A Low-Loss Flywheel Battery

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First and second prototype flywheel batteries were constructed (right: first prototype flywheel assembly image). Electronics to control its magnetic bearing, mounted on the top and bottom decks, are not shown here. Nor are connections to external power interface electronics.

Prototype tests did not include a vacuum enclosure. A space satellite version, that provides power storage/regeneration while also providing satellite pitch, yaw, and roll attitude control, may resemble this prototype image.

Flywheel battery commercial versions, mostly for underground installations, will resemble the left image.

The flywheel, inside a cylindrical vacuum enclosure, will hang fixed to the rigid cylinder, from a 2-axis gimbal which can be seen above it.

This mounting structure will provide self-leveling and accommodate ground that may settle or shift over time.

Flywheel power and feedback signals will be connected to external electronics via a hermetic connector to a flexible cable, which will pass through the rigid cylinder.

The external electronics is enclosed within a sturdy metal box, that minimizes electromagnetic radiation from its high-frequency pulse-width-modulation switching, and facilitates easy flywheel status monitoring and control.

Monitored flywheel status variables may include magnetic bearing readiness for rotor spin-up, DC interface power, rotor rpm, available energy, etc.

Before we started constructing our first prototype flywheel, finite-element-analysis and Spice models predicted our servo-stabilized magnetic bearings that levitate and center the flywheel rotor would be difficult to stabilize and would require considerable power until they achieve a rotor steady-state operating position. Despite these and other known difficulties, we proceeded with our prototype development, because we believe this technology is much needed.

From our detailed design analysis, we knew before we constructed the prototype shown above, that its magnetic bearings would be difficult to stabilize and would require about 2kw before all magnetic bearing servos reach minimal power steady-state operation. We decided that far simpler, lower cost, magnetic bearings were needed, which did not need such high power at startup and if the flywheel assembly is disturbed by movement of its mounting structure.

So we developed a flywheel battery assembly having magnetic bearings that support its rotor by axial repulsion force of permanent magnets and axial preload to ceramic ball bearings that maintain rotor centering.

This second version is less expensive, does not require startup power to position the flywheel rotor, and is far easier to implement.

But it has limitations we knew about before we started prototype construction and testing:

Its rotor balance is very critical, because if rotor center of mass and the ball bearing centers differ by more than 0.005" the ball bearings will be destroyed by vibration.

Ball bearings speed limits, which are lower for larger ball bearings, limit rotor speed.

A photo of this second prototype lower cost version, partially assembled to show its essential features, is shown below:

The 2 flywheel battery versions shown above are more completely described in Fradella's U.S. Patents 6566775 and 6794777 and 8242649.

We are developing third and fourth flywheel battery versions, which will cost about the same as the version shown above, and not incur its limitations. Detailed descriptions will be shown in this webpage, after patent coverage is established.

There is currently a poorly met need for reliable no-maintenance UPS (Uninterruptible Power Supplies), for critical manufacturing, hospitals, cell phone repeater sites, computing centers, datacom, telecom, and intranet/internet servers and routers. The Electric Power Research Institute estimates that central utility grid power outages, sags, and spikes, cause losses amounting to $100 billion yearly. These losses are rising with increased dependence on quality power. Also, chemical battery power storage and fuel-burning generators can't provide practical UPS in remote locations, due to their maintenance and fuel logistics needs. With virtually no losses, unlimited service life without need for maintenance, and lower annual cost than existing options, we expect RPM's systems can, in 10 years, grow a $10 billion yearly US market for UPS to $200 billion.

Solar power for a space-craft, from photovoltaic panels, is very practical -- but not if chemical batteries are needed to supply power during extended periods when sunlight is blocked (e.g., by the earth): Satellite battery maintenance and replacement is not feasible, and their poor reliability is inconsistent with mission-critical applications. Also, long-term space-craft attitude control is limited by existing jet thrustors with finite fuel supply. RPM's power storage/regeneration systems can power low-earth-orbit and geostationary communications satellites during their dark hours, re-energized by photovoltaic panels during sunlight hours. Virtually unlimited service life with no maintenance, ultra-high efficiency, zero idling losses, and ultra-high reliability, are major advantages RPM's systems have for space vehicles, over all other UPS. And besides uninterruptible power, RPM's systems can provide inertial pitch/roll/yaw attitude control. That would eliminate the need for gyros and jet thrustors, and most importantly, loss of attitude control due to exhausted fuel consumed by the thrustors.

