Flywheel Energy Storage

Flywheel energy storage (FES): a flywheel is a rotating mechanical device that is used to store rotational energy.

From: Comprehensive Energy Systems , 2018

Flywheel energy storage

Keith R. Pullen , in Storing Energy (Second Edition), 2022

Abstract

Flywheels are one of the earliest forms of energy storage and have found widespread applications particularly in smoothing uneven torque in engines and machinery. More recently flywheels have been developed to store electrical energy, made possible by use of directly mounted brushless electrical machines and power conversion electronics. This chapter takes the reader from the fundamentals of flywheel energy storage through to discussion of the components which make up a flywheel energy storage system. The place of flywheel energy storage in the storage landscape is explained and its attributes are compared in particular with lithium-ion batteries. It is shown that flywheels have great potential for rapid response, short duration, high cycle applications, many of which are listed and described. For flywheels to succeed beyond niche applications, cost reduction is necessary but certainly possible by use of low-cost materials and innovative design.

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Wind Power Peak-Valley Regulation and Frequency Control Technology

Kun Ding , Jing Zhi , in Large-Scale Wind Power Grid Integration, 2016

6.3.1.3 Flywheel energy storage

Flywheel energy storage uses electric motors to drive the flywheel to rotate at a high speed so that the electrical power is transformed into mechanical power and stored, and when necessary, flywheels drive generators to generate power. The flywheel system operates in the high vacuum environment. Characterized by no friction loss, small wind resistance, long life, no impact on the environment, and needing no maintenance, this flywheel system is applicable to power grid frequency modulation and power quality guarantee. However, it also has some shortcomings such as low energy density and the high cost of ensuring the system's security. Its advantages cannot be manifested on a small scale. At present it is mainly used to supplement the battery system.

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Technologies of energy storage systems

In Grid-scale Energy Storage Systems and Applications, 2019

2.2.3.2 Key technologies

Flywheel energy storage is now at the experimental stage, and there are still five main technical problems: the flywheel rotor, bearing, energy conversion system, motor/generator, and vacuum chamber.

1.

Flywheel rotor. The flywheel rotor is the most important part of the flywheel energy storage system. The transformation of energy of the whole system depends on the rotation of the flywheel. It is necessary to develop the flywheel with high strength and suitable structure based on rotor dynamics design.

2.

Support bearing. Bearing technology supporting a high-speed flywheel is one of the key factors restricting the efficiency and service life of the flywheel.

3.

Energy conversion system. The core of the flywheel energy storage system is the conversion between power and mechanical energy, which adjusts energy input and output of the conversion process to coordinate the frequency and phase. The energy conversion unit determines the efficiency of the system and governs the operation of the flywheel system.

4.

Generator/motor. The high speed of the flywheel energy storage rotor leads to the high speed of the flywheel motor, which requires high efficiency, low power consumption, and high reliability of the flywheel motor system. The current research on permanent magnet motors focuses on reducing loss and resolving the temperature sensitivity of a permanent magnet.

5.

Vacuum chamber. The vacuum chamber is the auxiliary system of the flywheel energy storage system that makes the system independent from the outside environment.

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Classification of energy storage systems

Ahmad Arabkoohsar , in Mechanical Energy Storage Technologies, 2021

1.5.2 Flywheel

Flywheel energy storage is a smart method for storing electricity in the form of kinetic energy. The idea behind this technology is that the surplus electricity to be stored drives a motor that spins a flywheel thousands of rounds per minute to store kinetic energy. The flywheel moves easily because of being levitated in an evacuated chamber with magnets and highly efficient bearings. The stored kinetic energy is the momentum of the flywheel and can actuate an electricity generator as another part of the system to produce power. Low maintenance costs, a long expected lifetime, fast response, and roundtrip efficiency of about 90% are of the main advantages of flywheel systems. The main disadvantages are high cost, self-discharge risk, and appropriateness for smaller capacities only (from 3 kWh to 130 kWh) [18].

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Basic Concepts and Control Architecture of Microgrids

David Wenzhong Gao , in Energy Storage for Sustainable Microgrid, 2015

1.7.2 Flywheels

Flywheel energy storage unit (FESU) can supply immediate active power support for a renewable energy based microgrid. It has numerous merits such as high power density, high conversion efficiency and long life-span. In the past few decades, it has been used in uninterruptible power supplies where the short-duration power changes reduce the battery lifetime. In the context of autonomous energy production, flywheels are used in the field of transportation and in space applications for energy transfer and, particularly, to stabilize or drive satellites (gyroscopic effect) [35]. Flywheel energy storage is characterized by its long lifetime (typically 20 years) [36,37].

