Source-linked AI summary
A review of flywheel energy storage systems: state of the art and opportunities
Xiaojun Li, Alan Palazzolo
TL;DR
Wind and solar power’s intermittent nature creates a need for energy-storage infrastructure, while FESS analysis is highly interdisciplinary. This paper reviews commissioned and prototyped FESS technologies, examining subsystem choices, their impacts on performance, and future opportunities.
Problem
Wind and solar power’s intermittent nature prevents reliable energy infrastructure, while FESS analysis requires an interdisciplinary focus.
Method
The paper reviews state-of-the-art FESS technologies, emphasizing commissioned or prototyped systems and comparing subsystem choices and their implications for performance.
Results
The review identifies state-of-the-art FESS technologies and future opportunities while relating subsystem choices to overall system performance.
Takeaways & Limitations
FESS development should consider different subsystem choices together with their impacts on system performance.
Takeaways & Limitations
Work remains limited on AMB- or converter-related losses, and the largest existing deployment is geographically constrained.
Abstract
from arXiv · showhide
Thanks to the unique advantages such as long life cycles, high power density, minimal environmental impact, and high power quality such as fast response and voltage stability, the flywheel/kinetic energy storage system (FESS) is gaining attention recently. There is noticeable progress made in FESS, especially in utility, large-scale deployment for the electrical grid, and renewable energy applications. This paper gives a review of the recent developments in FESS technologies. Due to the highly interdisciplinary nature of FESSs, we survey different design approaches, choices of subsystems, and the effects on performance, cost, and applications. This review focuses on the state of the art of FESS technologies, especially for those who have been commissioned or prototyped. We also highlighted the opportunities and potential directions for the future development of FESS technologies.
1. Introduction
Renewable energy’s intermittency creates a need for reliable, high-quality power, while existing storage options face geographic, duration, self-discharge, cost, or environmental trade-offs. This review addresses the comparatively limited technological coverage of FESS by surveying designs, subsystem choices, applications, and future research directions.
- Motivation: Wind and solar intermittency prevents them from serving as independent and reliable micro-grid energy sources.Energy storage helps provide continuous, high-quality power by storing intermittent renewable energy.
- Storage alternatives: Pumped hydro has the largest deployment but is geographically constrained, narrowing scalable large-deployment candidates to Li-ion batteries, supercapacitors, and flywheels.
- Storage alternatives: Li-ion batteries offer high energy density and lower cost per energy capacity but lower power density and higher cost per power capacity.These characteristics suit applications requiring high energy capacities.
- Storage alternatives: Flywheels and supercapacitors provide high power density and lower cost per power capacity, but supercapacitors have narrower discharge duration and significant self-discharge.
- FESS rationale: Flywheels remain competitive for frequent large-scale charge/discharge cycles and have the least environmental impact among the three technologies because they contain no chemicals.Their high power density and fast response also support grid regulation, renewable-output smoothing, UPS, locomotives, and power-pulsation applications.
- Review scope: The review focuses on technological aspects of this interdisciplinary field, including materials, bearings, subsystem effects on design and performance, commissioned or prototyped systems, and future research directions.
2. Working principles and technologies
A flywheel energy storage system is presented through its overall system components and a typical configuration. The system combines mechanical storage with an electric machine, bearings, and power electronics.
- System architecture: The figures provide an overview of flywheel energy storage system components and a typical system configuration.
- System architecture: A FESS includes a flywheel/rotor, a bearing system, and a power converter system incorporating an electric machine and power electronics.
2.1. Overview
FESS stores rotational kinetic energy in a rotor and exchanges it with electrical energy through an energy converter. Its architecture combines rotor, bearing, conversion, containment, and auxiliary subsystems selected for capacity, loss, maintenance, and operating requirements.
- Energy storage principle: The stored energy is a function of rotational speed and the rotor’s primary moment of inertia, expressed as E = 1/2 Ipω^2.
- Energy storage principle: Higher energy capacity requires either a rotor with significant moment of inertia or operation at a fast spinning speed.
- Rotor and bearings: FESS rotors use composite or metallic materials; a cited composite rotor operates above 15,000 RPM and is supported by operational bearings.
- Rotor and bearings: Bearings may be mechanical or magnetic, with magnetic bearings preferred for minimal standby loss and reduced maintenance requirements.
- Energy conversion: An electric motor/generator converts kinetic energy to electricity and vice versa, while magnetic or mechanical gears can directly couple the flywheel to an external load.
- Containment and auxiliaries: A vacuum enclosure reduces standby loss, and auxiliary components include a vacuum pump, catcher bearings, and a cooling system.
2.2. Flywheel/Rotor
The flywheel rotor stores most of a FESS’s kinetic energy, making its material choice and design central to energy capacity and specific energy. Composite and steel rotors involve different performance, manufacturing, and integration trade-offs.
- The rotor stores most of the kinetic energy and is usually the starting point for the entire FESS design.Its design is critical for energy capacity.
- A flywheel’s specific energy increases with lower density and higher tensile strength, while volumetric energy density is unaffected by material density.The energy-capacity equations use safety factor, depth of discharge, rotating-mass ratio, tensile strength, shape factor, and density.
