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Using Flywheels for Energy Storage

Spinning up carbon fibre discs instead of burning petrol.

By Charles Bakis*

Click on pics to view larger images

This article was first published in 2000.

One of the proposed alternatives to the internal combustion engine is to use energy-storing flywheels. Here one of the experts in the field takes us on a quick guided tour of the current state of the art.

Flywheels that store and release energy are commonly encountered, though seldom considered. Petrol engines that power our cars and electric motors that run our air conditioners all use some sort of rotating mass - a flywheel to smooth out rapid fluctuations in power production and loadings. As one of the great names in mechanics - Isaac Newton - once said, objects at rest tend to remain at rest and objects in motion tend to remain in motion, unless acted upon by external forces. It is this phenomenon that results in a need for a large amount of energy to be added or subtracted from a massive flywheel to impart a noticeable change in speed. Hence, our car engines run smoothly rather than in the bone-jarring fashion that would otherwise result from the discrete power pulses of each piston firing. Also, the heavily flywheeled (and low-geared) engines of farm tractors are very hard to stall, despite the worst heavy-footed clutching.

Saving For A Rainy Moment

Industrialists have known for years that flywheels can be used to store a large reserve of energy over a long period of time by slowly adding energy from a limited power supply. The reserve energy can be drawn down rather rapidly if the need arises - more rapidly than the original power supply permitted if unassisted by the flywheel. This is a little like filling a large balloon by exhaling into it one puff at a time.

Examples of this concept in current industrial practice include the several-ton flywheels sometimes used to provide vast amounts of current to melt steel in electric arc furnaces. The transfer of energy to the flywheel is done slowly by an electric motor, which draws only a limited amount of power from the town grid - and therefore does not dim the lights in neighbouring homes every time it is activated! Then, when the time comes to zap the steel, the primary energy is drawn from the flywheel via the same motor now being driven by the rotating flywheel and thereby acting as a generator. These dual-purpose motors are called motor/generators since they can be used to either add or take energy from the flywheel.

Why All The Fuss?

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Back in the late 1970's and early 80's, many research programs were aimed at flywheels for energy storage aboard hybrid internal combustion/electric or purely electric cars. The impetus for this activity was the series of oil shortages and price hikes around that time. Despite formidable gains in fundamental knowledge and technology achieved during those years, the basic motivation for alternative energy storage devices lost its lustre as fuel prices stabilised and then dropped during the 80's. For the ensuing decade, flywheel research continued at a relatively slow pace.

In the early 1990's, a new race emerged among many of the major industrialised nations of North America, Europe and the Far East to build the first economically viable flywheel battery that could replace conventional chemical batteries for certain applications. This ongoing race actually involves more than just flywheels - superconducting magnetic energy storage devices, ultracapacitors, and advanced chemical batteries are all in the running.

However, flywheels made of engineered materials provide one of the leading high-risk, high-payoff approaches for achieving a desired ten-fold increase in energy storage or power delivery capability per unit weight. Other attributes that are desired in future energy storage devices are that they be compact, lightweight, reliable, long-lived, affordable, safe, and good for the environment.

Engineered Materials

Engineered, or advanced, materials are one of the keys to success in the race for a feasible flywheel energy system. While it may seem at first a simple matter to add a lot of low-cost mass to a shaft to manufacture a flywheel, the overall performance of that system is likely to be quite uncompetitive. The paramount performance parameter for flywheel rotor materials (ie the part that rotates and stores the energy) is strength per unit density.

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As the rotor spins, the outermost material that travels at the highest peripheral speed wants to fling itself away from the innermost, slowest material. This self-destructing tendency is commonly called "centrifugal loading" and it is related to rotational speed and density of the rotor material. In the final analysis, less-dense but stronger materials can be spun faster than denser but weaker materials and can also store more energy per unit weight or volume as well. Reducing the weight and volume of the system is important so that related expenses of the flywheel energy system such as bearings, vacuum pump (most flywheels are spun in a vacuum) and overall space needs are reduced.

New generation flywheel rotors are made of the same ultra-high strength, lightweight fibre composite materials that were originally developed for top secret defence applications like the stealth bomber and are now commonly seen in sporting goods like fishing rods, golf clubs, skis, and tennis rackets. The potential benefits of lightweight, high-strength, fibre-reinforced composite materials versus conventional materials for flywheel applications are tremendous, as shown by the comparison (below) of the amount of energy that can be stored per unit rotor mass.

