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Photovoltaic Cells
Name: Christopher
Status: student
Grade: 9-12
Location: MO
Question:
Dear Scientist, I am a senior at Cass
Midway High School in Cleveland, Missouri. Our
school is very small and when I was presented with
the task of conducting a science project relating
to energy I have found it difficult to gather
information since we do not have any teachers that
are knowledgeable enough in the details of energy. I
need to know more about photovoltaic cells. I would
like to understand photovoltaic cells better, how to
make one, how to increase efficiency of a
photovoltaic cell and how to use the energy
generated by a photovoltaic cell.
Replies:
Hi Christopher,
Let me begin by addressing one of your last points, where you asked how you
could make a photovoltaic ("PV") cell. The manufacturing process to make a PV
cell is breathtakingly expensive and requires toxic materials you will not have
access to. Most photocells start with a thin wafer of either single-crystal
silicon, or a deposited layer of polysilicon. Neither of these basic materials
are available to you or I. This is just the starting point; from there you need
tens of millions of dollars of equipment to form the P-N junctions that are at
the heart of operation of any PV cell. In essence, a PV cell is a type of diode
(hence the "P-N" junction that all diodes have) that generates electrical
current when exposed to light.
By the way, as a demonstration of this, consider the ordinary Light Emitting
Diode (LED). As you know, if you feed current to an LED, it produces light.
The reverse is also true. Shine light on an LED, and it will generate a
miniscule amount of electrical current!
The 50-year race to improve the efficiency of PV cells started with the simple
first-generation silicon cell that had an efficiency of around 6%, and has
resulted in modern PV cells that have very complex internal structures with an
efficiency of around 40%. The story behind how the efficiency has increased, is
extremely complex and highly technical, but you can get an excellent general
summary here: http://en.wikipedia.org/wiki/Solar_cell
Note that efficiency is not the real problem here. Cost is. Cost can be lowered
by increasing the PV cell's efficiency (thus delivering more power from the same
PV cell), but there are other ways to do this too. The single crystal silicon
wafers mentioned above are an extremely expensive starting point. A long silicon
ingot is "grown" slowly in a furnace from a small silicon crystal, in a process
that takes weeks. This ingot is then sawn into thin wafers. One alternative
process is to use polysilicon (multicrystalline silicon) that is deposited (often
using vapor deposition) in a very thin layer onto a cheap substrate. This results
in lower efficiency, but it so much cheaper that the Cost-per-Watt is often
lower, even though more polycrystalline PV cells are needed for the same power
output.
An new, radical, and promising development is organic PV cells, but so far this
is still in its infancy.
Finally, your question about how to use the power generated. PV cells are a very
inconvenient source of power. A monocrystalline PV cell (made from a single
crystal silicon wafer) has an output voltage of only about 0.6 Volts at room
temperature, and with no load. But this voltage falls significantly when the
cell is heated by the sun's energy. Much more troublesome, is that output voltage
also falls dramatically as increasing current being delivered. The more current
is drawn from the cell, the lower its output voltage falls. This is in contrast
to an AC plug in your home (for example) that maintains a nearly constant 120
Volt output, whether you plug in a small load like a radio, or a heavy load like
a toaster.
In order to be useful, many PV cells are usually connected in series to increase
the output voltage, and many of these series "stacks" are connected in parallel
to increase the output current. The output power from this array is then fed to
a device called an inverter, which is capable of accepting the varying output
voltage that this array generates under varying load conditions, and converts
this varying, relatively low DC voltage into a constant voltage that can be used
by existing electrical equipment, such as a constant 12 Volts DC, or 120 Volts
AC. Without this additional power conversion provided by the inverter, the
output of a PV cell array is practically useless.
Regards,
Bob Wilson
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Update: June 2012
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