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Green Braking

October 20, 2010

Thirty years ago, a company for which I worked was investigating flywheels as a way to store the energy lost in vehicle braking so that it could be used for subsequent propulsion. This technique is now known as regenerative braking. That was the time of the second energy crisis, and industry was scrambling to do its patriotic duty, and make a little money at the same time. Any cursory analysis will show that city buses are an ideal candidate for flywheel braking, since they make frequent stops, they are on the road for extended periods, and their high initial cost makes it easy to hide the additional expense of a flywheel. There's another property of flywheels that adds an additional constraint. To eliminate the gyroscope effect, you need two wheels rotating in opposite directions with a lot of electronics to make certain that the rotation rates are equal.

Flywheel technology is quite ancient, going back at least to the potter's wheel. The physics behind a flywheel is quite simple, and it's usually introduced as a topic in mechanics in introductory physics courses. The energy E stored in a flywheel is given as
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. The rotating body of a flywheel is usually a solid cylinder, the moment of inertia of which is given by
I = ½ m r2
where m is the rotor mass, and r is its radius.

Scanning these equations reveals two important facts. If you want to store a lot of energy in a flywheel, you need a large flywheel that spins very fast. For a vehicle application, your flywheel needs to be somewhat compact, so you're left with a rapidly spinning body, and that has some problems. First, you don't want the flywheel to lose energy by heating the air around it, so you'll need to put it in a vacuum. Second, the flywheel material will be under considerable tensile stress (called hoop stress for this cylinder geometry), so you need a high strength material, preferably one that is not prone to fracture. Unfortunately, spinning flywheels up, then down, repeatedly, leads to material fatigue, and fracture does occur even with careful engineering. It's best to encase the flywheel in a bullet-proof shroud to contain flying schrapnel.

spinning head

organic flywheel.

All of this seems like a lot of work, and it would only apply to buses. Is there some way to harvest some braking energy from all vehicles without adding expensive subsystems to the vehicles themselves? It's actually possible to transfer the energy harvesting from the vehicle to the roadway. In the case for recovering braking energy, we don't need to retrofit the entire roadway, as I wrote about in a previous article (The High Road, September 22, 2010). We have an advantage that we know where the vehicles will stop; namely, at stop signs, stop lights and some toll plazas (the ones without E-ZPass). Some companies have been developing systems that harvest wasted energy from roadways, and they were highlighted in a recent article in the IEEE Spectrum.[1]

It's usually quite easy to transform energy from one form to another; e.g., mechanical to electrical. The important principle in these automotive systems is that we're not just stealing fuel from automobiles by having them run on a treadmill for our own nefarious ends. This is energy that would have been wasted as heat. An Israeli company, Innowattech,[2] a spin-off of the Israel Institute of Technology ("Technion," Haifa), started doing research into using piezoelectrics to harvest environmental energy for wearable electronics. Now, it's developing devices that can be buried under a road surface to harvest energy from moving vehicles. Its devices are claimed to recover 80 kilowatt-hours per kilometer for normal road traffic. The energy is actually gravitational, since it comes from the weight of the vehicles as they deform the roadway. The devices are placed about two inches below the roadway, where the compressive stress on the device is greatest. Although more power can be gained if these devices make the roadway more spongy, Innowattech is careful to make its devices stiffer than the roadway, so it can't be claimed that they've ventured into the energy-stealing mode. The devices would actually make the vehicles very slightly more fuel efficient.

Innowattech's stem would be embedded in large portions of roadway, but New Energy Technologies[3] is looking at the actual braking application. Its system is a mat that's placed over the roadway. The New Energy Technologies web site claims that capturing the kinetic energy generated by moving vehicles just twice per day produces enough electricity to power a half million homes. Although New Energy Technologies has filed nine US patent applications, my search reveals that none have yet been published. There is no technical information on the web site, but the IEEE Spectrum article states that mechanical or hydraulic cells are used to generate electricity. A short video on the web site shows vehicle wheels deforming a trackway quite a few inches, and it's most likely that a lot of power can be generated this way.

The major problem, of course is durability. Highway Energy Systems (Henstridge, England), tested its system based on articulated plates at a grocery parking lot. The system was a technical success, generating up to 40 kWh, but it couldn't handle the environment and heavy traffic. Like any good engineers, they're working to solve these problems.[4]

As I was finalizing this article, one of my former colleagues told me about an invention of his that would render flywheels safe if they undergo catastrophic failure.[5] The idea is to form the flywheel from a brittle, high specific strength material. Failure will result in the fragmentation of the rotor into many small particles, essentially dust, that would be easily stopped by a light shroud. One advantage of this scheme is that the more energy you have stored in the flywheel, the smaller the particles will be.

As my own contribution to mechanical energy harvesting, wouldn't it be nice to attach generators to some of these? We would fight childhood obesity at the same time.

Alice and Lorina Liddell, 1860

Alice Liddell and her sister, Lorina, on a see-saw (1860). This photograph is by Lewis Carroll, and Alice is the Alice of Alice's Adventures in Wonderland.


  1. Lucas Laursen, "Start-ups Try to Capture Road Traffic's Excess Energy," IEEE Spectrum, October, 2010.
  2. Technical Information section of Innowattech Ltd. web site.
  3. Kinetic Energy: An Electrifying Technology, New Energy Technologies web site.
  4. Highway Energy Systems web site.
  5. Robert C. Morris, Dave Narasimhan, John Moody and Patrick Coronato, "Isotropic energy storage flywheel rotor," US Patent No. 6,122,993, September 26, 2000.

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Linked Keywords: Flywheels; regenerative braking; second energy crisis; city buses; gyroscope; potter's wheel; physics; mechanics; moment of inertia; angular velocity; radians per second; vacuum; tensile stress; hoop stress; fracture; material fatigue; Kevlar; schrapnel; E-ZPass; IEEE Spectrum; energy transformation; Dr._Evil; Israel Institute of Technology ("Technion," Haifa); piezoelectrics; gravitational; elasticity; Highway Energy Systems; Henstridge, England; childhood obesity; Alice Liddell; Lorina Liddell; see-saw; Lewis Carroll; Alice's Adventures in Wonderland.