The fly wheel floats using two magnet rings. The fly wheel itself (not shown) is mounted on the top magnet ring which floats over the lower ring. The arrows show the direction of the magnetization of the individual magnets in the rings. The color scale shows the resulting magnetic field (red corresponds to high field). Illustration: DTU Energy

Kick-starting electrical trains using floating magnets

Thursday 26 Jan 17


Rasmus Bjørk
Professor, Head of Section
DTU Energy
+45 46 77 58 95

3118 km electrified railway

The Danish railway system is split between Banedanmark operating and maintaining the tracks and infrastructure, and the actual train operators.

Banedanmark currently operates 1756 km of electrified railroad tracks. In 2014, the Danish government decided to electrify the lines Roskilde-Kalundborg, Fredericia-Aarhus, Aarhus-Aalborg, Aalborg-Frederikshavn and Vejle-Struer, adding additional 1362 km of railroad tracks, bringing the total to 3118 km.

Contact info

For further information, please contact:

DTU Energy, Associate Professor Rasmus Bjørk,, phone: +45 4677 5895

WattsUp Power, Martin Speiermann, phone: +45 4026 2610

A novel floating magnetic flywheel will reduce peak loads on the electricity grid when the train services in Denmark become fully electrified. The flywheel and the integrated management systems will be developed by DTU Energy, DTU Electrical Engineering, Banedanmark and the company WattsUp Power A/S.

Electric trains starting from a standstill have the same challenge as trucks and supertankers: it requires large amounts of energy to put a stationary object in motion, whereas the energy demand decreases when the object is underway. Electric trains have the additional challenge that while trucks and supertankers carry their own fuel, the electric train draws power from the power grid with a brief but heavy load during startup.

"It is bad for both the individual substation and the overall power grid when there is a very heavy but rather short-lived peak, and the problem will be significantly increased due to the comprehensive electrification of the Danish railway network," says Associate Professor at DTU Energy, Rasmus Bjørk

Heavily oversized

"Based on the test results we will be able to evaluate the technology's suitability, if it is able to solve the problems with peaks on the grid, and whether it is economically sensible "
Associate Professor Rasmus Bjørk, DTU Energy

The transformer stations of Banedanmark, who are responsible for the Danish railroad tracks, are currently heavily oversized to provide the necessary power for the electric trains to start and to soften the influence of the short-lived peaks on the overall grid. With Banedanmark being tasked to electrify an additional 1362 km  of the Danish railway network, huge expenses to build new oversized transformer stations can be foreseen, unless a cheaper and equally effective replacement is found.

Now, collaboration between DTU Energy, DTU Electrical Engineering, Banedanmark and the Danish company WattsUp Power A/S, supported by the Danish Energy Technology Development and Demonstration Program (EUDP), will develop an energy storage system based on a magnetic flywheel, designed and produced by WattsUp Power. This flywheel will continuously draw a steady but small power load from the power grid, storing the energy in the system as kinetic energy to be released in a high output of power when a starting electric train needs a helping hand.

Energy storage by means of a flywheel is a well-known technology, working by accelerating a rotor (flywheel) to a very high speed and maintaining the energy in the system as rotational kinetic energy. Once the wheel rotates, it only needs minimal amounts of energy to keep rotating. Normally some energy would be lost through friction with the surroundings, but this loss can be minimized by letting the flywheel float in a magnetic field in vacuum, saving all the energy in the system for use by, e.g., electric trains.

"Power WattsUp already has a magnetic flywheel which functions quite well in smaller systems in private households. But substations are located right next to the rail tracks and a floating flywheel is quite sensitive to the vibrations from heavy trains passing by. Additionally, the electronics of the flywheel doesn’t function properly with the electric grid to trains yet," says Rasmus Bjørk.

New concepts needed

This is where new concepts are needed. DTU Energy has for years conducted research in magnetic materials and design of magnetic fields for use in magnetic refrigeration.

Read about DTU Energy’s research in magnetocaloric materials

This unique expertise is essential when designing a magnetic bearing able to keep the flywheel from WattsUp Power hovering and functional despite vibrations from nearby heavy traffic. DTU Electrical Engineering will be designing and optimizing the electronics allowing the flywheel to plug into the grid, enabling it to provide the desired power when needed. The new optimized magnetic bearing is expected to increase the lifespan of the flywheel to more than 20 years in the electrical environment of a transformer.

The optimized flywheel will be tested on a small scale at an electric transformer substation near Kolding.

"Based on the test results we will be able to evaluate the technology's suitability, if it is able to solve the problems with peaks on the grid, and whether it is economically sensible and thus worth integrating in the 2026 plan for electrification of the Danish railway network," says Rasmus Bjørk. If the design behind the magnetic flywheel works out, it can potentially also be used to limit fluctuations in the power produced from renewable energy sources, like wind turbines.

The 2.5-year project is supported by EUDP with 9.5 million DKK.

Using flywheel technology as a mechanical battery

Flywheel systems acts as a kind of mechanical battery based on the principle of conservation of energy: supplying energy to the system increases the speed of the flywheel, or rotor, to high speed, after which the energy is maintained as rotational kinetic energy. When energy is extracted from the system, the rotational speed of the flywheel is reduced. WattsUp Power has developed a flywheel able to rotate more than 10,000 revolutions per minute, equivalent to the storage of 10-100 kWh energy.

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