09-02-2013, 10:15 AM
Third Generation Flywheels For High Power Electricity Storage
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ABSTRACT:
First generation flywheels of bulk material such as steel can mass tens of tons, but have low energy
storage density. Second generation flywheels of composite materials have higher energy storage density
but limited mass due to structural and stability limitations. LaunchPoint is developing high energy third generation
flywheels – "Power Rings" – using radial gap magnetic bearings to levitate thin-walled composite
hoops rotated at high speed to store kinetic energy. Power levels exceeding 50 megawatts and electricity storage
capacities exceeding 5 megawatt-hours appear technically feasible and economically attractive. Power
Rings can be used to decrease the peak power requirements of electric transportation systems by supplying intermittent
high power for vehicles such as maglev trains. They can also store braking energy, isolate the
power grid from surges and spikes, reduce the incidence of transportation system power outages, and provide
back-up power in case of blackouts.
INTRODUCTION
In 1973, Dr. Richard Post of Lawrence Livermore
National Laboratory proposed the construction of
200-ton, 10-megawatt-hour composite flywheels
(Post 1973) to provide electricity storage for the US
power grid. Unfortunately, achieving dynamic stability
and structural integrity in composite flywheels
proved far more difficult and costly than expected.
The largest commercial units constructed to date are
400 times smaller than those Dr. Post envisioned —
in spite of a critical need and a huge potential market.
Now, discoveries made during the course of a
magnetic levitation transportation project have finally
opened the door to construction of utility-scale
flywheel electricity storage systems. We call these
devices “Power Rings”. Figure 1 shows what a 400
kilowatt-hour Power Ring might look like.
ELECTRICITY STORAGE TECHNOLOGIES
Technologies such as pumped hydro, compressed air
energy storage (CAES), batteries, fuel cells, superconducting
magnetic energy storage (SMES), ultracapacitors
(or supercapacitors), and flywheels can all
be used to provide electricity storage. Flywheels,
SMES, and ultracapacitors all cost too much for use
in large installations. Fuel cells and flow batteries
show promise but are also projected to be expensive.
A new class of magnetic bearing
In a permanent magnet Halbach array (Halbach
1985), the field produced by each magnet reinforces
the fields of all the other magnets on the “active”
side of the array, and cancels them on the other side.
The result is, in essence, a one-sided permanent
magnet with an intense field. When two identical
Halbach arrays are placed with their active sides facing
each other, they produce powerful repulsive, attractive,
or shear forces, depending on alignment.
As compared to simple opposed pole faces, a 5-
element Halbach array provides more than three
times as much force per unit volume of magnet.
This provides the basis for the “shear-force levitator”,
shown in Figure 3.
CONCLUSIONS
The Power Ring third generation flywheel design
has been extensively analyzed and promises to
transcend the barriers that have constrained construction
of compact, high power flywheel electricity
storage systems. As a result of support from
four government agencies in the US, the first
hardware prototype began development in 2006.
Further development could lead to production of
commercial units for a wide variety of applications
within three to five years.