Thermosyphons

The Process
In gasses and liquids, there is a natural process known as convection that follows the principal of flotation. Circulation occurs because hotter molecules are lighter than colder ones. Hotter molecules take the shortest path to the top displacing colder ones.

As the hottest molecules arrive at the top they distribute themselves as evenly as possible over the available surface area. In practice, in a tank of water, each molecule imparts, by conduction, some of it's heat to it's neighbours and together they rise until they reach equilibrium. When heating is interrupted, such as when an electric element is switched off by a thermostat, circulation ceases and the body of water becomes layered or stratafied. The hottest molecules are now at the top and the coldest at the bottom.

Forced Convection
Heat energy is applied near the bottom of the tank, which transfers heat to the molecules closest to it. They, in turn, rush toward the top where they evenly distribute themselves across the surface area. The displaced molecules are replaced and the process repeats until the tank, and its contents, reach the temperature of the heat source.

Thermosyphon Convection
There is no direct heat energy applied to the tank, instead heat is applied to an element, called the collector or absorber, which is remote from the tank. No pumps are involved. Heat is transferred to the tank using only naturally occurring convection heating. For this to be effective, the tank must be above the collector.

Next: The parallel Collector Array