Avoiding stagnation can be done a few ways. First, is to angle the panels at 80-90 deg, next is to have panels with low terminal temps anyway. For example, most German panels (Viessmann for example) have highly selective absorbers such as "sunselect" or "bluetec". These panels can get up to 200C+ if left in full sun.
Thermo-Dynamics from N.S. has two different panels (as does Viessmann) with the highly selective surface or a painted surface. The difference in stagnation temps could be as much as 40C.
The other thing to think about is the insulation. The better the enclosure the higher the stagnation temp. Many people will decide to have a moderately insulated (lesser cost) case and more panel area to get more total heat out.
Most good controllers will limit the pump operation to collector temps below 130C as the pump is typically not approved for temps above that. After that, the collector will steam the antifreeze out of the panel which will then go into the expansion tank. If the panel temp can be limited to below 100C by angle and a large or multiple tanks, stagnation will be kept to a minimum. Good glycols, made for solar, are designed to take the high temps but standard glycols are not.
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