thermosiphon

A thermosiphon is a passive heat-transfer loop that uses natural convection to move a fluid between a heat source and a heat sink without the aid of pumps. When a portion of the fluid is heated, its density decreases and it rises, while cooler fluid sinks. The buoyancy-driven flow creates a continuous circulation as long as the system is arranged so that the heated region is below the sink and both are vertically separated.

In solar thermal installations, the solar collector is usually located below a storage tank. The sun heats the water or heat-transfer fluid in the collector; the warmed fluid rises to the storage tank, while cooler water in the tank returns down to the collector to be reheated. For reliable circulation, the tank is typically placed above the collector, and piping is laid out to avoid air locks and excessive bends that could impede flow.

Thermosiphon systems can operate in direct configurations, where the working fluid circulates through the collector, or indirect configurations, where a heat exchanger coil inside a storage tank is heated by a separate loop. They are commonly used in domestic solar-water heaters and some space-heating installations, offering simplicity and quiet operation but requiring adequate vertical height and a predictable climate to maintain effective flow.

Advantages of thermosiphons include simplicity, low maintenance, and the absence of moving parts or electric pumps. Limitations include dependence on gravity and installation orientation, reduced performance with small temperature differences or limited vertical space, potential heat losses when idle, and freezing risk in cold climates without proper antifreeze or drain-down provisions. They are generally best for moderate hot-water demand and installations that can guarantee a reliable buoyancy-driven circulation path.