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Salinity gradient power or osmotic 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 ¹ (RED), and Pressure retarded osmosis ² (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 ³

Basics of Salinity Gradient Power

In this day and age in which global warming and carbon dioxide emissions are at an all time high, alternatives must be thoroughly researched. Salinity Gradient Power is a specific alternative that creates renewable and sustainable power by using naturally occurring processes. This practice does not contaminate or release CO2 emissions. 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 of resources of “osmotic pressure difference between fresh water and sea water.” ⁴ 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.” ⁵ 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 then 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 Brauns 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” ⁶ The pressure drives the turbines and power the generator that produces the electrical energy.

Another technique as described in Jones and Finley is Vapor Compression. The practice uses the difference in vapor pressure between fresh and salt water. Freshwater becomes evaporated an then condensed in sea water. The vapor that is created through this pairing then drives a turbine.

Within "Integrated power, water and salt generation: a discussion paper" Ahmed et al. researcher speak of the developing potential of salinity gradient power from water dispersed in the desalination process. It describes the greater treatment and usage of brine. Solar Ponds used within the desalination process can be a new alternative resource for energy. As stated within this paper. "Solar ponds become attractive as a source of renewable energy where water, salt, solar radiation and flat land are readily available and there is a valuable use for thermal energy"⁷. A salt-gradient non-convective solar pond has three layers. The heat produced is the convection for the energy. The top is the Upper Convective Zone ⁸. Here is fresh water 30 cm thick. Next is the Non-Convective Zone which is an insulator and can be .5 to 1.5m thick. The heat store or lower convective zone is a saturated saline solution⁹. It has been stated that in Victoria Australia a 10,000 meters squared solar pond could potentially produce the equivalent of more then $130,000 per year of low-grade heat or also generate 2000,000kWh of electricity per year ¹⁰.

Salinity Gradient Energy must be further studied and in fact implemented to begin the process of use of renewable, non polluting and sustainable energy that will allow us to combat global warming.

Methods

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. ¹¹

Another method to utilize salinity gradient is called pressure-retarded osmosis. ¹² 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 a company in Norway called Statkraft, which has calculated that up to 25TWh/yr would be available from this process in Norway. ¹³

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.” A company utilizing this technology in Vladivostok, Russia produces 4V of energy per year or 0.15 kWh/m3, though the system can theoretically produce close to 0.7 kWh/m3.

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.

References

See also

Energy portal

Sustainable development portal

• Electrodialysis reversal (EDR)
• Reverse osmosis
• Semipermeable membrane
• Green energy
• Renewable energy

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