Stagnation:  When the sun is shining on a collector, and the heat is not being taken away, the collector temperature will rise to the stagnation point, usually 300 degrees F - 400 degrees F. 

Tilt Angle:  the angle above the horizontal at which the collector is lifted to face the sun.

How heat moves...

Free Convection, Heat Radiation, Conduction, Forced Convection