Study of the Capacitive Mixing Process for Energy Production

The mixing process of fresh water and seawater releases a significant amount of energy and is potentially an enormous source of renewable energy. The so called ‘blue energy’ or salinity-gradient energy can be harvested by a device consisting of carbon electrodes immersed in an electrolyte solution, based on the principle of capacitive double layer expansion (CDLE). In this study, we have investigated the feasibility of energy production based on the CDLE principle. Experiments and computer simulations were used to study the process. Mesoporous carbon materials synthesized at the Oak Ridge National Laboratory (ORNL), were used as electrode materials in our experiments. 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. A new model that calculates concentration profiles of individual ions has been derived to model the charging/discharging steps. Ionic transport by diffusion and electrical double layer expansion processes were considered to simulate the two alternating steps. Neutron imaging of the blue energy cycle was conducted with cylindrical mesoporous carbon electrodes and 0.5 M lithium chloride as the electrolyte solution. For experiments conducted at 0.6 V and 0.9 V applied potential, a voltage increase of 0.061 V and 0.054V was observed respectively. Measured experimental data and computer simulations agree well for similar operating conditions. The model presented in this work can be used to improved energy production.