Uncertainty Analysis of an Optoelectronic Strain Measurement System for Flywheel Rotors

The strain in a fast spinning carbon fiber flywheel rotor is of great interest for condition monitoring, as well as for studying long-term aging effects in the carbon fiber matrix. Optoelectronic strain measurement is a contactless measurement principle where a special reflective pattern is applied to the rotor which is scanned by a stationary optical setup. It does not require any active electronic components on the rotor and is suited for operation in a vacuum. In this paper, the influences of the key parts comprising the optoelectronic strain measurement are analyzed. The influence of each part on the measurement result including the uncertainty is modeled. The total uncertainty, as well as each part’s contribution is calculated. This provides a valuable assessment of requirements for component selection, as well as tolerances of mechanical parts and processes to reach a final target measurement uncertainty or to estimate the uncertainty of a given setup. We have shown that the edge quality of the special reflective pattern has the strongest influence, and how to improve it. Considering all influences, it is possible to measure strain with an uncertainty of less than 1% at a rotation speed of 500Hz.

Effects of Viscoelasticity on the Stress Evolution over the Lifetime of Filament-Wound Composite Flywheel Rotors for Energy Storage

High-velocity and long-lifetime operating conditions of modern high-speed energy storage flywheel rotors may create the necessary conditions for failure modes not included in current quasi-static failure analyses. In the present study, a computational algorithm based on an accepted analytical model was developed to investigate the viscoelastic behavior of carbon fiber reinforced polymer composite flywheel rotors with an aluminum hub assembled via a press-fit. The Tsai‑Wu failure criterion was applied to assess failure. Two simulation cases were developed to explore the effects of viscoelasticity on composite flywheel rotors, i.e., a worst-case operating condition and a case akin to realistic flywheel operations. The simulations indicate that viscoelastic effects are likely to reduce peak stresses in the composite rim over time. However, viscoelasticity also affects stresses in the hub and the hub-rim interface in ways that may cause rotor failure. It was further found that charge-discharge cycles of the flywheel energy storage device may create significant fatigue loading conditions. It was therefore concluded that the design of composite flywheel rotors should include viscoelastic and fatigue analyses to ensure safe operation.

Influence of resin curing cycle on the deformation of filament wound composites by in situ strain monitoring

The residual stress of metal liners wrapped by composite materials has a significant influence on the service performance of rotating parts, such as flywheel rotors and motor jackets. However, the deformation of the liners, the flow of resins, and the temperature variation during the winding process make it difficult to predict and control this residual stress. In this paper, the process-induced strains were monitored online by a strain gauge with the help of a wireless strain meter. The evolution of this strain during the manufacturing process was fully discussed. A rapid curing resin system was used and its curing properties were tested by differential scanning calorimetry. The mechanical properties of the resin matrix and its composite were characterized. The effect of the curing cycle on the evolution of the residual strain was discussed in detail through comparative experiments. The experimental results show that the use of infrared radiation has a significant advantage regarding residual stress accumulation. This advantage is greater when carbon fiber is used than when glass fiber is used. The prestress in composites of glass fiber and carbon fiber increases by 5.9% and 41.7%, respectively, after cooling.

Improving the Energy Capacity and Cost Effectiveness of Flywheel Rotors in Grid-Scale Energy Storage Systems by Varying Their Shape, Speed and Size

Flywheel energy storage systems (FESS) are an excellent short duration grid energy storage solution; however, their cost and energy storage capacity are typical barriers to their widespread commercialization. FESS can be designed by optimizing the shape of the flywheel rotor, choice of rotor material, operating speed and rotor radius. This study optimizes the flywheel rotor shape at various operating speeds and outer radii. It is found that the energy capacity of the rotor can be improved by choosing an ideal combination of operating speed and rotor radius. Our earlier work showed that including the cost of the FESS as an optimization objective could significantly alter the FESS design [1]. Therefore, the cost effectiveness of the FESS is also studied by comparing rotors made from different materials on an energy-per-cost basis, while the cost ratio of the materials is varied.

Design and Multi-Objective Optimization of Fiber-Reinforced Polymer Composite Flywheel Rotors

A multi-objective optimization strategy to find optimal designs of composite multi-rim flywheel rotors is presented. Flywheel energy storage systems have been expanding into applications such as rail and automotive transportation, where the construction volume is limited. Common flywheel rotor optimization approaches for these applications are single-objective, aiming to increase the stored energy or stored energy density. The proposed multi-objective optimization offers more information for decision-makers optimizing three objectives separately: stored energy, cost and productivity. A novel approach to model the manufacturing of multi-rim composite rotors facilitates the consideration of manufacturing cost and time within the optimization. An analytical stress calculation for multi-rim rotors is used, which also takes interference fits and residual stresses into account. Constrained by a failure prediction based on the Maximum Strength, Maximum Strain and Tsai-Wu criterion, the discrete and nonlinear optimization was solved. A hybrid optimization strategy is presented that combines a genetic algorithm with a local improvement executed by a sequential quadratic program. The problem was solved for two rotor geometries used for light rail transit applications showing similar design results as in industry.

Feasibility Study for Small Scaling Flywheel-Energy-Storage Systems in Energy Harvesting Systems

Two concepts of scaled micro-flywheel-energy-storage systems (FESSs): a flat disk-shaped and a thin ring-shaped (outer diameter equal to height) flywheel rotors were examined in this study, focusing on material selection, energy content, losses due to air friction and motor loss. For the disk-shape micro-FESS, isotropic materials like titanium, aluminum, steel and wolfram are shown to be suitable as a flywheel rotor. Wound fiber reinforced composite plastics (T1000-, T300-carbon fibers and carbon nanotubes “CNTs”) were investigated for the flywheel in a ring shape. It was shown that isotropic materials reach the highest energy densities in the shape of a Laval disk with a rim. A micro-FESS with wolfram flywheel would reach the highest half-time-periods due to its high density, and thus, it is the favored material to design a flat disk-shaped micro-FESS with low standby-losses. Fiber reinforced plastic flywheels in ring shape reach the highest energy densities, from 150 W h/kg (T300) to 2,600 W h/kg (CNT), but display higher standby-losses as well. A scaling of the rotors was done within this study and showed that air friction is influenced by the shape of the examined flywheel rotors and the material. A linear correlation of down scaling and air friction losses was shown. As a motor/generator type, an ironless air coil Halbach array motor was suggested. Motor losses due to eddy currents in the stator coil were estimated. Losses correlated in square with downscaling. FESSs with wolfram and CNT showed the lowest standby-losses due to eddy currents.