Norway looking to osmotic power generation

are currently under way in the Netherlands, where the meeting of seawater and fresh water has been a fact of life for centuries. Energy consultancy KEMA is leading research into preventing the biofouling of membranes using a 50kW installation, with plans to scale up in subsequent projects.

PRO

PRO uses a form of osmotic generation more closely related to Nollet’s original experiments. Seawater and fresh water enter the system through separate pipes, filtered along the way to remove sediments and other particles. They are fed into opposite sides of a chamber separated by a membrane that is permeable to water but not to the dissolved salt in the seawater.

In accordance with the laws of osmosis discovered by Nollet, the system tries to equalize the concentration of salt, and the only way this can happen is by fresh water diffusing through the membrane into the salty side. As this happens, pressure builds up on the seawater side. This pressure is relieved by letting water escape, routing it through a turbine. The result is brackish water — saltier than seawater but less salty than river water — discharged back to the sea and a supply of carbon-free electricity.

Membranes

There are drawbacks. The membranes that power this process have always been far too expensive to make commercial-scale power plants a practical proposition. Torleif Holt and Thor Thorsen, working for Norwegian research organisation SINTEF, began looking into this problem during the 1980s and became involved with Statkraft in 1997, aiming to develop lower-cost membranes based on electrically modified polyethylene. Funding from the European Union opened up in 2001 and finally — with a contribution from NASA — put the technology into the realms of possibility.

The Statkraft plant uses a polyimide membrane capable of generating a watt per square meter of membrane, which requires a flow of 10 liters of water per second flowing through the membrane at a pressure of 10bar. The capacity of the plant is tiny: just 4kW. Yet, according to Statkraft, the potential for osmotic power is huge. The company claims that 1,600-1,700TWh of electricity could be generated globally each year.  “In Norway alone, we will be able to generate 12TWh per year, equivalent to 10 per cent of our total power consumption,” it said.

These figures might seem fanciful, until you consider what historically happens where big rivers meet the sea: people tend to build ports and large cities grow up around them. Estuaries and river mouths are very often the site of major industrial and urban development, creating demand for power in the very place that osmotic power could provide a supply. “A power plant the size of a football stadium could supply 15,000 households with electricity,” said Statkraft, adding that such stations could be built underneath parks or municipal buildings, reducing their visual impact.

The cost of the membranes is, although reduced, still significant, especially if the process is to be scaled up; the pilot plant has 2,000m2 of membrane and a full-scale 25MW plant would need five million square meters, according to Stein Eric Skillhagen, Statkraft’s head of osmotic power.

Statkraft is continuing to develop the technology. “Only once membranes with a particular efficiency have been developed will osmotic power become a competitive source of renewable energy,” said the company, which has the ambitious aim of developing a high-performance, low-cost membrane to enable commercial-scale osmotic power by 2015. Its performance target is 5W/m2 of membrane — five times the performance of the pilot plant.

To this end, Statkraft is investigating membranes and is also testing samples developed by other organizations, including cellulose-based materials and biomimetic, protein-based substances. The company recently went on a ’membrane-hunting expedition’ to the United States, organizing an osmotic power membrane conference in San Diego along with the American Membrane Technology Association. “We will meet players such as General Electric, Hydranautics, Dow and Toray,” said Skillhagen. “It is very important to engage in a dialogue with these players and it is also important for us to demonstrate that we are at the forefront of development of osmotic power.”

The Engineer quotes Skillhagen to say that so far, he added, Statkraft’s membranes are yet to exceed 3W/m2 in laboratory tests. The company is continuing trials, though, with membrane manufacturers looking at a potential new source of high demand for their products.