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Solar Power Design Manual

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The sun in the morning, and... solar power... can the sun provide?

For atmospheric scientists, the sun may be a globe of gas, but that globe, that star, around which our planet wanders, is our source of light, heat, and life. Undeniably, the light and heat from that fiery furnace whose surface tempera ture reaches 9.981°F (55.27°C) has made life on earth sublimely comfortable, as only 1 kW of energy, a minuscule amount of its radiant heat, falls on a square yard of earth each day. Over the course of a year, this 1 kW contributes a thousand times more energy than the combined sources of energy produced by coal, oil, gas, nuclear, wind, hydroelectricity, and other sources of heat/electricity - producing power plants.

Most of that 1 kW of solar energy is absorbed by the oceans—which cover 72% of the planet's surface—and the belt of lands circling the equator. Nonetheless, sunlight is everywhere, is inherently clean, and is infinitely renewable; yet only within the past 50 years has it began to be seriously exploited.

With the availability of lenses, it was quickly learned that the sun' s rays could be focused and concentrated, producing sufficient heat at the focal point for paper to burst into flame.

A solar-powered water heater was invented by Clarence M. Kemp, and was offered to the public in 1891. Many are in use in the US West and Southwest, but the heat generated is insufficient to boil water, which means that if solar energy is to be used to produce electricity, it must develop temperatures of at least 212°F (100°C). To do so, solar energy must be concentrated—focused.

Parabolic trough systems can concentrate the sun's energy via long, rectangular U-shaped mirrors. The mirrors are tilted toward the sun, focusing sunlight on a pipe running along the center of the trough. This heats the medium—oil or molten salt (liquid sodium)—flowing through the pipe. The heated medium transfers its heat to water in a conventional steam generator, which spins, producing an electric current [16].

A second type of concentrator is the dish/engine system, which uses a mirrored dish, similar to a large satellite dish. This highly polished and reflecting surface—the concentrator—collects and focuses the sun's heat onto a receiver that absorbs the heat and transfers it to a fluid within the engine. The heat causes the fluid to expand against a piston or turbine that spins, producing the mechanical energy needed to run a generator or alternator.

Yet a third type of concentrator, the solar power tower system (see Fig. 7.5), employs a huge field of highly reflecting mirrors (1800-2000 total) heliostats, each measuring 7 x 7 meters (22 x 22 feet), that concentrate sunlight onto the top of a tower situated in the center of the field. As the sun rises and crosses the sky, the mirrors tilt and swivel to continuously harvest the rays and focus them onto the tower, or receiver, where molten salt, a mixture of sodium and potassium nitrate, reaches a temperature of 1000°F. Unlike water, molten salt retains heat efficiently so that it can be stored for days before being converted to electricity, which means electricity can be produced on cloudy days or hours after sunset. Ergo, storage is a key element in the alternative energy equation. Unfortunately thousands of heliostats laid out in circles require 120-130 acres of land, which can be found only in barren deserts or on the great plains (see Ref. 18 or 19).

Concentrating collectors are good as far as they go, but are impractical for generating the levels of electricity demanded currently and projected for the

Figure 7.5. A solar power tower showing the hundreds of mobile, highly polished helio-stats—concentrating tracking mirrors—arrayed around the Tower, and the white-hot central power receiver at the top of the tower. Heat energy directed to the receiver is absorbed by molten salt, which generates steam that drives the generator. (Figure courtesy of Sandia National Labs.)

Figure 7.5. A solar power tower showing the hundreds of mobile, highly polished helio-stats—concentrating tracking mirrors—arrayed around the Tower, and the white-hot central power receiver at the top of the tower. Heat energy directed to the receiver is absorbed by molten salt, which generates steam that drives the generator. (Figure courtesy of Sandia National Labs.)

future. A device that can convert or change sunlight directly into electricity is needed.

With no moving parts, no mirrors, no heat transfer fluid or generators, no polluting chemicals or particles, and no land-use constraints, photovoltaic cells may be too good to be true. To understand photovoltaics, it may be helpful to go back a few years, to 1839, when Edmund Becquerel, son of Henri Becquerel, discovered the process of using sunlight to produce an electric current in a solid material. Without knowing it, Edmund had discovered the photoelectric effect, but which is attributed to Heinrich Hertz in 1897, and for which Albert Einstein won the Nobel Prize in Physics in 1905—for explaining the theory behind it. With these great minds at our service, comprehension may not be difficult. So, for example, Einstein showed that the kinetic energy of an ejected electron was equal to the energy of the incident photon minus the energy required to remove the electron from the surface. Thus, a photon of light hits a surface, transfers almost all its energy to an electron, and the electron is ejected with that energy less whatever energy is required to get it out of the atom and away from the surface. The photoelectric effect has many practical applications including the photocell and solar cells.

Solar cells, usually made from specially prepared silicon, act like a battery when exposed to light. Individual solar cells produce voltages of abut 0.6-

0.7 V, but higher voltages can be obtained by connecting many cells together. The photovoltaic effect is the conversion of radiation, sunlight, into electric power via absorption of a semiconducting material. How do they work?

When sunlight shines on a photovoltaic (PV) cell, it may be reflected, be absorbed, or pass through. Only the absorbed light generates electricity. The energy of the absorbed light is transferred to electrons in the atoms of the PV cell. With this new energy the electrons escape from their normal positions in the atoms of the semiconductor-PV material and become part of the electric flow or current.

A "built-in" electric field provides the force, a voltage needed to drive the current. To induce the built-in electric field within a PV cell, two layers of differing semiconductor materials are placed in contact with one another.

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