Earth Beat, 21 April 2009: The IJsselmeer is a feat of Dutch engineering. The largest lake in Western Europe, it was created by building the Afsluitdijk, a dyke closing off a vast expanse of water from the North Sea. It is also a huge source of untapped energy that can provide energy for thousands of homes without damaging the environment. It's called Blue Energy.

A short distance from the Afsluitdijk ('enclosing dyke') lies the Dutch city of Leeuwarden, home to the labs of Wetsus, the Centre for Excellence in Sustainable Water Management. A team from Wetsus is working to harness this "blue energy" and PhD student, Jan Post explains how:

 

"Blue Energy is just a name we gave to salinity gradient energy, and that term should explain a bit more. Basically, you can harvest energy from mixing salt and fresh water.

 

It's a bit complicated to explain but ... if you have a waterfall, for example, then everybody knows the water is falling down, and you can harvest energy from that by placing a turbine in between.

 

Well, what we are doing is placing the technology between salt and fresh water, allowing the mixing to occur but we try to get energy - electricity out of it."

 

Membrane battery
Wetsus are proposing a kind of battery, using salt and fresh water streams, and channelling them over stacks of membranes. The salt water contains positively charged sodium ions, and negatively charged chloride ions. By using two types of membranes, one permeable only to the chloride ions, the other which takes the sodium ions, Wetsus set up a potential difference between a pair of electrodes. Bingo, they have a battery and power.

It's not a new idea; salinity power came out of attempts, in the 1950s, to extract drinking water from the sea. Fifty years on and the technology is still at the start-up stage, but Jan Post has hopes for a power plant on the IJsselmeer:

"We are in the research phase but we are aiming to do this before 2020.We are aiming for a two hundred megawatt power plant and that will be enough for the households of the three northern provinces of the Netherlands. And I don't know exactly how much that is, but it's about 300,000 households."

 

Any estuary will do
The Afsluitdijk isn't the only place suitable for blue energy harvesting; any estuary where salt and fresh water mix will do. There are, theoretically, huge amounts of energy available; the Rhine has a flow of 2000m³ per second, producing five gigawatts of energy.

However, it's also a busy shipping lane, and diverting the entire river flow into a fleet of salt-water batteries would be an environmental disaster. Wetsus reckon using up to a fifth of the flow would minimise environmental impact.

Cost is another factor against blue energy. Building the salinity power plant at the Afsluitdijk dam would cost hundreds of millions of euros, and once up and running, the cost of producing a megawatt-hour of electricity would be almost double a MWh from a fossil-fuel power plant.

The membranes are also expensive to produce, but Wetsus are optimistic that this technology will, in a decade or so, provide a reliable renewable green energy that offers the world another alternative to fossil fuels. Now who was it that said, "Blue and green should never be seen?"