We are back with a topic that is guaranteed to make your wheels spin. Today we are covering all things flywheel energy storage!

Flywheel Energy Storage is a form of kinetic energy storage that uses rotating discs to store and release rotational energy. While the technology has been around for decades as a form of Uninterrupted Power Supply (UPS) to provide power when main sources fail, it has more recently begun to be refined and developed. Flywheels can be used to supply short bursts of power for tasks, such as smoothing fast fluctuations in grid voltage or power output from renewable sources, regulating the frequency of alternating current as generators may briefly operate out of sync with the grid, spinning up/down turbines to maintain constant generator speed (frequency), storing surplus electricity generated by wind turbines or solar photovoltaic systems at periods of low demand, powering mobile machinery during peak load periods and fine tuning the profile of energy produced by intermittent energy sources to match usage. Flywheels are being used to improve power quality for renewable power projects, making the devices of more interest and use in today’s greener world.

How Does Flywheel Energy Storage Work?

The flywheel energy storage system is useful in converting mechanical energy to electric energy and back again with the help of fast-spinning flywheels. This system is composed of four key parts: a solid cylinder, bearings, a motor/generator and a vacuum sealed casing. To create kinetic energy, the motor derives energy from the electric grid to power the cylinder or disk to spin at a rate of up to 60,000 RPM. Because a flywheel must be accelerated by an external force before it will store energy, it is considered a “dynamic” storage system. The rate at which the flywheel spins remains nearly constant because of the vacuum-like container, which prevents friction from slowing the revolution. Once there is a need for electricity, the motor converts to a generator and releases the energy that has been stored. This device is illustrated below.

Video Credit: NAVAJO Company on YouTube

The Pros and Cons of Flywheel Energy Storage

Flywheels are an excellent mechanism of energy storage for a range of reasons, starting with their high efficiency level of 90% and estimated long lifespan. Flywheels can be expected to last upwards of 20 years and cycle more than 20,000 times, which is high in comparison to lead-acid (2,000 cycles), lithium-ion (<10,000 cycles) and sodium-sulfur batteries (2,500-6,000 cycles). Another advantage is the flywheel energy storage system’s ability to provide energy with little start up or transition time. They do not emit any toxic material, making this energy storage method an environmentally friendly concept. These systems are capable of providing short-term energy storage to the electrical grid and are more efficient than compressed air or pumped hydropower for this application. Flywheels are considered tertiary systems in the context of sustainable development, but flywheel energy storage systems can contribute significantly to a more flexible power grid based on renewable sources.

Just like with all things, there are drawbacks to using the flywheel for energy storage. The flywheels have a low energy density of 5-30Wh/kg and high power loss due to self-discharge. Flywheels also cannot provide continuous base load supply, unlike batteries or conventional pressurized fluid system energy storage machines, such as pumped-storage hydroelectricity. However, there is active research to continue development in these areas for better system results. Studies have also been conducted to investigate possibilities of coupling flywheels with other storage systems. The first study combined flywheels with lead-acid batteries to store energy from a wind power system. This combination utilized the quick response time of a flywheel and the longer discharge duration of a battery. This prompted common use of flywheels in conjunction with batteries as a quick-burst power option. The second study focused on creating a flywheel storage system that was reduced in size and weight. This was done by placing a supercapacitor inside of the disk, which increased the instantaneous power release.

Are Flywheels in Use?

Not only is research in the area expanding, but possible use in residential spheres is as well, with designs from Bill Gray, Silicon Valley inventor. Gray’s creation is coined Velkess, for VEry Large Kinetic Energy Storage System. The Velkess flywheel’s design allows for more than 80 percent efficiency and is expected to store 15 kilowatts per hour, which is enough to run an average home for one day. The cost of a flywheel energy storage system is $6,000. Each kilowatt is priced at $1,333 a kilowatt. This flywheel energy storage design is a viable electricity source in homes. It functions to meet peak power demands within 25 seconds, allowing for significant savings in energy costs.



Although small in relation to other systems, the use of flywheel energy storage is expanding. Flywheels are now being designed to take on a growing role in renewable energy integration. As research on this type of storage continues, we should see conceptual improvements, including increased energy density and decreased power loss. With improvements of the flywheel energy storage design, use could increase in both industrial and residential domains.

Andrew Schaper is a professional engineer and principal of Schaper Energy Consulting.  His practice focuses on advisory in oil and gas, sustainable energy and carbon strategies.

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