The solar tube consist of a copper heat pipe. A black copper absorber is attached to the heat pipe. These parts are in a vacuum-sealed solar tube. The heat pipe is hollow and the space inside, like that of the solar tube, is evacuated. The reason for evacuating the heat pipe is not insulation but to promote a change of state of the liquid it contains. Inside the heat pipe is a small quantity of liquid, such as alcohol or purified water plus special additives. The vacuum enables the liquid to boil at a much lower temperature than it would at normal atmospheric pressure. When solar radiation falls the surface of the absorber, the liquid within the heat tube quickly turns to hot vapor rises to the top of the pipe. Water, or glycol, flows through a manifold and picks up the heat, while the fluid in the heat pipe condenses and flows back down the tube for the process to be repeated.
An advantage of heat pipes over direct-flow evacuated-tubes is the "dry" connection between the absorber and the header, which makes installation easier and also means that individual tubes can be exchanged without emptying the entire system of its fluid.
Some heat pipe collectors are also supplied with a built in overheat protection – when a programmed temperature has been reached, a "memory metal" spring expands and pushes a plug against the neck of the heat pipe. This blocks the return of the condensed fluid and stops the heat transfer.
A drawback of heat pipe collectors is that they must be mounted with a minimum tilt angle of around 25° in order to allow the internal fluid of the heat pipe to return to the hot absorber.
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