A flywheel is a disk with a certain amount of mass that can spin to store energy in kinetic form. To prevent the influence of gravity, the disk in flywheel ESS are built in perpendicular position of the rotor. Flywheels can be charged by electric motors when there is excessive electricity. It can also act as a generator when discharging.

Due to the existence of friction, eventually flywheels will lose some energy. Hence, minimizing friction can help to improve their efficiency. This goal can be realized through two approaches: the first one is to make a vacuum environment for the flywheel to spin in, ensuring there will be no air resistance. The second approach is to install a permanent magnet or electromagnetic bearing to make the spinning rotor float. The spinning speed of modern flywheel energy storage system can reach up to 16,000   rpm with a capacity of up to 25   kWh.

Flywheel have low maintenance costs, and their life-span can be long. There is no greenhouse emission or toxic material produced when flywheels are working, so it is very environment-friendly. The response time is very short. The drawbacks of flywheels are the small capacity and high power loss, ranging from 3% to 20% per hour.

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Smart grid power system

Neeraj Gupta , ... Karan Singh Joshal , in Advances in Smart Grid Power System, 2021

7.4 Flywheel energy storage

Flywheel energy storage (FES) is one of the most efficient technologies for storing electric energy in the form of kinetic energy by constantly spinning a disk or the rotor of a flywheel. The key components of FES systems are a rotating cylinder, bearings, a generator or motor, and a container to accommodate the flywheel [ 9]. The charging process requires high acceleration for spinning of the rotor by acquiring the electrical energy given to the motor. This electrical energy is stored in the flywheel by keeping the body rotating at a constant speed. During the discharge process—i.e., when electrical energy is required, the disk rotates the shaft connected to the generator to produce electricity. An FES system can be used as another option for batteries. An FES system comes with a long life, low maintenance, and high power density. The short duration of operation and high losses due to self-discharge are the major drawbacks of this system. Compared with batteries, this storage system can operate under a wider range of temperatures and has no toxic impact on the environment.

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Historical background

G. Genta , in Kinetic Energy Storage, 1985

1.7 Future developments

It is still a matter of discussion whether flywheel energy storage will, in the future, become very common. This issue depends on many factors—the concentrations of effort, the short-term progress in other types of energy storage devices and, perhaps more important, the political and economic importance given to energy conservation and ecological problems.

There are two schools of thought: one is mainly based on the advantages of flywheel devices—their very high power density, high energy density, high efficiency, ability to be built in a wide range of sizes from a few kilograms to hundreds of tons, their conceptual simplicity and the availability of output energy directly in mechanical form.

Those who follow this opinion think that flywheel systems will be built at low cost and with great reliability within a short time and that the flywheel will take over in a large variety of applications.

On the other hand, the drawbacks are the complexity of the design, the need of high angular velocities and the related dynamic problems, the frequent need for continuously variable transmission, the high cost of current systems, the need for complicated and heavy ancillary equipment which lowers the overall energy density and, sometimes, the efficiency. It is not worthwhile to devote time and money to develop systems which have never demonstrated great economic advantages, when other energy storage systems are already available.

The author holds that both points of view are valid and does not share either the optimistic views of the first, or the pessimistic opinions of the second. If the efforts in their development are maintained, flywheel energy storage systems may come to be used in industrial practice among the other energy storage devices because their advantages can be exploited in certain applications. However, they are far from being 'the ultimate energy accumulator' that some people seem to think. It is also the opinion of the author that it is useless, for many applications, to seek for the 'optimum flywheel' or the 'very-high-performance flywheel'. A reliable, safe, well-designed and well-built medium-energy density rotor is enough for most applications. The maximum effort should now be devoted to the design of sound, reliable and cheap kinetic accumulators with all the necessary devices to give them a good all-round performance.

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A FLYWHEEL ENERGY STORAGE AND CONVERSION SYSTEM FOR PHOTO-VOLTAIC APPLICATIONS*

A.R. Millner , in Energy Storage, 1980

SYSTEM CONCEPT

Such a device would have a DC motor, an energy storage flywheel and a 60-Hz AC generator. It would be supported on magnetic bearings in a vacuum housing, which would do double duty as safety containment for the flywheel rotor. The motor-generator would in fact be the same device with separate input and output electronics. The electronics are shown schematically in Figs. 2 and 3 and are very simple and inexpensive because the permanent magnet motor-generator concept is used.

