Evacuated Heat Tube Solar Water Heater
This system by Thermomax uses heat pipes and a small amount of liquid that boils in sunlight (at low pressure). Heat flows up metal assemblies to the top of the tube where the metal pieces are ganged together and the heat is transferred to antifreeze or some other conducting fluid.
A heat pipe employs an evaporating-condensing cycle, which accepts heat from an external
source, uses this heat to evaporate the liquid (latent heat) and then releases latent heat by
reverse transformation (condensation) at a heat sink region.
A residential system sized for 60 gallons per day would cost about $5,000.