Optimizing the energetic performance of capacitive deionization devices with unipolar and bipolar connections under constant current charging

Abstract

Abstract In this study, the desalination performance and energy efficiency of a scaled-up capacitive deionization (CDI) device, in which a one-unit cell was composed of a pair of 20 cm × 20 cm activated-carbon electrodes, was evaluated at constant voltage and constant current charging in a single-pass mode. Moreover, a three-cell CDI device was operated with unipolar and bipolar connections under constant current charging. The CDI performance was evaluated in terms of the desalination capacity (DC), mean deionization rate (MDR), charge efficiency, specific energy input (Esalt), specific energy efficiency (SEE), and optimization product. A comparison of the optimal operation points for the desalination of 0.01M NaCl revealed that the bipolar CDI had an MDR of 0.085 mg/g/min, which was 37% higher than that with the unipolar connection, and the SEE for a bipolar connection was 4.92 mg/g/kJ, which was 10% larger than that for a unipolar connection. These differences are due to the non-uniform voltage distribution among three-pair electrodes connected in series. Consequently, a bipolar CDI stack has a higher salt-removal rate and energy efficiency for desalination. These findings constitute significant contributions to the promotion of a more relevant CDI configuration for real water-desalination operating systems.