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Williams Hybrid Power and Alstom cooperate to develop flywheel energy storage technology for Citadis trams

Williams flywheel unit. Click to enlarge.

Williams Hybrid Power, a division of the Williams group of companies that includes the Williams F1 Team, and Alstom Transport have signed an exclusive agreement that will see Williams Hybrid Power’s composite flywheel energy storage technology (earlier post) applied to Alstom’s Citadis trams by 2014.

After several years of research into energy storage, Alstom teamed up with Williams Hybrid Power to trial its composite MLC (magnetically loaded composite) flywheel energy storage technology which offers potential fuel savings of 15% when installed in public transport applications.

There is no mechanical connection from Williams’ flywheel storage; the unit connects using only electrical cables to transmit the energy back and forth, allowing the same vehicle packaging freedom as a traction battery. The company suggests thinking of its electric flywheel energy storage as an electro-mechanical battery or as an ultra high-speed electric motor/generator having a high inertia, composite rotor.

Originally developed for the 2009 Williams Formula One car, Williams Hybrid Power’s energy storage technology has since been introduced into applications such as London buses and the Le Mans-winning Audi R18 e-tron quattro.

The technology offers fuel savings and emissions reductions by harvesting the energy that is normally lost as heat when braking and turning it into additional power.

Flywheel energy storage technology offers higher specific energy than ultracapacitor technologies, but compared to batteries, the energy capacity is much lower. Thus, the flywheel is a very efficient short-duration energy accumulator that can capture and re-deploy large cumulative amounts of energy with little waste, Williams notes—i.e., it is ideally suited to kinetic energy recovery on trams because of their stop-start nature and high mass.

Flywheels also can operate efficiently at extreme ambient temperatures, unlike chemical batteries and capacitors. As a result, in vehicle installations less attention is required to control ambient temperature, ultimately allowing a less complicated, lower cost installation.

The flywheel rotor assembly is entirely composite apart from the rotor shaft at the neutral axis. An all-composite rotor is inherently safer than a steel hub and composite overlay construction, Williams says, as there is no metallic structure travelling at very high speed.

At the rotor’s maximum operating speed there is redundancy in design to prevent electrical and therefore rotational overcharge. This redundant, positive over-speed prevention is inherently safer than mechanically driven flywheel over-speed prevention methods, Williams says, where there is always a risk of over-speed via a malfunctioning mechanical drive.

From the very beginning we highlighted trams as an ideal application for our technology and to be collaborating with the market leader on this project is very exciting. We both share a common goal—developing the next generation of green transport technologies—and this agreement will hopefully prove pivotal in finding a solution that not only cuts carbon emissions but crucially cuts costs for the end user.

—Ian Foley, Managing Director of Williams Hybrid Power



All frequent stop or negative-positive load vehicles-machines could benefit from energy storage units. The efficiency of this mechanical-electrical unit seems to be between current super-caps and current low performance batteries.

However, by 2020 or so, both improved super-caps and higher performance batteries may be superior.


Could be very nice on diesels, where you don't want the extra expense of hybridisation, but want more than stop-start for urban use.


It is a very interim solution at best.

Fortunately, Coal and NG/SG fired power stations, ICE, ICEVs and flywheels mechanical contraptions will certainly be replaced by common sense Hydro-Wind-Solar-Nuclear power plants and light weight high performance e-storage and electrical traction units in the next 10 to 30 years.

That's what happened to most mechanical time keeping machines many years ago and many other early technologies.


Aluminum price has dropped below $1/lb (2013 $) and as such could become a choice replacement for heavier steel/iron car parts.

A weight reduction of 25% to 40% is possible.

Much lighter e-vehicles together with higher performance-lighter batteries could increase EVs range from 100 miles to well over 200 miles by 2015 and to well over 300 miles by 2020 or so.


Aluminum car parts can be recycled over and over again at a reasonable cost.

A mix of composites and aluminum parts could replace most current steel parts.


This is obsolete from the start, it add weight and cost for few benefits and it raise dramatically garentee cost for manufacturers. Nobody will buy that as also lithium batteries. Hydrogen is a 100% better solution.


Aluminum in cars will increase the price, but probably less than a fancy set of 20" wheels.. the fuel savings over the life of the car would be massive.

What are the details on this flywheel, how much energy can it store and how much power can it put out?.. the previous article says it can put out an impressive 120kW

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