Many storage technologies have been considered in the context of utility-scale energy storage systems. These include:
- Pumped Hydro
- Batteries (including conventional and advanced technologies)
- Superconducting magnetic energy storage (SMES)
- Compressed Air Energy Storage (CAES)
- Electrochemical capacitors
This CTW description focuses on flywheels. Flywheels rank among the earliest mechanical energy storage mechanisms discovered by mankind. The principle was probably first applied in the potter’s wheel, a device used to produce symmetrical ceramic containers. The millstone, a contrivance used to grind grain into flour, is another form of flywheel. Beginning in the early years of the Industrial Revolution, flywheels found their way into various contrivances to smooth the delivery of mechanical power. In handlooms, for instance, flywheels were used to store mechanical energy applied in pulses by the operator. Flywheels allowed the development of more complex power machines such as steam engines and internal combustion engines by enabling the delivery of constant, continuous power from a pulsating power source. Flywheels continue to have a broad variety of applications in mechanical systems.
In energy storage, the principle of the flywheel can be used. Flywheels store energy in the form of the angular momentum of a spinning mass, called a rotor. The work done to spin the mass is stored in the form of kinetic energy. Video 1 is a simple video that illustrates the concept of flywheel electrical energy storage. The image above is an artist's impression of a energy storage facility that uses flywheels.
In electromechanical systems, the kinetic energy of a moving mass stores electrical energy. The most prevalent type of mass in an electromechanical storage system is a rotating mass, or flywheel. Like electrochemical batteries, flywheels must be part of a fully integrated system that includes sophisticated solid-state power conversion devices, monitors, controls, climate controls, utility and user interface equipment, safety devices and transportation features to be useful in electric power applications (Butler, Miller and Taylor, 2003)..
The mechanics of energy storage in a flywheel system are common to both steel- and composite-rotor flywheels. In both systems, the momentum (the product of mass times velocity) of the moving rotor stores energy. In both types of systems, the rotor operates in a vacuum and spins on bearings to reduce friction and increase efficiency. The rotor, loaded with magnets, is effectively part of an electromagnetic motor/generator that converts energy between electrical and mechanical forms. Steel-rotor systems rely mostly on the mass of the rotor to store energy and composite flywheels rely mostly on speed (Butler, Miller and Taylor, 2003).
During charging, an electric current flows through an electromagnetic coil and creates a magnetic field that interacts with the magnets loaded on the rotor, causing it to spin. During discharge, the spinning magnets on the rotor induce a current in the electromagnet and generate current flow out of the system (Butler, Miller and Taylor, 2003)..
Flywheel technology is characterized by relatively short discharge times and a limited system power rating as illustrated in Figure 1. The short discharge times can be seen as both an advantage and as a disadvantage: short discharge times allows the technology to be used for power quaility applications, but limits its use in large scale applications. However, aggregating several flywheels in a larger installation, as illustrated in the artists' impression above, covers part of this problem. In this case, the second flywheel picks up when the first one is done discharging and is followed by the third, etc.
Comparison with other energy storage technologies.
To use flywheel technology as an electrical energy storage medium offers several advantages and disadvantages compared to the other energy storage technologies. These are summarized in Table 1.
|Power and energy are nearly independent||Complexity of durable and low loss bearings|
|Fast power response||Mechanical stress and fatigue limits|
|Potentially high specific energy||Material limits at around 700M/sec tip speed|
|High cycle and calendar life||Potentially hazardous failure modes|
|Relatively high round-trip efficiency||Relatively high parasitic and intrinsic losses|
|Short recharge time||Short discharge times|
Components of a flywheel energy storage system
A flywheel has several critical components.
a) Rotor – a spinning mass that stores energy in the form of momentum (EPRI, 2002)
The rotor, as the energy storage mechanism, is the most important component of the flywheel energy storage system. The design of the rotor is the most significant contributor to the effectiveness and efficiency of the system. Rotors are designed to maximize energy density at a given rotational speed, while maintaining structural intergrity in the face of rotational and thermal stresses. Rotor designs can be divided into two broad categories of low-speed, vertical or horizontal shaft and high-speed, usually vertical shaft rotors. Both types of rotors have advantages and disadvantages, and the two find uses in different applications.
b) Bearings– pivots on which the rotor rests
The bearings support the flywheel rotor and keep it in position to freely rotate. The bearings must constrain five of the six degrees of freedom for rigid bodies, allowing only rotation around the axis of the rotor. The construction of the bearings is important in flywheel performance. Speed of the flywheel is limited in large part by the friction on the bearings, and the resulting wear on the bearings often defines the maintenance schedule for the system.
