New design reduces system costs
Flywheel energy storage systems can play an important role in stabilising the electricity grid. When required, they provide high outputs within milliseconds. The short-term electricity storage systems also act as active voltage filters for optimising the electricity quality. In the KoREV-SMS project, scientists are developing a flywheel energy storage system with an external rotor design. This results in a higher energy density with lower acquisition costs compared to conventional flywheel energy storage systems.
|Project status||Component tests|
|Typical system size – energy [MWh]||0.001-0.01|
|Typical system size – output [MW]||0.05-0.2|
|Gravimetric energy density [Wh/kg]||10-30 (rotor mass only)|
|Gravimetric power density [Wh/kg]||300-1,200 (rotor mass only)|
|Storage loss [1/d]||> 100 %, therefore deployment as short-term storage system|
|Cycle durability||> 100,000, irrespective of the discharge depth|
|Service life of the system (1 cycle/day)||Only depends on the lifetime of the power electronics (approx. 10 years)|
|Typical discharge time||1-5 min|
|Response time for supplying energy||< 100 ms|
|Typical period between storage and withdrawal||Max. 10 min|
|Example application areas||Load profile smoothing, grid stabilisation, uninterrupted power supply, isolated networks with high proportions of renewable energies|
|Project duration||November 2015 until November 2018|
New, high-strength fibre-reinforced composites now enable flywheel energy storage systems to attain speeds and designs that would have cracked the steel or titanium rotors previously used. Since the kinetic energy increases quadratically with the speed of the rotating mass, much greater performances can be achieved with a smaller weight and a compact size.
In the KoREV-SMS project, the scientists want to continue developing this technology for competitive, long-lasting energy storage systems that provide system services for ensuring a stable European power grid. In particular, the research intends to reduce the specific acquisition costs and to ensure the system availability during maintenance intervals of three to five years. This could reduce the life cycle costs below those of competing storage devices such as lithium-ion batteries or supercaps.
The researchers can build on a previous research project that provided the first worldwide verification that flywheel energy storage systems with an external rotor design work. In this design, the flywheel is realised as a hollow cylinder. This rotates around a fixed stator. A shaft through which the flywheel is coupled to conventional internal rotor components does not exist. The flywheel, which is mounted with non-contact magnetic bearings in a high vacuum, is accelerated up to 21,000 rpm by means of an integrated electric, high-rev motor.
Higher speeds enable the researchers to increase the energy density of the storage system and thus reduce the specific acquisition costs. They want to maximise the strength of the flywheel made of fibre-reinforced plastic (FRP) without compromising the reliability. Since many different fibre-matrix combinations and different production methods are available for the rotor, they are determining their strength values experimentally. The operating strength as a function of the relevant operating parameters (temperature, load cycle number, load duration) will be determined experimentally for each material candidate. For this purpose, an appropriate test method must first be developed and qualified. Statistically validated series investigations of relevant fibre matrix combinations are being used to determine which materials and production methods are suitable for significantly increasing the energy content and density. The results will then be transferred to fully scaled up FRP flywheels.
Testing the operational strength
The scientists are testing the operational strength of the flywheel made from FRP under operating conditions. In order to reduce both the test rig costs and the test duration, they are using a scaled, representative test rig. The test rig is designed so that the life cycle of a flywheel energy storage system can complete more than 200,000 load cycles in an accelerated manner within two to three months. This makes it possible to investigate the operational strength of at least six different rotors during the project running time. Finally, the rotors will be loaded on the test rig until they crack. The findings from these tests will be used to optimise the protection devices.
Robust, electronically reduced magnetic bearing sensors
In the active magnetic bearing, a sensor system continuously measures the exact position of the rotor. The necessary high resolution and bandwidth make the sensor system expensive. The researchers therefore want to reduce this expenditure to the minimum necessary structure. For this purpose they are pursuing two approaches. On the one hand, they can further develop the sensor concept into a minimised sensor system that uses only passive sensor components within the vacuum. On the other hand, the further development of the active magnetic bearing to form a self-sensing bearing would also be possible. The latter enables the position to be determined via the indirect measurement of the bearing inductance. Both sensor concepts reduce not only the costs but also the probability of failure, since fewer components are necessary.
Redundant magnetic bearings
In recent years, magnetic bearing systems have shown very impressively that they achieve longer service lives and reduced downtimes than conventionally mounted systems. However with regard to the overall flywheel energy storage system with an external rotor design, the power electronics for the magnetic bearing nevertheless has the highest failure probability. In a sub-project, the scientists are therefore developing a redundant magnetic bearing system. It incurs only slight additional costs, but in the event of damage it ensures safe operation or at least a safe shut down of the entire system. The likelihood that the energy storage device fails is thereby further reduced and its availability is ensured.
Planetary retainer bearings
Retainer bearings stabilise the rotor in the event of overloads or a defect in the magnetic bearing. Retainer bearings therefore ensure a high availability of the system, but are usually subjected to extreme loads and often fail after only a few operations. In the case of the external rotor, a conventional retainer bearing is not possible. The geometry as well as the highly dynamic processes require alternative concepts. The researchers are therefore developing an innovative planetary retainer bearing concept and are testing it on a representative test rig. The findings from these tests will be used for designing retainer bearings on complete systems. The aim is to construct a viable, near-series retainer bearing system that demonstrably increases the availability of flywheel energy storage systems with the external rotor design. Conclusions about the lifespan of the retainer bearing are of crucial importance and need to be proven experimentally as realistically as possible.
System integration of the individual results
In the concluding sub-project, the researchers will collate the results from the various other sub-projects in order to investigate and verify their functionality in the overall system. In addition to increasing the operationally stable energy density, the intention is to show that the developed magnetic bearing system can be adapted to the real system. Likewise, the developed retainer bearing system shall be used on a real flywheel energy storage system with an external rotor design. In line with the overall project objective, it is also intended to verify that the partial results significantly reduce the costs per energy content and at the same time increase the availability of the system.
First of all, individual components used in the previous energy storage concept shall be tested on representative test rigs. In most cases, the test rigs will have to be specially designed and constructed for this purpose. Based on the test results, the components will then be revised and adapted. Finally, the adapted components will be integrated into an already existing energy storage unit and tested in the overall system. Several representative test rigs for components are currently being designed and constructed. The tests will start at the end of 2016.