A supplier has stated that one must protect against overheating of solar systems-
"Basically, stagnation occurs when the solar storage tank heats up to maximum temperature early in the day; movement through the solar collector stops, and the fluid in the system sits under the sun getting hotter and hotter. The result is a high pressure, high temperature condition that can damage the system over time by subjecting it to extremes of expansion and contraction. In addition, when anti-freeze is superheated every day for weeks at a time, it tends to break down and become acidic, thereby transforming into a corrosive substance that circulates through your system slowly damaging its components."
I wonder why should antifreeze break down and become acidic? isn't that what it's doing every day in the car?

The reason might be that propylene glycol is used for solar collectors and ethylene glycol is used for our cars.  The reason that propylene glycol is used for collectors is that it is non toxic whereas ethylene glycol is toxic.  This is why it's so important if one drains their radiator they make sure and dispose of it properly.  It tastes sweet apparently and there have been numerous cases of dogs lapping it up and dying.