Tidal and Wave Energy

The Physics of Ocean Energy

Tides are caused primarily by the gravitational attraction between the Earth and the Moon, with a smaller contribution from the Sun. The Moon's gravity creates a bulge of water on the side of Earth facing it and a corresponding bulge on the opposite side due to centrifugal effects of the Earth-Moon orbital system. As Earth rotates through these tidal bulges roughly every 12.4 hours, coastal areas experience two high tides and two low tides per day (semidiurnal tides in most locations). The height difference between high and low tide, called the tidal range, varies from less than one meter in open ocean to over 16 meters in the Bay of Fundy, Canada, where the funnel-shaped coastline amplifies the tidal wave.

The energy available in tides is proportional to the square of the tidal range and the area of the tidal basin. Sites with tidal ranges exceeding 5 meters, large water volumes, and narrow channels that concentrate tidal flow are most suitable for electricity generation. The total global tidal energy resource is estimated at approximately 3 terawatts, though only a fraction of this is technically and economically extractable. Tidal currents in straits and channels can reach speeds of 3 to 5 meters per second during peak flow, carrying enormous kinetic energy that tidal stream turbines can capture.

Waves are generated by wind blowing across the ocean surface, transferring kinetic energy into circular orbital motions of water particles that propagate as surface waves. Wave energy depends on wave height, wavelength, and period, with more energetic waves produced by stronger winds blowing over longer distances (fetch) for longer durations. The most energetic wave resources are found on western coastlines at temperate latitudes (40 to 60 degrees) where prevailing westerly winds cross vast ocean expanses. Wave power density, measured in kilowatts per meter of wave front, ranges from 10 to 30 kW/m in moderate locations to 60 to 100 kW/m in the most energetic sites off Scotland, Norway, Chile, and southern Australia.

Tidal Energy Technologies

Tidal barrage systems dam an estuary or inlet, trapping water at high tide and releasing it through turbines as the tide falls. The La Rance tidal barrage in France, operational since 1966, was the world's first large-scale tidal power plant, generating 240 MW with 24 bulb turbines across an 800-meter barrage. The Sihwa Lake tidal power station in South Korea (254 MW, operational since 2011) is currently the world's largest. Barrages produce predictable, dispatchable electricity at high capacity factors (25 to 35%), but their large-scale damming of estuaries raises significant ecological concerns including altered sediment transport, changes to intertidal habitat, and disruption of fish migration patterns.

Tidal stream turbines, sometimes called underwater wind turbines, extract kinetic energy from flowing tidal currents without impounding water. These devices are installed on the seabed in channels, straits, and headlands where tidal currents are strongest. Horizontal-axis designs resembling underwater wind turbines are the most common configuration, with rotors of 15 to 25 meters in diameter turning slowly (10 to 15 RPM) in the dense water flow (seawater is roughly 800 times denser than air, so much smaller rotors capture equivalent energy). MeyGen in Scotland's Pentland Firth is the world's largest tidal stream array, with 6 MW of operational capacity and plans for 398 MW at full buildout.

Tidal lagoon designs create an artificial enclosed body of water in the nearshore environment, separate from the natural estuary, mitigating the ecological impacts of full barrages while still generating power from the tidal range. The proposed Swansea Bay Tidal Lagoon in Wales, though ultimately not approved for funding, spurred interest in artificial lagoon concepts that could generate power on both the incoming and outgoing tides with minimal ecological disruption. Dynamic tidal power, a more speculative concept, envisions very long dams (30 to 60 kilometers) extending from the coast that exploit the phase difference between tidal waves traveling along the dam's length.

Wave Energy Technologies

Wave energy converter (WEC) designs number in the hundreds, reflecting the challenge of efficiently capturing energy from the complex, multi-directional, and variable motion of ocean waves. Point absorbers are floating buoys that bob up and down with passing waves, driving a linear generator or hydraulic pump through their vertical motion. Oscillating water columns (OWCs) capture waves in a partially submerged chamber, where the rising and falling water column alternately compresses and decompresses an air pocket that drives a Wells turbine or other bidirectional air turbine. Overtopping devices channel waves up a ramp into a raised reservoir, then release the water through low-head turbines as it returns to sea level, similar to a miniature hydroelectric dam.

Attenuator devices, long floating structures oriented parallel to the wave direction, flex at hinged joints as waves pass along their length, and this flexing motion drives hydraulic cylinders that pump fluid to power generators. Pelamis Wave Power developed the most prominent attenuator design, tested at the European Marine Energy Centre (EMEC) in Orkney, Scotland, before the company ceased operations in 2014 due to funding challenges. Oscillating wave surge converters are bottom-mounted flaps that swing back and forth as waves pass overhead, driving hydraulic systems, with the Oyster device developed by Aquamarine Power being the best-known example. Submerged pressure differential devices sit on the seabed and generate power from the oscillating pressure changes caused by passing waves above.

The primary challenge for wave energy is reducing the cost of electricity to competitive levels while ensuring devices survive the harsh ocean environment for 20 or more years. The marine environment subjects devices to corrosion from saltwater, biofouling from marine organisms, extreme storm loads (wave forces during severe storms can exceed operational loads by 10 to 100 times), and the logistical difficulty and expense of installation and maintenance in offshore locations. Current wave energy costs are estimated at $0.20 to $0.50/kWh, far above grid parity, though proponents argue that costs will decline along learning curves similar to those of wind and solar if deployment scale increases.

Current Status and Future Potential

Global installed ocean energy capacity remains small, roughly 530 MW, dominated by the La Rance and Sihwa Lake tidal barrages. The pipeline of tidal stream and wave projects is growing, with significant activity in the UK, France, Canada, South Korea, and China. The European Marine Energy Centre in Orkney, Scotland, has served as the primary testing ground for ocean energy devices, with over 30 different tidal and wave prototypes deployed and tested since 2003. Canada's Bay of Fundy, home to the world's highest tides, hosts the Fundy Ocean Research Centre for Energy (FORCE) where several tidal stream devices are undergoing trials.

The theoretical global wave energy resource exceeds 2 terawatts, and extractable tidal energy is estimated at approximately 100 GW, representing a significant potential contribution to global electricity supply. Ocean energy's greatest value may be its complementarity with other renewables: tidal energy is highly predictable and can provide generation during calm, cloudy periods when solar and wind output is low. Wave energy, while more variable than tidal, correlates with wind patterns but with a time lag (waves continue after wind dies), providing generation that partially fills gaps in wind power output.

Cost reduction pathways for ocean energy mirror the early development phases of wind and solar: standardized designs, automated manufacturing, array-scale deployment (multiple devices sharing infrastructure), improved materials and coatings to extend maintenance intervals, and shared installation vessels and equipment. Industry projections suggest tidal stream energy could reach $0.10 to $0.15/kWh at scale (comparable to current offshore wind costs), while wave energy will likely require longer development timelines and more radical innovation in device design and survivability to reach competitive costs. Government support through feed-in tariffs, contracts for difference, and research funding remains essential for carrying ocean energy technologies through this early commercial phase.