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Energy Efficiency - Data Centers



On 2019-12-18 20:06 CET, Rod Beck wrote:

> I was reasoning from the analogy that an incandescent bulb is less
> efficient than a LED bulb because more it generates more heat - more
> of the electricity goes into the infrared spectrum than the useful
> visible spectrum. Similar to the way that an electric motor is more
> efficient than a combustion engine.

Still, you should not look at how much heat you get, but how much
utility you get.  Which for a lighting source would be measured in
lumens within the visible spectrum.

If you put in 300 watt of electricity into a computer server, you
will get somewhere between 290 and 299 watts of heat from the server
itself.  The second largest power output will be the kinetic energy
of the air the fans in the server pushes; I'm guestimating that to
be somewhere between 1 and 10 watts (and thus my uncertainty of the
direct heat output above).  Then you get maybe 0.1 watts of sound
energy (noise) and other vibrations in the rack.  And finally, less
than 0.01 watts of light in the network fibers from the server
(assuming dual 40G or dual 100G network connections, i.e. 8 lasers).

Every microwatt of electricity put into the server in order to toggle
bits, keeping bits at their current value, transporting bits within
and between CPU, RAM, motherboard, disks, and so on, will turn into
heat *before* leaving the server.  The only exception being the light
put into the network fibers, and that will be less than 10 milliwatts
for a server.

All inefficiencies in power supplies, power regulators, fans, and
other stuff in the server, will become heat, within the server.

So your estimate of 60% heat, i.e. 40% *non*-heat, is off by at
least a factor ten.  And the majority of the kinetic energy of
the air pushed by the server will have turned into heat after just
a few meters...

So, if you look at how much heat is given off by a server compared
to how much power is put into it, then it is 99.99% inefficient. :-)

But that's just the wrong way to look at it.

In a lighting source, you can measure the amount of visible light
given off in watts.  In an engine (electrical, combustion or other-
wise), you can measure the amount of output in watts.  So in those
cases, efficiency can be measured in percent, as the input and the
output are measured in the same units (watts).

But often a light source is better measured in lumens, not watts.
Sometimes, the torque, measured in Newton-meters, is more relevant
for an engine.  Or thrust, measured in Newtons, for a rocket engine.
Then, dividing the input (W) with the output (lm, Nm, N) does not
give a percentage.

Similarly, the relevant output of a computer is not measured in
watts, but in FLOPS, database transactions/second, or web pages
served per hour.

Basically, the only time the amount of heat given off by a computer
is relevant, is when you are designing and dimensioning the cooling
system.  And then the answer is always "exactly as much as the power
you put *into* the computer". :-)


	/Bellman

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