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Energy from Salt Meeting Fresh Water

Page history last edited by Malcolm 11 years, 3 months ago

Salt solution: Cheap power from the river's mouth

25 February 2009 by Kate Ravilious

Magazine issue 2697. Subscribe and get 4 free issues.

 

STAND on the banks of the Rhine where it flows into the North Sea, near the port of Rotterdam in the Netherlands, and you'll witness a vast, untapped source of energy swirling in the estuary. According to Dutch engineer Joost Veerman, it's possible to tap this energy without damaging the environment or disrupting the river's busy shipping. For rather than constructing a huge barrage or dotting the river bed with turbines, Veerman and his colleagues at Wetsus, the Dutch Centre for Sustainable Water Technology in Leeuwarden, believe they can tap energy locked up in the North Sea's saltwater by channelling it, along with fresh water from the Rhine, into a novel kind of battery. With a large enough array of these batteries, he says, the estuary could easily provide over a gigawatt of electricity by a process they've called Blue Energy - enough to supply about 650,000 homes.

 

"Salinity power" exploits the chemical differences between salt and fresh water, and this project only hints at the technology's potential: from the mouth of the Ganges to the Mississippi delta, almost every large estuary could produce a constant flow of green electricity, day and night, rain or shine, without damaging sensitive ecosystems or threatening fisheries (see map). One estimate has it that salinity power could eventually become a serious power player, supplying as much as 7 per cent of today's global energy needs.

 

In an attempt to prove that this isn't just a pipe dream, Veerman's team has done lab tests on a prototype salinity power generator, and are now planning to scale it up. Yet a group of Norwegian engineers have gone one stage further, with their own twist on salinity power.

 

In the next few months, engineers at Norwegian power company Statkraft plan to throw the switch on the world's first salinity power station. Though their prototype is small, its impact could be huge. So what are these rival technologies, how do they stack up, and what are the obstacles to making electricity wherever rivers meet the sea?

 

Salinity power emerged from a rather different use for sea water. In the late 1950s, Sidney Loeb and Srinivasa Sourirajan, then working at the University of California, Los Angeles, came up with a new trick to extract drinking water from the sea. Their idea was based on osmosis, a natural process in which water passes spontaneously from a dilute to a concentrated solution through a semipermeable membrane. The pair realised that by using a synthetic membrane and high pressure pumps, they could run osmosis in reverse and literally squeeze fresh water from sea water. This approach is now used in desalination plants worldwide.

 

About 15 years later, Loeb had another brain wave. He realised that their design could be exploited to generate power. Working at Ben-Gurion University of the Negev, in Beer Sheva, Israel, he envisaged a tank with two chambers separated by a semipermeable membrane. With saltwater on one side and fresh on the other, osmosis would draw fresh water into the salty side, raising its pressure. This pressurised saltwater could then be piped through a turbine to generate electricity (see diagram). Loeb named this process pressure retarded osmosis (PRO) and patented it in 1973.

 

His plan was to harvest power where rivers meet the ocean, close to the point where fresh water meets salt. Fresh water would be piped to a generating plant from upstream and saltwater from downstream. Inside the plant, the fresh and saltwater would be channelled along either side of a membrane. Osmosis would then provide sufficient water pressure on the salty side of the membrane - up to 12 atmospheres, Loeb reckoned - to make electricity generation profitable.

 

The key lay in finding the right membrane. It would have to be permeable to water but not salt, and very thin yet extremely durable. This proved too tall an order and Loeb retired in 1986, his dream unrealised.

 

The concept was revived in 1997, when Thor Thorsen and Torleif Holt, working in Trondheim at the Norwegian research organisation SINTEF, became convinced that membrane technology was finally advanced enough to make Loeb's idea feasible. With their enthusiasm, and detailed calculations, they convinced Statkraft that salinity power could pay off in Norway. Using a design much like Loeb's original, they now believe they are close to their goal.

 

Membrane development remains the biggest headache, says Stein Erik Skilhagen, manager of the PRO project at Statkraft. Unfortunately, membranes used in desalination plants are too thick, he says, and cannot draw enough water through. So Statkraft's engineers have been working with membrane developers to improve designs. While their first membranes generated about 100 milliwatts per square metre, the latest version generates over 3 watts per square metre, close to their target of 5 watts.

