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Study Finds High-Speed Flywheels a Competitive Energy Storage System for Fuel Cell Hybrids

Reed Doucette and Malcolm McCulloch at the University of Oxford (UK) have compared high-speed flywheels, batteries, and ultracapacitors on the bases of cost and fuel economy as the energy storage system in a fuel cell based hybrid electric vehicle. Their paper is in press in the Journal of Power Sources.

They built computer models to simulate the powertrain of a fuel cell based HEV where high-speed flywheels, batteries, and ultracapacitors of a range of sizes were used as the ESS (energy storage system). A simulated vehicle with a powertrain using each of these technologies was run over two different drive cycles in order to see how the different ESSs performed under different driving patterns.

The results showed that when cost and fuel economy were both considered, high-speed flywheels were competitive with batteries and ultracapacitors.


  • Reed T. Doucette and Malcolm D. McCulloch (2010) A comparison of high-speed flywheels; batteries; and ultracapacitors on the bases of cost and fuel economy as the energy storage system in a fuel cell based hybrid electric vehicle. Journal of Power Sources doi: 10.1016/j.jpowsour.2010.08.100



Too bad the ESStor ESSU wasn't out yet. It could have been the winner. May be next time.


"High-speed flywheels are an emerging technology with traits that have the potential to make them competitive with more established battery and ultracapacitor technologies in certain vehicular applications"
"The results showed that when cost and fuel economy were both considered, high-speed flywheels were competitive with batteries and ultracapacitors."

without purchasing this article, the above two sentences tell a lot.

The advantage of battery or capacitor storage over mechanical flywheels, taking conversion losses into account is primarily that electricity is fungible.
I'm sure there are many 'more energy efficient means but that must be discounted when.
1; extra systems are employed for the reason that it adds complication.
2; that it is inevitably heavier.
3; In integrating the new system versatility and opportunity is lost.IE the large battery storage can absorb up to the ~ 100klm range storage limit.
A flywheel cannot do that.Also if the electrical system includes capacitors, then we could expect the electrical efficiencies to near equal that of flywheels, increasing as a factor of time (delay from storage to use) as well electrical storage has only a small incremental weight penalty as the capacitor is used in other modes(ossibly including charging and boost output.) That weight increase could be in the order of grams rather than kilograms.

I cant really see a future for these systems in ordinary motoring as electrical system (even those systems not directly required for propulsion ) are increasing in market penetration.

Electrical machines are ubiquitous so mainstream and well understood, well resourced as stock components and - well that seems to be the only objection, IE the auto makers loose a lot of control over patent and propriety components and services.


Even if High-Speed Flywheels are currently price competitive with batteries and ultracapacitors, it's not likely that they'll stay competitive.

Traditionally the price of mechanical devices was going down much slower than most other devices based on other technologies. There were far fewer breakthroughs in mechanical engineering, unless based on some other technologies, like microprocessor controll or lasers.

Will S

It doesn't say if the viability comes from direct drivetrain interaction (clutch or CVT), or whether it would be indirect (motor/generator, which then drove drivetrain motor).

I could easily see this in a HEV. A pure EV implementation would require quite a bit of specialized materials, which I'd like to see the assumptions/costs on.


@ HarveyD
"Too bad the ESStor ESSU wasn't out yet"...

In US there is "Invention Secrecy Act". (
Even if EESTOR company succeeds in developing their product, with claimed energy density, it may be classified and reserved for military use.
EESTOR already work with big military contractor Lockheed-Martin.
If they succeeded we may not even know it for many years, if ever.

I think that in France there is some similar legislation, that requires that all patent applications, submitted by French nationals, be first examined by Ministry of Defence, and they can prevent their use in public domain.
I know that in US many of innovations out of public domain are in the fields of cryptography, and weapons technology.
Who knows what inventions related to energy storage may have already been classified as secret, and prevented from public use.


MG, if that was the case, China (and many other countries) would be producing ESStor ESSu already. Many well intentioned dollar loving patriots would have sold the 'secret' for a few thousand $$ to pay the mansion bank loan.


Flywheels in vehicles are only usefull as energy generators for storage. Come to a stop, your stop/start kicks in, the regenerative braking kicks in, and a little flywheel in each hub starts spinning crazy, using brushings to generate and electric charge which is sent to the batteries.
They came up with something like this in the mid-to-late seventies, but it never took off.


For personal transportation, I don't see flywheels as the range extender of choice. We need something more solid state. Supercaps are electrostatic, batteries are electro chemical, this is closer to a solution.

I predict that we will have a break through in rechargeable metal/air batteries that will be range extenders for lithium ion. No flywheels nor draining the sludge out of zinc "fuel cells". Business needs to get real about the possibilities and stop wasting money.

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