From Paul Morrice, who has kindly shared with me details of the Black Pipe collector he has built (in England) and will be installing at a house in France very shortly to help heat the pool. He's using a 12v controller and low power 12v pump ....
"...this is my prototype, there is 55metres of half inch pipe. I have a controller.... and will be sealing the probes in 6 mm stainless tube with epoxy 50mm long.... I have now retained the pipe with some ply and fitted a 2mm thick transparent lid so it looks the business. I reckon it holds nearly 6.6 litres. I will fit a bleed valve on the output side to allow me to purge it. "
Paul is making a st st tube at the exit pipe from the collector and attaching the probe to that, to get good thermal conduction, in the hope that this will be better than attaching the probe directly to the plastic pipe inside the collector.
What a professional looking panel he's made. Please let us now how you get along Paul.