The minimum work requirements for atmospheric water harvesting

Area of Science:

• Thermodynamics
• Environmental Engineering
• Materials Science

Background:

• Atmospheric water harvesting (AWH) is crucial for water-scarce regions.
• Common AWH methods include membrane, desiccant, and condenser systems.
• Understanding the thermodynamic efficiency of these systems is vital for optimization.

Purpose of the Study:

• To analyze and compare the idealized thermodynamic cycles of three common AWH methods.
• To determine the efficiency of membrane, desiccant, and condenser systems as a function of water removal fraction.
• To identify the sources of thermodynamic work requirements in AWH processes.

Main Methods:

• Thermodynamic analysis of idealized cycles for membrane, desiccant, and condenser AWH.
• Evaluation of system efficiency based on the water removal fraction.
• Calculation of minimum thermodynamic work requirements and their origins.

Main Results:

• All three analyzed AWH methods exhibit comparable efficiencies relative to their water removal fraction.
• For small water removal fractions, all systems approach the theoretical minimum thermodynamic work.
• This minimum work is attributed to the entropy of mixing at the water-atmosphere interface.
• Higher water removal fractions necessitate additional work due to the mixing of drier output air with ambient air.

Conclusions:

• The thermodynamic performance of common AWH methods is fundamentally similar.
• The entropy of mixing is a key factor limiting efficiency, especially at low water extraction rates.
• Optimizing AWH systems requires careful consideration of mixing processes for enhanced water recovery.