Tidal & Marine Energy
Why Tides Are Different
Unlike solar and wind, tidal energy is fully predictable. Tides result from the gravitational interaction between Earth, the Moon, and the Sun. The periodic forcing is so well-understood that tide tables can be — and have been — calculated centuries into the future with high accuracy.
This predictability is a profound economic advantage. A tidal power plant operator knows exactly when it will produce and at what rate, allowing power system operators to plan without the forecast uncertainty inherent in wind and solar. Tidal generation follows a known 12.4-hour semidiurnal cycle (or 24.8-hour diurnal in some locations), with spring-neap modulation over ~14.8-day cycles.
The global tidal resource is vast — theoretically ~900 GW of economically accessible tidal power — but concentrated in specific locations with large tidal ranges or strong tidal currents. Coastal geometry amplifies tides: the Bay of Fundy (Canada) has tidal ranges up to 16.3 m, the largest in the world.
Tidal Physics
Two fundamental mechanisms drive tidal energy extraction:
1. Tidal range (potential energy): The height difference between high and low tide creates a hydraulic head that can drive turbines through a barrage. Power potential scales with the square of tidal range: P ∝ A × R², where A is basin area and R is range. This is why a doubling of range quadruples power potential.
2. Tidal flow (kinetic energy): Where tidal currents are forced through narrow channels, high-velocity flows (3–5 m/s) contain significant kinetic energy. Power scales with velocity cubed (P ∝ v³), so a 10% increase in current speed increases power by 33%. Optimal sites have peak currents exceeding 2.5 m/s.
The interplay of basin resonance, coastline geometry, and bathymetry creates globally rare but highly concentrated tidal energy hotspots — the Pentland Firth (Scotland), Bay of Fundy (Canada), Alderney Race (Channel Islands), and Severn Estuary (UK).
Marine Energy Resource by Type
Source: Ocean Energy Systems (OES) Annual Report 2023; IRENA Untapped Potential of Ocean Energy 2023.
Historical Tide Mills
Tidal power extraction is ancient — horizontal tide mills have been documented in tidal estuaries from Ireland to the Basque Coast since the 11th century. A mill built at Woodbridge, England in 1170 AD still exists. These mills impounded tidal water at high tide behind a dam, then released it through millstones as the tide ebbed.
This same principle — creating a head difference to drive a turbine — underpins modern tidal barrages, making tidal energy one of the oldest renewable energy concepts now being reimagined with 21st-century engineering.
Ocean Energy Spectrum
Marine renewable energy encompasses several distinct resources:
- Tidal range — potential energy from tidal height differences (barrages, lagoons)
- Tidal stream — kinetic energy of tidal currents (underwater turbines)
- Wave energy — kinetic + potential energy in ocean surface waves
- Ocean thermal energy conversion (OTEC) — temperature difference between surface and deep water
- Salinity gradient / osmotic energy — chemical potential at river-sea boundaries
- Ocean current — persistent unidirectional currents (Gulf Stream, Kuroshio)
La Rance Tidal Power Station (France)
Built across the Rance estuary in Brittany, La Rance opened in 1966 and for 40 years was the world's largest tidal power station. It remains the most successful tidal barrage ever built:
- Capacity: 240 MW (24 × 10 MW units)
- Annual generation: ~540 GWh
- Tidal range: 8.2 m mean, 13.5 m maximum
- Operating mode: Both ebb and flood generation
- Capacity factor: ~26% (4,000+ hours/year)
- Cost (original): Equivalent ~$2B in 2024 terms; fully amortized and highly profitable today
La Rance demonstrated that tidal barrages can be reliable, long-lived (>50 years), and economically attractive once construction costs are recovered. Its turbines were reversed to generate on both flood and ebb tides — an operational innovation that improved capacity factor.
Sihwa Lake Tidal Power Station (South Korea)
Opened in 2011, Sihwa overtook La Rance as the world's largest tidal power station by capacity:
- Capacity: 254 MW (10 × 25.4 MW units)
- Annual generation: ~552 GWh
- Tidal range: 7.8 m mean
- Operating mode: Ebb generation only (flood sluices fill lagoon)
- Special feature: Built into an existing seawall, reducing construction cost vs. greenfield barrage
Sihwa's construction cost was approximately $355M, significantly below La Rance per MW by leveraging the existing seawall. It supplies electricity to ~500,000 households and has dramatically improved water quality in the formerly polluted Sihwa Lake by increasing tidal flushing.