- 2.2.1. Composite flywheel: Composite anisotropy gives high longitudinal but weak radial tensile strength, which limits energy capacity; multiple thin shrink-fit rims can reduce radial stresses.The Tsai-Wu failure criterion is a popular design criterion for composite flywheels.
- 2.2.1. Composite flywheel: Composite flywheels can achieve 50-100 Wh/kg specific energy, but metallic shafts and interfaces with bearings and motor/generators reduce whole-system specific energy.Composite designs commonly use carbon fiber, glass fiber, and epoxy, with optimization of rim thickness, shrink-fit allowances, and material combinations.
- 2.2.2. Steel flywheel: Steel flywheels are regaining interest because of low cost, easy fabrication, and better recyclability, with designs targeting improved specific energy and energy density.Recent development includes FESSs based on high-strength steel flywheels.
2.3. Operational Bearings
Operational bearings support the rotor during normal operation, and modern FESS designs increasingly use magnetic or hybrid bearing systems. Bearing selection balances friction loss, speed capability, control complexity, power consumption, and auxiliary requirements.
- 2.3.1. Magnetic bearing: Magnetic bearings let the rotor spin without physical contact, eliminating friction loss that is inevitable with mechanical bearings.Mechanical-bearing power loss is roughly proportional to rotor spinning speed, restricting higher-speed operation.
- Fluid-film bearings may reduce power loss but require an extra lubrication system, making them inapplicable in a vacuumed FESS.
- 2.3.1. Magnetic bearing: Active magnetic bearings require power electronics and feedback control, while passive magnetic bearings have more complex designs but do not require feedback control.Commercial magnetic bearings are often heteropolar active magnetic bearings because of their lower cost.
- 2.3.1. Magnetic bearing: Active magnetic-bearing control algorithms address system couplings, gyroscopic effects, and synchronous vibrations.
- 2.3.1. Magnetic bearing: Superconducting magnetic bearings offer superior performance but most designs are complicated and require cryogenic equipment.
- 2.3.2. Hybrid bearing: Hybrid bearing systems combine mechanical and magnetic bearings or passive and active magnetic bearings to support flywheel operation.One reported arrangement uses passive magnetic bearings for axial support and tilting stiffness with active radial magnetic bearings.
2.4. Energy converter
FESS energy converters use electric machines or magnetic gears to exchange electrical and kinetic energy, with designs balancing efficiency, power density, robustness, cost, and control performance.
- Electric machines operate as both motors and generators, converting electrical energy to kinetic energy and vice versa.
- Electric machine designs: PMSMs are popular for FESS because of their high efficiency, high performance, compact size, and high power density.Their design must account for magnet size, grade, pole count, and heat dissipation in vacuum operation.
- Electric machine designs: Induction machines offer lower cost and high robustness but are generally less efficient than PMSMs.They have been modeled and controlled for frequency regulation and wind-power intermittency applications.
- Electric machine designs: SRMs avoid permanent magnets and provide high-speed efficiency, acceleration capability, simple converters, and fault tolerance, but remain less mature than PMSMs.
- Bearingless machine: Bearingless machines integrate magnetic suspension and motor/generator functions, requiring simultaneous consideration of torque and suspension-force performance.A five-phase bearingless flux-switching permanent-magnet machine improved torque and suspension-force amplitude while reducing fluctuation.
2.5. Auxiliary components
Auxiliary FESS components protect high-speed operation and accommodate application-specific mechanical requirements, including backup bearings, vacuum enclosures, cooling, low weight, and vibration absorption.
- Catcher bearings: Catcher bearings provide backup support when magnetic bearings fail because of power shortages or excessive external disturbances.They are not designed to replace magnetic bearings during normal operation.
- Catcher bearings: Double-decker catcher bearings are more resistant to temperature rise than single-decker catcher bearings.The comparison is supported by both theoretical and experimental results.
- Housing and casing: Vacuum enclosures reduce windage loss during high-speed flywheel rotation.Systems may also require an air pump to maintain vacuum and active cooling for the magnetic bearings and motor/generator.
- Housing and casing: Mobile FESS housings must reduce overall weight and absorb vibration to prevent failures caused by excessive external vibrations.
3. Applications
FESS applications span grid services, renewable-energy smoothing, UPS, vehicles, rail, ships, and pulsed-power systems, leveraging fast response, high power density, and long life cycles.
- Grid and renewable energy: FESSs smooth fluctuating wind and solar output, while newer systems can operate for multiple hours rather than only minutes or seconds.
- Frequency regulation: Frequency regulation uses FESS to add or subtract power rapidly so line frequency remains constant despite real-time generation–load imbalance.FESS responds almost instantly, has a high power-to-mass ratio, and offers a long life cycle compared with Li-ion batteries.
- UPS: Flywheel UPS systems provide high power quality, long life cycles, and low maintenance, with reported systems ranging from 450 kW to 675 kW.
