Today’s Solutions: October 28, 2021

What do you get when saltwater and fresh water meet? A clean, renewable source of power called blue energy.

Diane Daniel | March 2009 issue

The blades stutter a bit at first, but after a couple of halting starts, the tiny propeller on the miniature windmill is soon turning at top speed. The fuel: a tank containing saltwater and fresh water. “Here’s your proof,” says Jan Post, a Ph.D. student at the Dutch water technology research institute Wetsus. “Blue energy works.”
This little windmill is just a laboratory demonstration model, but Post and his colleagues have big ambitions for blue energy, a process that derives clean power from the mixing of saltwater and fresh water. Within a decade or so, they hope blue energy will produce a significant share of the Netherlands’ electricity. But if the technology that powers the windmill can be scaled up, blue energy could do a lot more. Along with other renewable sources of power like solar and wind, it could help make sustainable energy a reality.
Blue energy—or reverse electrodialysis (RED), its technical name—derives power from an unlikely source: energy released from the process of osmosis. Whenever two solutions of differing concentrations meet—tea and coffee, for example, or saltwater and fresh water—they blend so the concentration becomes equally distributed throughout. Pour tea and coffee into a single cup and you get, well, “toffee”; do the same with saltwater and fresh water and you get brackish water, in which salt is spread evenly within the solution. This process releases energy.
In a RED system, saltwater and fresh water are brought together through an alternating series of ion-exchange membranes, which harvest the energy released as the fresh water is drawn towards the saltwater. And Post’s miniature windmill isn’t the only device running on blue energy. Last June, Redstack, a company affiliated with Wetsus, began a trial at a salt factory in the Dutch town of Harlingen. So far, the experiment is only yielding enough power to run a vacuum cleaner, but it’s a start. “The trial has to run trouble-free for a couple of years before you can consider a real power station, but it looks good,” says Post. “Investors are ready to step in.”
One of the attractions of blue energy is that power plants can be situated wherever saltwater and fresh water meet—in other words, wherever rivers flow into the sea, from the fjords of Norway to Asia’s estuaries. The plants could even be located underground, placing minimal impact on communities and land. Westus researchers believe the 19-mile Closure Dike in the northern Netherlands, which divides the fresh water Lake IJssel from the saltwater Wadden Sea, is the perfect place for a blue energy power station.
The Norwegians, like the Dutch, are rich in regions where saltwater and fresh water meet, and they too are experimenting with blue energy. Statkraft, the electricity company owned by the Norwegian government, hopes to complete its own blue energy power plant in Hurum, about an hour south of Oslo, soon. To date, this prototype facility has a capacity of only 2 to 4 kilowatts, just enough to power a refrigerator. “Two to four kilowatts is almost nothing,” says Stein Erik Skilhagen, vice-president of the Osmotic Power project at Statkraft. “The important part is not how much electricity we generate, but that we can. We’ve been working so long that now it’s time to show that the process works.”
While the Dutch project uses RED, the Norwegian one uses an alternative approach to blue energy: pressure-retarded osmosis (PRO). The main difference is that instead of using energy released by osmosis, the PRO system uses the hydrostatic pressure created when fresh water passes through the membrane to the saltwater side. The pressure spins a turbine, which is plugged into a generator to produce electricity. The Hurum prototype is next to the sea, and the fresh water is drawn from a nearby lake.
Skilhagen sees lots of advantages to blue energy—”There’s a continuous flow of fresh water; [unlike wind farms and hydroelectric plants,] it doesn’t require a lot of area; and it’s renewable”—but concedes that the technology has to become more cost-effective. “To make this profitable, we need to reach 5 watts per square meter of membrane,” he says. They’re now at 3 watts per square meter. At full capacity, though, Statkraft predicts blue energy will be able to produce 25 terrawatt hours, which is equivalent to 20 percent of Norway’s power production.
Cost-effectiveness also worries Hans de Wit, professor of electrochemistry at the Delft University of Technology. In 2007, De Wit and other scientists wrote an energy report for the Royal Dutch Academy of Science entitled “Sustainability Lasts Longest.” The report was critical of blue energy. “An enormous effort is required to get [blue energy] to work effectively,” the authors stated. “Such a huge effort for such a small contribution simply makes no sense.”
Despite this criticism, De Wit still sees potential in blue energy. “It presents real opportunities,” he says. “My only point is, let’s not make it more than it is. I see very interesting, small-scale applications, primarily in areas where there are no power plants nearby.” On that, De Wit and Post agree. “Our motto is, ‘Use it where you can,'” Post says. “If blue energy can be used with existing infrastructure, or if the necessary infrastructure can be relatively easily built, it’s a great supplement to other sustainable energy sources.” That little windmill in the Wetsus lab might yet create quite a stir.
Arnoud Veilbrief, who only likes saltwater when it’s found in the sea, writes about economics and energy.

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