[AR] Re: Peltier specifications
- From: Peter Fairbrother <zenadsl6186@xxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Tue, 04 Aug 2015 16:31:04 +0100
On 04/08/15 03:12, Troy Prideaux wrote:
I once worked out that to cool to LOX temps I would need about 511
cascaded Peltiers, only one of which would be actually working at
proper efficiency ... ouch.
I was under the impression that only specialised peltiers are
suitable for cryo temps.
Yes indeed.
Do you have any links for readily available
peltiers that are suitable?
No, sorry.
That "once" was in about 1974, I was doing a thermodynamic study of
theoretical but non-existent Peltiers - and I do not know whether any
suitable Peltiers even exist nowadays.
Though a few years ago I read something about developments in low
temperature Peltiers - unfortunately, while the operating temperature
range was interestingly low, the Modified Figure of Merit (which
determines the number of Peltiers needed in the chain) wasn't that
interesting.
I don't think they were commercially made, though perhaps they might use
some in spacecraft or the like. Useful for a second stage cooler, eg
getting to say 40K from 80K if you have lots of available power and lots
of 80K sink and a very low cooling power requirement - but no use for
getting to 80K from RT (and no use for getting to 20K LH2 or 4K LHe from
80K either).
BTW, that 511 is the length of the chain, not necessarily the number of
Peltiers used.
I made the 511 up as I couldn't remember my original figure, but afair
it was something like that. However I just read you can in theory do
something useful with about 30 steps nowadays (the length of the chain
is highly dependant on the MFoM, which has improved since 1974).
Supposing you were using only one size of Peltier, you might have three
Peltiers in one step per Peltier in the previous step; you would then
need about 3^30 or 200 trillion individual Peltiers in total.
The point of all this being that there is no sensible way to get to LOX
temperatures from room temperature using Peltiers.
You can't get there from here.
-- Peter Fairbrother
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