# back to the issue of cooling

Robert G. Brown rgb at phy.duke.edu
Tue Apr 22 09:36:52 EDT 2003

On Mon, 21 Apr 2003, jbassett wrote:

> Sorry to keep kicking a dead horse guys, but the issue of increasing
> thermal efficiency in large clusters and data centers has been keeping
> me awake at nights. Has anyone tried to use a stirling engine or other
> system
> for instance:
>
> http://www.stmpower.com/Technology/Technology.asp
>
> that can take as input pure heat, not just potential in the form of btus, in
> order to recover some of the heat energy that would simply be wasted at a
> large facility. Could this be economically viable?

<sigh> In almost all cases, no.  It's the problem with heat -- you have
to pay for it to get it where you want it, then you have to pay for it
again to get rid of it when it is where you DON'T want it.

I won't inflict a full review of the laws of thermodynamics on the list,
but the relevant one (second) here says that you can only extract work
when running e.g. a heat engine between two reservoirs, one "hot", one
"cold(er)".  Even then, one can only extract strictly less than

\eta = \frac{T_h - T_c}{T_h}

(degrees kelvin only) of the energy in the heat that flows from hot to
worth it.  You're trying NOT to allow T_h to exceed 340K at the CPU (the
room itself would need to be far colder or you'd be in deep trouble out
of the gate); you'd have to work very hard (and spend a lot of energy
and money!) to come up with a "free" cold reservoir at T_c = 290K (got a
glacier or springfed lake handy?).  So you could recover at most 12% of
the heat energy from the CPUs themselves, probably not enough to run the
pumps from your "free" cold reservoir.

The only way to recover any fraction at all, is to use your A/C as a
"heat pump" in the wintertime and pump the heat elsewhere in your
building where it could be of use.  A modern new building facility might
well do that, if the architect designed things appropriately for that
purpose from the beginning.  It would be quite difficult to
cost-effectively retrofit in most other environments.  This would still
cost money to operate, but you'd get a gain on the investment as the
coefficient of performance of your heat pump/AC could be 3-5 (giving you
a solid gain on the energy used).

The BEST way to remember the second law is that it says that "There
ain't no such thing as a free lunch" (tanstaafl).  So any clever scheme
to get something for nothing will almost certainly fail.

rgb

--
Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu

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