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Electricity and Basic Electronics
When a PV system is used to supply
energy to the grid, it is a two-way system.
When there is a surplus of energy in the PV
system, the electricity is fed into the grid.
When there is a shortage of energy in the PV
system, the grid feeds it electricity. If a storm
causes an outage on the grid, people repairing
the electrical lines could be shocked from the
PV system unless it is automatically discon-
nected. A PV system that performs all of these
functions is called a line-tied or grid-connected
installation.
If a PV system is installed on a home, the
owner can sell electricity to the local power
company. Any excess photovoltaic energy that
is not needed to run the home would be fed
into the power grid. This spins the home’s
electric meter backward. The excess energy
provides a credit against the electricity from
the power company that is used in the home.
This is known as net metering.
However, the homeowner may choose not
to sell the electricity to the power company.
Stand-alone PV units provide the owner with
an off-the-grid power supply that is indepen-
dent from the power company.
Uses and examples
PV systems can be used in many different
applications. They are ideal for areas where it
is diffi cult or impossible to run power lines,
Figure 24-15. For example, farmers may
use photovoltaics to power electric fences in
pasture areas. Some mountain homes or other
isolated vacation areas use PV systems for fans,
computers, lights, Figure 24-16, and radios.
Bigger PV systems can be used to operate
appliances such as furnace fans, lights, refrig-
erators, and security systems.
The portability of PV systems gives them
another advantage and opens new possi-
bilities for their uses. Many smaller portable
devices can run using photovoltaics. There
have been cell phones, calculators, watches,
games, or toys that have been designed to run
on solar power. PV systems can be used to run
devices on boats and recreational vehicles so
that those on board do not have to rely on a
generator during the day.
Photovoltaic locations
Photovoltaics are used in more than a
dozen countries around the world. Since
photovoltaics depend on sunlight, it can be
used just about anywhere on this planet. At
the present time, there are more than 300 PV
systems in the world. Germany has more than
any other country. A useful source for the
most current information can be found on the
Internet at www.pvresources.com. This site
provides a list of the top 300 PV systems in the
world. Updates are provided as new plants
start producing.
Photovoltaic devices are most cost effi cient
in areas where the sun shines most of each
day. This includes places where the weather is
cold. Locations with the most hours of sunlight
provide a higher savings potential. However,
in cloudy conditions, PV systems still function
but at a lower rate.
Figure 24-15. Crossing gates can be powered by
photovoltaic systems.