Tofte (Norway): Only up to powering light bulbs so far, “salt power” is a tantalizing if distant prospect as high oil prices make alternative energy sources look more economical.
Two tiny projects to mix sea and river water—one by the fjord south of Oslo, the other at a Dutch seaside lake—are due on stream this year and may point to a new source of clean energy in estuaries from the Mississippi to the Yangtze.
The experiments, which seek to capture the energy released when fresh and salt water are mixed, build on knowledge that has been around for centuries—in one case imitating the process of osmosis used by trees to suck water from their roots. Although they are far from being economically viable, if eventually successful they might help a long-term quest to diversify away from fossil fuels such as coal and oil, widely blamed for stoking global warming.
“We might well be able to find new promising solutions such as generating power naturally from osmotic forces occurring when salt and fresh water are mixing,” Norwegian deputy energy minister Liv Monica Stubholt said in a speech earlier this month.
Oil, currently trading not far from a record $112 (Rs4,536) a barrel, is forecast to peak this year as a US slowdown reduces demand, but analysts polled by Reuters in February still saw the average price above $80 in 2010.
The science at the heart of the projects is the fact that when salt and fresh water mix at river mouths, they are typically warmed by 0.1 degree Celsius. Dutch scientists say such energy at all the world’s estuaries is equivalent to 20% of world electricity demand.
The plants may support hopes that the technology can overcome hurdles, the most significant of which is poor cost-effectiveness of the membranes used in the process.
In Norway, Statkraft Group, which says it is Europe’s top producer of hydro and wind energy alongside Electricite de France, is building a test plant costing $20 million.
“Ours will be the world’s first saline power plant based on osmosis,” said Stein Erik Skilhagen of the state-owned firm. The plant, at Tofte on the Oslo fjord, will have an output of up to about 5kW—enough to run household appliances such as washing machines or heaters or a few dozen lightbulbs.
The Dutch Centre for Sustainable Water Technology (Wetsus) will also in three to four months start a pilot “blue power” test at IJsselmeer in the Netherlands, from where water flows into the sea. “At the start, it will be on the scale of 100W...but we aim at this salt factory to obtain 1-5kW within one year,” said Jan Post, a researcher at Wetsus.
The Achilles heel
The Norwegian and Dutch plants use different systems but both depend on membranes placed between the salt and fresh water, which are currently prohibitively expensive and highly energy-intensive to produce.
“The Achilles heel for this process is that there is no commercial membrane,” said Menachim Elimelech, a professor of chemical and environmental engineering at Yale University in the US. “It’s not even close to being economical.”
The membranes are similar to, but thinner, than those used at many desalination plants, when sea water is pressed against membranes that allow only fresh water through in a process known as reverse osmosis.
Makers of membranes such as General Electric Co., Dow Chemical Co., Hydranautics or Japan’s Toray Industries Inc. focus most on membrane technology for desalination—a market growing by about 15% a year worldwide.
Ellen Mellody of GE Infrastructure, Water and Process Technology said the company has “an aspirational goal” of producing fresh water from salt through membranes at a cost of 10 cents per cu. m, down from 70 cents to a dollar.
Asked about prospects for a separate market for power-generating membranes, she saw one “potentially, but not for about 5-10 years”.
The Norwegian project will include 2,000 sq. m of plastic membranes, through which fresh water will be sucked into salt water by osmosis.
Osmosis’ power was shown in 1748 when French physicist Jean-Antoine Nollet put a pig’s bladder filled with alcohol in a trough of water. The bladder swelled and burst—the more concentrated liquid draws pure water into it.
At Tofte, the power exerted by salt water sucking in fresh water is equivalent to water falling 270m in a waterfall. The only emissions are brackish water.
Unlike the osmosis of the Norwegian system, the Dutch scheme captures salt particles which give off electrical currents. “In theory, both techniques use the same energy source and you could in theory get the same amount of energy out,” said Sybrand Metz, project leader at Wetsus.