IEEE Spectrum - North American - March 2017 - 30

chilling the air mechanically, they simply use outside air. This is far cheaper,
with a cooling overhead of just 10 to
30 percent, but it means the computers are subject to outside air temperatures, which can get quite warm in
some locations. It also often means
putting the centers at high latitudes,
far from population centers.
What's more, these facilities can consume a lot of water. That's because they
often use evaporation to cool the air
somewhat before blowing it over the
servers. This can be a problem in areas
subject to droughts, such as California,
or where a growing population depletes
the local aquifers, as is happening in
many developing countries. Even if
water is abundant, adding it in the air
makes the electronic equipment more
prone to corrosion.
Our Natick architecture sidesteps
all these problems. The interior of
the data-center pod consists of standard computer racks with attached
heat exchangers, which transfer the
heat from the air to some liquid, likely
ordinary water. That liquid is then
pumped to heat exchangers on the
outside of the pod, which in turn
transfer the heat to the surrounding
ocean. The cooled transfer liquid then
returns to the internal heat exchangers to repeat the cycle.
Of course, the colder the surrounding
ocean, the better this scheme will work.
To get access to chilly seawater even
during the summer or in the tropics,
you need only put the pods sufficiently
deep. For example, at 200 meters' depth
off the east coast of Florida, the water
remains below 15 °C all year round.
Our tests with a prototype Natick pod,
dubbed the "Leona Philpot" (named
for an Xbox game character), began
in August 2015. We submerged it at just
11 meters' depth in the Pacific near San
Luis Obispo, Calif., where the water
ranged between 14 and 18 °C.
Over the course of this 105-day experiment, we showed that we could keep
the submerged computers at temperatures that were at least as cold as
mechanical cooling can achieve and
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with even lower energy overhead than
the free-air approach-just 3 percent.
That energy-overhead value is lower
than any production or experimental data center of which we are aware.
Because there was no need to provide
an on-site staff with lights to see, air to
breathe, parking spaces to fight over,
or big red buttons to press in case of
emergency, we made the atmosphere
in the data-center pod oxygen free. (Our
employees managed the prototype
Natick pod from the comfort of their
Microsoft offices.) We also removed all
water vapor and dust. That made for a
very benign environment for the electronics, minimizing problems with heat
dissipation and connector corrosion.

icrosoft is committed to
protecting the environment. In satisfying its
elec t r ic it y need s, for
example, the company uses renewable
sources as much as possible. To the
extent that it can't do that, it purchases
carbon offsets. Consistent with that
philosophy, we are looking to deploy
our future underwater data centers
near offshore sources of renewable
energy-be it an offshore wind farm
or some marine-based form of power
generation that exploits the force of
tides, waves, or currents.
These sources of energy are typically
plentiful offshore, which means we
should be able to match where people are with where we can place our
energy-efficient underwater equipment and where we would have access
to lots of green energy. Much as data
centers today sometimes act as anchor
tenants for new land-based renewableenergy farms, the same may hold true
for marine energy farms in the future.
Another factor to consider is that
conventionally generated electricity
is not always easily available, particularly in the developing world. For
example, 70 percent of the population
of sub-Saharan Africa has no access
to an electric grid. So if you want to
build a data center to bring cloud
services closer to such a population,

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you'd probably need to provide electricity for it, too.
Typically, electricity is carried long
distances at 100,000 volts or higher, but
ultimately servers use the same kinds of
low voltages as your PC does. To drop the
grid power to a voltage that the servers
can consume generally requires three
separate pieces of equipment. You also
need backup generators and banks of
batteries in case grid power fails.
Locating underwater data centers
alongside offshore sources of power
would allow engineers to simplify
things. First, by generating power at
voltages closer to what the servers
require, we could eliminate some of
the voltage conversions. Second, by
powering the computers with a collection of independent wind or marine turbines, we could automatically build in
redundancy. This would reduce both
electrical losses and the capital cost
(and complexity) associated with the
usual data-center architecture, which
is designed to protect against failure of
the local power grid.
An added benefit of this approach is
that the only real impact on the land is a
fiber-optic cable or two for carrying data.

he first question everyone
asks when we tell them about
this idea is: How will you keep
the electronics dry? The truth
is that keeping things dry isn't hard.
The marine industry has been keeping
equipment dry in the ocean since long
before computers even existed, often
in far more challenging contexts than
anything we have done or plan to do.
The second question-one we asked
ourselves early on-is how to cool
the computers most efficiently. We
explored a range of exotic approaches,
including the use of special dielectric
liquids and phase-change materials
as well as unusual heat-transfer media
such as high-pressure helium gas and
supercritical carbon dioxide. While
such approaches have their benefits,
they raise thorny problems as well.
While we continue to investigate
the use of exotic materials for cool-

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Table of Contents for the Digital Edition of IEEE Spectrum - North American - March 2017