Flywheel energy storage

Smart grids, clean renewable-energy power plants, and distributed generation, which are the main pillars of future clean energy systems, strongly require various types of energy storage units as part of their hardware chain. As discussed in the first chapter of the book, energy storage devices with a mechanical operation basis are typically expected to have a longer life span and the capability to operate on larger scales than those using electrical or electrochemical energy, though they will surely suffer from some limitations and shortcomings too. FESS has been used on various scales as a mechanical energy storage system so far (which is the focus of this chapter) (Arabkoohsar & Sadi, 2020). The main part, let's say the heart, of a FESS is the rotating disk ( called the flywheel), of which there are many examples of applications today that could help us understand the operating principle of a FESS. As one of the earliest applications, James Watt used a flywheel in his steam engine and did a physical examination of its behavior. Today's other applications of flywheels in industrial uses are in spinning machines, pottery wheels, windmills, watermills, etc. Indeed, almost all wheel-based methods of using natural energy for applications, such as agriculture, pottery, and spinning, use flywheels as a source of energy or energy transfer medium (Ostergard, 2011). All combustion engines, including those of planes, trains, ships, and vehicles, use flywheels to smooth out the delivery of power from the motor to the machine (Stodola, 1927).

For FESS itself, however, the most important milestone was met when NASA investigated this technology for space applications in the 1960s and concluded that it was a promising solution for space missions back in the 1970s (Bitterly, 1998). In the beginning, they considered FESS as one of the storage candidates; however, due to practical and technological limitations, and thus too low efficiency, there was a need for further research and development to make it a feasible solution. Finally, with the development of electronic converters, electric machines, high-strength com­posites, and magnetic bearing systems over the years, FESS was able to compete with conventional storage technologies such as batteries and supercapacitors.  Flywheels (the disk) are generally used for three mechanical purposes, all of which are kinds of energy applications, but only one is specifically about energy storage. These are:

•    In the absence of smooth continuous energy, to provide continuous smooth energy. For example, in reciprocating motors, flywheels are used because the torque produced by the motor is discontinuous.
•    A flywheel is used to store energy and then release it. In some cases, energy is released at a speed that the energy source cannot.
•    Measure or maintain direction in navigation. In this application, the principle of conserva­tion of angular momentum is used (Meijneke et al., 2021).