### Flywheel Energy Storage

July 21, 2011

In a previous article (Green Braking, October 20, 2010), I discussed the use of flywheels as a means of harvesting braking energy in vehicles. Flywheel technology has been with us for millennia, going back to the potter's wheel. The energy E stored in a flywheel is given by the simple equation,
E = ½ I ω2
where I is the moment of inertia of the rotating body and ω is the angular velocity, usually expressed in radians per second. If the rotating body is a solid cylinder, the moment of inertia is given as
I = ½ m r2
where m is the rotor mass, and r is its radius.

These equations reveal that you can store a lot of energy in a flywheel if it's large and spins very fast. If you want to do this, you need to keep several things in mind:
• To prevent the flywheel from losing energy by heating the air around it, it needs to operate in a vacuum.

• Since the flywheel material will be under considerable tensile stress, it should be fabricated from a high strength material, preferably one that is not prone to fracture.

• Since flywheels spin up, then down, repeatedly, there the possibility of material fatigue that will lead to fracture, so flywheels should be enclosed in a protective shroud to contain flying schrapnel.

Because of its manifold advantages, the alternating current electrical power distribution system of George Westinghouse won the War of Currents against Thomas Edison's direct current. The biggest advantage of alternating current is that transformers can be used to convert low current, high voltages to higher current, low voltage. This is important when conducting electrical power over long distances, since there is less loss in low current lines. Power loss in a wire of resistance R is I2R, where I is the current.

A typical US residential power distribution transformer mounted on a utility pole.

This transformer sits on a pole at the left of my house as a reminder that we live in a culture that's evolved technically, but not aesthetically.

Photo by author (Via Wikimedia Commons)

The power line frequency in the US is 60 Hz. This frequency was selected by Westinghouse Electric as a compromise for operating induction motors and lighting on the same electrical circuit. Too low a frequency would cause a noticeable flicker in lights, and too high a frequency increases the impedance of inductance circuits. Other countries use a 220-240 volt, 50 Hz system, and not the 100-120 volt, 60 Hz system used in the US. The essential fact is that once a frequency is defined, everything from the transformers on utility poles to refrigerator motors are designed to give best performance at that frequency.

A recent article in the IEEE Spectrum reviews the use of flywheels as a means of regulating the frequency of electrical generators that are used to supply electrical power.[1] Just as an automobile will slow as it climbs a hill and speed up while going downhill, the same thing happens to electrical generators in response to changing consumer power demand during the day and supply generators being added or removed from the electrical grid. As reported in Spectrum, utility frequency regulation, which is becoming more important as non-predictable power such as photovoltaic and wind power is added to our power palette, is no small market. It was nearly half a billion dollars in 2010.[1]

Performance of a four-section Midwestern wind plant over a two day interval. Wind speed is also shown.

From an Oak Ridge National Laboratory Report.[2]

Such control was somewhat easier in the past, since it could be achieved by adjusting the mechanical speed of generators, but you can't tweak wind speed or passing clouds to give more power. In the case of an extremely variable supply, power must often come from a stored source, or be easily dumped into a storage device. Flywheels are like capacitors for the AC power grid. As always, there are competing technologies, such as electrochemical batteries. The only technologies that are viable in this frequency leveling application are ones that can respond in seconds.

The problem with batteries, according to the Spectrum article, is that they will wear out with the constant charging and discharging. Beacon Power Corporation has a twenty megawatt flywheel facility in Stephentown, New York, with more than 160 metric ton sized carbon fiber composite flywheels spinning hundreds of time per second.[1,3] Beacon says that its flywheels have a twenty year life expectancy in the utility frequency-leveling application.[1] You can view an "Environmental Assessment" for this type of a facility in Ref. 4.

The US Department of Energy's Pacific Northwest National Laboratory has been exploring a different type of control in which commands are sent to homes to turn off heating elements in clothes dryers and water heaters. How much load capacity is there in such devices? The water heater in my house uses natural gas. Although our clothes dryer is electric, I wouldn't want to head to the supermarket in wet clothing during a New Jersey winter!

### References:

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