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Accurate & intuitive mechanical representation of how PV works

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Posted by David Austin on June 29, 2011, 12:58 am
 
Okay, this is really a Eureka moment I just wanted to share, and see
if someone could do better.  I used to train people on solar PV always
dissatisfied with the various descriptions for non-technical people.
Now I'm building a touchscreen kiosk that does it so I wanted to make
it intuitive so I've been mulling it over.  Perhaps it might be useful
for others.  If anyone tries it it would be nice to know what
dimensions worked best (including water depth, and what your photons
were).  If you have a better idea please let me know.

DEMONSTRATION:
1) Fill your "PV-system" (a purse-shaped reservoir, i.e.-a water
channel where the handle would be) with "substrate" (water), nearly to
the top of the "junction" (a wall down the middle of the reservoir
that has a specially designed cross-section to look like the
electrostatic distribution of a depletion region).  Explain that
"substrate" is where the electrons flow (metal in the channel, silicon
in the reservoir, etc).
2) Sprinkle into the water, especially the water channel, a few
"electrons" (foam beads or anything small that will float), just
enough to show "electron flow" in the channel when things start
happening.
3) Have a number of students softly drop "Photons" (marbles) on either
side of the "PN-junction" (the a fore mentioned wall with a x-section
shaped like the electrostatic potential of the depletion region - see
below).   You might need 5-6 students doing this, enough to generate
waves that will flow over from the upwardly sloping top (labled "p-
type"), but not so aggressive that they splash up from the abrupt side
("n-type").  "Photons" might need to be dropped an inch or more from
the wall for best effect (which, yes, I know is contrary to the ideal
case for a real PV diode).
4) watch as the "electrons" (foam beads) in the "electric circuit"
start to flow from the N-type side to the P-type side.  Explain that
is "electric" current which runs our electronics, etc.
5) lift the wall out and re-insert it so the abrupt side is facing the
other direction, and drop "photons" again and watch the "electric"
current change direction.
6) Discuss.  Why do electrons flow when photons hit the substrate
around the PN junction? [because of the electrostatic shape of the PN-
junction traps electrons as they flow from one direction only].  What
gives it that shape [filled outer electron shells at the interface of
the PN junction due to the immediate proximity of p-doped (holes) and
n-doped (electron) regions].  Explain that the reservoir is actually a
cross-section view of a PV cell, with the PN Junction separating
layers on the PV wafer.  Explain that the photons come through the N-
type because most material is made of empty space, make their "splash"
into the P-type.  Discuss how to increase electron current.

MAKING THE CONTRAPTION:
1) From the helicopter view you make a reservoir of water with a
stream on the side ... something shaped like a woman's purse, at least
1 inch deep or deeper.  For example, possibly a large cookie-tin with
a "D" shaped styrofoam piece to separate the purse-portion of the
reservoir from the handle part so the stream of water will follow the
path of the purse-handle shape.
2) The width and the depth of the stream should be small enough so it
will be sensitive to small changes in the reservoir (smaller channel =
larger flow-rate).  If the foam beads don't move too much it's because
the channel's x-section is too wide and/or deep and you'll want to do
something ...for example fill the bottom of the channel with something
to decrease it's cross-section.
3) make a wall out of  something waterproof, like aluminum flashing
(at home depot) that will split the reservoir into two sides and have
a special cross-sectional shape.
 - X-Section Shape: The shape of the cross-section shape of the wall
is what's important and is shaped like the distribution of the
electrostatic potential accross the entire depletion region (not just
the fully-depleted region - which is what EE's use in band-diagrams).
I know I've lost you here so see below where I provide detailed
instructions on how to make the shape.
 - hotglue styrofoam on each end, making it wide enough that stays
pinned in place with little waves going back and forth.
 - The sloped-up side says "P-type".  The bottom of the abrupt side
will say "N-type" although it will be underwater.
5) Somehow make slots on the sides (styrofoam?), in which the ends of
the wall can slide in and out.

Depletion Region Electrostatic Distribution Shape:
It's the electrostatic shape (how much charge it has at each point of
depth) that allows electrons to flow from the P-layer to the N-layer
with just 0.6 volts, and not back without 20V or more.  Assuming the p-
type is on your left, it's like an "N" tilted 45 degrees with the top
and bottom of it flattened (fully depleted regions).  We're going to
simulate it's shape as follows:  If I made the reservoir with a 12"
tin, then I'd make the wall about 2.5 inches wide at the base... cross-
section like so: 2.5 inches wide on the bottom from right to left,
then back right just past the center (1.5in. back), bent there to be a
verticle wall going straight up by 1 inch, then bending left by
90degrees(now horizontal) for 0.5in., then 45degree bend downward for
1 inch.  Then hotglue ends so it fits tight in the reservoir.  You may
need to experiment with widths to maximize effectiveness.  Note that
band diagrams that EE's use who do device physics for PN Junctions
(used to be me) don't show this shape because they only show the fully
depleted region (which ignores the sloped portions leading up to the
depletion layer), whereas we must model the entire depletion region
for the "PV System" to work right.


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