There are several types of bearings used in flywheel construction. Mechanical bearings are the simplest form of flywheel bearings. These might be ball, sleeve, roller, or other type of mechanical bearing. These bearings are well understood, reliable, and inexpensive, but also suffer the most wear and tear, and produce the largest frictional forces, inhibiting high rates of rotation.
Magnetic bearings are required for high-speed flywheel systems. These bearings reduce or eliminate frictional force between the rotor and its supports, significantly reducing the intrinsic losses. There are several types of magnetic bearings. Passive magnetic bearings are simply permanent magnets, which support all, or part of the loads on the flywheel. Active magnetic bearings, on the other hand, use controlled magnetic fields, where field strength on the bearing axes is varied to account for the effect of external forces on the rotor. Superconducting bearings are passive magnetic bearings, which use superconducting materials to produce the magnetic repulsive force to support the rotor assembly. These materials operate at very low temperatures, and so require cryogenic cooling systems to maintain.
c) Motor-Generator – a device that converts stored mechanical energy into electrical energy, or vice versa
Motors convert electrical energy into the rotational mechanical energy stored in the flywheel rotor during charge, and generators reverse the process during discharge. In many modern flywheels the same rotating machine serves both functions. The machine is called a motor alternator or motor generator and consists of a wound- or permanent magnet rotor, usually revolving within a stator containing electrical winding through which charge (or discharge) current flows. Note that this machine, along with any power electronics, limits the power rating of the flywheel system. And in some practical systems the generator for discharging the wheel is higher power than the recharging motor. Thus at full power charging the wheel will require more time than discharging.The starter motor and alternator or generator are connected to the flywheel via the same steel shaft and may be either a single machine or two different machines. In both cases the rotor becomes part of the flywheel mass. When separate, the starter motor is typically a simple induction motor that is able to produce starting torque.
d) Power Electronics – an inverter and rectifier that convert the raw electrical power output of the motor/generator into conditioned electrical power with the appropriate voltage and frequency
Most flywheel energy systems have some form of power electronics that convert and regulate the power output from the flywheel. As the motor-generator or alternator draws on mechanical energy in the rotor, the rotor slows, changing the frequency of the AC electrical output.
The main function of these devices is to allow energy to be taken from the wheel before its frequency and power output drop below usable levels. In fact the low-end (i.e., end-of-discharge) cutout speed at which the flywheel is considered discharged is primarily dependent on the current carrying capability of the electronics (or electromechanical coupling) and the size of the load. For example, most flywheels have output current proportional to load and inversely proportional to speed. This means a lighter load can go to a lower speed before the system cuts out on maximum current. T
e) Controls and Instrumentation – electronics which monitor and control the flywheel to ensure that the system operates within design parameters
Flywheel systems require some controls and instrumentation to operate properly. Instrumentation is used to monitor critical variables such as rotor speed, temperature, and alignment. Rotor speed and alignment are also often controlled variables, through activefeedback loops. The latter is especially important for systems with magnetic bearings, and most magnetic systems have complex controls to reduce precession and other potentially negative effects on the rotor. In many systems, other instrumentation is used to monitor performance or design parameters related to failure modes. In some composite flywheel systems, for example, instrumentation is used to measure deformation of the rotor over time, alerting operatorswhen the rotor shape indicates possible failure in the future.
f) Housing – Containment around the flywheel system, used to protect against hazardous failure modes. It is sometimes also used to maintain a vacuum around the rotor to reduce atmospheric friction.
Theory of flywheel operation
a) Energy storage capacity
The amount of kinetic energy stored in a spinning object is a function of its mass and rotational velocity, which can be expressed through the following formula:
Kinetic Energy = 1/2 x moment of inertia x rotational velocity
The moment of inertia is dependent on the mass and geometry of the spinning object. Increasing the rim speed, which is the speed at the outer end of the wheel, is more effective in order to store additional kinetic energy than increasing the mass of the flywheel.
