Science - March 26, 2009


In 2030 the afsluitdijk – the long dyke that separates the IJsselmeer from the sea – will supply ‘blue energy’ to almost a million households. At least, that is if it’s up to PhD researcher Jan Post, who is researching the production of energy by mixing salt water with fresh water. This is more efficient than solar and wind energy and it generates no waste products apart from brackish water. The perfect sustainable energy then? ‘As long as you take care where you put the installations, yes.’

Design for a blue energy installation.
River water that enters the sea and gets mixed with salt seawater is a huge potential source of sustainable energy. This is not news in itself, since it was reported by scientists as far back as the nineteen fifties. But how do you extract the energy? With this question in mind, Jan Post embarked on his research project with the Environmental technology section of the Wetsus research institute. The publication of his first results in Environmental Science and Technology mid-2008 won him the prize for the best paper in 2008.

Bert Hamelers, one of the initiators of the project, says, ‘Seawater contains a lot of dissolved salts which float around as charged particles – ions – in the water.’ The difference in ion concentrations between sea and river water is the basis for energy production. In order to convert this difference to energy, Post, Hamelers and their colleagues designed a sort of electrochemical cell made up of a series of compartments separated by wafer-thin membranes. These are filled alternately with sea and river water and through controlled mixing, the differences in ion concentrations between the compartments can be converted into electric energy.

And how! Hamers claims that the efficiency of this system is about eighty percent, incredibly high compared to other sources of energy. A coal-fired power station achieves a maximum of about fifty percent, and wind and solar energy score even lower with twenty to thirty percent. ‘Our research has now shown for the first time that you really can exploit the huge potential energy source of rivers flowing into the sea’, says Hamelers. And because all that is left over is mixed, brackish water, there is no waste problem.

Kitty Nijmeijer of the University of Twente is closely involved in developing the membranes for the electrochemical cell. She is just as enthusiastic about the new technique. 'It is really fantastic, we must absolutely go for it!' is her response.
‘We started four years ago and so much has been achieved already, even though we still need to perfect the system further.’
At present, Post is working on optimizing the system. One problem that remains to be solved is what is called biofouling - the growth of organisms, such as the formation of biofilms on the membranes. ‘So you first have to purify the water in some way’, explains Post. ‘With new purification techniques we can already run the system four times longer than we can without purification.’ A new research project will focus on a whole new design in which the vulnerable membranes are no longer necessary and fouling is no longer an issue.

The researchers think there is a big chance that this new technology will provide a substantial number of Dutch households with ‘blue energy’ in the future. They are focusing on the IJsselmeer, where large quantities of fresh water are drained into the sea. ‘The afsluitdijk is due for renovation and this is the ideal moment to build our technology into the dyke’, Hamelers explains. He reckons that this could supply almost one million houses with energy. Big energy companies such as Eneco and Nuon are involved in the Wageningen/ Friesland research and are in discussions with Rijkswaterstaat (the Directorate-General for Public Works and Water Management) about the feasibility of building the new technology into the dyke. And the afsluitdijk project is just the start. Hamelers says that the Rhine, for example, would be able to supply ten times as much energy.

The Dutch discovery seems almost too good to be true and one cannot help being reminded of the ‘white energy’ generated by hydroelectric dams. Entire valleys were flooded for the sake of this ‘sustainable’ electricity, nature gave way to dams and fish species such as salmon and trout practically disappeared from the dammed rivers. If we are going to exploit blue energy on a large scale it will be very tempting to locate the energy installations in ecologically rich but fragile deltas and estuaries. Precisely because the technology is not suitable for undeveloped areas, there is a danger that people will construct new dykes in order to be able to install the technology.

Hamelers doesn't think it will go that far. ‘Building a dyke just for this purpose would be much too expensive’, he says. ‘Our technology will mainly be used in places where there are dykes already.’

Nijmeijer agrees that there are some potential disadvantages to blue energy. ‘There could indeed be some problems for shipping and for the environment’, she says. ‘You can’t very well just dam an entire river.’ But she sees technical solutions to these problems, for example by placing the electrochemical cell in an underground container. 'We are far from finished with developing this', she concludes.

So is blue energy the ultimate sustainable energy? For Post it certainly comes pretty close. ‘I am tremendously keen on blue energy’, he responds. ‘I have few reservations, as long as you take good care where you locate the installations.’

And sometimes nature and energy production can even be combined. One of the plans is to build a circular dyke in the IJsselmeer next to the afsluitdijk, to catch the salt water that washes over during a storm. This will create a unique saltwater lake within the IJsselmeer. ‘This would be the perfect solution for us’, says Hamelers. ‘We could get rid of our ‘waste’ – brackish water – in the lake, and we would be developing a nature feature at the same time.'