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Mark Delcambre: Oh no love for the flywheel group. This is a very interesting way to store energy. I know that there are a number of data centers that use the low rpm flywheels to store energy. In the case of a electrical failure the flywheel provides about 20 seconds of power to the facility while the diesel generator starts up. This could be used to store energy in all sorts of places.

Alex Lawson: This is definitely a promising technology, but I don't see it being applied to many things. For facility backup generation, like the data center Mark mentioned, it seems like the ideal system to deliver power instantly but for a very short time. I have a hard time believing that, even with future developments, flywheels would ever be an effective replacement for batteries, capacitors, or other means of direct electrical storage. I'd like to know, when I read statements like “When compared to lead-acid batteries a high speed flywheel weighs just 20% as much as the equivalent number of batteries,” what the measure of 'equivalence' is - 'equivalent number' is pretty meaningless in this context. Regardless, I think this storage method could be good for integrating into mechanical systems where a relatively high power output is required for a short time on short notice.

Adam Reienr: I must say that this was a very well put together page, because one of the biggest problems with flywheels at lest in the past is the dangerous aspect with flywheels, which they addressed nicely. But I do want to add the problem wasn’t always the flywheel broke apart, but also in a crash because of its mass it would keep on moving and go right through what ever was in its path. I will make a comment which was stated above, that a flywheel does not just last a few minutes to power stuff. As a mechanical engineer I have studied similar systems, and you would be surprised what a flywheel can do, and especially with good bearings the wheels won’t slow down to quickly. However for long term storage over a period of days that isn’t always the case. For use in balancing out the power on wind and/or solar would defiantly smooth out the power generation, and because weight isn’t an issue it’s just a matter of how big is the company willing to make it.

Colin Podelnyk: Flywheel technology is surprisingly fascinating. As someone who rarely encounters information about flywheels, I was really surprised to even see flywheels in the energy alternatives wiki, but after reading this cite I now realize what a great storage device flywheels can be. On that note, if all the information presented in this article is correct, I really don't see why batteries are used in applications where flywheels could be employed. As we've all heard Dr. Hetherington say, electromagnetic energy is very organized and so the conversion to mechanical energy leads to very little power loss. With this in mind along with the comparisons between batteries and flywheels, it seems like flywheels are the obvious choice. Not only does it make sense in terms of space and energy density, but when you consider the harmful effects that batteries have on the environment, this technology begins to seem too good to be true. So what part of this argument are we not hearing?

Richard Holton- You have to take into account the engineering that would be needed to produce large storage flywheels, even with levitating flywheels any surface imperfections will result in eddy currents and the wheel becoming less stable with more angular momentum reducing the amount of energy that can be stored. If you were to build the magnets large enough to levitate the enormous flywheels it would probably be enough to just store the energy in those magnetic fields, in superconducting materials at least. So I think flywheel technology has its place in storing energy for backup generators, and smoothing energy conversion from certain non linear extraction methods but the engineering complications with large scale flywheel storage are not as productive to long term planning as other potential storage systems. Also they do pollute albeit indirectly the chemical, and mechanical processes to refine the large solid chunks of metal are enormous undertakings, the better balanced and pure the metal the better flywheel so the more chemical purification and smelting processes the more energy put in to creating the flywheels, plus their nearly inconceivable transport at the size or quantity they would need to be produced in for large scale energy storage.


Flywheel technology is a very promising future for storing energy. Delving into the topic, a conceptual analysis will explain how flywheels work. How they can store energy, possible future applications, and possible limitations will then wrap up flywheel technology.

Flywheels are very “green” technology, which do not have carbon footprints. Replacing chemical batteries with flywheels will result in fewer chemical hazards that revolve around chemical battery use. Large battery systems used for power quality disturbances become very toxic areas with the large amount of chemical batteries needed.

Image provided from [14]

Conceptual Analysis


The modern flywheel energy system design has three major components: a rotor, the rotor’s bearings, and an electrical motor or generator to transfer energy. The rotor makes use of its low friction bearings to rotate at high RPMs creating rotational kinetic energy. This is harnessed and transferred to an electrical motor or generator, which make the exact efficiency (from eddy currents, [9]) of the process. The design portion is maximizing this efficiency by modifying size and materials used in making the rotors and bearings.

The rotor disc requires much attention to detail in design due to friction and rotational orientation. To correct drifting from rotational axis vibrations, magnets are used to increase system stability (flux pinning, [9]). In an application sense, an important factor to the system’s design is what load the flywheel is expected to provide for and power. For these reasons, engineers will continue to improve efficiency by analyzing frictional losses and maximizing the power transfer.


