Clean Energy from First Principles

Tidal Stream Energy

Extracts kinetic energy from horizontal tidal currents using underwater turbines, functioning like underwater wind turbines in the ocean.

Fundamental Principle

Kinetic Energy Extraction

Tidal stream energy extracts kinetic energy from the horizontal flow of water driven by tidal cycles. Unlike tidal barrages (which exploit potential energy from vertical head differences), tidal stream turbines work like underwater wind turbines, converting the kinetic energy of moving water directly into rotational mechanical energy and then electricity.

The fundamental physics is identical to wind energy: a fluid flows through a rotor, momentum is transferred to the blades, and the rotor spins a generator. The key equation for power available in a flowing fluid is:

P=12ρAV3P = \frac{1}{2} \rho A V^3

Where:

The critical insight is the cubic relationship with velocity: doubling flow speed increases available power eightfold. This makes site selection paramount, as small differences in current speed translate to large differences in energy yield.

Water vs. Wind: The Density Advantage

Seawater is approximately 800 times denser than air:

This density ratio means that for identical swept areas and velocities, a tidal turbine can extract roughly 800 times more power than a wind turbine. In practice, tidal currents are much slower than wind speeds, but the net result is still favorable:

Fluid Typical Speed Density Power Density at Speed
Wind 12 m/s 1.225 kg/m³ 1,058 W/m²
Tidal current 2.5 m/s 1,025 kg/m³ 8,008 W/m²
Tidal current 3.0 m/s 1,025 kg/m³ 13,838 W/m²

At typical operating speeds, a tidal turbine accesses 4-8 times more power per square meter of swept area than a wind turbine, allowing much smaller, more compact devices for equivalent power output.

Rule of thumb: A tidal current of ~1/10th wind speed delivers comparable power density. A 3 m/s tidal current is roughly equivalent to a 30 m/s wind (hurricane force) in terms of energy flux.

Tidal Current Origins

Tidal currents arise from the horizontal movement of water responding to tidal range changes. As the tide rises and falls, water must flow into and out of bays, estuaries, and channels. Where geography constricts this flow, currents accelerate:

Flow acceleration mechanisms:

The strongest tidal currents occur where large tidal ranges combine with constrained geography. The Pentland Firth between mainland Scotland and Orkney experiences currents exceeding 5 m/s (10 knots), making it one of the world's premier tidal stream sites.

Tidal current characteristics:

Conversion Mechanism

Turbine Types

The European Marine Energy Centre (EMEC) recognizes six principal tidal energy converter types:

1. Horizontal Axis Turbines (HAT)

Most common design, analogous to wind turbines. Rotor axis aligned with flow direction. Examples: Atlantis AR1500, Andritz Hydro Hammerfest AH1000, Orbital O2.

Characteristics:

2. Vertical Axis Turbines (VAT)

Rotor axis perpendicular to flow. Accepts flow from any horizontal direction without yawing.

Characteristics:

3. Oscillating Hydrofoils

Hydrofoil attached to oscillating arm; lift forces cause reciprocating motion converted to rotary motion or hydraulic pressure.

Characteristics:

4. Ducted/Venturi Devices

Turbine housed in a duct that concentrates flow, increasing velocity through the rotor.

Characteristics:

5. Archimedes Screws

Helical screw that rotates as water flows through it.

Characteristics:

6. Tidal Kites

Underwater "kite" carrying a small turbine, tethered to seabed, flying figure-eight patterns through the water to amplify relative velocity.

Characteristics:

Power Take-Off Systems

Direct drive: Rotor directly coupled to permanent magnet generator. Eliminates gearbox but requires large, slow-speed generator. Reduces maintenance but increases generator cost.

Geared systems: Gearbox steps up rotor speed (typically 10-20 rpm) to generator speed (hundreds to thousands of rpm). Standard approach but gearbox is a maintenance concern in marine environment.

Hydraulic: Rotor drives hydraulic pump; hydraulic fluid powers generator. Allows flexible placement of generator (potentially above water). Higher losses but easier maintenance.

Mounting Configurations

Seabed-mounted (gravity base):

Seabed-mounted (monopile/pin-pile):

Floating:

Floating systems offer significant advantages: installation without heavy-lift vessels, maintenance without diving or ROVs, and flexibility to relocate. The Orbital O2 demonstrated that its turbines can be raised above water for inspection without specialized equipment.

