# Another Take On Harvesting Energy While Walking

Harvesting energy from the human body may sound scary, but fortunately a Matrix-style setup exists only as a cinematic fiction. Instead a typical path lies in external contraptions that use the body’s natural motions to drive a small generator, a bit of flexible piezo material, and so on. A popular target for harvesting the body’s kinetic energy is the knee joint, as this has a comparatively large range of motion and is fairly easy to use.

Thus a team from Hong Kong university opted to pick this part of the human anatomy for their experiment as well. While at first glance their results do not seem particularly impressive, with up to 1.6 mW of power generated, a look at their published results in the Applied Physics Letters journal showed their reasoning behind this setup. While one generator-based setup referenced produces on average 4.8 Watt of power, the device itself weighs 1.6 kg and increases the rate at which the person wearing it burns calories by a significant amount.

The goal for this device was to have a way to generate significant amounts of power without having the user exerting themselves more than usual. This led to them using flexible piezoelectric composites, resulting in a weight of just 307 grams, based upon two M8514-P2 pieces (Smart Materials Corp. manufacturer). Tests with volunteers on a treadmill show that users do not burn more calories than without.

As with all piezo materials, they can flex a bit, but not too much, so a lot of time and effort went into calculating the optimal bend radius in different usage scenarios. While around 1 mW of power is not massive, it is a reliable source of power for individuals who do any amount of walking during the day and doesn’t require any effort beyond strapping the device onto one’s legs.

## 17 thoughts on “Another Take On Harvesting Energy While Walking”

1. steves says:

How much is that in Mars Bars per day?

2. Vinalon says:

Let’s see, 1 Watt-second = 1 Joule and 1 Mars Bar = 230 kcal = 962320 Joules?

So if you walk for say, 4 hours throughout the day, that’s 14400 seconds, and 14400 * 0.001 = 14.4 Joules per day.

And: ( ( 1 / 962320 ) Mars Bars / Joule ) * ( 14.4 Joules / day ) = 0.000015 Mars Bars / day

If that’s not too far off, then you could generate one Mars Bar of energy in only 182 years, give or take.

1. Foobacca says:

So if we make a micro turbine engine that runs on liquid chocolate it will run for longer than we will XD

2. Luke says:

Remember that the average human metabolic efficiency is about 20-25% from food to limb motion. One extra milliWatt generated means four extra milliwatts consumed, so you’re actually consuming Mars bars at a rate of 1 per 45 years.

3. helge says:

Absolute weight matters, but it’s also evident that the piezo device only manages to deliver a power-to-weight ratio that is 1/1000 of that of the bulky reference.

regarding the generator mode implemented here, it might be worthwhile to look into other directions as well.
Incorporating e.g. a piezo bimorph element in an oscillating structure makes multiple actuations per ringdown possible.
Cyclic load fatigue increases exponentially with amplitude (Stress-Cycle (S-N) curve), so generating energy over mutliple actuations per motion seems preferable while potentially allowing higher power output.

4. Garrett Mace says:

By comparison, someone who lives in a cave walking on a treadmill 16 hours a day would come out ahead by going outside the cave for one hour of sun with a 10 watt solar panel instead.

1. steves says:

A month ago I started winding my automatic watch by cycling 3km per day. I am now pondering how best to make the 90km trip to get back home.

2. Jan says:

ehmm can you explain that:
knee generator: 16x60x60x1.6mW = 92.16J
solar panel: 1x60x60x10W = 36000J

But I was wondering, if the knee is such an interesting part of our body, why isn’t the foot.
A piezo element ion a shoe could be very interesting. It experiences the same amount of movement, BUT you have the pressure of the human body. For every step is at least 30Kg is pressing on that piezo element and when you move you other foot, you are also putting pressure on the non moving foot when you lift the other one, resulting in your full weight pressing on the “generator”.
But mostly from a potential consumer point of view, I rather put something on my foot (I do that every day) then strapping something on my top and bottom leg.

So… I really don’t understand the focus of this “research” and 1.6mW isn’t very much…

Although I like complicated things, this is going nowhere. Harvesting works best if you use continuous waste products. Body heat is one of them. A simple peltier element must be doing something, and you don’t even need to move for that. The beauty of a peltier element is that it has no moving parts. I imagine a simple array of these things on the back of my coat or if not much power is needed in my shoes. consideringthe contact with the “cold” floor might be more efficient then contact with the air.

a 5 second google came up with this:

1. Jan says:

oops (sorry Garret), never, mind, I misread the “come out ahead” part by overlooking the “by going outside”. So no need to explain further.

2. Joey Bloggs says:

“But I was wondering, if the knee is such an interesting part of our body, why isn’t the foot?”

Purely from an energy standpoint:
assuming 50kg weight per step, a 2mm compressive element in the shoe working at 100% efficiency, you could extract 50*9.81*0.002 = 0.981 Joules per step. At a stepping rate of 2 steps per second (1 step per second per foot) that would be almost one Watt.

More surprisingly: I guess that the padding in my shoes easily compresses 2mm per step. Does that mean that my shoes are heated by about 1 Watt while I am walking?

1. Luke says:

Yes, and that’s why shoes wear out.

5. Mongrel Shark says:

This would still be adding load to the exercise. Its just too small an amount to detect the extra calories used. Its no different than using a dynamo on a push bike.

Energy harvesting is about using waste energy. This device is more of a weight loss / fitness gadget thats turned down so low its ineffective.

There are only really 2 good sources to tap on a human. Waste heat, and gravity/body weight. Both are nearly impossible to tap into without adding inefficiency to the bodys function.

1. Birdy says:

“Energy Scavenging” is the phrase you’re looking for. It’s a subset of “Energy Harvesting” but focused on wasted energy specifically.

“Energy Harvesting” by itself makes no distinction as to the source. The dynamo on a push bike is still energy harvesting.

6. foxpup says:

Anyone remember those self-winding watches that stayed wound and consequently running as a result of the wearer moving about? That would classify as energy harvesting from it’s human operator.

7. ALEJANDRO says:

Needed very much humans walking in the big matrix, with their brains connected too.

8. bird says:

I LOVE energy harvesting.

The reason they’re typically larger is because a battery is better at this scale.

A 1.6kg battery would last nearly twenty years at that 1.6mW output, be 10x cheaper, significantly more reliable, less prone to failure and be less of a nuisance to the wearer.

9. Alan says:

For energy harvesting, I’d like to see a small motor (used as a dynamo) in the tip of a walking cane. This would a variation of the spinning spiral plunger, to harvest energy from the act of putting your weight on it.
A walking cane that shortens by about 2cm due to this action should still be usable. A light source is what I’d use it for – if you need a walking aid, the dark can be extra hazardous.

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