Utilizing salinity gradients where freshwater rivers meet saltwater to generate electricity.
Salinity gradient technology is also known as "blue energy" or "osmotic power". The difference in salt concentration between freshwater and seawater creates a pressure differential, most often observed when a river flows into the sea. Seawater has higher osmotic pressure because of the salt, compared to the low pressure of freshwater. Two methods have been used to convert this pressure into energy, though more are in development. Reverse electrodialysis (RED) is essentially the creation of a salt battery, where ions are transported through membranes to generate power. Pressure retarded osmosis (PRO) involves pumping freshwater through a semipermeable membrane towards a pressurized chamber full of seawater. This increases the pressure, which spins a turbine to generate power.
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 organisims and are both biologically and physically diverse. Most reports mention salinity gradient technolgies will have minimal environmental impacts. 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 salinty 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 uncertainity about environmental impacts and more research is needed to fully understand potential impacts.
Photo Credit: "Blue Energy Mechanism" by Nein Arimasen