A
solar cell is a photovoltaic device; as the name
implies, it converts light (“photo”) into electricity
(“volt”). The cost of a solar array may seem
prohibitive, but its long-term financial advantages and
environmental benefits can be attractive to people
concerned with their effect on the global environment.
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MATERIAL
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Silicon
is widely used for solar cells because of its atomic
structure. Regular silicon lacks a charge because the
number of positively charged protons (16) equals the
number of negatively charged electrons; in effect, they
cancel each other out. The electrons orbit the core of
protons and neutrons in three different layers, with two
electrons lying closest to the center, eight on the
middle layer and four on the outer layer. (See
illustration)
The outer layer can hold a total of eight electrons. To
fill the gaps, silicon atoms join up and share electrons
with one another, creating a crystalline pattern. This
crystal is a stable system of interlocked atoms, with
each electron fitting neatly into the overall structure.
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DOPING
Electricity is essentially a flow of free electrons, but
since the electrons in silicon fit into the crystal
structure not enough of them break free to be useful. To
encourage free electron flow, two silicon panels are
doped.
Doping is a process by which foreign atoms are added to
the silicon crystal, disrupting its neat order.
Phosphorus and boron are popular chemicals for doping;
phosphorous has five electrons in its outer layer
(compared to silicon’s four), which leaves one electron
out of the crystal grid. Boron, on the other hand, has
only three electrons in its outer layer, effectively
creating a hole where an electron could go.
Phosphorus-doped silicon is called n-silicon (n stands
for negative) because of its extra electrons. Boron
doping creates p-silicon, or positive silicon.
JUNCTION
The
doped panels are still electrically neutral (despite
their misleading names) because the foreign chemicals
have brought in an equal number of protons and
electrons, maintaining the balance. However, when the
panels come together this equilibrium is disrupted near
where they join. Extra electrons from the n-silicon jump
to the p-silicon to fill the holes in the p-side’s
crystal.
Since the electrons on the n-side have jumped ship, the
n-side is left with more protons than electrons near the
junction, creating a positive charge. The converse is
true on the p-side. This static charge creates a barrier
between the two sides of the cell, making it difficult
for more electrons to cross over. (See illustration)
SUNLIGHT
Sunshine is comprised of photons, little particles with
a lot of energy. When a photon with the right amount of
energy hits an electron, it can knock the electron free
from its atomic orbit. This is much more likely to
happen on the n-side, where there are electrons that are
left out of the interlocked crystal. However, it happens
on a smaller scale on the p-side too.
BARRIER
CROSSING
The
static barrier is strong, but it isn’t impenetrable. If
a free electron can work up a high enough velocity, it
can break through to the other side. This isn’t likely
to happen on the n-side, because there’s a bigger crowd
of free electrons and it’s difficult for one to work up
enough energy to break through. However, there are so
few free electrons on the p-side that they’re better
equipped to work up the speed to cross over and join the
crowded n-side.
FLOW
With
the n-side getting more and more crowded with free
electrons, a highly conductive wire provides them the
opportunity to spill outside of the solar cell. The
overflow of free electrons keeps getting bigger, which
pushes the crowd farther along the wire until they reach
a load.
REUNION
Once
the electrons have powered the load, they keep going
along the wire until they get back to the p-side of the
solar cell. Some electrons break through the barrier to
the n-side; others fill the gaps in the boron-doped
p-silicon.
COST
EFFECTIVENESS
Solar
power is still more expensive than power from the
utility grid. If you calculate the amount of electricity
you should be able to produce with a solar cell over its
30-plus year lifespan, then divide that number by the
initial cost of installation, the cost per kilowatt hour
of a solar cell is about 30 cents. The power grid, on
the other hand, sells electricity at a rate of about
four cents per kilowatt hour.
Moreover, the solar cell is unlikely to meet all of your
electricity needs, which means you’ll still need to be
hooked up to the utility grid. This can also work to
your advantage, because when your solar cell is
producing more than you need you can pour your extra
electricity into the grid and effectively turn your
meter back. This is a more efficient option than storing
the extra energy in a battery.
If the cost seems prohibitive, consider the redeeming
benefits of solar energy. First, it’s clean energy. It
doesn’t produce any greenhouse gases or other
environment-damaging byproducts. Second, solar energy
users are protected from inflation in energy cost. Once
the initial installation is paid for, the cost of
maintaining a solar cell is minimal. If inflation
continues to affect energy costs, as it almost certainly
will, electricity from the utility grid will become more
expensive and the solar array will eventually pay for
itself.
Third, since solar energy is clean, there are government
incentives in place to encourage people to use it. One
incentive is subsidies; another is the exchange of
renewable energy credits.
Companies are only allowed to put so many harmful gases
into the atmosphere. That amount is regulated through a
credit system. When a company runs out of credits, it
can essentially buy more by paying a person with a solar
array. While this is environmentally questionable, since
it allows polluters to put more toxins into the air,
the system is intended to make solar energy an
attractive alternative energy option.
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