Fridge evaporator plate painted black and flung on the roof. No problem, as long as you pull the heat out to keep it cool, and keep the snow and frost off in winter.

Solar heaters here can get up to boiling, when you see a little plume of steam wafting out the top of 1000 Jondolo roofs making a cloud. That is why there is a mixing valve installed ( regulations) to keep the output water under 55C.