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Mechanical Battery – Flywheel-Powered 90% Efficient Energy Storage


1097rosen_flywheelWe have been so focused on chemical storage systems lately, that some us forget other old, seemingly more efficient, mechanical batteries.

Such a battery is the flywheel. Several successful experiments have been carried out in the last 50 years, and the flywheel’s applications ranged from acting as a UPS for a hospital to putting an entire train to movement and then to cruise speed, only by its power.

How does a mechanical battery work? Did you ever repair your bike, and while it was sitting upside down, spinned the pedals, so the wheel got to a high rpm? If you did so, you may have observed that by trying to stop the wheel, you exert a force on it. What does that mean? There is energy stored in the rotational movement of the wheel, energy that loses itself through friction (and, possibly, heat).

Smart guys thought of using this rotational energy from a flywheel and make it do something useful. This is ancient technology, though, that it has been written about centuries ago.

So, scientists created steel tubes, put them on magnetic bearings (the tube’s connection to the stator was through a magnetic field, to reduce friction), and spinned the thing to up to 50,000 rpm. When needed, they used that rotational force to make electricity (the classic way), and decrease its speed, by extracting energy from it. Didn’t this raise your eyebrow by now? Well, you may think that flywheel stops quickly, but figures show that typical energy capacities range from 3 kWh to 133 kWh, with a storing efficiency of up to 90%.

There were experimental buses built in the 1950s, called “gyrobuses”, and were used in Yverdon, Switzerland. Also, prototype cars have been built on this principle. New materials, such as carbon fibers, make them more usable and potent. In fact, the stronger the flywheel’s material is, the higher the rotational speed and the energy it can store. That is the only serious limitation and danger of flywheels. It can break into pieces if spun too quickly.

Mechanical batteries are also time-resistant. An ECE reseacher, Dr. Mark Flynn, from the University of Texas at Austin, designed a flywheel system that could last 20 years of continuous usage.

NASA G2 Flywheel
NASA G2 Flywheel

“Flynn’s design captures the braking energy and uses it for the next hoist. More importantly, the addition of a flywheel energy storage system lowers the peak power requirements which saves energy during idle periods. Field tests in China showed that when operators used a genset appropriate for the reduced power requirements and added a mechanical battery, fuel consumption went down by 38%, with significant reductions in NOx and PM emissions.

Flynn’s flywheel motor controller is also replacing the industrial batteries used by mission-critical data centers and hospitals. “Industrial batteries are less expensive initially than a flywheel, but when you factor in maintenance and having to pay for more charge than you need to avoid frequent battery replacement a flywheel-based solution can be considerable less expensive,” says Flynn. “A VYCON flywheel will last 20 years and eliminates the problem of what to do with 200 large-scale toxic lead-acid batteries.”

Hospitals and data back-up centers cannot afford power outages. Lives and disaster recovery for businesses depend on an uninterrupted flow of energy. A typical power outage is very short and most hospitals and data centers have back-up diesel generators, meaning the extra energy storage of an industrial battery is never fully utilized. Most outages are well within a flywheel’s capability, but when the outage persists, the flywheel absorbs damaging power abnormalities then gracefully transfers to the generator-meeting emergency power regulations that stipulate gensets must be able to assume the load within 10 seconds. Mechanical batteries also have a higher tolerance for rapid cycling.”

Having flywheels around gives us an alternative to chemical batteries, and the impulse to continue development of this interesting technology. For example, I could use a pocket one to power my laptop’s just finishing battery. The possibility of making these things mobile, for usage in electric cars or other applications, has been tested, and it has been discovered that it would need special measures to not interfere with the car’s stability in curves. I’ll write about that in a future article.

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  1. In reponse to #4 and #5 see my comment #1 – the porche is a RACING car and bounces around like hell with no issues (in terms of shock proof). You just need a well designed bearing system. The vertical axis orientation deals with comment #5

  2. They are shockproof. Our systems are now tolerant of 16% grade, 1 Hz – 20 Hz random vibration and pitch or roll rates of 10 degrees per second with plans to make them more robust again (the above specs are good for a city bus).

    As for the moment induced with a flywheel, you install it with a vertical axis orientation so no issues with right or left turning. There is theoretically a moment when the car banks but as the ratio of the mass of the flywheel vs the mass of the vehicle is wide it actually has minimal measurable impact for a passenger vehicle. The larger the vehicle the less impact. We think there may be gyroscopic issues for F1 (light and FAST) but the Porsche GT3R Hybrid endurance racer uses a flywheel with no impact to handling.

    The wayside rail is a great application, just wish more cities would see the benefit. The big thing here about using a flywheel is that it has a very long life (> 5 x 10^6 cycles or > 20 years in-service with minimal maintenance), so is actually a very low cost option compared to chemical storage which has to be replaced every few years.

    Anyway my 2c

  3. Flywheels have been used for the 60 years, in the New York City subway system. Every time the train comes to a stop it stores energy and is used to get the train going. If not used the required power for startup would be tremendus. Lots of work was done on this technology in the sixtys and seventies. Just gimble the flyweel and it would have little gyroscopic effect on a car.

  4. Now these sound awesome. So how could you make one of these shock proof enough to use in an electric car? How long would it take to ‘recharge’ one? And is there much of a delay between asking for electricity and receiving it?

    • They tried placing the flywheel in a car, but the problem is that it destabilizes itself once the car turns (you can imagine why). One solution was to incline the flywheel opposite to the turning direction, so the perpendicular force on the flywheel’s ground remains the same.


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