Background

The "new breed" of flywheel under development at the Lawrence Livermore National Laboratory is aimed specifically at the "bulk storage" of electrical energy where the flywheel module may be charged at one time of the day, storing its energy for use many hours later. It may be necessary to insert an interval of many hours, or even many days, between the time that the EMB is "charged" and when it is discharged into a load. A flywheel-based bulk storage system could therefore be composed of a bank of many individual storage modules, each module of which might store, for example, 100 kWh of energy.

Bulk energy storage systems are needed, for example, for solar or wind power systems, or for "spinning reserve" for the utility network. At present virtually the only type of system that can meet these demands is of the "pumped storage" type, where water is pumped up into an upper reservoir in a mountainous area, to be used later to power hydroelectric generators. However, environmental constraints, and cost, construction time, and efficiency issues associated with the reservoirs, the pumps and the generators, together with the necessarily long transmission lines between the storage system and populated areas, have meant that there are only a few such systems in use in this country.

Electrochemical batteries in principle could fill the bulk storage demand, but they fall short in two areas. The first is their limited cycle life under deep discharge conditions. Despite many decades of development the best of such batteries typically has a deep-discharge cycle life of about 1000 cycles, i.e. about three years in a diurnal-use cycle, where decades of life, together with low maintenance requirements, is needed and expected for utility applications. Second, the "turnaround" energy efficiency of electrochemical batteries under deep discharge use is typically of order 80 percent, presenting an additional economical burden of up to 20 percent loss in energy.

The Laboratory has several decades of experience in the development of EMBs. During the course of this work carbon-fiber and glass-fiber composite rotors were developed and prototype flywheel systems were developed that utilized conventional bearings and employed special-design permanent-magnet generator/ motors. These EMB designs were subsequently licensed to a manufacturer who employed them to produce flywheel modules suitable for short-pulse (tens of seconds) output, such as that required for UPS (uninterruptable power supply) service.

During the latter years of this period the concepts behind simple, low-cost, and low-loss "passive magnetic bearings were proven out in prototype systems, and a totally new approach to the generator/motor design, involving a new type of generator/motor that is based on electrostatics rather than electromagnetism was conceived. The new generator/motor was computer-modeled, and its basic concepts demonstrated by tests carried out with small models. Both of the above new technologies are covered by issued and/or pending patents.

It is the integration of these three technologies, namely the composite rotor, the passive magnetic bearing system, and the new version of the electrostatic generator/motor, that distinguishes the "new-breed" LLNL EMB from all other commercially available flywheel-based energy storage systems. This combination of technologies also makes the new LLNL system uniquely suited for bulk-storage systems, where decades-long, maintenance-free service, high efficiency, and low parasitic losses between charging and discharging are critically important. The overall simplicity of the new system also augers well for lowering its cost relative to other systems.

A summary of attributes of LLNL's new EMB includes:
  • Projected 95% storage efficiency.
  • Simple and straightforward architecture promotes low cost, high reliability and decades of lifetime.
  • Long energy storage lifetimes (hours) due to extremely low parasitic loses.
  • Long device lifetime (10 to 20 years - a consequence of 100% passive magnetic bearing design (no electromagnets, amplifiers or sensors).
  • Ease of scaling; a few kwh (~12" diameter) to several hundred kwh per EMB unit (~72" diameter).
  • Different EMB unit sizes can be combined into an integrated system to promote both short timeframe operation (minutes) and long term usage (hours).
Description

A CAD image of an EMB concept is shown below. Identified are several of the key components showing their relationship to one another.

LLNL's new EMB designs are intended to answer to all of the new requirements for bulk energy storage systems, including very low parasitic losses and high turnaround efficiency. The new systems are designed for low capital and maintenance cost, and long (decades) service lifetime. The size of the modules will be such as to make them useful in a wide variety of applications, all the way from single-use in residential settings, to use in "battery banks" at substations and/or alternate-energy generating plants.

Feature

Benefit

Decades-long service life with minimal maintenance. Lower operating costs and greater reliability
High turnaround efficiency Better ability to take advantage of peak/off-peak rate structures
Simple design/modularity Better suited for mass-production; broadens fields of application

To achieve the above-mentioned requirements, LLNL has incorporated the following key new technologies into their bulk storage flywheel system, technologies that we believe are unique to flywheel energy storage. These stringent requirements for bulk storage mean that the new-generation flywheel must break with past tradition in two critical technological areas:

The first of these is the generator/motor. In all commercially available flywheel storage systems built to date, this component consists of an electromagnetic-type generator. This generator must operate within the evacuated housing of the flywheel rotor, using internally mounted high-field permanent magnets and internally located windings that require cooling. In addition to the mechanical issue of the high centrifugal force load caused by the magnets, even when no power is drawn from the system there exist parasitic losses associated with eddy currents induced in the windings and in other metallic parts. When power is drawn, the ohmic power losses in the windings lower the system efficiency and represent a substantial heat source that requires an active cooling system.