Flywheels for Energy Storage

A flywheel is a simple way to store kinetic mechanical energy using a disk or rotor rotating on an axis. Flywheels use the momentum of this rotation to store energy which may be converted into work or electricity by slowing the flywheel's rotational speed down with a generator or gear system. Flywheels have been used by humans for a very long time with one of the earliest known applications being the potter's wheel. Artisans would start a heavy stone spinning on an axle and use the spinning surface of the wheel to shape clay into many types of pottery. The energy stored in a spinning flywheel is a function of the flywheels mass and the square of its rotational speed. Thus, as is described in more detail later in this report, the rotational speed of a flywheel has a greater impact on the energy density than does the mass. Flywheels are perhaps most promising as a direct alternative to chemical batteries in cases where uninterrupted direct current power is required. They could be used in situations as backup power sources in emergency situations for critical services. Major advantages of flywheels as opposed to batteries include lower annual costs for upkeep, smaller overall environmental footprint, and higher power density (often by a factor of 5-10). This means that flywheels are easier on the environment to construct and maintain and take up less valuable floor space than chemical battery alternatives. [7]

Flywheels can be separated into low speed and high speed groups which have important differences. A low speed flywheel is generally thought of as one with a maximum rpm under 10,000 while high speed flywheels may have rpm's much greater than 10,000. Low speed flywheels may be very dense and large and can deliver a large amount of power for a short period of time. These designs are ideal for emergency backup power sources. A high speed flywheel is generally lighter and smaller and may produce usable work or electrical energy for hours but in smaller quantities than a low speed type. When compared to lead-acid batteries a high speed flywheel weighs just 20% as much as the equivalent number of batteries. A modern flywheel also has a lifetime of 100,000 charge and discharge cycles while lead-acid batteries only last through 1,000 such cycles, not to mention the slow decline in capacity and efficiency with time in chemical batteries. [8]


Possible Future Applications for Flywheels

Flywheel energy storage (FES) systems have many uses in the future. Applications being looked at include the International space system, low earth orbits, hybrid electric vehicles, power quality events, and a few other scenarios.

Flywheels are a great fit for power storage in satellites and spacecraft. They can store energy captured by PV solar cells for use when solar energy is unavailable and they can also offer a gyroscopic function in controlling the movements of the satellites or spacecraft. Flywheels are also more reliable and last much longer than batteries, along with requiring less maintenance, these are very important features for space flight especially with unmanned probes that cannot be repaired once launched. Frictional resistance would also be minimal in space so the added cost of a vacuum enclosure might not be needed in this application. [8]

Flywheels have great potential to advance technology for acceleration of motor vehicles. The modern lightweight flywheel decreases the interior mass of the engine, which decreases the amount of work components of the engine do. The result is more power output, because the components will not waste as much energy driving the flywheel, and thus greater acceleration occurs. The advantage of flywheel engines, is they act as mechanical batteries, storing energy more efficiently than conventional chemical batteries. Typical mechanical batteries consist of a high speed inertial composite rotor, magnetic bearing support and control system, integral drive motor/generator, vacuum support housing and containment, compact heat removal and exchangers, instrumentation monitoring and control, and power electronics for electrical conversion.[1] This type of energy storage now has many applications, from stationary to hybrid electric vehicles. The outset was to produce “a flywheel that could be used to power pollution-free electric and hybrid vehicles.”[2]

Replacing the current first generation chemical batteries with the space station could save more than $200 million if replaced by flywheels [14]. The space station’s primary power source is the sun. During eclipse, it needs to be powered by batteries, and flywheel’s available efficiency would benefit the system. Flywheels can cause spin problems due to its own spin, but this can be reduced by counter rotating flywheels. With hybrid electric vehicle prototypes emerging, flywheels offer very efficient solutions. With regenerative braking, energy is put into the flywheel to be used later instead of turning the energy into heat while clasping brake rotors. Stored energy in flywheels also helps the engine by allowing it to keep a nearly constant speed which reduces fuel, wear, and tear by eliminating the engines short burst needed to accelerate the vehicle. High speed trains are also looking into the technology. New York has fitted two subway cars with flywheel units spinning at 15,000 rpms and ended up saving 25% to 40% on formally consumed energy per car [12]. Power quality events include power outages, voltage lags, and harmonic distortions. The use of flywheels can help eliminate all of these. Flywheels also pair together well with wind turbines and photovoltaic arrays by buffering out the variability of power produced. With wind turbines, paring flywheels improves overall efficiency [13]. Yet another potential for flywheels is to replace the steam systems on aircraft carriers that launch aircraft. Hydraulics being paired with flywheels opens the door for lifting equipment by harnessing potential energy during drops where gravity pulls the objects down. This could include oil pump lift equipment, cranes, hoists, and hydraulic elevators.


