From: John De Armond
Newsgroups: rec.outdoors.rv-travel,alt.rv
Subject: Re: 12V thermoelectric cooler/warmer
Date: Wed, 28 Jun 2000 14:03:18 -0400

Larry wrote:

> I have to differ with your comment. We have a Coleman 12V cooler and have
> used it in conjunction with lights and a furnace (needed heat a few times a
> night) over a four day period with a pop-up and did not drain the 12V
> battery. No we didn't have shore power hook-up, did have a large deep cycle
> battery.

Peltier junction heat pumps have a Coefficient of Performance (COP)
of 1 or less, usually less.  That is, it pumps 1 watt or less for
every watt consumed.  Most of these coolers are either around 50
watts (~4 amps) or 100 watts (~8 amps).  One can easily determine
how long a battery will last from that.

Though not exhaustive, I have either owned or closely examined many
of the units available on the market.  Every one I've looked at
either has no thermostat and runs continuously or has a simple
ON-OFF thermostat that interrupts the main power.  The former will
obviously use its rated power continuously.  The BIG problem with
all these  units is that the Peltier junction heat pump is also an
excellent Seebeck junction*.  The problem comes when the thermostat
opens (or the power switch is thrown.)  The junctions start
generating voltage, in most cases, enough to continue running the
cooling fan for awhile.  The problem is, the junctions are pumping
heat back into the cold space in the process, and about as fast as
it removed it when powered up.  So the box either must be powered
all the time or it will rapidly get hot when disconnected.

I've been through several of these things in a stubborn belief that
the next one would work better than the last :-(  I invariably
return to the good old ice box.  The one place where I found one of
these to be useful was in a motel room on an extended business
trip.  It was powered from a 12 volt power supply, remained on all
the the time and kept cokes and cold cuts nicely cool.  In a 70 deg
room, of course.

* I once bought a large number of Peltier junctions surplus and used
some of them to make a propane-powered battery charger to use as the
power source for a ham radio Field Day station.  Had to be the most
unusual power source for that contest.  They were arraigned so that
a small burner would heat a slab of aluminum attached to one side
while the other side got a large heat sink.  Because of the soft
solder used to fasten the individual junctions to the Peltier pile,
I limited the temperature on the hot side to about 300 deg.

John

From: John De Armond
Newsgroups: rec.outdoors.rv-travel,alt.rv
Subject: Re: 12V thermoelectric cooler/warmer
Date: Thu, 29 Jun 2000 19:48:01 -0400

JOE NEAR wrote:

> John:
>
> Thanks. Knew about the COP but not about the Seebeck junction. Ever try
> to run two in series (cold side of one to hot side of another)? Can you
> get the cold end of the series 80deg below ambient?

Yes!  As a matter of fact, that is exactly what is used to cool
infrared detectors in precision guided munitions (replaced liquid
nitrogen) and closer to my professional home, high performance
nuclear radiation detectors.  Six of them stacked in series could
achieve the temperature of liquid nitrogen IFF the heat load was
teeny, tiny.  The HPGe (High purity germanium) detectors used in
high performance gamma spectroscopy are normally inside vacuum
dewars anyway so the heat load matches the devices.  They've kinda
fallen out of favor, however, except for portable units because of
that nasty Seebeck effect.  Allowing an HPGe detector crystal to get
warm won't instantly destroy it like it would the GeLi crystals they
replaced but it does them no good.  Lose power and the crystal gets
warm REAL quickly.  Little micro-sized helium refrigerators have
replaced them for that application.

If you want to gang 'em, Each pack going toward the cold end has to
be 1/2 the capacity of the one above it toward the hot side.  This
is because the one on the hot side must pump not only the heat that
the smaller unit pumps but all the heat it dissipates.

A less satisfactory alternative if you happen to find a pile of
these things cheap like I did is to run the units toward the cold
end at progressively lower current.  This is not as good a using
smaller piles because the large pile running at low current still
has the same heat leakage as it would at full power.  But they still
will get amazingly cold.  I put a stack of five in a small bell jar,
evacuated it for insulation and achieved about -120 deg F.  I could
have done better with optical shielding and such but I was just
playing around.


John