Environmental Engineering Reference
In-Depth Information
Solar Power Tower
Concentrating solar power plants generate electricity by using the heat from solar thermal col-
lectors to heat a fluid which produces steam used to power a generator. A solar power tower or
central receiver generates electricity from sunlight by focusing concentrated solar energy on a
tower-mounted heat exchanger (receiver). This system uses hundreds or thousands of flat sun-
tracking mirrors called heliostats to reflect and concentrate the sun's energy onto a central receiver
tower. The energy can be concentrated as much as 1,500 times that of the energy coming in from
the sun (USEIA 2011f). Power towers must be large to be economical. Though power towers are
in the early stages of development compared with parabolic trough technology, a number of test
facilities have been constructed around the world. Of thirteen large concentrating solar power
generating units operating in the United States at the end of 2008, eleven were in California, one
in Arizona, and one in Nevada (USEIA 2011c).
SOLAR PHOTOVOLTAIC TECHNOLOGIES
Photovoltaic (PV) solar cells change sunlight directly into electricity. Individual PV cells are
grouped into interconnected panels and arrays of panels that can be used in a wide range of ap-
plications, ranging from single small cells that charge calculator and watch batteries, to systems
that power single homes, to large power plants covering many acres.
French physicist Edmond Becquerel discovered the process of using sunlight to produce an
electric current in a solid material as early as 1839. More than a century later, scientists eventu-
ally learned that the photoelectric or photovoltaic effect caused certain materials to convert light
energy into electrical energy at the atomic level (USDOE 2011b). The first practical photovoltaic
cell was developed in 1954 by Bell Telephone researchers examining the sensitivity of a silicon
wafer to sunlight. Beginning in the late 1950s, PV cells were used to power U.S. space satellites.
PV cells were next widely used for small consumer electronics like calculators and watches and
to provide electricity in remote or “off-grid” locations where there were no electric power lines.
More complicated systems provide electricity to pump water, power communications equipment,
and provide electricity to homes. Technology advances, government financial incentives, and in-
novations in financing them have helped to greatly expand PV use since the mid-1990s. Shipments
of PV cells and panels by U.S. manufacturers in 2006 were the equivalent of about 337 megawatts,
about twenty-five times greater than shipments of about thirteen megawatts in 1989. Since about
2004, most of the PV panels installed in the United States have been in grid-connected systems on
homes, buildings, and central-station power facilities. There are now PV products available that
can replace conventional roofing materials while generating electricity (USEIA 2011d).
Sunlight is composed of photons, or particles of solar energy. When photons strike a photovoltaic
cell, some are absorbed by a semiconductor material, and electrons are dislodged from the material's
atoms. Special treatment of the material surface during manufacturing makes the front surface of
the cell more receptive to free electrons, so the electrons naturally migrate to the front surface.
When the dislodged electrons, each carrying a negative charge, travel toward the front surface
of the cell, the resulting imbalance of charge between the cell's front and back surfaces creates a
voltage potential like the negative and positive terminals of a battery. When the two surfaces are
connected through an external load, such as an appliance, electricity flows (USEIA 2011d).
The photovoltaic cell is the basic building block of a photovoltaic system. Individual cells can
vary in size from about 0.5 inches to about 4 inches across. One cell produces one or two watts,
not enough power for most applications. To increase power output, cells are electrically connected
 
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