Salinity gradient technologies generate electricity from the chemical pressure differential created by differences in ionic concentration between freshwater and seawater. Seawater has a higher osmotic pressure than freshwater due to its high concentration of salt. Two main technology types, Reverse Electrodialysis (RED) and Pressure-Retarded Osmosis (PRO), make use of semi-permeable membranes which generate an osmotic potential that can be used to generate electricity using turbines in deltas or fjords.
- Pressure retarded osmosis (PRO): Converts the osmotic pressure of saline solutions to hydraulic pressure, which is then used to drive a turbine and generate electricity. Similar to reverse electrodialysis technologies, PRO technologies generate energy from the difference in salt concentration between saltwater and freshwater.
- Reverse electrodialysis (RED): Generates electricity from the controlled mixing of two water bodies with different salinities. RED technology typically consists of several cation and anion exchange membranes assembled to form high and low salinity compartments. When saltwater and freshwater are fed through these membranes, the opposing transport of positively and negatively charged ions creates charged poles similar to a battery.
The primary environmental concerns associated with salinity gradient technologies typically encompass changes to water quality and impacts on the physical environment. The natural process of mixing freshwater and seawater flushes nutrient poor water and brings in nutrient and oxygen rich water, creating a unique brackish water habitat that leads to some of the most productive ecosystems. These areas are used by many organisms and are both biologically and physically diverse. Potential impacts could arise from speeding up the mixing process, altering the balance of freshwater and saltwater, or risks to organisms at intake or release points. These impacts could be reduced by releasing the resulting brackish water into the middle of the water column and using screens to cover the intake tubes. The main socio-economic concern with salinity gradient technology is diverting fresh water resources for power generation, which can be negated by avoiding water stressed or scare regions. Due to limited deployments and information on these technologies, there is much uncertainty about environmental impacts and more research is needed to fully understand potential impacts.