Capacitive Mixing in Porous Electrodes for Energy Production

Energy obtained by salinity-gradients (blue energy) is a promising renewable energy source. Mixing of a high-concentration saline solution, such as seawater, with fresh water (river water) to produce a brackish solution dissipates more than 2.2 MJ of free energy per m³ of fresh water.  The charging-discharging cycle inside the pores of the solid electrodes is strongly influenced by the pore size distribution.  Macroporous materials behave like standard plate capacitors, while mesoporous materials behave like cylindrical capacitors. The goal of this project was to optimize the recovery efficiency of free energy change by using tailored-design carbon electrodes.  Morphology, pore size, and carbon electrode composition were chosen to optimize the charge-discharge steps of the reversible cycle.  The physicochemical processes inside the electrode and the corresponding effects on the reversible energy cycle were investigated.  A proposed energy cycle comprises a charging step, a discharging step and two switching steps that replace a saltwater solution inside the pores by a fresh water solution and vice versa. Equations derived using the volume averaging method have been employed to simulate the charging-discharging cycle in homogeneous porous materials of a macropore size distribution. Ionic transport by diffusion and electrical double layer expansion processes were considered to simulate the two alternating steps. Improved energy production was achieved by optimizing the duration of each step and variation of the operating variables.