Energy transfer for storage or recovery in capacitive deionization using a DC-DC converter

Energy recovery from capacitive deionization (CDI) has the potential to increase overall desalination efficiency. We here define the storage (one-way) and utilization (round-trip) efficiencies between a CDI cell and an energy storage device using a generic direct current/direct current (DC/DC) converter circuit. Presented is a closed-form analytical model for the case of a supercapacitor (SC) as the energy storage device and a buck-boost converter as the DC/DC converter. The model is benchmarked with a numerical model, showing good agreement. Also presented is a comparison among energy transfer methods wherein desalination productivity is fixed. For constant current operation, this condition requires higher currents to compensate for the inactive time introduced by the converter. For fixed productivity, the mean current of the converter circuit approaches the constant current value for higher initial voltages and lower capacitances in the supercapacitor. Finally, we show the effect of relevant parameters of the SC in the storage and utilization efficiency. The model predicts storage and utilization efficiencies of at least 90% for an initial voltage of 1 V or higher and reasonable CDI and SC parameters. Lastly, we provide engineering operational parameters to maximize the efficiency of energy transfer and guidance in the selection of electronic components.