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| Fig. 1: Beacon Power, LLC's 20MW flywheel energy storage plant in Stephentown, New York (Source: Wikimedia Commons) |
As the world's energy demand increases, reliable energy storage technologies are becoming more important. While batteries have gained a lot of popularity for their compatibility with electric vehicles and smaller devices, there has also been interest in mechanical energy storage systems and how they can help create a more robust power grid.
Flywheel Energy Storage (FES) is a method to store kinetic energy in the form of rotational energy. The simplest design of a FES system is a cylindrical rotating mass connected to a rotor that is supported by bearings. A low friction environment ensures that there is minimal energy loss, and the rotor of the flywheel is connected to a motor-generator which allows energy to be transferred to and from the flywheel. When energy needs to be stored, electric energy powering the motor gets transferred to the rotational energy of the flywheel, and when energy is needed, the energy stored in the flywheel will be converted to electrical energy by the generator.
The energy that a flywheel can store is
where I is the moment of inertia of the flywheel's rotor and ω is its rotation speed. In order to increase the amount of energy stored either the speed or or the moment of inertia of the flywheel must be increased. The speed of the flywheel is limited by the strength of the material. The maximum stress occurs near the center of the flywheel, and for a simple flywheel where the rotor is a homogeneous cylindrical disk, the maximum stress is
| σmax | = | (3+ν)ρ ω2 8 |
where ν is poisson's ratio, ρ is the density of the material, and r is the radius of the rotor. The maximum stress is proportional to the square of the speed of the rotor. This holds true even when the rotor is not a perfect cylinder. For rotors of different shapes, a shape factor, K can be assigned. This helps account for how the moment of inertia and the maximum stress are influenced by the distribution of mass. Using this factor, the maximum energy stored per unit mass is
where E is the energy that a flywheel stores, m is its mass, and σ is the yield strength of the material. From this equation it is clear that flywheels with stronger materials can store more energy. [1]
Common choices for flywheel rotor material are high-strength steel alloys or composite materials. While both steel alloys and composites provide high tensile strength, the isotropic properties of alloys allows for rotors to be made as constant stress disks. On the other hand composite materials are orthotropic and cannot be made into constant stress disks. This means the shape factor for steel rotors is generally higher than that of composite rotors. Different materials used for flywheel rotors are compared in Table 1, where metal rotors are estimated to have a shape factor of 1, and composite rotors are estimated to have a shape factor of 0.5. [1]
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| Table 1: Rotor Materials. [1] |
Composite materials can be made to have very high yield strengths, but although they outperform steel in strength, they are very expensive to manufacture. Metal FES systems are a more mature technology that have a better energy per cost of materials. Since metal is more dense, metal FES systems also take up less volume than composite FES systems. [1] Many companies looking to use utility scale FES systems opt for steel rotors for a more cost effective product. Table 2 lists different FES models, their rotor types, their power capacity, energy storage, specific energy, speed, and efficiency. Despite composites have much higher yield strengths, the specific energy of steel rotor FES systems is comparable to that of a composite rotor system.
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| Table 2: Performance of FES Models [3] |
The efficiency losses for FES systems come from friction losses and the efficiency of the motor-generator. Friction losses come from the rotor's windage losses and the performance of the bearings supporting the rotor. Windage losses can be very small due to the flywheel operating in a vacuumed container. Bearings in FES systems are mechanical or magnetic and result in losses around 0.1-0.3 W per kg supported by the bearings. There are different types of motor-generators used in FES systems, but they must all be brushless and categorized as high speed, as even low speed FES systems can operate in the 10s of thousands of rpm. The choices of motor-generators for FES systems are permanent magnet synchronous, switched reluctance, induction asynchronous, and homopolar, and they all have electrical conversion losses. With losses from friction and from the motor-generator, round trip efficiencies of FES systems are typically 85%-90%. Due to these losses, the energy stored by a FES system is less than the ideal value calculated from the equation for the kinetic energy of a rotating mass. [2]
Commercial FES systems supporting the power grid have energy capacities between 25 kWh - 50 kWh. Beacon Power, LLC's BP400 has a mass of 1133kg, a diameter of 3 feet, a maximum rotational speed of 15,500 rpm, and an efficiency of 85%, and it is rated to have a 25kWh charge and discharge. This was the FES system model they used to build their FES plant that supports Pennsylvania Light and Powers transmission system in Hazle Township, PA. [3,4]
There are multiple FES plants that support the power grid. In addition to the plant in Hazle Township, Beacon Power, LLC also has a 20MW plant in Stephentown, NY (Fig. 1) that supports New York Independent System Operator. [4] Amber Kinetics has partnered with Hawaiian Electric Company and installed a 8kW FES storage system to test its compatibility with a simulated PV array. [5] National Renewable Storage Clear Creek LP runs a 5MW flywheel and battery hybrid storage facility. [6]
As the power grid shifts to renewable sources of energy, the inertia of the grid decreases due to the intermittent nature of solar and wind power. Lithium-ion batteries are beginning to be used to create synthetic inertia in energy grids, and due to their low costs, it is difficult for other energy storage methods to compete. However, FES systems offer distinct advantages for dedicated grid stabilization applications. FES systems have a rapid response giving them the ability to absorb or discharge power nearly instantaneously and stabilize grid frequency. In addition, Lithium-ion batteries chemically degrade after a certain amount of cycles, whereas FES systems do not experience chemical degradation. When lifecycle costs are considered, including battery replacement due to degradation, FES systems prove more cost-effective than batteries for high-cycling grid stabilization applications. As a result, although batteries remain the dominant energy storage technology due to their versatility and low manufacturing costs, flywheel energy storage systems provide an economic advantage for high-cycling grid stabilization applications, offering essential inertial support as power grids transition to renewable energy sources. [2]
© Kimia Sattary. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] H. Dongxu et al., "A Review of Flywheel Energy Storage Rotor Materials and Structures," J. Energy Storage 74A, 109076 (2023).
[2] K. R. Pullen, "The Status and Future of Flywheel Energy Storage," Joule 3, 1394 (2013).
[3] V. Kale and M. Secanell, "A Comparative Study Between Optimal Metal and Composite Rotors For Flywheel Energy Storage Systems," Energy Rep. 4, 576 (2018).
[4] D. Bender,R. Burns, and D. Borneo, "ARRA Energy Storage Demonstration Projects: Lessons Learned and Recommendations," Sandia Natonal Laboratory, SAND2015-5242, JUne 2015.
[5] "Flywheel Systems for Utility Scale Energy Storage," California Energy Commission CEC-500-2019-012, January 2019.
[6] "Decision and Order: EB-2019-0158 - NRStor Clear Creek LP," Ontario Energy Board, September 2019.
