Flywheel Design for Energy Storage

Understanding Flywheel Energy Storage Systems

Flywheel energy storage systems (FESS) represent one of the most elegant solutions in mechanical engineering for storing and releasing energy on demand. Unlike chemical batteries or fuel-based storage methods, flywheels store energy kinetically through the rotation of a massive disc or cylinder, offering distinct advantages in applications requiring rapid charge-discharge cycles, long operational lifespans, and minimal environmental impact.

For industries across Atlantic Canada, from offshore energy operations to manufacturing facilities in Nova Scotia, flywheel technology presents compelling opportunities for grid stabilization, backup power systems, and renewable energy integration. As the Maritime provinces continue their transition toward sustainable energy sources, understanding the principles of flywheel design becomes increasingly valuable for engineers and technical decision-makers.

The fundamental principle behind flywheel energy storage is remarkably straightforward: energy is stored as rotational kinetic energy according to the equation E = ½Iω², where E represents stored energy, I is the moment of inertia, and ω is the angular velocity. However, translating this simple concept into a reliable, efficient, and safe engineering system requires careful consideration of materials science, mechanical design, power electronics, and system integration.

Core Design Parameters and Material Selection

The design of an effective flywheel energy storage system begins with understanding the critical parameters that govern performance. The energy storage capacity depends primarily on two factors: the rotor's moment of inertia and its rotational speed. Engineers must carefully balance these parameters against practical constraints including material strength limits, bearing capabilities, and safety considerations.

Rotor Geometry and Mass Distribution

Flywheel rotors typically fall into two categories: low-speed steel rotors and high-speed composite rotors. Low-speed systems operate at rotational speeds between 5,000 and 10,000 RPM, utilizing steel or iron rotors with high mass concentrated at the outer radius to maximize moment of inertia. These systems are well-suited for applications requiring robustness and moderate energy density.

High-speed composite flywheels, conversely, operate at speeds exceeding 50,000 RPM and sometimes reaching 100,000 RPM. These systems employ advanced materials such as carbon fibre reinforced polymers (CFRP) or glass fibre composites, which offer superior specific strength—the ratio of tensile strength to density. The specific energy storage capacity of a flywheel is directly proportional to this specific strength, making material selection crucial.

• Steel rotors: Typical specific energy of 5-30 Wh/kg, tensile strength approximately 550-700 MPa, suitable for stationary industrial applications

• Glass fibre composites: Specific energy of 15-50 Wh/kg, excellent cost-effectiveness for mid-range applications

• Carbon fibre composites: Specific energy of 50-200 Wh/kg, tensile strength exceeding 3,000 MPa, ideal for aerospace and high-performance applications

• Hybrid designs: Steel hub with composite rim, balancing cost and performance for commercial installations

Stress Analysis and Safety Factors

The primary stress in a rotating flywheel is tensile stress caused by centrifugal forces. For a thin rotating ring, this stress can be approximated as σ = ρr²ω², where ρ is material density, r is radius, and ω is angular velocity. This relationship reveals why lower-density materials enable higher rotational speeds and greater energy density.

Canadian engineering standards require comprehensive stress analysis including finite element modelling to account for complex geometries, temperature variations, and fatigue considerations. Safety factors typically range from 2.0 to 4.0 depending on the application, with critical infrastructure installations in Nova Scotia and throughout the Maritimes requiring additional considerations for seismic events and extreme weather conditions.

Bearing Systems and Friction Management

The bearing system represents one of the most critical subsystems in flywheel design, directly impacting efficiency, maintenance requirements, and operational lifespan. Friction losses in bearings can significantly reduce round-trip efficiency if not properly addressed through careful engineering design.

Mechanical Bearing Options

Traditional mechanical bearings, including angular contact ball bearings and cylindrical roller bearings, remain common in lower-speed applications. These bearings offer reliability and lower initial costs but introduce friction losses typically ranging from 1-3% of stored energy per hour. For applications in Atlantic Canada's industrial sector, mechanical bearings often provide the most practical solution when maintenance infrastructure is readily available.

Magnetic Bearing Technology

Active magnetic bearings (AMB) eliminate mechanical contact entirely, suspending the rotor using electromagnetic forces controlled by sophisticated feedback systems. These systems offer several advantages:

• Virtually zero friction, enabling standby losses below 0.1% per hour

• No lubricant requirements, simplifying maintenance and eliminating contamination concerns

• Integrated monitoring capabilities through bearing control systems

• Extended operational life exceeding 20 years with minimal maintenance

The complexity and cost of magnetic bearing systems have decreased substantially over the past decade, making them increasingly viable for commercial installations. Modern AMB controllers utilise digital signal processors capable of adjusting bearing currents thousands of times per second, maintaining rotor position within micrometres of the optimal centreline.

Superconducting Magnetic Bearings

For the highest-performance applications, superconducting magnetic bearings offer passive levitation through the Meissner effect, requiring no active control systems once the superconductor is cooled below its critical temperature. High-temperature superconductors (HTS) such as YBCO have reduced cooling requirements, making these systems more practical than earlier designs requiring liquid helium temperatures.

Vacuum Enclosure and Thermal Management

Aerodynamic drag presents a significant challenge for high-speed flywheel systems. At rotational speeds of 50,000 RPM and above, air resistance can consume substantial energy, necessitating vacuum enclosures to minimize these losses. Most commercial flywheel systems operate at pressures below 1 millibar, reducing aerodynamic losses to negligible levels.

The vacuum containment vessel must be engineered to withstand atmospheric pressure while providing adequate structural integrity to contain rotor fragments in the unlikely event of catastrophic failure. Steel containment vessels with wall thicknesses of 25-50 millimetres are typical for systems storing 5-50 kWh of energy.

