What is flywheel energy storage?

Energy storage is the process of capturing and storing energy from various sources, such as solar, wind, or nuclear, and releasing it when needed, such as during peak demand, power outages, or emergencies. Energy storage can improve the reliability, efficiency, and sustainability of the power grid, as well as reduce greenhouse gas emissions and fossil fuel dependence.

There are many types of energy storage systems, such as batteries, capacitors, pumped hydro, compressed air, thermal, and kinetic. In this blog, we will focus on one of the most promising and innovative forms of kinetic energy storage: flywheel energy storage (FES).

Introduction

!A low-speed flywheel)

• What is flywheel energy storage (FES)?
  • FES is a technology that uses a rotating device, called a flywheel, to store and release energy as rotational kinetic energy.
  • The basic principle of FES is to accelerate a flywheel to a high speed, using an electric motor or generator, and then decelerate it when energy is needed, using the same device as a generator or motor.
  • The amount of energy stored in a flywheel depends on its mass, shape, and rotational speed. The faster the flywheel spins, the more energy it can store.
• How does FES work?
  • A typical FES system consists of four main components:
    • A flywheel, which is a cylindrical or disc-shaped device made of steel, composite, or other materials, mounted on a shaft and enclosed in a vacuum chamber to reduce air friction.
    • A motor/generator, which is an electric device that can either spin the flywheel using electricity from the grid or produce electricity from the flywheel’s rotation.
    • A power electronics unit, which is a device that controls the speed, direction, and power flow of the motor/generator, as well as converts the alternating current (AC) from the grid to direct current (DC) for the motor/generator, and vice versa.
    • A bearing system, which is a device that supports the flywheel’s shaft and allows it to rotate smoothly and with minimal friction. There are two main types of bearings: mechanical and magnetic. Mechanical bearings use physical contact, such as ball or roller bearings, to support the shaft. Magnetic bearings use magnetic fields, such as permanent magnets or electromagnets, to levitate the shaft without contact.
  • The operation of a FES system can be divided into two modes: charging and discharging:
    • In the charging mode, the power electronics unit converts the AC from the grid to DC and sends it to the motor/generator, which spins the flywheel to a high speed, storing energy as rotational kinetic energy.
    • In the discharging mode, the power electronics unit reverses the process and converts the DC from the motor/generator to AC and sends it back to the grid, as the flywheel slows down, releasing energy as electrical energy.
• What are the advantages and disadvantages of FES?
  • FES has many advantages over other energy storage systems, such as :
    • High efficiency: FES can achieve up to 90% round-trip efficiency, meaning that only 10% of the energy is lost during the charging and discharging cycles.
    • Long lifespan: FES can last for more than 20 years and withstand thousands of charge-discharge cycles without significant degradation.
    • Fast response: FES can charge and discharge energy in milliseconds, making it suitable for applications that require high power and fast frequency regulation.
    • Low maintenance: FES has few moving parts and does not require cooling, heating, or chemical replenishment, unlike batteries or capacitors.
    • Environmental friendliness: FES does not use toxic or hazardous materials, such as lead, acid, or lithium, and does not emit greenhouse gases or pollutants, unlike fossil fuels or nuclear power.
  • FES also has some disadvantages and challenges, such as :
    • High cost: FES is still relatively expensive compared to other energy storage systems, especially for large-scale applications. The main cost drivers are the materials, manufacturing, and installation of the flywheel, motor/generator, power electronics, and bearing system.
    • Limited energy density: FES can store only a limited amount of energy per unit mass or volume, compared to batteries or capacitors. This means that FES requires more space and weight for the same amount of energy storage.
    • Safety risks: FES involves high-speed rotation of heavy objects, which can pose potential hazards in case of failure, such as flywheel rupture, bearing failure, or fire. Therefore, FES requires proper design, protection, and monitoring to ensure safe operation.

Types of flywheels

!A high-speed flywheel)

• Low-speed flywheels
  • Material and design
    • Low-speed flywheels are flywheels that operate at speeds below 10,000 revolutions per minute (rpm).
    • Low-speed flywheels are usually made of steel or other metals, which have high strength and durability.
    • Low-speed flywheels are typically cylindrical or disc-shaped, with a large diameter and a small thickness, to maximize the moment of inertia and minimize the stress.
  • Applications and examples
    • Low-speed flywheels are mainly used for short-term energy storage, such as power quality improvement, uninterruptible power supply (UPS), and load leveling.
    • Low-speed flywheels can provide power for a few seconds to a few minutes, depending on the size and speed of the flywheel.
    • Some examples of low-speed flywheel applications are:
      • The Beacon Power 20 kW flywheel, which is a modular and scalable system that can provide frequency regulation and voltage support for the grid.
      • The Active Power CleanSource UPS, which is a flywheel-based UPS that can provide backup power for data centers, hospitals, and industrial facilities.
      • The Vycon REGEN, which is a flywheel system that can capture and reuse the braking energy of cranes, trains, and elevators.
• High-speed flywheels
  • Material and design
    • High-speed flywheels are flywheels that operate at speeds above 10,000 rpm, up to 100,000 rpm or more.
    • High-speed flywheels are usually made of composite materials, such as carbon fiber or fiberglass, which have high stiffness and low density.
    • High-speed flywheels are typically ring-shaped, with a small diameter and a large thickness, to minimize the centrifugal force and maximize the energy density.
  • Applications and examples
    • High-speed flywheels are mainly used for long-term energy storage, such as renewable integration, peak shaving, and grid stabilization.
    • High-speed flywheels can provide power for a few minutes to a few hours, depending on the size and speed of the flywheel.
    • Some examples of high-speed flywheel applications are:
      • The Amber Kinetics M32 flywheel, which is a 32 kWh flywheel that can provide four hours of discharge at 8 kW, suitable for solar and wind integration.
      • The Temporal Power 2 MW flywheel, which is a large-scale flywheel that can provide frequency regulation and ancillary services for the grid.
      • The NASA Glenn flywheel, which is a 500 Wh flywheel that can provide power and attitude control for spacecraft.

