Well, I've run my panels at night when the water got too warm. But my panels are elevated, not on the ground. If your panels are directly on the ground, they bake the dirt and infuse heat into the surrounding ground that will return heat energy to the panels by conduction, but only for a while. If they are elevated you'll get less of that. Al should be up on that part.

Heat transfers in 3 ways, our old friends radiation, conduction and convection. Horizontal panels, at night, should give off heat mainly by radiation and convection. The more water you can run through them without causing it to spring leaks, the more heat will dissipate. Then turn off the feed to the panel when it's sunny. Pressure won't build up because the panel's own return remains open.

From a practical standpoint, solar panels that are close by, at ground or deck level, are really simple to plumb in. I tee off the return, with a ball valve to the panels. I have a separate return from them (actually, two returns as I split my system into two circuits after the main return).

What complications did you have in mind, Ben?

Carl