Energy Out of the Blue: Generating Electric Power from the Clash of River and Sea Water

Two pilot projects are testing the potential of "salt power," a renewable energy that relies on the differing salinities at river mouths to make watts

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   Freshwater, Saltwater, Salt Power
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   In the hunt for alternatives to polluting and climate-warming fossil fuels, attention has turned to where rivers meet the sea. Here, freshwater and saltwater naturally settle their salinity difference, a phenomenon that two pioneering projects in Europe will try to harness to generate clean energy.
   This concept of "salt power"—also known as osmotic, or salinity-gradient, power—has been kicked around for decades, and now, proponents hope, technology has advanced enough to make it economically competitive.
   On November 24, the world's first large-scale prototype facility for developing a form of salt power called pressure-retarded osmosis is expected to begin fully operating in Norway. "The big reason to build this thing is to answer important questions [about osmotic power], and while we've done a lot of theoretical studies, we need live experience," says Stein Erik Skilhagen, vice president of osmotic power at Statkraft, Norway's state-owned power utility that built the plant. The prototype will have no customers, although the very small amount of electricity it generates will technically be directed into the power grid.
   Statkraft's approximately $5-million prototype plant is a converted paper mill in the seaside village of Tofte, about 60 kilometers south of Oslo. The plant's pressure-retarded osmosis setup will place freshwater and brine on either side of a semipermeable membrane that prevents the passage of salt particles but allows water through. Water from the fresh side naturally flows into the salty side, generating pressure equivalent to a column of water 120 meters high. This pressurized water can be used to turn a turbine to make electricity. Statkraft's goal is to yield five watts per square meter of membrane, although current capacity is about three watts. If successful, the utility hopes to build a commercial salt power plant for paying customers around 2015 with a targeted cost ranging from seven to 14 cents per kilowatt-hour (pdf) (at current euro–dollar conversion rates), which at the low end would be competitive with coal and natural gas prices.
   To the south in the Netherlands, a Dutch research firm called Wetsus has fired up its own salt power experiment to evaluate what is essentially a saltwater–freshwater battery.
   Wetsus, with the collaboration of a spin-off company called Redstack, is pursuing a version of salt power dubbed "blue energy". A pilot-scale installation that is about two times the size of a big American refrigerator is up and running in Harlingen, by the Wadden Sea, says Gert Jan Euverink, Wetsus's deputy scientific director. The technology relies on reverse electrodialysis, wherein a series of fresh and saltwater streams are diverted via underground pipes to opposite sides of two kinds of membranes. These let sodium or chlorine ions—the constituent elements of salt—dissolved in the water to pass into separated freshwater streams. This builds an electrical potential across the membranes, like a battery, and this charge reacts with iron to form an electric current. Joost Veerman, a researcher at Wetsus, says the company aims to get five watts per square meter of membrane, the same result as Statkraft's process.
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