Green machine is our weekly column on the latest advances in environmental technologies

We've already tried to generate power from ocean waves and tides. Now engineers are trying to tap energy from another of the sea's abundant resources: salt.

Efforts to generate power from the salinity difference between seawater and fresh water from rivers and lakes are already well advanced in Europe, where two alternative approaches are being tried out.

Last week Redstack, based in Sneek in the Netherlands, was granted permits to build a pilot salt battery at the Afsluitdijk dyke in the north of the country, fuelled by waters from an inland lake and the North Sea. The plant should initially be able to deliver 5 kilowatts, but the company wants to increase this to 50 kilowatts over the next few years if funding is secured, says director Pieter Hack.

The company already has a small pilot plant operating on waste water from a salt mine in the same area.

Red sea power

Its salt battery is based on a process called reverse electrodialysis (RED) and consists of a stack of membranes. Each one is waterproof but allows either positive or negative ions to pass through, with "positive" and "negative" types alternating in the stack.

Salt and fresh water are pumped into chambers in the stack that are sealed off from one another by the membranes. 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.

Redstack calculates that there is enough water available at the Afsluitdijk dyke site to support a 200-megawatt plant.

Pressure drop

Meanwhile power company Statkraft in Lilleaker, Norway, is testing a different approach to osmotic power, known as pressure-retarded osmosis (PRO). Last November the company opened a prototype plant on the Oslo fjord at Tofte in southern Norway. Here a water-permeable membrane is used to draw fresh water on one side to salty water on the other. This creates a current that drives a turbine.

Whatever the choice of system, there is theoretically enough energy available across the globe in the estuaries where rivers and seas meet to generate more than 2 terawatts of energy, enough to meet all of the world's electricity needs, claims Bert Hamelers, an environmental engineer at Wageningen University in the Netherlands. Better still, osmotic power plants would provide a continuous source of electricity, whatever the weather – unlike wind, wave and solar energy.

But the technology is expensive. Hack has estimated that a 200-megawatt salinity power plant would cost up to $600 million to construct. As a result electricity from the plant would cost around $90 per megawatt-hour – almost twice that of fossil fuel-generated electricity at $50 per megawatt-hour.

So Hamelers and colleagues are developing a new type of salt battery that he claims will be able to generate power from a salinity difference much more cheaply.

Saline solution

In Hamelers's design, seawater is fed into a chamber containing a sandwich of two ion-permeable membranes between two electrodes. Like the RED device, the 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.

Electrons from the now negatively charged chloride electrode then begin flowing to the positively charged sodium electrode, generating a current between the two. Once the seawater electrons are exhausted, fresh water is pumped into the chamber, the sodium ions are drawn back through the membrane to the water, and the electrons begin to flow in the opposite direction. By constantly switching between salty and fresh water in this way, the device generates an alternating current (Environmental Science and Technology, DOI: 10.1021/es100852a).

The device should be cheap to produce, as unlike the RED process it needs only inexpensive carbon electrodes, says Hamelers. And unlike the PRO device, it does not need turbines and pressure exchangers to drive the turbine. "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.

Hamelers believes the system could be made even simpler, by moving the electrodes between tanks of fresh and salty water, rather than alternately pumping fresh and salty water into a stationary battery.