Salinity gradient
Energy released where fresh river water mixes with salty seawater ('blue energy').
Natural sources · Solar-derived flows · No. 7
- Natural sources
- Origin: Indirect solar
- Renewable: Yes
Every river mouth hides a waterfall: fresh water meeting the sea pushes like a column of water 270 metres tall.
Chart: Illustration: Salinity gradient
Where it comes from
Chart: Map of where it comes from
When fresh and salt water mix, the osmotic pull is about 26 bar, the pressure at the foot of a 270 m water column, and membranes can harvest it.
Three things to know
- Asia holds about 208 GW of technical potential and South America about 199 GW.
- Saturated brine can raise the energy to about 15 MJ per cubic metre, against 1.4 MJ in an ordinary estuary.
- Japan's Fukuoka plant uses concentrated seawater from a desalination works and should give about 0.88 GWh a year.
The journey
- 1 River waterFresh water flows towards the coast
- 2 SeawaterSalty water waiting at the estuary
- 3 MembraneOsmosis or ion flow across a barrier
- 4 Pressure or currentAbout 26 bar of osmotic push
- 5 ElectricityOnly pilot plants so far, like Fukuoka