Solar Energy
Noteworthy Keyword
【Solar Energy】

Solar Energy

Takuya Homma
Professor Emeritus, The University of Tsukuba
Advisor, New Energy Research Institute

1. Solar energy
Solar energy is made up of the electromagnetic radiation emitted by the sun, and about 99% of solar radiation is contained in the wavelength region from about 300 nm(ultraviolet) to 3,000 nm(near-infrared). In other words, solar energy is useful energy that is immediately derived from the sun, as opposed to fossil fuels being ultimately derived from it. Therefore, it is renewable source of energy that does not create green-house gas emissions or non-degradable toxic wastes.

Solar radiant intensity we can receive on the ground is usually less than 1 kW/m2, although it varies depending on the latitude, season, angle, and the weather conditions at the given site. The solar constant is defined as the total radiant energy density received vertically from the sun per unit area and unit time, just outside the earth’s atmosphere, when the earth is at its average distance from the sun. Its value is approximately 1,368W/m2, within an accuracy of about 0.2%.

2  Photovoltaic system
A solar cell is any material that converts sunlight directly into electricity. This property of materials is known as the photoelectric effect, first described in 1905 by Albert Einstein. The first solar cell that was capable of generating a substantial amount of power emerged from pioneering semiconductor research at Bell Laboratories in the 1940s and early 1950s.

A photovoltaic cell can generate electricity based upon the phenomenon that occurs when photons strike electrons. The electrons free themselves from the valence band, leave holes behind them and jump up into the conduction band over the forbidden band. When an incident light is combined with this property in a junction diode, the electrons and holes tend to flow in opposite directions along the potential gradient across the junction, setting up a voltage. With the addition of circuitry, current flows and then electric power become available.

A photovoltaic system is a complete set for photovoltaic power generation, including the array and the balance of system constituents. An array is an interconnected system of photovoltaic modules that consist of plural photovoltaic cells or solar cells. These cells are made of various semiconductor materials including silicon, cadmium sulfide, cadmium telluride, gallium arsenide, Cu(In⋅Ga)Se2 and so on, in single crystalline, polycrystalline, or amorphous forms. A polycrystalline silicon is composed of many multi-oriented individual crystals, and widely used for solar cells, because its fabrication can be easier than that of a single-crystalline type. An amorphous silicon is an alloy of silica and hydrogen with a disordered, non-crystalline internal atomic arrangement. It can be deposited in thin-film layers using a number of deposition methods to produce photovoltaic cells on glass, metal, or plastic substrates.

A Solar Power Satellite system(SPS) is a grand system to supply power from space for use on the earth. A proposed SPS system would have a huge array of solar cells that would generate electric power to be beamed and sent to a central receiver on the earth in the form of microwave energy.

3. Solar thermal energy conversion
Solar thermal energy conversion is a technological method of converting the radiant energy of the sun into heat, which can then be used for such purposes as space heating, air conditioning, hot water heating, industrial process heat, drying, distillation,  desalination, and power generation. When a dark surface is placed in sunshine, it absorbs solar energy and heats up. A simple solar thermal collector, working on this principle, consists of a sun-facing surface which transfers part of the energy to a working fluid like water or air. To reduce heat losses to the atmosphere and to improve its efficiency, one or two sheets of glass are usually placed over the absorber's surface and insulation is placed behind the absorber. This type of solar collector is called a flat plate collector, which can achieve temperatures up to about 100℃. To achieve higher temperatures, as high as 1000℃ or even higher, a system of concentrating collectors is used, where a parabolic trough, parabolic dish, and central receiver tower are arranged.

“Dictionary of Energy” by Cutler J. Cleveland and Christopher Morris, Elsevier Ltd. (2006)

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