The heat required to freeze water is about 80 cal/g, whereas the heat required to vaporize is above 560 cal/g. However, freezing processes do not require less energy than evaporative processes. All desalination processes are essentially heat-pumping processes, and the work requirement depends on the quantity of heat pumped and the temperature difference over which the heat is pumped. The work requirement for sea water desalination is about the same regardless of whether freezing or evaporation methods are used.
The following equation describes a reversible heat pump:
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