Ahhh! There is nothing like a tall, cool drink of water when thirsty. Not surprisingly, computers also prefer liquid refreshment as opposed to air cooling when hot. The choice for the technologist
resides in when to make the move to liquid cooling and in
what type of liquid cooling system is most appropriate.
Anyone who has been cautioned by their parents not to
use an electrical appliance when taking a bath — especially
one that plugs into the wall — has a visceral reaction against
mixing electronics and water. Usually the combination
results in disaster, otherwise referred to as letting the magic
smoke out of the electrical device so it no longer works.
(Better that than you, should your bath water come into
contact with house current!)
The motivation in moving to liquid-cooled hardware is
silent cooling at far greater density than would be possible with air-cooling. In some cases, the extra cost of the
water-cooling system can be offset by a lower operating
cost. Cooling and moving large amounts of air is expensive! For example, the 1.2 petaflop/s Peregrine supercomputer at the National Renewable Energy Laboratory
(NREL) built in conjunction with Intel and HP has a
near-perfect annualized average power usage effectiveness
(PUE) of 1.06. An ideal PUE is 1.0, where the total energy
utilized by the computer exactly equals the power delivered to the computer center. The Peregrine warm-water
cooling architecture is part of the reason for the extreme
annualized average efficiency, because the waste heat
from the supercomputer also is used to heat the building.
In contrast, Jeff Vetter notes in his book that air cooling
the TSUBAME 1.0 supercomputer had a PUE of 1.44, as
the air cooling system alone required 44 percent of the
energy consumed by the supercomputer. When a system
consumes megawatts of power, a lower PUE translates to
big money savings!
While water is used in many liquid cooling systems, other
systems utilize electrically insulating liquids such as flouri-
nert, which eliminates concerns about the liquid coming into
contact with the electronics. It really is counterintuitive to
submerge your cellphone or a piece of expensive — and like-
ly critical — computer equipment into a vat of liquid and
then apply power. Both water and flourinert are examples of
a single-phase liquid cooling system, where the temperature
is kept below the boiling point of the liquid.
Many elementary school students perform a physics ex-
periment where they put a thermometer into a pot of water
and then apply heat. They observe that the temperature rises
until the liquid starts boiling, at which time the temperature
of the liquid stabilizes. This is a crude example of phase-
change cooling where a liquid can carry away much more
heat through evaporation than by simply absorbing heat.
Try pouring some rubbing alcohol on your hand and blow-
ing on it to observe the efficiency of this phase-change effect.
Phase-change cooling systems are utilized to provide very
compact and efficient cooling systems for building very
dense and compact computational systems. Essentially,
phase-change systems work by pumping a liquid to a
heat sink mounted on the CPU and other heat-gener-ating components such as GPUs, coprocessors, bus
and network components. The heat from these
devices causes the liquid to evaporate. Tubes
and a pump are used to move the resulting
vapor to a remote condenser, where the
waste heat is removed or used to heat
water for the building, while also
reverting the vapor to a liquid. The
idea is very similar to how the air
conditioner works in your car.
The cost of both liquid and
phase-change cooling systems
is rapidly dropping. This is
a good thing, as one of my
favorite Seymour Cray
expressions highlights the
cost of the engineering
put into these cooling
systems, “When I first
started out, I was an
Is Your Computer Thirsty?
Liquid-cooled hardware is silent cooling at far greater density than
would be possible with air-cooling