Energy harvesting with PV indoors, for IoT

[DamonHD]

Hi,

We're having a little fun, aiming to remove the need for batteries in our smart radiator valve.  (How much nicer NOT to have to remember to replace a couple of AA cells every year or two.)

https://github.com/opentrv/OTWiki/wiki/Energy-Harvesting-Feasibility

Anyway, I have these 5F and 15F supercaps on my desk, about the size of a normal ~470uF cap, which makes me feel like I have some sort of superpower.  Yes, still probably thousands of times less energy dense than the NiMH AA cells next to them, but, well, it still feels like fun!

Anyway, it looks plausible, and I'll try this simple circuit out first for the sensor in my porch:



It doesn't need to drive a motor, and gets more light than anything inside.

I know that I may still have a problem with the AVR not starting properly if the supply is ramped up *really* slowly with the low brown-out voltage that we have set, but I can deal with that later.

Rgds

Damon

[OperaHouse]

My home system works directly off solar and that has a really slow ramp up.  The A/D converter gives strange numbers for a while. Never had it hang up.  Had to disable certain functions for a period of time using a loop counter.  Thought of using a TL431 as a reference and when that input read correctly allow voltage based decisions.   

[george65]

My wife bought an LED solar light a while back which we put in the bathroom. It charges a AA cell and has an auto on off function which works real well.
the thing won't last all night though so I replaced the 600 Mah battery with a 2400 unit.  Worked fine for a couple of nights than started running out again.
I tested the thing and concluded the solar cell is too underpowered to supply enough charge to the battery to get it though the night.

I was given a small panel, I estimate about 6W a while back that is too small for anything practical I could use it for.
I rigged it up to this light and just soldered the output wires to the connection where the inbuilt panel is.  I put the panel in the top of the window where no one can see out anyway. It only gets direct sun for about 2 hours a day but that seems to be enough as the thing provides light all night now. Last week we had 5 very cloudy and overcast days on the trot and it still made it through the night.  My wife gets up just after daybreak at this time of year and said the light was still going in the morning and she saw it turn off as the sun came up.

I'd like to do a few of these around the house.  Just get something like a 7Ah battery with a cheap solar charger and a bigger panel and put them strategically around the house.  Could wire them up with thin and cheap speaker wire as the current to the things would only be MA anyhow.   
They are very sensitive to light and work perfectly indoors as well.

Maybe charging a NIMH battery would be cheaper than a super cap but then again if you have them and aren't ding anything with them....... Just make sure your panel can generate enough power on the cloudy days as well as the sunny ones.