Fig. 2. Motor power electronics

Fig. 3. Generator power electronics

All solar/electric power would go through the DC motor to spin up the rotor. The motor would be of a DC brushless, ironless armature-type design controlled as a maximum power tracker for the solar array. This is important because the varying electrical output of a solar array is generally mismatched to the characteristics of the storage system and load, causing inefficient operation. The flywheel rotor could be an advanced design of one or another of the types presently being tested for DOE by a number of organizations. The electrical output would be accomplished by use of a permanent magnet brushless generator and a silicon-controlled rectifier (SCR) cyclo-converter. The generator would be sized to accept the large surge demands of many candidate loads.

The entire rotating unit would be supported on a DC magnetic bearing. This could be powered from windings on the motor, which would allow fail-safe, spin-down operation. The resulting rotating unit would have no brushes or physical contact with the rotor, allowing very long life and high reliability. Also, the higher speeds possible with such an assembly (perhaps 20,000 rpm) allow smaller rotor size and enhance the quality of available AC power. Mechanical touchdown bearings would be included for cold start/stop conditions. No rotating seals would be required.

The vacuum chamber ensures a relatively long energy storage time before aerodynamic losses become a problem. The unit can be placed underground to provide safety confinement. The system would be run over a 2:1 speed range with the output held at 60 Hz, generated independently or synchronized to an external line. This corresponds to a 75% depth of discharge for the energy storage function. It is intended that the flywheel unit shall be able to "cold start" from solar PV input power. This can be done by deriving DC control power from the DC input until the wheel is up to speed, and using the maximum power tracking feature of the input power circuit to avoid pulling the input voltage down too low.

The terminals of the motor-generator serve as a summing junction for motoring and generating currents, both kept synchronous by the electronics. This allows arbitrary combinations of charging and discharging power levels within the design range of the unit.

An attractive practical application for this device is to utilize it with a PV-powered single-family residence operating in either a stand-alone mode or coupled to a utility, with utility power drawn only during off-peak hours and only when the energy reserve in the flywheel is low. It can be scaled up to larger sizes associated with multiple dwelling units or commercial PV applications. The residential system concept is illustrated in Fig. 4.

Fig. 4. Artist's concept of residential PV system

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Microgrids architectures

Vijay K. Sood , Haytham Abdelgawad , in Distributed Energy Resources in Microgrids, 2019

1.4.10 Microgrid implementation at the University of Manchester, United Kingdom

The hardware topology used in the University of Manchester microgrid/flywheel energy storage laboratory prototype is shown in Fig. 1.11. The overall microgrid test system is nominally rated at a 20   kVA, although the flywheel and power electronics are rated much higher at 100   kW connected with 0.4   kV mains supply of the laboratory, which is considered as the main grid. A synchronous generator coupled with an induction motor acts as the micro-source. A three-phase balanced load of 12   kW is connected at the end of the feeder. Control systems for real-time control of the microgrid hardware, using the Simulink/dSPACE control environment, was developed in 2005. The test-rig was designed to allow the investigation of power electronic interfaces for generation, loads, or energy storage. The AC/DC inverter, labeled 'flywheel inverter', can be configured in software to allow the interfacing of the flywheel storage system to the remaining microgrid unit. The microgrid may be operated in an islanded operation. Breaker 1 is opened to emulate a loss of mains condition as shown in the Fig. 1.11. Breaker 2 is under the control of the microgrid controller system. Thus the onset of islanding, and resynchronization to mains, as well as mains-connected operation may be tested [43].

Figure 1.11. University of Manchester microgrid/flywheel energy storage laboratory prototype [43].

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Energy Storage

Asmae Berrada PhD , Khalid Loudiyi PhD , in Gravity Energy Storage, 2019

Flywheel Energy Storage

Although it is being integrated in the energy market at slow pace, flywheel ES is considered as a very interesting technology. It is one of the first mechanical storage methods. Flywheel ES uses kinetic energy as a form of storage. This technology has many advantages such as its high efficiencies ranging from 90% to 95%, its long-life cycles and long lifetime (15–20   years) [45]. Issues associated with this storage technology include the high capital cost of $1000–$5000/kWh and the high self-discharge rates that can go from 50% up to 100% [46]. Therefore, flywheel storage can be used especially for large storage capacities. In addition, because of its high self-discharge rates, this technology can be seen as effective only when storing energy for short periods of time. It is also used to regulate current fluctuations in power output from RE sources.

Flywheel stores energy in the form of rotational kinetic energy. It could be expressed as Eq. (1.2):

(1.2) E K = 1 2 I m ω f 2 with I = 1 2 m f r f 2

where E k is kinetic energy; I m is the moment of inertia; m f is the flywheel mass; ω f is the flywheel speed; and r is the radius of the flywheel. Increasing the speed of the flywheel and its movement of inertia increases the energy production.

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