The Severn Barrage — Proposed & Contested
The Severn Estuary (UK/Wales border) has a tidal range of up to 14.5 m — second only to Bay of Fundy globally. Multiple studies since the 1970s have examined a barrage across the estuary. The most recent major study (2010 UK government review) assessed a ~8.6 GW Cardiff-Weston barrage costing ~£30 billion capable of generating ~14 TWh/year (5% of UK electricity).
The project has never been built due to:
- Enormous upfront capital (30–40-year payback at realistic electricity prices)
- Severe environmental concerns: loss of ~50,000 ha of intertidal mudflats critical for migratory birds (EU Habitats Directive)
- Navigation and flood risk changes
- Competing alternatives: Swansea Bay tidal lagoon proposal, offshore wind
Tidal Lagoons — A Gentler Alternative
Instead of blocking an entire estuary, tidal lagoons impound a defined offshore area using a U-shaped breakwater. Water flows in and out through turbines. Lagoons avoid many barrage environmental impacts (they don't block fish migration or dramatically alter estuarine hydrodynamics) while still capturing tidal range energy.
The Swansea Bay Tidal Lagoon (Wales) proposal — a 320 MW, 11.5 km breakwater lagoon — was granted planning permission in 2015 but rejected for subsidy in 2018 on cost grounds (~£89.90/MWh strike price vs. offshore wind at £57.50/MWh). Proponents argue costs would fall dramatically at larger scale. The developer Tidal Lagoon Power filed for administration in 2018 but the technology concept remains live.
Barrage Economics & Environmental Trade-offs
Indicative cost ranges. Source: Carbon Trust Marine Energy 2023; IEA Ocean Energy Report 2023.
How Tidal Stream Works
Tidal stream turbines extract kinetic energy from moving tidal currents — conceptually identical to wind turbines, but operating in water ~800× denser than air. This density advantage means a much smaller rotor can generate the same power as a large wind turbine. A 20 m diameter tidal turbine in a 3 m/s current generates comparable power to a ~100 m wind turbine in a 10 m/s wind.
The theoretical maximum efficiency of any turbine in uniform flow is ~59.3% (Betz limit). Modern tidal turbines achieve 40–50% in practice. Turbines are typically mounted on monopile foundations or gravity bases, with rotors 15–30 m in diameter.
Tidal Stream Device Types
| Type | Description | Status | Developer |
|---|---|---|---|
| Horizontal Axis | 3-blade rotor, like subsea wind turbine | Commercial | Orbital Marine, Atlantis, Nova Innovation |
| Vertical Axis (Darrieus) | Blades rotate around vertical shaft | Demo | DesignPro, Tocardo |
| Ducted / Shrouded | Cowling concentrates flow, boosts output | Demo/Pre-commercial | OpenHydro (defunct), RivGen |
| Oscillating Hydrofoil | Wing pitches up/down, drives hydraulic pump | Demo | Pulse Tidal |
| Floating Platform | Turbines hang beneath surface vessel | Commercial | Orbital Marine Power (O2) |
MeyGen — World's Largest Tidal Stream Array
The MeyGen project in the Pentland Firth, Scotland (Kyle of Tongue area) is the world's most advanced commercial tidal stream array. Phase 1A (4 turbines, 6 MW) began generating in 2016–17, becoming the world's first utility-scale tidal stream array connected to the grid. Each Atlantis AR1500 turbine is 1.5 MW with an 18 m rotor.
MeyGen demonstrates what's possible: the Pentland Firth is one of the world's strongest tidal flows (peak 4–5 m/s), and the full project Phase 3 could reach 400 MW. Annual generation from Phase 1A (~6 MW) has consistently exceeded expectations at ~4,200 full-load hours per year — a capacity factor of ~48%, more than double most offshore wind farms.
Source: Simec Atlantis Energy 2024 project reports.
Orbital Marine Power O2
The Orbital O2, launched in 2021 in Orkney (Scotland), is the world's most powerful floating tidal turbine at 2 MW. It uses a surface-floating platform with two 1 MW turbines on retractable arms, allowing maintenance without dry-docking. The O2 connects to the European Marine Energy Centre (EMEC) test facility and has supplied electricity to the Orkney grid.
Floating designs like the O2 avoid the expensive subsea foundation installation that makes fixed-bottom tidal turbines costly. They can be towed to maintenance berths, dramatically reducing operational costs — a key advantage for tidal stream economics.
Nova Innovation — Tidal Micro-grids
Nova Innovation operates a tidal array in Bluemull Sound, Shetland (Scotland) — the world's first fully operational offshore tidal array, generating since 2016. At modest scale (six 100 kW turbines), it demonstrates consistent generation in one of the UK's most exposed locations, supplying Shetland's grid.
