The small amount of liquid in the bottom of the copper tube is heated by the sun's energy falling on the inner absorber tube until it vaporizes and rises to the top of the copper tube's bulb. As the vapor condenses, it delivers its heat to the water that flows arond the bulb in the collector's manifold.   The resulting liquid recycles back down to the bottom of the copper tube to be reheated, so long as the sun is shining and delivering its energy to the glass tubes.

A vacuum exists between the inner glass tube, which has super absorber material deposited on its surface and the outside glass tube that largely prevents the heat from exiting back out of the tube.