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Learner's
Notes
Electronics
is the study of electrons. Every element in this world is made up
of atoms, and atoms
in turn are made up of protons, neutrons and electrons. In
olden days, the shape of an atom and their
constituents were unknown. Without knowing the shape of the basic
element of every element,
experiments on these elements could not be studied.
 Hence,
a huge number of experiments on
various
elements were performed, which showed specific characteristics
of an
atom. This led to the scientists worldwide to agree
upon a standard shape for the
atom. The centre of the atom is the nucleas made up of the
protons (+ve charge)
and neutrons (neutral charge). Electrons revolve around the
nucleas in fixed orbits.
They are negatively charged. They are the ones
that conduct electricity. Electric
energy is transported by free electrons, that are in the conducting
band. Electrons
in the valence band of an atom are also free but have to be
brought to the
conducting band to make them conduct or transfer electric energy.
The further details of the structure of atoms and bonding are all
beyond the scope of this notes.
Elements
are classified into conducting, non-conducting and semiconducting.
Semiconductors
are those elements that conduct electricity
when energised by an external energy source. A P-N
junction semiconductor is formed by sandwitching a p-type semiconductor
and an n-type
semiconductor. The junction formed inbetween, is the barrier between
the electrons and holes
(positively charged particles).
SOLAR CELL
A solar cell is
formed by a light sensitive P-N juction semiconductor, which when
exposed to
sunlight are bombarded by the photons in light (Photons are particles
of light, like we have atoms as
particles of an element). Every photon has a energy.
 If
the energy of the photon, hitting an electron
of an atom in the semiconductor, is greater than or equal to the
energy required to release the electron from its non-conducting
position in the atom to the free conducting state, it will contribute
to
the output of the solar cell. The
free electrons generated, find a path
towards the P-type semiconductor (As the rule goes " unlike
charges
attract each other " ), through an external path. If an external
path is
not there, the process of generating free electrons stops. The more
the amount of light falling on the cell's surface, more is the probability
of photons releasing electrons, and hence more electricity generated.
SOLAR PANEL
An average
sized solar cell with sufficient light, produces around 0.3V. Solar
cells are generally
stacked up together in series or parallel, and sold as solar panels.
A solar panel may consist of 300
or more solar cells. If these cells are arranged in series combination,
they yield a larger voltage(sum
of cell voltages). If they are arranged in parallel combination,
they yield a larger current (sum of cell
currents).
In the panel connections
shown, the top portion of the cells(represented by circles) is marked
as
positive, and the bottom portion is marked as negative. In the series
connection, if one cell can
generate v volts, then the total or rated voltage available at the
terminals is v x n volts; where
n - no. of cells.
In the parallel connection, if one cell can supply I amps to the external
circuit, then the total or rated
current that can be supplied is I x n amps; where n - no. of cells.
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ENERGY WASTED
When
the cell is exposed to the solar spectrum, a photon that has an
energy less
than the band gap Eg. makes no contribution to the cell output.
A photon that has an energy greater
than Eg contributes an energy Eg. to the cell output. A photon that
has an energy greater than Eg.
contributes an energy Eg. to the cell output. Energy greater than
Eg. is wasted as heat.
TODAY'S SCENE
IN THE INDUSTRY
Efforts
are being made to reduce the cost of electricity produced by using
solar panels, by
reducing
the wastage, and by making more efficient cells. Also, there are
several other approaches
towards making solar power economical compared to other sources
of power.
I Approach : By using solar concentrators, more light can
be focussed on to a cell thereby increasing
the output. (Note that this approach is already being used for solar
heating.)
II Approach : By using tracking system, which track the relative
movement of the sun to the earth
and tilt the solar module towards the sun.
III Approach : The solar beam is split into two or more parts
of different wavelengths (i.e., of
different colours). Each of this spectrum of light is then foccussed
on a separate solar cell most
sensitive to it. IV Approach : The sunlight is allowed to pass through
thin cells of different materials
lying above one another. The topmost cell absorbs and is activated
by photons of higher energy. The
next cell is activated by photons of lesser energy, and the last
cell absorbs light rays of greater
wavelength.
V Approach : Concentrated solar energy is used to heat a
radiator surface to a very high
temperature of 18000C at this temperature, the hot radiator surface
emits photons to a range where
most of them are close to minimum excitation threshold of the silicon
cell. Kindly note that by the time
you may be reading this line, the above approaches may have been
modified, bettered, and put to
commercial usage.
CONCLUSION : Solar cells have become much more feasible,
due to enormous development in electronics industry. It is expected
that solar power will cost lesser than the conventional energy sources
in a few more years.
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