This was a fun project, but it was probably more trouble than it was worth.

It was built around a Raspberry Pi Zero. 3 temp sensors controlled a small circulation pump which circulated water in a closed loop between my home's 40 gallon water tank, and my solar collector on the roof.

In direct sunlight it would raise the temperature of 40 gallons of water by about 3 degrees. This was great as long as it wasn't cold outside. Otherewise, 3 degrees an hour would still be an unpleasant shower by the time the sunset.

I built the collector out of a 2x4 and plywood frame, covered with 2 layers of glass from an old sliding door. It was filled with cpvc pipe painted flat black. It worked, but If I were to build another one, I probably wouldn't use anything but copper for the internals. (I chose cpvc for budgetary reasons) On more than one occasion, the pump stopped for various reasons. It only takes a few minutes on a hot summer day to overheat the cpvc enough until the top most joint (where the box is the hottest) fails. I added a feature to send me an emergency text if the internal temperature exceeded a certain threshold. Which was great, except for that time the whole unit got unplugged, and that other time I forgot to pay my electric bill. Long story short, use copper. I would also insist on a drain-back manifold like design, instead of the neandrolithic "s" configuration I went with.