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Storing energy in massive underground rings
Citation-> Popular Science, Jan 1989 v234 n1 p66(3)
COPYRIGHT Times Mirror Magazines Inc. 1989
Rings of power
There's been a nuclear attack. An incoming warhead has just struck a key
U.S. defense site and knocked out the region's power grid, leaving the
country's ground-based missile-defense system without power. In the
meantime, the United States has less than 10 seconds to fire laser weapons
at the next round of incoming missiles. Where will the power come from? An
emergency electrical generator takes 15 minutes to spin up, and power is
required instantly. The solution: tap into giant energy-storage rings that
can deliver an awesome 1,000 megawatts to the lasers within 30 milliseconds.
These storage rings do not exist yet, but the Strategic Defense Initiative
Organization (SDIO) plans to build a football-field sized prototype by the
end of 1994.
The heart of the undergound ring, called a superconducting magnetic
energy-storage (SMES) ring, is a superconducting coil cooled by liquid
helium to temperatures nearing absolute zero. AT those temperatures the
coil will have no electrical resistance, and the current can be stored
almost indefinitely.
Energy storage rings could also serve an important role for utilities. By
storing excess power generated during off-peak hours and releasing it into
the grid during peak hours, the ring could increase utility efficiency.
Experts believe that a SMES unit could be built almost anywhere, and because
it would have no moving parts, it would be easy to maintain. And if efforts
succeed in developing high-temperature superconductors [July '87, April
'88], refrigeration costs will drop substantially, making a SMES system more
practical in smaller sizes.
"I've been trying for eighteen years to get funding to build a SMES for a
large utility," says Roger Boom, director of the University of Wisconsin's
Applied Superconducting Center in Madison. A portly man with glasses and a
fringe of gray-white hair, he was not dissuaded when the funds did not
materialize. But then came SDIO with new reasons for investing in the
technology.
"We need a system that will provide about one gigawatt of power available
immediately when the battle starts," explains Capt. Paul Filios, technical
adviser to the Defense Nuclear Agency (DNA), the agency overseeing the SMES
project for SDIO. "We can't get that kind of power straight from the power
grid. For one, the grid might not be there after an attack. And secondly,
we need the energy equivalent of what a nuclear power plant produces in a
fraction of a second." The actual figures are classified, but a
free-electron laser weapon [Dec. 87] would require at least 1,000 megawatts
of electricity in 3/100 of a second.
The heart of a SMES is the current-carrying conductor that is coiled around
the ring 550 times. The number of coiled conductors varies from one to
four, depending on the design. Each conductor is composed of tens of
thousands of 27-millimeter-wide filaments made from superconducting
niobium-titanium alloy and embedded in copper. In Boom's design, these
filaments are embedded in unalloyed, extremely pure aluminum, a soft
material about the consistency of toothpaste that is an extremely good
conductor. If the superconductor warms up slightly and becomes resistant,
the high-purity aluminum will carry the current.
Keeping it cool
Another conductor concept being developed by Bechtel National in San
Francisco uses a ropelike cable of superconducting strands contained in a
stainless-steel tube. The voids in the tube are filled with liquid helium
to keep the conductor at 1.8 degrees Kelvin (minus 456 degrees F).
To ward off warm-up in Boom's design, the coil will be surrounded by a
vacuum vessel that acts like a giant thermos bottle with superfluid liquid
helium inside. Moving outward from the thermos, several heat shields are
cooled to temperatures progressively higher than 1.8 degrees K and further
protect the coil.
In November 1987 SDIO (through DNA) awarded two $15-million contracts to
develop plans for a test SMES unit. One contract went to Ebasco Services in
New York and the other to Bechtel. One company will be chosen in the spring
of 1990 to build the device. It will have a 300-foot diameter and be capable
of storing about 20 megawatt-hours of electricity--enough to power 1,000
100-watt light bulbs for 200 hours. The model must operate in two modes:
fast discharge for the ground-based laser (400 to 1,000 megawatts for 100
seconds), and slow discharge for utility applications (10 to 25 megawatts
for two hours).
The design must also solve less obvious problems. When the SMES is first
cooled down to super-low operating temperatures, the entire ring, including
the coil and aluminum support structure, will shrink several feet and tend
to move inward toward the ring's center. But if the ring needs maintenance,
the liquid helium will be drained out and the ring will warm up, causing it
to expand. To accomodate strain on the coil, the ring must have some radial
leeway.
One idea championed by Boom's group is to have two conductors and an
aluminum support structure between them ripple radially around the ring (see
drawing). "The nodes of the ripple will be fastened through struts to the
rock wall," says Boom. "When the coil cools down, those nodes stay fixed
while the radius between the ripples contracts."
Terry Walsh, Bechtel's project manager, finds fault with this rippled
support design. "As the coil cools down it's going to be pulling against
the point where it's restrained," he says. Walsh is concerned that these
high-stress points will cause strain on the conductor as well as dangerous
hot spots from friction. He says that Bechtel is studying an alternative
approach in which a telescoping support structure absorbs coil shrinkage and
expansion.
Ring sharing
The benefits associated with SMES are wide ranging. "With SMES, the utility
can operate all plants continuously at one hundred percent efficiency," says
Boom. And in the event of a generator failure, "the power control equipment
can prevent a blackout by switching the unit from charging to discharging in
thirty milliseconds," he adds. Boom also says that SMES could make it
simpler for utilities to buy power from intermittent alternate energy
sources, such as cogeneators and wind and solar power plants.
Where would a SMES site be located? "It depends on what the facility is
going to be used for," says Filios. Although a laser weapon probably would
not be located right next to an electric utility, it would be more
economical if the two shared a SMES. The reason: A SMES is only economical
if it's used continuously. Otherwise, the savings from generating efficiency
are lost to the costs of refrigerating the coil. a large
5,000-megawatt-hour facility would require a 1-1/2-mile-diameter exclusion
zone, so populated areas are out. This is because as the superconductor
cools down it generates strong magnetic forces. The aluminum ring must also
be buried in a bedrock trench to counterract these forces.
Although the free-spending military has firm plans for SMES, the trick in
persuading a frugal utility to build is to reduce operating costs. One way
of achieving this is with the new generation of high-temperature ceramic
superconductors. "Without refrigerating cost constraints, smaller SMES
units could be economical," says Boom. So far the current-carrying capacity
of these warmer superconductors is not high enough to be useful in a SMES
facility. But the pace of developments in the field lends credence to their
possible use in the future.
Boom is optimistic that economical SMES rings could have even more exotic
applications: storing energy for space platforms or powering rail guns or
tanks.
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