Blue energy, also known as salinity gradient power, is a novel form of renewable energy derived from the difference in salt concentration between two bodies of water. This typically occurs where freshwater rivers meet the saltwater ocean in estuarine mixing zones. The immense contrast in salinity holds a significant amount of stored chemical potential energy.
The power generated from this process is predictable and continuous, operating independently of weather conditions like wind or sunlight. This non-intermittent characteristic makes it a valuable candidate for providing baseload power, which is the minimum electric power needed for the electrical grid at any given time.
The Fundamental Principle of Salinity Gradients
The existence of blue energy is rooted in the natural drive of water to equalize its salt concentration, a process known as osmosis. When water with low salt concentration (river water) is separated from water with high salt concentration (seawater) by a semi-permeable membrane, osmotic pressure is created. This membrane allows water molecules to pass through but blocks the dissolved salt ions.
This pressure drives the movement of water molecules from the dilute (freshwater) side to the concentrated (saltwater) side. The chemical potential energy stored in this salinity difference is what scientists seek to convert into usable electricity. If allowed to mix naturally, this energy is dissipated as heat, but technology aims to capture the work performed by the water.
The thermodynamic concept behind this is the Gibbs free energy difference, representing the maximum work that can be extracted from the mixing process. For typical river water and seawater mixing, the theoretical osmotic pressure difference is substantial, often equivalent to a hydraulic head of a 270-meter waterfall.
Converting Salinity Gradients into Electricity
Harnessing the chemical potential energy requires specialized membrane technologies that manage the controlled mixing of the two water streams. The two primary methods developed are Pressure Retarded Osmosis (PRO) and Reverse Electrodialysis (RED).
Pressure Retarded Osmosis (PRO)
PRO works by leveraging osmotic pressure to generate mechanical energy. In a PRO system, freshwater flows across a semi-permeable membrane into a chamber containing pressurized saltwater. The water naturally migrates to the saltwater side, driven by osmotic pressure greater than the applied hydrostatic pressure in the chamber.
This influx increases the volume and pressure of the saltwater stream, effectively diluting it. This pressurized, diluted stream is then directed through a hydro-turbine, which spins a generator to produce electricity. For a typical seawater-freshwater gradient, the saltwater side is pressurized to about 12 to 14 bars, approximately half of the total osmotic pressure difference.
Reverse Electrodialysis (RED)
RED generates an electrical current directly from the flow of ions rather than water. A RED system uses a stack of alternating anion exchange membranes and cation exchange membranes placed between two electrodes. These membranes are selectively permeable, allowing only positive ions (cations) or negative ions (anions) to pass through.
The freshwater and saltwater flow through alternating compartments in the stack. Driven by the salinity gradient, positive ions move through the cation exchange membranes, and negative ions move through the anion exchange membranes, flowing toward the freshwater compartments. This selective movement of charged ions creates an electrical voltage across each membrane pair, and the sum of these voltages produces a direct current between the end electrodes.
Current Status and Real-World Applications
Blue energy technology remains largely in the development and pilot phase, with commercial viability hinging on advancements in membrane performance. The Norwegian utility company Statkraft pioneered the world’s first PRO osmotic power prototype, which opened in Tofte, Norway, in 2009. This small-scale plant produced 2 to 4 kilowatts of power, serving as a testbed for the technology.
The primary obstacle to widespread commercialization is the efficiency and cost of the membranes, which account for a substantial part of the plant’s expense. Statkraft determined that for osmotic power to be economically competitive, the membrane power density needed to increase from 1 watt per square meter to at least 5 watts per square meter. This technical hurdle led Statkraft to discontinue its work on large-scale PRO in 2013.
Despite the challenges, the global potential for blue energy is estimated to be enormous, with projections suggesting a technical potential of up to 2.6 terawatts. This capacity is comparable to the world’s total electricity production capacity. Research continues globally, focusing on RED technology development in places like the Netherlands. These efforts aim to improve the conductivity of ion exchange membranes and optimize system design, pushing the technology closer to a practical reality.