I thought I’d share this project, nothing very ground breaking in the config but I’m quite pleased with the way its worked out. The basic idea is to use the suns energy to help keep the spa pool temperature as high as possible and reduce the electricity bill.
I made a solar heater using mostly scrap material. Its a ply backing board 80cm square onto which I fastened some metal sheet (back of some old dexion shelving) painted black. Used some wood for the sides I then coiled up 20m of black pond then fastened some acrylic roofing sheet onto the front. On a test with a bucket of water and a pond pump it raised the temperature of the bucket from 19 degrees to over 40 degrees in 3 hours although that was on a perfect cloudless day. The pond pump struggled a bit with the length of hose so I switched to using a 12v motorhome type whale pump.
To add some automation I used an ESP32 flashed with ESPhome and then connected three DS18B20 sensors; one to measure the temp inside the box, one to measure the input (pool) temp and one to measure the water temp output to see what the gain is. The ESP32 also controls a relay to switch power to the pump.
I used a template to create a sensor which is the calculated difference between the box temp and the pool temp. I then set an automation to run the pump if this difference is greater than 7 degrees. I might adjust this but on tests so far with any less of a difference there’s no real gain to the output.
So far its surprised me to see the box temp climb well above ambient temp even on fairly cloudy days and the pump runs more often than I expected. I know its never going to heat the 1100 litres in the pool to the required temp but it is helping to keep the base level high so it should save me money in the long run. I realise “proper” materials such as solar evacuated tubes etc would greatly increase the efficiency but I don’t want to spend hundreds to save tens 
Overall this was a bit of fun using mostly scrap materials. If I can scrounge a solar panel cheap enough it would be nice to use that to power the pump.