4. Trends and future topics
FESS adoption is constrained by application-specific designs, specialized manufacturing and maintenance, high capital cost, and uncertainty in mechanical failure modes; future work targets these barriers.
- FESSs are not yet widely adopted, and most existing systems are designed for specific applications such as frequency regulation or UPS.
- Cost: FESSs have higher capital costs than electrochemical batteries, although high cycle counts can yield competitive cost per (kWh*cycles).Beacon Power’s flywheel costs almost ten times more than a Li-ion system with similar energy capacity.
- Reliability: Moving parts increase uncertainty in failure modes, particularly for composite flywheels operating at higher speeds with less predictable mechanical properties.
- Specialized manufacturing, assembly, and maintenance requirements limit large-scale production and efforts to reduce unit cost.
- Future topics: The review identifies emerging efforts addressing these adoption, manufacturing, cost, and reliability challenges.
4.1. New Technologies
Recent FESS development combines material, rotor-geometry, bearing, machine, and integration innovations to improve energy performance, compactness, cost, and reliability. High-strength steel is favored for fixed, ground-based systems, while composites remain attractive where low mass is critical.
- Materials: Composite flywheels provide low density, high tensile strength, and high specific energy, making them important for aerospace and mobile applications.Reported composite specific energies of 50–150 Wh/kg may include only the composite rim, excluding the metallic shaft and other system components.
- Materials: 42KJ/kg is the reported specific energy of one composite FESS rotor, but it falls to 5.6Wh/kg when the whole system weight is included.The system-level figure illustrates the effect of including components beyond the rotor.
- Materials: T1000 has 20% of steel’s density and 26% higher tensile strength, but its cost is almost 100 times greater.The review therefore reports metals as superior where energy-per-cost must be maximized.
- Materials: High-strength steel flywheels offer high volume-based energy density, thermal conductivity, design-data availability, and lower cost for fixed, ground-based, large-capacity applications.They are described as suitable for mass production and competitive cost.
- Subsystem integration: Integrated and bearingless designs reduce component count or package size, while magnetic gears can make FESS powertrains more compact without extra power electronics.Further work is needed to establish whether magnetic gears meet FESS power, torque, speed, and efficiency requirements.
- Emerging materials: New rotor and subsystem materials are proposed to increase specific energy and lower cost, but low-tensile-strength concrete and e-glass concepts remain uncertain in cost and performance.Graphene offers theoretical specific energy over 15 kWh/kg, although how its energy can be harvested remains unclear.
- Reliability and losses: FESS loss and failure research remains uneven, with fewer studies addressing active magnetic-bearing or converter-related losses than windage or motor-related losses.Failure containment and prognosis are also identified as important research needs for complex electromechanical FESSs.
4.2. New Application
New applications extend FESS beyond conventional storage to regenerative industrial motion, energy harvesting, hybrid battery systems, fast charging, and multifunctional control. These applications exploit high-power cycling, buffering, compact integration, and the ability to provide secondary functions.
- Regenerative industrial motion: Repeated crane, truck, and robot-arm motions can recover energy for reuse in subsequent cycles, improving efficiency where charge/discharge events require high power.Commercial deployments exist for large industrial devices such as cranes, while smaller devices remain less studied.
- Energy harvesting: Flywheels can store intermittent energy harvested from triboelectric nanogenerators, opening an energy-harvesting application for FESS.Future work includes source-integration topologies, energy-saving and harvesting control, and coordination among multiple FESSs.
- Hybrid storage: In a cascaded storage architecture, FESS acts as a fast buffer that provides high-quality power while protecting batteries from regular charge/discharge cycling.The review compares this relationship to fast memory layered with slower, higher-capacity storage.
- Hybrid storage: A hybrid FESS–battery configuration greatly slowed the battery aging process by a factor of 300%.The review notes that relatively few studies address the design and control of flywheel-based hybrid energy-storage systems.
- Electric vehicles: Flywheel-based fast charging for electric vehicles is gaining attention because of high power density and long life cycles.Reported advantages also include operation across low and high temperatures, state-of-charge precision, and recyclability.
- Beyond storage: FESS can provide secondary functions such as vehicle stabilization and satellite attitude control while serving as an energy-storage device.Simulation reported attitude accuracy can be controlled up to an unspecified value in the supplied passage.
5. Conclusion
The review synthesizes recent FESS technologies across electrical, mechanical, and magnetic subsystems, emphasizing how subsystem choices affect performance. It identifies new materials, compact designs, energy harvesting, hybrid systems, and secondary functions as future opportunities.
- Conclusion: The review covers FESS as an interdisciplinary technology involving electrical, mechanical, and magnetic subsystems, and discusses how subsystem choices affect system performance.Its focus is on recently developed systems, including utility-scale and low-cost steel flywheels.
- Conclusion: New materials and compact designs are identified as routes toward higher specific energy and energy density, improving flywheel competitiveness with batteries.The conclusion specifically highlights recently developed utility-scale and low-cost steel flywheels.
- Conclusion: Future opportunities include energy harvesting, hybrid energy systems, and secondary flywheel functionality beyond energy storage.These directions extend FESS applications beyond storing energy alone.