Reversed electrodialysis (RED) is the salinity gradient energy retrieved from the difference in the salt concentration between seawater and river water.[1]
In reversed electrodialysis a salt solution and fresh water are let through 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 through difference in ion concentration instead of an electric field, which has implications for the type of membrane needed.[2]
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.
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 approximately 135 meters. The energy potential is therefore e=mgΔh=3.3*106 kg/s*10 m/s2*135 meters ca.= 4.5*109 Joule per second, Power=4.5 gigawatts.
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In 2006 a 50 kW plant is planned located at a coastal test site in Harlingen, the Netherlands,[3] the focus is on prevention of biofouling on the anode, cathode and membranes and increasing the membrane performance.[4][5] 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 [6] 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.
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