readings with assessment in Alternative Energy -- Wind
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Alternative Energy Basics
FROM THE SUN
The sun has
produced energy for billions of years. Solar energy is the
sun’s rays (solar radiation) that reach the earth.
can be converted into other forms of energy, such as heat and electricity.
In the 1830s, the British astronomer John Herschel used a solar
thermal collector box (a device that absorbs sunlight to collect
heat) to cook food during an expedition to Africa. Today, people
use the sun's energy for lots of things.
can be converted to thermal (or heat) energy and
– for use in homes, buildings, or swimming pools.
– inside greenhouses, homes, and other buildings.
can be converted to electricity in two ways:
(PV devices) or “solar cells” – change sunlight directly into
electricity. PV systems are often used in remote locations that
are not connected to the electric grid. They are also
used to power watches, calculators, and lighted road signs.
Power Plants - indirectly generate electricity
when the heat from solar thermal collectors is used to heat
a fluid which produces steam that is used to power generator.
Out of the 15 known solar electric generating units operating
in the United States at the end of 2006, 10 of these are in
California, and 5 in Arizona. No statistics are being collected
on solar plants that produce less than 1 megawatt of electricity,
so there may be smaller solar plants in a number of other states.
The major disadvantages
of solar energy are:
of sunlight that arrives at the earth's surface is not constant.
It depends on location, time of day, time of year, and weather
the sun doesn't deliver that much energy to any one place at
any one time, a large surface area is required to collect the
energy at a useful rate.
energy is the conversion of sunlight into electricity. A photovoltaic
cell, commonly called a solar cell or PV, is the technology used
to convert solar energy directly into electrical power. A photovoltaic
cell is a nonmechanical device usually made from silicon alloys.
Sunlight is composed of photons, or particles of solar energy.
These photons contain various amounts of energy corresponding to
the different wavelengths of the solar spectrum. When photons
strike a photovoltaic cell, they may be reflected, pass right through,
or be absorbed. Only the absorbed photons provide energy to
generate electricity. When enough sunlight (energy) is absorbed
by the material (a semiconductor), 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 surface.
When the electrons leave their position, holes are formed.
When many 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, electricity
cell is the basic building block of a photovoltaic system.
Individual cells can vary in size from about 1 centimeter (1/2 inch)
to about 10 centimeter (4 inches) across. However, one cell
only produces 1 or 2 watts, which isn't enough power for most applications.
To increase power output, cells are electrically connected into
a packaged weather-tight module. Modules can be further connected
to form an array. The term array refers to the entire generating
plant, whether it is made up of one or several thousand modules.
The number of modules connected together in an array depends on
the amount of power output needed.
of a photovoltaic array is dependent upon sunlight. Climate
conditions (e.g., clouds, fog) have a significant effect on the
amount of solar energy received by a photovoltaic array and, in
turn, its performance. Most current technology photovoltaic
modules are about 10 percent efficient in converting sunlight. Further
research is being conducted to raise this efficiency to 20 percent.
cell was discovered in 1954 by Bell Telephone researchers examining
the sensitivity of a properly prepared silicon wafer to sunlight.
Beginning in the late 1950s, photovoltaic cells were used to power
U.S. space satellites (learn more about the history
of photovaltaic cells). The success of PV in space generated
commercial applications for this technology. The simplest
photovoltaic systems power many of the small calculators and wrist
watches used everyday. More complicated systems provide electricity
to pump water, power communications equipment, and even provide
electricity to our homes.
of photovoltaic systems are:
from sunlight to electricity is direct, so that bulky mechanical
generator systems are unnecessary.
can be installed quickly and in any size required or allowed.
impact is minimal, requiring no water for system cooling and
generating no by-products.
cells, like batteries, generate direct current (DC)
which is generally used for small loads (electronic equipment).
When DC from photovoltaic cells is used for commercial applications
or sold to electric utilities using the electric grid, it must be
converted to alternating current
(AC) using inverters, solid state devices that convert DC power
PV has been used at remote sites to provide electricity. In
the future PV arrays may be located at sites that are also connected
to the electric grid enhancing the reliability of the distribution
energy is often used for heating swimming pools, heating water used
in homes, and space heating of buildings. Solar space heating systems
can be classified as passive or active.
space heating is what happens to your car on a hot summer
day. In buildings, the air is circulated past a solar heat surface(s)
and through the building by convection (i.e. less dense warm air
tends to rise while more dense cooler air moves downward) .
No mechanical equipment is needed for passive solar heating.
heating systems require a collector to
absorb and collect solar radiation. Fans or pumps are
used to circulate the heated air or heat absorbing fluid. Active
systems often include some type of energy storage system.
can be either nonconcentrating or concentrating.
collectors – have a collector area (i.e. the area that
intercepts the solar radiation) that is the same as the absorber
area (i.e., the area absorbing the radiation). Flat-plate
collectors are the most common and are used when temperatures
below about 200o degrees F are sufficient, such as for space heating.
collectors – where the area intercepting the solar radiation
is greater, sometimes hundreds of times greater, than the absorber
power plants use the sun's rays to heat a fluid, from which heat
transfer systems may be used to produce steam. The steam, in turn,
is converted into mechanical energy in a turbine and into electricity
from a conventional generator coupled to the turbine.
Solar thermal power generation works essentially the same as
generation from fossil fuels except that instead of using steam
produced from the combustion of fossil fuels, the steam is produced
by the heat collected from sunlight. Solar thermal technologies
use concentrator systems due to the high temperatures needed to
heat the fluid. The three main types of solar-thermal
power systems are:
AND THE ENVIRONMENT
is free, and its supplies are unlimited. Using solar energy produces
no air or water pollution but does have some indirect impacts on
the environment. For example, manufacturing the photovoltaic cells
used to convert sunlight into electricity, consumes silicon and
produces some waste products. In addition, large solar thermal farms
can also harm desert ecosystems if not properly managed.
SOURCE : ENERGY
INFORMATION ADMINISTRATION -- DEPARTMENT OF ENERGY -- KIDS HOME
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1) The photovoltaic
cell was invented in what year?
enter your answer in the space provided:
much power can one photocell generate?
3) The current efficiency of most photocells is about ___