Getting a Taste for Salt and Seawater in the Energy Menu
When the mighty Amazon finally meets the sea, millions of gallons of fresh water mingle with the salt water of the Atlantic Ocean. It could be the start for a new form of simple battery that uses the difference in saltiness between fresh water and seawater to produce electricity, say researchers at Stanford University in Palo Alto, California, capable of producing as much as 13% of the world’s energy. Meanwhile, the Southern Hemisphere’s first commercial-scale wave power unit has begun producing energy as part of a trial project, says Carnegie Wave Energy.
In New Scientist (21 April 2011):
When the mighty Amazon finally meets the sea at Belem in Brazil, millions of gallons of fresh water mingle with the salt water of the Atlantic Ocean.
It’s the end of an epic journey across South America – but it could be the start for a new form of simple battery that uses the difference in saltiness between fresh water and seawater to produce electricity.
Researchers at Stanford University in Palo Alto, California, say these sorts of meeting points could be used to produce as much as 13 per cent of the world’s energy if electricity from all of the world’s rivers were harvested in this way.
It’s quite a claim, but not an entirely new idea. Scientists have long known you can produce electricity using the difference in saltiness of water. Other methods of using difference in salinity to create electricity have usually relied upon membranes which the ions pass through. But these are costly as well as fragile and are unsuitable for large-scale energy production.
The battery itself, known as a “mixing entropy battery” is relatively simple, and consists of two electrodes – one which contains positive sodium ions, one that has negative chlorine ions – which are immersed in fresh river water.
The battery slowly charges as the river water, which is low in salinity, and a small electrical charge pull the ions from the electrodes, increasing the voltage between the two.
The fresh water is then drained and replaced with seawater. The seawater is full of ions, which rush back to the electrodes, and produce electricity that can be drawn off from the battery. The saltwater is then drained and replaced with river water again, and the process starts over.
In tests the team managed to achieve 74 per cent efficiency in converting the potential energy in the battery to electrical current. During tests they found that 2.2 kilojoules of free energy could be harvested from every litre of fresh water.
Nice idea – but good luck telling environmental groups you’re planning on building giant power plants at some of the worlds most picturesque estuaries, however efficient.
Sydney Morning Herald (21 April 2011):
The Southern Hemisphere’s first commercial-scale wave power unit has begun producing energy as part of a trial project.
Carnegie Wave Energy said its CETO 3 unit on Garden Island, south of Perth, had begun producing hydraulic power and was performing to expectations.
Managing director Michael Ottaviano said the trial would be followed by a grid-connected, two-to-five-megawatt demonstration project.
The system is made up of submerged buoys tethered to pump units anchored to the ocean floor at about 25 metres depth. The buoys move with the motion of the waves, driving the pumps, which in turn pressurise water that is delivered to shore via a pipeline to drive hydroelectric turbines, producing zero-emission electricity.
Waves in excess of one metre are needed to produce reliable energy, which makes the southern and western coasts of Australia ideal, according to Mr. Ottaviano. Most of the southern half of Australia receives two-metre swells for at least 90 per cent of the year.
Mr. Ottaviano said the purpose of the trial project was to demonstrate Carnegie’s pumping technology, not the power off-take technology, which was ”off the shelf”.
Power from the trial project was not being used, he said. The company was instead focused on collecting performance data.
”We’ll have a full idea of this unit’s performance in a matter of weeks,” he said.
Power from the forthcoming grid demonstration project could be sold to state electricity retailer Synergy or the Department of Defense for use on Garden Island, which is Australia’s largest naval base, he said.
Carnegie has signed a memorandum of understanding with both parties.
”We’ve also been short-listed by WaterCorp to sell them power for the new desalination plant being built south of Perth down at Bunbury,” Mr. Ottaviano said.