Russell McMahon <apptech@paradise.net.nz> wrote:
> >>If you need to go VERY low then a Peltier device may not have enough
> >>power. What is your target temperature range?
> >
> > If you need more delta T then stacked Peltiers are a possiblity. 
> > This is what we use for over temperature testing in production (way 
> > faster than the environmental chambers we used to use).
> 
> Stacked Peltiers do give extra temperature differential but at a 
> substantial decrease in Watts of cooling as you use N layers and a 
> reducing delta T per layer. For cooling you can get up to 6 layers (I 
> think I've seen) but the stack is shaped something like a Christmas 
> tree with a very small unit at the top and very small cooling watts 
> overall.

The problem is that Peltier devices are not all that efficient overall.
Something like 30-40% IIRC. In other words, for every Watt of heat coming
out of the hot side, 300-400 mW of that came from the cold side, the rest
was the electrical energy you put into the device.

Even with stacked devices, you eventually get to the point where all of
the heat coming off the hot side is the electrical energy you're putting
into the stack, and there's zero remaining capacity to remove heat from
the target.

-- Dave Tweed
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