Comparison with other energy storage systems

!A comparison of different energy storage systems)

• Batteries
  • Pros and cons
    • Batteries are devices that store and release energy as chemical energy, using electrochemical reactions between electrodes and electrolytes.
    • Batteries have many advantages, such as:
      • High energy density: Batteries can store a large amount of energy per unit mass or volume, compared to flywheels or capacitors.
      • Low self-discharge: Batteries can retain their stored energy for a long time, without significant losses, unlike flywheels or capacitors.
      • Scalability: Batteries can be easily connected in series or parallel to increase the voltage or capacity, respectively, of the energy storage system.
    • Batteries also have some disadvantages, such as:
      • Low efficiency: Batteries have lower round-trip efficiency than flywheels or capacitors, due to the internal resistance and heat generation of the electrochemical reactions.
      • Short lifespan: Batteries have limited cycle life and calendar life, due to the degradation of the electrodes and electrolytes over time and use.
      • High maintenance: Batteries require cooling, heating, or chemical replenishment, depending on the type and condition of the battery, to ensure optimal performance and safety.
      • Environmental issues: Batteries use toxic or hazardous materials, such as lead, acid, or lithium, which can cause pollution or health risks.

      Comparison with other energy storage systems

      !A comparison of different energy storage systems)

      • Batteries
        • Pros and cons
          • Batteries are devices that store and release energy as chemical energy, using electrochemical reactions between electrodes and electrolytes.
          • Batteries have many advantages, such as:
            • High energy density: Batteries can store a large amount of energy per unit mass or volume, compared to flywheels or capacitors.
            • Low self-discharge: Batteries can retain their stored energy for a long time, without significant losses, unlike flywheels or capacitors.
            • Scalability: Batteries can be easily connected in series or parallel to increase the voltage or capacity, respectively, of the energy storage system.
          • Batteries also have some disadvantages, such as:
            • Low efficiency: Batteries have lower round-trip efficiency than flywheels or capacitors, due to the internal resistance and heat generation of the electrochemical reactions.
            • Short lifespan: Batteries have limited cycle life and calendar life, due to the degradation of the electrodes and electrolytes over time and use.
            • High maintenance: Batteries require cooling, heating, or chemical replenishment, depending on the type and condition of the battery, to ensure optimal performance and safety.
            • Environmental issues: Batteries use toxic or hazardous materials, such as lead, acid, or lithium, which can cause pollution or health risks
        • Use cases and scenarios
          • Batteries are suitable for applications that require high energy and low power, such as energy arbitrage, peak shaving, and backup power.
          • Batteries can provide energy for hours or days, depending on the size and type of the battery.
          • Some examples of battery applications are:
            • The Tesla Powerwall, which is a lithium-ion battery that can store excess solar energy for residential use or grid services.
            • The Hornsdale Power Reserve, which is a 100 MW/129 MWh lithium-ion battery that can provide frequency regulation and grid stability for South Australia.
            • The Vanadium Redox Flow Battery, which is a liquid battery that can store large amounts of energy for long durations, suitable for renewable integration and microgrids.
      • Capacitors
        • Pros and cons
          • Capacitors are devices that store and release energy as electric charge, using two conductive plates separated by a dielectric material.
          • Capacitors have many advantages, such as:
            • High efficiency: Capacitors have higher round-trip efficiency than batteries or flywheels, due to the low resistance and heat generation of the electric charge transfer.
            • Long lifespan: Capacitors have unlimited cycle life and calendar life, as they do not degrade over time and use.
            • Fast response: Capacitors can charge and discharge energy in microseconds, making them suitable for applications that require high power and fast voltage regulation.
          • Capacitors also have some disadvantages, such as:
            • Low energy density: Capacitors can store only a small amount of energy per unit mass or volume, compared to batteries or flywheels.
            • High self-discharge: Capacitors can lose their stored energy quickly, due to the leakage of the electric charge through the dielectric material or the external circuit.
            • Cost: Capacitors are still relatively expensive compared to other energy storage systems, especially for large-scale applications. The main cost drivers are the materials, manufacturing, and installation of the capacitor plates and the dielectric material.
        • Use cases and scenarios
          • Capacitors are suitable for applications that require low energy and high power, such as power quality improvement, transient load support, and electric vehicle acceleration.
          • Capacitors can provide power for milliseconds to seconds, depending on the size and type of the capacitor.
          • Some examples of capacitor applications are:
            • The Maxwell Ultracapacitor, which is an electrostatic capacitor that can provide power for hybrid buses, trains, and trucks, as well as wind turbines and cranes.
            • The Skeleton Technologies SkelCap, which is a graphene-based supercapacitor that can provide power for automotive, aerospace, and industrial applications, as well as grid services.
            • The EEStor EESU, which is a ceramic-based capacitor that claims to have higher energy density than lithium-ion batteries, suitable for electric vehicles and grid storage.
      • Other kinetic energy storage systems
        • Pros and cons
          • Other kinetic energy storage systems are systems that store and release energy as mechanical energy, using different forms of motion, such as linear, rotational, or vibrational.
          • Other kinetic energy storage systems have some advantages, such as:
            • High efficiency: Other kinetic energy storage systems can achieve high round-trip efficiency, similar to flywheels, depending on the type and design of the system.
            • Long lifespan: Other kinetic energy storage systems can have long cycle life and calendar life, similar to flywheels, depending on the type and design of the system.
            • Low maintenance: Other kinetic energy storage systems can have low maintenance requirements, similar to flywheels, depending on the type and design of the system.
          • Other kinetic energy storage systems also have some disadvantages, such as:
            • Low energy density: Other kinetic energy storage systems can have low energy density, similar to flywheels, depending on the type and design of the system.
            • Safety risks: Other kinetic energy storage systems can pose safety risks, similar to flywheels, depending on the type and design of the system.
            • Technical challenges: Other kinetic energy storage systems can face technical challenges, such as material selection, friction reduction, and vibration control, depending on the type and design of the system.
        • Use cases and scenarios
          • Other kinetic energy storage systems are suitable for applications that require medium energy and medium power, such as renewable integration, grid stabilization, and load leveling.
          • Other kinetic energy storage systems can provide power for seconds to minutes, depending on the size and type of the system.
          • Some examples of other kinetic energy storage systems are:
            • The Gravity Power Module, which is a system that uses a large piston and a water-filled shaft to store and release energy as gravitational potential energy.
            • The Advanced Rail Energy Storage, which is a system that uses electric locomotives and rail tracks to store and release energy as gravitational potential energy.
            • The Velkess Flywheel, which is a system that uses a flexible rotor and a magnetic bearing to store and release energy as rotational kinetic energy.

