Harvesting Natural Salinity Gradient Energy for Hydrogen Production Through Reverse Electrodialysis (RED) Power Generation

There is an enormous potential for energy generation from the mixing of sea and river water at global estuaries. If technologies are developed which are capable of converting this energy into a usable form (electricity or fuels), salinity gradient energy may be able to dramatically increase the worlds supply of renewable energy. Here we present a novel approach to convert this source of energy directly into hydrogen and electricity using Reverse Electrodialysis (RED). RED relies on converting ionic current to electric current using multiple membranes and redox based electrodes. A thermodynamic model for RED is created to evaluate the electricity and hydrogen which can be extracted from natural mixing processes. With equal volumes of HC and LC solutions (0.001m³), the maximum energy extracted is found to occur with 5 number of membrane pairs. At this operating point, 0.4 kWh/m³ can be extracted as electrical energy and 0.95 kWh/m³ of energy is extracted as hydrogen energy. The electrical energy conversion efficiency approaches 15%, whereas the hydrogen energy efficiency is 35%. Overall, the maximum system conversion of Gibbs free energy to electrical and hydrogen energy approaches 50%. The results show that as the number of membrane pairs increases from 5 to 20, the hydrogen power density decreases from 13.2 W/m² to 3.7 W/m². Likewise, the power density from electrical energy decreases from 1 W/m² to 0.3 W/m². This is because of increase in the total membrane area as increasing the number of membrane pairs. The stack voltage increased from 1.5V to 6V as the number of membrane pairs is increased from 5 to 20. This corresponds to an increase in internal resistance from 600 Ω.cm² to 2400 Ω.cm². Long term trade-off between improving the system voltage, while decreasing the system resistance will be crucial for improved long term RED performance. Furthermore, optimum operation of RED, depends on proper selection of external resistance. A small external resistance will increase hydrogen energy and decrease electrical energy, particularly using a small number of membrane pairs. With the fixed small external resistance, as increasing the number of membrane pairs, the difference between internal and external resistance increases. Therefore, the load potential and current density do not increase considerably. For the cases analyzed with 8.29 Ω.cm² external resistance, the maximum current density increases from 11.1 mA/cm² to 12.4 mA/cm² as the number of membrane pairs increases from 5 to 20. Likewise, the load potential rises from 92 mV to 102 mV.