Computing stuff tied to the physical world

Solar… again

In Hardware on Jan 20, 2013 at 00:01

These dark and cold winter days are not really effective for solar energy: the entire month of January will probably not generate more power energy than two peak days in July!

Still, it’s a good baseline to try things with. And one of the experiments I haven’t given up on is making nodes run off solar power, using a supercap to hold the charge. Maybe they’ll not last through the night, but that’s fine – there are still various uses (especially outdoor) where being able to run during daytime with nodes that never need to have their batteries changed would be really nice.

In previous attempts, I’ve always immediately tried to power the actual node, but now I’d like to try something simpler: a solar cell, a supercap, and a resistor as load. Like this:

I’m using a tiny solar cell by Clare again, the CPC1824, with the following specs:

Not much, but then again, it’s a cell which is just the size of a fingernail. As SOIC-16 package, and with the specs of the available current next to it:

In fact, I’d like to try this out with 4 different resistive loads:

• one cell, 2x R = 20 MΩ = 0.2 µA current draw at full output
• one cell, 2x R = 2 MΩ = 2 µA current draw at full output
• one cell, 2x R = 200 KΩ = 20 µA current draw at full output
• two cells in paralel, 2x R = 200 KΩ = 20 µA current draw at full output

There may be a flaw in this approach, in that the leakage of the supercap could completely overshadow the current draw from the resistors. But my hope is that supercaps get better over time when kept charged. Hmmm… not sure it applies if they run down every night.

So the second part of the idea, is to alternate solar cell use and dumb charging – just to measure how that affects output voltage over time. One hour, DIO will be on, and put the supercap on about 2.7V, the other hour it’ll be off and the solar cell takes over. With a bit of luck, the output voltage changes might show a pattern, right?

I think it’s worth a try and have made a setup with 4x the above – more tomorrow…

1. \$4025/W! Are you kidding? You know you can get panels for £0.51/W? They need to be a bit bigger, though…

That’s the DigiKey price for the CPC1824 vs the Navitron price for a 235 W non-MCS-approved Scheuten panel. Not an entirely fair comparison, I admit. ;-)

• Heh :) – and to spice it up, you used different currencies…

This is just a test to see how indoor light levels relate to usable energy. These fingernail-sized chips are probably not an effective option, but I have a couple of ’em lying around. I’d definitely be interested in a 5 cm^2 panel or so which delivers about 4V and works well with low light levels. Apparently mono-crystalline cells work better indoor.

2. I’ve been wondering recently if one of the cheap solar garden lights would make a good donar for solar panel, charging circuit and maybe even the battery. Last one I opened had a singe AA battery inside. I was hoping that the panel could just top up the battery to make manual recharging a rare event.

3. Cheap unfortunately often maps to ‘nasty’. Getting a decent life out of a rechargeable requires a good charge monitor/inhibit circuit (as Boeing may well discover). Many of those garden lights slap the solar panel directly across the battery and pray for cloudy days – it gets it out of the Garden Center and through the 30days warranty..

4. Those specs do not seem to match. The table lists 100 uA at 6000 lux and the graph lists 1 uA at 6000 lux. Or am I reading that wrong?

• That’s normalised, i.e. 6000 lux is set to 1.0 for reference.

Note also that light levels are way below 6000 lux here, at the moment. I’m seeing 500..1200 lux at best, at 15:30 (cloudy with snow). And then there’s that nasty supercap self-discharge… will be able to show some first results soon.

5. “the entire month of January will probably not generate more power than two peak days in July!” That should be energy, not power.