We used to use peltier devices as a passive cooling system for the
military back in the 60's. We built the first ones where I used to
work; they were heat pipes in effect. They are not very efficient from
an electricity usage standpoint, are affected by ambient conditions;
they are rated by temperature across the faces at a given current.

You did hit the nail on the head when you questioned the heat transfer
required if placed into a carboy. Your surface to mass ratio with any
material is too low if you intend to insert it into a carboy. You
can't use copper or copper bearing alloys in winemaking as you know
from others so the thermal conductivity will be lower to make matters
worse.

If you wrap the tubing around the carboy that might be another matter
but all of this would turn out to be an expensive proposition.

If you live in an area where the relative humidity is low the
evaporative cooling idea sounds good, you could wrap a towel around the
carboy and drip water onto it and blow a fan across it to improve the
efficiency.

If you are willing to pump fluid you may be able to place a tank in a
refrigerator or freezer and pump fluid out through copper tubing
wrapped around a carboy foamed over with an insulation package.

I apologize for not giving you values but I just can't see it working;
you can gather thermal conductivity data from the CRC handbooks, any
version would do. That won't be the issue. The available surface area
of the tubing inserted and in contact with the wine will be the issue
no matter how cold you can get the liquid in the tubing or how fast it
flows.

Joe

> So long as the reduced thermal conductivity doesn't present a problem,
> stainless would be fine in place of copper.
>
> When Scott pointed out that, without a pump, the amount of pipe within
> the fermenter may not be adequate for the amount of heat to be moved, he
> may well have hit the nail on the head, but I'd like to see if I can
> locate some precise figures or a formula or something like that. That
> was one of the main questions that I'm researching