Figure 3 briefly illustrates several of the main factors in a flywheel system.
b) Rotor stresses and failure modes
Flywheels with a large radius rotating at very high speed, in order to maximize rim velocity, are favored according to the formula above. However, flywheel design is limited by the strength of the rotor material to withstand the stresses caused by rotation. If the rotor spins too quickly, it will fly apart, ending the useful life of the flywheel and possibly causing harm to personnel and damage to nearby equipment in the process.
c) Energy conversion
Flywheels store kinetic energy while the end-use applications for which the energy is stored require electrical energy. Conversion from kinetic to electric energy is simply accomplished via electromechanical machines. Many different types of machines are being used in available flywheel systems. The key is to match the decreasing speed of the flywheel during discharge and the acceleration when recharged with a fixed frequency electrical system. Along with electromechanical machines, two methods are used to match system frequencies, mechanical clutches and power electronics. The trend is toward a power electronic frequency conversion, with mechanical clutches only seen in the larger low-speed machines.
d) Energy losses and friction
In any real flywheel system, there are forces that act against the spinning wheel, causing it to slow down and lose energy. These forces arise from friction between the rotor and surrounding environment, between the rotor bearing and its support, and from the stresses and strains within the rotor itself. In addition to these energy losses through friction, the minute stress differentials within the spinning rotor and induced magnetic currents in the motor/generator can also cause energy losses.The mechanical bearings, which support the flywheel rotor, are a significant source of friction. Many developers have introduced magnetic bearings into the flywheel system, which remove load from mechanical bearings and reduce frictional losses.The fluid surrounding the rotor is also a source of frictional loss. At higher speeds, this loss can be very large indeed. Most developers have addressed this problem by enclosing the rotor within a vacuum or low-viscosity fluid.
e) Thermal effects
The energy lost during rotation is transformed into heat, which raises the temperature of the flywheel rotor. If heat accumulates it must somehow be removed to prevent damage to the rotor and other components. Material considerations will define a maximum temperature for the rotor. One way to reduce heat is to limit the operating speed of the flywheel system so that the steady-state temperature of the rotor is within a safe margin of the maximum temperature. This speed limitation will also reduce the energy density of the flywheel system.
The answer to this problem has been low-loss bearing technology, which has kept thermal effects from being a limiting factor in most practical flywheel systems. Vacuum containment and magnetic bearings can significantly reduce friction, and therefore reduce the amount of heat that must be removed. The trade-off is that they also can make it difficult to remove the heat that remains.
There is an ever-growing selection of new flywheel products on the emerging on thecoattails of advances in technology. Consequently there are also a number ofapplications that now propose using flywheels as the energy storage medium. Theseinclude inrush control, voltage regulation and stabilization in substations for light rail,trolley, microturbine and wind generation. Still the majority of products currently beingmarketed by national and international-based companies are targeted for power quality(PQ) applications. And the number one application in power quality is short-termbridging through power disturbances or from one power source to an alternate source.
Flywheels are being marketed as environmentally safe, reliable, modular, and high-cyclelife alternatives to lead-acid batteries for uninterruptible power supplies (UPSs) and otherpower-conditioning equipment designed to improve the quality of power delivered tocritical or protected loads.
Three energy market support contributions of this technology are identified in the 2002 EPRI study:
a) System stability and damping
Large power systems may experience instabilities associated with the delivery of power over long distances when there are abrupt changes in operating conditions. Such instabilities can cause high economic damage to installations. An example of such a instability is the north-south power corridor on the West Coast of the United States. A great deal of power is generated in the Pacific Northwest and is delivered to middle and Southern California via multiple transmission lines. Oscillations in power through this corridor are generally insignificant but can be, under certain conditions, damaging. The conventional method to address this issue is to install additional transmission capacity in order to reduce the sensitivity. Therefore, stability can be achieved by adding additional transmission lines, but this is at the expense of a variety of other issues such as environmental degradation.
Renewable energy technologies are often located far from the location where the electricity is required. This is for instance the case with large scale solar and wind farms. To maintain system stability without energy storage with a high discharge rate, implementing additional transmission lines would be necessary.
