Stare out at a river as it meets the sea and you'll probably miss the chemical reaction taking place beneath the surface.

It's not as flamboyant as, say, a wave or a giant waterfall. But some European scientists think it can become an equally-vital source of environmentally friendly power.

In fact, scientists are testing whether or not salt - already a fairly useful resource that regulates water in the body and makes food tastier - can be upgraded into an energy source.

Experiments in "osmotic power" are built around the energy produced when freshwater passes through a membrane and then merges into saltwater. The pressure built up from that process, called osmosis, gets captured through a turbine, which turns it into a power source that's renewable, clean and leaves little impact on its surrounding environs.

At least, that's the idea.

The research has been quietly emerging over decades, and it received a little bounce in attention this week when the Norwegian state-owned power company Statkraft announced it plans by the summer of 2008 to build the world's first significant prototype osmotic power plant.

Just outside of Oslo, Norway, where a river flows into the sea, Statkraft is converting an old paper mill into the scene of the experiment.

"You don't have to build big buildings in the mountains, and you don't have to have a great storage of water," said Statkraft spokesperson Knut Fjerdingstad. "You just have to have ocean water and fresh water from a river. You can just put the plant where you already have buildings, as long as they're on the coast."

Sounds pretty good, but scientists working in osmosis and salt-related technologies say that what Statkraft aims to do is very tricky, and could be very costly. Another potentially promising ocean-related energy technology - wave power - has, for example, struggled after decades of research and has not yet produced a commercial plant.

"Theoretically, osmotic power may be possible, but I'm very skeptical," said James Klausner, a University of Florida professor of mechanical and aerospace engineering who works on desalination (essentially the opposite process as osmotic power).

"If you combine sea water and fresh water you can create a pressure difference. And if they developed a device that converts and stores that pressure difference, that could potentially work, but it's something that's very difficult to do."

So far, Statkraft has focused on locating and developing the right membrane - the thin sheet that creates energy as the freshwater passes through it to meet the salt water. Advancements in desalination have spurred research in synthetic membranes, which helped provide a head start.

But Statkraft has had to do much work tweaking the polymer membranes to come up with a usable version, and it's still not sure whether or not its membranes can be scaled to commercial production.

"This next step will make it possible to test out our membranes in the larger scale," said Fjerdingstad.

The $18.4 million test project - about the size of a desk, producing around 2-4 kilowatt hours of energy or enough to power around 3-4 electric ovens - has been funded by the company and through governmental grants.

Statkraft will decide in the next five years or so whether or not its technology can be built to commercial scale, said Stein Erik Skilhagen, a Statkraft scientist.

But to do so would take a lot more public awareness and governmental support, with costs about on par with other expensive emerging technologies, such as offshore wind power or tidal energy plants.

Skilhagen figures that based on the number of places in the world where a river meets the ocean - the required location for an osmotic power plant - the system could potentially power 1600 terabytes of power globally, and one-third of household power use in Norway alone.

The plant's discharge will get dumped into the ocean, but it will be the same as the freshwater-saltwater mixture that naturally occurs when a river flows out into the ocean, said Skilhagen. And the company argues that osmotic power is both renewable and discreet (a plant could be located in any industrial building, or even underground).

Without knowing or factoring in cost, that low environmental impact could theoretically make it more attractive to build an osmotic power plant than, say, a hydropower plant that floods an area and leaves a significant impact, say analysts.

"With hydropower, at the end of the day you have to justify that producing that amount of energy is worth disrupting the natural ecosystem, and there's always a give and take," said Frost & Sullivan analyst Renee Chu. "There's a lot of interest right now in technologies that are progressively trying to reduce our impact on the environment, and if this is something that actually works, I think it could certainly have potential."