The LLNL-proposed answer to the first of the two technological areas is to replace the electromagnetic generator with an electrostatic (E-S) one, building on the pioneering work of John Trump at M.I.T. in the 1950's. In this type of generator/motor the heavy magnets located on the inner surface of the flywheel rotor are replaced by lightweight electrodes (the generator/motor "rotor"). These electrodes face another set of electrodes (the "stators") that, together with the rotor electrodes, form a time-varying capacitor. The system is charged through special charging circuits to power up the generator or motor functions of the system. In this type of generator the internal efficiency of the generator is essentially 100 percent (the only significant losses are in the external power electronic components that permit generator (discharge) and/or motor (charge) functions. When not generating or motoring, the parasitic losses of this system are zero. Preliminary experimental model tests together with computer simulations have shown that the generator output and motor power of this new E-S generator/motor should be entirely adequate for the bulk-storage application.

The second key technological innovation is the bearing system. Because mechanical bearings operating in a vacuum have limited lifetimes and significant parasitic losses, present-day commercial flywheel storage systems have adopted the use of magnetic bearings to support the spinning rotor. However, all magnetic levitation systems, including maglev trains, must deal with the constraints imposed by Earnshaw's theorem. This theorem, introduced in the early 1800's, shows that it is impossible to stably levitate any array of permanent magnets by the magnetic forces exerted by another, stationary, array of permanent magnets. The loophole for the magnetic bearing problem is that Earnshaw's Theorem only applies to static systems. Stable levitation is possible if dynamic effects can be employed. The commercial flywheel systems therefore employ what is known as "active" magnetic bearings. In an active bearing system, electromagnets are driven by power circuits that are controlled by sensors that detect unstable motion and, in turn, control the magnet power. Such bearing systems are expensive, require maintenance, and represent an internal parasitic power loss (in the electromagnets) that requires active cooling, and can lead to appreciable losses during the "off" times of the flywheel system.

Given the above-listed negative features of active magnetic bearing systems the LLNL approach is to replace the stabilizing elements of the active magnetic bearing by a purely "passive" stabilizer. That is to say the entire magnetic bearing system is composed simply of annular arrays of permanent magnets that provide contactless levitation. Their Earnshaw-based instability is overcome by a passive system that uses flywheel rotation to generate a stabilizing force. Specifically, such a stabilizer consists of a set of magnets on the inner surface of the rotor (typically using "Halbach arrays" of permanent-magnet bars). These rotating arrays interact with a stationary set of windings to produce a restoring force that overcomes the inherent instability of the levitating permanent magnets. In a typical form of the stabilizer the windings are located at a "null" point in the Halbach array magnetic fields so that current is only induced in the stabilizer windings upon motion away from the force equilibrium plane of the levitating magnets. In operation, therefore, the losses of the stabilizer approach zero, except in those cases (such as seismic events) when there exist accelerations of external origin.

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Potential Applications

The new EMB technology has many potential commercial applications, both within and without the electrical utilities industry. In the utilities, the availability of cost-effective distributed storage with minimal maintenance and decades-long service life would fill a need for which there is presently no viable solution. Another example is the telecommunications industry, where the electrochemical (lead-acid) back-up power system are notoriously poor in terms of high maintenance requirements and short service life in warm-climate situations. Stand-alone solar and wind-power systems represent another example of an application where present systems are inadequate. Office buildings could take advantage of the differences between peak and off-peak electrical rates by having energy storage systems in their basements. A short list of potential applications include:

  • Electrical grid energy storage: long and short-term bulk energy.
  • Utility distribution energy storage systems.
  • Uninterruptible power supplies (UPS).
  • Back-up power for servers or for power-interruption-sensitive industrial sites, hospitals, etc.
  • Stationary charging stations for electrical cars, buses, etc.
  • On-demand energy source for electrified locomotives or trucks (allowing use of regenerative energy recovery and eliminating need for electrification of track systems).
  • Auxiliary energy source for trucks, tractors, heavy equipment/earth movers.
  • Unmanned underwater vessels.
  • Solar-powered aircraft (UAV's) designed for round-the-clock operation.
  • Remote-site solar-powered 24-hour-operation monitoring systems (weather, etc.).
  • Load-shifting systems for office buildings (to take advantage of utility day/night and peak-demand rate structures).
  • Solar-powered navigation aids (e.g. buoys).
  • Solar-powered cell-phone towers.
  • Telephone central systems (now using lead-acid batteries).
  • Regenerative power recovery for elevator systems.
  • Energy storage for military remote site operations, utilizing solar or wind power sources.
  • Energy storage for spacecraft utilizing solar power.
  • Directed energy weapon/rail gun energy source.