      Future prospects and challenges

      • Technological innovations and developments
        • FES is a rapidly evolving technology that has the potential to improve its performance, cost, and reliability, through various technological innovations and developments, such as:
          • New materials and designs: FES can benefit from the development of new materials and designs that can increase the strength, stiffness, and durability of the flywheel, as well as reduce the weight, size, and cost of the system.
          • Advanced power electronics and control systems: FES can benefit from the development of advanced power electronics and control systems that can optimize the speed, direction, and power flow of the motor/generator, as well as enhance the efficiency, stability, and safety of the system.
          • Hybrid and integrated systems: FES can benefit from the development of hybrid and integrated systems that can combine FES with other energy storage systems, such as batteries or capacitors, to achieve higher energy density, lower self-discharge, and better performance.
      • Market trends and opportunities
        • FES is a growing market that has the opportunity to expand its applications, customers, and regions, through various market trends and opportunities, such as:
          • Increasing demand for energy storage: FES can meet the increasing demand for energy storage, driven by the growth of renewable energy, electric vehicles, and smart grids, as well as the need for power quality, reliability, and security.
          • Favorable policies and regulations: FES can benefit from the favorable policies and regulations that support the development and deployment of energy storage, such as subsidies, incentives, standards, and mandates, at the local, national, and international levels.
          • Competitive advantages and differentiation: FES can leverage its competitive advantages and differentiation, such as high efficiency, long lifespan, fast response, low maintenance, and environmental friendliness, to attract and retain customers, as well as to create and capture value.
      • Environmental and social impacts
        • FES is a sustainable technology that has the potential to create positive environmental and social impacts, such as:
          • Reducing greenhouse gas emissions and fossil fuel dependence: FES can reduce greenhouse gas emissions and fossil fuel dependence, by enabling the integration of renewable energy, improving the efficiency of the power grid, and providing clean and reliable power for various applications.
          • Enhancing energy access and affordability: FES can enhance energy access and affordability, by providing distributed and decentralized energy storage solutions, especially for remote and rural areas, as well as for developing and emerging countries.
          • Creating jobs and economic growth: FES can create jobs and economic growth, by stimulating the innovation and development of the energy storage industry, as well as by supporting the deployment and operation of the energy storage systems.

      Conclusion

      • Summary of main points
        • In this blog, we have discussed what is flywheel energy storage, how does it work, what are its advantages and disadvantages, how does it compare with other energy storage systems, and what are its future prospects and challenges.
        • We have learned that flywheel energy storage is a promising and innovative technology that can store and release energy as rotational kinetic energy, using a rotating device called a flywheel.