 

Skilhagen reckons these membranes are now efficient enough to be worth testing beyond the lab, and in the next few months the company plans to turn on the world's first prototype PRO plant at the Södra Cell paper pulp factory in Tofte, alongside a fjord 60 kilometres from Oslo.

 

The prototype will provide crucial experience in scaling up the system. The new plant fits inside a room no bigger than a tennis court. "In the lab, our membrane had the footprint of a coffee cup. At Tofte we will be using 2000 square metres of the stuff," says Holt. A full-scale plant will need millions of square metres of membrane, so maximising the surface area for exchange is crucial, he says. The team is testing two designs. In the first, a long membrane tube is rolled up lengthways into a spiral something like a Swiss roll. Fresh water is pumped through the tube, while saltwater is pumped around the tube on the outside. Each spiral roll is less than 1 millimetre in diameter, and hundreds are arranged in parallel inside a pipe about 20 centimetres across. In the alternative design, the membrane is made into straight tubes which run through a tank of saltwater. Fresh water is pumped along the membrane tubes.

 

Help from gravity

As well as finding the optimal way to pack the membranes together, the researchers must work out how to prevent the delicate pores from clogging with silt and algae in the water. They are looking at an anti-fouling coating to put on the membrane, and experimenting with reversing the flow periodically to flush silt out, says Holt.

 

The other challenge will be to minimise the energy used to bring the water into the plant. Lab tests show a fifth of the electricity generated was expended on pumping the water in. However, many of Norway's rivers drop steeply from the mountains, so in future it should be possible to pipe in the fresh water using gravity alone. Build the power plant underground or on the river bed and gravity will also bring in the saltwater, says Skilhagen.

 

The Tofte plant will generate about 4 kilowatts, though a fifth will be used for pumping the water. The rest - just over 3 kilowatts - is only enough to power a couple of kettles, but Statkraft hopes to construct a large scale salinity power plant by around 2015. This will be about the size of a football stadium, contain 5 million square metres of membrane and generate about 25 megawatts of electricity, they say, incorporating a new membrane and efficiencies of scale. It should power more than 15,000 households.

 

Statkraft calculates that salinity power could eventually provide Norway with up to 12 terrawatt-hours of electricity annually, roughly 10 per cent of the country's consumption. "We estimate the global potential to be 1600 to 1700 terrawatt-hours annually," says Skilhagen, about 1 per cent of the world's annual energy needs. This would mean using about half of the fresh water flowing through every large estuary.

 

There is some scepticism that Statkraft's technology can be rolled out globally. Norwegian rivers are relatively clear of mud and silt, says Veerman. "In other parts of the world such as the Netherlands and the UK there is lots of silt and bacteria in the rivers." The cost of cleaning up this water makes PRO a non-starter, he says.

 

So Veerman and his colleagues at Wetsus have devised a rival system - a salt-based battery. Dubbed Blue Energy, it generates electricity by moving ions rather than water molecules across membranes. Their membranes are along the same lines as those used in kidney dialysis machines. In fact, their system requires two kinds of membrane - one permeable to positive ions, the other to negative ions. Both are impermeable to water.

 

Typical sea water contains about 35 grams of salt per litre, so compared with fresh water it is packed with positively charged sodium ions and negatively charged chloride ions. The team placed alternating layers of their two membranes in a stack to create separate chambers. When fresh water and saltwater flows simultaneously across alternate chambers, chloride ions flow spontaneously from the saltwater through one membrane into the fresh water, while sodium ions flow through the other membrane in the opposite direction (see diagram). This movement of ions generates a potential difference between a pair of electrodes, placed at either end of the cell.

 

Veerman and his colleagues have already switched on a small prototype Blue Energy generator in their lab. Though it only produces 20 watts of power, this convinced Pieter Hack, director of Dutch company Magneto, to form a new company called Redstack, which will commercialise the technology.

 

Now Redstack and Wetsus are collaborating on a pilot project, at a salt mine in Harlingen in the northern Netherlands. Salty waste water from the mine and fresh water from the local river are piped into the pilot plant, with each pipe feeding the rows of salt or fresh water channels inside the salt batteries. The unit - the size of three washing machines - is due to be switched on in weeks, and should produce several kilowatts of power. Unlike Statkraft's Tofte plant, this one isn't linked to the electricity grid, but will help the researchers assess how Blue Energy can be scaled up.

