From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Generator or Solar Panels
Date: Fri, 11 Feb 2005 22:54:16 -0500
Message-ID: <gctq01lhd6parmgqju8s63fb9ju015t5at@4ax.com>

On Fri, 11 Feb 2005 14:00:41 -0800, "Evad" <evad@dodgeit.com> wrote:

>Personally, I'm looking for a 100 watt propane turbine generator or fuel
>cell that I can run 24 hours a day and make almost zero noise. 100 watt
>continuous would keep my batteries up and supply all my boondocking needs.

100 watts?  Child's play and with no unobtanium involved.  Something you
can make in your very own home workshop.

Needed:

A small propane burner.  A torch burner will do.
Half a dozen or so 12 volt Peltier junction modules
A hunk of aluminum to mount them to.
A couple more hunks of aluminum to drill and make a water cooled heat
exchange.
Water cooling stuff.  Small pump, radiator, tubing, etc.  A PC CPU water
cooling kit would probably work.

Take the first hunk of aluminum, the width of the Peltier junctions and
about 3/4" thick and drill a series of diagonal holes as follows:

  Flue
+|  |------+
| \  \     |
|  \  \    |
|   \  \   |
|    \  \  |
|     \  \ |
|      \  \|
|       >  | NPT plug or weld shut.
|       /  /
|      /  /|
|     /  / |
|    /  /  |
|   /  /   |
|  /  /    |
| /  /     |
|/  /      |
|  /       |
|  |       |
+  |-------+

Propane torch nozzle inserts here.

Ziz-zag several times.  This is the hot side.  The Peltier junctions
preferably are fastened with thermally conductive high temperature epoxy.

Drill the other pieces similarly and outfit with hose nipples for the
water cooling.

Stack the assembly as such

Cold side
------------
Peltier pile
------------
hot side
------------
Peltier pile
------------
Cold side

Fasten the pile together with multiple thru-studs equipped with Belville
washers to maintain constant pressure.

With 12 volt piles, hook them in parallel.  With lower voltage piles, hook
them in series/parallel to achieve the desired output voltage.

Arrange the cooling system, preferably where natural convection will move
the water.  A small fan on the radiator will use only a few hundred ma and
will greatly improve performance.

A Peltier junction operated in reverse is known as a Seebeck junction or
more simply, a thermocouple.  The materials used in Peltier junctions make
particularly good Seebeck generators.  A 12 volt rated pile will generate
that at about 100 degrees F delta-T across it.  With the cold side at say,
90 degrees and the hot side at 250 degrees, the output will be in the 15
volt range, perfect for battery charging.  A 1.5" square pile should be
good for 2 amps or so.

Simply fire off the propane torch and adjust it to keep the hot side at
between about 250 and 300 degrees.  Commence charging.  An overtemperature
cutoff should be fitted that turns off the propane at about 350 degrees.
The limitation of these commercially available piles is the soft Indium
based solder used to assemble the pile that melts at around 400 degs.

The control valve and pilot assembly out of a gas oven would be a great
enhancement.  The pilot on high (calling for heat) would make plenty of
heat and the operating temperature can be set directly on the control
valve dial.

I have built one of these and can attest that it works fine.

A commercial version has been available for decades.  I became familiar
with one version when I moonlighted with the Forest Service radio shop
back in the mid 70s.  The FS used these at remote radio repeater sites to
power the radios.  The commercial version uses different materials in the
thermal pile and the hot side runs at a dull red heat. The cold side is
convection air cooled.  Much more efficient than my design but also much
more costly.  We don't care about efficiency because we're carrying a
relatively huge amount of fuel and because we're generating so little
power.

The commercial one is arranged as concentric tubular assemblies.  In the
center is the flue.  Then the hot side of the thermopiles, insulation,
cold side of the piles and a large cylindrical finned aluminum heat sink,
obviously cast for the application.

I don't recall the power rating but it was >1kw.  The radios still had
vacuum tubes in the finals so significant power was required.  Each site
was equipped with a 500 gallon propane tank which would run the site for
months, maybe a year.

A clever RV adaptation of this would be one mounted in the water heater.
The hot side is heated by oil or some other suitable liquid from a loop
mounted in the heater flame.  Any time the water heater runs electricity
is generated.  A similar adaptation could be done with the reefer with a
slightly boosted burner output.  That might be accomplished by simply
raising the gas pressure.

My original design could be implemented for well under $150, depending on
the price of the surplus Peltier piles one buys.  Be sure to put a diode
in the output lead or else the thing will act as a refrigerator again,
drawing at least as much current as it was producing.  A suitably rated
Schottky diode is the best since it has the lowest voltage drop.

John