Thermal versus membrane desalination

Two ways to separate salt from water: boil the water off, or push it through a membrane. Membranes won on energy, which is why almost all new capacity is reverse osmosis.

Why thermal persists: Thermal plants tolerate poor feedwater that would foul membranes, and make sense where waste heat from power generation is free.
The theoretical floor: Separating salt from seawater cannot take less than about 1 kWh/m³ thermodynamically. Modern RO at 3 kWh/m³ is already within a factor of three of that limit.
Brine: Roughly twice the salinity of the feed, and often warmer. Discharge design determines whether it disperses or forms a dense layer on the seabed.

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Two parallel routes from seawater to fresh water. The thermal route heats seawater so it flashes to vapour in a series of chambers at falling pressure, condensing to distillate, and uses roughly 10 to 25 kilowatt-hours per cubic metre equivalent. The membrane route pressurises seawater through reverse osmosis membranes with energy recovery, using roughly 3 to 4 kilowatt-hours per cubic metre. Both produce a concentrated brine that must be disposed of.