Harvesting Energy from Humans in Motion Researchers have built a device resembling a knee brace that can generate usable amounts of electrical energy as a person walks. The brace, described in an article in the journal Science, stores the energy lost when a human brakes the knee after swinging the leg forward to take a step.
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Harvesting Energy from Humans in Motion

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Harvesting Energy from Humans in Motion

Harvesting Energy from Humans in Motion

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This is TALK OF THE NATION Science Friday. I'm Ira Flatow.

A little bit later in the hour, carbon electronics, tornadoes and getting your head wrapped around a trillion, how big is a trillion?

But first, how many times have you found your cell phone, your iPod, your BlackBerry on its last few minutes of juice? No place to plug it in. Maybe you're out in the middle of a forest, some place or on a hike or even in your car. Now, imagine being able to supply that charge with energy from your own body, simply by walking. That's the idea behind a new energy-harvesting device unveiled this week in the journal Science. It's a little generator mounted on a knee brace that captures energy that's usually wasted when you walk. For you drivers of hybrid cars, well, it's like the same kind of concept as regenerative braking where your car captures energy that's usually wasted when you put on the brakes. Researchers still need to figure out how to store that energy or how to hook up to the device that needs charging. But once they do, they say their energy harvester could power more than personal electronics. It could be used to power portable medical devices, soldiers' instruments or even basic laptops in places far from traditional energy sources.

And joining me now to talk about this invention, how it works and its potential applications is my guest Max Donelan, chief science officer at Bionic Power. He's the director of the locomotion laboratory at Simon Fraser University in Burnaby, British Columbia. He joins us today from his office.

Welcome to Science Friday.

Dr. MAX DONELAN (Chief Science Officer, Bionic Power; Director of Locomotion Laboratory, Simon Fraser University): Thanks very much, Ira.

FLATOW: How do you get - don't you have to put work in to get work out?

Dr. DONELAN: Yeah, absolutely. I mean, we're not violating any laws of thermodynamics. But the basic idea is that walking is inherently uneconomical. And so, when you're walking along, it's a bit like stop and go driving where the muscles of the body, even within a stride, are constantly accelerating and then decelerating your body. And hybrid electric cars take advantage of that stop and go driving that whenever you normally push on the brakes, you instead now brake with a generator. And that generator takes the energy that's in the car and turns it into electricity, unlike a traditional car which takes the energy in the car and just dissipates it as heat. So we took to the same sort of thing, but for walking, so that when the muscles at the knee are decelerating the leg, we help those muscles in doing the deceleration using a generator, and the generator, at the same time, produces electricity.

FLATOW: So there's a period in your leg where you are decelerating, where you're not putting out the energy but trying to stop from walking.

Dr. DONELAN: Yeah. Well, there's actually more than one period. If you just think about the body as sort of an arbitrary mechanical system, then if you're walking at a constant speed on the level, then you have no net change in your kinetic energy and you have no net change in your potential energy. So that means you have to have an equal amount of positive work that accelerates or lifts your body and negative work that slows down or lowers your body. And so - and all that positive work comes from muscles, and most of that negative work comes from muscles as well. So the muscles - the main part of their job is to, you know, a coordinated way slow your body down periodically and take the energy that it put in and take it away and dissipated as heat. And that happens on the level at a constant speed, but it may be easier to understand if you think about walking down a hill where the main job of your muscles in that case are to make sure you don't reach the bottom at the speed that gravity wants you to.

FLATOW: Is there one muscle more than the other that you used for that?

Dr. DONELAN: Yeah. In our case, we're targeting the muscles that cross the knee. And so in this case, it's the hamstring muscles, the ones that run down the back of your leg. And towards the end of the swing phase, which is when, you know, you take your foot from a backwards position, swing it forward to being another step. Towards the end there, those muscles turn on and their job is to slow down the extension of the knee. And if you - I mean, your muscle is doing it in normal walking but it could be done by something else, and in our case, it's a generator. And so the generator - part of the trick with this device - is to know when that phase occurs and engage the generator only at that time assisting the muscles in doing their - and slowing the leg down while generating electricity. And then once that phase is over, it disengages and tries to get out of the way of the muscles for the rest of the stride.

FLATOW: So is there some sort of accelerometer on it to know when things are changing or changing direction?

Dr. DONELAN: Yeah, there is. In our case, the sensor that we - that is described in this paper is just a simple potentiometer that measures knee angle. And then that signal sent back to a computer that runs a real time control system that senses the knee angle a thousand times a second. And it uses that signal to determine where in the gait cycle you are, where in the walking cycle you are. And when it determines that this is the right phase in which the hamstring muscles are normally dissipating that energy and engages the generator and then disengages it once it thinks that that period is over. And by engaging and disengaging the generator, what I mean is that it actually just opens and closes a switch between the circuit that allows it to generate power. And so whenever the knee's extending, it's always swinging the gear - it's always moving the gears and the generator. But only at that one period does the control system allow the generator to generate power. And now the consequence is a much bigger - well, there's a big back electromagnetic force that provides resistance to the knee, which is a consequence of allowing current to flow in that circuit.

FLATOW: So how does it feel to wear one?