Nova has also deployed the first tidal-powered micro-grid in Coratoe (Scotland), and has contracts for systems in Canada, Mexico, and Rwanda (run-of-river). Their scalable 100 kW units address both large tidal array markets and remote community power applications — a business model that parallels early solar.
Global Tidal Stream Installed Capacity (Cumulative, MW)
Source: Ocean Energy Systems (OES) Annual Report 2024; Carbon Trust Tidal Stream Industry Prospectus 2023.
Wave Energy Physics
Ocean waves are generated by wind transferring momentum to the sea surface. Wave energy is a combination of potential energy (water height) and kinetic energy (water particle motion). Wave power per unit of crest width (J/m per second = W/m) is proportional to wave height squared and period:
P ≈ (ρ × g² × H² × T) / (32π) W/m
Where H is significant wave height and T is energy period. In practice, Atlantic swells off Ireland or the Hebrides deliver 50–80 kW/m of wave front — enough that capturing even a fraction could supply large amounts of electricity. The UK wave resource exceeds its current total electricity consumption.
Wave Energy Converter (WEC) Types
| Type | Mechanism | Example |
|---|---|---|
| Point Absorber | Buoy oscillates vertically on waves; linear generator | CorPower, AW-Energy |
| Attenuator | Elongated structure aligned with waves; bends at joints driving hydraulics | Pelamis (defunct), Wave Dragon |
| Oscillating Water Column (OWC) | Waves pressurize air column in chamber; drives Wells turbine | NEREIDA (Spain), Mutriku (Spain) |
| Overtopping | Waves spill into elevated reservoir; low-head turbines | Wave Dragon |
| Submerged Pressure Differential | Seabed device flexes with wave pressure variations | CETO (Carnegie Clean Energy) |
Mutriku Wave Plant (Spain)
The Mutriku breakwater wave power plant in the Basque Country, Spain opened in 2011 — the world's first commercial multi-turbine oscillating water column wave farm. Sixteen OWC chambers are integrated into a harbour breakwater, each driving a 18.5 kW Wells turbine (total 296 kW).
Despite its modest scale, Mutriku has generated valuable long-term operational data. It has supplied electricity to ~250 homes and recorded one of the longest track records of any wave energy device. Its grid-connected breakwater integration model is seen as the most near-term commercial pathway for OWC technology.
CorPower Ocean (Sweden)
CorPower is developing a point absorber WEC that uses a phase control mechanism — a phased resonance principle inspired by human heart muscle — to amplify power capture 5–10× relative to passive devices. Their C4 unit (100 kW) underwent extensive testing at EMEC Scotland.
A 3-unit 300 kW pre-commercial array was deployed off Portugal in 2024 under the HiWave-5 project. CorPower's wave-to-wire efficiency target of >50% would make it competitive with offshore wind in high-resource locations if achieved at commercial scale.
Why Wave Energy Hasn't Scaled
Wave energy has attracted billions in R&D funding since the 1970s yet remains pre-commercial. The reasons are instructive:
Extreme marine environment: Devices must survive 100-year storm waves while remaining economical. The forces involved are orders of magnitude beyond those in a normal operating condition — designing for survival dominates cost.
Device diversity problem: Unlike wind (consolidated around 3-blade HAWT) or solar (silicon PV), wave energy has never converged on a dominant design. Dozens of radically different concepts have been developed, each requiring full development from scratch. This fragmentation has prevented manufacturing scale.
Non-linear loads: Irregular wave forcing creates extreme variability in power output and mechanical loads. Power take-off systems (hydraulics, linear generators) have struggled to handle this efficiently at low cost.
Grid connection: High-resource locations (exposed Atlantic coasts) are far from population centers and grid infrastructure. Offshore cable costs add substantially to project LCOE.
Source: IRENA Innovation Outlook: Ocean Energy Technologies 2024; IEA Ocean Energy Report 2023.
World Tidal Energy Hotspots
Resource estimates from OES Annual Report 2024; Carbon Trust Marine Energy Atlas 2023; IRENA Ocean Energy.
United Kingdom
Resource: UK has ~50% of Europe's tidal stream resource. Key sites:
- Pentland Firth — 1.9 GW practical; MeyGen Phase 3 target 400 MW
- Alderney Race — peak 3–4 m/s; 3 GW theoretical (Channel Islands)
- Severn Estuary — 14.5 m tidal range; Swansea Bay lagoon proposals
- Orkney/Shetland — EMEC test site; Nova Innovation arrays
The UK's Crown Estate held its first tidal stream leasing round in 2023, awarding 4 projects totalling 65 MW in Scotland.
