The boundary zone between salt water and fresh water could produce enough electricity to cover ten percent of the Dutch demand. Producing this so-called ‘blue’ energy has not yet been optimized though. Shortening streams and adjusting the flow direction of the fresh water and salt water could make electrical installations more efficient. Chemist Joost Veerman will be awarded a PhD on 1 October 2010 for his research into optimizing the process of generating such sustainable energy.
A difference in salt concentration can yield energy – known as blue energy. Veerman studied how reverse electro-dialysis (RED) – a system where salt and fresh water flows between stacks of ion-exchange membranes – could be optimized. Due to the difference in salt concentration between fresh and salt water, positive sodium ions and negative chloride ions pass through the membranes towards the fresh water, creating flow. The same principle applies in electro-dialysis, a technique where salts are removed from water using electricity. In RED the opposite occurs and it’s electricity that is generated.
Using computer models of the RED system as well as demo models, Veerman investigated which adjustments lead to higher efficiency and more power. One of the ways he was able to produce a better yield was by changing the distance between the membranes. Stacking the membranes as closely as possible has the advantage that electrical resistance is kept as low as possible. However, the flow resistance of the water is then very high. Veerman developed a branched system for the water to flow through. Such a fractal system leads to both lower electrical and flow resistance, with a seventy percent higher power yield. Veerman also discovered that the water channel should be as short as possible, with a maximum of ten centimetres.
In existing installations salt and fresh water flow past each other in opposite directions. While investigating what the optimal flow rate and direction were, Veerman discovered that it’s much better to have them flowing in the same direction. Apparently, having flows in both directions results in high pressure on the membranes lying in between. When flow is in the same direction the forces working on the membranes are much lower. This in turn means that thinner membranes can be used – perhaps as thin as ten micrometres – which are much more efficient.
A ferric solution (Fe2+/Fe3+) is most suitable for the electrode system, Veerman concluded. Electrodes convert ion flow into electron flow, which make them important components of the RED system. Veerman investigated various solutions, including ferric solutions, hexacyanoferrate, sodium chloride and sodium sulphate. Veerman compared the economic, environmental and safety aspects of these electrode systems. He considers the widely-used sodium sulphate to be unsuitable for the RED system because of the explosive hydrogen-oxygen mix that is created, which of course is a safety issue.
The installations studied by Veerman should ultimately be scaled up to the size of a sea container and have a 0.5 Megawatt capacity. The containers should be modular and as such connectable.
Next year the first experimental installation will be built on the north shore of the IJsselmeer, near the Afsluitdijk. The Netherlands has a very long sea coast and many rivers, thus a vast potential of salinity gradients. Theoretically, this could be enough to meet 50 percent of Dutch electricity needs. Veerman, however, expects that an energy yield of 1 Gigawatt is much more realistic, which would cover about 10 percent of the demand. On a world scale there is a potential 2.6 Terawatt of salinity gradient energy to be had, which is more energy than is being used worldwide at present.
Joost Veerman (Zwolle, 1945) studied physical chemistry at the University of Groningen. He conducted his research at the Wetsus Centre for Sustainable Water Technology. It was funded by NHL University of Applied Sciences in Leeuwarden, Wetsus, The Zestor promotion fund, SenterNovem and the company RED-stack. Veerman will be awarded a PhD in Mathematics and Natural Sciences and was supervised by Prof. G.J. Harmsen and Dr S.J. Metz. His thesis is entitled ‘Reverse Electro-dialysis – Design and optimization by modeling and experimentation’.
More information: Joost Veerman, e-mail: j.veerman tech.nhl.nl
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