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Solar Cells
A photovoltaic cell (sometimes spelled photo-voltaic)
or shortened to PV are used to
convert solar energy into electrical energy. Solar cells are the basic elements
of a solar module. Silicon is by far the most common material of semiconductors
from which solar cells are made. Unfortunately pure silicon, while the most
prevalent media, is expensive to manufacture and requires a lot of sunlight to
push electrons around.
The amount of the current
produced is proportional to the strength of the sun on the solar panel. When a cell is
exposed to the solar energy, photons with energy equal to the material bandgap
are converted into electrical energy. When an electron (negative) is
knocked away from an atom, it leaves behind a
positive charge or hole. A proportion of charges that cross the bandgap
can be harvested by completing a circuit from a grid on the cell's surface to a
collector on the backplane. Simplified, think of sunlight as a photon
knocking an electron off one media to another to create the electrical
potential of solar panels.
The usable voltage that a solar
cell produces depends on what semiconductor material it's made from. In the case
of silicon-based cells, the output is approximately 0.5 V per cell.
Although the current increases
with increasing luminosity, the terminal voltage is mostly dependant on
semi-conductor material and only slightly dependent on the amount of light falling
on the cell.
Obviously PV energy is only
available while the sun shines. The electrical energy needs to be stored and
recovered for later use. Usually battery storage is used. An
alternative is returning power back to the electrical generating grid.
If you use batteries
to store electricity you some energy is lost during the charge and discharge
cycles due to
conversion inefficiency. Total loss using battery storage can bring the overall
efficiency down to 25%.
Energy Loss within the Solar Module
Heat:
An amount of surplus photon energy is transformed into heat rather than into
electrical energy. Heat adversely affects solar cell performance, 20% at
operating temperature.
Optical: Losses
resulting from shadowing of the cell surface glass
surface or reflection of incoming rays on the cell surface.
Resistance:
losses through electrical resistance in the semiconductor ,diode, internal
wiring and the connecting
cable.
Downstream Energy Loss
Resistance: Electricity
traveling through wires encounters resistance. The lower the voltage and higher
amperage and distance, the higher the
electrical loss.
Conversion: The solar array
cannot be directly to the battery but an intermediary to regulate battery charging
is required. Battery charging
electronics are ~75 to 98% efficient.
Battery: Power is lost
to heat generated while loading electricity into and extracting
it out of the battery. This is ~85% efficient overall.
In Overview there is loss due to wiring and
electrical connections. The end-to-end efficiency of the system is therefore
degraded by approximately 7% to 8%.
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