The world’s oceans represent a vast, untapped source of renewable energy. Beyond the power of waves and tides, the saltiness of seawater holds energy potential. This field, known as salinity gradient power or blue energy, generates electricity from the meeting of freshwater and saltwater. While the technology is in its early stages, it offers a consistent and reliable power source.
The Science of Saltwater and Electricity
The ability to generate electricity from saltwater stems from its nature as an electrolyte solution. The salt in seawater, primarily sodium chloride (NaCl), separates into positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-) when dissolved. These free-floating charged particles allow the water to conduct an electrical current. While different from the flow of electrons in a metal wire, the movement of these ions creates a current. The chemical potential stored in the salinity difference between saltwater and freshwater is known as osmotic pressure. Solutions with a higher salt concentration have a higher osmotic pressure, and this pressure difference is the resource that saltwater energy technologies are designed to harness.
Methods of Generating Power from Saltwater
Two main techniques have been developed to capture energy from the salinity gradient: Pressure Retarded Osmosis (PRO) and Reverse Electrodialysis (RED). Both methods rely on special membranes to control the movement of water and ions.
Pressure Retarded Osmosis works by separating freshwater and saltwater with a semi-permeable membrane that allows water molecules to pass through but blocks salt ions. The freshwater is naturally drawn across the membrane into the saltwater compartment through osmosis, a process that increases the pressure on the saltwater side. In a PRO facility, seawater is pumped into a chamber at a pressure that is lower than the osmotic pressure difference. As freshwater flows across the membrane, it increases the volume and pressure in the chamber, and this pressurized water is then used to spin a turbine and generate electricity.
Reverse Electrodialysis operates differently, using the flow of ions rather than water to create a current. A RED system consists of a stack of alternating cation-exchange and anion-exchange membranes. When saltwater and freshwater flow through alternating channels in this stack, the membranes separate the sodium and chloride ions. The positively charged sodium ions are pulled toward the cathode and the negatively charged chloride ions toward the anode, creating an electrical voltage over each membrane. The sum of these small voltages across the entire stack can produce a substantial electrical current.
A separate application is found in saltwater batteries, which focus on energy storage rather than direct generation. These batteries use saltwater as the electrolyte. In one design, an entropy-mixed battery, electrodes release sodium ions into a tank with wastewater effluent to create a current. When freshwater is replaced with saltwater, the ions flow back to the electrodes, reversing the current. This process allows the battery to be charged and discharged by switching water flows.
Current and Potential Applications
The application of saltwater power is in its early stages, with most projects operating on a pilot or experimental scale. One of the first blue energy plants opened in Norway in 2009, based on the Pressure Retarded Osmosis method. This project by the utility company Statkraft confirmed the technology’s viability, calculating that Norway alone had a potential of 2.85 GW from this process.
In the Netherlands, research and development have focused on Reverse Electrodialysis. These pilot plants aim to optimize membrane performance and energy efficiency. Such locations could provide consistent, baseload power that does not depend on weather conditions like solar or wind energy.
On a smaller scale, saltwater is being used to power remote devices where other energy sources are impractical. This includes oceanographic sensors, monitoring buoys, and autonomous underwater vehicles. Researchers have also developed nanofluidic devices that generate electricity from the ionic flow at the interface between saltwater and freshwater. These devices use a phenomenon called “Coulomb drag,” where moving ions pull charges within a tiny semiconductor, creating a voltage and current. In the future, this technology could be integrated with coastal wastewater treatment plants, using their effluent as the freshwater source to generate power.
Environmental Considerations and Technical Hurdles
As a carbon-free energy source, saltwater power offers environmental benefits over fossil fuels. The process is driven by the natural mixing of freshwater and saltwater and does not produce greenhouse gas emissions. The main byproduct is brackish water, which is simply a mixture of the two water sources and is naturally created where rivers meet the sea.
However, the technology is not without its environmental and technical challenges. The construction of osmotic power plants requires infrastructure such as tanks and pipelines, which can impact local river mouth ecosystems. A concern is the large volume of water required for the process, which could lead to the impingement and entrainment of marine organisms at intake points. The discharge of large quantities of brackish water could also alter local salinity levels, potentially affecting aquatic life in the immediate vicinity.
A primary technical hurdle has been the development of efficient and durable membranes. These membranes are the core component of both PRO and RED systems, but they are expensive and susceptible to fouling, where microorganisms and sediment clog their pores, reducing performance. The corrosive nature of saltwater also poses a challenge, requiring the use of expensive, corrosion-resistant materials for all equipment, which further drives up the cost of implementation. Overcoming these cost and durability issues is the primary focus of ongoing research and development.