A flywheel is a mechanical device with a significant moment of inertia used as a storage device for rotational energy. Flywheels resist changes in their rotational speed, which helps steady the rotation of the shaft when a fluctuating torque is exerted on it by its power source such as that caused by a piston-based (reciprocating) engine, or when an intermittent load, such as the motion of a piston pump, is placed on it.

Flywheel energy storage works by accelerating a rotor-flywheel to a very high speed and maintaining the energy in the system as rotational energy. When energy is extracted from the system, the flywheel's rotational speed is reduced as a consequence of the principle of conservation of energy; adding energy to the system correspondingly results in an increase in the speed of the flywheel.

MESSIB advanced flywheel energy storage systems have rotors made of high strength carbon filaments, suspended by magnetic bearings, and spinning at speeds up to 50,000 rpm in a vacuum enclosure. Such flywheels can come up to speed in a matter of minutes — much quicker than some other forms of energy storage.

The design parameters which had the greater influence on the FW configuration were:

·     the working/maximum angular speed;

·     the FW dimensions;

·     the rotor material.

The speed influences the energy storage, as higher speed means higher energy stored but it is limited by vibration problems, system performance and materials strength.

The high energy to be stored in the FW is one of the primary goals of the project, that could be achieved increasing the dimension of the wheel, its angular speed or its density. Energy constraints, FW dimensions and maximum angular speed are strictly related: high angular speed and big rotor height mean a little outer diameter and so vibration and eccentricity related problems are lower. On the other hand, big rotor dimensions (outer diameter and height) mean low angular speed with a consequent reduction of safety related problems and less strict requirements for the mechanical performance of the rotor material.

Summarizing, the energy to be stored influences the dimensions and the angular speed of the FW rotor and in turn it is limited by the mechanical characteristics of the material used.

According to the Description of Work, the FW for the MESSIB project should be able to store 300MJ.  This energy requirement would lead to a high rotational speed and a big rotor if stored in a single FW, which means problems in terms of safety, materials choice, vibration and eccentricity of the wheel as, well as instability of the shaft. To avoid all these problems, during the Steering Committee it was decided to design a FW system in series constituted by FW of 4÷30 MJ each.

Image 1: Flywheel

Video presentation of flywheel