"SLT Energy offers Solar water heating systems using vacuum tubes which are made of borosilicate glass with special coating to absorb the solar energy are called as Evacuated Tube Collector system (ETC Systems).Vacuum tube is the main component which absorbs solar energy. The vacuum tube is an assembly of two concentric, borosilicate glass tubes. Features: All glass evacuated Borosilicate solar tubeHigh Solar-thermal conversion and low heat lossStable advanced selective coatingHigh level vacuum ensure operation under cold weatherTube material: Special grade Borosilicate glassouter tube diameter: 58 mmInner tube diameter: 47 mm