Osmotic power plans scrutinised

‘Blue energy’ from mixing salt and fresh water is said to have a great potential. A young civil engineer however has recently voiced his doubts, stating that the power cost needed will be four times higher than claimed.

The world’s first osmotic power plant in the Oslofjord in Norway. (Photo’s: Koos Termorshuizen)

Following up on the Norwegians, who in 2007 made the world’s first osmotic power plant, the Dutch are now also planning to build a pilot-scale osmotic power plant and generate electricity by mixing freshwater and seawater.

A company called Redstack has received subsidies to build a pilot plant on the Afsluitdijk, which separates the fresh water lake IJsselmeer from the salty Waddenzee.
The potential of osmotic energy – also called blue energy – could be phenomenal. The state-owned Norwegian energy company Statkraft – whose pilot plant currently produces only just enough energy for one person to do some ironing (about 2 to 4 kilowatt) - calculated that osmotic power has a global potential of 1600 to 1700 terawatt hours annually, which equates to 50 percent of current power production in the EU. With the help of special ion membranes, blue energy can basically be harvested anywhere where clean freshwater runs into the sea.

As a rule of thumb, a continuous flow of 1000 liters of freshwater per second mixed with an equal amount of seawater represents a capacity of approximately 1 megawatt. With an average flow of freshwater from the IJsselmeer to Waddenzee measuring 200,000 liters per second, an osmotic power plant on the Afsluitdijk could produce 200 megawatt.

“A 200 megawatt plant is what we’re ultimately aiming at,” says Simon Grasman, project leader of Redstack. “With that amount of energy we could provide the energy demand for all the households in the three northern provinces of the Netherlands.”
Redstack will start with a much smaller plant however, one which the company hopes, after four years of research and fine tuning, will produce 50 kilowatts.

The price of the energy, Redstack believes, will amount to as little as 8 cents per kilowatt hour only. That is cheaper than energy produced by conventional power plants that use oil, coal or gas, with this energy costing around 10 cents per kilowatt hour.
However, according to civil engineer Rick Kleiterp, the price of blue energy comes nowhere near the one calculated by Redstack. As it turns out, Kleiterp defended his Master’s thesis last month on the economic viability of osmotic power plants.
The young engineer believes Redstack’s energy will be four times more expensive. What’s more, by scaling up the plant to 200 megawatts, the price will even rise further, to more than 1 euro per kilowatt hour.

“The oil price will have to rise significantly for blue energy to become interesting,” says Wilfred Molenaar, one of Kleiterp’s supervisors, understatedly.
One of the major issues with blue energy is brackish water. The power plant’s effluent is a brackish solution that should be discharged away from the power plant in such a way that it doesn’t contaminate the supplies of fresh water and salt water, since this would decrease the salinity difference between the two types of water and thus decrease the energy output. This problem increases exponentially with the size of the power plant.

In other words, one would need a lot of infrastructure, with pipes or long groins, if one were to make a large osmotic power plant.
Grasman believes the costs have been grossly overestimated in Kleiterp’s report, yet he also admits that the brackish water can be a tough problem to deal with and that Redstack has not taken into account the costs of constructions, like large groins to separate brackish and salt water over great distances.

However, Grasman envisions other solutions. He believes tides might solve the discharge problem. “If we discharge the brackish water at the western end of the Afsluitdijk while the tide is going out, currents will take the brackish water out into the open sea. The salt water intake will then occur at the eastern end of the Afsluitdijk. And during high tide we turn this process around. I’m not sure it will work. But the principle is worth studying. At TU Delft a lot of people could help us with this, which would be nice, instead of only criticizing.”