Elsevier

Water Research

Volume 143, 15 October 2018, Pages 367-375
Water Research

Energy consumption in capacitive deionization – Constant current versus constant voltage operation

https://doi.org/10.1016/j.watres.2018.06.034Get rights and content
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Highlights

  • Comparing energy consumption in CDI in constant current and constant voltage mode.

  • Energy consumption is calculated as function of the desalination objective.

  • The desalination objective is defined by water recovery and desalination.

  • With energy recovery during discharge, constant current shows a lower consumption.

Abstract

In the field of Capacitive Deionization (CDI), it has become a common notion that constant current (CC) operation consumes significantly less energy than constant voltage operation (CV). Arguments in support of this claim are that in CC operation the endpoint voltage is reached only at the end of the charging step, and thus the average cell voltage during charging is lower than the endpoint voltage, and that in CC operation we can recover part of the invested energy during discharge. Though these arguments are correct, in the present work based on experiments and theory, we conclude that in operation of a well-defined CDI cycle, this does not lead, for the case we analyze, to the general conclusion that CC operation is more energy efficient. Instead, we find that without energy recovery there is no difference in energy consumption between CC and CV operation. Including 50% energy recovery, we find that indeed CC is more energy efficient, but also in CV much energy can be recovered. Important in the analysis is to precisely define the desalination objective function, such as that per unit total operational time –including both the charge and discharge steps– a certain desalination quantity and water recovery must be achieved. Another point is that also in CV operation energy recovery is possible by discharge at a non-zero cell voltage. To aid the analysis we present a new method of data representation where energy consumption is plotted against desalination. In addition, we propose that one must analyze the full range of combinations of cycle times, voltages and currents, and only compare the best cycles, to be able to conclude which operational mode is optimal for a given desalination objective. We discuss three methods to make this analysis in a rigorous way, two experimental and one combining experiments and theory. We use the last method and present results of this analysis.

Keywords

Capacitive deionization
Constant current and constant voltage operation
Optimizing salt adsorption
Minimizing energy consumption

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