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FAQ: Kinetic Energy Storage



Why use light composite materials instead of heavy materials like steel ?

What's the advantage of a flywheel compared to an ordinary lead acid battery ?

Is such a high speed rotor not very dangerous ?

What kind of bearings are you using for such a high speed application ?

How is the energy put into and taken out of the flywheel ?

How much does your system cost ?

Are you working in other fields than high speed rotor systems ?

I like this idea, can I place an order right now ?

I'm a student writing a class work about kinetic energy storage, where do I get more info's ?
 
 
 
 
 
 


Why use light composite materials instead of heavy materials like steel ?Top

The energy density of a flywheel is probably the first criteria for the selection of a material. Imagine a rotating thin ring the maximal energy density is only dependent from the specific strength of the material, not from the mass anymore. Or as a formula

With specific strength composite materials have big advantages compared to metallic materials.

 
 
Density 
[kg/m3
Strength 
[MN/m2
Specific strength 
[MNm/kg] 
Steel 
(AISI 4340)
7800 
 1800  0.22
Alloy 
(AlMnMg)
2700 
 600  0.22
Titanium 
(TiAl6Zr5)
  4500 1200  0.27
GFRP 
(60 Vol% E-Glass)
2000 1600  0.80
CFRP 
(60 Vol% HT Carbon)
1500   2400 1.60
Furthermore the burst behavior is a deciding factor for choosing a specific material. With circumferential speeds up to 2000m/s you wouldn't like to have big sharp edged hard fragments flying around witch would happen in case of a bursting a metallic rotor. Composite material rotors can be designed to have a benign failure mode with almost no penetration load to the housing.
 
 


What's the advantage of a flywheel compared to an ordinary lead acid battery ?Top

The price is higher ...

No, seriously. The advantage begins where conventional batteries begin to fail. For example experiencing lots of charge and discharge cycles within one minute. In general we think that flywheels are well suited for a power to energy ratio of 1 second to 10 minutes where batteries prefer 1 hour and more. Capacitors on the other side are a perfect storage solution below the 0.1 seconds range. Therefore the power density of a flywheel system is higher than with ordinary batteries.

A deciding advantage is the unlimited number of charge and discharge cycles. Batteries have a typical live cycle time below 1000 charge/discharge cycles.
 
 


Is such a high speed rotor not very dangerous ?Top

Generally spoken, like any other energy storage device, the stored energy in flywheels has a destructive potential when released uncontrolled.

We put great efforts to design our rotors in such a way, that in case of a failure a lot of thin an long fragments occur. These fragments have almost no translateral energy and the rotor burst is therefore relative benign.

Burst debris of ASPES flywheel

However be warned, not every composite rotor must fail harmless. They can fail quite unfriendly and dangerous as well.

destroyed flywheel test pit

The safety of the system is not only related to the rotor alone. The housing, and all other parts and materials within influence the result of a burst quite significantly. A flywheel of the size we are using is not a toy to play with, like a car speeding with 200mph witch isn't meant for children anymore.

We have experience with flywheel bursts from more than 20 bursts of different rotor sizes. And no, the above picture is not the result of one of those bursts. But it shows a picture after a test with a rotor of comparable energy content. Don't try this at home anymore.


What kind of bearings are you using for such a high speed application ?Top

We used all possible kind of bearing already running under vacuum conditions. Precision ball bearings are probably the most economic solution right now. Active magnetic bearing have lot's of technical benefits but will hardly ever meet the price of a ball bearing. Passive permanent magnet bearings in combination with another bearing can expand the limits of conventional bearings without the cost penalize of active magnetic bearings. Passive magnetic bearings using superconducting materials may have a bright future for such high speed applications.

Therefore choosing the best bearing system depends very much on the specific application. Aspects like life cycle time or maintenance interval, rotational speed and rotor weight, vibration monitoring needs, cost etc.
 
 


How is the energy put into and taken out of the flywheel ?Top

As we only deal with high speed flywheels running under vacuum conditions mechanical shafts with gearboxes are out of discussion. We couple the flywheel with an electrical motor/generator on the same shaft and then just deal with electrical energy.

In general we are using synchronous electrical machines, with permanent or dynamic excitation. The according to the rotational speed variable frequency is then converted with an inverter to the constant grid frequency.


How much does your system cost ?Top

How much does a car cost ?

We will not compete lead acid batteries regarding cost per stored energy. But, depending on the specific application we know, that our solution CAN  be competitive against all other possible solutions on the market.

In general, if you can live with the limits a today's solutions (e.g. Batteries) we don't even consider to calculate an offer. But if today's technology causes sever additional costs like

then it might make sense to consider a flywheel energy storage device.
 


Are you working in other fields than high speed rotor systems ?Top

In general, no. The field of high speed rotors together with all related specific problems like bearings, high speed electrical machines, power electronics, vacuum systems, safety aspects, controls etc. is big enough for us to keep us busy all day.

We are not particular interested in low speed rotor applications unless the technology for that special application is similar to high speed rotors. And, we are specialized in composite rotors, not steel or alloy materials.


I like this, can I place an order right now ?Top

I'm sorry, I think we run out of order forms right now, please call tomorrow again.

No seriously. We think the time of selling kinetic energy storage system out from the shelf is not yet ready. We might have THE system for you if you have the exact same application in mind we have developed in the last few years. But calling for a quotation of a kinetic energy unit right now is like going to your next car dealer asking the price for A CAR neglecting that there are quite some differences between a Rolls Royce and a Pick-up truck.

If you have an application in mind, please ask yourself first

In most cases we would have to develop or adapt our systems to meet the specifications of your application. This means that there will be time and money involved BEFORE you have a system in your hands. In most cases we are looking for a partnership between the party having the problem or the market, a possible manufacturer and us as the developer. We have limited manufacturing capabilities, but right now we are an engineering company not a mass producer.
 
 


I'm a student writing a class work about kinetic energy storage, where do I get more info's ?Top

Please, check our pages first before calling us to ask questions answered here. We have a quite complete list of books on our literature page, please check your library first. Additional information can be found in our publication list.

We are happy to give additional information to anybody who is interested in this field, but our time is limited as well. The most chances to receive a detailed answer has a caller who


 
 


Updated: 2/99  .