It would enable ultra-reliable care-free on-site electricity, and non-polluting sustainable on-site alternative energy options.
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, would 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 would be enclosed within a sturdy and easily accessible metal box, that has display gages.
That enclosure would probably be aluminum, for its high electric and thermal conductivity, to meet EMI (electro-magnetic-interference) requirements and conduct heat from its circuits.
An accessible electronics enclosure is needed to facilitate practical 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.
Control settings include startup, discharge for shutdown, etc.
Left photo: 1st version prototype flywheel assembly.
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 axial and radial operating position.
Despite these and other known difficulties, we proceeded with our prototype development.
We believe better stationary power storage technology is much needed, which provides a practical alternative to chemical batteries.
From our detailed design analysis, we knew the prototype magnetic bearings would be difficult to stabilize and would require about 2kw before all magnetic bearing servos reach minimal power steady-state operation.
All predictions from our analysis were confirmed by our prototype tests.
We did not have enough resources to construct a similar prototype of a re-design mitigating its difficulties.
Instead, we decided that simpler, lower cost, magnetic bearings were needed, which did not need high power at startup.
The 2nd version was needed to demonstrate that a useful flywheel battery could store and regenerate DC power with minimal power loss.
Left: 2nd version prototype lower cost version, partially disassembled to show its essential features:
This 2nd version assembly has magnetic bearings that support its rotor by axial repulsion force of permanent ring magnets and axial preload to ceramic ball bearings that maintain rotor centering.
It would be less expensive, does not require startup power to position the flywheel rotor, and is far easier to implement.
Its ultra-efficient regenerative motor connects to external power interface electronics we built (compatible with all 3 flywheel battery versions). via the 4 stator and 4 sensor conductors shown. The electronics also connect to a 48vdc battery that supplies DC current, to spin the rotor at high speed. Power is then regenerated that charges the battery.
It performs as predicted.
But it has limitations we knew about before we started prototype construction and testing:
Its rotor balance is very critical --
If rotor center of mass relative to the ball bearing centers differ by more than 0.005" the ball bearings would be destroyed by vibration.
Ceramic ball bearings were selected, to minimize eddy loss in balls spinning in a strong magnetic field.
Ball bearings speed limits, which are lower for larger ball bearings, limit rotor speed. So energy storage capacity is limited.
Ball bearings have a limited service life.
Even with its known limitations, this 2nd version flywheel battery provides performance that has been demonstrated many times.
The 2 flywheel battery versions shown above are described in U.S. Patents 6566775 and 6794777 and 8242649.
Left: 3rd flywheel battery version cross-section view. It would cost little more than the 2nd version, but will not incur its limitations.
Concentric ring magnets (a pair at the top near the top plate, and a pair at the bottom near the bottom plate - purple color) provide stable centering force, plus lift force stabilized by a pair of axial servo electromagnets (each shown by orange color coil in gray iron). The electromagnets are controlled by axial servo electronics, responsive to axial position and rate sensors shown, which sense magnetic field between opposing axial-field magnets.
The ring magnets that provide centering forces and lift when axially offset as shown, are axially magnetized neodymium-iron-boron. The outer ring magnet is glued to its support cylinder. Centering forces they produce are proportional to radial off-center distance between the inner and outer ring magnets.
For a prototype with 40-pound rotor, axial servo maximum power drain is less than 200watts; its steady-state power drain about 5watts.
For a practical size flywheel battery, predicted energy storage capacity is about 10-kwh. Maximum power of about 5kw with DC current and voltage control, depends mainly on its power interface electronics.
Radial vibration, especially at resonant rotor spin speeds, is limited by current in conductors near the ring magnets. The top and bottom plates (shown by gray color) are (high current conductivity) aluminum. Along with (high current conductivity) copper rings (shown by orange color) near the ring magnets, any radial motion is opposed by forces from induced current in the aluminum and copper.
No current is induced when the rotor spins with no radial motion. Our analysis and experiments predict that radial motion damping forces from induced current, which increase with radial vibration frequency, will be able to damp vibrations. These damping forces should effectively control radial vibration, plus rotor tilt, plus motion from earthquakes.
The regenerative motor shown in this 3rd version, is basically the same as our 1st and 2nd versions.
Like our 1st and 2nd versions, power and signal conductors emerge from the top of the 3rd version center shaft.
Height-to-diameter ratio is higher than prior versions.
This results in less spin-axis precession torque from earth rotation, and more spin-axis tilt-control torque with less ring magnet off-center distance.
This 3rd version includes a low-friction radial touch-down surface near the top and another near the bottom, with cylindrical contact surfaces near the center shaft, plus top and bottom axial touchdown surfaces. Touch-down surface gaps are small; with the radial gap limiting off-center distance so no parts are damaged from earthquakes, and the axial gap likewise preventing damage from earthquakes - also resulting in maximum forces needed from the axial electromagnets of less than half the rotor weight (for low maximum axial servo power).