Limitations of Flywheels

Flywheels are used as another means of storing energy. They are being implemented in many different ways including space applications, hybrid vehicles, wind and solar energy application as well. As with any technology, there are always limitations to its use. For flywheels, one of the biggest limitations is materials. The basic principle behind the flywheel is that the wheel spins and the energy that was used to make it spin is stored as it rotates. By slowing the flywheel down, energy can be taken from the flywheel and put to use in the applications mentioned above [4].

The reason for the limitations of material is because that the momentum that the flywheel can maintain limits the amount of energy the flywheel can store. In the past, steel was the material of choice for flywheels. This was because steel had a high density. Having a high density meant that, with other parameters equal, steel would store mechanical energy better, creating a more efficient energy storage device. However, even though a higher density material could store more mechanical energy, the higher density materials created higher centrifugal forces at the rim edge of the flywheel. With these higher centrifugal forces at the rim of the flywheel, many flywheels would break apart and be destroyed. This was very dangerous as the flywheel is usually spinning at very fast speeds. The destruction of the flywheel would send pieces of the flywheel flying at very high speeds. Because of the high speeds that flywheels would self-destruct at, engineers had to limit the operational speed of flywheels to about 50 meters per second [4].

With the development of new engineered composite materials, it was learned that lighter flywheels might actually be better. Some of these new flywheels were made from materials such as Kevlar. These new flywheels could spin at much higher speeds of up to 1,000 meters per second. Because the materials were much stronger they would not fall apart at these high speeds. Engineers determined that doubling a flywheels speed quadrupled the amount of energy that it could store, while doubling a flywheel’s density only doubled the amount of energy it could store [4]. Even the Green Car Congress states that it has developed a Flybrid Flywheel that rotates at 60,000 revolutions per minute [5].

Another limitation to flywheels is friction. As the flywheel spins on an axle or any other way it is designed it has physical contact to transmit power. This physical contact between rotating part creates friction which is an energy loss in the entire system. By reducing frictional losses the efficiency of using flywheels as energy storage systems could be significantly increased. Some of the technology that has been implemented to reduce friction includes using electromagnetic bearings in place of mechanical bearings. Also, engineers designed the flywheels to be enclosed in a vacuum which would reduce the drag that the flywheel experienced while rotating.

The primary cost of flywheels is based on the material used to create the flywheel. The engineered composite materials are more expensive than materials like steel, however as stated above the efficiency is better in these composite materials. In comparing flywheels with lead acid batteries over a 20 year design lifetime the saving is anywhere from $100,000 to $250,000 [6].

As explained above there are a few limitations to flywheel energy storage, however these limitations are being reduced with expanded technology making flywheels a safe, clean and efficient way to store energy.

The chief engineering trade offs in hybrid vehicle design involve the drive capabilities of the gasoline engine and electric motor and the electric storage capacity.[10] This requirement is specific to the hybrid electric vehicle application because of the high-rate energy storage requirement that would need to come from batteries or capacitors. However, the application is not always associated with the drive train and energy storage. A German hybrid design utilizes energy storage from a flywheel to provide acceleration and braking. With an efficient controller from energy storage, all the components of a hybrid electric vehicle could see higher efficiency.[11] Designers need to make consideration of how to balance the necessity for various components, and note what parts will be the most expensive, and how to maximize efficiency while reducing cost.

Another major limitation on design is cost. The modern flywheel uses high energy magnets, which can present as potential hazards considering corrosion and possible demagnetization.[11] As technology progresses, whatever alloy used for the rotor will become a cost issue because of weight limitation and strength requirement to improve efficiency. Collectively, engineers must design within limits a product that will be feasible for consumers to buy. Hopefully the high prices of oil and petroleum will increase interest in alternative energy vehicle designs.

Concerns with flywheels include breaching a maximum speed that causes failure. Being able to slow and stop the flywheel in emergency is then critical. There are three ways to control these concerns that include having a design margin that keeps the use speed below a maximum allowed speed. Fault protection is another which can be monitored by computers. The third is containment for a failure.

Advancements with Flywheel Technology

Flywheel technology is very practical once they are refined, but much advancing still needs to be completed. Advancements include the use of fiber composites instead of steel. Steel is the older design that weighs more. Fiber composites are better with their high tensile strengths and low densities. These then weigh less which make them more applicable to more designs. Using fiber composites is also a safer alternative since they disintegrate gradually when failing as opposed to steel’s trait of bursting apart.

