Flywheel storage

NASA G2 flywheel, speed 60,000 / min, amount of energy 525 Wh, power 1 kW

Flywheel storage is a method of mechanical energy storage in which a flywheel (also called “ rotor ” in this context ) is accelerated to a high speed and energy is stored as rotational energy . The energy is recovered by inductively coupling the rotor to an electrical generator and thereby slowing it down.

They are mostly used to balance peak loads , smooth power peaks , recuperation in electric vehicles and also as a system for uninterrupted power supply in hospitals and industrial plants.

Working principle

A typical system consists of a flywheel (rotor) that is connected to an electric motor - generator combination.

To charge the accumulator, the flywheel is set in motion, for example by means of an electric motor. A high speed corresponds to a high rotational energy. With a connected generator, this energy can be converted back into electrical energy if required. The flywheel transfers its rotational energy to the generator.

Type of stored energy

Mass moment of inertia  :	${\displaystyle J_{m}=\int _{m}r^{2}\,\mathrm {d} m}$
Angular velocity  :	${\displaystyle \omega _{m}=2\pi \cdot n_{m}}$
Stored rotational energy  :	${\displaystyle W_{\text{kin}}={\frac {1}{2}}J_{m}\omega ^{2}}$

where stands for the rotating body or in the integral for its volume and for the speed or frequency (= number of revolutions / time) of this body. ${\displaystyle m}$ ${\displaystyle n_{m}}$ ${\displaystyle f}$

Practical technique

Flywheel accumulator used in Formula 1

Most flywheel storage systems work electrically to accelerate the rotor with an electric motor and to decelerate it with an electric generator. However, systems are also being developed that use mechanical energy directly .

The rotors of systems with high speeds are made of carbon fiber reinforced plastics (CFRP) and rotate at 20,000 to over 50,000 revolutions per minute. To keep the friction losses low, evacuated housings and magnetic bearings are used. Such systems can be brought up to nominal speed in a few minutes, in contrast to the minutes to hours that are required for charging batteries .

Some higher-performance emergency power generators also contain a flywheel, which is kept rotating by an electric motor. In the event of a power failure, the crankshaft of a preheated diesel engine is quickly set to rotate from a standstill via an electromechanical clutch. The flywheel supplies the energy to start the diesel engine and at the same time to bridge the time until the combustion engine can deliver full power.

In the 1950s, so-called gyrobuses with flywheel storage were used in Switzerland and Belgium.

In Formula 1 with FIA rules, torque accumulators are charged when braking and discharged again when accelerating ( KERS ); however, so far (2012) no team has used a flywheel KERS on a racing weekend; Until now, only battery KER systems have been used. An (electro) mechanical solution, on the other hand, can be found in the Porsche 911 GT3 R Hybrid and the Audi R18 e-tron quattro .

At the Max Planck Institute for Plasma Physics in Garching, a flywheel is accelerated within 20 minutes. It can then deliver 150 MW or 580 MVA for the ASDEX upgrade fusion experiment for 10 seconds . Other areas of application in the form of special rotating converters are the power supply of test systems in the field of electrical energy technology such as high-voltage laboratories and test fields. In this way, disruptive repercussions on the public power grid can be avoided in high-voltage tests in which greater shock loads such as those that occur in short-circuit tests .

Advantages and disadvantages

The advantages include short access times, possible deep discharge , good efficiency as a short-term storage device for energy storage in the seconds to minutes (loading and unloading with an efficiency of 90%), and a high number of cycles. For a short time, a very high performance can be called up with many storage systems.

A disadvantage is the self-discharge (3–20% per hour) caused by air friction and losses in the bearing. The losses can be minimized by a magnetic bearing, operating the flywheel in an evacuated housing and, if necessary, other measures. The shape (cross-section) of the flywheel can also have an impact on the efficiency. However, the performance for the operation of the vacuum pump and the magnetic bearings increase the self-discharge of the entire system (except in the case of hermetically sealed vacuum containers and permanent magnetic bearings).

Another disadvantage, especially in mobile applications, is the high weight. For the storage of only 10 kWh, about 200-2000 kg flywheel mass are required. In the event of the rotor bursting or tearing loose, a massive protective cover is required for mobile use or accessible installation, which makes up a large part of the mass of the overall system.

In the case of moving flywheel accumulators, as in vehicles, changes in the direction of the axis of rotation can cause gyroscopic effects that affect vehicle behavior, e.g. B. when cornering, can affect.

Flywheel storage power plant

→ Main article : Flywheel storage power plant

Flywheels can also be used as comparatively small storage power plants to stabilize power grids in terms of frequency and serve as short-term compensation storage. Other areas of application for flywheel storage systems are island networks, hybrid systems (combination of flywheel storage systems with combined heat and power plants or batteries), wind energy, the recovery of braking energy from rail vehicles and fast charging stations for electric vehicles.

See also

• Gyro drive
• Gyrobus

Web links

Commons : Flywheel Storage  - Collection of images, videos and audio files

• Book of Synergy, Achmed AW Khammas 2007
• Kinetic storage of electricity - Dynastore project
• Power storage in comparison

literature

• Flywheel energy storage on buses and trains yesterday and today . In: Eisenbahn-Revue International , Issues 11/2001 and 12/2001, ISSN  1421-2811 , pp. 512-515, 563-567.

Individual evidence

1. ↑ Torotrak Toroidal variable drive CVT ( Memento of the original from May 16, 2011 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , accessed June 7, 2007. 
2. ↑ ^{a b} Castelvecchi, D. (2007). Spinning into control. Science News , vol. 171, pp. 312-313
3. ↑ Pulsed power supply system of the ASDEX upgrade Tokamak research facility 2015, doi : 10.1109 / EEEIC.2015.7165545 .
4. ↑ ^{a b} Hans-Hermann Braess (Ed.), Ulrich Seiffert: Vieweg Handbook Automotive Technology. Springer Vieweg; 7th edition 2013, ISBN 978-3658016906 . Digitizedhttp: //vorlage_digitalisat.test/1%3D~GB%3DwxLxx_iSKiEC~IA%3D~MDZ%3D%0A~SZ%3DPA152~ double-sided%3D~LT%3D~PUR%3D
5. ↑ http://www.bine.info/publikationen/projektinfos/publikation/kinetisch-speicherung-von-elektizitaet/
6. ↑ RWTH ISEA script for the 2005 course (archived copy of the table of contents) ( memento of the original from March 12, 2017 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. 
7. ↑ https://stornetic.com/news_de.html
8. ↑ Chakratec. Retrieved November 5, 2018 .