Performance Evaluation of a Solar-powered Membrane Capacitive Deionization System Considering Energy Recovery

Abstract

Membrane Capacitive Deionization (MCDI) is characterized by low energy consumption, making this technology suitable to be powered by renewable energies. This study experimentally evaluated performance of a photovoltaic-powered MCDI system (PV-MCDI) in three main steps. In the first step, the effect of the charging current of the MCDI module and the feedwater flow rate on the water production rate was investigated. The results showed that the desalination rate increased by 49% when the supplied current increased from 2 A to 8 A. Similarly, the desalination rate improved by 25% when the flow rate decreased from 1.4 l/min to 0.2 l/min. The results also showed that the water recovery (WR) index does not depend on the feedwater salinity, amount of the supplied charging current, or feedwater flow rate rather than the module's operation, ranging from 47% to 71%. The second step studied the amount of energy available for recovery. The amount of supplied charging current, the discharging method, and the remaining charge in the MCDI module were evaluated. The results of the experiments indicated that the maximum energy consumed for the deionization of 1 liter of saline feedwater is 0.962 kJ (at a charging current of 10 A and a feedwater salinity of 1300 ppm). Moreover, under the specified operating conditions of a 4-A supplied charging current and a feedwater salinity of 1300 ppm, the maximum recovered energy from the MCDI module was obtained at 17.55%. The MCDI module was integrated with a small off-grid PV system in the third phase. The results of the economic analysis indicated that the financial payback decreases to less than one year when the treated water price is above 0.034 USD/l. Therefore, it was found that the PV-MCDI system, especially when combined with energy recovery, can compete in energy consumption with conventional electric desalination technologies such as electrodialysis (ED) and reverse osmosis (RO) when brackish water is desalinated.