Cell phone and com tower UPS, remote site UPS, and satellite UPS + pitch/roll/yaw control, will be RPM's most viable near-term market, during production ramp-up and expected product cost reductions.

Ultimately, RPM's flywheel batteries can enable a global $200 billion new building-integral solar/wind power industry, and can even enable dual-mode high-performance electric highway vehicles. These clean, sustainable technologies will afford profound environmental benefits.

Conventional UPS is mainly a combination of high-maintenance diesel-generators and lead-acid batteries. Other flywheel storage systems offer only short-term (most "tens of seconds") ride-through power, during utility line outages; and while the utility or on-site generator supplies power, they constantly consume typically kilowatts while idling. That's over 1000x more standby losses than RPM's flywheel power storage system; which runs far cooler, will have far longer service life, negligible self-discharge, far higher reliability, far lower life-cycle cost, no wear-out, and will not need maintenance.

For on-site generated solar or wind power, that is available on demand, or distributed power storage for load-leveling, other available on-site options require tons of lead-acid batteries, that have troublesome limits on numbers of charge/discharge cycles, plus service, replacement, reliability, siting, and toxic materials disposal problems. In such applications, requiring daily or even more frequent charge/discharge cycles, annualized life-cycle cost is higher than RPM's flywheel battery.

Other flywheel energy storage systems have been developed for different purposes, and don't meet needs for practical carefree UPS -- and certainly not for on-site solar/wind power systems (which we view as our ultimate market) -- mainly because their idling losses and resulting failure modes are intolerable.

RPM's flywheel power storage system can provide on-site (underground flywheel for most urban installations) long-term (days) uninterruptible power. It can store energy for months, without significant self-discharge. It will afford safe, care-free, clean, quiet, no-maintenance, environment-friendly UPS and on-site power storage with virtually no charge/discharge cycling limits, at lower life-cycle cost, than all other options (including fuel-burning generators). Weight of our total flywheel power storage system will be less than that of lead-acid batteries; commercial versions, with carbon fiber-composite rotor rims, will have lower bearing loads and will be easier to install.

Expected competitive advantages, and performance not available from any other:

This is a zero-maintenance design, that makes safe underground installation practical.

Due mainly to its exclusive zero idling losses, the system uses under 2-watts to maintain full energy, and all its on-line monitoring, safety, and display functions.

Electronics, that initiate power-down on sensing abnormal magnetic bearing servo activity; concentric rims, that prevent total flywheel disintegration; and underground siting, that can absorb rapid energy discharge, which prevents damage outside the flywheel site, from possible flywheel disintegration.

It uses a standard reinforced concrete slab floor, and flush bolt-down steel cover enclosing the flywheel site, that protect building occupants from possible blast and explosion fragments.

All of these proprietary features are obtained at lower life-cycle cost than any other power storage device, mainly due to thorough integrated system design expressly dedicated to stationary on-site applications.

RPM's flywheel batteries are not intended for onboard applications in road vehicles, nor onboard any vehicles that need to make fast maneuvers such as high accelerations and abrupt stops plus fast direction changes.

Right: Block diagram of electrical system of a building, with RPM flywheel battery and multiple power sources.

RPM created the only flywheel UPS that will accommodate on-site solar and wind power, utility-line-powered UPS (switchgear at U.P. for utility power, P.F. during utility power failure outage), and any combination of power inputs.

It can have adequate power generation and storage capability, plus discretionary loads, so that it does not require utilities to buy excess power generated on-site. Also, it does not subject utility lines to hazardous "live" loads, which have killed workers performing otherwise routine line repairs.

