Evacuated tubes are a modern version of the flat plate panel for a thermal system. They capture the energy from the sun's rays and transfer it to a passing fluid. The vaccum between two pieces of glass minimizes heat loss as the solar fluid passes through a complicated manifold.


These cylindrical tubes look cool, but unless you're in a cloudy environment they aren't the most productive way to heat water for typical thermal systems. They can make systems very hot and often have a heat dump for excessive energy.