EfficiencyEvacuated Tube solar water heaters are made up of rows of parallel, glass tubes. There are several types of evacuated tubes (sometimes also referred to as Solar Tubes).Type 1 (Glass-Glass) tubes consists of two glass tubes which are fused together at one end. The inner tube is coated with a selective surface that absorbs solar energy well but inhibits radiative heat loss. The air is withdrawn ("evacuated") from the space between the two glass tubes to form a vacuum, which eliminates conductive and convective heat loss. These tubes perform very well in overcast conditions as well as l...