A thermosiphon system uses the natural tendency of hot liquids to expand and rise to circulate a fluid. This only works in stationary application where the entire receiver pipe and mirrors can be inclined. It works best 20 degree or more below or above the equator and the raised position then allows for more solar energy to be captured when faced towards the equator. Inclining the mirror can be difficult but some house roofs have the correct inclination for their location. A very low drag check valve can be used to prevent back flow. An electric valve can even be used to restrict the flow in order to get the desired temperatures. In the drawing below a double walled heat exchanger is used to heat water and produce steam to run a steam turbine. 

On the left side of the drawing the sun's heat is focused on the pipe and heats up the oil. The hot oil becomes less dense and rises. It then enters the double walled heat exchanger and transfers some of its heat to the water. As it cools it sinks down and the cycle begins again. So only the temperature difference in the oil drives it to circulate without a pump. Inclining the receiver pipe can be difficult for longer mirrors. The mirrors can be mounted on a hill, a support structure can be built or they can be installed on a roof facing the correct direction and with a close to correct roof angle.


A thermosiphon system does not require a pump and can reduce cost, complexity and power required to operate the solar thermal unit. The flow speed also increases as the temperature between the receiver pipe oil and heat exchanger oil and water become larger.

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