Also called a thermosyphon, the thermosiphon is used to passively pump liquids through a physical phenomenon known as convection.
A thermosiphon uses free convection, which means that the liquid being pumped suffers from an increase in temperature thereby becoming more buoyant rising upward.
As the temperature of the rising liquid begins to cool down, it gains density and will thus fall downward, influenced by gravity.
What is clear in this process is that a common thermosiphon does not push hot liquid up and cool liquid down, but rather the force of gravity pulls cool (or far denser) liquids down (toward the center of the earth).
While the consequent vacuum is what creates the siphon effect pulling hot liquids up (or in whatever direction the piping points, generally away from the center of the earth).
The upward movement of heated liquids such as water is one of the most common uses for a thermosiphon, usually in the design of a solar water heater box panel with the reservoir located directly above the solar collector.
Solartrope Supply Corporation makes an interesting passive design that fits the average home-owners needs, providing both hot water and water pressure.
But a thermosiphon can be used anywhere there is a lot of heat being generated, such as on the computer processor of a CPU, where the cold liquid is used as a coolant for the whole casing.
Anything called noiseless usually follows the thermosiphon profile and the simpler designs just cool the insides of any CPU, usually blowing a fan through a thermosiphon radiator.
Being creative with this process is a tradition in the field of thermosiphoning. One such variant on the thermosiphon design are the variothermal wall and floor heating/cooling modules.
As the world of sustainable and passive solar products start to get more and more recognition from the new world economy, thermosiphons get more and more credit from the investing community.
Thermosiphons create whole new realms of products and designs that make the future look ever brighter.
Themosiphon Systems Diagram