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Homemade Thermopile

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Posted by Dunc on December 2, 2007, 12:44 pm
 
There are two major problems with thermopiles as tool to generate
electricity. Very low efficiency and high initial cost.  It appears
that the only way to attack efficiency is to utilize more exotic (read
expensive) materials.

What I was wondering is could a "homemade" thermopile be constructed
using very cheap or recycled materials (nails, aluminum cans, aluminum
foil, metal scrap, etc.).  If you consider sweat equity to be free you
might be able to produce a viable installation.

If possible, it would be best to utilize a natural occurring
temperature differential such as air to ground, air to water, or dry
bulb to wet bulb. I recognize that these are relatively small delta
T's, but they are completely free and universally available.

My questions are as follows:

Which, if any, commonly available materials would be best for such a
device?

What design parameters should be considered?
For example:
Do the cross-sectional area, distance between or shapes of the
junctions effect the output?
Are there an optimal number of junctions to be placed in series?

Posted by Neon John on December 2, 2007, 1:52 pm
 
Short answer: No

Long answer:  Spend some time here looking over thermocouple tables:

http://www.temperatures.com/tctables.html

Look at the voltage developed between some common TC materials such as type T,
copper
vs Constantine (copper-nickel alloy).  Let's say that you could achieve a 100
deg F
difference between cold and hot junctions.  The output would be 1.519 mV.  In
other
words, you'd need about 750 of these junction pairs to make one volt.

Another common thermocouple, Type J, Iron vs Constantine, produces 1.942 mV at
100
deg differential.  That gets you down to around 500 junction pairs to make a
volt.

These thermocouple metals weren't chosen arbitrarily.  One of the man objectives
was
to achieve as much emf output as possible, consistent with other required
physical
properties.  Common metal pairs make less voltage.

Unless they've eliminated it in the shuffling of mission, there is a section on
the
NIST website that tabulates TC voltages of most of the common metals and alloys.
 You
won't find anything much better than type J.

There is a reason why exotic metals like indium are used in commercial Peltier
devices and it's not that there is a surplus of those metals laying around :-)
Peltier and Seebeck effects are reciprocal functions.  That means that a good
Peltier
pile is also a good Seebeck pile.  IOW, a TEC pile is also a relatively good
generator.

Several years ago I built a thermoelectric battery charger to use while camping
in my
MH.  It was designed to be a last-ditch rescue device in the event I flattened
all my
batteries to the point where I could not crank either the engine or the
generator.

It consisted of several surplus Peltier piles arranged between an aluminum bus
and
several large heat sinks.  The bus was heated with a propane torch.  It generated
about 4 amps across a 12 volt battery.

A similar system could be put together from surplus modules to use "free" heat.
Flue
heat, engine exhaust heat, etc.  That's about as cheap as you can get, as a
practical
matter.

John




John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com  <-- best little blog on the net!
Tellico Plains, Occupied TN
It isn't Global Warming.... It's Jerry Falwell arriving in hell.


Posted by Neon John on December 2, 2007, 2:00 pm
 

copper

deg F

other

100

volt.

To correct myself, those voltages are for the hot junction at 100 deg and the
cold
junction at 32.  Too used to working with centigrade where the reference is 0
deg C.

--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com  <-- best little blog on the net!
Tellico Plains, Occupied TN
What do you call 4 Blondes in an Abrams?  Air Tank.


Posted by BobG on December 2, 2007, 5:32 pm
 Honeywell make a thermopile that sits in the flame of the pilot light
on a gas water heater and produces a volt or so to hold the gas relay
open. Flame goes out, gas shuts off. These things are sold as
replacement parts for gas water heaters... about $0... about the size
of a cigarette butt on a long wire.

Posted by Eeyore on December 2, 2007, 6:04 pm
 

BobG wrote:


And the conversion efficiency is ?

Graham



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