Theoretical Limits

The Betz Limit and Its Application

The Betz limit (59.3% or 16/27) represents the maximum fraction of kinetic energy that an ideal turbine can extract from an unbounded flow. Derived from conservation of mass and momentum, it applies when flow is steady and uniform, the turbine operates in open (unbounded) flow, and there are no energy losses.

Wind turbines typically achieve 75-80% of the Betz limit in practice, giving power coefficients (Cp) of 0.45-0.50.

Tidal turbines in unbounded flow face similar constraints. However, tidal channels present a fundamentally different situation: flow is bounded by the seabed, surface, and often by channel walls. This confinement enables power coefficients exceeding the classical Betz limit.

Channel Blockage Effects

When turbines occupy a significant fraction of a channel's cross-section (high "blockage ratio"), the flow cannot simply divert around them. The partial ducting effect forces more water through the turbine plane, increasing the effective power coefficient.

Research by Vennell (2013) showed that tidal turbines in channels can theoretically achieve Cp values several times higher than 16/27. However, this comes with complications:

The optimal extraction from a tidal channel is typically 10-25% of the undisturbed kinetic energy flux. Extracting more degrades the resource faster than power increases.

Practical Power Coefficients

Real-world tidal turbines achieve:

The Orbital O2 achieves rated power (2 MW) at a current speed of 2.5 m/s. For 20 m diameter rotors (628 m² total swept area):

Theoretical power at 2.5 m/s = ½ × 1,025 × 628 × 2.5³ = 5.03 MW

Actual power = 2 MW

Implied Cp = 2.0 / 5.03 = 0.40

This is consistent with well-designed horizontal axis turbines operating near their design point.

Practical Limitations

Site Constraints

Economically viable tidal stream sites require:

Minimum current speed: >2 m/s mean spring peak current (ideally >2.5 m/s)

Appropriate water depth: 25-50 m typical

Seabed conditions: Stable substrate for foundations

Grid access: Reasonable distance to onshore connection

Environmental constraints: Avoiding sensitive habitats

These constraints dramatically limit suitable sites. A comprehensive 2024 review identified only 426 potentially suitable sites globally, concentrated in:

Intermittency Pattern

Tidal stream generation is predictable but intermittent. Unlike solar or wind, the pattern is known precisely:

Within each tidal cycle (~12.4 hours):

Daily pattern:

Spring-neap cycle (~14 days):

Annual variation:

Capacity factors: Due to these patterns, tidal stream capacity factors are typically:

For comparison: onshore wind 25-35%, offshore wind 35-50%, solar PV 10-25%, nuclear 85-95%.

Environmental Considerations

Tidal stream devices have lower environmental impact than barrages but still raise concerns:

Collision risk: Marine mammals and fish may collide with rotating blades

Electromagnetic fields (EMF): Subsea cables produce EMFs that may affect fish navigation

Acoustic emissions: Operational noise may disturb marine life

Habitat alteration: Foundations and cables change local seabed

Flow modification: Large arrays may alter local hydrodynamics

Overall, tidal stream has significantly lower environmental impact than tidal barrages, which is why development focus has shifted to stream technology despite higher per-MW costs.

Installation and Maintenance Challenges

The marine environment presents formidable operational challenges:

Installation:

Maintenance:

Floating systems (like Orbital O2) address many challenges:

MeyGen achieved a milestone in 2025 when one turbine operated for 6.5 years without unplanned maintenance, demonstrating that reliability targets are achievable.

Scaling Characteristics

Cost Reduction Trajectory

Tidal stream energy has demonstrated rapid cost reduction:

Year Representative LCOE Notes
2016-2018 £300+/MWh Early demonstration projects
2018 £359/MWh ORE Catapult estimate
2022 £213/MWh UK CfD AR4 strike price
2023 £198/MWh UK CfD AR5 strike price
2024 £178/MWh UK CfD AR6 strike price
Target 2030 £90-150/MWh Industry projections

This represents >40% cost reduction in 6 years with minimal deployment. The trajectory mirrors early offshore wind, which achieved 60%+ cost reduction from 2012 to 2020 as deployment scaled from GW to tens of GW.

Economies of Scale

Cost reduction pathways include:

Device scaling: Larger turbines reduce cost per MW

Array effects: Multi-turbine projects reduce per-unit costs

Supply chain maturation: Volume enables specialization

Technology improvement:

ORE Catapult projects 70% cost reduction from current levels to 1 GW installed capacity, bringing LCOE to ~£90/MWh, competitive with offshore wind a decade ago.

