Flywheels For Short Duration / High Power Energy Storage Applications

I first read about the possibility of using composites flywheel suspended in a vacuum as high energy storage devices nearly 30 years ago in an article in Physics Today. Since two of the companies (Pentadyne andd Beacon Power) at the forefront of this field have declared bankrupty within the last year and a half, one would have to say the progress in the commercialization of this technology has been glacial.

Flywheel With Steel Rims Find a Market Niche in Uniterruptible Power Supplies

However vaccum sealed steel flywheels with magnetic bearing seem to be finding a significant market niche in Uniterruptable Power Supplies (UPS) system which are designed to protective sensisitve electrical systms from power interruptions. The flywheels provide protection again power glitches up to about fivteen seconds. This time range protects many systems from nearly 99% of all power interruptions. For long interruption 10 to 15 seconds is sufficient time to power up a diesel generator. The upfront cost flywheels are more expensive than lead acid batteries, but they have very long cycle life and low maintenance costs, so that life time costs are lower. A partial list of companies selling flywheel based UPS sytems is given below:

  1. Active Power (
  2. Vycon (
  3. Caterpillar Flywheel UPS (
  4. Powerware Flywheel UPS (
  5. PCI (
  6. CM Technololgy (
  7. KST Rotary Solutions (
  8. Precise Power Corporation (
  9. PowerThru (

This list is probably not complete. I just got tired of searching web. I had been aware of Active Power and Vycon for a long time, but it now appears that flyweel UPS is a tecnology which has found a siginficant market niche. Which the exception of PowerThru all of the above companies which bother listing the material used for the flywheel rim are using steel. PowerThru, which is a reincarnation of Pentadyne whose assets were acquired by Phillips Service Industries after the bankruptcy, is continuing to use carbon fiber composites for their flywheel rims. I am sceptical that they can compete with steel flywheels in this stationary storage market where total cost matters more than energy density.

Most of the above companies are using flywheels for the single use of power conditioning in UPS systems. The one exception is Vycon which sells flywheels energy storage systems for use in loading cranes in rail and ship yards. When the diesel electric cranes lower heavy loads the flywheels recover and store gravitational potential energy which the cranes utilize when they raise their next load. Vycon is also proposing using their flywheels for recovering braking energy in rail applications, but I do not believe they have installed any systems of this type.

In the mean time composite flywheels are still being developed by various companies. The technology of Beacon Power has not yet disappeared since their assets have been acquired by Rockland Capital who has created two new companies, Beacon Power LLC and Spindle Grid Regulation LLC. Rockland is continuing to run the 20MW Stephentown NY flywheel frequency regulation plant which came on line in January 2011. A second 20MW plant in Hazle Township, PA is being planned according to this announcement. The decison to split the company in two in interesting. Presumbly Beacon Power LLC is a flywheel technology company whil Spindle Grid Regulation LLC is a system integrator. Conceivably Rockland could drop Beacon and buy flywheels from another supplier if someone else provides a superior product. Since grid regulation, like UPS systems, is a stationary storage application where cost more important than energy density, it might very well turn out that cheaper steel flywheel rims will become the technology of choice.

Development of Flywheel With Carbon Fiber Rims Continues for Rengerative Braking Applications

The target market of choice for the developers of carbon fiber composite flywheels is regenerative braking. For such a mobile appication energy density is important. The main competitor of flywheels in this field are supercapacitors which have already found a niche in hybrid buses. Maxwell Technologies claims that there are more than 5000 hybrid buses on the road using their capacitors to capture and reuse braking energy. When composite flywheels were first proposed several decades ago they were touted as a pontential rival of chemical batteries in energy density, but in actual implementations the energy density is significantly lower than batteries, and the real market focus is on high power applications with short storage time and high cyling rates. This is exactly the market segment targeted by supercapacitors. The high surface area carbon material, which is the key technology enabling the production of supercapacitors, is relatively expensive to manufacture. The hope of composite flywheel developers is that they will be able to rival supercapcitor performance at a lower cost.

Flybrid Automotive Limited

I have previously written about the company Flybrid Automotive Limited which has developed a composite flywheel energy storage system for use in Formula 1 racing cars. This socalled KERS (kinetic energy recovery system) stores and return braking energy via a mechanical transmission, thus avoiding the need for an electrical motor. The F1 version of the flywheel is targeted at cars particpating in endurance races like the le Mans 24 hour race where increased milage is important. Flybrid hopes to get in to the hybrid bus market where they claim they can deliver regenerative braking systems at one third of the upfront cost of pure elecrical system utilizing supercapacitors. Flybrid has recently undergone a 'restructuring' in an effort to enable the flow of outside capital to allow expansion of their production capacity. Whether or not the desired capital is actually going to flow in remains to be seen.

Ricardo Kinergy

Kinergy is the name of a composite flyweel storage product being developed by Ricardo a British engineering consulting firm. Their product is differentiated from Flybrid's by the fact the that the flywheel housing is hermetically sealed and the flwheel is magnetically coupled through the flywheel housing to the exterior drive mechanism. This deisgn, like Flybrid's can be directly coupled into the drive train without the need of separate electric motor. Ricardo is targeting a hybrid bus application for this technology, partnering with Torotrak, a developer of gearless transmissions, and Optare Solo, a manufactuer of low carbon emission buses.

Ricardo is also working on a project to create a regenerative braking system for diesel commuter trains partnering with Artemis Intelligent Power Ltd, a developer of Digital DisplacementŠ hydraulic pumps and motors, and Bombadier Transportation, a railroad engineering company.

Williams Hybrid Power

Williams Hybrid Power is another company developing composite flywheels for transportation applications. Their flywheel is coupled to the outside world by electrical wires only and they the unit as a high speed electric motor/generator with a high interia composite rotor. The rotor design is decribed as a magnetically loaded composite, implying that the magnetic portion of the rotor is composed of small particles of magnetic material enclosed within the carbon fiber matrix. They claim that this design has lower losses due to eddy currents and is safer in the event of a rotor failure compared to a design using solid metal magnetic components.

Since the input and output of this storage device is an electric current, it can provide regenerative braking only in a true hyprid disign with an electric motor connected to the drive train. Therefore this sort of flywheel is competing directly with ultracapacitors. The hope is, of course, that their flywheel will be able to supply equal or superior performance at a reduced cost. William Hybrid Power has signed an agreement with the Britsh Go Head Group, a provided of bus and rail transporation to develop and produce six hybrid buses utilizing flywheel energy storage.

Flywheel Energy System Inc.

The Canadian company, Flywheel Energy Systems Inc., is also developing composite flywheels for transportation applications. The table below compares the characteristics of their currently existing flywheel demonstration technology to the charateristics of a proposed commerical flywheel product.

Parameter Units Existing Flywheel Commerical Flywheel
Power kW 120 120
Energy kWh 0.77 0.75
Mass kg 150 230
Volume liters 103 70
Specific power W/kg 800 520
Power density W/liter 1,165 1,715
Specific energy Wh/kg 5.1 3.3
Energy density Wh/liter 7.5 10.7
Outside diameter mm 610 400
Length mm 460 540
Average round-trip efficiency N/A 86% 94%

The energy density of 10.7 Wh/kg is low compared to batteries (Lead Acid: 25Wh/Kg, Lithium Ion: 110Wh/kg) but better than ultracapacitors (3Wh/Kg). The intended market is for short duration high power applications where Flywheel Energy Systems hopes to beat the costs of ultra capacitors.

Mar 01, 2013

Energy Storage News

rogerkb [at] energystoragenews [dot] com