Chapter 24 Energy Conservation
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sun’s energy, and turbines are used to harness
the wind’s energy. This section discusses these
systems.
Photovoltaics
The total amount of energy the earth
receives from the sun is estimated to be over
1,500,000,000,000,000,000 kWh per year. That
is many thousand times more than all the
energy consumed by all the countries on
this planet. The sun represents a tremendous
reserve of energy, surpassing that of all the
earth’s energy sources.
The use of solar energy to produce elec-
tricity is called photovoltaics. Photovoltaic
(PV) cells, or solar cells, convert sunlight
directly into electrical energy. This technology
was initially developed in the 1950s at Bell
Laboratories. Sunlight causes the electrons to
move about in the solar cell. If the solar cell is
connected to a circuit, an electrical current is
produced.
Since there are no moving parts, these
units are very reliable, operate silently, and
have an expected life of 20 to 30 years. Since
the sun serves as the energy source, there is
no pollution.
When a number of the solar cells are con-
nected together they form a photovoltaic mod-
ule, Figure 24-14. Depending on the number
of solar cells used, PV modules are sized by
the number of watts they produce. A PV mod-
ule that is roughly 20″ by 20″ might produce
1.63 A at 17.1 V and would be rated at 27.9 W
(P = EI).
A larger size might vary the number
of solar cells and be able to produce 190 W.
A 1 kWp (kilowatt peak output) module
would take up a space of 10 square meters.
This unit would be expected to generate
roughly 1000 kWh in a year. The size of the
unit selected for a particular task is based
on the power needed. Some PV modules
are large enough to cover the entire roof of
a building, Figure 24-15. Others are large
enough to produce over one million watts
of electricity.
Photovoltaic systems
PV systems produce dc. Small PV systems
can be used to charge batteries or run a small
piece of electrical equipment. Larger systems
use an inverter to change their output to ac
and connect them to the grid. The grid is a
wiring network that is spread across most of
the United States. In the event of an electrical
outage in one area, electrical power can be
provided from a substation from another loca-
tion. The grid provides ac electrical power,
while the PV system produces dc. Therefore,
Figure 24-14. Photovoltaic modules.
PV module