Blue energy
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Blue energy, 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 [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]
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[edit] Availabilty and status
It is estimated that 1600 TWh could be generated world wide, and 12 TWh in Norway, sufficient to meet 10% of Norway's total demand for electricity. A prototype 2-4 kW plant is being constructed in Buskerud, Norway in 2008.[4]
[edit] 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. [5] However, the optimal working pressure is only half of this, 11 to 15 bar. [6]
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.
[edit] 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[7]
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.
[edit] 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 [8]. 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. [9] [10]
[edit] References
- ^ Power generation by reverse electrodialysis[1]
- ^ How does PRO work?[2]
- ^ History of osomtic power[3]
- ^ Statkraft to build world's first osmotic power plant
- ^ How does it work? - Statkraft
- ^ Osmoosivoimalan toiminta - Tekniikka & Talous (Finnish)
- ^ See note 2
- ^ Friese wereldprimeur met stroomproductie uit zoet-zout water
- ^ Statkraft to build world's first osmotic power plant
- ^ Norway gets the world's first salt power plant
- Wetsus
- KEMA/VolkerWessels/Velsen Flexoplast
- KEMA
- Dutch Research Database
- Osmotic Energy (1995)
- Salinity Power UN Report
[edit] Further reading
- Loeb S., Norman R. S. (1975). "Osmotic Power Plants". Science 189: 654–655. doi:.
- Loeb S. (1998). "Energy Production at the Dead Sea by Pressure-Retarded Osmosis: Challenge or Chimera?". Desalination 120: 247–262. doi:.
- Norman R. S. (1974). "Water Salination: A Source of Energy". Science 186. doi:.
- Cath T. Y., Childress A. E., Elimelech M. (2006). "Forward osmosis: Principles, applications, and recent developments (Review)". Journal of Membrane Science 281: 70–87. doi:.
- Loeb S. (1988). "Comments on the suitability of reverse osmosis membranes for energy recover by submarine osmotic power plants Desalination (Review)". Journal of Membrane Science 68: 75–76. doi:.
- Loeb S. (2002). "Large-scale power production by pressure-retarded osmosis, using river water and sea water passing through spiral modules desalination (Review)". Journal of Membrane Science 143: 115–122. doi:.
[edit] See also
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