# [Beowulf] Re:running hot?

Lux, James P james.p.lux at jpl.nasa.gov
Thu Mar 19 14:17:23 PDT 2009

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On 3/19/09 12:13 PM, "David Mathog" <mathog at caltech.edu> wrote:

> Mark Hahn <hahn at mcmaster.ca> wrote:
>
>> are you running your machinerooms warm to save power on cooling?
>
> How much would that really save?  Is there a study somewhere
> demonstrating substantial power savings?
>
> Whatever the steady state temperature in the room the AC still has to
> pump out heat at the same rate it is generated.   Raising the room
> temperature could affect heat exchange slightly because of a
> steeper/shallower T gradient across the walls/floor/ceiling.  For
> instance, increasing RT would let a little more power drain out through
> the walls instead of the AC, assuming it is cooler outside than inside.

The efficiency of a heat pump changes a lot as the differential temperature
changes. COP = Qc/Work (Qc total heat pulled out of cold side, W work done
to do it, Qh = Qc+W, of course)
Carnot says COP<= Tc/(Th-Tc), so the closer Th and Tc are, the higher the
"best" COP can be

This is true in practice as well as theory. There's a certain "base" power
consumption and a part that's dependent on the delta T.
EER is the BTU/Hr per Watt  assuming the condenser is at 95F (lots of good
British units there..).  EER/3.413 = COP.
The SEER is calculated using a seasonal pattern of condenser temps.. And
will always be higher than the EER.
A typical EER might be 10-12, corresponding to a COP of around 3. That is,
to remove a 100kW heat load from your machine room will take 30kW.

Home AC is usually in the SEER around 13 or higher range (13 is the lowest
you can sell today, in the US), but industrial systems can do much better
(but probably not 2 or 3 times better): Variable compressor speeds and
variable fan speeds are two ways to get there.

> Does a PC doing the same work uses more or less power at 70 or 80 degrees?

Probably not a big difference. The resistance of the wiring and components
goes up as the temperature goes up, but it's a small effect (ppm sort of
scale) and there's enough other things happening that it would be hard to
predict.

>
> Off the top of my head I wouldn't expect a huge change in either number
> for a 10 degree RT change.  Maybe it makes a big difference if the
> machine room is very poorly insulated.

It could make a pretty large difference, especially if your room temp is
close to outside temp, because the denominator of that COP fraction is
small, so COP changes fast.  Going from a delta T of 10 to 5 could double
the COP, halving the energy required to pump the same amount of heat.

Jim

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