Harvesting energy from walking humans

WilliamE.

So, this may sound like a homework question but I promise it's not.

How much energy do you think could be harvested per human step? For instance, if you had "buttons" in the bottom of shoes which compressed a spring and somehow moved that energy to some sort of energy storage device, how much could be stored, do you think (using whatever you think are realistic numbers).

I am asking this for this sort of product idea competition. My thought is that something could be attached to the bottom of a shoe which could be used to generate energy when it's wearer walked. From the sound of it, it should seem like an obviously bad idea due to what energy actually costs, and what such a device must at minimum cost, but I'm just trying to throw some sort of idea out there.

Thanks.

berkeman

Welcome to the PF.

Here is some introductory reading for you, with links to other information sources:

http://en.wikipedia.org/wiki/Energy_harvesting

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russ_watters

Work is force times distance, so if you weigh 150lb, take two steps per second and compress a piston 1" each time, how many ft-lb of work could you do per second?

cesiumfrog

You measure work in ft-lb? Even if you insist on (middle-north) American units, just type the numbers into google..

Hence, I estimate anywhere from 10-100W of power. Still, might be a plausible way of keeping something like a netbook charged in certain circumstances. In fact there exist areas of the world where I think you'd have a market for a phone charger on this principle, depending on the technology of the button.

It would be like constantly walking up stairs. In the developed world there's probably a place for this kind of thing, just because we are generally so lacking in exercise. And we already eat too much (i.e., more calories than we use, and afterward our nutrient saturated waste streams tend to remain a liability rather than a resource) so by harnessing some of this excess energy from our decadent diets we would be getting this power for free (or for negative cost accounting for health benefits).

NobodySpecial

Thats about about 16micro furlong-firkins if you want it in real units

Probably nearer 10W, 100W is quite a lot of effort.
A decent competition cyclist can sustain 250W over long periods.

russ_watters

ft-lb was the first thing that occurred to me - you can also do a 2.5 cm compression and 70 kg if you like.....though that was meant more for the OP to solve...

...and calculating the number, regardless of the units, is better than pulling it out of thin air.

NobodySpecial

You can't extract too much energy from walking or it would become very difficult to walk (ain't no such thing as a free lunch)
The reason you can walk easily is that you recover a lot of the energy in each step from the springs in your tendons (and slightly from the rubber soles in your shoes)
A system that extracts lots of energy would be like walking in soft sand or mud - or on those stepper things at the gym where your foot drops away from under you.

cesiumfrog

is that intentionally omitting a factor of acceleration then?

mheslep

There's been quite a bit work on this beginning I believe with MIT's wearable computer work. We have a prototype device here, and the Army has been paying attention for years - anything to help with their exploding battery and battery life problem.

MIT Media Lab came up with a working maximum power of ~67W (68kg, 5cm vertical movement, 2Hz), but concedes an 'unobtrusive' maximum power of only about 1W (enough to keep cell phone going all day w/ an hour per day walking), and working models so far obtaining mW ranges. Piezoelectrics seem to be the leading candidate for the most unobtrusive, and miniature magnetic generators for the most power (tens of mW).

http://www.rst2.edu/njheps/resources...scavenging.pdf

Academic

How much extra food is this gonna cost me? :p

sophiecentaur

If you have to work too hard whilst walking around the office, you will work up a lather and need a shower - there goes any net energy savings. You can't win, you know.

JDługosz

You can get enough energy to wind a wristwatch, without bothering the wearer.

Phrak

Yeah, 100 Watts is asking too much. It would be a major effort making the round trip from couch to frig.

mishrashubham

A 70 kg person walking at a speed of 5 kmph uses 1300 joules of energy. Practical machines however might be able to capture only a small amount of that energy.

berkeman

How did you calculate that? And 1300 joules of energy in how much time? A second? An hour?

mheslep

Check your figures. Only power, Watts, makes sense in that statement with out a time limit.

mishrashubham

Oh sorry for that, I forgot to write the time period which is 60 minutes. Walking at that speed a person of that weight on average in 1 hour uses approx 290-300 calories which is approximately 1300 joules.