Osmotic power, also called salinity gradient power or blue energy, is the energy that comes from the difference in salt levels between seawater and river water. Two practical ways to use this energy are reverse electrodialysis (RED) and pressure retarded osmosis (PRO). Both methods use osmosis with special membranes. The main waste product from these processes is brackish water. This byproduct forms naturally when fresh water mixes with salt water.
In 1954, Pattle proposed that energy could be created when a river flows into the sea, due to the loss of osmotic pressure. However, it was not until the mid-1970s that a practical way to use this energy with selectively permeable membranes was described by Loeb.
The method of generating power through pressure retarded osmosis was developed by Sidney Loeb in 1973 at the Ben-Gurion University of the Negev, Beersheba, Israel. Loeb was inspired by observing the Jordan River flowing into the Dead Sea. He wanted to capture the energy from the mixing of these two water sources, which was previously wasted. In 1977, Loeb created a method to produce power using reverse electrodialysis with a heat engine.
These technologies have been tested in laboratories and are being improved for use in real-world applications. RED is being developed in the Netherlands, and PRO is being developed in Norway. The cost of membranes has been a challenge. A new, less expensive membrane made from a type of plastic modified with electricity has made commercial use possible. Other methods are also being studied, such as those using electric double-layer capacitor technology and those based on vapor pressure differences.
Basics of salinity gradient power
Salinity gradient power is a type of renewable energy that uses natural processes to create electricity. This method does not pollute the environment or release carbon dioxide (CO₂) emissions. However, some methods, like vapor pressure techniques, may release gases that contain CO₂ at low pressures. These gases can be reabsorbed, but the process requires extra energy. According to Jones and Finley in their article "Recent Development in Salinity Gradient Power," this energy source typically does not require fuel costs.
Salinity gradient energy works by using the difference in salt concentration between fresh water and seawater. This difference creates a pressure called osmotic pressure. When fresh water and salt water are separated by a special membrane, water moves from the side with less salt to the side with more salt. This movement happens because of the difference in pressure caused by the salt concentration. Solutions with higher salt levels have higher osmotic pressure.
One common method for generating salinity gradient power is called pressure-retarded osmosis (PRO). In PRO, seawater is pumped into a chamber where the pressure is lower than the osmotic pressure difference between fresh and salt water. Fresh water passes through a semipermeable membrane into the chamber, increasing its volume. As the pressure in the chamber rises, it turns a turbine to produce electricity. Braun explains this process by comparing two solutions: one with salt water (solution A) and one with fresh water (solution B). He notes that only water molecules can pass through the membrane, moving from solution B to solution A to balance the salt concentration. The pressure from this movement drives the turbine and generates electrical energy.
In some places, like the Netherlands, osmosis could be used to move fresh water from land into the sea. This process is currently done using electric pumps, but using osmosis might offer a more efficient alternative in the future.
Efficiency
A 2012 study from Yale University found that the maximum energy that can be produced using pressure retarded osmosis with seawater and river water is 0.75 kWh/m (2.7 kJ/L). The energy available from mixing these waters is 0.81 kWh/m (2.9 kJ/L). This means that 91.0% of the available energy can be used effectively.
Methods
Salinity gradient power is a type of energy that scientists are still studying, but it has already been used in some places. Most of these uses are experiments, but they have mostly worked well so far. Different companies have used this energy in various ways because there are many methods to capture it.
One way to use salinity gradient energy is called pressure-retarded osmosis (PRO). In this method, seawater is pumped into a chamber that has lower pressure than the difference between seawater and freshwater. Freshwater is also pumped into the same chamber through a special membrane, which increases the chamber’s volume and pressure. When the pressure differences are balanced, a turbine spins, creating energy. A company in Norway named Statkraft is studying this method. They estimate that up to 2.85 gigawatts of energy could be produced in Norway using this process. Statkraft built the first PRO power plant on the Oslo fjord in 2009. The plant aimed to produce enough electricity to light and heat a small town in five years. At first, it produced only 4 kilowatts of power—enough to heat a large electric kettle—but by 2015, the goal was to reach 25 megawatts, similar to a small wind farm. However, in 2014, Statkraft decided to stop the project because the salt difference was not strong enough to make the process cost-effective. Other studies have also found this issue. Higher salt differences are found in geothermal brines and desalination plant brines. A Danish company named SaltPower is now building its first commercial plant using high-salt brine. Some experts believe combining PRO with reverse osmosis might be more practical than using it alone.
