# Vibration Based Energy Harvesting

1. Aug 27, 2015

### HYPETRAIN

So, personally, I'm really excited about having wireless energy harvesting. I was looking up articles on piezoelectrics, since that seems to be the leading thing these days, and I encountered an article that designed a MEMS circuit (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3958224/). It's fairly recent, so I assume it's atleast close to modern piezoelectric tech.
Basically, I'm wondering how long it would take for these systems to really charge a phone. The values seem quite small..
" 66.75 μW, or power density of 5.19 μW·mm−3·g−2 with an optimal resistive load of 220 kΩ from 5 m/s2 acceleration"
But I'm not sure what it means in practical terms. How long would it take for a device like this to charge, say, a 3.6V smartphone with a 2000 mAh battery if it was operating at peak excitation?

2. Aug 27, 2015

### Staff: Mentor

About 100000 hours. Round the battery energy off to 7 W h and round the power off to 70 uW, gives about 100000 h.

3. Aug 27, 2015

### HYPETRAIN

Wow, really? Is the power output not like per second or something? Shouldn't there be vibration mechanisms that charge it in a few hours?

4. Aug 27, 2015

### Staff: Mentor

Maybe there are such mechanisms, but the one you quoted isn't one of them.

Regarding the units, power is energy per time, so 1 W is 1 J/s. For some reason battery energy and energy from power companies is typically quoted in units of power times time which is energy. So 1 Wh = 1 J/s 3600 s = 3600 J

5. Aug 27, 2015

### HYPETRAIN

Oh okay, I understand. Are there any human powered generators that are generating anywhere near Watts? (I'm googling this as I type this btw)

6. Aug 27, 2015

### Staff: Mentor

Strong cyclists can generate a couple hundred watts for a couple hours... Even standard cycling enthusiasts can generate over 100W for an hour or so...

7. Aug 27, 2015

### Staff: Mentor

Sure, an adult pedaling a bicycle can produce about 100 W. To make the math easy estimate 70 W accounting for losses, so you could charge that phone in 6 min.

8. Aug 27, 2015

### HYPETRAIN

I was looking into some small scale harvesting devices and I found one about an MIT shoe energy harvesting thing that generated 250 mW as you walked. Are there any newer versions of such tech? I saw a company called Instep nanopower, but I'm not sure if their original experiment succeeded in scaling up to full size from their original design.

9. Aug 27, 2015

### sophiecentaur

I have to point out that taking significant energy from every step you take would actually make walking around hard work and contribute to getting tired earlier in your working day. This energy that is claimed to be 'harvested' would not go to waste if it were not used. The body would use it elsewhere. The energy would need to be paid for in the form of food.
Does your car 'harvest' energy from its fuel in order to run your inc car entertainment? No, it just uses the energy.

To make any serious contribution to the World's energy situation (or our electricity bills) you have to find a source of energy that would otherwise be going to waste. Solar, wind and ground/air sourced heat pumps are workable solutions. Sticking an antenna up and expecting to run any device from it is not workable. Using the vibrations from vehicles going over bridges and other schemes that have been considered would actually be costing the drivers due to extra fuel being used if the amount of energy were to be significant.

Increasing the efficiency of cars, buildings and machines is the way forward - and that is exactly what is being done by serious engineers these days. Show me a 'harvesting' scheme that is not either a scam or invented by an unrealistic enthusiast. Dalespam's figures, above, say it all. Did anyone ever consider the energy needed to implement a device that could generate just a few kWh in its lifetime? It used to be said that making a motor car consumed the same sort of energy as the car will use for fuel in its lifetime. As their efficiency increases, this could be even more true nowadays.

10. Aug 27, 2015

### HYPETRAIN

While I agree with most of what you said sophie, I actually found a pretty promising startup related to the shoe thing. While this may not be "the future" since it's sort of impractical, what do you think about the tech they use? Apparently it's not piezo but it works like this (similar to a handcrank):
"1) A drivetrain converts the energy of heel strikes into rotational energy, spinning magnetic rotors.

2) The motion of the rotors induces an electrical current within coils of wire.

3) Electricity travels along a wire and into a lithium-ion polymer battery pack on a wearer’s shoelaces."

I'm sort of confused about the first step though, does this mean it's more like a lever or does the force of the heel actually matter or is it just the displacement?

11. Aug 27, 2015

### HYPETRAIN

Sort of adding on to what I was asking, it says "form" matters but I'm wondering if weight does. Like if someone weighs 100 lb, will they generate 1/2 the energy of a 200 lb person? Or is that not how this works? It isn't piezo so I'm not sure...

12. Aug 27, 2015

### billy_joule

The force and the displacement matter, you need both to generate some power.
Work is force x displacement
W=Fd
and power is work done over time
P=W/t
So
P=Fd/t
This applies regardless of how the mechanism works.

This sort of energy capture could generate energy that a normal shoe dissipates as heat from the shock absorption material, that is, it doesn't make walking more tiresome than regular shoes while still generating power.
The question is, if the lifetime power output of the shoe is more than the increased energy requirement of manufacturing such a high tech shoe. Probably not.

If you are interested in reducing energy use, don't walk at all, driving is generally much more energy efficient
The human body is a machine with very poor efficiency, the fact that most food production is also energy intensive makes matters worse.

