Solar Power
Photovoltaic
solar cells, which directly convert sunlight into electricity, are made of
semiconducting materials. The simplest photovoltaic cells power watches and
calculators and the like, while more complex systems can light houses and
provide power to the electrical grid.
Technologies
Common techniques for the production of crystalline silicon include the Czochralski
(CZ) method, float-zone
(FZ) method, and other methods such as casting
and die or wire pulling. The removal of impurities
and defects in the silicon is of critical importance, and is addressed with
techniques such as surface passivation
(reacting the surface with hydrogen) and gettering
(a chemical heat treatment that causes impurities to diffuse out of the
silicon). Also at issue as the industry grows is the availability and purity of
the solar-grade
silicon feedstock.
Thin
Films
Thin film photovoltaic cells use layers of semiconductor materials only a few
micrometers thick, attached to an inexpensive backing such as glass, flexible
plastic, or stainless steel. Semiconductor materials for use in thin films
include amorphous
silicon (a-Si), copper
indium diselenide (CIS), and cadmium
telluride (CdTe). Amorphous silicon has no crystal structure and is
gradually degraded by exposure to light through the Staebler-Wronski
Effect. Hydrogen
passivation can reduce this effect. Because the quantity of semiconductor
material required for thin films is far smaller than for traditional PV cells,
the cost
of thin film manufacturing is far less than for crystalline silicon solar
cells.
Group
III-V Technologies
These photovoltaic technologies, based on Group III and V elements in the
Periodic Table, show very high conversion efficiencies under either normal
sunlight or sunlight that is concentrated (see "Concentrating
Collectors" below). Single-crystal cells of this type are usually made of
gallium arsenide (GaAs). Gallium arsenide can be alloyed
with elements such as indium, phosphorus, and aluminum to create semiconductors
that respond to different energies of sunlight.
High-Efficiency
Multijunction Devices
Multijunction devices stack individual solar cells on top of each other to
maximize the capture and conversion of solar energy. The top layer (or junction)
captures the highest-energy light and passes the rest on to be absorbed by the
lower layers. Much of the work in this area uses gallium
arsenide and its alloys, as well as using amorphous
silicon, copper indium diselenide, and gallium
indium phosphide. Although two-junction cells have been built, most research
is focusing on three-junction (thyristor) and four-junction devices, using
materials such as germanium
(Ge) to capture the lowest-energy light in the lowest layer.
Fabricating
Solar Cells and Modules
A variety of technical issues are involved in the fabrication of solar cells.
The semiconductor material is often doped
with impurities such as boron or phosphorus to tweak the frequencies of light
that it responds to. Other treatments include surface
passivation of the material and application of antireflection
coatings. The encapsulation
of the complete PV module in a protective shell is another important step in the
fabrication process.
Advanced
Solar Cells
A variety of advanced approaches to solar cells are under investigation. Dye-sensitized
solar cells use a dye-impregnated layer of titanium dioxide to generate a
voltage, rather than the semiconducting materials used in most solar cells.
Because titanium dioxide is relatively inexpensive, they offer the potential to
significantly cut the cost of solar cells. Other advanced approaches include polymer
(or plastic) solar cells (which may include large carbon molecules called
fullerenes) and photoelectrochemical
cells, which produce hydrogen directly from water in the presence of
sunlight.
Balance
of System (BOS) Components
The balance of system (BOS) components include everything in a photovoltaic
system other than the photovoltaic modules. BOS components may include mounting
structures, tracking devices, batteries, power electronics (including an
inverter, a charge controller, and a grid interconnection), and other devices.
Applications
Concentrator
Collectors
Concentrating photovoltaic collectors use devices such as Fresnel lenses,
mirrors, and mirrored dishes to concentrate sunlight onto a solar cell. Certain
solar cells, such as gallium arsenide cells, can efficiently convert
concentrated solar energy into electricity, allowing the use of only a small
amount of semiconducting material per square foot of solar collector.
Concentrating collectors are usually mounted on a two-axis
tracking system to keep the collector pointed toward the sun.
Building-Integrated
Photovoltaics (BIPV)
Building-integrated photovoltaic materials are manufactured with the double
purpose of producing electricity and serving as construction materials. They can
replace traditional building components, including curtain walls, skylights,
atrium roofs, awnings, roof
tiles and shingles, and windows.
Stand-Alone
Photovoltaic Systems
Stand-alone systems produce power independently of the utility grid. In some off-the-grid
locations as near as one-quarter mile from the power lines, stand-alone
photovoltaic systems can be more cost-effective than extending power lines. They
are especially appropriate for remote, environmentally sensitive areas, such as national
parks, cabins, and remote homes. In rural
areas, small stand-alone solar arrays often power farm lighting, fence chargers,
and solar
water pumps, which provide water for livestock. Direct-coupled systems need
no electrical storage because they operate only during daylight hours, but most
systems rely on battery
storage so that energy produced during the day can be used at night. Some
systems, called hybrid
systems, combine solar power with additional power sources such as wind or
diesel.
Grid-connected
Photovoltaic Systems
Grid-connected photovoltaic systems, also called grid interface systems, supply
surplus power back through the grid to the utility, and take from the utility
grid when the home system's power supply is low. These systems remove the need
for battery storage, although arranging for the grid
interconnection can be difficult. In some cases, utilities allow net
metering, which allows the owner to sell excess power back to the utility.
Space
Applications
Solar arrays work well for generating power in space and power virtually all
satellites. Most satellites and spacecraft are equipped with crystalline silicon
or high-efficiency Group III-IV cells, but recently satellites have begun using
thin-film amorphous-silicon-based solar panels.
Issues
Cost
Issues
Photovoltaics are expensive to produce because of the high cost of
semiconducting materials. Cost reductions can be achieved by reducing manufacturing
costs. As manufacturing
capacity increases, costs of manufacturing decrease. Manufacturers aim to
achieve the break-even
cost for a photovoltaic system, at which the cost of the electricity it
produces is equal to the cost of electricity from an alternative source plus the
cost of delivering this electricity to the site. The distance a power line needs
to be extended to equal the installation cost of a photovoltaic system is called
the break-even distance.
Incentives
Programs
Regulatory and financial incentives, such as tax credits, low interest loans,
grants, special utility rates, and technical assistance to encourage the
installation of photovoltaic systems are all available, though they vary from
region to region.
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