# Noob Question: Energy harvesting circuitry

Discussion in 'Physics' started by ahash31, May 15, 2014.

1. ### ahash31 Thread Starter New Member

Apr 30, 2014
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Hi Guys,

I'm wondering how high impedance AC voltage which comes from piezoelectric material gets turned into WATTS through circuitry?

What are the fundamental concepts going on?

2. ### kubeek AAC Fanatic!

Sep 20, 2005
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First you need to have watts going in to get watts out. So if you attached a piezo for example to a diesel engine and harvested the vibrations you could get quite a lot of power from it. If you attach it to your fingers while typing you wouldn´t get enough to power even a digital watch.

3. ### Brownout Well-Known Member

Jan 10, 2012
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It doesn't get turned into watts through circuitry. The piezo turns motion into watts. The circuitry has nothing to do with the conversion. The mechanics at work are the crystals in the piezo material and their inherent charge. The crystals/charges move and disrupt the electric field, which induces a voltage on the metal plates attached to the material.

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4. ### alfacliff Well-Known Member

Dec 13, 2013
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how does voltage get turned into watts? by drawing current is how. watts = voltage times current.

5. ### ahash31 Thread Starter New Member

Apr 30, 2014
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So when the piezo is squeezed, it's electrical field changes, inducing a voltage AND a current from the piece of metal it is attached to via electromagnetism?

6. ### Brownout Well-Known Member

Jan 10, 2012
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The piezo converts the "squeeze" into stored energy. From that point, it's the same as any other energy source (eg. battery, etc)

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7. ### alfacliff Well-Known Member

Dec 13, 2013
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Not electromagnetism, piezeo electric effect. certain crystals give off elctricity when deformed (bent or squeezed) usually high voltage and low current. it still takes some energy to deform them, so its isnt free energy, just makes use of "wasted" energy like viberations and motion.
another source would be magnetohydrodynamics, where electrical current can be pulled from flame sources by magnetic fields. ocasionally used to recover energy from stack gas in power plants.

8. ### studiot AAC Fanatic!

Nov 9, 2007
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There are several mixed up concepts in this thread.

Watts are a measure of power, not energy.

Power is the rate of doing work or the rate of change of energy. That is it involves both energy and time.

The measure of energy, without regard to time is the Joule.

The piezoelectric effect generates a voltage directly from rate of mechanical strain. There is no intermediate energy storage involved. As soon as the strain stops changing the voltage stops.

The piezo effect normally generates unidirectional voltage. You have asked about alternating voltage. This can only be done if the mechanical driving system is reversing.

Voltage is not a measure of power or energy.
In order to derive power from the effect we have to connect the gnerated voltage to circuitry that can draw current from this voltage and utilise it to generate power either resistively or (very inefficiently) magnetically.

So you are correct in saying that it is the associated circuitry that generates/ dissipates the power.

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9. ### ahash31 Thread Starter New Member

Apr 30, 2014
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Thanks again, just to further my understanding. Like any battery that piezo will have stored energy represented as a voltage with a current. So a piezo crystal at 4v 1amp is the same as a battery 4v 1amp?

Thanks for your patience with my ignorance! This is all very new!

Sep 9, 2010
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11. ### studiot AAC Fanatic!

Nov 9, 2007
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Go on.

I hope you are not suggesting that a piezo device could contravene the first law?

12. ### Brownout Well-Known Member

Jan 10, 2012
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There is no current involved until an external circuit is connected. However, energy doesn't require a current. When the device is 'squeezed', the mechanical energy is converted and stored as potential energy within the crystal lattice. Thus the conversion process doesn't depend on any connected circuit. Now, in order to put the energy to work, current must be extracted from the device via the external circuit. That is not any different from any other energy source.

13. ### MrChips Moderator

Oct 2, 2009
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Energy = power x time

power = voltage x current

Hence, energy = voltage x current x time

In a piezo device such as a gas igniter, current and time are low, but the voltage is high.

14. ### Brownout Well-Known Member

Jan 10, 2012
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Energy doesn't need current. For example, in a capacitor E=CV. So, for devices like the piezo, energy exists without current.

15. ### studiot AAC Fanatic!

Nov 9, 2007
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Only if the voltage and current are constant over time.

16. ### MrChips Moderator

Oct 2, 2009
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What? Can we not integrate over time?

17. ### studiot AAC Fanatic!

Nov 9, 2007
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Of course, but you didn't say that.

Furthermore, we need to distinguish carefully between Power and Energy.

It take a fixed amount of energy to distort a lattice a specific amount.

It doesn't matter if you do it slowly or quickly the energy is the same.

But if you do it quickly the rate of doing work is greater (but for a shorter period).

This is why the harder you hit a piezo lighter the bigger the spark.

But you cannot get out any more than the lattice strain energy overall.

That is a consequence of the First Law.

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18. ### ahash31 Thread Starter New Member

Apr 30, 2014
9
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So...

It IS a circuitry issue converting the raw voltage that comes out of a piezo crystal into workable energy. What is the way to efficiently turn this raw voltage into energy through "resistive means"?

Thanks for this discussion it has been very helpful.

19. ### Brownout Well-Known Member

Jan 10, 2012
2,375
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Nope. The conversion does not require a circuitry, as I already stated.

20. ### alfacliff Well-Known Member

Dec 13, 2013
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the potential energy of a piezo crystal that has been deformed is the same as the potential energy when it is relaxed after deforming. squeeze or bend it, voltage apears, relax it, and the voltage dissapears. bend it one way, and the voltage apears, relax it, and the voltage goes away. bend it the other way, and oposite voltage apears, relax it and the voltage goes away.