Installations requiring more energy storage and power can connect these flywheel battery DC power interface electronics in parallel, with each assembly that stores its kinetic energy preferably housed in a separate safe enclosure. This should provide far greater safety, plus redundancy for greater UPS reliability, at lowest cost.
Fradella helped a University of California at Berkeley engineering student and faculty team, who built a prototype that demonstrates magnetic bearings similar to the 3rd version described above, levitating a 40-pound rotor. The team spun its levitated rotor by contact with a spinning wheel. Their magnetic bearings perform as predicted by our analysis. They made videos of their demo.
Below: UC team magnetic bearing prototype image.
RPM Flywheel Battery Applications:
Conventional UPS is mainly a combination of high-maintenance diesel-generators and lead-acid batteries. They provide 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 the RPM flywheel battery 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 predicted for RPM flywheel batteries.
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 the ultimate RPM flywheel battery application).
RPM flywheel power storage systems 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 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 magnetic bearing loads and will be easier to install.
RPM 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.
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 can be replaced with LED and DC interface electronics.
Clearly, there are enough advantages for most DC appliances, to replace 60hz within a 1-year transition period.
Left: Underground installation, made practical by zero-maintenance. 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.
Sand would be a suitable backfill material.
Shredded tires, carpets, etc. are also good backfill options.
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.
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 flywheel battery DC electric power storage system can ultimately enable, to meet vast global power needs:
By enabling clean renewable energy use, it can help meet vast global power needs, besides current need in remote homes, military posts, scientific field stations, etc.
These remote applications are expected to be a substantial part of RPM flywheel battery early uses.
PV solar panels and windmills are increasingly used to provide power for remote buildings.
RPM has developed wind-powered brushless DC generators, which can produce 100x the energy yield, having far better power quality, compared to conventional wind-farm 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
Building-integral PV panels are the basis for a very high growth industry.
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 flywheel batteries.
Buildings like this could enable profound environmental
and energy conservation benefits.
Examples of attractive homes by pioneering architects, with handsome solar tile roofing and lead-acid battery power storage. RPM flywheel batteries 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 .
Since they were built, PV yield vs. cost and area has increased about 4x.
Buildings can have power-generating PV panels instead of conventional roofing, skylights, windows, and walls.
RPM brushless regenerative ultra-efficient DC motor:
About 40 years ago, Fradella built and demonstrated the brushless regenerative DC motor prototype shown below. Hydropower, US Navy, Aerojet, JPL and Parker-Hannifin wanted to place purchase orders for these motors. But not enough funds were available for their needed manufacturing facilities.
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 EV motor-wheel developments that followed.
Broad-speed-range brushless ultra-efficient DC generator:
This broad-speed-range generator can produce 100x more and far better quality DC electric power from wind turbines, compared to other generators. It is tested and proven; described in US Patent 7646178 and published application US20120256422A1.
It generates regulated DC current and voltage, with power proportional to wind speed cubed (for maximum power available from wind turbines).
Left: Vertical-axis generator prototype we built and tested:
DC output recorded at wide range of selected shaft speed.
Efficient integral power interface electronics and generator maximize speed range of generated power.
Boost-regulated integral electronics controls generator current to batteries, for maximum power from a wind turbine attached to its shaft.
RPM generator cogging torque is zero.
Tests show it very successfully performs as predicted by analysis.
Tests we also conducted show that shaft coupling of conventional wind turbine shafts to generators results in high mechanical cogging torque.
Laborious and unreliable shaft alignment, plus flexible couplings, are needed.
Besides their high mechanical cogging torques, most conventional generators also incur high magnetic cogging torque.
Conventional turbines with gears incur additional gear cogging. No power is generated, unless all cogging is overcome and conventional generator output voltage exceeds load voltage.
The RPM generator is described in published pending patent application US20120256422A1.
Onboard Batteries, PV, Pedal Power EV:
The RPM generator and regenerative motor-wheel images below can enable ultra-low-cost battery/solar/pedal power EVs. They include tested and proven regenerative DC motor technology.
RPM Flywheel Battery Comparison with Others
Flywheel energy storage tutorial (basic flywheel physics)
On-site Solar and Wind Power Tutorial and Examples
Dual-mode electric Vehicles with In-transit Power from Electric Highways
EV with Batteries, PV, Motor-wheels, Pedal Power
UPS+CMG for satellite power and angular orientation: Illustrated analysis
Future environmentally responsible and sustainable electric power options
Broad-speed-range Generator and its wind power
Bleak future of business-as-usual coal, oil, and nuclear policies