Marine Energy

Marine energy, also called ocean energy, covers technologies that draw power from the movement, temperature or salinity of seawater: tides, tidal currents, waves, thermal gradients and the mixing of fresh and salt water. On paper the resource is enormous. The IPCC’s Special Report on Renewable Energy Sources (2011) put the global theoretical potential at several thousand to tens of thousands of TWh per year for wave and thermal energy alone, and tides are among the most predictable energy flows on Earth. In practice the sector is tiny. About 0.5 GW of capacity runs worldwide, producing on the order of 1 TWh a year, less than 0.01% of world electricity, and two tidal barrages account for nearly all of it. Below we go through each technology, why progress has been slow, and where costs and deployment stand.

How marine energy works

Every marine technology faces the same engineering problem: the sea is a hostile place. Equipment has to withstand extreme wave loads, saltwater corrosion, biofouling and storms, and installation and maintenance depend on short weather windows and specialised vessels. The physics behind each technology is different, though.

Tidal range (barrages and lagoons)

Tidal range power is basically low-head hydropower. A barrage across an estuary, or a lagoon wall out at sea, traps water at high tide. Once the sea outside has dropped by several metres, the water flows back through low-head bulb turbines. Some schemes also generate on the incoming tide (two-way generation) or pump at high tide to add head. The energy available per tide rises with the square of the tidal range, so only sites with very large tides, generally above about 5 m, are worth developing.

Real plants recover only part of the theoretical figure, typically a third or less. Turbines can’t run over the full range of head, and operation has to respect navigation and ecological limits. The tidal energy calculator estimates output for both approaches.

Tidal stream

Tidal stream turbines sit in fast currents in straits and off headlands, where peak flow regularly tops 2–2.5 m/s. Seawater is about 840 times denser than air, so a tidal turbine needs a much smaller rotor than a wind turbine to produce the same power. A 2.5 m/s current carries roughly the power density of a wind of about 24 m/s. The cube law and the Betz limit apply just as they do for wind power. Designs include horizontal-axis turbines fixed to the seabed, floating platforms with turbines hung beneath them (such as Orbital Marine Power’s 2 MW O2), and kite-like devices that fly through the current.

Wave energy

Wave energy converters capture the motion of surface waves, which get their energy from wind blowing across long stretches of ocean. The best resources, often 40–70 kW per metre of wave crest, lie on west-facing coasts at mid to high latitudes: Scotland, Ireland, Portugal, Chile, southern Australia and the US Pacific Northwest. Wave power density is proportional to the square of wave height and to the wave period. Wind power settled on the three-bladed turbine; wave energy has not settled on any one design:

• Point absorbers: buoys whose up-and-down motion drives a hydraulic or linear electric generator.
• Oscillating water columns (OWC): a partly submerged chamber where waves compress and decompress air, driving an air turbine. The Mutriku breakwater plant in Spain (about 0.3 MW) has run since 2011.
• Attenuators: long, jointed floating structures that flex as waves pass, as in the Pelamis device.
• Oscillating wave surge converters: hinged flaps on the seabed near shore that waves push back and forth, as in Aquamarine’s Oyster.
• Overtopping devices: waves spill into a raised reservoir and drain back through low-head turbines.

Ocean thermal energy conversion

OTEC runs a heat engine between warm tropical surface water and cold water pumped up from about 1,000 m. Closed-cycle systems boil a working fluid such as ammonia. Open-cycle systems flash warm seawater into low-pressure steam and can make desalinated water on the side. With a temperature difference of only about 20–24 °C, the Carnot efficiency is around 7% and net efficiency after pumping roughly 2–3%, so plants need very large water flows and cold-water pipes. Demonstration plants of around 100 kW run in Hawaii (Makai Ocean Engineering) and Okinawa, Japan. OTEC appeals mainly to tropical islands that depend on expensive diesel generation.

Salinity gradient

Where river water meets the sea, the difference in salt concentration can be turned into electricity. Pressure-retarded osmosis lets fresh water flow through membranes into pressurised seawater; reverse electrodialysis uses ion-selective membranes to create a voltage. Statkraft ran a small osmotic prototype at Tofte, Norway, from 2009 to 2013 and then concluded the membranes performed too poorly for commercial use. The technology is still at laboratory and pilot scale.

Major projects

Bigger tidal range proposals in the UK, including the Severn Barrage (around 8.6 GW) and the Swansea Bay Tidal Lagoon (320 MW), have been turned down again and again on cost and environmental grounds.

Efficiency and capacity factor

Tidal stream turbines usually reach a power coefficient of 0.35–0.45 and a capacity factor of about 25–45%, depending on how the rated power is set against peak current speed. MeyGen’s turbines have reported availability above 90% in good years. Tidal barrages run at around 20–30% because they generate during only part of each tidal cycle; La Rance gets roughly 500 GWh a year from 240 MW, about 24%. Mature wave devices are expected to reach 20–35% at good sites, but few have run long enough to give reliable figures. Because ocean temperatures are stable, OTEC could in principle run as baseload at capacity factors above 90%.

