How Energy Is Produced - The Renaissance of Renewable Energy - page 160

Salinity Gradients and Osmotic Power
The reason it is dangerous to use a hair dryer or other electrical appliance in the bath is not
because water conducts electricity (it doesn't), but because the salts present in freshwater
do. In a battery or between the poles of a socket, electrons flow from an area of high
electrondensity to an area of low electron density. Something similar happens if we add salt
to a pot of boiling water. As the grains of salt dissolve, the molecules of sodium chloride
split into ions (molecules with an electric charge); in this case, positive sodium ions and
negative chloride ions.
When a river flows into the sea, freshwater and saltwater mix. So-called reversed
electro dialysis (RED) systems can exploit the difference in electric potential between two
solutions with different concentrations of ions. Saltwater and freshwater are brought into
contact through an exchange membrane, 31 and the difference of charges that results may be
used to generate a current. The first prototype to test this concept is currently being built in
the Netherlands.
Another way to take advantage of different concentrations of salt in water is osmotic
power. In this case, the osmotic potential rather than the electric potential of ions dissolved
into the seawater is harnessed and a different type of exchange membrane is used. A
semipermeable membrane allows water molecules to pass through but prevents salt
molecules from doing the same. For physical and chemical reasons, the concentration
levels at the two sides of the membrane tends to balance out and water molecules naturally
move through the membrane from the region of low to high concentration of salts (from
freshwater to salt water). The osmotic pressure difference may be up to 26 bar, roughly
ten times the pressure in a car tire. This process produces a small but consistent stream
of water moving from one side of the membrane to the other. This flow can then be
harnessed to drive a hydropower turbine. The world's first 5-kilowatt osmosis power plant
was commissioned in Norway in 2009 (Lewis et al. 2011 ) .
The Benefits and Costs of Ocean Energy
Although more than 100 different technologies are under development in more than thirty
countries, ocean energy systems are at a very early stage and will not have much impact
on energy markets for many years. Some governments, particularly in countries along the
northeastern Atlantic, are promoting ocean energy technologies.
With the exception of tidal power, which basically adapts the technology of
hydroelectric dams to estuarine situations, most ocean energy technologies have not yet
progressed beyond the prototype stage. 32 Even tidal power is in its infancy. A small
conventional coal plant generates more electricity than the world's total installed capacity