Designing, Building and Using Larger Flywheels

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The U.S. Navy is presently pursuing electromagnetic launch technology to replace the existing steam catapults on current and future aircraft carriers.

The present EMALS design centers around a linear synchronous motor, supplied power from pulsed disk alternators through a cycloconverter. Average power, obtained from an independent source on the host platform, is stored kinetically in the rotors of the disk alternators. It is then released in a 2-3 second pulse during a launch. This high frequency power is fed to the cycloconverter which acts as a rising voltage, rising frequency source to the launch motor. The linear synchronous motor takes the power from the cycloconverter and accelerates the aircraft down the launch stroke, all the while providing “real time” closed loop control.

The introduction of EMALS would have an overall positive impact on the ship. The launch engine is capable of a high thrust density, as shown by the half scale model that demonstrated 1322 psi over its cross section. This is compared to the relatively low 450 psi of the steam catapult. The same is true with energy storage devices, which would be analogous to the steam catapult’s steam accumulator. The low energy density of the steam accumulator would be replaced by high energy density flywheels. These flywheels provide energy densities of 28 KJ/KG. The increased densities would reduce the system’s volume and would allow for more room for vital support equipment on the host platform.

The EMALS offers the increased energy capability necessary to launch the next generation of carrier based aircraft. The steam catapult is presently operating near its design limit of approximately 95 MJ. The EMALS has a delivered energy capability of 122 MJ, a 29% increase. This will provide a means of launching all present naval carrier based aircraft and those in the foreseeable future.

The so-called Electromagnetic Aircraft Launch System, or EMALS, is now under development in a shore-based test facility at Lakehurst naval air station in New Jersey. However, according to May 12, 2010 reports, the test mass-driver installation suffered serious damage earlier this year in a mishap blamed on a “software malfunction”. Apparently the “shuttle” – which moves along the catapult track to accelerate a plane to flying speed – went the wrong way in a test shot and smashed into important equipment. The accident has delayed the shore-based testing by several months. It had been planned to commence launching aircraft – as opposed to test loads – this summer, but that will not now happen until autumn. The next US supercarrier, CVN 78, aka USS Gerald R Ford, is now under construction and intended to join the fleet in 2015. Navy officials confirmed last year that it is now too late to amend the ship’s design and revert to steam catapults: EMALS must be made to work or the US Navy will receive the largest and most expensive helicopter carrier ever.

Energy Storage Overview

There are currently (2009 study) six technologies for electricity storage that are under active consideration for commercial deployment: pumped hydropower, compressed air storage (CAES), batteries, flywheels, superconducting magnetic energy storage (SMES) and “super” capacitors.
They are in various stages of development and commercialization and offer differing advantages.

Pumped hydropower storage uses off-peak electric power to operate pumps that fill a water reservoir. At peak demand, the stored water is released through a hydroelectric generating plant. The technology is well understood and has been commercially deployed, for example, by the TVA at the Raccoon Mountain Plant which has a generating capacity of 1600 megawatts. Hydropower responds quickly to changes in demand and can generate high levels of power for long times. The difficulty with pumped hydropower is that it requires a large reservoir with attendant environmental problems, and systems are very expensive to construct. Projected improvements rely on variable speed pumps and turbines which can lead to at least a 3% increase in efficiency.

Compressed air storage uses off-peak power to pump compressed air into a storage container. On a commercial scale, the container will probably be a limestone cavity. Should CAES be used to support distributed generation, the container will a pressurized tank. There are two large CAES facilities built as demonstration plants although there are no commercial facilities as yet. CAES is less environmentally damaging than pumped hydro and as a distributed system is projected to work as a natural partner with wind generation. Large scale systems require a reservoir to store the compressed air, and small scale systems have safety problems with the possibility of exploding containers. Technical advances include development of small scale systems for distributed generation and better storage containers for the compressed air.

Batteries are a major technology for portable energy storage and find wide application in transportation and portable appliances. Here we consider only their application to the storage of electric power. In these applications which are primarily commercialized at facilities like the Fabs where power outages are disastrous, battery banks are located close to the facility that is being protected. Local power companies also use battery banks to supply emergency power in areas where power demand has rapidly growing peak demand. Batteries offer high energy storage densities, rapid response times, and they are portable. However, they are very expensive and have limited life times. The materials of which they are made pose environmental hazards. There are major research efforts underway to develop batteries that cost less and have longer life times. The research is creative but there is a long way to go before batteries will be an affordable option for electricity storage on a residential or industrial scale.

Flywheels store energy as rotational kinetic energy. They can store more energy if they operate at greater rotational velocities or if they are larger. They are limited by the properties of the materials of which they are made since large wheels tend to break apart at high angular velocities and by dynamical instabilities in rotation. Flywheels respond very quickly and can be connected in “farms” for large energy storage. At present, they are in a prototype phase and are very expensive. The obvious research needs are in materials science.

Superconducting magnetic energy storage uses high currents in superconducting coils to store electrical energy. SMES systems offer the possibility of very fast response with discharge of high power. For large scale energy storage, they can be networked, and they have long lifetimes because they have no moving parts. However, they require cryogenic systems which do wear out, and they are very expensive and currently experimental. However, it is possible that developments in materials such as high Tc superconductors could make this appealing technology a practical method of storing electrical energy.

Conventional capacitors store energy as a charge on electrodes separated by a dielectric material. Charge storage depends on the area of the electrodes. “Super” capacitors increase the electrode area by using porous electrodes and vary materials to increase operating voltages. They are potentially capable of rapid and high power discharges. Like SMES systems, they have no moving parts and potentially long lifetimes. At present, they are experimental, expensive and able to store little energy.

New Way Air Bearings 1 MWh Flywheel

Flywheels are not a new technology, they have been employed in trains and amusement park rides for over a hundred years.

The principal behind the flywheel is you can have a relatively small generator spinning up or charging a flywheel over a period of say a minute and then take the power off the flywheel over a period of several seconds to initiate acceleration of the train or amusement park ride at a much higher power than is available from the generator. This is the same way the flywheel would be used by the Navy, it takes about a minute between planes to get them staged and ready to be launched and the flywheel is charged during this time. What is impressive is that utility scale power has been demonstrated even if for only short periods of time.

Energy storage systems with the capacity to supply large power ratings for short periods of time, like our 1 MW hour capacity flywheel (New Way Air Bearings) that could supply 30 MWh of power for two minutes, are a perfect way to make up for instantaneous (power grid) outages and so giving time to get other generators started.

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