Concluding: system stabilization, as provided by the distributed form of SMES, provides benefits primarily through the avoidance or deferral of new transmission requirements. In other words, the higher reliability of the system removes the need to construct additional transmission lines. As such, SMES is a technology that supports the deployment of renewable energy technologies.
b) Power quality
Power quality and back-up power is necessary at industrial installations substations of the electricity grid. The power quality and back-up energy is used for a variety of conditions such as when momentary disturbances require real power injection to avoid power interruptions. In the case of industrial customers, a local source of power may be required when there is an interruption of power from the utility. This power source may need to function until power from the utility is restored, until a reserve generator is started, or until critical loads are shut down in a safe manner. As such, SMES technology can provide this role of power quality and back-up power source. In the case of a substation, a variety of momentary disturbances such as lightning strikes or transmission flash overs cause power trips or low voltages.
Therefore: power quality benefits result primarily from reliable service. Issues such as outages and voltage sag can be avoided which can have high economic benefits.
c) Load levelling
Demands for electric power vary both randomly and with predictable variations. Perhaps the most significant variation of power demand is the diurnal change associated with the functioning of an industrial society. Both commercial and residential demands are greater during the day than at night. On the other hand, many power plants operate most efficiently and have longer lives if they operate continuously near their maximum power output. One method of accommodating users’ power demands and the characteristics of these plants is to install an energy storage system that can accept energy at night and can deliver it back to the grid during periods of high demand. The value of this type of storage is based on the difference in marginal cost of off-peak power and the price paid for power during the peak. An additional impact of diurnal storage is that it can replace or defer the installation of extra generation capacity to accommodate.
Therefore: load- levelling benefits, or arbitrage benefits, result from the differential between the cost of on-peak and off-peak power.
Schoenung and Hasselzahn (2003) studied the life cycle costs of various energy storage technologies, including flywheel technologies. They differentiated between high speed and low speed flywheels. In addition the results include three different high-speed flywheel systems. The three are quite different in cost as one is designed for longer output, and the other two are optimized for a 20-second output. It is not possible to show a "generic" high-speed flywheel system (Schoenung and Hasselzahn,, 2003).
The annual costs for several energy storage technologies is displayed in Figure 4. This Figure relates to power quality applications of the energy storage technologies, and it can be seen that flywheel costs increase relatively marginally with longer discharge times compared to some of the other energy storage technologies.
The components of annual costfor one-second systems are shown in Figure ... . All technologies are dominated by capital cost. As shown in Figure .. ,micro-SMES and supercapacitors are particularly attractive for one-second discharges. Lithium-ion batteries also have potential for this application because of the potential for long life.
Battery systems and some flywheels can beoptimized for greater storage capacity. Abattery sized for one-second of discharge isthe same as a battery sized for 20 or 30seconds of discharge. This is not true formicro-SMES or supercapacitors. So thecomparison changes dramatically for 20-second systems, as shown in Figure 14. Notealso that the cost of a SMES systemincreases due to the large electrical powerrequirement for refrigeration.
At short discharge times, the costs are similar for all systems except the hydrogen fuel cell, whichis more expensive. With increasing storagetime, the curves diverge.
APS, 2007. Challenges of Electric Energy Storage Technologies: A Report from the APS Panel on Public Affairs Committee on Energy and Environment. Document can be found online at: www.aps.org/policy/reports/popa-reports/upload/Energy_2007_Report_ElectricityStorageReport.pdf 
EPRI, 2002. Handbook for Energy Storage for Transmission or Distribution Applications. Report No. 1007189. Technical Update December 2002. Document can be found at: www.epri.com 
Schoenung, S., M., & Hassenzahn, W., V., 2002. Long- vs Short-Term Energy Storage Technology Analysis: A life cycle cost study. A study for the Department of Energy (DOE) Energy Storage Systems Program. Document can be found online at: http://www.ntis.gov/help/ordermethods.asp?loc=7-4-0#online 
Butler, P., Miller, J. L., Taylor, P. A., 2002. Energy Storage Opportunities Analysis Phase II Final Report A Study for the DOE Energy Storage Systems Program. Document can be found online at: prod.sandia.gov/techlib/access-control.cgi/2002/021314.pdf