The PWM regulator interface, to solar tiles and windmill generators, maximizes their energy yields and prevents dc line over-voltage when the flywheel reaches maximum energy capacity.

At first glance, except for reducing 60-Hz inverter cost, it may seem that including dc power outlets does not make sense, because it requires additional wiring and another type of socket, to prevent users from plugging in electric appliances that could be damaged by dc. But that rationale neglects these facts:

Most consumer electronics could be produced 10-60% smaller and lighter, 10-40% lower cost, and even more reliable, if designed for dc. It would eliminate need for rectified 60-Hz hold-up capacitors in all, 60-Hz power transformers and rectifiers in many. That's ample incentive for their producers to make the straightforward changes needed within a year, for their products to work from dc power outlets.

Electric ovens, cook-tops, toasters, and incandescent lights can use ac and dc interchangeably.

Power tools like drills, saws, etc. have universal motors. They are more efficient with dc, because ac causes more core loss.

Most induction motors; for fans, blowers, refrigerators, washers, and dryers, run at less than 50% efficiency on single-phase 60-Hz power. Brushless dc motors that run at more than 90% efficiency, and are smaller and lighter, could replace them.

Conventional 60-Hz inductive ballast, for fluorescent lights, could be replaced with smaller instant-starting electronic ballast. SCR light dimmers and speed control can be replaced with numerous types of PWM controls that run on dc power.

Clearly, there are enough advantages for most dc appliances, to replace ac within a 1-year transition period.

Left: Underground installation, made practical by our zero-maintenance design. It can absorb a maximum fast-release energy discharge (exploding flywheel) with minimal pressure and temperature rise.

It uses a standard reinforced concrete slab floor (except for the flywheel siting installation) of a garage or storage area for a safety barrier between the flywheel and other parts of the building.

The backfill is permeable, and preferably filled with energy-absorbing material, to absorb energy over a volume far larger than the flywheel enclosure.

Besides its safety advantages, this underground flywheel siting design does not take up valuable space.

This feature further reduces overall cost of the building UPS (Uninterruptible Power Supply).

Existing UPS chemical batteries, housed on multiple-level shelves, and fuel-powered generators, need a ventilated off-limits area protected from weather, that is a major additional site expense.

Examples of clean, cost-competitive, convenient, care-free, renewable on-site power that RPM's flywheel electric power storage system can ultimately enable, to meet vast global power needs ...

Right: Proposed remote UPS application, integrated with photovoltaic solar tiles and windmill power generator.

By enabling clean renewable energy use, it can help meet vast global power needs, besides current need in remote homesites, military posts, scientific field stations, etc.

These remote applications are expected to be a substantial part of RPM's early markets.

PV (photo-voltaic) solar panels and windmills are increasingly used to provide power for remote buildings.

RPM has developed wind-powered brushless DC generators, which can produce 2x to 10x the energy yield, having far better power quality, from any wind turbine, compared to most existing generators.

Lead-acid batteries are currently the only real available option for power storage.

They are not widely acceptable in millions of US buildings that need UPS (e.g., medical, dental, critical manufacturing, banks, etc.) due to their high maintenance, replacement, and life-cycle cost, plus housing and toxic waste disposal problems.

Right: Proposed urban building design, suitable for mini-malls, offices, manufacturing and commercial facilities; with solar tiles generating power, providing translucent skylight and window areas, and funneling wind to increase windmill power.

However, wind turbines and generators are presently rarely integrated with them.

Combining solar and wind power can provide higher energy yields, with lower peak-to-average power ratio; and thus lower power electronics cost.

It will encourage innovative new architecture, of cost-effective and attractive buildings, with stand-alone power capability.

UPS and load leveling (with lower off-peak utility rates) are afforded to buildings connected to utility power lines.

Benefits from UPS depend upon criticality of on-site power. Off-peak rate savings alone could result in typical payback periods of 10 to 20 years for RPM's flywheel battery.

