# Harnessing energy from piezioelectric roadways

1. Feb 26, 2009

### fireseed

I am trying to determine how much energy can be, in ideal circumstances, harvested from piezoelectric crystals that draw pressure from being buried in roadways.
To start, we found a basic fact on Wikipedia;
http://en.wikipedia.org/wiki/Piezoelectric

"...a 1 cm cube of quartz with 2 kN (500 lbf) of correctly applied force can produce a voltage of 12,500 V."

Using what means and understanding we possess, we determined so far that;
-that the average wheel on a car would be roughly 5 kN
-that .20m of a wheel touches the ground on average with an area of 0.034m^2

Now based on my understanding of piezoelectricity, I am not sure as to whether time would have to be factored in as well or not. So basically if we could come up with some kind of a unit measurement/equation to determine how much voltage a 1 cm cube of quartz would put off with the force of the average vehicle going at an average speed, we could then extrapolate that to conclude exactly how much power could be generated from a roadway of any given length and thus the overall energy potential of piezoelectricity as a whole.

Overall, we'll have one more sound fact to give hope to the world that could be.
But I seem to have taken this equation as far as I can on my own at the time, so I need someone's help.
Anything at all would be greatly appreciated.

2. Feb 26, 2009

### Staff: Mentor

Voltage alone doesn't really tell you anything useful about the energy available - you'd need a current. And note, conservation of energy applies: any energy generated by the crystals has to come from the car engine and not much of a car's energy is lost as rolling resistance on a road.

3. Feb 26, 2009

### fireseed

So then is there any direction you could suggest that I look in to further my progress with this question?

4. Feb 26, 2009

### mgb_phys

From talking to people who use the piezeo sensors for speed/traffic measurements - very little signal is generated because of the road surface material over the top of the sensors.
They have to do some pretty complex signal processing to even detect a car.

5. Feb 26, 2009

### Staff: Mentor

Well, I guess you could pick an arbitrary deflection (it's up to you to design it, right?) and calculate based on that. Ie, try these numbers:

A car weighs 5000N
You design your device to deflect 1m, like a spring
The contact patch is 20 cm long and the car is moving at 10 m/sec (so the contact area is changed 50 times a second)

6. Feb 27, 2009

### nottheone

You don't get something for nothing. I am guessing that the energy you get would be offset by losses elsewhere like loss of gas mileage in all the vehicles that travel the road. So if you expect to harvest free energy I doubt it will work.

7. Feb 27, 2009

### zoobyshoe

The cars are already deforming the road surface without any reclamation of that energy. It would be no different than trying to generate energy from the sound the tires produce: they're going to be making noise whether you do something with that noise or not. The real problem is whether the cost of engineering and installing any such system is paid for by the energy produced.

8. Feb 28, 2009

### fireseed

What about something even simpler like specially designed 1/2 inch air hoses, that when ran over, take the mechanical pressure applied, and force air out, and transfer that into electric energy? Wouldn't that be simple enough to work?

9. Feb 28, 2009

### nottheone

The roads have to handle relatively heavy loads like tractor trailers, they aren't deforming much with a car or it would really sag under a truck. And if they are already sagging then they would have to sag an additional amount for the piezo wouldn't they?

10. Feb 28, 2009

### nottheone

That would definitely kill gas mileage, no net gain.

11. Mar 1, 2009

### Cthugha

Well, this is not quite right as you would just exchange some part of the road for piezos, so you just switch from deforming concrete, where most energy used for the deformation will finally end up as a slight increase in temperature towards deforming piezos, where you can harvest a little bit of the energy. Nevertheless I think the amount of energy you can get is far too small to make the use of expensive piezos sensible. Also the change of temperature between summer and winter should be a great problem at most locations.

However piezos have been used to harvest energy in other locations. Although I think this is more or less a marketing gag, there is a club in the netherlands, which uses a dancefloor with piezos: http://www.nytimes.com/2008/10/24/w...html?_r=3&pagewanted=2&partner=rssnyt&emc=rss

12. Mar 1, 2009

### zoobyshoe

Yes. The amount of deformation, the actual deflection from the previous dimension required to produce voltage, is very slight but requires a lot of pressure, proportionately, to accomplish. Crystals are very hard. You can scratch glass with a quartz crystal.

13. Mar 1, 2009

### Redbelly98

Staff Emeritus
Do I understand this correctly -- a 1 m deflection? Wouldn't a cm be more reasonable? The car has to push the device down by this distance, and then "climb" up out of the depression in the road that has been created. As a driver, I doubt I'm willing to tolerate more than a cm or 2.

Of course, the car must consume more gas to climb out of the dip in the road, which I guess is the whole point. If you really want to burn gasoline/petrol to generate electricity, you might as well buy a generator.

14. Mar 1, 2009

### zoobyshoe

I'd be amazed if the deflection required anything on the order of a milimeter much less a centimeter.