Abstract: A solar energy collector element having an inner tube which carries a heat exchange fluid, an outer glass tube, an evacuated space between the two tubes and a solar selective surface coating in thermal contact with the outer surface of the inner tube. A gas is admitted to the evacuated space for the purpose of degrading the vacuum and limiting the stagnation temperature of the collector element. The gas is selected to exhibit hydrophobic characteristics, to absorb onto the selective surface coating at temperatures less than a predetermined temperature to desorb into the evacuated space at temperatures greater than the predetermined temperature but not adsorb significantly onto the outer glass tube.