A new approach to generate electricity comes from engineers from Europe’s northern countries of the Netherlands and Norway. They want to use the difference between salty and fresh water through osmosis in two different manners and say that their solutions could suffice the entire world’s energy needs. The New Scientist joined them both in an interesting case study.
The first is a company named Redstack, from Sneek, the Netherlands (nice city – been there), who wants to build a “salt battery” fueled by freshwater from an inland lake and the salty water of the North Sea, at the Afsluitdijk dyke. Their initial plan is that the power plant should produce 5 kW. If everything works as on paper, the company wants to extend the production facility to 50 kW and even more, as they say that there is enough water at the Afsluitdijk dyke to produce 200 megawatts (huge numbers!), since in the Netherlands, more than 3,300 m³ fresh water runs into the sea per second on average.
Redstack’s technology uses a process called reverse electrodialysis (RED), which consists of a stack of membranes that allows either positive or negative ions to pass through. Positive and negative-permitting membranes will be alternated inside the stack.
Fresh and salted water would be pumped alternatively between the membranes, which would seal the two. The positive sodium ions in the seawater flow across one membrane to the fresh water, while the negative chloride ions from the seawater flow across a membrane in the other direction. This process generates a potential difference between titanium electrodes coated in a precious metal placed at either end of the cell.
Another company that dreams of using waters’ salt difference is Statkraft in Lilleaker, Norway. They use pressure-retarded osmosis (PRO), and even opened up a prototype on the Oslo fjord at Tofte, in southern Norway. Their membranes are water-permeable and draw fresh water on one side and salted water on the other side, creating a current which drives a turbine.
Both technologies are expensive to implement, though. Bert Hamelers, from the Wageningen University in the Netherlands, estimated the companies’ two methods of producing power and says they would need $600 million to build a 200 MW plant, which would equal a production cost of electricity that would be double than that obtained from fossil fuels. Instead, he proposes a much cheaper osmosis-based approach.
Hameler designed a scheme in which saltwater is pumped into a chamber that contains a sandwich of two ion-permeable membranes, between two electrodes. Positive sodium ions are drawn through a membrane that allows only positive ions to pass, and are then attracted to the electrode. The negative chloride ions are drawn through a membrane that only allows anions to pass, and are attracted to another electrode on the opposite side.
He says that his device should be cheap to produce, because it only needs cheap carbon electrodes. “You can produce the electrode and membrane materials very easily, because you can produce them on rolls on a large scale, and you can directly use them in a simple set-up to generate electricity,” he says. Even more simple, Hamelers proposes to move the electrodes between tanks of fresh and saltwater, to create an alternative current.
Two terawatts of electricity could be produced if osmotic power plants would be installed in all of the world’s estuaries where freshwater meets saltwater, covering all of the world’s current need of electricity. Further development has to be done in this field, money has to be invested and time has to be spent to make the technology as cheap as possible. The big advantage is that the osmotic power plant would generate electricity continuously, unlike other forms of renewable energy like wind, solar, tidal etc.