Salinity Power

It has been known for centuries that the mixing of freshwater and seawater releases energy. For example will a river flowing into the salty ocean releases large amounts of energy. The challenge is to utilise this energy, since the energy released from the occurring mixing only gives a very small increase in the local temperature of the water.

Authors: Øystein S. Skråmestø & Stein Erik Skilhagen, Statkraft AS,  Norway

 

The power of osmosis

During the last few decades at least two concepts for converting this energy into electricity instead of heat has been identified. These are Reversed Electro dialysis and Pressure Retarded Osmosis. With the use of one or both these technology one might be able to utilise the enormous potential of a new, renewable energy source. On global basis this potential represents the production of more than 1600 TWh of electricity per year.

The Reversed Electro dialysis (RED) is a concept where the difference in chemical potential between both solutions is the driving force of the process. The chemical potential difference generates a voltage that with the use of membranes for electro dialysis is converted into electrical current. This concept is under development in the Netherlands and there are preparations for the first prototype to be build.

For Pressure Retarded Osmosis (PRO), also known as osmotic power, the released chemical energy is transferred into pressure instead of heat. This was first considered by Prof Sidney Loeb in the early 70’s, when he designed the world first semi permeable membrane for use in desalination trough reverse osmosis. In osmotic power one can utilise the natural occurring osmosis, which relates to the difference in concentration of salt between two liquid, for example sea water and sweet water. Sea water and sweet water have a strong force towards mixing, and this will occur as long as the pressure difference between the liquids is less than the osmotic pressure difference. For sea water and sweet water this would be in the range of 24 to 26 bars based on the salt concentration of sea water.

In a PRO system filtered sweet water and sea water are led into the system. Before entering the membrane modules the sea water is pressurised to approximately half the osmotic pressure, approximately 12-14 bars. In the module sweet water migrates through the membrane and into pressurised seawater. This results in an excess of diluted and pressurised seawater which is then split in two streams. One third is used for power generation in a hydropower turbine, and the remaining part passes through a pressure exchanger in order to pressurise the incoming seawater. The drain from a plant will to the main extent be diluted seawater that will be led either back to the river mouth or into the sea.

An osmotic power plant will to a large degree be designed of existing “off the shelf” technology. The two unique components are the pressure exchanger and the membrane. The majority efforts in order to commercialize osmotic power are the improvement and scale up of these components.

 

Environment and market potential

Osmotic powers excellent environmental performance and CO2-free power production will most likely qualify for green certificates and other supportive policy measures for renewable energy. The estimated energy cost is comparable and competitive with the other new renewable energy sources, such as wave, tidal and offshore wind being in the range of 50-100 €/MWh. 

With a potential of more than 1600 TWh a year world wide, where 170 TWh a year is in Europe, this will likely prove to be a major contribution to the growth of renewable energy, and to represent a new attractive business potential for both the commercial power companies and technology suppliers. 

 

References

Thorsen T, Holt T. Semipermeabel membran og fremgangsmåte for tilveiebringelse av elektrisk kraft samt en anordning. Patent No 314575 B, 1 Assigned Statkraft AS.

Loeb S, Method and apparatus for generating power utilizing pressure-retarded osmosis, Patent US 4,193,267, Assigned Ben-Gurion University of the Negev, Research and Development Authority, Beersheba, Israel.

Markhus E (2006). The potential for salinity power in the World. Norconsult.