By Mark Gregory
BBC News technology correspondent
It is hoped the plant will produce enough electricity for a small town
The world's first power project that generates energy by mixing fresh water with sea water has opened in Norway.
The Norwegian renewable power company Statkraft has built a prototype osmotic power plant on the Oslo fiord.
It aims to produce enough electricity to light and heat a small town within five years by osmosis, the process that allows plants to absorb water.
At first it will produce a minuscule 4 kilowatts - enough to heat a large electric kettle.
But by 2015 the target is 25 megawatts - the same as a small wind farm.
With the Copenhagen climate summit just weeks away, the hunt is on for ways of producing electricity that do not put greenhouse gases into the atmosphere.
In theory at least, osmotic power ticks all the boxes. It is emission-free, renewable and - unlike wind or solar power - works whatever the weather.
Osmotic power works by putting sea water next to fresh water separated by a membrane thin enough to allow small fresh-water molecules through but not the larger sea-water molecules laden with salt.
If a concentrated solution is in close proximity to a less concentrated solution the liquids will naturally mingle to reach the same level of dilution.
The pressure of the fresh water driving through the membrane to dilute the sea water drives a turbine that generates electricity.
HAVE YOUR SAY
Is this our last best hope to save the world?
Supporters say the technology has almost unlimited potential. It just needs a coastline.
But detractors say osmotic power is likely to be prohibitively expensive.
One big issue is coming up with a membrane of exactly the right thickness, capable of withstanding huge pressures that does not constantly get clogged up with salt.
Many experts say tidal power is much more promising as a possible solution to the world's energy problems.