Material Specific Energy
High Strength Aluminium 41.5 W-h/kg
High Strength Steel 54.7 W-h/kg
Glass/Epoxy 144.4 W-h/kg
High Strength Aramid/Epoxy 193.7 W-h/kg
High Strength Glass/Epoxy 213.3 W-h/kg
High Strength Carbon/Epoxy 356.5 W-h/kg

Designed For Speed

The fundamental problem to overcome in flywheel rotor design is the tendency of the rotor to fling itself apart by centrifugal forces. In one design, six conventional fibre-reinforced composite rings were arranged concentrically with the stronger, more expensive materials towards the outer periphery where the stresses are the highest. Some of the rings were press-fitted together in pairs to impose some pre-compression in the radial direction and thereby achieve some extra speed. These ring-pairs were separated from each other by compliant elastomeric interlayers that prevented the transmission of radial stresses from one to the next. The highly stressed outermost ring was made with the strongest carbon fibre commercially available and its shape was optimised for the highest possible speed. In addition, this ring was isolated from the rest of the rotor by an elastomeric interlayer. Speeds of 66,000 rpm (1100 m/s rim speed, 82 W-h/kg specific energy) were achieved with this design.

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When a rigid matrix composite disk fails, it is generally catastrophic. A second design used a hoop-wound elastomeric matrix composite (EMC) disk to eliminate the possibility of catastrophic failure. If the EMC disk is spun too fast, only a small region of material could fail and a catastrophic failure event would not occur. The rotor must then be spun even faster to fail additional material at the outside diameter - a "fail-safe" situation if the dynamic imbalance caused by the first failure event is not catastrophic.

The Long Haul

Although the experience available to date suggests that fibre composite rotors fail in a less catastrophic mode than metallic rotors do, the chance of an accidental rotor burst must be minimized. At operational speeds of upwards of 75,000 rpm, you can easily appreciate that a failure event could launch fragments of composite material at ballistic rates. In order to design a rotor for, say, 15 years of service in the field, the designer must know how well the material will hold up after being subjected to many cyclic variations of speed and stress.

This must be determined in a realistic environment which includes (i) a moderately strong vacuum because flywheels cannot store energy for long in the presence of air drag and (ii) potential temperature fluctuations experienced by flywheels located in small, outdoor bunkers without the benefit of temperature control.

The Future

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The viability of all this research and development effort depends very much on engineers making creative, intelligent use of advanced materials to achieve performance and economic targets established directly by businesses who, in turn, reflect the expectations of consumers. Flywheel energy storage systems must be made more safe, reliable, compact, and efficient than competing systems. The current state of flywheel technology is very close to commercialisation. Most of the necessary technological pieces of the puzzle are in place: high performance rotors, low friction bearings, low-loss vacuum systems, and high efficiency motor/generators. Containment requirements remain a major unknown, prices are still formidable, and long-term durability has yet to be proven, however.

* Professor Charles E. Bakis
Dept of Engineering Science & Mechanics
The Pennsylvania State University
University Park, PA 16802-6812, USA

Another Perspective

The US Office of Transportation Technologies has extensive documentation on all alternative forms of car propulsion. Here's what they have to say on the subject of flywheels...

Although flywheels are being used in some bus applications today, more work needs to be done to make flywheels safe and effective for automotive applications. Current flywheels are still very complex, heavy, and large for personal vehicles. In addition, there are some concerns regarding the safety of a device that spins mass at high speeds.

Flywheels store kinetic energy within a rapidly spinning wheel-like rotor or disk. Ultimately, flywheels could store amounts of energy comparable to batteries. They contain no acids or other potentially hazardous materials. Flywheels are not affected by temperature extremes, as most batteries are.

Flywheels have been used in various forms for centuries, and have a long history of use in automotive applications. Early cars used a hand crank connected to a flywheel to start the engine, and all of today's internal combustion engines use flywheels to store energy and deliver a smooth flow of power from the abrupt power pulses of the engine.

Modern flywheels employ a high-strength composite rotor, which rotates in a vacuum chamber to minimize aerodynamic losses. A motor/generator is mounted on the rotor's shaft both to spin the rotor up to speed (charging) and to convert the rotor's kinetic energy to electrical energy (discharging). A high-strength containment structure houses the rotating elements and low-energy-loss bearings stabilize the shaft. Interface electronics are needed to convert the alternating current to direct current, condition the power, and monitor and control the flywheel. Flywheels could be used in several ways, and all of them exploit the ability to deliver very high power pulses.

One concept combines a flywheel with a standard engine, providing a power assist. Another concept employs a flywheel to load-level chemical batteries. Still another uses a large or multiple flywheels to replace chemical batteries entirely (in some uses, a flywheel is referred to as an "electromechanical battery"). For flywheels to have success, however, they would need to provide higher energy densities than is now available.

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