# Flywheel energy storage compared to batteries and other means



## PStechPaul (May 1, 2012)

I've been looking into flywheel energy storage as a possible alternative to various types of batteries and other means such as compressed air and hydrogen. I've come across some interesting facts and this may be more practical than I first thought. Here is an article that provides a good comparison of various energy storage technologies and their relative merits:
http://www.resilience.org/stories/2011-10-05/energy-storage-flywheel

















But I have not found any definitive estimation of cost, although the Wiki states that UPS systems have an installed cost of about $350/kW. If this is a one hour rating, then $350/kWh is very impressive and certainly competitive with most batteries. But if one uses the 5000W/kg figure and the 120 Wh/kg, then the ratio of power to energy is about 40, and the cost becomes $14k/kWh, which is totally unaffordable. 

There are also other issues such as the 20%/hr energy loss, and drivability issues due to gyroscopic effect.
http://en.wikipedia.org/wiki/Flywheel_energy_storage

Other links that provide some more information:
http://flywheelenergysystems.com/flywheel-faq.html
http://www.gizmag.com/velkess-flywheel-technology-large-scale-energy-storage/27088/
http://www.vyconenergy.com/
http://www.beaconpower.com/files/FESS_Tech_Data_Sheet.pdf


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## Woodsmith (Jun 5, 2008)

Have a look at this:
http://www.time.com/time/magazine/article/0,9171,136542,00.html

And this:
Swiss Gyrobus 1951.

I did a whole load of research into flywheel powered vehicles in preparation for a thesis about 8 years ago. I was refused permission to submit it as a subject but I found out an awful lot about the possibilities of flywheels for transport.

IIRC General Motors built a flywheel powered car as a test and it used two counter rotating flywheels to reduce the gyroscopic effects. It worked well but the idea was shelved.

Many of the links I had book marked at the time are now dead though, however, I had found that many bus companies were testing flywheel power, and some still are for hybrid operation along the same lines as the F1 KERS systems.


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## kennybobby (Aug 10, 2012)

Woodsmith said:


> ...
> I did a whole load of research into flywheel powered vehicles in preparation for a thesis about 8 years ago. I was refused permission to submit it as a subject...along the same lines as the F1 KERS systems.


Sounds like you were ahead of the times, and they were short-sighted to not allow you to document that research.


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## Siwastaja (Aug 1, 2012)

Just as a general note, when doing fair comparisons, double check the numbers! (Remember: garbage in garbage out.)

The mystical "lithium ion" for example shows a power density of 300 W/kg. I have lithium ion cells here that do almost 3000 W/kg! The cheap prismatic LiFePO4s everybody uses do approx 400 W/kg. And these are continuous ratings. OTOH, as far as I know, there are no LiFePO4 cells capable of 3000 W/kg available despite the table. (Small power tool cells may be near that, but they are quite irrelevant for most of us.)

I really question the flywheel rating of 5000 W/kg. For example, the motor/generator has specific power of approx. 1000W/kg so it's impossible to achieve anything better as you also need the gearing, the flywheel itself, casing etc, so 500 W/kg might be nearer to the truth. This would mean that the specific energy table is probably wrong by the same factor!

So, as usual, someone has just thrown random numbers to make a nice table without references. It happens all the time. I also tend to do it. The difference is that I'm always right


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## PStechPaul (May 1, 2012)

I also thought some of those figures were questionable. It would be helpful and valuable to make a new table with better numbers and keep it updated, and should also have a cost range. There should also be some distinction among different types of Li-Ion, and perhaps some actual commercially available devices. And it should also include lead-acid SLAs and FLAs. I might start such a table and include it in my EV calculator.

The very high power density of flywheels is based on their mechanical energy and may be somewhat of a theoretical maximum. Conversion to electrical power would have limitations, and ideally a flywheel would convert its energy directly to vehicle motion in an EV, where the high power may be best suited to acceleration and braking. Also the relatively short "self-discharge" makes them more suited to a type of hybrid.

I would assume that this does not include the potential power of a destructive event, which would show much higher power levels for lithium cells when they self-destruct in a fire or explosion. 

Thanks again for the double check and heads up.


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## Duncan (Dec 8, 2008)

Siwastaja said:


> I also tend to do it. The difference is that I'm always right


This from the guy who thinks serial hybrids are a good idea!!


