The minimum work requirements for atmospheric water harvesting

Idealized cycles for the three most common methods of atmospheric water harvesting, membrane, desiccant, and condenser, are analyzed. It is found that they all have substantially the same efficiency as a function of water removal fraction. In addition, for small removal fractions they all approach the minimum thermodynamic work requirement. This minimum is shown to come from the entropy of mixing at the water-atmosphere boundary. For larger removal fractions, additional work is required, which is shown to come from mixing the drier output air with ambient air.