Solar Hot Water Heater Vacuum Glass Tubes.The principle behind solar vacuum tubes is simple. A solar vacuum tube works similar in design to a coffee thermos. It consists of two layers of glass with a vacuum in between the layers. The outer layer of the solar tube is Borosilicate glass which is very low in iron and allows 98% of light energy to pass through. The absorbed solar radiation is then transferred to a heat transfer fluid with in the tube (liquid inside the thermos), This liquid quickly heats up and rises to the top. In an open vacuum tube system the potable water is heated directly with in the vacuum tube. Thermosiphon causes the hot water to rise and be replaced by colder water. The hot water is the collected in the top collection chamber and pumped though the system.