Sensors Mag

Good Vibrations

July 13, 2007 By: Melanie Martella, Sensors

E-mail Melanie Martella

The flurry over the Orbo, the most recent entry in the list of perpetual motion machines ("Perpetual motion machine hits 'technical' snag," courtesy of Vnunet) highlights our desire to get free energy. Unfortunately for us, the pesky laws of thermodynamics regularly thwart these endeavors. Energy harvesting, however, makes the second law work for us.

Thermodynamics Makes the World Go Round

Just to refresh your memory, Ginsberg's Theorem (The modern statement of the three laws of thermodynamics) says the following:

  1. You can't win.
  2. You can't break even.
  3. You can't get out of the game.
  4. The law of entropy: The perversity of the universe tends towards a maximum.

Under the second law, there's no such thing as 100% efficiency (not in reality, anyway). So motors lose energy in noise, heat, and vibration, and wherever you have something moving, you've got a source of waste energy. There are a number of different approaches to converting waste energy into a useful energy source.

MicroStrain, for instance, uses a piezoelectric film to convert the vibration from a helicopter rotor into electricity to power its wireless strain transmitter. The recent article on the New Scientist Web site ("Microgenerator feeds on good vibrations") highlights a technology developed at the University of Southampton that uses magnets on a cantilevered oscillator to convert vibration to electricity. Perpetuum, a spin-off from the University of Southampton, sells a larger version, which couples the magnetic microgenerator to a capacitor.

Vibration isn't the only waste energy source being tapped—companies are converting light and heat into electricity, too. EnOcean, for instance, uses solar cells coupled with a storage device to power wireless sensors from ambient light. For links to other companies working on energy harvesting, the Energy Harvesting Forum is a good place to start.

No Free Lunch

As Robert Heinlein observed, there ain't no such thing as a free lunch (frequently shortened to TANSTAAFL). While perpetual motion machines constantly try to buck that fact, the people working in energy harvesting use that to their advantage. Put simply, you can get a lot more done if you work with the universe rather than against it.

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