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Osmotic power or salinity gradient power is the energy retrieved from the difference in the salt concentration between seawater and river water. Two practical methods for this are Reverse electrodialysis  (RED), and Pressure retarded osmosis  (PRO).

Both processes rely on osmosis with ion specific membranes. The key waste product is brackish water. This byproduct is the result of natural forces that are being harnessed: the flow of fresh water into seas that are made up of salt water.The technologies have been confirmed in laboratory conditions.

They are being developed into commercial use in the Netherlands (RED) and Norway (PRO). The cost of the membrane has been an obstacle. A new, cheap membrane, based on an electrically modified polyethylene plastic, made it fit for potential commercial use Other methods have been proposed and are currently under development.

Among them, a method based on electric double-layer capacitor technology  and a method based on vapor pressure difference. Salinity Gradient Power is a specific renewable energy alternative that creates renewable and sustainable power by using naturally occurring processes.

This practice does not contaminate or release CO2 emissions (vapor pressure methods will release dissolved air containing CO2 at low pressures—these non-condensable gases can be re-dissolved of course, but with an energy penalty). Also as stated by Jones and Finley within their article “Recent Development in Salinity Gradient Power”, there is basically no fuel cost.

Salinity Gradient Energy is based on using the resources of “osmotic pressure difference between fresh water and sea water.”  All energy that is proposed to use salinity gradient technology relies on the evaporation to separate water from salt.

Osmotic pressure is the "chemical potential of concentrated and dilute solutions of salt". When looking at relations between high osmotic pressure and low, solutions with higher concentrations of salt have higher pressure.Differing salinity gradient power generations exist but one of the most commonly discussed is Pressure Retarded Osmosis (PRO).

Within PRO seawater is pumped into a pressure chamber where the pressure is lower than the difference between fresh and salt water pressure. Fresh water moves in a semipermeable membrane and increases its volume in the chamber. As the pressure in the chamber is compensated a turbine spins to generate electricity. In Braun's article he states that this process is easy to understand in a more broken down manner.

Two solutions,

A being salt water and

 B being fresh water are separated by a membrane. He states "only water molecules can pass the semipermeable membrane. As a result of the osmotic pressure difference between both solutions, the water from solution B thus will diffuse through the membrane in order to dilute the solution".

The pressure drives the turbines and power the generator that produces the electrical energy.While the mechanics and concepts of salinity gradient power are still being studied, the power source has been implemented in several different locations. Most of these are experimental, but thus far they have been predominantly successful.

The various companies that have utilized this power have also done so in many different ways as there are several concepts and processes that harness the power from salinity gradient.At the Eddy Potash Mine in New Mexico, the technology of a salinity gradient solar pond (SGSP) is being utilized to provide the energy needed by the mine.

The pond collects and stores thermal energy due to density differences between the three layers that make up the pond. The upper convection zone is the uppermost zone, followed by the stable gradient zone, then the bottom thermal zone. The stable gradient zone is the most important.

Water in this layer can not rise to the higher zone because the water above has lower salinity and is therefore lighter and it can not sink to the lower level because this water is denser. This middle zone, the stable gradient zone, becomes an insulator for the bottom layer, exponentially increasing its temperature. This water from the lower layer, the storage zone, is pumped out and the heat is used to produce energy, usually by turbine.

Another method to utilize salinity gradient is called pressure-retarded osmosis. In this method, seawater is pumped into a pressure chamber that is at a pressure lower than the difference between the pressures of saline water and fresh water. Freshwater is also pumped into the pressure chamber through a membrane, which increase both the volume and pressure of the chamber.

As the pressure differences are compensated, a turbine is spun creating energy. This method is being specifically studied by the Norwegian utility Statkraft, which has calculated that up to 25TWh/yr would be available from this process in Norway. A third method being developed and studied is reversed electrodialysis or reverse dialysis, which is essentially the creation of a salt battery.

This method was described by Weinstein and Leitz as “an array of alternating anion and cation exchange membranes can be used to generate electric power from the free energy of river and sea water.”The technology related to this type of power is still very much in its infant stages, even though it was suggested over 30 years ago.

Standards and a complete understanding of all the ways salinity gradients can be utilized are important goals to strive for in order make this clean energy source more viable in the future.

 Possible Negative Environmental Impact

Although the use of salinity gradients is a considerably environmentally friendly method of obtaining electrical power, there are possible negative effects to be considered. There is at least one way in which the use of this method of power production could harm the environment; the impact of the brackish water waste on the local marine and river environment.Marine and river environments have obvious differences in water quality, namely salinity.

Each species of aquatic plant and animal is adapted to survive in either marine, brackish, or freshwater environments. There are species that can tolerate both, but these species usually thrive best in a specific water environment. The main waste product of salinity gradient technology is brackish water.

The discharge of brackish water into the surrounding waters, if done in large quantities and with any regularity, may alter the aquatic environment significantly. Fluctuations in salinity will result in changes in the community of animals and plants living in that location.

However, while some variation in salinity is usual, particularly where fresh water (rivers) empties into an ocean or sea anyway, these variations become less important for both bodies of water with the addition of brackish waste waters. Extreme salinity changes in an aquatic environment may result in findings of low densities of both animals and plants due to intolerance of sudden severe salinity drops or spikes.   

The disappearance or multiplication of one or more aquatic organisms as a result of an influx of brackish water has the potential to cause ecosystem imbalance. The possibility of these negative effects should be considered by the operators of future large blue energy establishments. In general, the estimated impact of this technology is very limited to non-existent in comparison with current power production methods.

 
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