Tidal energy’s big strength is predictability, not efficiency. Output can be forecast years ahead, and tides peak at different times along a coastline, so sites spread out geographically can smooth the combined output. It is still variable renewable energy, but without weather-driven forecast errors.

Costs

Marine energy is nowhere near cost-competitive with established renewables, mostly because too little has been built to drive the learning effects that brought down wind and solar costs. Published LCOE estimates vary widely:

• Tidal stream: roughly EUR 0.20–0.45/kWh for today’s pilot arrays, according to IRENA’s Innovation Outlook: Ocean Energy Technologies (2020) and Ocean Energy Europe. The UK’s Contracts for Difference auctions gave tidal stream projects strike prices of about £178/MWh in Allocation Round 4 (2022) and around £170–200/MWh in later rounds (all in 2012 prices), more than three times offshore wind in the same rounds.
• Wave energy: higher again, commonly put at EUR 0.30–0.55/kWh or more for early arrays, with large uncertainty.
• Tidal range: barrages and lagoons have very high upfront CAPEX, but civil works last 100 years and running costs are low. La Rance paid off its costs long ago; new lagoon proposals have had trouble showing a competitive LCOE at commercial discount rates.
• OTEC: estimates for commercial plants of 10–100 MW range very widely, often USD 0.15–0.40/kWh, and no commercial-scale plant exists to check them.

The European Commission’s SET Plan targets tidal stream at EUR 0.15/kWh by 2025 and EUR 0.10/kWh by 2030, and wave at EUR 0.20/kWh by 2025 and EUR 0.15/kWh by 2030. Hitting those numbers depends on cumulative deployment growing from tens to hundreds of megawatts. The LCOE calculator lets you compare such costs with other options.

Land, water and materials

Marine devices take up sea space, not land. Tidal stream and wave arrays need exclusion zones that can clash with shipping, fishing and recreation, though the area per MW is similar to offshore wind. Barrages and lagoons are a different case: they permanently change large intertidal areas and estuaries. The devices are built mostly from steel, concrete and composites, with copper subsea cables, and corrosion protection and anti-fouling coatings bring chemical issues of their own. No freshwater is consumed, and open-cycle OTEC plants can actually produce it.

Lifecycle CO2 emissions

Life-cycle assessments of marine energy rest on a handful of installations, mostly prototypes, so treat the numbers as indicative. The IPCC AR5 WGIII Annex III (2014) gives a range of roughly 5.6–28 gCO2e/kWh for ocean energy (tidal and wave), with a median of about 17 gCO2e/kWh, similar to wind and solar. Steel-heavy prototypes and low capacity factors put early devices at the top of the range; larger, better-designed arrays should move toward the bottom. For tidal barrages, as for hydro dams, the embodied carbon of concrete dominates, spread over a very long life. See lifecycle emissions for how these figures are worked out.

Safety and environmental impacts

Marine energy has a good record on human safety. The risks are mostly the occupational ones of working offshore. Environmental questions depend on the site:

• Barrages change tidal flows, sediment movement, water quality and the intertidal mudflats where birds feed. After construction, La Rance’s estuary took about a decade to settle into a new ecological balance. Fish that pass through low-head turbines get injured, and concern about fish deaths helped close Annapolis Royal.
• Tidal stream turbines raise concerns about collisions with marine mammals, diving birds and fish. Monitoring at MeyGen and in the US and Canada has recorded very few encounters so far, and blade tips move much more slowly than ship propellers, but the data are still thin.
• Underwater noise during installation (pile driving above all) and electromagnetic fields from cables have been studied and are generally judged minor for small arrays.
• OTEC discharges large volumes of cold, nutrient-rich deep water, which could disturb local ecosystems if not managed.

Outlook

Marine energy is roughly where offshore wind was in the early 2000s: proven in principle, expensive, and dependent on public support. Among the newer technologies, tidal stream is furthest along, led by the UK, France, Canada and China. Dedicated revenue support has given the UK a pipeline of over 100 MW of contracted tidal stream projects, and French projects in the Raz Blanchard hope to follow. Wave energy is still pre-commercial after several high-profile company failures; programmes such as Wave Energy Scotland and the US Department of Energy’s PacWave test site are trying to steer designs toward a common approach. Tidal range has huge potential at a handful of sites but faces tough environmental and financing obstacles. OTEC and salinity gradient remain niche.

The long-term case rests on predictability, on output that peaks at different times from wind and solar, and on the value of local power for islands and remote coastal communities. Ocean Energy Europe and the IEA’s Ocean Energy Systems programme see tens of gigawatts in Europe by 2050 if costs fall as projected, which would still be small next to wind and solar. To see how marine energy stacks up against other renewables, try the technology comparison, offshore wind and the world energy map.