Question about the State of Piezoelectronics

[BillIsTheDill]

So another thread here got me thinking about piezoelectronics.
From what I saw there and from what I can find, people have only been able to produce voltages capable of charging 1.2V batteries. Are there any that are capable of charging 3.7V batteries or has our technology not progressed that far?

 

[Bystander]

Wiki, "... 1 cm3 cube of quartz with 2 kN (500 lbf) of correctly applied force can produce a voltage of 12500 V ..."

technology not progressed that far?

More a matter of what's marketable.

 

[BillIsTheDill]

Wiki, "... 1 cm3 cube of quartz with 2 kN (500 lbf) of correctly applied force can produce a voltage of 12500 V ..."

More a matter of what's marketable.

Oops, I meant in terms of recharging batteries, not in general >_<
For example, in this (2005) article they recharge a 1.2V battery https://institutes.lanl.gov/ei/pdf_files/JIMSS2005.pdf.
Any similar studies also use 1.2V, one of which stated that the root-mean-square voltage produced was 1.18, so they used a 1.2V battery. Of course, both studies are fairly old so I was looking for an update (a piezo plate, that when excited can recharge a 3.7V battery).

 

[dlgoff]

Here's some equations to play with from http://www.piezo.com/tech2intropiezotrans.html

 

[berkeman]

I googled Piezoelectric Energy Harvesting, and got some promising hits. Maybe try that and see if you find some battery charging arrangements that work for you...

 

[Alec Dacyczyn]

Output voltage isn't the issue. Voltages can be converted quite easily. Since piezoelectric transducer output is intrinsically AC you could simply run it through a audio transformer (if it's in suitable frequency range, it likely will be) or a charge pump to get a more convenient voltage.

The real issue is joules and watts. Real outputs are tiny. To get anything useful you'd have to use one specifically designed for this application. Those little disk piezo buzzers you tear out of old electronics wouldn't give you much.

Here are some YouTube videos:

 

[BillIsTheDill]

Ah okay, I understand alec.

So, in this article, https://institutes.lanl.gov/ei/pdf_files/JIMSS2005.pdf
A 750 mAh battery charges in 7 hours. Could the PZT output be run through a transformer that would increase the volts to 3.7? In theory, wouldn't that make the battery charge in 21 hours?

 

[Alec Dacyczyn]

Ah okay, I understand alec.

So, in this article, https://institutes.lanl.gov/ei/pdf_files/JIMSS2005.pdf
A 750 mAh battery charges in 7 hours. Could the PZT output be run through a transformer that would increase the volts to 3.7? In theory, wouldn't that make the battery charge in 21 hours?

They charged a 750mAh battery from a piezo source in only 7 hours! Wow, give me a second to read through that...

Okay. They strapped a device specifically designed for energy harvesting to the vibrating engine of a Mitsubishi eclipse... yup.

*shrug* I suppose it could. If the power required to charge a 1.2V 750mAh battery in seven hours was converted to 3x voltage and 1/3 current then, yes, it would charge a 3.7V (or.. 3.6V) 750mAh battery on about 3x the time.

It may even be slightly better than that because of lower power losses in the AC-DC conversion. They show that they're using a full-wave silicon rectifier. That shaved 1.2V off what the piezo was outputting. So if you pass it through the reciter after boosting the voltage than the same 1.2V loss would represent a smaller fraction of total power. But it's hard to say without knowing more about the actual output of the device. Their charging circuit was rather simplistic, and they were making no attempt to optimize power extraction or do any efficient DC-DC conversion. It's also possible that the device was naturally capable of higher voltages (I suspect it would have been); but they chose to throw away some of the energy rather than increase complexity of the charging circuit.

In my earlier post I was assuming you were playing with salvaged piezo disks to harvest a tiny amount of power from ambient sound energy. My suggestion to use a small audio transformer was based on the assumption that you'd be dealing with audio frequencies. But those might not be effective at the much lower frequencies that this large device would typically be used at. If you find that you do need a voltage boost then a charge pump would be the next best very-simple option (they don't care about frequency, but efficiency isn't great).

https://en.wikipedia.org/wiki/Charge_pump

 

[BillIsTheDill]

They show that they're using a full-wave silicon rectifier. That shaved 1.2V off what the piezo was outputting. So if you pass it through the reciter after boosting the voltage than the same 1.2V loss would represent a smaller fraction of total power.

https://en.wikipedia.org/wiki/Charge_pump

Amazing!
You mentioned a DC-DC conversion, but wouldn't you need an AC-DC conversion since I thought the output was AC?

Also, where can I get the PZT they used, I'm trying to find similar ones online but there's lots of terms I don't understand :P

 

[Alec Dacyczyn]

Amazing!
You mentioned a DC-DC conversion, but wouldn't you need an AC-DC conversion since I thought the output was AC?

Also, where can I get the PZT they used, I'm trying to find similar ones online but there's lots of terms I don't understand :P

If you wanted to get every last bit of power from the device you would have to measure the power output for a given amount of flexing and across a range of voltages. Of course, you'd have to rectify it to get DC. But then, to maximize efficiency, to could do a DC-DC conversion from the voltage of max output to the battery's charging voltage.

Never mind what I said about transformers and charge pumps. These things actually develop much higher voltages than you need. They don't work like batteries, with specific output voltages. You can think of them as capacitors which develop a charge between the plates when flexed. The voltage is a consequence of charge (coulombs) and capacitance (farads). Hooking it to a battery through a full wave rectifier allows current to leave the device as soon a the voltage is high enough to overcome the batteries opposition. This prevents it from getting much higher than the battery voltage.

If you are serious about this then you should use the supplier's own power harvesting solution. MIDE doesn't seem to have a transducer currently available that looks like the one in the paper. But they're currently running a clearance sale on another model. (http://www.mide.com/products/quickpack/quickpack-piezoelectric-actuators-and-sensors.php. First on the list.) Its datasheet mentions that they also sell a matching energy harvesting conditioning circuit, the "EHE004". I found it on Digikey. Not cheap, but you know that it will work as advertised and better than anything you could cobble together. It even has a 3.6V output.

 

[BillIsTheDill]

Thanks for the explanation!
I think I understand now, but I still don't understand the exact power output of it.
However, I am serious about "recreating" the experiment in the paper.
Looking at the Piezo you linked, do you think it would charge at the same rate (or even outperform) the one in the paper?

 

[anorlunda]

Also, where can I get the PZT they used, I'm trying to find similar ones online but there's lots of terms I don't understand :P

You should try to contact the authors of that paper and ask them. They may have built it themselves in the laboratory.

 

[BillIsTheDill]

I have contacted them via email, but in case they don't respond, what do you guys think the specs of the PZT were given the results? I'm wondering if what Alec linked would work at an equal/better pace on a battery.