I built an inexpensive DIY solar heater for my 15' above ground pool.
We get cold night winds where I live that just chill the heck out of the pool water, so that even though it's warm during the day, the water is often pretty chilly.
I modeled my heater after this:
But, since it's an AGP, I had to build an elevated frame for it, which I built out of 2x4. I also did not use a solar pump, I am using the original Intex filter pump that I got with the pool (I upgraded to a big sand filter/pump for use on the pool).
So currently I have to remember to turn on the pump for sunny days and remember to turn it off, otherwise it becomes a very effective water chiller at night.
So my plan is to automate this with a couple of temperature sensors, one mounted on the solar surface and one either in the air or possibly in the water, and to have a relay turn the pump on when the temperature difference grows big enough (that is, the sun is heating the panels).
Anybody interested in this project? If so I will try to document it.