Blue Energy: Can We Get Our Future Energy from Salt Water?

Photo by James Boreham on Unsplash

Forget Nuclear Fusion. There’s a future energy source that has been proven, piloted, and is accessible to any coastal nation. Salinity gradient power — more simply known as ‘blue energy’ — is an idea that has been kicking around since the 1950s. When the mouth of a river meets the sea, huge volumes of freshwater meet salty seawater. As the salinity of the seawater mixes into the freshwater, the two waters diffuse quickly to reach an equilibrium. This mixing process releases energy: as much, in fact, as the same volume of water falling from a 250 meter high hydroelectric dam.

The potential power available was estimated in the 1970s (based on average ocean salinity and global river discharges) to be between 1.4 and 2.6TW a year: enough to meet almost all the entire global electricity demand.

If you place a membrane between the two bodies of water as they meet, you can capture the energy. There are currently two competing methods to do so. One, Pressure Retarded Osmosis (PRO), is similar to a hydroelectric plant: with river water on one side of the membrane and seawater to the other, the flow between the membrane builds up pressure on the seawater side, which can be used to power a turbine (click the video below). In 2009, Norwegian company Statkraft opened a pilot PRO plant in the Oslofjord with a 2000 metre squared membrane, achieving a modest proof-of-concept output of 5KW.

The second method is Reverse Electrodialysis (RED). This doesn’t harness energy from pressure, but instead diverts the ions in the water, creating an ionic current which is converted by the electrodes to electricity. This requires two membranes, effectively creating a seawater sandwich (with freshwater as the bread, and saltwater as the filling). The salt in the water, sodium chloride, is composed of two ions: sodium ion (or Na+, which has a positive charge) and chloride ion (Cl-, which has a negative charge). By sending negative and positive ions in opposite directions, you in effect form a saline battery. Create a big enough bank of these…