Evacuated Tube Collector

 Evacuated tube collectors are made of a series of modular tubes, mounted in parallel, whose number can be added to or reduced as hot water delivery needs change. This type of collector consists of rows of parallel transparent glass tubes, each of which contains an absorber tube (in place of the absorber plate to which metal tubes are attached in a flat-plate collector). In some cases, the tubes are covered with a special light-modulating coating. In an evacuated tube collector, sunlight passing through an outer glass tube heats the absorber tube contained within it. The absorber can either consist of copper (glass-metal) or specially-coated glass tubing (glass-glass). The glass-metal evacuated tubes are typically sealed at the manifold end, and the absorber is actually sealed in the vacuum, thus the fact that the absorber and heat pipe are dissimilar metals creates no corrosion problems. Some systems use foam insulation in the manifold. Soda-lime glass is used in the higher quality evacuated tubes manufacture.

Later technology evacuated tube systems use the glass coated absorber. The glass is a boron silicate material and the aluminum absorber and copper heat pipe are slid down inside the open top end of the tube. In lower quality systems moisture can enter the manifold around the sheet metal casing is eventually absorbed by the glass fibre insulation and then finds its way down into the tubes. This leads to corrosion at the absorber/heat pipe interface area, also freeze ruptures of the tube itself if the tube fills sufficiently with water