 

Membrane design is still an issue. The water-flow rate must also be optimised. But since only ions cross the membrane, there is less mass flowing across the membrane than with their rival's technology, and so silting problems are reduced, Veerman says.

 

Veerman and his colleagues calculate that if all the rivers of sufficient size in the Netherlands were utilised, Blue Energy could provide as much as 75 per cent of the country's electricity needs. By exploiting around half of the water flowing in the world's largest rivers, they estimate that Blue Energy could provide up to 7 per cent of the world's energy needs.

 

Blue Energy could provide up to 7 per cent of global energy needs by exploiting around half the flow in the world's largest rivers

Skilhagen thinks this figure is optimistic. "They haven't accounted for the seasonal variations in river flow, and environmental considerations." In theory, the Rhine can deliver 5 gigawatts of electricity. In practice, Skilhagen says, it would be impossible to block or divert the river's entire flow without doing serious environmental and economic damage. Yet Veerman estimates that using around one-fifth of its flow would be acceptable, providing around 7000 gigawatt-hours of electricity annually through Blue Energy.

 

Keep it clean

What of other impacts on the environment? The process generates brackish water, but this could simply be pumped or channelled into the sea. And each plant requires pipelines to collect and discharge water, as well as pylons to carry electricity to the grid. Large rivers often have industrial ports where they meet the coast and plants could be built in such areas, says Skilhagen. They already have much of the necessary infrastructure, too. "I'd be surprised if there are no environmental problems, but we are not aware of any right now," he says. "This is why it is important for us to build this prototype and ensure that it has minimal impact on the environment."

 

Both teams aim to learn these lessons quickly; though outwardly complimentary about each other's work, there is a clear element of competition. Both technologies are at a similar stage and the first to prove their design can be profitable without damaging the environment could have a serious advantage in the marketplace.

 

River estuaries are not the only place where salinity power can be set to work. Power could be generated at desalination plants using leftover brine, or with waste brine from industrial processes or salt mines. The Dutch team's master plan, though, is to use the Afsluitdijk dam which separates the Ijsselmeer from the North Sea in the central Netherlands.

 

The IJsselmeer is the largest lake in western Europe, covering an area of 1100 square kilometres. Fed by the river IJssel, its level rises by around 4 centimetres each day. This is insufficient for hydroelectric power, and the excess is currently emptied into the North Sea. However, the sluice gates on the Afsluitdijk dam are perfect for feeding water past ionic membranes in a salinity power plant, says Veerman. This could create perhaps hundreds of megawatts for short periods, at times of peak demand. "We could use this lake as an energy buffer, in combination with wind energy," says Veerman. The Dutch team are applying for permits to begin this project.

 

Compared to conventional energy generators, the capital cost of a salinity power plant is high, but likely to drop significantly once the technology is proven. Hack estimates that it would cost over $600 million to construct a 200-megawatt salinity power plant covering the area of two soccer pitches at the Afsluitdijk dam, and that this plant would produce electricity at a retail cost of $90 per megawatt-hour. Statkraft won't reveal detailed figures, but are aiming to produce electricity at a retail price of between $65 and $125 per megawatt hour by 2015. By comparison, modern fossil-fuel power stations churn out electricity at a cost of about $50 per MWh.

 

Roelof Schuiling, a geoengineer at Utrecht University in the Netherlands, believes that both types of salinity power projects are feasible and could play a valuable global role. "If realised, it is more dependable than wind energy and could have a big impact on our energy sources," he says. For example, a typical wind turbine is reckoned to generate electricity for an average of 3500 hours each year. A salinity power plant, on the other hand, could operate close to full capacity for more than 7000 hours a year.

 

Will salinity power ever pulse down the power lines? By May this year, when Statkraft flicks the switch at their pilot plant, the technology's true potential should become clearer. "We don't claim that salinity power will be the global energy solution," says Skilhagen, "but it could play an important role, and ensure that we hand over a better world to our children."

 

Unfortunately, Loeb won't be there to witness the event: he died in December 2008, aged 92. "He had a lot of interest in our work until his last days, and his wife still follows our efforts," says Skilhagen.

 

Kate Ravilious is a science writer based in York, UK

 

 

From issue 2697 of New Scientist magazine, page 40-43. Subscribe and get 4 free issues.

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