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Electrical Storage
Energy storage flywheels 12.4.2017

The figure shows the test rig for the superconducting bearings during commissioning.
© Babcock Noell GmbH

Faster power storage with high cycle rate

Energy storage flywheels as short-term intermediate storage for regenerative energy generation systems can help keep the power grid stable. With good efficiency, they absorb or emit high power levels – while at the same time having an extremely durable cycle. In the ENERSPHERE project, scientists are working on a flywheel rotor with a superconducting bearing. This makes it extremely intrinsically safe and robust.

Project status Near completion
Typical system size – energy [MWh] several kWh to several 10 kWh
Typical system size – output [MW] 0.5 to 1
Volumetric energy density [Wh/l] 80 (related to the rotating mass respectively)
Gravimetric energy density [Wh/kg] 50
Volumetric power density [W/l] 9,000
Gravimetric power density [W/kg] 6,000
Efficiency AC/AC to be measured later
Storage loss [1/d] to be measured later
Cycle durability (80% discharge level) anticipated > 200,000
Service life of the system (1 cycle/day) 20 to 40 years
Typical discharge time Minutes
Response time when preparing the energy Several ms
Typical period between storage and withdrawal Minutes to hours
Areas of application All applications in which a higher cycle rate and higher power are to be used to offset fluctuating generation or load.
Examples Reactive power compensation is also possible due to the inverter.
Project duration April 2014 until September 2017

The project partners are developing a first demonstrator for energy storage flywheels with superconducting bearings. The reference application for this development is its use as an energy storage system in combination with regenerative energy generators, such as photovoltaic (PV) systems. The storage system offsets deviations between weather forecasts and the actual energy produced. Additionally, it provides system services such as second reserve, reactive power provision in order to achieve voltage stability, and active filtering of harmonics. The researchers base their specified goals on this reference application: The storage system should provide between 250 and 500 kW. In order for the storage system to be able to offset short-term, weather-related power fluctuations of PV systems, it must have an energy content of at least 3 kWh, while the aim is to achieve 6 kWh.

The development and design work for the components and the overall construction of the flywheel and the inverters have largely been completed. Whenever necessary, the scientists have produced test rigs or models in order to ensure the correct design. They are currently building two flywheel systems as demonstrators. The tests are due to begin at the start of 2017.

  • The figure shows the test rig for the superconducting bearings during commissioning. © Babcock Noell GmbH
  • Schematic view of the grid involvement, for example parallel to photovoltaics and battery storage systems. © Babcock Noell GmbH
  • Here, the stator of the motor or generator can be seen. © IMAB, TU Braunschweig

Long-term stability and low loss

The flywheel offers a very high power density. However, the particular advantage is that the storage capacity does not change with either the cycle rate or duration of use. Unlike electrochemical storage systems, flywheels also need no air conditioning. As a result, the storage concept is highly attractive, particularly in the short-term range of minutes. The superconductor bearings developed in the project operate in a non-contact, maintenance-free way. The bearing is also intrinsically stable and requires no active regulation.

Alongside the innovative bearing of the flywheel rotor, the scientists are also optimising the additional components such as the fibre composite rotor, the motor generator and the inverter in order to meet the specific requirements. Despite the use of superconductors at temperatures of around minus 200 degrees Celsius, the cooling of the bearings requires very little power. The main losses occur, as is also the case with other storage concepts, in the MW class inverters, which typically contribute several percentage points of the power.


Development of the superconducting bearings and the rotating mass:

The work package comprises the design and production of the superconducting bearing, the electrodynamic absorber and the construction of the entire rotor. Due to the soft bearing of the rotor, the rotor dynamics are designed very differently to classic bearings: The working range of the rotor is within the so-called over-critical range.

Energy conversion:

In the flywheel, a new-style, electrically regulated permanent magnet synchronous machine is used as a motor or generator. The machine is fully integrated in the flywheel. In the project, one drive inverter and one grid inverter are constructed from commercial components for extracting and introducing energy.

Containment and cryosystem:

In order to reduce gas friction and for thermal insulation of the cryogenic bearing, the flywheel rotor is housed in an evacuated container which at the same time serves as a safety container against mechanical damage. In order to cool the system, durable and maintenance-free cryocoolers are used.

System integration:

This package contains the control technology of the components, as well as the interfaces to the power grid. The systems should be tested under realistic conditions, as they are required by the distribution grid or suppliers of regenerative generation.

Supported by: The Federal Government on the basis of a decision by the German Bundestag


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Highly dynamic energy storage flywheels for industrial and grid applications (in German) (PDF, 2.1 MB)

Research funding

The information system EnArgus provides information on research funding, including on this project (German only).