Blue energy, osmotic power plant 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 [1] (RED), or Pressure Retarded Osmosis [2] (PRO) Both processes rely on osmosis with ion specific membranes. The key waste product in these processes is brackish water. This byproduct is the result of natural forces that are being harnessed: the flow of precipitated water (rivers) into seas that are made up of salt water. The technology of both have been confirmed in laboratory conditions. Both are being developed in the Netherlands and Norway into commercial use. The cost of the membrane was an obstacle. A new, cheap membrane, based on an electrically modified polyethylene plastic, made it fit for potential commercial use . In PRO, the water potential between fresh water and sea water corresponds to a pressure of 26 bars. This pressure is equivalent to a column of water 270 metres high. [1] However, the optimal working pressure is only half of this, 11 to 15 bar. [2] In the Netherlands, for example, more than 3,300 m³ fresh water runs into the sea per second on average. The membran halves the pressure differences which results in water colomn of approx. 135 meters. The energy potential is therefore e=mgΔh=3.3*10^6 kg/s*10 m/s^2*135 meters ca.= 4.5*10^9 Joule, Power=4.5*10^9 watt. So the power 3,300 MW seems reasonable, based on an output of 1 MW/m³ fresh water.
In RED. as in a fuel cell, the cells are stacked. A module with a capacity of 250 kW has the size of a shipping container.
Testing
With RED, in 2005 a 50 kW plant was located at a coastal test site in Harlingen, the Netherlands, the focus was on prevention of biofouling on the anode, cathode and membranes and increasing the membrane performance . In 2007 the Directorate for Public Works and Water Management, Redstack and ENECO signed a declaration of intent for development of a clean plant in Afsluitdijk dam in Netherlands [3]. To start with, the conditions for the construction of an installation in or on the Afsluitdijk dam will be investigated. This study should be concluded by the middle of 2008. Subsequently a small 10-50 kiloWatt installation will be built to test the RED technology under actual working conditions. These tests will run from mid 2008 until 2010. After that, the capacity will be expanded to 1000 kW and the system will be optimized. It is to be expected that after this phase the installation will be further expanded to a final capacity of 200 MW. In the case of PRO, Statkraft in Norway has announced that it will build an osmotic power plant prototype in Hurum in Buskerud. The prototype is planned to produce 2-4 kW at the start in 2008.Statkraft to build world's first osmotic power plant
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Cath T. Y., Childress A. E., Elimelech M. (2006). "Forward osmosis: Principles, applications, and recent developments (Review)". Journal of Membrane Science281: 70-87.
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