I had thought of the irrigation valves but the problem is that irrigation valves are meant for a smaller line such as 1/2 inch. My pool pump uses 1.5 inch lines, so I wonder if there is some way to pull off part of the flow (that is on its way to the pool return lines) to go up to the solar panel and then bring the flow back to the pool. My home built heat sensor should be able to detect a higher temperature on the roof and open the irrigation valve which should allow minimal flow up to the roof so long as there is sufficient back pressure. I cannot see how it would be possible to reintroduce that return flow into the return line going to the pool returns, so maybe an option is to have a separate line going back to the pool from the solar panels which will trickle out a very slow flow of hot water. I shall have to get on and build this and see if it works. Thanks for your suggestions.

I have standard 1.5 inch electric irrigation valves. I may have gotten lucky and only paid like $15 or so each off ebay plus some shipping. About a pound of pressure drop at 40gpm.

Here is a link that might be useful: irritrol valves