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.If they can get the price of those membranes down, this looks like a promising way to produce electricity.
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.
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 (0.2 Fahrenheit). Dutch scientists say such energy at all the world's estuaries is equivalent to 20 percent of world electricity demand.
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 Norwegian project will include 2,000 square meters (21,530 sq ft) of plastic membranes, through which fresh water will be sucked into salt water by osmosis.
At Tofte, the power exerted by salt water sucking in fresh water is equivalent to water falling 270 meters in a waterfall. The only emissions are brackish water.