Comparison with Wind Energy Development

Tidal stream in 2024 is roughly where offshore wind was in 2008-2010:

Offshore wind subsequently achieved:

If tidal stream follows a similar trajectory (uncertain but plausible given the technology similarities), it could achieve competitive costs by the mid-2030s with appropriate policy support.

Current Status

Global Installed Capacity

Tidal energy total installed capacity (including barrages): ~530 MW globally

Tidal barrage capacity: ~520 MW

Tidal stream capacity: ~10-15 MW operational (as of late 2024)

Tidal stream represents <3% of total tidal capacity but >90% of current development activity.

Major Tidal Stream Projects

MeyGen (Scotland) - World's largest tidal stream project

Orbital O2 (Scotland) - Most powerful single tidal turbine

Nova Innovation Shetland Tidal Array (Scotland)

Sabella D10 (France)

FloWatt (France) - Under development

Normandie Hydroliennes NH1 (France) - Under development

Minesto Dragon (Faroe Islands)

Pipeline and Projections

UK tidal stream pipeline: 121 MW contracted under CfD (AR4-AR6) for delivery by 2029

European pipeline: 165 MW of publicly funded projects planned over next 5 years (152 MW tidal stream)

Global projections:

Market and Investment

Market size: ~1.8billion(2024),projectedtoreach1.8 billion (2024), projected to reach10-14 billion by 2030s

Investment trends:

Key policy mechanisms:

Technology Readiness

Tidal stream technology has reached TRL 7-8 (system prototype demonstration in operational environment), transitioning toward TRL 9 (commercial operation):

Technology Aspect Status
Horizontal axis turbines TRL 8-9 (multiple operational systems)
Floating platforms TRL 8 (Orbital O2 operational)
Seabed foundations TRL 9 (proven at MeyGen)
Tidal kites TRL 7-8 (Minesto Dragon operational)
Multi-turbine arrays TRL 7 (MeyGen Phase 1, limited scale)
Large-scale arrays (>50 MW) TRL 5-6 (designed, not deployed)

Global Resource Potential

Theoretical Resource

Global tidal stream theoretical resource estimates vary widely:

For context, global electricity consumption is ~30,000 TWh/year. Even optimistic estimates suggest tidal stream could supply at most 10% of global demand.

Practical/Technical Resource

After accounting for depth constraints, minimum current speeds, and environmental exclusions:

Region Practical Resource Estimate
UK 34 TWh/year (10% of theoretical)
France 10-20 TWh/year
Canada 17-25 TWh/year (Minas Passage alone: 11 TWh/year)
Indonesia 10-20 TWh/year
New Zealand 5-10 TWh/year
USA 5-15 TWh/year
Global total 100-150 TWh/year

This practical resource represents <1% of global electricity demand. Tidal stream will remain a niche technology, valuable in specific locations but not a major contributor to global decarbonization.

UK Potential

The UK has the best tidal stream resource in Europe:

Scotland's target of sourcing significant electricity from tidal stream is technically achievable. The Pentland Firth could "provide half of Scotland's electricity" according to Oxford University studies, though this would require tens of GW of installed capacity and significant cost reduction.

Strategic Outlook

Near-term (2025-2030)

Expected developments:

Key milestones needed:

Medium-term (2030-2040)

Possible trajectory:

Barriers:

Long-term Potential

Tidal stream's ultimate role depends on:

  1. Cost trajectory: If costs reach £80-100/MWh, tidal stream becomes competitive in suitable locations. If costs stall at £150+/MWh, deployment will remain limited to subsidized projects.

  2. Value of predictability: Tides are perfectly predictable, unlike wind and solar. As grids incorporate more variable renewables, the value of predictable generation may increase, providing additional revenue streams.

  3. Complementarity: Tidal generation often peaks when wind is weak (calm weather often correlates with strong tides). This complementarity could enhance system value.

  4. Geographic concentration: Benefits will be concentrated in a few favorable locations. Countries like UK, France, Canada, and Indonesia stand to gain most.

Realistic long-term contribution:

Comparison with Other Marine Energy

Technology Global Potential Current Status Outlook
Tidal barrage 100-200 GW 520 MW installed Limited by site constraints and environmental impact
Tidal stream 50-150 GW 10-15 MW installed Best prospects; technology maturing
Wave energy 500-2,000 GW <5 MW installed Earlier stage; higher technical challenges
OTEC 10-80 GW <1 MW operational Very early stage; limited to tropics

Tidal stream is the most promising marine energy technology for near-term deployment, combining:

The technology has moved beyond "promising concept" to "early commercial deployment." Whether it scales to meaningful contribution depends on sustained policy support through the critical next decade of development.