Energy-Recovery Optimization of an Experimental CDI Desalination System

Currently, most of the capacitive deionization (CDI) research is oriented to improving the electrode materials. However, if the CDI overall efficiency is to be improved, it is necessary to optimize the CDI cell geometry and the charge/discharge current used during the deionization process. In this paper, an experimental CDI module is electrically characterized, and its performance is derived by solving the differential equations that represent the behavior of a total system. The solving method, which provides a detailed description of energy losses, is applied to a set of theoretical CDI modules whose properties are related to those of actual modules. This allows the extrapolation of the performance obtained to that of other modules with different geometries. From this information, it is possible to derive the optimum geometry that gives rise to the highest efficiency in the whole system for each charging current. The system equations allow an optimal charging current to be derived and an optimal energy-recovery performance to be achieved for any CDI cell. This charging current is a function of parasitic resistances and voltages on the cells along the energy transfer. The theoretical results are compared to experimental measurements conducted on actual CDI cells; good agreement with theoretical predictions was observed.