In article ,
Derek Geldard writes:
On Fri, 03 Aug 2007 10:28:16 -0000, "
wrote:

Wow - that's about 2-3 units a day just for a coolbox! I use about 3.5
units a day for everything - including a fullsize Bosch A-rated
fridge.

I think those coolboxes use Peltier cooling (a weird diode that gets
cool one side and hot the other when a current is passed through it).
Peltiers are very inefficient. http://en.wikipedia.org/wiki/Thermoe...ng#Performance
- says conventional refrigeration is many times more efficient.

The reason they are inefficient is mainly that the diode proper is
very thin and heat from the hot side leaks back to the cool side,
simply by conduction. However I don't know what their thermodynamic
efficiency is like.

At the temperature differentials you normally find in a fridge,
they consume about the same power that they pump, i.e. a 50W
peltier will pump about 50W too (giving off 100W from the hot side).
Compressor based refrigeration does very much better than this.
The inefficiency is not so much due to heat conduction as you imply,
just a 5-10% duty cycle will maintain the temperature difference when
there is no heat to pump across, so only 5-10% of the power is lost
to conduction.

BTW, it's not a diode -- peltier devices are a series of semiconductor
junctions which can be powered either way around, depending which way
you want it to pump heat. Many cool boxes have a heating option which
simply powers the peltier device the over way round.

Their performance gets rapidly worse if the heat is not removed from
the hot side effectively enough, the temperature can rapidly go up and
cause melting of a cheap Chinese tat coolbox if the vents are covered
up in a car boot and it does nothing for your ice cream. With better
cooling (implies a more powerful noisier fan than the 12v plastic one
in a CCT Coolbox.) useful results can be obtained.

I've had a couple of peltier cool boxes since 1981. I ran them
continuously which they weren't designed for, and the fan motor
brushes died after a few months. Replaced them with proper equipment
cooling fans, and the units lasted for years. They both still work
but aren't in constant use anymore. I repaired one peltier element
after about 10 years, and replaced the other a couple of years
later.

In serious applications they are handy for cooling small components
which aren't of themselves generating heat. An instrument we sold Ca.
1970 used them with success to cool an antibody chamber in an
Automated RadioImmunoassay Anayser.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]