As long as rivers of freshwater flow into a salty sea, rivers of electricity could flow from estuaries into the power grid.
Scientists from the Netherlands have found a way to harvest electricity from estuaries using devices similar to a fuel cell and they say that a full-scale production plant could provide significant amounts of power to nearby cities.
These small stacks have no moving parts to endanger wildlife or break, and could add electricity to the grid as fast as freshwater is dumped into saltwater.
"Wind energy is only available when the wind is blowing," said Sybrand Metz, a scientist at the Dutch Center for Sustainable Water Technology or Wetsus, who, working with colleagues at the University of Groningen and the University of Twente, co-authored a new study in the journal Environmental Science and Technology. "You can use this technology to generate electricity all the time."
The process is called reverse electrodialysis. In normal electrodialysis an electric current creates freshwater from saltwater by removing the dissolved charges. In reverse electrodialysis (RED) the mixing of two different solutions with different salinity produces a mild electric current. The larger the difference in salinity the greater the power that can be generated.
The key to RED is a special membrane that separates the freshwater from the saltwater and gathers the electric charge. One cell can harvest between 70 and 80 millivolts of electricity, not a significant amount. Line up one cell after another, however, and the electricity starts to add up.
A sea container 40 feet long, containing several thousand feet of membrane and expected to be installed in a Dutch estuary in 2012, should generate approximately 200 kilowatts of electricity.
Right now that electricity would be relatively expensive; the membrane that lifts electricity from separate streams of water is expensive. As demand for the membrane eclipses the supply, however, the cost for RED electricity should drop to about the same price as wind power, said Metz.
"The process itself is fairly efficient at converting the energy potential that exists into electricity," said Glenn Lipscomb, a scientist at the University of Toledo familiar with the European work. RED power has great potential, provided the cost of the membrane comes down and that suitable sites can be found.
Like wind power, not just any estuary will do. A good site will have a large and steady supply of freshwater, minimally disturb the seabed, and not interfere with fish migrations, said Lipscomb.
Years of work remain before estuary-generated power becomes commercially viable, but there is a lot of available energy flowing into the ocean every day, ready to be harvested.