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## shortbus (Sep 27, 2011)

They used this(or maybe still are) in Formula 1 racing. Called a FES device. And a company in the UK is doing it now - http://www.williamshybridpower.com/#/technology


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## dtbaker (Jan 5, 2008)

PStechPaul said:


> I've been looking into flywheel energy storage as a possible alternative to various types of batteries and other means such as compressed air and hydrogen.


mechanical density is one thing.... SAFETY and durability is another. Even w counter -rotation to cancel gyroscopic effect, that much mass spinning at high rpm turns into a bomb in an accident. Also, has to have significant mass, which impacts accel/decel performance, and takes a while to add energy to spin it back up.

My opinion is that flywheels are better suited for stationary applications.... perhaps for PV feed-in to the grid or something.

pressurized air with a compressor/turbine on board to capture braking energy and return for accelleration seems like a cool direction to investigate.


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## Woodsmith (Jun 5, 2008)

dtbaker said:


> mechanical density is one thing.... SAFETY and durability is another. Even w counter -rotation to cancel gyroscopic effect, that much mass spinning at high rpm turns into a bomb in an accident. Also, has to have significant mass, which impacts accel/decel performance, and takes a while to add energy to spin it back up.
> 
> My opinion is that flywheels are better suited for stationary applications.... perhaps for PV feed-in to the grid or something.
> 
> pressurized air with a compressor/turbine on board to capture braking energy and return for accelleration seems like a cool direction to investigate.


From what I found the earlier heavy metal flywheels were prone to explosive over speed failure leading to shrapnel breaking out of the flywheel's container.

The later designs were based on heavy rimmed cylinders of carbon fibre composites spinning inside a protective cylindrical container with minimal clearances. 
The over speed failure mode was that the fibres would slowly (relatively speaking) de-laminate causing the rotating cylinder to expand, like a brake shoe, against the inside of the protective container. That would bring it to a more controlled stop without the 'explosive' element of the failure.

I did also find an anecdotal account of German automotive (IIRC) engineers being killed by the failure of a carbon fibre energy storage flywheel under test. According to the account the engineers were trying to 'fail' the flywheel by throwing things in it until it exploded!


I can't find the source of that anecdote anymore so treat that as no more then that.


I always figured that flywheel storage would be good for city buses. A short range, heavy vehicle with regular stops, and charging built in under the road at each bus stop. The bus would then only need a few miles range as it would be able to recharge while picking up passengers.

The charger would be another flywheel under the road at the bus stop that is spun up to speed between buses and then rapidly discharged via an electrical connection to spin up the flywheel in the bus while passengers are loading. Using the flywheel transfer would reduce the peak loading to the power grid that is often already found under the road surface anyway.

Sensors betwee the bus and the bus stop charger would allow the energy company to 'read' the bus and charge each bus company according to the energy demand.

A small 'donkey' engine would be the emergency energy source should a bus be stranded without charge. It would only need to propel the bus to the next bus stop for a charge, or the edge of the road in case of breakdown.


I don't think very high pressure air as a storage medium is any safer in a crash then a well designed flywheel, but I could be wrong.


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## Siwastaja (Aug 1, 2012)

Ultracapacitors show similar or better performance over flywheel or compressed air, with additional benefits such as better efficiency. The only real argument for mechanical system is supposedly price, but with both batteries and capacitors getting cheaper all the time (mechanical systems are not), I think these solutions do not have any real future and it's a waste of time to study them.

I wouldn't say so if the batteries and capacitors didn't show the great results they are getting now. Just 10-15 years ago, it was a mystery whether battery or capacitor technology will have any future, and this is why mechanical solutions were studied. But the battery and capacitor field of study has changed with the recent generations of technology.

It seems that high-discharge batteries are the way to go in most cases, and ultracaps will have some niche use as a solid-state, high-efficiency replacement for flywheels. City busses / metro trains might be the case for ultracaps, but only within very densely populated areas with very frequent stops equipped with charging infrastructure. This would mean something like a 1 kWh "pack" giving 1 km of range and 10-second quick charging. This is possible with the current ultracap technology and it's not that expensive. And it's simple as hell with no problems with safety, durability or anything else.


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## PStechPaul (May 1, 2012)

I think buses should have all seats equipped with rowing machine type pedals and levers so that passengers could be the power source. Plus they get a free workout and it would give teenagers something better to do than harass the bus driver. 

This could be used to charge up capacitors, batteries, or a flywheel. There could be individual watt-hour sensors for each seat and the passenger could be refunded for their efforts. Special awards could be granted for the highest peak power to make it a competition.

This project could be funded by our health care system as it would pay off in reduced obesity and diabetes and heart disease.


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