# Homework Help: How to harness current from piezoelectric plates to charge a phone?

1. Jan 16, 2014

### cnichols1999

Hello! We're in 8th grade and our doing a project-not homework. Our goal is to put piezoelectric plates inside a pillow and harvest the energy from the pressure of the head on the plate during the night and use it to charge a phone. However, from this point we are a little confused. We've been doing a lot of research but there's not a ton of information we've found so far and it's been conflicting. We know it would be best to experiment, but we don't want to buy unnecessary materials as we have a tight budget. So far we are think that the piezoelectric plates have wires-can't think of the technical name-that lead to a portable outlet that charges the phone. But how many amps and volts would we need? We found the number of volts necessary (we think), but not the amps. Also, we saw in some of our research about capacitors and direct vs. inverse current? Where would we get a capacitor and do we need one? How do you change a current? We have a lot of questions... any advice someone with expertise in the subject could give us on what materials we need, where to get the materials, how to build it, and the science behind it would be terrific! We are sort of new to this, so please explain any terminology beyond the basics. Thanks!

2. Jan 16, 2014

### Staff: Mentor

Welcome to PF!

I'm sorry, but I think you have the wrong idea about piezoelectricity and more generally about conservation of energy. There is no energy generated by anything under constant pressure. A piezoelectric device generates a voltage when deformed, but not continuously unless the energy absorbed during the deformation is not released. In other words, continuous power generation requires repeated deforming and releasing of the deformation - a vibration.

3. Jan 16, 2014

### cnichols1999

Sorry, we weren't assuming constant pressure...we assumed from tossing and turning during the course of the night it wouldn't be constant. So the pressure of the head wouldn't be fast enough deforming and releasing? We did know it had to release, just not the speed of. So this wouldn't be a way to harvest kinetic energy? Thank you!

4. Jan 16, 2014

### mishima

I know one small disc can create a potential difference (voltage) of at least 1.7 V, because I have lit normal LEDs with them before. You can just grasp the two leads of the LED tightly against the two sides of the disc and flick it with your forefinger to see a flash.

As mentioned above, you really would need some kind of continuous vibration. One person used rainfall dripping through a long pipe, you can look at his project here:

There is some good links there as well. As for where to purchase, I like Digi-key.com. These discs are quite cheap, less than a dollar. You can also salvage them from many electronic devices that make sound.

In my opinion (not expert) the tossing and turning would not be enough. Perhaps over many nights it would work.

To use it, I think you will have to solder. One side has a white conductive paint covering the crystal, the other side is bare brass. You will need to put one wire on each side.

Last edited by a moderator: Sep 25, 2014
5. Jan 17, 2014

### dlgoff

Personally, I think this is a good idea for a project. You may not be able to charge your phone overnight but would certainly be able to get some charging depending on the mechanics of your "pillow". I say give it a try and collect data from different configurations. Presenting the results of the experiment would be a good learning experience.

If you haven't already found this, take a peek: http://www.piezo.com/tech2intropiezotrans.html

6. Jan 17, 2014

### Staff: Mentor

Welcome to the PF.

If you do use a piezo transducer, you will follow that with a Schottky diiode to rectify the output AC voltage spikes, and store that energy on a capacitor. You would then use a DC-DC converter to convert that stored voltage to the 5V that you want to have as the output.

But, from an overall engineering point of view (at least my point of view), this is a very inefficient way to try to harvest energy from a person moving about in a bed. Have you brainstormed other ways to harvest energy for this purpose? I can think of several better ways to do it, but before giving you some hints, I'd like to hear if you and your team have thought of other ways to harvest energy.

7. Jan 17, 2014

### cnichols1999

Yes, we have. However, they had already been done we learned upon googling and we wanted to do something original. We understand that it has to be a quick "squeeze and release" for lack of a better terms so we thought of a computer keyboard, a mouse, shoes, and the floor. All had been done. What is a Schottky diiode? Thank you!

8. Jan 17, 2014

### Staff: Mentor

A Schottky diode is a circuit component that is used to "rectify" an AC waveform into a DC waveform. The output from a piezo sensor is AC (voltage spikes from pressure changes), and you need a DC voltage to be the source of the energy that you use to make a battery charger.

To illustrate some typical circuits that are used with piezo transducers to harvest energy, I used a Google Images search on Piezo Energy Harvesting Circuit, and got lots of good results:

And for more information of rectification and DC-DC converters, you can search for those on wikipedia to get introductory articles.

9. Jan 17, 2014

### Staff: Mentor

Also, from your reply, it sounds like the main focus of your project is to use a piezo transducer for the source of the energy harvesting. Can you consider other means for converting mechanical energy into electrical energy?