Most importantly, buildings like this could enable profound environmental and energy conservation benefits.

Left: Proposed urban or remote building design, with solar tiles providing roofing, skylights, windows, and exterior walls.

This is a cost-competitive, functional, attractive, integrated-power building, with important energy and environmental benefits.

The building provides high elevation mounting, without incurring the cost of towers, for the integrated windmills, and funnels wind through them, to increase power generated by 10x or more. It also can limit maximum wind speed at the turbines, and protect the turbines and integral broad-speed-range DC generators from weather damage.

Through-flow (possibly hemispheric shape) small-mesh (.5"x.5") grills on each side of the windmills provide a shield around them, to protect people and other parts of the building from harm of possible blade disintegration; eliminate bird kill; and shield blades from sun damage (particularly ultra-violet on fiber-composite blades).

It should result in far lower total wind-power system cost, much higher reliability, much longer service life without maintenance, with less cyclic blade and stem stress, and "swooshing sound" compared to typical towers.

Birds also are killed, that fly into building walls and windows, perceiving them as open space, if they reflect light like a mirror or are clear.

PV panels are far better in that regard, because birds perceive them as solid obstacles.

Examples of attractive homes by pioneering architects, with handsome solar tile roofing and lead-acid battery power storage. RPM's Flywheel Battery would be ideal for, and encourage future projects like these, and enable far greater use of clean, renewable, cost-competitive, ubiquitous, environmentally compatible energy sources ...

Global markets for PV panels is now over $1 billion yearly, and growing about 50% yearly. It would grow much faster with RPM's on-site flywheel battery, providing carefree, lower total life-cycle cost power storage.

About 35 years ago, Fradella and two of his brothers who owned a machine shop developed the brushless regenerative DC motor shown below. Fradella very successfully demonstrated it to colleagues where he worked as a consultant, at the Navy weapons development center and at Hydropower in San Diego, also JPL and Aerojet; each issued purchase orders for various diverse applications. It was also demonstrated at Parker-Hannifin. Fradella was not able to finance motor manufacturing, and therefore could not accept the motor purchase orders.

The left photo shows the portable motor demo, with its 48vdc battery pack, power interface electronics (including a plug-in battery charger), the motor assembly, and user control box. The next photo shows the motor prior to assembly, with its rotor and stator disks and supporting parts.

This motor technology is closely related to the DC generators and flywheel batteries, plus the RPM EV motor-wheel developments that followed. It is tested and proven.

RPM has also developed a broad-speed-range generator that produces at least 2x more and better quality electric power from wind turbines, compared to most other generators. It is tested and proven. It is described in our US Patent 7646178. Improvements that reduce its cost and increase its reliability thru fewer parts are described in a pending patent.

Our generators produce regulated DC current and voltage, with power proportional to wind speed cubed.

RPM's generator and regenerative motor-wheel can enable no-gas ultra-low-cost battery/solar/pedal power EVs. They are enabled by our proprietary tested and proven regenerative DC motor technology. Essential features are shown in the image below from 3D CAD models of RPM's solar/fitness EV and 1 of its 2 rear motor-wheels:

They can provide safe, convenient, very low cost transportation. About 94% of the DC electric power from its onboard batteries, photovoltaic panels on top surfaces and amorphous thin-film photovoltaic coating on front and rear windows and each side of its body, and its optional pedal-powered generators, is converted to mechanical power at its 2 rear motor-wheels.

We hope you share our vision. For more examples of projects by pioneering architects and builders, and
renewable power research programs, visit these links:

RPM's Motor, Generator, Storage/Regeneration, EV

Left: Proposed urban or remote building design, with solar tiles providing roofing, skylights, windows, and exterior walls.

This is a cost-competitive, functional, attractive, integrated-power building, with important energy and environmental benefits.

It should result in far lower total wind-power system cost, much higher reliability, much longer service life without maintenance, with less cyclic blade and stem stress, and "swooshing sound" compared to typical towers.

Featured Solar Building Construction Projects (RPM hopes to play a pivotal future role)