Industry Article

Flywheel Energy Storage System Basics

September 23, 2021 by John Jeter, VYCON

Today, flywheel energy storage systems are used for ride-through energy for a variety of demanding applications surpassing chemical batteries.

Flywheels are among the oldest machines known to man, using momentum and rotation to store energy, deployed as far back as Neolithic times for tools such as spindles, potter's wheels and sharpening stones. Today, flywheel energy storage systems are used for ride-through energy for a variety of demanding applications surpassing chemical batteries. 

A flywheel system stores energy mechanically in the form of kinetic energy by spinning a mass at high speed. Electrical inputs spin the flywheel rotor and keep it spinning until called upon to release the stored energy. The amount of energy available and its duration is controlled by the mass and speed of the flywheel. 

In a rotating flywheel, kinetic energy is a function of the flywheel's rotational speed and the mass momentum of inertia. The inertial momentum relates to the mass and diameter of the flywheel. The kinetic energy of a high-speed flywheel takes advantage of the physics involved resulting in exponential amounts of stored energy for increases in the flywheel rotational speed.

Kinetic energy is the energy of motion as quantified by the amount of work an object can do as a result of its motion, expressed by the formula: Kinetic Energy = 1/2mv2

Anatomy of a High-Speed Flywheel

The main components of a flywheel are a high-speed permanent magnet motor/generator, fully active magnetic bearings, and rotor assembly construction (Figure 1).

1. A high-speed permanent magnet motor/generator incorporates specialized rare earth magnets to minimize rotor heating and maximize efficiency and reliability, allowing flywheel systems to cycle quickly without overheating. This facilitates use in demanding applications with high cycling and long-life requirements. The flywheel's rotor assembly operates in a vacuum provided by an external vacuum pump. By removing air from the rotating area of the motor, all windage losses from the system are eliminated, thereby increasing electrical efficiency.

2. The flywheel incorporates a steel mass for storage. Because steel is a well-understood, well-supported material, it avoids the technology risks associated with other materials such as composites that may offer higher energy densities but with greater risks of temperature changes and creep that can cause unbalanced loads and degrade operation over time. 

Flywheel components. Image courtesy of VYCON
3. Based on a permanent magnet motor design, flywheels can continuously cycle rapidly with minimal heat. In contrast, other motor technologies generate significantly more heat during a discharge.

4. A magnetic bearing/levitation system allows the motor rotor assembly to rotate at very high speeds with no physical contact with stationary components, optimizing efficiency and product life. Magnetic bearings virtually eliminate the need for maintenance as there are no contact points within the flywheel – no bearings to replace or repack with lubricant. 

5. A built-in power conversion module controller provides high efficiency and maximizes reliability over the flywheel’s operating life with self-diagnostic tools that can proactively prevent failures. For each application, flywheel rotational speed limits can be modified for appropriate cycling demands and other specific conditions. 

6. Real-time display provides users with views of the flywheel status, including vital parameters such as rotor speed, charged capacity, discharge event history, and adjustable voltage settings. Additional monitoring and control capabilities are available through a serial interface, alarm status contacts, soft-start pre-charge from the DC bus and push-button shutdown. 

Prime applications that benefit from flywheel energy storage systems include:

Data Centers

The power-hungry nature of data centers make them prime candidates for energy-efficient and green power solutions. Reliability, efficiency, cooling issues, space constraints and environmental issues are the prime drivers for implementing flywheel energy storage. Flywheels paired with a data center's three-phase UPS units provide instantaneous and cost-efficient backup power.

Flywheel battery. Image courtesy of VYCON

During a power disruption, the flywheel will provide backup power instantly. When flywheels are used with UPS systems (instead of batteries), they provide reliable protection against damaging voltage sags and brief outages. During power disruptions and outages, the flywheel provides the energy required to maintain the load allowing enough time for the emergency generator to start and take on the load. At this time, the flywheel recharges back up to full speed ready for the next event. The leading cause of a UPS failing to support the load is battery failure. Battery life is impacted by the number of cycles, temperature and maintenance. To improve battery life and system availability, flywheels can be combined with batteries to extend battery run time and reduce the number of yearly battery discharges that reduce battery life (Figure 2).

Medical Diagnostics

Many types of medical imaging equipment, such as CT or MRI machines can also benefit from flywheel energy storage systems. Power brownouts, surges and outages can have devastating effects on MRI equipment. Often, electricity from the power substation to a hospital is not consistent for MRI and CT operations as voltage drops or surges in power can damage the unit's refrigeration systems and prompt a hard shutdown of the MRI equipment.

Flywheels paired with the facility's three-phase UPS systems deliver clean, reliable power to the imaging suite. If there is a power outage or the power coming in from the utility is "dirty," the UPS will generate smooth, high-quality power from the flywheels. Besides needing the highest power reliability, space is often a concern. Due to the flywheel's small footprint and no requirement for dedicated cooling, the UPS and flywheels can reside in the radiology suite. Conversely, a UPS with a bank of batteries would need to be located in a larger environmentally cooled area.

Renewable Microgrids

Microgrids deployed in remote installations such as islands face daunting fuel costs if diesel generators are the power source. Photovoltaic solar panels are typically employed to minimize the need for engine generators to save costs while providing cleaner, quieter power in areas such as remote resorts requiring 200 to 300kW power sources. While solar power has many advantages, solar-powered microgrids are subject to problems during demand surges as well as sags in power due to cloud cover. Adding flywheels to this type of installation can support the entire microgrid or just the solar system to prevent power quality problems resulting from sags and surges. The fluctuating nature of power problems on an unprotected solar installation can cause damage to the connected equipment, sensitive electronics such as computers and various appliances. Because the flywheel will serve as a power conditioner, absorbing these fluctuations, operators will find that connected equipment will be far less likely to fail prematurely.

A Greener Approach to Energy

As energy needs in a broad range of applications become more complex, those responsible for assuring reliable, clean, cost-effective energy supplies within their organizations are constantly looking for solutions that can increase efficiencies while enhancing energy reliability. In many cases, incorporating flywheel technology in a new or retrofit electrical system design can serve as an excellent foundation for achieving the sometimes-conflicting goals of maximizing dependability and reducing operating costs. With the added benefit of providing an environmentally friendly energy source, flywheels with a typical 20-year service life, are a clean, cost-effective solution for any application requiring "always on" power.


About the Author

John Jeter is the Director of Sales for VYCON, Inc. in Cerritos, Calif. John received his electronics training in the US Navy and holds a B.S. in Business from San Diego State University.  He has been involved with power quality solutions for over 40 years with domestic and international experience.