A flywheel energy storage (FES) system is an electricity storage technology under the category of mechanical energy storage (MES) systems that is most appropriate for shorter period applications. An FES system works with kinetic energy, where the surplus electricity is stored in a high rotational velocity disk-shaped flywheel. The stored energy will be later used to drive a generator and thereby produce electrical power.

Typically, users of high-speed flywheels must choose between two types of rims: solid steel or carbon composite. The choice of rim material will determine the system cost, weight, size, and performance. Historically, flywheels are typically made of steel and rotate on conventional bearings; these are generally limited to a maximum revolution of up to 10,000 RPM.

High energy density or high-speed flywheels can be made of carbon fiber composites and employ magnetic bearings, these rims are both lighter and stronger than steel, which means that they can achieve much higher rotational speed, enabling them to revolve at a rotational frequency in excess of 100,000 RPM. Still, the amount of energy that can be stored is proportional to the object’s moment of inertia times the square of its angular velocity. To optimize the energy-to-mass ratio, the flywheel must spin at the maximum possible speed. Rapidly rotating objects are subject to significant centrifugal forces . So, while dense materials can store more energy, they are also subject to higher centrifugal force and thus may be more prone to failure at lower rotational speeds than low-density materials.

To calculate the maximum energy to be stored with each material you can use the following equation.

E-energy. I-inertia. M-mass. r-radius. w-angular velocity.

Flywheels are hollow cylinders, which gives us Mr2 for moment of inertia. Second, determine the limits to angular velocity due to material used: ρ = density, r = radius, ω = angular velocity, σ = tensile stress (maximum before breaking).

Third, substitute the maximum angular velocity into energy equation.

Table 1. Maximum flywheel energy storage of various materials. (Material properties produced from commercial material suppliers.

More advanced FESS achieve attractive energy density, high efficiency and low standby losses (over periods of many minutes to several hours) by employing four key features:

• rotating mass made of fiber glass resins or polymer materials with a high strength-to-weight ratio,
• a mass that operates in a vacuum to minimize aerodynamic drag,
• mass that rotates at high frequency, and
• the rotating mass is supported by air or magnetic suppression bearing technology to accommodate high rotational speed.

These conditions will allow flywheels to go from full discharge to full charge within a few seconds or less (Figure 1). Some other key advantages of FES are:

• Temperature range: Flywheels are not as adversely affected by temperature changes, can operate at a much wider temperature range, and are not subject to many of the common failures of chemical rechargeable batteries.
• Unlimited Lifecycle: a flywheel potentially has an indefinite working lifespan, can perform for decades at full capacity.
• Low Maintenance: Most modern flywheels are typically sealed devices that need minimal maintenance throughout their service lives. Magnetic bearing flywheels in vacuum enclosures, do not need any bearing maintenance.
• Low Environmental Impact: being largely made of inert or benign materials, with no use of any chemicals and with no toxic products and easily recyclable.
• Fast-response: they have a very fast response and ramp rates.

As has been discussed on our previous article on supercapacitors (Supercapacitors…What's super about them?) and the supercapacitos Forum ("Supercapacitors: Providing fast-response to LDES"), Flywheels can bridge the gap between short-term ride-through power and long-term energy storage with excellent cyclic and load following characteristics as can be seen in Figure 1.

Figure 1. Energy storage technologies,comparison as a function of specific power & energy.

Some considerations to take into account of FES:

• High performance flywheels can explode, so they are not as safe as other energy storage systems (e.g. iron flow batteries, some new battery chemistries or other mechanical energy storage systems).
• They have lower energy density than most electrochemical batteries
• The physical arrangement of batteries can be designed to match a wide variety of configurations, whereas a flywheel at a minimum must occupy a certain area and volume, because the energy it stores is proportional to its angular mass and to the square of its rotational speed. As a flywheel gets smaller, its mass also decreases, so the speed must increase, and so the stress on the materials increases. Where dimensions are a constraint, (e.g. under the chassis of a train), a flywheel may not be a viable solution.
• They are not a solution for LDES (>4h) requirements
• To avoid low efficiencies of the system flywheels requires an active magnetic levitation, these systems may be complex and expensive.

Market & Applications

These characteristics make flywheels especially attractive for applications requiring frequent cycling given that they incur limited life reduction if used extensively. FES are especially well-suited to several applications including:

• electric service power quality and reliability
• ride-through while gen-sets start-up for longer term backup
• frequency response
• FESS may also be valuable as a subsystem in hybrid vehicles that stop and start frequently as a component of track-side or on-board regenerative braking systems.
• In advanced locomotive propulsion systems & advanced technology transit buses
• In satellites to control direction
• In wind turbines

The global flywheel energy storage system market size was valued at USD 326.43 Million in 2021 and is expected to expand at a compound annual growth rate (CAGR) of 9.8% from 2022 to 2030. The growing energy storage market and automobile industry, globally, have provided a boost to the market. Increasing demand from UPS and data center application segments has driven the market for flywheel energy storage systems, within this region. Similarly, distributed energy generation, which involves the generation of power at the place of consumption, is expected to result in augmented demand for the flywheel energy storage systems, within this region. The U.S. held the largest share among all countries in the North American flywheel energy storage market in 2021, and accounted for the largest revenue share of more than 78.22%

Figure 2. U.S. Flywheel Energy Storage System Market Size, By Application, 2020 - 2030 (USD Million).

The North American flywheel energy storage market is characterized by growing demand for UPS systems, to maintain a continuous supply of power intended for commercial and industrial applications. While the utility-scale market is still developing, flywheels are a inherently safe solution for fast-response and frequency and voltage regulation in the grid. Companies like Amber Kinetics are developing solutions with a focus on utility-scale systems utilizing flywheels, to learn more about Flywheel technology a join us on our next Power & Utilities Event, next Thursday 28 at 10 AM CT.

If you have any questions or would like us to address a certain aspect of energy storage please leave your comments here, on the event page or email us directly. At this Forum we will also have a special guest from Salt River Project (SRP) to share their thoughts on their strategy towards LDES. See you there!

References & Further Reading

• Flywheel Energy Storage. Benjamin Wheeler. October 24, 2010. Submitted as coursework for Physics 240, Stanford University, Fall 2010.
• Flywheel Energy Storage Systems (FESS). Energy Storage Association - American Clean Power.
• Flywheel Energy Storage System Market Size, Share & Trends Analysis Report By Application (UPS, Distributed Energy Generation, Transport, Data Centers), By Region, And Segment Forecasts, 2022 - 2030.. Grand View Research. 2022.