Statkraft's plant in Tofte in Norway
At first sight, the small blue building seems unremarkable. It stands on its own on a broad and long dyke that connects the village of Den Oever in North Holland with the even tinier village of Zurich in the neighbouring Dutch province of Friesland. It could very well be a random warehouse, or maybe just a garage. Instead, it’s the home of what may turn out to be a technology and energy revolution, tapping one of the ocean’s most abundant resources: salt.
The building stands on the Afsluitdijk, a 32km seawall built in the 1920s and 1930s, cutting across a saltwater bay – the Zuiderzee. However, the river IJssel, a branch of the Rhine, kept flowing into the artificial inland bay, thereby steadily decreasing its salinity. Today, this bay is called the IJsselmeer, and is Western Europe’s largest freshwater lake. On one side of the Afsluitdijk are the salty waters of the North Sea; walk 90m across to the IJsselmeer, and you look across freshwater. It’s this difference in salinity that REDstack, the company that built the blue building, wants
to exploit.
REDstack is based in the nearby town of Sneek. The firm is a spin-off from Wetsus, the Dutch centre for sustainable water technology in Leeuwarden, which facilitates collaboration between universities and companies, taking their research from the laboratory to the pilot phase and getting it ready for the market.
Just a few more months, and REDstack will start to produce energy. Its name indicates the technology it uses: RED stands for reverse-electro dialysis. The plant on Afsluitdijk is a pilot project, which received planning permission in 2010. At first the plant is expected to generate 5kW. The aim is to increase capacity to 50kW over the next few years. The company says it is possible to tap salt-powered energy without damaging the environment – simply by channelling both saltwater and freshwater into a new kind of battery.
REDstack calls its process Blue Energy, and it offers considerable power generation potential, according to Simon Grasman, the company’s project engineer. “From every cubic metre per second of freshwater flowing into the sea, you could generate 1MW of power,” he says. “For example, the average discharge of freshwater from the IJsselmeer into the Waddenzee – the mud flats – is 200m3/s, so there’s an estimated 200MW of potential power.” An average household in the Netherlands consumes 3,500kWh of electrical power a year, so the 200MW would be enough to support 500,000 households, he says, adding: “The global potential, of course, is far bigger.”
Although the technology being used by REDstack is new, it is not the first time that Europe has tried to generate power by exploiting the difference in salinity between seawater and freshwater rivers or lakes. In 2009, Norwegian firm Statkraft, Europe’s largest generator of renewable power, opened a plant at Tofte in Norway. The facility on the Oslo Fjord used ‘pressure-retarded osmosis’ (PRO) to generate 2-4kW from the water’s salinity difference.
Although PRO is very different from RED, both technologies make use of the salinity difference between types of water. When two solutions with different salt concentrations come into contact through a semipermeable membrane, these solutions ‘want’ to mix so that the concentration of salt is the same on both sides of the membrane – a natural process called osmosis. The membrane allows control of the flow between the solutions. There are two options: the membrane either allows the flow of water and retains the salt, or it blocks the water, but lets the salt through.
The concept evolved from attempts to make seawater drinkable. In the late 1950s, researchers Sidney Loeb and Srinivasa Sourirajan at the University of California, Los Angeles developed a way of desalinating seawater through a process called ‘reverse osmosis’. Using high-pressure pumps, they managed to run osmosis in reverse, extracting freshwater from seawater.
The Statkraft plant used a variation of this process to generate power from the flow of saltwater. Its PRO process involves pumping seawater at 60-85% of the osmotic pressure against one side of a semi-permeable membrane that lets water through but not salt. On the other side of the membrane there is freshwater. Because of osmosis, freshwater flows across the membrane, diluting the saltwater and increasing its volume and the pressure in the saltwater container. Turbines then convert the water’s movement to electricity.
However, after about four years of operation, Statkraft announced last January that it would scuttle the project and concentrate on other technologies instead. While the prototype proved that the PRO process worked, the company admitted that the technology could not be sufficiently developed to become competitive any time soon.
Despite the osmotic power plant’s failure – at least on the energy market – proponents of salt power are convinced that the concept has matured enough to prove itself – albeit using a different technology, a salt-based battery. With this system, reverse-electro dialysis (RED), energy is generated through the transport of ions. “It is not the water that is moving
through the membranes but the ions,” says Olivier Schaetzle, a researcher at Wetsus.
Can this process succeed where PRO failed? Kitty Nijmeijer, of the membrane science and technology department at the University of Twente in the Netherlands, thinks that comparing the two plants is like comparing pears and oranges. “The Afsluitdijk plant is a different technology, so in that sense the developments in Norway do not predict the future of RED,” she says.
The Blue Energy system exploits the chemical differences between saltwater and freshwater, generating electricity by transporting ions across membranes. Seawater normally contains about 35g of salt per litre. And salt contains ions – many more positively charged sodium ions and negatively charged chloride ions than those in freshwater.
