Energy harvesting

Posted on by crackingcontraptions_3voi0v

I don’t see a point in using a super-cap for energy harvesting apps.  May as well use a lipo, which has better power density.

Slow charge a LiPo from harvested power

For battery charging purposes maximum power-point tracking seems to be a good thing.


Looking at the energy harvesting regulators that David found they seem to be specialized for different applications:

Thermoelectric Regulators

I am assuming that the options here are a Peltier, or a thermopile.  I have not seen a thermopile that even approaches wearability, so I’m going to assume that a Peltier is the only feasible wearable device.

This is an article about Peltier modules that I can understand: https://www.cui.com/blog/how-to-select-a-peltier-module.   Unfortunately it only deals with the case where the Peltier is used as a heat pump, not where it is used as a TEG.  

Using it backwards to generate electricity is called the Seebeck Effect.  From the examples on this video it looks like you need fire on one side, and ice on the other to generate enough current to charge a phone!
Here’s a 12V Peltier plate spec

Model: TEC1-12706.
Size: 40mm x 40mm x 3.6mm.
Working current: 4.3-4.6 A (rated 12 v); Imax: 6A.
Rated voltage: DC12V (Vmax: 15 v starting current 5.8 A).
Operates Temperature: -30℃ to 70℃.
Refrigeration power: Qcmax 50-60 w.

They come in sizes as small as 15x15mm from Digikey.

Have to get good contact between the heat source/sink and the Peltier.  Here’s some heat conducting glue.  Also probably need a heat-sink.

Digression – Peltier watch…

Here is a proof-of-concept Peltier watch which at least shows it’s feasible.

Latest update is not great news.   I wonder how often you have to get your wrist out in the sun to be effective.

Cool Video – https://www.indiegogo.com/projects/smartwatch-powered-by-you-matrix-powerwatch-2#/
I wonder where the solar panel is?

Back to the plot…

So the other thing I might consider is placing a Peltier atop a radiator, or somewhere where it would gather direct sunlight.

LTC3106
LTC3107
LTC3108-1
LTC3108
LTC3109
ADP5091

ADP5092

Piezo

Piezo seems easily capable of illuminating a LED with a relatively small force, deflected over a significant angle.   Here’s a crappy quality video, but the demo is really simple and obvious.
LED illuminates momentarily each time the piezo strip is flexed
I decided to do my own experiment with some stuff I had to hand.  Suppose I wanted to harvest some energy from a wearable.  What sort of piezo could I use?  Would need to be small!  I got a piezo beeper that I salvaged from somewhere, and simply taped it to my Bip watch.   
Then I attached a voltmeter to it and walked up and down.., viz.

On the 300mA setting I got nothing, so I guess uA is the best case!  I connected the currentRanger and got a tiny nA reading which I assume was AC hum.  I stuck a diode on one terminal and tried again.  By tapping the piezo pretty hard with my finger nail I managed to see a peak of 16nA.  16nA at 40mV… hmm?
So I’m not all that encouraged by this either.

Solar

Solar seems like by far the most encouraging.  The question is how much energy can be generated behind a window.  I know that window glass attenuates the received energy very substantially.
ADP5092

Slow charge a supercap

Here is an app schematic that seems to handle both solar and capacitor.

Quick Charge

And here is one designed just to charge a cap.


But the MAX14575 requires that the input voltage is the same as the desired output voltage.  Also it seems to have some limit on the size of capacitor it can handle.

For a 100F SuperCap    100,000,000 = (250 * tBlank) / 2.7.  
tBlank = 1080 seconds!

So it seems like it will turn the output on/off until the overcurrent condition goes away.  The duty cycle will be about 3%.  The alternative is to use the 14575C which does not duty cycle, but it might get hot!



 The MAX667 seems to limit the voltage and the current (to 250 rather than 240, but hey).  But perhaps it doesn’t limit the current drawn, and simply get’s hot and angry.

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