Dr. DONELAN: Well, so if you compare normal walking without the device to walking with the device on so that you have the mass on or anything but it's not generating electricity, then you certainly know it's there. And, you know, it weighs about three pounds on each leg and you feel that mass. It's not super comfortable at this stage, but we do have people walking for four hours at a time. And the, you know, that mass is - the reason for - part of the reason for that heavy mass is because it's designed for a convenient experimentation that we pull gears…

FLATOW: I see.

Dr. DONELAN: …in and out of the generator, in and out and so on. But one of the interesting things is if you ask many of the subjects how it feels, if they have it on and they're walking and they have the weight on but it's not generating any electricity, and they do that for, say, let's say 10 minutes. And then without telling them, you engage the control system, so now it's being smart and engaging the generator at the right time, they don't know that we've turned it on. So they've started to generate electricity without realizing it. Until we take it away and then they miss it.

(Soundbite of laughter)

FLATOW: They miss it.

Dr. DONELAN: And so what happens is is when you take it away, now they've actually, immediately start to swing their leg a little faster than they'd like to because the muscles that normally cause that deceleration have decreased their activity a little bit. In fact, we can measure that they've decreased their activity. And so it takes them, you know, three to 10 strides to readjust and increase the activity again back to normal so that their extension of their knee is more controlled.

FLATOW: And so how much electricity can you get out of one of these? And can you just store them up - store the electricity up in a battery that you might use later for your laptop or something?

Dr. DONELAN: Yeah. Right now, we don't store it in a battery, we're just dissipate it into resistors so we can measure it. And to store it in a battery, you know, we really need to hear from those who want to use this technology what sort of battery they want it in and how they want the power. But to give you some context about the amount of power, I think it's helpful to use the cell phone example. So in this mode where there's no additional increase in your effort to generate this electricity, we get about five watts total between the two legs. And five watts is enough to give you 10 minutes of talk time on a cell phone for one minute of walking. So a typical cell phone consumes about 500 milliwatts when you're using it, so 10 cell phones at the same time. And then there's a second mode where there is if you walk a bit faster and you allow for a moderate increase in your effort, you can get as much as 13 watts. And 13 watts will give you half an hour of talk time for one minute of walking. Now, I don't imagine in the near future that people are going to use their - use this device to charge their cell phones. But it just helps give some context about what sort of power we're talking about.

FLATOW: What would they use the device for then?

Dr. DONELAN: Well, I think the people who would really be interested in it are the people whose lives depend upon portable power. So, on the medical side of things, people who use power prosthetic limbs or powered orthoses. Orthoses are devices that help people walk after they're recovering from stroke or spinal cord injury.

FLATOW: Mm-hmm.

Dr. DONELAN: And these are basically robotic limbs these days, like the new - the future and the current cutting edge prosthetic limbs and orthoses are really these wearable robots.

And incredible devices, but they require battery power. So if you could use this basic principle to help charge these devices, they could walk longer and further and faster. And there's also another kind of more subtle point. And it's not just about battery life. It's also about enabling new technologies that wouldn't normally be possible because of the constraints of batteries.

Just like your desktop is more powerful than your BlackBerry, you know, medical devices have the same sort of limitations that the kind of devices you can get working in the lab aren't necessarily the devices that people are going to wear around.

So that's one group that, you know, whose lives depend upon portable power. And if you think further into the future, this idea of going about energy harvesting, about joints and about taking advantage of the inherent economical nature of walking, it could also be implied - applied to a fully implantable device, to charge implanted medical devices like drug pumps and neuro-prostheses.

Another group that depends critically on power is the military. Soldiers today treat batteries like they treat food and water. So they need it to power navigation and communication. Those things allow them to talk to each other and allow them to get back to base safely.

And to do so, they can carry as much as 13 kilograms of batteries. Well, that's about 30 pounds for a 24-hour mission. So you can substantially lighten their load and power their devices by harnessing energy from their motion.

FLATOW: Yeah. And if you've go - get in a situation where you've run out of batteries, you could…

Dr. DONELAN: Exactly. I mean, in this case, it's - it's not exactly a wearable battery because, in our case, the battery is the person, right?

FLATOW: Mm-hmm.

Dr. DONELAN: And you can think of least or as much energy in fat as a thousand kilogram battery. And - or another way to say that is that there's as much energy, chemical energy in a 35 gram granola bar as a 3.5 kilogram of a lithium ion battery. So - I'm sorry for the metric.

FLATOW: Well, listen, if you can start getting power out of fat, now we're talking.

(Soundbite of laughter)

Dr. DONELAN: Yeah. Well, I mean, in the mode where I mention that it increases your effort a little bit. That's basically what's happening, right…


Dr. DONELAN: …is it's coming from your fat stores. In the mode where it doesn't increase your effort, then it's not like exercise at all. And you can imagine that would be really beneficial for the medical uses, where you don't want to make walking any harder.


Dr. DONELAN: You want to make it easier, if you could.

FLATOW: Well, Max, I want to thank you for taking time to talk with us. And good luck to you. We'll be following your progress.

Dr. DONELAN: Oh, it's absolutely my pleasure, Ira.

FLATOW: You're welcome.

Max Donelan is chief science officer at bionic power. He's the director of the locomotion laboratory at Simon and Fraser University in Burnaby, British Columbia.

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