Flywheel advancement also concentrates on the bearing mechanisms. Ball bearings are better suited for lower speed flywheels. Higher revolution speeds result in higher temperatures due to friction of the bearings. This is also due to the actual lubricants themselves. Magnetic levitation cuts out this specific friction. Magnetic levitation also helps with reducing vibrations within the flywheel. Other advancements include reducing drag on the actual wheel. Use of vacuums can help out, but is an expensive alternative that needs time to become more cost efficient. Using a mixture of air and helium together helps cut out windage loss as well with a lower cost.


In conclusion, flywheels are very practical and environmentally friendly energy storage systems. The concepts are simple, energy storage can be efficient, there are many future applications, and known limitations help with efficient design.


[1] Bitterly, J. (1998). Flywheel Technology: Past, Present,and 21st Century Projections. Retrieved May 21, 2008, IEEE. Web site: http://ieeexplore.ieee.org/iel4/62/15286/00707557.pdf?isnumber=15286&prod=JNL&arnumber=707557&arSt=13&ared=16&arAuthor=Bitterly%2C+J.G.

[2] Steven, A. (1993) Flywheels put a new spin on electric vehicles. Retrieved May 21, 2008, Mechanical Engineering-CIME. Web site: http://www.allbusiness.com/professional-scientific/scientific-research-development/384726-1.html

[3] NASA's Aeronautics Mission Directorates. (2004). Reinventing the Wheel. Web Site: http://nasaexplores.com/show2_articlea.php?id=04-015

[4] Castelvecchi, Davide. (2007). Spinning Into Control. Science News. Vol. 171 Issue 20, p312-313, 2p

[5] Hilton, Jon. (2007). Flybrid Flywheel Hybrid System Passes First Crash Test; Developing for Road Cars as Well. Green Car Congress. Web Site: http://www.greencarcongress.com/2007/10/flybrid-flywhee.html

[6] Blankinship, Steve. (2007). Energy Storage Systems Get a New Spin. Power Engineering. Vol. 111 Issue 2, p72-73, 2p

[7] U.S. Department of Energy. Federal Energy Management Program. (2003). Federal Technology Alert: Flywheel Energy Storage. Web Site: http://www1.eere.energy.gov/femp/pdfs/fta_flywheel.pdf.

[8] Regenerative Power and Motion. (2002). Flywheel Facts and Fallacies. Web Site: http://flywheel.esmartbiz.com/facts.htm

[9] Energy Topics. (2008) Flywheel Energy Storage. Web site: http://en.wikipedia.org/wiki/Flywheel_energy_storage

[10] Bates, B. IEEE Spectrum. (1995). Getting a Ford HEV on the road. Accessed May 28th 2008. Web site: http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=392799

[11] West, J. Power Engineering Journal. (1993) DC induction, reluctance, and PM motors for electric vehicles. Accessed May 28th, 2008. Web site: http://ieeexplore.ieee.org/iel3/1425/5973/00231118.pdf?arnumber=231118

[12] Access to Energy Newsletter. (2004). The Flywheel Advances. Retrieved May 26th, 2008. Web site: http://www.accesstoenergy.com/view/atearchive/s76a3956.htm

[13] Ghedamsi, K. (2007). Control of Wind Generator Associated to a Flywheel Energy Storage System. Retrieved May 26th, 2008, Science Direct. Web site: http://0-www.sciencedirect.com.oasis.oregonstate.edu/science?_ob=ArticleURL&_udi=B6V4S-4RW4RWH-2&_user=576687&_coverDate=09%2F30%2F2008&_alid=746213065&_rdoc=2&_fmt=high&_orig=search&_cdi=5766&_sort=d&_docanchor=&view=c&_ct=295&_acct=C000029364&_version=1&_urlVersion=0&_userid=576687&md5=063310b45cdd2bb656f335ddbb993a66 or from (www.sciencedirect.com)

[14] Liu, Haichang. (2006). Flywheel Energy Storage – An Upswing Technology for Energy Sustainability. Retrieved May 26th, 2008, Science Direct. http://0-www.sciencedirect.com.oasis.oregonstate.edu/science?_ob=ArticleURL&_udi=B6V2V-4MG6P8C-1&_user=576687&_coverDate=05%2F31%2F2007&_alid=746213065&_rdoc=22&_fmt=high&_orig=search&_cdi=5712&_sort=d&_docanchor=&view=c&_ct=295&_acct=C000029364&_version=1&_urlVersion=0&_userid=576687&md5=46f5f5fa253242c018451b6edf649554 or from (www.sciencedirect.com)

modern_flywheel_technology.txt · Last modified: 2008/06/05 23:22 by holtonr
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