10. Jan 17, 2014

### CWatters

How about working some numbers to see if it's feasible? Start by working out how much energy you might expend raising and lowering your head against gravity in a typical night. Then assume you can capture some percentage of that. Multiply by the efficiency of the piezo etc.

11. Jan 17, 2014

### cnichols1999

Thank you, this is very helpful. We will take advantage of this info and hopefully find another way to harness kinetic energy.

12. Jan 17, 2014

### cnichols1999

Is it against gravity or the force of it hitting the piezo? Sorry, little confused. Is their a particular equation to use? I found multiple on google...

13. Jan 17, 2014

### cnichols1999

We could. From our limited knowledge and google we have not found much in our price range but we will continue to search.

14. Jan 17, 2014

### cnichols1999

15. Jan 17, 2014

### cnichols1999

If we need about 5 v to charge a phone that would be three discs...is there a certain way to arrange the discs so they don't cancel one another out or is that not a potential problem with piezoelectric discs? Thanks

Last edited by a moderator: Sep 25, 2014
16. Jan 17, 2014

### Staff: Mentor

17. Jan 17, 2014

### cnichols1999

I thought you meant a different type of generator, those were more expensive. Our goal is to harvest kinetic energy from something no one has done yet, but perhaps we can build off this. Thanks! :)

18. Jan 17, 2014

### cnichols1999

As an alternate way to harness energy, does anyone know what type of small generator is used for capturing motion, like that of an arm swinging?

19. Jan 17, 2014

### Staff: Mentor

What have you found so far on this question via Google?

20. Jan 17, 2014

### cnichols1999

So far I have found not a specific name for anything we could either A, build, or B, buy. They have nanowires, but that is a recent development. Some websites also said that piezoelectric discs did that, but from my previous research I believed that piezoelectric discs released energy from mechanical pressure, not motion so I am somewhat confused on which it is for, or if it is for both? I have also found something that says motion-like an arm swinging-is vibrations and could therefor be used by anything that picks up on vibrations, but I don't know if thats true. There is a myriad of large generators, but not many small generators for our intents. In most cases they were given general terms, such as just generator and nothing very specific as to help us find it. There is also something called microgenerators, but I only found that on wikipedia and seems to be a broad term, so again not specific. I suppose I am asking if there is anyone who knows if A, piezoelectric works for motion or B, if anyone knows of anything else. I will continue to research the manner myself also obviously. My apologies, should have included this info in my original question.

21. Jan 17, 2014

### Staff: Mentor

No need to apologize. As you can tell, we are trying to help you learn how to learn, in addition to helping you a bit in your thinking about the project. It's all good.

The energy harvesting clothing Google link that I posted earlier should help to answer many of your questions. Arm swinging is part of what they will be addressing with anything wearable.

The problem with piezo energy conversion from mechanical to electrical energy, is that piezo transducers are constrained to very small mechanical movement/displacement. That doesn't match head movements on a pillow. It's closer to the keyboard key strikes that you mentioned, but definitely not a good match for longer displacement, slow mechanical movements.

I'd recommend spending a bit more time with Google and Google Images. Try some search terms like biomechanical, energy harvesting, arm, etc. TBH I don't know the best answer to your question of what is the best way to harvest energy from arm movements, but I'm pretty sure that I could come up with some good answers using Google and refining my searches with some search terms. Pretty amazing what the modern search engines enable us to do...

22. Jan 17, 2014

### Staff: Mentor

Full disclosure -- I do not own stock in Google...

23. Jan 17, 2014

### cnichols1999

Alright. Thank you!

24. Jan 18, 2014

### CWatters

I was suggesting you estimate the available energy that could possibly be harvested...

Lets assume that people lift their head 0.1m and allow it to drop back onto the pillow once every 5mins. Lets say the average head weighs around 5kg. If these figures sound unreasonable feel free to substitute your own estimate.

When it's raised 0.1m the potential energy of a head would be

PE = mgh = 5*9.8*0.1 = 5 Joules.

That would be converted to Kinetic Energy as the head falls. If they do that every 5mins then the available power is..

Power = energy/time = 5/(5*60) = 0.01W.

Some of that energy will be absorbed by the pillow rather than captured by the piezo electric energy harvesting device. I've no idea how to estimate that so we will assume all the energy is applied to the Piezo device somehow.

The next question is how much of the energy does the Piezo convert into electricity. I couldn't immediately google up a figure for the efficiency of a Piezo electric device but eventually found this reference from a 2010 paper...

http://www.mne.psu.edu/me345/Lectures/Piezoelectric_Energy.pdf [Broken]

So lets assume currently available models are 20% efficient. Again feel free to substitute your own figure.

That means you might be able to harvest..

0.01 * 20/100 = 0.002W

Which really isn't very much. I would also suggest it's an optimistic estimate.

Last edited by a moderator: May 6, 2017