A thermosiphon is a device that uses natural convective currents to move a fluid from a hot state to a cold state. This movement is achieved without a conventional pump. Rather, a differential pressure between the hot and cold state causes movement. Gravity acts on the fluid when it is in a colder more dense state causes movement towards a region at a hotter state. And conversely, thermal energy is transfer to the cold fluid to warm it, then its density can decrease and current reverses direction. In this way a convective current from hot to cold back to hot can flow in the system.
A thermosiphon typically includes a thermal energy source heat exchanger like a solar collector which warms returning cold fluid and a reservoir and thermal load such as domestic hot water that exchanges hot for cold fluid. Once the fluid decreases energy it returns to the thermal source heat exchanger and increase its energy. At the reservoir the hot fluid can be stored. When hot fluid is demanded by a thermal load it is supplied from the reservoir. The reservoir should be higher with regard to gravitational potential than the source heat exchanger to create natural convection. If the reservoir is below the source, a pump would be needed to recirculate.
Water thermosiphons and solar collectors are commonly available commercial devices. These are often used for domestic hot water heating or possibly radiant floor heating. However, it is also possible to use air as the working fluid. For example consider a solar collector coupled with a porous thermal mass or similar heat exchanger. Air flowing through or across the thermal mass or heat exchange can heat the air. This air can circulate a building to provide convection heating. If the air enters the near side of the building and returns from the far side then the temperature will be uniform across the space.
Moreover, the circulation can be stopped for example by a valve or damper. This will provide some control to and from the thermal mass. Additionally the thermal mass can be insulated from the building mass. This is an advantage over simple indirect solar thermal where the absorbing thermal mass such as a Trombe wall exists within the building thermal envelop. Without insulation and without circulation control an interior thermal mass can cause uncomfortable cooling drafts at night.