Another method is reversed electrodialysis (RED), also called reverse dialysis. This method works like a salt battery. It uses alternating membranes that allow positive and negative ions to pass through, creating electricity from the difference in salt levels between seawater and freshwater. This idea was first described in the 1950s, but the technology is still in early development. Scientists agree that understanding how to use salinity gradients fully is important for making this energy source more practical in the future.
A third method is the capacitive method, developed by Doriano Brogioli. This method is new and has only been tested in labs so far. It uses electrodes that are charged in saltwater and then discharged in freshwater. During the charging step, the saltwater’s ions help neutralize the electrode’s charge, keeping the voltage low and making charging easier. When the electrodes are moved to freshwater, fewer ions are available to neutralize the charge, causing the voltage to rise. This allows the discharge step to produce more energy. This process works because the ions in saltwater create an electric double layer near the electrode, reducing the energy needed to charge it. When the electrodes are in freshwater, the lack of ions increases the voltage, making the discharge more powerful.
Unlike PRO and RED, these methods do not require membranes, so filtration is less important.
Similar to open-cycle ocean thermal energy conversion (OTEC), this method uses a large turbine to extract energy from the difference in salinity between two water sources. However, the turbine must operate at below atmospheric pressure, which is a challenge.
In some systems, osmotic power is used to help dehumidify air in a water-spray absorption refrigeration system. This process uses heat differences as its main energy source, part of a thermodynamic cycle.
At the Eddy Potash Mine in New Mexico, a technology called a salinity gradient solar pond (SGSP) is used to provide energy. This method uses sunlight absorbed by a saltwater pond to generate heat. The pond has three layers: an upper layer where heat rises, a middle layer that blocks heat from escaping, and a bottom layer where heat is stored. The middle layer is crucial because it prevents heat from moving upward. Water from the bottom layer is pumped out and used to produce energy, often through a turbine in an organic Rankine cycle.
In theory, a solar pond could also generate osmotic power if evaporation from sunlight creates a salt difference. This salt difference could then be used with methods like the capacitive method to produce energy.
A research team tested a new system using boron nitride nanotubes. They created a membrane with a single nanotube that separated saltwater and freshwater. The team measured the electric current passing through the membrane using electrodes in both fluids. Their results showed the device produced a current about 1,000 times greater than other methods. They claimed a 1-square-meter membrane could generate around 4 kilowatts of power and produce up to 30 megawatt-hours of energy per year.
At the 2019 fall meeting of the Materials Research Society, a team from Rutgers University reported creating a membrane containing about 10 million boron nitride nanotubes per cubic centimeter. Researchers at Pennsylvania State University are also studying similar technologies.
Possible negative environmental impact
Marine and river environments have clear differences in water quality, especially in salt levels, called salinity. Each type of aquatic plant and animal is adapted to live in either marine, brackish, or freshwater environments. Some species can survive in both, but they usually grow best in one specific type of water. The main waste product from salinity gradient technology is brackish water. If large amounts of brackish water are regularly released into nearby waters, it can change the salt levels. While some changes in salt levels are normal, especially where rivers meet the ocean, these changes become more significant when brackish waste water is added. Sudden and extreme changes in salt levels in an aquatic environment may lead to fewer plants and animals because they cannot survive sudden drops or increases in salt. Many environmental experts believe that the possible harm from these changes should be considered by those who operate large blue energy facilities in the future.
The impact of brackish water on ecosystems can be reduced by pumping it into the ocean and releasing it into the middle layer of water, away from the top and bottom ecosystems.
Using large amounts of river and sea water in PRO and RED systems can cause problems for organisms near intake structures. Permits for building intake structures must follow strict environmental rules. Desalination and power plants that use surface water often work with local, state, and federal agencies to get approval, which can take up to 18 months.
The Tethys database offers information from scientific studies and general details about the possible environmental effects of salinity gradient power.