THE SOLAR GUIDE

Environmental concerns and desire for energy independence drive increasing interest in solar power. There is plenty information on this subject scattered throughout the web. It is just not always easy to quickly find what you are looking for among the numerous web sites. This site is designed as an online reference guide for anyone looking for practical information on the sun's energy usage and its applications.

Photo courtesy of NASA Gateway to Astronaut Photography of Earth

FACTS ABOUT THE SUN AND ITS RADIATED ENERGY

The word "solar" originated from the Latin "solaris," which means "of the sun." Technically speaking, the sun is a star, which is a body of hot gases that emits radiation from thermonuclear reactions.

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Just like probably all stars, the sun is composed mainly of hydrogen (70-74% of its mass) and helium (25-28% of its mass). However, astronomers refer to it as heavy element-rich because it has a relatively high content (up to 2%) of other elements. All bodies with temperatures above absolute zero radiate certain energy in the form of electromagnetic waves whose amount depends on the body's temperature. As the temperature rises this thermal radiation rapidly goes up: according to Stefan–Boltzmann law the electromagnetic radiation per unit surface area is proportional to the fourth power of the absolute temperature. The scientists estimate that the temperature at the sun's radiative surface is about 5500°C (9900°F) and the total amount of power radiated in all directions called solar luminosity, is 3.84x10²⁶ watts, which is about 63,000 kW per square meter of sun's surface. According to science, this power is produced at the core of the sun by nuclear fusion. The intensity of this radiation per unit area drops with the squared distance. The Earth intercepts only a small fraction of the sun's radiation, determined by its cross section area and the Earth-Sun distance (for more information see Calculation of the incident energy that reaches the Earth).

PRACTICAL USES OF SOLAR ENERGY

In a broad sense, sunlight along with gravitational energy, is the fundamental physical source for all renewable 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 wind generators. 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 use" usually refers to the applications in which the sun's radiation is converted directly to usable products. Currently the resulting products of solar conversion systems fall into three basic categories: electricity, fuels, and heat. The first major application of photovoltaic technology was launched in 1958 with powering orbital satellites. Nowadays this technology is utilized in both large-scale and small-scale applications, such as central power stations, space installations, home systems, and portable consumer electronic devices. In residential houses sunlight's power can be used today for heating water and swimming pools and for generating electricity (both in off-grid and on-grid homes).

HOW DOES SOLAR POWER WORK

Electromagnetic radiation in general interacts with matter in various ways in different parts of its spectrum: it can be reflected, absorbed or transmitted. When electromagnetic waves strike an object they can cause various effects, such as excitation of the molecules, excitation and ejection of electrons, and oscillation of charge carriers in the object. As the result, a portion of radiated energy can convert to heat, electricity or chemical energy. The systems that utilize these effects can be active and passive. In active solar 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 convert sunlight into electricity. In PV systems, the electromagnetic radiation is transferred to electricity in the form of DC voltage by using PV cells. This voltage can then be transformed by power electronics devices to other voltages, such as conventional household AC voltage. In the power plants, the heat from the solar thermal collectors heats a fluid, which produces steam that turns the blades of a turbine of an electric generator. 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 sun's electromagnetic radiation into chemical energy through a chemical reaction (such as photoelectrolysis that dissociates water into hydrogen and oxygen). Active solar hot water systems utilize pipes and pumps or fans to transport heated water.

In the passive designs, the sun's radiation is converted to thermal energy that can heat, for example, water and buildings. This heat is transported passively by natural convection without use of external energy.

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