Piezoelectric Project

I am currently working on project involving piezoelectricity and I'm in need of some advice. I'm pretty much a beginner, but I've been doing lots of research. I just need input to help fill some gaps. The question I’m trying to answer is, “How much energy is produced when piezoelectric elements are placed on top of the keys of a laptop and what percent of the energy required to charge a battery does this represent?”

I’m going to go about answering this by collecting the following measurements:

What is the average amount of energy generated per keystroke on the piezoelectric-equipped key?

How much energy is stored per average keystroke on a piezoelectric-equipped key wired to a capacitor?

How much energy is generated and stored when the same procedure is used but with multiple keys wired in a circuit?

How does this amount of energy compare to the amount of energy required to charge a battery?

My first step will be determining the average energy generated per keystroke by a piezoelectric-equipped key. Next, I will wire an element on 1 key and connect this to a capacitor in order to measure the energy stored under these conditions. I will then link multiple keys with elements to a capacitor in order to see how they function as a unit. From there, I will build an entire prototype with piezoelectric elements embedded with the keyboard of a laptop. At the end of this, I will compare how much energy is generated to the amount of energy needed to recharge a battery to see how much potential there is for recharging it.

Some of my questions are:

What units should I be using to measure “energy”?

How can I ensure that each time an element is struck, it is with the exact same amount of force?

How can I directly compare the number of keystrokes it takes to charge a capacitor to how many it would take to recharge a battery?

What is a good type of battery to compare to given the scale of what I'm working with?

Thank you so much for getting this far, I really appreciate any help :D

• You will find that this is not a viable idea. Best to drop it quickly and pick a different project before you've boxed yourself into a corner by wasting the available project time pursuing something that won't work. – Chris Stratton Dec 3 '18 at 19:29
• Thanks- would you mind elaborating on why it's not viable? – Vanessa Dec 3 '18 at 19:35
• The amount of energy per keystroke is going to be very small, making it difficult to design a system to harvest it. Once you have your system, you're not going to be able to sell it because the amount of energy extracted is not worth the cost of the device or the hassle of setting it up. – C_Elegans Dec 3 '18 at 20:16
• relevant xkcd: what-if.xkcd.com/102 – C_Elegans Dec 3 '18 at 20:17
• @C_Elegans Ok I see what you're saying. I'm not trying to sell it or anything, I was just doing this as a fun project. Even if the results are negligible, I'd still like to try and get them. If you've got any ideas, I'm all ears! – Vanessa Dec 5 '18 at 3:50

I'm assisting another engineer on a sensor project. With moderate taps on some round piezo, he observes 2volt PP across the piezo, with a 100Kohm resistor in parallel and with about 20nanoFarad capacitance.

Given the energy stored in a capacitor is

0.5 * C * V^2,

the energy is 0.5 * 20nF * 1^2

The energy is 10 nanoJoule.

• you could also add comparison how much energy have common AA battery. For example around 10 kJ. – Rokta Dec 4 '18 at 8:58
• Average AA battery has about 2 A*h capacity and 1.5V voltage, so it has 3600 sec/hour * 1 hour * 1.5 V = 10800 J of energy. – Eugene K Dec 7 '18 at 20:33

A piezoelectric element will generate a charge when you deform it. The deformations are tiny, so this is pretty equivalent to generating a charge proportional to the force applied to the element.

This charge is amplified with a charge amplifier to convert it to a voltage. Energy is not directly related to force: See Spencer (https://physics.stackexchange.com/users/34252/spencer), Relation between Force, Time, and Energy, URL (version: 2014-04-04): https://physics.stackexchange.com/q/106865

Perhaps what you are actually trying to calculate, though, is the integral of force with respect to time.