Estuaries are the locations where river water mixes with seawater. They are found along coastlines throughout the world, but what most people don’t realize is the significant potential to produce clean energy from these mixing water streams. The salinity gradient between the two streams of water contains a large amount of osmotic power, which can be thought of as the available energy (or chemical potential) from the differences in salt concentration between the fresh water and seawater. The enormous amounts of energy released where freshwater and seawater meet “can be utilized for the generation of power through osmosis”, which is “defined as the transport of water through a semi-permeable membrane.” 
The idea of obtaining energy from osmosis, or salinity gradient power, has been studied for decades, but in early 2009 two teams were racing to be the first to build a working prototype power plant making salinity gradient power a feasible method for renewable energy generation. Both teams have been working on the development and implementation of a membrane based osmotic process, but their approaches for generating the electricity are very different . Westus (The Centre for Sustainable Water Technology), located in the Netherlands, is focusing on the Reverse Electrodialysis (RED) method to produce electricity. They claim that they will utilize fresh water from the Rhine river and saltwater from the North Sea to construct a type of battery by employing two membranes permeable to ions, but not to water. Utilizing the saltwater, one membrane will allow the passage of positively charged sodium ions into a stream of fresh water and the other membrane will allow the passage of negatively charged chloride ions into another channel of freshwater. The separated charged particles with electrodes placed in both streams makes up the chemical battery, which directly produces electricity.  Statkraft, a leading renewable energy group located in Norway, is focusing on Pressure Retarded Osmosis (PRO) as their method to extract electricity from salinity gradients. This method utilizes a membrane, permeable to water, to draw fresh water into the concentrated salt water, thus increasing the pressure in the salt water chamber. The resulting pressure can then be used to drive a turbine to produce electricity. In November of 2009, Statkraft opened the world’s first prototype osmotic power plant in Tofte Norway.  This facility had a limited production capacity (of around 4 kW) and is mainly used for testing and validation of data, hopefully leading to the construction of a large commercial power plant by 2015.
While the idea of harvesting free energy from estuaries and salinity gradients may seem like a flawless idea, it comes with both pros and cons. On one side, the technology is considered “as green as it gets”, with the only waste product being brackish water, which flows into the sea mixing with the sea water . It is a constantly flowing source of renewable energy, unlike the intermittent energy provided by sources such as solar or wind power. It can also easily be combined with existing power plants and industries and can be built underground, thus reducing costs and visual pollution . On the other side, the membrane technology still has a long way to go. The membranes are prone to bio-fouling from algae and silt, which reduce the membrane’s lifetime and efficiency. Salinity gradient power is mainly suitable only for places where there is an abundant supply of freshwater meeting saltwater, which clearly favors countries with a large coastline . Also, the environmental impact and environmental policy should be considered for future plants of this type. First off, there are many species of aquatic life that are adapted to survive in waters with a specific range of salinity concentrations, and these power plants could affect the salinity of an area of water. It has been found that large salinity changes in aquatic environments can result in low densities of plants and animals . Then one must consider the environmental policy and impact of structures that intake such large volumes of river water and sea water. These power plants must conform to strict construction permits and environmental regulations.
While many issues dealing with the viability of salinity gradient power as a renewable energy source are still being addressed, advancements in the technology are constantly being made. In June of 2012, Statkraft’s head of osmotic power said “We see that the development of technology is accelerating and that an industry is emerging. The membranes we are testing at Tofte this summer are ten times more efficient than the ones we installed during the opening of the prototype in the autumn of 2009” . Earlier this year, researchers discovered a new more efficient way to harness osmotic power utilizing Boron nitride nano tubes . They claim that a 1 meter-squared membrane using this technology could have the same 4 kW capacity as the whole Statkraft prototype power plant. This experimental device, which is three orders of magnitude more efficient than the current system, could significantly enhance the commercial viability of salinity gradient power as a realistic source of energy . While the ultimate future of salinity gradient power is unknown, it has the potential to be one of the prominent renewable energy sources on the planet.
 Montague, C., Ley, J. A Possible Effect of Salinity Fluctuation on Abundance of Benthic Vegetation and Associated Fauna in Northeastern Florida Bay. Estuaries and Coasts. 1993. Springer New York. Vol. 15 No. 4. Pg. 703-717