Osmotic power
In 1954, Pattle[1] suggested that there was an untapped source of power when a river mixes with the sea, in terms of the lost osmotic pressure, however it was not until the mid ‘70s where a practical method of harnessing it using selectively permeable membranes by Loeb [2] was outlined.
The method of generating power by pressure retarded osmosis was invented by Prof. Sidney Loeb in 1973 at the Ben-Gurion University of the Negev, Beersheba, Israel.
A new, lower cost membrane, based on an electrically modified polyethylene plastic, made it fit for potential commercial use.
Also as stated by Jones and Finley within their article “Recent Development in Salinity Gradient Power”, there is basically no fuel cost.
Differing salinity gradient power generations exist but one of the most commonly discussed is pressure-retarded osmosis (PRO).
A 2012 study on efficiency from Yale University concluded that the highest extractable work in constant-pressure PRO with a seawater draw solution and river water feed solution is 0.75 kWh/m3 (2.7 kJ/L) while the free energy of mixing is 0.81 kWh/m3 (2.9 kJ/L) — a thermodynamic extraction efficiency of 91.0%.
This method is being specifically studied by the Norwegian utility Statkraft, which has calculated that up to 2.85 GW would be available from this process in Norway.
[13] Statkraft has built the world's first prototype PRO power plant on the Oslo fjord which was opened by Princess Mette-Marit of Norway[14] on November 24, 2009.
At first, it did produce a minuscule 4 kilowatts – enough to heat a large electric kettle, but by 2015 the target was 25 megawatts – the same as a small wind farm.
[16] Statkraft found that with existing technology, the salt gradient was not high enough to be economic, which other studies have agreed on.
Standards and a complete understanding of all the ways salinity gradients can be utilized are important goals to strive for in order to make this clean energy source more viable in the future.
The primary power source originates from a thermal difference, as part of a thermodynamic heat engine cycle.
This water from the lower layer, the storage zone, is pumped out and the heat is used to produce energy, usually by turbine in an organic Rankine cycle.
A research team built an experimental system using boron nitride that produced much greater power than the Statkraft prototype.
It used an impermeable and electrically insulating membrane that was pierced by a single boron nitride nanotube with an external diameter of a few dozen nanometers.
[22] At the 2019 fall meeting of the Materials Research Society a team from Rutgers University reported creating a membrane that contained around 10 million BNNTs per cubic centimeter.
Each species of aquatic plant and animal is adapted to survive in either marine, brackish, or freshwater environments.
[26] According to the prevailing environmentalist opinions, the possibility of these negative effects should be considered by the operators of future large blue energy establishments.
Impingement and entrainment at intake structures are a concern due to large volumes of both river and sea water utilized in both PRO and RED schemes.
The Tethys database provides access to scientific literature and general information on the potential environmental effects of salinity gradient power.
