SCIENCE OBSERVER
Getting All Revved Up
David Schneider
Cruising along on the highway, you spot a traffic jam ahead and hit
the brakes. In seconds, most of the kinetic energy of your car is
transformed to heat on the brake pads and rotors, slowing you
(thankfully), but at a cost: An enormous amount of energy is wasted.
If you're driving a hybrid, your car recaptures a portion of this
kinetic energy by using it to generate electricity, which in turn
goes into a large battery for later use. But that approach isn't
very efficient, because chemical batteries are not particularly good
at taking a charge in large bursts.
So imagine how difficult the situation becomes when one tries to
perform this kind of regenerative braking on something as big and
energetic as a diesel locomotive. The solution, at least according
to some, is to adopt an entirely different kind of energy-storage
technology, one that appears poised to serve for this and other
novel applications: the flywheel.
Using a flywheel to store energy is nothing new—potters have
been doing it for millennia. Modern high-speed flywheels are,
however, much more sophisticated than the massive stone wheels used
in ancient times to turn clay vessels. Instead of using rock or
metal, today's designs use comparatively light composite materials,
which are stronger and can be spun at high speeds without coming
apart. This approach is advantageous because the amount of energy
stored in a flywheel scales linearly with mass but increases with
the square of the rotational speed.
But getting a large object to turn freely at many thousands of
rotations per minute is not without its difficulties. The standard
solution is to evacuate the chamber holding the flywheel, so there
is no air resistance, and to support the spinning mass using
magnetic bearings, again to eliminate friction. The exchange between
electrical and mechanical energy takes place by electromagnetic
induction, as it does in an ordinary electric motor or generator.
Such flywheel energy-storage systems have been built for many years,
but being considerably more expensive than conventional batteries,
they have had limited application.
One place that flywheels might eventually find a niche is space.
NASA has contemplated using flywheel energy storage for the
International Space Station and has funded considerable
research in this area (although so far this technology has not been
adopted). The impetus was to find a way to hold the electrical
energy generated by the station's solar panels, in darkness a good
fraction of each orbit, without having to suffer the vagaries of
chemical batteries, which tend to wear out after many
charge-discharge cycles. In space, flywheels could serve double
duty, replacing both the batteries that would otherwise have to be
carried and the "reaction wheels" that are often used to
adjust attitude by taking up or giving back angular momentum. For
such control, one would install several flywheels at different
orientations and then move energy among them to obtain the desired
angular momentum for the set.
Although they have worked on space flywheels, engineers at the
University of Texas at Austin are also employing this technology in
what they call the Advanced Locomotive Propulsion System. The
hardware they have been building in some ways resembles what one
finds today in hybrid cars. But instead of an internal combustion
engine, their system uses a gas turbine, and in place of a chemical
battery, it uses a flywheel—perhaps the largest high-speed
flywheel in the world in terms of the energy it can store: 133
kilowatt-hours, when it operates at its maximum design speed of
15,000 rotations per minute. At that rate, the perimeter of the
rotor moves at approximately 1,000 meters per second, which is
faster than a round from an AK-47 assault rifle. So far this
flywheel has been run only with a down-sized rotor. The full-size
one has just been assembled, and spin tests will begin shortly.
Why build one huge flywheel instead of many small ones? "On a
cost per mega-joule basis, a single, large flywheel was the most
efficient," explains John D. Herbst, who is co-principal
investigator on the project. Robert E. Hebner, who shares the lead
with Herbst, says, "We're not aware of anything larger than
ours," but quickly adds, "and right now, we don't know a
market for one as big as ours," referring to the fact that
their cutting-edge work might or might not ultimately lead to commercialization.
But there is another large flywheel now undergoing tests that
is being readied for commercialization—and soon,
perhaps during 2007. Although smaller than the University of Texas
flywheel, it will be able to hold 25 kilowatt-hours of energy when
operating at its maximum speed of around 16,000 rotations per
minute. What is more, its designers plan to link multiple flywheels
together in what they term an "energy matrix," which will
be able to store greater amounts of energy, absorbing and releasing
it much faster than can normal batteries. Its developer, Beacon
Power Corporation of Wilmington, Massachusetts, plans to use its
flywheels for a rather novel application: stabilizing the frequency
of electric-power grids.
The connection between energy storage and frequency regulation
requires some explanation. The alternating current carried by
electric utilities in the United States nominally oscillates at 60
hertz, but when demand exceeds supply, the many whirling armatures
generating electricity for the grid slow, and the frequency drops
slightly. Conversely, when demand is less than supply, the extra
energy goes into spinning those generators slightly faster than
normal, raising the AC frequency. Grid operators try to maintain a
stable frequency, which is to say that they seek to balance supply
and demand at all times. That's tricky, of course, because they are
not in control of demand, which changes every time someone flicks on
a light.
Having many people connected to the grid, some switching things on
while others turn things off, evens out demand to a great extent.
But there are still unpredictable changes that occur minute by
minute. To cope with such short-term variations, which might amount
to something like 1 percent of the total, grid operators arrange
with certain power generators to reserve a small portion of their
capacity, so that they can adjust the supply of electricity on the
fly. Such regulation service commands a hefty price tag, because it
typically comes from turbines burning expensive fuel (natural gas)
and because it requires these generators to operate at something
other than their most efficient power levels.
Here Beacon Power believes it can compete profitably using an array
of flywheels to absorb the excess energy when demand falls short of
supply and to return that energy to the grid when the reverse
happens. Equipment for doing just that with an array of
6-kilowatt-hour flywheels is now operating on a small scale in both
California and New York in an effort, supported by the Department of
Energy, to iron out technical wrinkles and to obtain certification
from the grid operators.
The management of Beacon Power believes they are tapping into a market
that will only grow—not because people are becoming more prone to
turning their washing machines on or off at the same time but because
wind power is likely to increase in significance as utilities embrace
this and other sources of renewable energy. Even modest changes in the
speed of the air flowing over the blades of a wind turbine cause
substantial swings in energy output. So grid operators will increasingly
have to struggle with unpredictable variations in supply as well as
those that have always existed in demand. Bill Capp, chief executive
officer, says, "We think this is going to be a large issue that we
can help solve." If Beacon or other flywheel makers can eventually
do that with devices that are both faster than a speeding bullet and
more powerful than a locomotive, that would, of course, be just super.
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