Could the place where a river meets the sea power your grid? Salinity gradient power uses membranes to turn the mixing of fresh and salt water into electricity, and it has so far been tested mainly in pilot plants.
This guide sits within the wider field of ocean renewable energy, and it explains how the technology works and where it stands.
| Item | Value |
|---|---|
| Global potential (Ocean Energy Systems estimate) | More than 1,600 TWh per year |
| European share of that potential | 170 TWh per year |
| Osmotic pressure between seawater and fresh water | 24 to 26 bars |
| Estimated energy cost | 50-100 €/MWh |
| Global potential after technical and physical limits (review) | Slightly below 1 TW |
| Trapani reverse electrodialysis prototype (commissioned 2014) | Up to 40 W |
These figures come from Ocean Energy Systems and a peer-reviewed membrane review. They can change as research moves on, so check the sources before you rely on them.
How does salinity gradient power work?
When fresh water and seawater mix, the mixing releases energy that would otherwise leave as heat. A river flowing into the sea releases large amounts of it, according to Ocean Energy Systems.
The same source names two concepts for turning that energy into electricity instead of heat:
- Pressure retarded osmosis (PRO): the released energy becomes pressure that drives a turbine.
- Reverse electrodialysis (RED): the difference between the two solutions creates a voltage across membranes.
The idea is not new. A 2021 review in the journal Membranes says it was first introduced by Pattle in the 1950s, and that the term blue energy has been coined for these technologies.
Pressure retarded osmosis: turning fresh water flow into pressure
In osmosis, water moves through a semi-permeable membrane toward the saltier side. Ocean Energy Systems says Prof. Sidney Loeb first considered using this in the early 1970s.
Seawater and fresh water keep trying to mix as long as the pressure difference between them stays below the osmotic pressure difference. For seawater and fresh water, the same source puts that figure at 24 to 26 bars.
A PRO plant works in a few steps, as Ocean Energy Systems describes them:
- Filtered fresh water and seawater are led into the system.
- The seawater is pressurized to approximately 12-14 bars before it reaches the membrane modules.
- Fresh water migrates through the membrane into the pressurized seawater.
- The excess, now diluted and pressurized, is split in two streams.
- One third drives a hydropower turbine, and the rest passes through a pressure exchanger that pressurizes the incoming seawater.
The drain from such a plant is mostly diluted seawater, which can be led back to the river mouth or into the sea. The same source adds that the two unique components are the pressure exchanger and the membrane, so most commercialization effort goes into improving and scaling them.
In its October 2009 report, Scientific American described a Norwegian prototype built by the utility Statkraft in a converted paper mill at Tofte, about 60 kilometers south of Oslo. The plant cost approximately $5 million, and its goal was five watts per square meter of membrane, against about three watts at the time.
Reverse electrodialysis: ions into current
In reverse electrodialysis, alternate layers of fresh water and salt water are separated by alternating cation and anion exchange membranes. The Membranes review explains that the result is a net flow of ions through the stack, and electrodes at each end turn that ionic current into electric current.
Two pilot efforts show how this has been tested:
- Netherlands: the research firm Wetsus ran a pilot installation in Harlingen, by the Wadden Sea, as Scientific American reported in October 2009.
- Italy: the EU-funded REAPower project launched in 2010. It commissioned a prototype in Trapani in 2014 with almost 50 square metres of membrane and a power output up to 40 W, according to the review.
REAPower used saturated brines from salt ponds and brackish water instead of seawater and fresh water. The same review says this reduces the electrical resistance of the dilute compartment and raises the achievable power output.
How much energy could it supply?
Estimates vary with the assumptions behind them. Ocean Energy Systems puts the global potential at more than 1,600 TWh of electricity per year, with 170 TWh per year in Europe.
Scientific American reported in October 2009 that Statkraft estimated up to 1,700 terawatt-hours, or about 10 percent of global demand. A 2021 review gives a different framing in terawatts:
- The theoretical potential from all river water reaching the sea was first estimated at 1.4 to 2.6 TW in the 1970s.
- More recent studies suggest slightly below 1 TW once technical and physical limits are counted.
- For comparison, global electricity demand in 2019 was around 2.85 TW.
On cost, Ocean Energy Systems estimates a range of 50-100 €/MWh. It calls that comparable with wave, tidal and offshore wind, so readers weighing those options can also read about offshore wind environmental impact.
For a different ocean resource that is not based on salt, you can read about ocean thermal energy conversion.
What still holds salinity gradient power back?
Membranes are the main hurdle. Scientific American reported in October 2009 that membrane design and performance remained the biggest hurdles for both Statkraft and Wetsus, and that membranes needed to become more efficient, durable and resistant to biofouling.
Pretreating the incoming water by filtering out organic matter and debris helps, but the same article says this step consumes energy and is expected to be expensive. One engineer quoted there said both methods might still need a breakthrough innovation before commercialization.
The environmental effect is the other open question. A Yale professor told the magazine that the process had not been tested at that scale and that he doubted there would be no environmental impact at all.
Closed designs offer one way around some of these problems. The Membranes review says a RED heat engine runs on a finite recirculated amount of salt solution, which can avoid fouling and the need for pretreatment.
Frequently asked questions
Is salinity gradient power free of carbon emissions?
Show answer
The Membranes review lists no emission of carbon dioxide and no consumption of the salts as the main benefits. Plant construction and operation are not covered by that statement.
Does it depend on the weather?
Show answer
According to Scientific American in October 2009, salt systems are not dependent on the weather, unlike solar or wind power. The report said they could provide baseload electricity because river water keeps flowing into the sea.
What is blue energy?
Show answer
Blue energy is a term coined for technologies that exploit salinity gradients to produce energy, according to the Membranes review. It covers approaches such as reverse electrodialysis.
Does it need a dam?
Show answer
No. Scientific American reported in October 2009 that, unlike conventional hydropower, a saline power plant does not require damming off a waterway.
