I wonder how it would pencil out if you just pumped groundwater through them and returned it to an adjacent well, without even trying to harvest the heat. It cools the panels in summer, and warms them in winter when snow coverage might otherwise be an issue. Melting the snow off can turn an otherwise-nearly-zero period into a productive one.

But this is way simpler than domestic water heating. You don't care about thermostats and storage tanks, you don't worry about overheating, you don't even necessarily have to care about leaks since it's all going the same place anyway. There's only one pump and no valves. The panels themselves could be made to a lower standard since the whole thing could run at nearly-zero pressure.

In colder climates maybe it makes sense to use mirrors to increase output since the panels are already cooled by the environment?

The term of art there is "concentrated PV solar". Big fresnel lenses concentrating light onto a postage stamp of photovoltaic material. Requires sun tracking to keep the light on the PV, so it's useless for rooftop solar.

If you look it up you'll notice all the citations are decades old. Designed for a world where PV was terribly expensive, so you'd trade a lot of mechanical and packaging complexity to use as little of it as possible.

Then silicon got 100x cheaper. Now every solar install uses fixed-angle racks or just plopping the panels flat on the ground. You lose some efficiency, but land is cheap, so who cares?