May 26: Statkraft builds osmostic test facility
Technicians in Tofte, Norway, build the world's first test plant to produce electricity from the osmotic difference between salt water from the fjord and sweet water from a river. The Statkraft test plant is expected to be operational by september.
Originally the plant was to produce its first power in May. But some delay has occurred, as Statkrafts spokesman Stefan Okstveit explains. “This is a new technique and that occasionally brings with it some surprises”, he says. The technicians at the site are reluctant to communicate, not sure of how much they can reveal about the technology. Taking the safest option, they prefer to remain silent.
Okstveit takes us inside the test plant in Tofte – a privilige that has thusfar been denied to all interested parties.

An howling engine-driven generator compensates for the tempory loss of power. Inside, a glass wall provides a view at the Oslo fjord just below the rocks. All over the place, a dozen of technicians work on the installation, consisting of an intricate network of pipes and valves and a rack with several shelfs containing rows of white cilinders. Behind it, there are four large generators mounted on vertical pipes – probably the generators to provide the osmotic power shortly once the plant is up and running.
Head of Osmotic Power at Statkraft, Stein Erik Skillhagen says the delay is the result of doing something completely new. “There are no managers, no consultants, no industrial services with any experience in this field. We really had to start from scratch, so that took more time than expected.”
The crucial part of the system is the membrane, a thin semi permeable sheet that lets water pass but no electrolytes (ions). As sweet water is passed at one side of the membrane and salt water at the other side, the salt concentration difference between the two draws water across the membrane to the salty side, thus increasing the pressure there.
The resulting osmotic pressure is considerable and expected to be equivalent to 120 meters of water column, equalling about 10 bar. This pressure, comparable to a medium sized waterfall, is high enough to drive a turbine, thus generating electricity.
At the test plant in Tofte, 2,000 square meters of membrane are rolled up in dozens of white cilinders. Through it flow about 10 liters of sweet water per second, Skillhagen says. He hopes the generated power will reach 2 to 4 kilowatts.

If the test plant performs well, and continues to do so over the next three years, a 25 megawatt demoplant should be the next step, working with five square kilometres of membrane as a promotion video says.
This would imply a considerable improvement in membrane performance from 1-2 watt per square meter to 5 watt/m2.
Eventually, the video says, osmotic power could provide up to 10 percent of Norwegian power demand.
There has been a long-standing interest in the technology, says Skillhagen. Already since 1997 there have been attempts to reverse the desalination process developed by Sindey Loup in the early 1970's known as 'reverse osmosis'. Loup demonstrated that fresh water could be squeezed out of salt water through a thin membrane if only the pressure was high enough. Reversal of this process would imply a build-up of high pressure over a membrane separating sweet and salt water. “It was called reverse reverse osmosis at first”, says Skillhagen. “Now we just call it osmosis.”
“What has been missing so far, despite all the interest, are the membranes”, Skillhagen concluded when he started the project. He contracted membrane researchers for the Statkraft project and started up collaborations with research institutes asGKSS in Germany and the European Membrane Institute from Twente University in the Netherlands and numerous other universities. “It's hard to keep the website updated on this”, he says. Also membrane producer Microdyne-Nadir from Germany is involved.
Skillhagen estimates that the 25 megawatt demo plant from the video will not be operational before 2015. Once the demo stage has been reached, the main purpose will be lowering the costs of power production to 50 – 100 euro per megawatthour to make osmose power a commercially viable sustainable power source.
In the Netherlands Wetsus works on an osmotic plant as well, be it based on a different principle. They use membranes that selectively permit ions to pass, thus generating a current that is closed by an external circuit acting as a power source. They have set up a pilot plant at the Frisia salt factory at Harlingen, making use of the extreme salty brine (30 times salter than sea water) to make the principle work.
Skillhagen, who actively focuses on the Netherlands as a target market, doesn't see Wetsus as a competitor. “We can't compete at this level of technology”, he says. Both technologies aim to gain energy from osmotic differences between salt and sweet water be it via different ways. “There might be room for both technologies”, says Skillhagen, “to exploit this great source of renewable energy.”
Statkraft's information page
Animation Osmotic Power by Statkraft: