Flywheel Energy I: Safety, Security, Reliability

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We “pay” close attention to the potential loss of our reputation, our money, our health, our lives, or even our soul. These are the important issues for which we then “pay” large sums of money to our lawyers, accountants, doctors, the church, and the state. Risk is a cost that is central to our lives and, naturally, a consideration in any technology like Flywheel Energy Storage (FES.) There are many aspects to risk. Sometimes it is a cost born by the general public and provides a reason for the “public’s right to know.”

Risk management is a little like managing grid electricity: it is difficult and costly to store a sufficient response, so we tend to spread it around. In a few days, I will examine implications of risk more closely in another industry. Here, I am looking at some areas of interest for risk management in flywheel energy technology.

Industry Cooperation

Energy is like a wild animal that can be contained and trained but never truly tamed. “Just as early auto engineers learned to minimize the substantial fire hazard inherent in a tank of gasoline, [we…] expect to address concerns about flywheel safety successfully.” For the safety of flywheels, we look at the “cage,” possible escape routes, and the potential effects of the energy released should the structure fail. To address these issues, DARPA (Defense Advanced Research Projects Agency) assembled the Flywheel Safety Project in 1995 and later the Flywheel Safety and Containment Consortium. The Consortium includes Trinity Flywheel Power, US Flywheel Systems, SATCON, the University of Texas Center for Electromechanics (UTCEM), Beacon Power, and some national laboratories. While bearings, rotors, and motors/generators were considered proprietary, containment issues were jointly considered.

Power Density and Flywheel Composition

We often (but not always) want our flywheels to spin fast and not come apart, so composite materials, like carbon fiber, are a common industry choice. Maximum speed control is also an important consideration. If we double the mass of a flywheel we can double its energy density, but if we double its speed, the energy density will increase four times. Speed, however, is limited by the tensile strength of the flywheel. Doubling the flywheel speed will also produce four times the centrifugal force. The maximum energy can be stored in materials that combine high tensile strength with low material density. Dense materials, like metals, are not often the best option. Composite materials, like carbon fiber, tend to be non-hazardous, easily recycled, and also tend to fail in a less damaging way than metals.

Beacon Power: Flywheel and Underground Concrete Containment


If a flywheel were to fail, the energy, like a wild animal, escapes to do damage. A manufacturer could reduce risk by operating the flywheel at several times below its failure speed, but this will also substantially reduce its energy density. Flywheels are tested to determine their maximum spin potential. As the operating speed comes closer to the failure speed, risk reduction shifts to containment systems. Three containment systems are:

  1. brute force: material of sufficient mass and thickness.
  2. rotating containment: an outer shell that absorbs energy, like a flywheel in a flywheel.
  3. soft catch: a material that spreads out energy impacts, like a net or a sponge.

Containment vs Power Density

We want a high power density. We also want to minimize our risk. Containment minimizes risk but at a cost to power density (KW/$, KW/# and KW/volume). Higher energy densities often require higher containment costs. Manufacturers strive for the best balance. A solution for stationary systems is the cheapest brute force method of concrete bunkers or tubes. For these systems, energy density by weight or size is not usually as much concern. This is why the first flywheel energy storage systems we are beginning to see are stationary systems. Efficient vehicles need containment systems that are lighter, smaller, and more likely in the near future.

Real World Lessons

Armed with a bit of knowledge, we can assess a real world example. Recently in the news was a flywheel bicycle: The flywheel was 15# of steel on a bicycle that may have been no more than twice that weight. It was massive rather than fast. The power density is likely somewhat low. The flywheel has no containment system but is operating well below its failure speed. A flywheel failure in this location would not be appreciated by the operator. The balance goes to cost.

Although flywheels are considered a safe technology, they have failed. A number of years ago a German engineer performing some testing on a flywheel engineered to fail died when the containment system proved inadequate for the low speed test. Cars also have flywheels, and you can find many examples of auto flywheel failures on the web.

Beacon Power (BCON) tends to be conservative. Its composite flywheels are not operating at the highest industry speeds. When just starting out as a private company, Charles Platt reported that one of the recipients of the first flywheels, “… broke that first flywheel several times …[because they]… wanted a catastrophic failure, so [they] could make it better.” In the same way, we crash test cars. Destructive testing is a part of the development process.


In an article published, July 29, 2011, the Eastwick Press wrote of a failure two days earlier at the Beacon Flywheel plant used for frequency generation. This caused an investor and financial adviser who had recommended Beacon stock to revise his analysis based upon some curious back-of-the-envelope math and citing that Beacon Power was not very forthcoming. The Eastwick Press updated its report with Beacon’s preliminary investigation suggesting that there may have been a manufacturing defect. The investigation continues.

From that time, the area has suffered an Earthquake and a large tropical storm (that had just been downgraded from a hurricane). Gene Hunt, Director of Corporate Communications for Beacon Power, explained that the damaged flywheel was replaced within 10 days and provided a response to inquires for this article:

“[Regarding the flywheel failure] The major stakeholders in our Stephentown plant – the NYISO (our customer), the U.S. DOE (a major underwriter and loan guarantor), NYSERDA (also a partial funding source), and NYSEG (the interconnecting utility) have been briefed and are satisfied that the plant is both safe and viable for the long term…

During the earthquake on August 23rd we continued to operate at full capacity without interruption.

Tropical Storm Irene led to an approximately 30-minute interruption over the weekend when the line voltage of the substation we connect to spiked and we automatically disconnected from the grid (i.e., it’s our equipment’s way of protecting itself). Once it normalized we reconnected. Apart from that we ran continuously. With all the load dropping, generator movement and other activity on the grid we were exercised quite heavily, all without any problem whatsoever.”

Beacon Power and others are training electricity, like wild animals, to perform tricks using flywheel energy storage. Beacon’s plant has now weathered a few storms. The technology appears safe, secure, and reliable. So, we are now able to enjoy some of its sometimes unseen benefits.

In the next article, Flywheel Energy 2: Applications, I will look at the advantages of flywheel energy storage, including civilian, military, and potential future directions.

Photo Credits: Top image via mikecogh; Lion via US Army Africa; Flywheel and concrete containment via Beacon Power; Flywheel Bicycle by Maxwell von Stein via Scientific American.


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