Knee Brace Generates Electricity From Walking

ktulus cry brings news of a device that can power portable gadgets, prosthetic joints, and other mobile appliances by harvesting energy generated by walking. Researchers are working on making the device — still a moderately cumbersome 3.5 pounds — smaller while maintaining its energy harvesting capacity. CNet has a write-up with more pictures and a diagram of the device. "In the mode in which the brace is only activated while the knee is braking, the subjects required less than one watt of extra metabolic power for each watt of electricity they generated. A typical hand-crank generator, for comparison, takes an average of 6.4 watts of metabolic power to generate one watt of electricity because of inefficiencies of muscles and generators. A lighter version would be helpful to hikers or soldiers who don't have easy access to electricity. And the scientists say similar mechanisms could be built into prosthetic knees other implantable devices such as pacemakers or neurotransmitters that today require a battery, and periodic surgery to replace that battery."

Re:less than one watt for one watt?

[Burdell]

It is less than one watt of extra metabolic power when braking. I would assume (without RTFA) that this is analogous to regenerative braking in electric/hybrid cars.

Re:less than one watt for one watt?

[TheEmptySet]

It generates energy as the knee is using energy to slow your downward motion, hence the statement "less than one watt of EXTRA metabolic power for each watt of electricity".

they had a bit about this on NPR

[Raleel]

it's so heavy right now because they made all the parts easily replaceable to the scientists working on it. The scientist they interviewed on it also mentioned that if you stop wearing it, you tend to swing your leg harder for the first 3-10 steps, unaccustomed to the now-unpresent braking by the device. Really neat idea... while it makes sense to me, I didn't realize we actually braked our legs as we walked forward.

Re:Considering the the potential energy stores in.

[ddrichardson]

A study in Holland [plosjournals.org] disagrees about the savings from obesity reduction:

Conclusions

Although effective obesity prevention leads to a decrease in costs of obesity-related diseases, this decrease is offset by cost increases due to diseases unrelated to obesity in life-years gained. Obesity prevention may be an important and cost-effective way of improving public health, but it is not a cure for increasing health expenditures.

Re:Perpetuum mobile?

[antifoidulus]

Someone needs to work on their reading comprehension skills:

In the mode in which the brace is only activated while the knee is braking, the subjects required less than one watt of extra metabolic power for each watt of electricity they generated.(emphasis mine)

That means that the system captures some of the energy that would normally be "wasted" and converts it into electricity instead....

Re:Lots of applications ...

[hazem]

Good joke, but just an FYI, privates don't "sir" sergeants, seeing that sergeants work for a living.

(In the US Army, at least, "sir" is reserved for male officers and warrant officers.)

Re:Exercise Power Plants

[Mendenhall]

On the efficiency of the human body as an engine, the number you quote is about right. However, assessing that as a terribly inefficient heat engine is a bit odd.

A really well-tuned automobile engine, running on pre-refined fuel, might get 40% thermal efficiency or so. The human body, of course, starts with rather unrefined fuel (food, to the non-techie :-) ), runs all the necessary chemical conversion machinery, and produces its output. It also expends a lot of energy in self-repair and maintenance, which for your car is separate. And if your car had to provide enough power for all the computational work we are doing, too, it would further increase the overall energy budget.

A full-cycle efficiency of the 33% or so (assuming your 2:1 ratio of waste heat to output) seems very good. Almost all the fuel conversion techniques we have (oil refining, fermentation to convert the stuff we eat to ethanol, coal gasification, etc.) lose more than this, I suspect.

Re:Perpetuum mobile?

[KiloByte]

Not exactly. The Maxwell demon takes what already is heat, these braces take what would turn to heat if unharvested.

Re:less than one watt for one watt?

[Yetihehe]

Yes, you are right. This brace helps you to brake your leg, when you straighten it for next step. It uses your lower leg momentum to generate electricity.

Audio interview about the "Energy Brace" from CBC

[ClarkMills]

The article with graphic:

    http://www.cbc.ca/quirks/archives/07-08/feb09.html [www.cbc.ca]

The Interview (in OGG & MP3 formats) :

    http://www.cbc.ca/quirks/media/2007-2008/ogg/qq-2008-02-09_01.ogg [www.cbc.ca]
    http://www.cbc.ca/quirks/media/2007-2008/mp3/qq-2008-02-09_01.mp3 [www.cbc.ca]

Re:less than one watt for one watt?

[heinousjay]

They're all controlled falls. Try doing any of those things without using gravity.