Photovoltaics is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect. Photovoltaic power generation employs solar panels comprising a number of cells containing a photovoltaic material. Materials presently used for photovoltaic include monocrystalline silicon, polycrystalline
silicon, amorphous silicon, cadmium telluride,
and copper indium selenide/sulfide.
Photovoltaic Effect
The
photovoltaic effect is the creation of a voltage (or a corresponding electric
current) in a material upon exposure to light. Though the photovoltaic effect
is directly related to the photoelectric
effect, the
two processes are different and should be distinguished. In the photoelectric
effect, electrons are ejected from a material's surface upon exposure to
radiation of sufficient energy. The photovoltaic effect is different in that
the generated electrons are transferred between different bands (i.e. from the
valence to conduction bands) within the material, resulting in the buildup of a
voltage between two electrodes.
Solar cells
Photovoltaics are
best known as a method for generating electric power by using solar cells to convert energy
from the sun into electricity. The photovoltaic effect refers to photons of light knocking
electrons into a higher state of energy to create electricity. The term
photovoltaic denotes the unbiased operating mode of a photodiode in which current
through the device is entirely due to the transduced light energy. Virtually
all photovoltaic devices are some type of photodiode.
•
Cover glass – required for protection
to the environment.
•
Anti-reflecting coating - a type of optical coating applied to the
surface of lenses and other optical devices to reduce reflection.
•
Contact grid – the front contact to the
load.
•
N-type SI - are a type of extrinsic semiconductor where the dopant atoms are capable of
providing extra conduction electrons to the host material (e.g. phosphorus in silicon). This creates an excess of
negative (n-type) electron charge carriers.
•
A P-type semiconductor (P for Positive) is
obtained by carrying out a process of doping, that is adding a certain type of atoms to the semiconductor in order
to increase the number of free charge carriers (in this case
positive). (e.g.boron or aluminium)
·
Back contact – the back contact to the load
Applications
Photovoltaic arrays are
often associated with buildings: either integrated into them, mounted on them
or mounted nearby on the ground.
Arrays are most often
retrofitted into existing buildings, usually mounted on top of the existing
roof structure or on the existing walls. Alternatively, an array can be located
separately from the building but connected by cable to supply power for the
building. In 2010, more than four-fifths of the 9,000 MW of solar PV operating
in Germany was installed on rooftops.[18]
Photovoltaic solar panels on
a house roof. Solar
Parking Meter
Building-integrated
photovoltaics (BIPV) are increasingly
incorporated into new domestic and industrial buildings as a principal or
ancillary source of electrical power. Typically, an array is incorporated into
the roof or walls of a building. Roof tiles with integrated PV cells are also
common.
The power output of
photovoltaic systems for installation in buildings is usually described in kilowatt-peak units (kWp).
Advantages
The 89 petawatts of sunlight reaching the Earth's surface is
plentiful - almost 6,000 times more than the 15 terawatts equivalent of average power consumed by
humans. Additionally, solar electric generation has the highest power density
(global mean of 170 W/m²) among renewable energies.
Solar power is
pollution-free during use. Production end-wastes and emissions are manageable
using existing pollution controls. End-of-use recycling technologies are under
development and policies are being produced that encourage recycling from
producers. PV installations can operate for many years with little maintenance
or intervention after their initial set-up, so after the initial capital cost of building any solar power plant, operating costs are extremely low compared to existing power
technologies.
Solar electric generation
is economically superior where grid connection or fuel transport is difficult,
costly or impossible. Long-standing examples include satellites, island communities,
remote locations and ocean vessels.
When grid-connected, solar
electric generation replaces some or all of the highest-cost electricity used
during times of peak demand (in most climatic regions). This can reduce grid
loading, and can eliminate the need for local battery power to provide for use
in times of darkness. These features are enabled by net metering. Time-of-use net metering can be highly
favorable, but requires newer electronic metering, which may still be
impractical for some users.
Disadvantages
Photovoltaics are costly to
install. While the modules are often warranted for upwards of 20 years, much of
the investment in a home-mounted system may be lost if the home-owner moves and
the buyer puts less value on the system than the seller.
Solar electricity is seen
to be expensive. With the UK Feed-In Tariff for green solar energy, Solar PV
has been made more accessible to homeowners. Under the scheme, homeowners can
generate both free electricity, and a fee per kWh sold to the grid "Solar PV as a Domestic Investment
Opportunity: The Options".
Solar electricity is not
produced at night and is much reduced in cloudy conditions. Therefore, a storage or complementary
power system is required.
Solar electricity production depends on the
limited power density of the location's insolation. Average daily output of a
flat plate collector at latitude tilt in the contiguous US is 3-7 kilowatt·h/m²
and on average lower in Europe.
Solar cells produce DC which must be converted to
AC (using a grid tie inverter) when used in existing distribution grids.
This incurs an energy loss of 4-12%.
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