Heat pipes which do not have a wick structure, and rely on gravity to return the fluid to the evaporator end are known as thermal siphons or thermo-siphons. (Single-phase) thermal siphons can only transfer heat “upward”, or away from the acceleration vector. Thus, orientation is much more important for thermal siphons than for conventional heat pipes. At the same time, thermal siphons are simpler and less expensive to manufacture or purchase.
A fast evolving application of thermal siphons is in solar water heaters, where the solar energy is harnessed and this heat is transported to water in a tank by usage of a particular type of thermosiphon arrangement in vacuum tubes. This is seen to reduce electricity consumption and the demand for solar water heaters is increasing every day. In developed countries, the usage of solar water heaters is very high due to the power savings, and some countries have also gone so far as to make this mandatory for every household.