Large metallic flywheels have long been used in many applications as a means of storing and smoothing energy. Flywheels store energy through accelerating a rotor up to a high rate of speed and maintaining the energy in the system as kinetic energy. The flywheel releases its energy by reversing the charging process and slowing down. A flywheel can be designed to either release a large amount of energy in a very short period, or a small amount of energy over a longer period. Flywheels also may be divided into two main types, low speed and high speed.
Development of flywheels as a stand-alone energy storage unit for electrical power was made possible by advances in power electronics that allowed for the efficient voltage and frequency control of the power output regardless of the rotational rate of the flywheel. However size and resultant bearing stresses limited the speed and thus the amount of energy that could be stored. Subsequent advances, notably in carbon fiber materials and magnetic bearings allowed for higher energy densities. Graphic courtesy of Vycon.
Flywheels store energy proportionally to the mass of the rotor and to the square of its rotational surface speed. Therefore the best way to make it store more energy is to make it spin faster, not to make it heavier. It is important to understand that it is the surface speed that is important--not simply rpm's--so a smaller diameter flywheel, rotating much faster, can have the same energy level as a larger one, rotating slower.
Low speed metallic flywheels are used to smooth out the speed of engines, other rotating machines and in uninteruptable power supplies (UPS). The flywheel attached to the rotating shaft of a engine, moderates fluctuation in the shaft's speed by temporarily storing excess energy created during the power stroke and releasing the energy during the non-powered stroke. Low speed flywheel systems, designed for the UPS market with short spurts of power, generally have a heavy solid steel rotor and rotate at speeds less that 10,000 rpm and a mass upwards of 5,000 pounds.
The basic components of a high speed flywheel energy system are the rotor motor/generator bearing system, vacuum housing and power electronics. The rotor is the most important part of the system, as its design determines the amount of energy that can be stored.
High speed flywheels systems usually spin a lighter rotor at much higher speeds, potentially up to 100,000 rpm but generally in the 20,000 to 60,000 rpm range and a mass of 1,500 pounds or less. Because of the increased stresses at these speeds, high speed flyweel rotors are normally constructed from composite materials, such as fiberglass or carbon fibers impregnated with epoxy, wound into a thick cylinder. These materials are of lower density and higher strength than steel and provide the best combination of properties for this application.
To reduce losses in high speed systems magnetic bearings are used and the system is enclosed in a vacuum chamber to reduce aerodynamic drag. Magnetic bearings use magnetic forces to levitate the rotor and eliminate frictional losses from rolling elements and lubrication. These two features, although they reduce heat build up in the system, do not eliminated it. Unfortunately they also minimize the removal of heat from the system and some type of cooling is required.
Flywheels store energy through accelerating the rotor up to its operating speed and maintaining it at that speed by the addition of a small constant amount of energy input to overcome bearing friction and aerodynamic drag. When power is needed, the process is reversed by using the motor as a generator. It is possible to deeply discharge the flywheel without any damage to the unit because the energy is stored mechanically, not chemically. Thus nearly an infinite number of cycles, considering a 20-25 year life of the device, can be obtained.
Since the power and energy components are decoupled in flywheels these systems can be loosely classified into two categories optimized for either power or energy. Optimizing for power requires a greater emphasis on the motor/generator and power electronics, while optimizing for higher energy densities requires a larger, high speed rotor. As improved power electronics, vacuum housings, and magnetic bearings become more widespread, round-trip efficiencies of flywheel systems have improved and many current production models are in the 70% to 80% range, with some of the new designs even higher.
APPLICATIONS:
UPS--The flywheel provides power during the period between the loss of utility power and either the return of utility power or the start of a sufficient back-up power system i.e. diesel power generator. Flywheels provide 1-30 seconds of ride-through time and back-up generators can typically be online within 5-20 seconds. More than 90% of power quality disturbances are short-term, lasting for less than 2 seconds, and 98% last for less than 30 seconds according to the Electric Power Research Institute (EPRI). Flywheels are sometimes used in conjunction with batteries in UPS applications with the flywheel providing power during the more frequent small disturbances and the batteries used during the longer outages. This dramatically extends the battery life and provides much better reliability and lower life-cycle costs.
