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A supercritical steam generator is a type of boiler that operates at supercritical pressure and temperature, frequently used in the production of electric power.
In contrast to a subcritical boiler in which steam bubbles form, a supercritical steam generator operates above the critical pressure – 22 megapascals (3,200 psi) and temperature 374 °C (705 °F). Under these conditions, the liquid water density decreases smoothly with no phase change, becoming indistinguishable from steam. The water temperature drops below the critical point as it does work in a high pressure turbine and enters the generator's condenser, resulting in slightly less fuel use. The efficiency of power plants with supercritical steam generators is higher than with subcritical steam because thermodynamic efficiency is directly related to the magnitude of their temperature drop. At supercritical pressure the higher temperature steam is converted more efficiently to mechanical energy in the turbine (as given by Carnot's theorem).
Technically, the term "boiler" should not be used for a supercritical pressure steam generator as boiling does not occur.