Its temp this morning was 7.3C, which is more than a full degree colder than yesterday morning. Despite the weather being a little warmer again. @IBob is right, it's still in cooldown.
A thermometer 2/3 of the way down (midpoint on the stack of water containers) was at the same time 3C!
So, the water is keeping a very good amount of cool overnight, the heating I'm seeing is mostly of the top layer of air. And I have a compartment already that is probably staying consistently below 4C (though also probably sometimes below 0C), in the "well" where the water tanks end. Currently I'm mostly only keeping some cold drinks and milk down there, but anything I want in that range (meat?) could go there.
My plans for the next week or so are mostly to observe it and do minor improvements to the control software. Longer term, I have a bunch of ideas to try..
- Reorganize so the food is stored below the level of some of the water, so that it sits in a pocket of cool air under a ceiling of warmer air. This probably involves water tanks up the side walls with the food in the middle. Also insulating from minus-degree walls while cooling. The disadvantage is it will be harder to access the food, while right now it's mostly a reach-in operation.
- Or add a small circulation fan to mix the air, instead of reorganizing the water tanks.
- Insulate the lid some more, but I doubt this will have a big effect.
- On a partly sunny day, there is still solar capacity going to waste in the half of the time when the fridge is too cold to run and batteries are charged. My old propane fridge can run on 12v DC or AC, so hook it up with a relay to turn on at those times, and try freeze water in it. If that works, I'd have a freezer compartment; I could also try to circulate the cold over to the fridge at night.
- Try switching it to freezer mode, so it can run all day long, and pipe the cold over to a free junk fridge or my old propane fridge.