An evacuated tube is most often encountered in solar hot water heating systems. This is where a glass tube is used, with a good heat conductor placed in the middle of it. All the air is removed from the tube, so that when the Sun light hits the heat conductor no heat is then lost out to the tube itself. This way the maximum amount of heat is caught and sent up the heat conductor to then heat the water passing over at the top of the tube.
The other advantage with this approach is that the tube will be placed to be always inline with the Sun as it move through the sky, i.e. no matter where the Sun is the central conductor will present the same surface area to heat.
Often these heating systems are so efficient, that over temperature valves and safety guards need to be put into the installed system.
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