Salinity gradient: How energy resides
How osmosis turns mixing into pressure
What this tab covers The physical form in which the energy is held or carried, where it comes from, the chain of transformations that put it there, and how concentrated it is.Chemical (osmotic) free energy released when fresh river water mixes with salty seawater. Mixing a cubic metre of each can in theory yield about 1.4 MJ; the osmotic pressure difference, about 26 bar, equals the pressure at the foot of a 270 m water column.
Form Free energy of mixing (osmotic pressure difference between fresh and salt water) Origin Indirect solarHow it got there
Fusion in the Sun Sunlight Evaporation of seawater leaves the salt behind Rain and snow on land Rivers carry fresh water back to the sea Fresh and salt water meet at river mouthsHow concentrated it is
| Measure | Value | Note |
|---|---|---|
| Theoretical energy from mixing 1 m3 river water with 1 m3 seawater | 1.4 MJ1 | Rises to about 15 MJ if river water is mixed with saturated brine. |
| Osmotic pressure difference | 26 bar (equal to the pressure of a 270 m water column)1 | |
| Net power density of membranes in use | up to 2.7 W per m2 of membrane1 | Laboratory pressure-retarded osmosis has reached 14.4 W/m2. |
When the Sun evaporates seawater it leaves the salt behind, and rain returns fresh water to the land. Rivers then deliver that fresh water to the sea, where it mixes and the stored free energy is lost as a trace of heat. A salinity gradient plant intercepts the mixing with a membrane.
There are two main designs. Pressure-retarded osmosis (PRO) lets fresh water seep through a membrane into pressurised seawater and runs the extra flow through a turbine. Reverse electrodialysis (RED) passes salt ions through alternating ion-selective membranes, producing a direct electric current, like a battery. Either way, energy per cubic metre is modest and output is limited by membrane area, so the cost and fouling of membranes (50-80% of capital cost in IRENA's assessment) decide whether the source is usable.
Sources
- Salinity Gradient Energy: Technology Brief IRENA, 2014