Evacuated tube solar collectors operate like a thermos bottle. The solar collecting surface is surrounded by a vacuum that is created inside the glass tube during the manufacturing process. Solar radiation (sunlight) passes through the glass tube and evacuated space, lands on the collection surface, generates thermal radiation which cannot pass back through the evacuated space. The evacuation acts as a check valve, allowing the sunlight past while keeping the heat inside. Also could be thought of as invisible insulation. As thermal energy builds inside the collector a cycle of boiling then condensing begins within the center of the collector. A small amount of distilled water boils to the top of the tube where it gives off its heat to the array and then condenses back to liquid and runs back down to start over again.