A comprehensive analysis of the minimum energy and thermodynamic efficiency of regenerating aqueous electrolyte solutions in air-conditioning systems

Abstract

The regeneration of aqueous electrolyte solutions is a critical and energy-intensive process in air-conditioning systems. In this work, the minimum energy required for regenerating aqueous electrolyte solutions was derived and analyzed from the perspective of the driving force. The thermodynamic efficiency of electrodialysis and thermal regeneration was evaluated and compared. The results show that the minimum energy consumption of the thermally-driven method involving water vapor removal, such as traditional packed-bed thermal regeneration, is lower than that of pressure- and electrically-driven methods that involve liquid water removal, such as electrodialysis and reverse osmosis. Utilizing dry air, such as indoor exhaust and return air, proves to be an effective approach to saving energy during thermal regeneration. For pressure- and electrically-driven regeneration methods, multi-stage processes are necessary to achieve feasibility at high concentrations. However, the complexity and cost of such systems would be considered unacceptable for air-conditioning applications. Instead, single-effect thermal processes can attain high thermodynamic efficiency at high concentrations. The analysis indicates that the thermodynamic efficiency of actual thermal processes studied ranges from 9.1 % to 40.1 %, which is significantly higher than the thermodynamic efficiency of actual electrodialysis processes (which is lower than 4.7 %).