the blades of an electric generator's turbine. A concentrating solar power system could also store captured sun rays energy as heat in a thermal medium (such as heating water or dry rocks) for later use. Today's photonic technology also allows to convert the electromagnetic radiation into chemical energy through a chemical reaction (such as photoelectrolysis that dissociates water into hydrogen and oxygen). Active solar hot water systems use pipes and pumps or fans to transport heated water.
In the passive designs, the sun's radiation is converted directly to thermal energy, which is then transported passively by natural convection without use of external energy. It can be used to heat, for example, water and buildings.