13. Aug 27, 2015

### HYPETRAIN

Excuse me for being naive, but how does this gather the heat? I thought it just rotated something that generated energy

14. Aug 27, 2015

### billy_joule

It doesn't, it harvest the kinetic energy of the walker that would otherwise be spent deforming the shoe sole (generating heat..)
The impact energy of each step has to go somewhere, if it isn't collected it is lost by other means.

You could liken it to regenerative braking in cars, where instead of using traditional brakes which simply use the cars kinetic energy to heat up the brakes, the energy is harvested and stored. Energy is still conserved, except in one case it heats the atmosphere and in the other it's stored and may do useful work later.

https://en.wikipedia.org/wiki/Regenerative_brake

15. Aug 27, 2015

### Staff: Mentor

And I think the main point that we are trying to make here is that there is usually no free energy to be gathered, other than the examples given so far like solar, wind, geothermal, etc. That energy is just being wasted if not gathered, so it makes sense to try to build systems that gather that energy for our use.

But just like sticking a windmill on top of a car, energy harvesting shoes takes more energy input to give you some harvested energy out. That harvested energy has to come from some mechanism that has an energy input that is not free. For the "energy harvesting shoes", you have to climb a small extra stair for each step in order to push some energy into the shoe generators. For energy harvesting clothing, you have to put a little extra muscle effort into each of your motions, so that the clothing can generate some extra energy from the motion.

It's much like the old bicycle light generators that pushed a pickup wheel up against your tire -- engaging them made pedaling a bike *much* harder.

Does that make sense?

16. Aug 27, 2015

### Staff: Mentor

So following that theme, harvesting vibration energy from light pole vibrations (due to the wind and vehicles driving by) to power a low-power RF link would be a good application of energy harvesting. That assumes that the RF transmission duty cycle is low enough and the TX power is low enough that the overall energy balance equation works.

Can you comment on where you would want to place the energy harvesting components on a light pole to get the most energy out?

17. Aug 28, 2015

### sophiecentaur

There will have been lots of nice looking and 'efficient' inventions, true. But we can't get away from the fact that humans have been evolving over millions of years to give them efficient propulsion. The 'design' of the leg, ankle and foot is based on what the shoe manufacturers call Energy Return. The Achilles tendon stores much of the energy of each step. People go the trouble of installing sprung floors for Squash Courts , Running Tracks and 'Ballrooms' () , carpet underlay for homes and offices and they're all there because we are aware of how much energy we are using to get around. My belief is that, if you managed to market shoes that would charge your personal electronics, people would visit their doctors more often because they were feeling unaccountably fatigued from the few Watts of extra energy drain. We are incredibly sensitive to this sort of thing. (With increasing age, I am very well aware of diminishing 'spring in my step')
Another thought: to connect the shoe to your device, would you have to dangle a wire down inside your trouser leg? I wear shorts for about 6 months of the year. Could I cope with the glances of people in the street, trying to make out what was going on down my leg. Bionic Pensioner?? Pervert with camera in shoes??
For our phones, I suggest we need contactless pads on all seats - public and private - so that we are topped up every time we take a few minutes rest (wearing shorts or longs). This would have to be paid for with an appropriate Tax!!! TNSTAAFL

18. Aug 28, 2015

### billy_joule

Most shoes are designed with shock absorption in mind rather than energy storage. There's a plethora of designs out there; air pockets, gels, foams, high tech rubbers etc where most energy is intentionally dissipated. I put shock absorbing gel inner soles in all my shoes, I prefer walking on clouds than trampolines.

I think there's definitely scope to harvest that otherwise lost energy (>95% apparently) without increasing the energy input from the wearer. With that said, I still think that micro power human energy harvesting is a gimmick.

Last edited by a moderator: May 7, 2017
19. Aug 31, 2015

### sophiecentaur

This figures (and some others I have now read) suggest that the energy return from shoes is probably not relevant. We're talking in terms of 1J being involved per running step.
But my point about a steady drain of even a small amount of Power being relevant, still deserves consideration. Cycling or running up or down a gentle slope will make a massive difference to the experience of the runner. A 1% hill will involve an input of, say 800J over 100m and this is not too different from 1J per step. Very obvious to any runner.
The fact that all sports are made a 'better' experience with the appropriate surface, surely implies that worthwhile output from a harvesting device will only be obtained at the expense of 'customer comfort'. I look forward to being proved wrong about this but technology has been available for some while and I have seen nothing (limited research) so far. I see lots of runners in my district but no wires down the legs yet.
When teachers, shop assistants and nurses start to use them, I will be convinced.

20. Sep 1, 2015

### nasu

I would rather charge my phone (in an emergency) by using my hand to turn some lever or handle, like in some radios or flashlights.
Walking is a very delicate process and messing with it is a very bad idea.
I see that usually young people are enthusiastic about this idea. Probably they don't realize yet how important is to have a walk in the best condition. It can be painful even as it is, especially as you age. Texting an extra hour may seem worth a little joint pain. :)

Unless you are on a deserted island, I think this is the worst idea for "collecting" energy. Assuming you will have cell phone signal.
After all, we are using energy from other sources than humans to increase our comfort. This will be against the trend.