Hi everyone! I am currently working on project involving piezoelectricity and would really appreciate any and all feedback/corrections/advice. 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?
  

Any general feedback/corrections/advice?


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

 

The units of energy is a joule.

1J = 1 kg m^2 s^-2

The power generated from a piezoelectric transducer continuously vibrating is very low, less than 1μW.
The energy generated from such a transducer struck once will be minuscule, less than 1μW-s
To charge a battery you will need to hit the key about a billion times.

 

Any general feedback/corrections/advice?

First: you cannot simply connect a capacitor across a piezo element to charge the capacitor; there's a LOT more to it than that. Check out the data sheet for this integrated circuit designed to handle the power management functions needed for energy harvesting with piezo transducers.

Secondly, I think you'll find that you're being extremely optimistic about the amount of energy that can be obtained by intercepting keystrokes on a laptop with piezo transducers; my guess would be you'll get somewhere between a millionth and a billionth of what you're hoping for.

Sorry to be a wet blanket, but reality bites sometimes.

 

Piezos are good at generating voltage but are pathetic at generating current, because their internal resistance is very high. Hence power = voltage x current = diddly-squat.

 

This is an interesting watch, you might ask the guy how many times he had to bend that thing to light an LED for a few seconds. Also that seems like a pretty beefy piezo element.

 

ceramic piezo elements produce a brief spike of voltage/current when struck. Strike it too hard and you risk breaking the ceramic. (Set your DMM to DC voltage and enable the HOLD function, connect the leads to your piezo element then tap the element with the plastic handle of a screwdriver) In your case it looks like you are trying to use it as a "Joule Thief." A steady, sharp vibration might work, but would be very inefficient in my opinion. For a steady vibration, try a vibration motor from a discarded cell phone or similar.

 

I assume this is a "could this be done and might it be useful" investigation project.

A digital oscilloscope would be a very helpful instrument since it would allow you to capture information on the output of piezo element. No other common instrument will be of much use.

You could do experiments looking directly at the voltage from the element with nothing but the oscilloscope as a load (around 10 to 15 picofarads in parallel with 10 megohms for a good quality probe) - careful - the voltage might be quite high. You could then do tests with various resistors for loads. With many digital scopes you can get directly displayed values such as the power put into the resistor. Many scopes also have the ability to write the data to a USB port or a memory stick so you can do additional analysis with a computer. Tests like this would allow you to gain some reasonably quantitative idea of output from the piezo element so you could determine if further work to try to capture that energy had any merit for the objective. Any method used to actually capture the energy is going to be less than 100% efficient.

My suspicion is that the energy produced by the piezo element probably wouldn't be enough to supply the processor in a computer with the energy to process the keystroke, much less what it does between keystrokes, but I might be wrong. You won't know until you do some tests.

 

I say no way. A very fast typist might sustain 10 keystrokes per second. A normal person would be less than half that. Each keystroke would be a few microwattseconds? I really don’t know. Even 1mW-second means the super typist would be topping out at 10mW. Trivial compared to the laptop. A cyclist on a generator would struggle to keep up with the laptop!