Canada
Bay of Fundy holds the world's highest tidal range (up to 16.3 m) and an estimated 2,500 MW of accessible tidal power. Key developments:
- Cape Sharp Tidal (OpenHydro/Emera) — deployed 2 MW turbine 2016; retrieved 2019 due to device issues
- FORCE (Fundy Ocean Research Centre) — active test berths; Black Rock Tidal Power, Minas Tidal projects
- Annapolis Royal Generating Station — 20 MW barrage operating since 1984
France & Channel Islands
Beyond La Rance, France has exceptional tidal resources:
- Raz Blanchard (Alderney Race) — 3–4 m/s; 3 GW potential; Normandie Hydro, HydroQuest projects
- Raz de Sein — 2 m/s+ currents; smaller but accessible
- Fromveur Passage — Bretagne; HydroQuest 1 MW grid-connected since 2019
France's tender for 30 MW commercial tidal stream arrays was launched in 2023, targeting the Raz Blanchard as primary site.
South Korea
Already home to the world's largest barrage (Sihwa), South Korea is developing the Incheon tidal barrage (~1.3 GW proposed) and multiple lagoon proposals in the Yellow Sea, which has tidal ranges up to 9 m. Government targets include 500 MW of marine energy by 2030.
Australia
Northwest Australia has exceptional tidal resources — the Kimberley Coast features tidal ranges up to 10 m and strong currents. Carnegie Clean Energy's CETO wave technology has been tested at the Garden Island wave energy test site in Western Australia. The Clarence Strait (Northern Territory) has been assessed at 2.5 GW tidal stream potential.
India & China
India has identified 12,455 MW of theoretical tidal potential along Gulf of Kutch, Gulf of Khambhat, and the Gangetic delta. China has operated a small tidal barrage at Jiangxia since 1980 (3.2 MW) and has assessed multiple sites in Zhejiang and Fujian provinces. Both nations are investing in tidal stream development under marine energy programs.
Current Cost & Cost Reduction Pathway
Tidal stream LCOE currently runs ~£200–300/MWh ($250–380/MWh) — roughly 4–6× offshore wind. This cost gap drives most of the skepticism about tidal's commercial future. However, the cost reduction pathway is well-understood:
- Technology learning: First arrays are expensive because they're first. Wind turbine costs fell 60% between first commercial arrays (1990s) and mass deployment. LCOE of tidal stream could fall 60–70% with 10 GW of deployment per learning curve models.
- Array effects: Turbines in an array share cables, installation, operations. MeyGen Phase 3 (400 MW) would be ~40% cheaper per MW than Phase 1A (6 MW).
- Supply chain industrialization: Dedicated manufacturing, standard vessels, trained installation crews — all dramatically reduce costs once achieved.
LCOE Trajectory Projections
Source: Carbon Trust Tidal Stream Cost Reduction Study 2023; IRENA Ocean Energy Cost Reduction Pathways 2023.
Policy Support Needed
The tidal stream industry needs targeted policy support to bridge the cost gap — similar to what early offshore wind received in the UK through Contracts for Difference with differentiated strike prices for emerging technologies.
The UK's AR5 CfD round (2023) allocated a £40/MWh cap specifically for tidal stream, enabling the first commercial-scale projects. Scotland's devolved energy policies have been particularly supportive through the Scottish Government's Wave and Tidal Energy: Scheme of Delegation.
Hybrid Marine Energy
Pairing tidal turbines with co-located offshore wind or wave energy on a shared platform and cable reduces marine spatial planning conflicts, shares grid connection costs, and provides more consistent power output. The "Blue Accelerator" concept (Carbon Trust) estimates 15–20% LCOE reduction for tidal through shared infrastructure with offshore wind in UK waters.
Emerging Technologies
Osmotic / salinity gradient energy — where rivers meet the sea, ~2,000 GW theoretical global resource. Statkraft's osmotic pilot plant (Norway, 2009) was the world's first but shut down in 2013. Pressure-retarded osmosis (PRO) and reverse electrodialysis (RED) are being refined.
OTEC — 25°C+ temperature differential between tropical surface and deep water drives a Rankine cycle. Net power requires deep-water pipe infrastructure (~1 km). Japan, Hawaii, and Martinique have demonstration projects. ~1 TW global resource.
Marine Energy Global Deployment Forecast
Source: OES Ocean Energy Systems Outlook 2024; IRENA Ocean Energy World Energy Transitions Outlook 2024.