Ocean energy is renewable electricity generated from the natural movement and thermal properties of ocean water. It includes power from waves, tides, ocean currents, temperature differences between deep and surface water, and even the mixing of salt and fresh water. The world’s oceans hold enough energy to theoretically produce between 45,000 and 130,000 terawatt-hours of electricity per year, more than double current global electricity demand. Despite that enormous potential, ocean energy remains one of the least developed renewable sources, with roughly 513 megawatts of total installed capacity worldwide as of 2024.
Wave Energy
Waves carry kinetic energy across the ocean surface, and several device designs exist to capture it. Point absorbers are buoy-like structures that bob up and down with passing waves, converting that vertical motion into electricity through hydraulic or mechanical systems. Attenuators take a different approach: these are long, flexible floating devices oriented parallel to the wave direction. As waves pass along their length, the flexing joints drive generators inside.
A third design, the oscillating water column, uses waves to push air back and forth through a turbine housed in a partially submerged chamber. Each design suits different wave climates and water depths. Recent cost analyses along Spain’s Galician coast found that the best-performing wave devices could produce electricity for around 50 to 77 euros per megawatt-hour, competitive with Spanish retail electricity prices. Even under conservative construction cost estimates, production costs stayed around 140 euros per megawatt-hour, suggesting wave energy is approaching economic viability in favorable locations.
Tidal Energy
Tidal power comes in two main varieties, and they work in fundamentally different ways.
Tidal stream systems place underwater turbines directly in fast-moving tidal currents. These currents are strongest where land narrows a channel, such as in straits or inlets, accelerating the water flow. The turbines look and work much like underwater wind turbines, and cables on the seafloor carry the electricity to shore.
Tidal range systems rely on the height difference between high and low tides. Tidal barrages are dam-like structures built across bays, rivers, or estuaries. As the tide comes in, water fills a basin behind the barrage. When the tide goes out, the trapped water flows back through turbines, generating electricity in both directions. Tidal lagoons work similarly but use man-made retaining walls to partially enclose a volume of tidal water rather than blocking an entire waterway. Both designs need locations with large tidal ranges to be effective.
The largest tidal power plant in the world is the Sihwa Lake Tidal Power Station in South Korea, with a capacity of 254 megawatts. The second largest, and the oldest still operating, is the La Rance plant in France at 240 megawatts. Smaller facilities operate in Canada, China, and Russia.
Ocean Thermal Energy Conversion
The ocean’s surface absorbs heat from the sun, while deep water stays cold year-round. Ocean thermal energy conversion, or OTEC, exploits this temperature gap. The process requires at least a 20°C (36°F) difference between warm surface water and cold deep water, which limits it mostly to tropical regions near the equator.
In a closed-cycle system, warm surface water heats a special fluid with a low boiling point, turning it into vapor. That vapor spins a turbine to generate electricity, then cold water pumped up from the deep ocean condenses the vapor back into liquid so the cycle can repeat. Open-cycle systems use the seawater itself as the working fluid, and the condensed water that results is desalinated, giving OTEC a secondary benefit: it can produce fresh drinking water alongside electricity.
Salinity Gradient Energy
Wherever a river meets the sea, energy is released as fresh water and salt water mix. Salinity gradient power captures this energy using specialized membranes. Two main technologies exist. Pressure-retarded osmosis uses a membrane that allows water molecules to pass through but blocks salt. Fresh water naturally flows toward the saltier side, building up pressure that drives a turbine. Reverse electrodialysis works differently: it uses membranes that allow charged salt ions to pass through, creating an electrical current directly.
Neither technology has reached commercial scale. The membranes remain expensive and degrade over time, but the resource is significant anywhere rivers discharge into oceans.
Environmental Considerations
Ocean energy devices interact with marine ecosystems in ways that researchers are still studying. Underwater turbines and support structures alter local sound levels, which can affect fish, marine mammals, and other species that rely on sound for navigation, communication, and finding mates. Cables running along the seafloor introduce electromagnetic fields that may interfere with how some fish detect predators or locate each other. Structures placed in the water can also change local water flow patterns, potentially shifting the distribution and behavior of marine life across a region.
There are some unexpected benefits, though. Hard surfaces in the water tend to create a “reef effect,” attracting marine organisms that cluster around structures. This can increase local biodiversity even as other impacts raise concerns.
Mitigation strategies in existing projects include timing construction to avoid sensitive periods for endangered species, using bubble curtains to dampen underwater noise, enforcing vessel speed limits to reduce the risk of striking marine mammals, and requiring acoustic monitoring during installation. These measures are evolving as more projects enter the water and generate real-world data.
Why Ocean Energy Is Still Small
At 513 megawatts of global capacity, ocean energy is a tiny fraction of what solar and wind produce. The ocean is a harsh environment for machinery: saltwater corrodes equipment, storms damage structures, and maintenance underwater is far more difficult and expensive than on land. Construction costs remain high, and most technologies are still in the demonstration or early commercial phase rather than mass production.
Cost is the central barrier. While the best wave energy sites are approaching price competitiveness with grid electricity, most ocean energy projects still produce power at higher costs than onshore wind or solar. Scaling up, as happened with those technologies over the past two decades, would drive costs down through manufacturing improvements and operational experience.
Government support is increasing, with targeted funding and streamlined permitting processes helping the sector move toward larger deployments. The physical resource is enormous and highly predictable (tides follow exact schedules, and wave patterns are forecastable days in advance), which makes ocean energy a potentially valuable complement to less predictable renewables like wind and solar.