Thermal Considerations

Despite the vacuum environment, thermal management remains essential. Heat generated by bearing losses, motor/generator inefficiencies, and eddy currents must be dissipated to maintain component temperatures within acceptable limits. Radiation becomes the primary heat transfer mechanism in vacuum, requiring careful attention to surface emissivity and thermal pathways.

For installations in Nova Scotia and throughout Atlantic Canada, seasonal temperature variations spanning from -25°C to +35°C must be accommodated in the thermal design. Outdoor installations require particular attention to condensation prevention and thermal cycling effects on seals and structural components.

Motor-Generator and Power Electronics

The motor-generator unit serves the dual purpose of accelerating the flywheel during charging and extracting energy during discharge. Most modern flywheel systems employ permanent magnet synchronous machines (PMSM) or switched reluctance motors (SRM), each offering distinct advantages.

Permanent magnet machines provide high efficiency—typically 95-97%—and excellent power density, making them suitable for applications where space and weight are constrained. Switched reluctance machines offer greater robustness, simplified construction, and inherent fault tolerance, valuable characteristics for critical infrastructure applications.

Power Conversion Systems

Bidirectional power converters manage the flow of energy between the flywheel and the electrical grid or load. These systems typically comprise:

• Machine-side converter: Controls motor-generator operation, managing torque and speed

• DC link: Provides energy buffering between converters

• Grid-side converter: Manages power quality and grid synchronisation

• Control systems: Coordinate overall system operation and protection functions

Modern silicon carbide (SiC) and gallium nitride (GaN) power semiconductors enable higher switching frequencies and reduced losses compared to traditional silicon devices. These advanced materials have improved round-trip efficiencies for commercial flywheel systems to 85-95%, depending on power levels and discharge duration.

Applications in Atlantic Canada

The unique characteristics of flywheel energy storage align well with several emerging needs across the Maritime provinces. As Nova Scotia advances toward its renewable energy targets, grid-scale energy storage becomes increasingly critical for managing the intermittent nature of wind and solar generation.

Grid Frequency Regulation

Flywheel systems excel at frequency regulation services, capable of responding to grid frequency deviations within milliseconds. The Nova Scotia Power grid, like other isolated or weakly interconnected systems, can benefit significantly from such rapid-response resources. A typical 20 MW flywheel installation can provide primary frequency response services while cycling thousands of times daily without degradation.

Industrial Applications

Manufacturing facilities across Nova Scotia, from the Michelin tire plant in Pictou County to various food processing operations throughout the Annapolis Valley, face power quality challenges that flywheel systems can address. Voltage sag compensation, momentary outage protection, and peak demand reduction represent practical applications where flywheels offer advantages over battery-based alternatives.

Renewable Energy Integration

Wind farms in Nova Scotia and throughout Atlantic Canada can utilise flywheel storage to smooth output variations and provide synthetic inertia to the grid. As conventional thermal generation is retired, maintaining adequate system inertia becomes critical for grid stability. Flywheel systems can provide this inertial response instantaneously, supporting the broader transition to renewable energy.

Marine and Offshore Applications

The growing offshore energy sector, including potential tidal energy developments in the Bay of Fundy, presents unique opportunities for flywheel technology. The inherent durability and long cycle life of flywheel systems suit the demanding marine environment, while their ability to handle rapid power fluctuations aligns with the variable nature of tidal generation.

Design Considerations and Engineering Standards

Successful flywheel system implementation requires adherence to relevant engineering standards and careful attention to site-specific factors. In Canada, flywheel installations must comply with CSA standards for electrical equipment, pressure vessel codes for vacuum containment systems, and applicable building codes for structural mounting.

Key design considerations include:

• Foundation design: Adequate mass and stiffness to prevent resonance with rotor critical speeds

• Vibration isolation: Preventing transmission of rotor vibrations to surrounding structures

• Safety containment: Engineering controls to manage rotor burst scenarios

• Environmental protection: Appropriate enclosures for Maritime climate conditions

• Accessibility: Maintenance access requirements for bearing replacement and system inspection

Professional engineering oversight ensures that all aspects of flywheel system design meet applicable codes and standards while optimising performance for the specific application. This includes detailed analysis of rotordynamics, structural integrity, electrical protection, and control system reliability.

Partner with Sangster Engineering Ltd. for Your Energy Storage Projects

As flywheel energy storage technology continues to mature and find broader application across Atlantic Canada, the importance of expert engineering support cannot be overstated. From conceptual design through detailed engineering and commissioning support, professional guidance ensures that flywheel systems deliver their promised benefits safely and reliably.

Sangster Engineering Ltd., based in Amherst, Nova Scotia, brings comprehensive mechanical engineering expertise to energy storage projects throughout the Maritime provinces. Our team understands the unique challenges and opportunities present in Atlantic Canada, from harsh winter conditions to the region's ambitious renewable energy goals.

Whether you are exploring flywheel technology for grid services, industrial power quality improvement, or renewable energy integration, we invite you to contact our engineering team to discuss your project requirements. Our experience in mechanical system design, coupled with our commitment to practical, cost-effective solutions, positions us to support your energy storage initiatives from concept through implementation.

Contact Sangster Engineering Ltd. today to learn how our professional engineering services can help you evaluate, design, and implement flywheel energy storage solutions tailored to your specific needs and the unique requirements of operating in Nova Scotia and Atlantic Canada.

Partner with Sangster Engineering

At Sangster Engineering Ltd. in Amherst, Nova Scotia, we bring decades of engineering experience to every project. Serving clients across Atlantic Canada and beyond.

Contact us today to discuss your engineering needs.