Introduction

Flywheel energy storage is an efficient technology that converts energy between a motor and a rotating flywheel. During energy storage, the motor accelerates the flywheel, transforming electrical energy into rotational kinetic energy. When releasing energy, the flywheel slows down, and the motor switches to generator mode, converting kinetic energy back into electrical energy. Modern flywheel energy storage systems achieve 85% to 90% energy efficiency with response times as low as milliseconds, making them ideal for transient power demand scenarios.

System Composition

A flywheel energy storage system comprises several key components, including the flywheel, motor/generator, bearings, vacuum chamber, power electronics, neodymium magnet materials, and auxiliary equipment.

Flywheel

The flywheel, typically designed as a disk or cylinder, is the core component for energy storage. Traditional flywheels use high-strength alloys, but modern advancements wrap metal rotors with carbon fiber, glass fiber, or graphene-based materials to enhance tensile strength. This enables flywheels to operate at extremely high rotational speeds, significantly increasing energy storage density.

Motor/Generator

The motor/generator facilitates energy conversion. It acts as a motor during storage to accelerate the flywheel and as a generator during discharge to convert the flywheel's kinetic energy into electrical power. Permanent magnet motors, known for high efficiency and low losses, dominate high-speed applications, while induction and variable reluctance motors serve specific use cases.

Bearings

Bearings support the flywheel’s rotating body, requiring low friction and high stability. Mechanical high-speed bearings are sufficient for low-speed scenarios, but magnetic levitation bearings, with their contactless and low-loss properties, are preferred for high speeds. Mechanical bearings often act as backups for magnetic bearing failures. Emerging research on superconducting magnetic levitation bearings shows promise with near-zero friction losses.

Vacuum Chamber

The vacuum chamber, typically a thin-walled cylindrical sealed container, is critical to the system. A vacuum pump reduces internal pressure to 10^-3 to 10^-6 Pa, minimizing frictional heat from the high-speed flywheel’s interaction with air molecules, thus ensuring high system efficiency.

Power Electronics

Power electronics, including signal detection circuits, information processing, control software, and signal output circuits, enable precise control of bidirectional converters’ current and voltage. This ensures smooth energy transfer between the motor, converter, grid, and load. Modular converters based on IGBT or SiC support a wide range of speeds, critical for grid integration and energy management.

Auxiliary Equipment

Auxiliary equipment includes vacuum pumps, cooling systems, monitoring sensors, and safety protection devices. Vacuum pumps maintain the chamber’s vacuum environment, cooling systems dissipate heat from the motor and converter, sensors monitor speed and temperature in real-time, and protective devices ensure safety under extreme conditions.

Electrical Topology

The electrical topology centers on the energy storage converter, linking the flywheel system to the AC grid. Machine-side filters reduce motor current harmonics to minimize heat, while grid-side filters ensure injected current meets grid standards. Both the energy storage and motor converters support bidirectional energy flow, enabling seamless charging and discharging.

Application Areas

Flywheel energy storage is suitable for power needs from hundreds of kilowatts to tens of megawatts, with durations from seconds to minutes and cycle counts exceeding a million. Its key applications include:

• Grid Frequency Regulation and Stabilization: Its rapid response excels in frequency regulation and voltage stabilization.
• Renewable Energy Integration: Smooths wind and solar power fluctuations, reducing curtailment rates. China’s 14th Five-Year Plan highlights flywheel storage for renewable energy bases.
• Rail Transit: Recovers train braking energy with over 80% efficiency. Dunshi Magnetic Energy has piloted this in some Chinese metro lines.
• Uninterruptible Power Supply (UPS): Delivers high-quality power to data centers for seconds to minutes, replacing traditional lead-acid batteries.
• Aerospace and Defense: Provides high-pulse power for satellites or supports electromagnetic launch systems.
• Electric Vehicles and Ships: Supplies instantaneous high power in hybrid systems, extending battery lifespan.