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What is a "Cell", "Module",
"Panel", and an "Array"?
This is an area of frequent confusion
and the terms are frequently misused unknowingly.
First of all, the basic building block
for energy collection is the individual "Photovoltaic"
or PV cell usually built using very high purity Silicon which
is the second most abundant element in the Earth's crust. In the
early days (1970's) cells were made from round (single crystal)
Silicon wafers processed for the Semiconductor Industry. The wafers
were usually round and 3 inches in diameter. Today Silicon PV cells
are of both single crystal and multi-crystalline types due to the
process utilized. Today cells may be Six inches square or even larger.
Amorphous (from the Greek meaning without form) Silicon, is also
processed using thin film techniques on a supporting substrate and
cut or etched into long rectangular cells. Single crystalline silicon
cells generate about .43 Volts when exposed to sunlight. Amorphous
cells are slightly higher at about .5 Vdc.
For practical loads, multiple cells
must be connected in "Series" in the laminated package
such that the cell voltages are additive. These multi-cell packages
then become the smallest modular building-block for a PV
system. The multi-cell laminates are usually provided with a mounting
frame, and simply called "Modules". Modules may
be obtained with output power levels ranging from a fraction of
a watt to over 240 watts. The size varies accordingly. Full sun
on a clear day at sea-level, provides about 1 kilowatt of power
per square meter or 93 watts per square foot. Depending upon the
technology used and the manufacturing processes employed, conversion
efficiencies from light to electrical energy vary from a few percent
to about 15 percent in commercial product. Like our eyes, Silicon
only "sees" the visible part of the solar spectrum; a
high quality cell will convert about two thirds of this energy into
usable power. Top quality modules will produce 10 or 11 watts per
square foot of area when in full sunlight. Since each cell in a
module is in series, the output current must pass through each cell
getting a boost in potential of less than .5 volt. Most modules
use about 36 cells in series giving approximately 15.5 volts at
their optimum output voltage. If any one of the cells in a module
is shaded from the sun, it will not generate its current, but block
most of the current generated by the unshaded cells. For this reason
PV modules need to be mounted in areas that are not shaded even
at the lowest sun angles.
In applications requiring more power
than that available from a single module, two or more modules are
mounted on a rack (usually Aluminum or Steel) to provide a larger
system building block. This is called a "Panel".
This panel may hold two, or as many as a dozen modules. Panels may
be mounted at a fixed or a variable tilt angle to the horizontal.
The tilt can be changed with the seasons to provide a higher daily
output. If the modules are not perpendicular to direct sunlight,
a reduction in output will be measured that follows the cosine of
the error angle. For this reason it is often cost effective to mount
the modules on a "tracker" that effectively follows
the sun across the sky. Trackers may employ reflectors and/or shading
bars that use solar heat to expand a gas and move liquid, which
in turn moves the center of gravity of the tracking system thus
following the sun. Various electro/mechanical means for tracking
the sun or positioning the tracker according to the time-of-day
may also be utilized. Means for tracking in one or two axes have
been used in the PV industry.
When panels are connected together
an Array is formed. Large systems are broken down into arrays
and Sub-Arrays. An even larger organization has been suggested
as a Field. Perhaps some day, we will be able to refer to
the "South Forty" field of PV, as usage continues to increase.
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