Sunday, September 25, 2005

Harvesting Power from Human Motion: Small to Large Scale?

You may have read about the backpack or combat boot that generates power from its wearer’s walking. These are small-scale examples of “power harvesting” from human motion. They work by clever use of devices and/or materials that convert motion to power. The power generated is small but potentially enough for a mobile phone or other personal electronics. And if we get a little futuristic, the power could be used for wearable computers or smart clothing.

For me, these technologies invite the question of scaling-up. For example, can a highway overpass be instrumented to harvest the vibrational motion from thousands of heavy, fast-moving cars and trucks? Can the overpass’s surface be adapted to harvest power directly from contact with those cars and trucks’ motion?

How would this work? A core technique of small-scale power harvesting is the use of piezoelectric materials. When these materials bend or stretch, they create power. Instead of having a small wafer of piezoelectric material in a shoe, how about thousands of wafers arrayed throughout an overpass? What if they were connected to the surface so that they bent and stretched with the forces of multi-ton vehicles constantly zipping by?

This isn’t my area of expertise, so I don’t know the answers. But the questions illustrate the more general opportunity of thinking bigger about harvesting (some might say recycling) energy that humans are already expending. From a societal point of view, it would be a welcome area for innovation.


  1. I've always wondered if there was a way to harness the power of the cold water 'upwellings' in the Arctic that, apparently, have so much to do with stabilizing the ocean's salinity and temperature.
    Underwater windmills were first introduced as 'real' solutions in 2003 off of the English Coast. I haven’t heard about recent success with this model, but it certainly seems like one of the most promising and viable options for harvesting the power expended on sea currents and turbulence.
    These links have some news and pics on it:,,895835,00.html

  2. Steve,
    You state that there's an opportunity for "harvesting" via this method, suggesting that there could be a net energy gain by putting energy absorbing materials on a road surface. I haven't worked out the equations but I think you'd end up with a 2nd law of thermo situation where the more you harvested from passing trucks, the more like running on sand driving those trucks would be. The real opportunities for power harvesting are twofold: 1) where the value from the harvested power in its new form is very high compared with the power in its old form (human food --> human motion --> battery power for communications which makes the human much more effective) and 2) the related sitution where you've got an abundance of one kind of energy (food energy) and a limited supply of another kind of energy (no power grid handy). I suppose that a third case could be when the hassle factor of recharge is high, and the ability exists to create an ambient/background recharge alternative. But in no case do I see this being a solution to overall societal energy usage.
    Very interesting blog, BTW!

  3. Good comment, Ethan. I should have been clearer that the goal is to harvest the energy that cars and trucks are already expending, not to make them expend more in the process.
    The overpass is already vibrating, and there is already frictional heat and displacing force at the road surface. By "connecting" energy-harvesting materials (not necessarily just piezoelectrics) to the surface, I was thinking of connecting to the underside of the surface--as opposed to something like piezoelectric micro speed bumps, which would have the second-law issue you mentioned.
    The idea is to optimize access to the energy already present--that is, get as close to the heat, vibration, and surface displacement as you can, using materials arrayed in the best configuration for harvesting. But, per your comment, this needs to be done passively; it can't actively steal extra energy (as would the micro speed bumps) that wasn't already there to be harvested.

  4. Isaac, as for your comment about harvesting power from the ocean, that's good stuff. I'd classify it as harnessing natural (as opposed to human) motion. The good thing about the natural approach is that it is often inherently large-scale to start with.

  5. Commenting as a layman (I do not have a mathematical or scientific background), wouldn't it be possible to convert the kinetic energy of motor vehicles passing over a point in the road into electrical energy? Every vehicle passing over a simple rocker plate in the road that is connected to a generator could convert the vehicle's kinetic energy. There could be many such 'energy trapping' points on a motorway so that a vehicle's kinetic energy could be used several times.
    E,g., a vehicle weighing 800 kilos and travelling at 50 mph would produce 199,485 joules (0.555 Kilowat hours approx.) of energy. I estimate that a single vehicle passing over 50 trapping points would provide a household's average daily use of electricity. I haven't looked into the economics of this and therefore cannot comment on how cost-effective the process would be.

  6. Tenjacks, I believe your idea runs afoul of Ethan's point in comment #2. Your rockers would transfer energy directly from cars, which would just make the cars expend more energy than they normally would. Per comment 3, the goal here is to harvest the energy cars are already giving the overpass, just by driving over it as-is.

  7. I've done a little bit of research and found that my idea is not an original one. Pity! J. Sainsbury plc is trialling a system at their Northampton, UK, depot using the Dragon Power System whereby "Vehicles simply drive over road plates placed in the road surface of the depot. Each road plate houses individual steps and each step captures the weight of the vehicle and initiates a series of pumping actions that turn a generator, ultimately producing electricity. A single lorry running over the DPS can produce approximately 3.3Kwh, and based on the number of vehicles entering and leaving the Northampton depot, this will help generate enough power to boil 5.7 million kettles or light 10,000 light bulbs a year." (Source: Also see The "2nd law of thermo" does not appear to be a problem here!

  8. It appears that the Dragon system transfers energy directly from the vehicles to the plates, which presumably cause extra resistance and thus require greater energy from the cars. This is different from passively harvesting the energy already transferred to the road in the form of vibrations.
    That said, whatever is being done by the Dragon system, I wish it the best.