Two kinds of membrane are used – one permeable to the positive cations, and the other to the negative anions. Both membranes are impermeable to water.
For RED to work, alternating layers of the two membranes are stacked, creating separate chambers. Freshwater and saltwater are made to flow across alternate chambers at the same time, and chloride ions then flow spontaneously from the saltwater through one membrane into the freshwater, while sodium ions flow through the other membrane in the opposite direction.
“Because of this split of charge, an electrical potential difference can be measured between one side of the membrane and the other,” says Schaetzle. “This potential is related to the concentration difference between the two sides, and in the case of sea and river water it is about 60-75mV.”
By stacking the membranes, the potential adds up, and tens of volts can be achieved, he says. “This potential is used to drive an electrochemical reaction that allows conversion of the ionic current into an electrical current that can be injected into the grid.”
Until recently, the biggest obstacle the technology faced was the manufacture of the membranes. Engineers had to design new types of membranes to improve efficiency. But now, says Schaetzle, “RED is reaching a point where affordable membranes are available.”
There have been many other technology advances, too, says the University of Twente’s Nijmeijer, mainly in terms of the membranes’ fouling control, stack design, hydrodynamics and mass transfer. She adds that “membrane production is in the scale-up phase, and research on fouling and fouling prevention has been performed that will be implemented in the Afsluitdijk”.
Already the RED pilot is causing much less fouling than in Norway’s PRO plant. For the reverse osmosis-based plant to work, all the water had to be pumped through the membranes, “so huge volume streams resulted in tremendous fouling, as that is directly related to the water flux”, says Nijmeijer. “In the Afsluitdijk case, salt transport is key. But the amount of salt to be transported is less than the amount of water, so the fluxes are much smaller, resulting in less fouling.”
Meanwhile, whether it’s RED or PRO, salinity gradient technologies are spreading. In Belgium, there is a lab prototype at the Flemish institute for technological research in Mol. In Singapore, a PRO pilot is under development by Nanyang Technological University, the National University of Singapore and Singapore’s Public Utilities Board.
In Quebec, Canada, another PRO pilot is being run by Hydro Quebec and Concordia University. And in South Korea, a prototype is being developed by the Korea Institute of Energy Research and the Gwangju Institute of Science and Technology.
Using salinity gradients to generate energy could also work in other environments, not just where sea and river water mix. The technology offers potential for other saline streams such as very saline brines from salt production facilities, desalination plants and other industries. For instance, the REAPower project, funded by the European Union’s Framework programme FP7, uses the RED process with brine and brackish water instead of seawater and freshwater.
“The freshwater has few ions, so its low conductivity is a limiting factor to the productivity of the Blue Energy system,” says Michael Papapetrou of WIP Renewable Energies, the company coordinating the REAPower project. “By replacing it with brackish water, we unlock this potential. And by replacing the seawater with brine, we increase
the salinity.”
This way, at least five times more energy per membrane area can be generated, claims Papapetrou. REAPower already has a prototype in a salt production facility in Sicily, Italy, which is being scaled up to 1kW to start working in June.
Frank Neumann of the Integrated Network for Salinity Gradient Energy (www.salinitygradientpower.eu), which brings together the main relevant pilot and research initiatives worldwide, says that using brine water from a desalination plant is particularly appealing.
“This application only needs highly saline wastewater and less saline water,” he says. “It has great potential as there are a lot of very highly saline waste streams – for example from desalination, chemical industry, and mining.”
There are other ideas around, too. At Wetsus, a technology is being developed to harvest energy from carbon dioxide emissions, by using the mixing energy of exhaust gases with air. The concentration of CO2 in the exhaust gases is much higher than in the air, so the same principle as with sea and river water can be applied, says Schaetzle.
“The trick consists in having the CO2 reacting with water to generate protons (H+) and bicarbonate ions (HCO3-). These ions are then used in the same way that sodium and chloride ions are used in the river-seawater process,” he says.
Yet another concept being developed at Wetsus is one of the newest in the field of salinity gradient energy. Called Capmix, this method uses capacitive electrodes made of activated carbon that can absorb ionic charges in the concentrated stream – the sea – and release these charges in the dilute stream – the river. It is a cyclic process whereby concentrate and dilute streams are alternatively fed to the system, making it possible to harvest electricity.
“This technology has two main forms – one that does not use membranes and another that uses ion exchange membranes like RED and that tends to perform better,” says Schaetzle. “The innovation in this technology is that the system can have a more flexible design that could allow operation with lower losses – for instance, losses linked to the pumping of the water.”
If some of these projects succeed, salt power could have huge potential. There are plenty of estuaries, and even more industrial plants with wastewater of different salinities. One day, perhaps, these methods will allow us to produce green electricity independently of the weather or time of day, and with little ecological damage.