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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 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]

PRO (Pressure Retarded Osmosis)

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 (Hydraulic head) 270 meters high. ^[4] However, the optimal working pressure is only half of this, 11 to 15 bar. ^[5]

In the Netherlands, for example, more than 3,300 m³ fresh water runs into the sea per second on average. The membrane halves the pressure differences which results in a water column 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 MWh seems reasonable, based on an output of 1 MW/m³ fresh water.

File:Reverse electrodialysis.png

An alternating series of anode and cathode exchange membranes bring fresh water and sea water into contact

RED (Reverse ElectroDialysis)

In reverse electrodialysis (RED) a salt solution and fresh water are let though a stack of alternating cathode and anode exchange membranes. The chemical potential difference between salt and fresh water generates a voltage over each membrane and the total potential of the system is the sum of the potential differences over all membranes. It is important to remember that the process works though difference in ion concentration instead of an electric field, this has implications for the needed properties for a suitable membrane^[6]

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 2006 a 50 kW plant in planned located at a coastal test site in Harlingen, the Netherlands, the focus is on prevention of biofouling on the anode, cathode and membranes and increasing the membrane performance ^[7]. 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 [4]. 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. ^[8] ^[9]

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References

^ Power generation by reverse electrodialysis[1]
^ How does PRO work?[2]
^ History of osomtic power[3]
^ How does it work? - Statkraft
^ Osmoosivoimalan toiminta - Tekniikka & Talous Template:Fi icon
^ See note 2|http://mtg.tnw.utwente.nl/teaching/assign/blue/
^ Friese wereldprimeur met stroomproductie uit zoet-zout water|http://www.netserver2.net/waterforum/index.asp?url=/template_a1.asp&que=paginanr=3716
^ Statkraft to build world's first osmotic power plant
^ Norway gets the worlds first salt power plant

Revision as of 18:17, 30 January 2008 edit 131.111.79.111 (talk) added left/right labels to water diagram. ← Previous edit		Revision as of 18:18, 30 January 2008 edit undo 131.111.79.111 (talk) added left/right labels to water diagram and got them the right way round. Next edit →
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	[[Image:Blue energy mechanism.png\|thumb\|right\|The [[water potential]] between [[fresh water]] (~~left~~) and [[sea water]] (~~right~~) corresponds to a [[hydraulic head]] of 270 metres]]		[[Image:Blue energy mechanism.png\|thumb\|right\|The [[water potential]] between [[fresh water]] (right) and [[sea water]] (left) corresponds to a [[hydraulic head]] of 270 metres]]

	==PRO (Pressure Retarded Osmosis)==		==PRO (Pressure Retarded Osmosis)==

v t e Fuel cells
By electrolyte	Alkaline fuel cell Molten carbonate fuel cell Phosphoric acid fuel cell Proton-exchange membrane fuel cell Solid oxide fuel cell
By fuel	Direct borohydride fuel cell Direct carbon fuel cell Direct-ethanol fuel cell Direct methanol fuel cell Formic acid fuel cell Metal hydride fuel cell Reformed methanol fuel cell Zinc–air battery
Biofuel cells	Enzymatic biofuel cell Microbial fuel cell
Others	Blue energy Electro-galvanic fuel cell Flow battery Membrane electrode assembly Membraneless Fuel Cells Photoelectrochemical cell Proton-exchange membrane Protonic ceramic fuel cell Regenerative fuel cell Solid oxide electrolyzer cell Unitized regenerative fuel cell
Hydrogen	Economy Station Storage Vehicle
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