Frequency Regulation--Modern high speed flywheels can be used in the electrical supply system to regulate frequency. Fluctuations in voltage or frequency—measured in cycles per second, or hertz—wreak havoc on the power grid. In matching the supply and demand of the grid, frequency varies as generating stations are adjusted or turned off and on to meet the demand. This creates inefficient use of generators and relatively poor frequency regulation. In this application, physical size is less of an issue, and for instance, Beacon Power has a system that puts 10 100-kilowatt flywheels in a shipping container. Each system can provide a megawatt of power for 15 minutes. By operating the system, rather than selling it Beacon will generate about $350,000 of revenue per year. Last year, U.S utilities paid $190 million for energy used for frequency regulation. Beacon has demonstration systems operating in California and New York.
Distributed Generation--The power generation technologies available today, such as microturbines, wind power, and diesel or natural gas generators, are not capable of replicating the power quality available through the existing power grid by themselves. By complementing these systems with a flywheel, the power quality of the system is increased as ride through during start up and voltage sag problems are both corrected. Flywheels can instantly absorb power when demand drops and deliver power when demand rises, allowing the distributed generation device to catch up with the load a few seconds later. This is called Load-Following.
Renewable energy sources, especially wind, suffer from intermittency that can be reduced through the use of flywheels. Through incorporating the flywheel-based energy storage unit into the installation of the wind turbines, three problems can be addressed: to stabilize the frequency variations stemming from the turbines, to capture excess energy from short-term wind gusts and to eliminate the need for spinning/standby generator reserve due to the introduction of the wind turbines.
Energy Recycling--Flywheels are capable of improving the efficiency of repetitive motion operations by capturing wasted energy by converting it to kinetic energy. Flywheel technologies with higher cycling capabilities fits in well with many operations in industrial and especially transportation applications where repetitive starts and stops produce very inefficient uses of energy. Applications include heavy lift operations, light-rail/subway systems and manufacturing operations with motors constantly turning off and on. An installation in a San Francisco rail system enabled a 20% reduction in energy purchases through capturing the braking energy of the trains. Besides the energy saved, the stored energy reduces the energy demand spike required during acceleration, creating less wear and tear on the equipment.
Pulse Power--Flywheels can be used to provide short burst of power in a variety of applications. Some include military applications, but much larger variety of applications can be found in a variety of industrial uses and engine starting in the transportation sector.
Future
Low lifetime costs and the ability to survive in harsh operating environments are the core strengths for the future success of flywheel energy storage technologies. Rapidly maturing from an already cost-effective base, life-cycle costs are already becoming a greater concern to owners--especially those who have already purchased earlier battery solutions for harsh applications. Flywheels currently represent 20% of the current $1billion market for the energy storage component of high-powered UPS market. With a greater understanding of space and environmental requirements of other technologies combined with a better understanding of the minimal space and space-conditioning requirements of flywheels, flywheel energy storage technologies provide a significant advantage in life-time costs and usability--leading to a growing market penetration in this existing market. However the emerging markets of distributed energy, frequency regulation, etc-- hold out the greatest opportunity in the long run for a much greater penetration of flywheel technology into industrial settings.
Suppliers
Steel rotor flyweels still dominate the market place and including the following suppliers: Active Power, Hitec Power Protection, Pillar Technologies, Precise Power Corporation, and Satcon Power Technologies.
Developers of higher-speed composite rotor based systems include: AFS Trinity, Beacon Power, Pentydyne Power Corp., and Vycon
Resources:
Flywheels, Electric Storage Association, Technologies & Applications, Technologies
Flywheel Energy Storage, EERE, Federal Energy Management Program
California Distributed Energy Resource, California Energy Commission
More blogs about flywheels, energy storage, energy, technology
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