Mechanic Energy Storage and High Speed Machines

An Introduction to Flywheel Energy Storage

Since 2010 the Institute of Electrical Measurement and Measurement Signal Processing has been working in the field of Flywheel Energy Storage Systems (FESS). An FESS is an energy storage device, similar to a battery, but instead of storing the energy chemically, it is stored in kinetic form in a rotating disc. Typically, the rotor is accelerated by an electric motor (charging process) and decelerated when the motor is switched to generative operation (discharging). The stored energy is a function of the angular velocity squared, which is why there is a trend towards faster and more compact flywheels. The following image shows the topology of various FESS concepts.

Previous Research Activities

In the past years a variety of scientific papers on FESS have been published, partially in cooperation with the Institute for Machine Elements and Methods of Development. Furthermore, 2 full scale prototypes of flywheel energy storage systems for automotive applications have been realized:

Clean Motion Offensive: A 40 kW / 100 Wh system for a parallel hybrid personal car
TSA HighFly: A 145 kW / 800 Wh system for a heavy commercial vehicle in urban traffic

FESS ca not only be used in automotive applications for break energy recuperation or load averaging, but are also suitable for stationary use. However, the requirements regarding overall weight, energy density, self-discharge and energy contend differ significantly from those of mobile systems. Within a research project a low cost flywheel energy storage system for decentralized photovoltaic systems has been developed and tested. Reducing self-discharge (=torque loss of the bearings and windage losses of the rotor) while keeping the costs low was the key challenge of the project. A novel bearing system was developed and tested on a special spin-down test rig, proofing that the torque loss of a 30 kg steel rotor supported by roller bearings was only a few Nmm at 6000 rpm. The bearing system is a key element not only in FESS, but any high speed rotating machinery. Hence, high speed bearing systems (both magnetic and rolling element) are among the fields of research of the institute. Aspects that have been investigated so far include:

Lifetime prolonging measures through active and passive vibration control
Reduction of frictional torque loss
Characterization of thermal properties of roller bearings
Special test rigs and measurement systems for high speed bearings

Antriebsstrank eines PKW mit Flywheel — © Armin Buchroithner

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