Clean Energy from First Principles

Tidal Barrage

Captures gravitational potential energy from the vertical displacement of water caused by tides using a dam-like structure with low-head turbines.

Fundamental Principle

Gravitational Origin of Tidal Energy

Tidal barrage energy exploits the potential energy stored in the vertical displacement of ocean water caused by gravitational interactions between Earth, Moon, and Sun. Unlike wave energy (wind-driven) or OTEC (solar thermal), tidal energy derives from the rotational kinetic energy of the Earth-Moon system, making it fundamentally astronomical rather than solar in origin.

The Moon's gravitational pull creates tidal bulges on opposite sides of Earth. As Earth rotates beneath these bulges, coastal locations experience two high tides and two low tides approximately every 24 hours and 50 minutes (the lunar day). The Sun contributes a smaller tidal force; when Sun and Moon align (spring tides), tidal ranges are maximized, while at right angles (neap tides), ranges are minimized.

The gravitational potential energy available in tides is ultimately extracted from Earth's rotational kinetic energy. Tidal friction gradually slows Earth's rotation (by about 2.3 milliseconds per century) while accelerating the Moon's orbital velocity, causing it to recede at approximately 3.8 cm/year. This process will continue for billions of years until Earth and Moon become tidally locked, but the effect of human tidal energy extraction on this timescale is utterly negligible.

Energy Content of Tidal Range

A tidal barrage captures the potential energy of water raised by the tide. For a basin of surface area A and tidal range R (difference between high and low water), the theoretical energy available per tidal cycle is:

E=12ρgAR2E = \frac{1}{2} \rho g A R^2

Where:

The critical insight is that energy scales with the square of tidal range. Doubling the tidal range quadruples the available energy. This is why tidal barrages are only economically viable at sites with exceptionally large tidal ranges (typically >5 m mean range, ideally >7 m).

Example calculation:

For a hypothetical basin:

Energy per tide = ½ × 1,025 × 9.81 × 10 × 10⁶ × 8² = 3.22 × 10¹² J = 3.22 TJ

With two tidal cycles per day, and accounting for conversion losses (~25-30% efficiency), this basin could generate approximately 50-60 MW average power.

Global Tidal Range Distribution

Tidal ranges vary enormously worldwide, from nearly zero in enclosed seas to over 16 m in exceptional locations. The highest tidal ranges occur where:

  1. Resonant amplification: Basin geometry creates standing wave resonance with tidal period
  2. Funneling effects: Converging coastlines concentrate tidal flow
  3. Shallow continental shelves: Amplify tidal wave height

Highest tidal ranges globally:

Location Mean Tidal Range Maximum Range
Bay of Fundy (Canada) 11-12 m 16.3 m
Ungava Bay (Canada) 9-10 m 17 m
Severn Estuary (UK) 8-9 m 14.5 m
Rance Estuary (France) 8 m 13.5 m
Gulf of St. Malo (France) 8-9 m 13.5 m
Bristol Channel (UK) 7-8 m 14 m
Cook Inlet (Alaska) 7-9 m 12 m
Gyeonggi Bay (South Korea) 6-7 m 9 m

These exceptional sites represent only a tiny fraction of global coastline, fundamentally limiting tidal barrage potential.

Conversion Mechanism

Barrage Structure and Components

A tidal barrage is a dam-like structure spanning an estuary, bay, or inlet. Key components include:

Embankments and caissons:

Sluice gates:

Turbine-generators:

Ship locks:

Operating Modes

Ebb generation (most common):

  1. Flood tide: Sluice gates open, basin fills to high tide level
  2. High tide: Gates close, trapping water at elevated level
  3. Ebb tide: Sea level falls, creating head differential
  4. Generation: When head reaches ~2-3 m, turbines open and generate as water flows out
  5. Continue generating until head drops below minimum (~1.5 m)
  6. Cycle repeats

This mode is most efficient because the upper half of the basin contains more volume than the lower half (for typical bathymetry), maximizing energy extraction per cycle.

Flood generation:

  1. Ebb tide: Sluice gates open, basin drains to low tide level
  2. Low tide: Gates close, maintaining low basin level
  3. Flood tide: Sea level rises, creating head differential
  4. Generation: Turbines generate as water flows into basin
  5. Less efficient than ebb generation (lower volume in lower basin half)

Two-way (dual-effect) generation:

Pumping-enhanced generation:

Efficiency Considerations

Theoretical maximum: The maximum extractable energy is limited by the need to maintain minimum head for turbine operation. Typically, only 25-35% of the theoretical potential energy can be captured:

Turbine efficiency:

System efficiency:

Capacity factor:

Due to the intermittent nature of tides (generation only during part of each cycle), tidal barrages have inherently limited capacity factors:

Plant Installed Capacity Annual Output Capacity Factor
La Rance 240 MW 500-540 GWh 24-26%
Sihwa Lake 254 MW 550 GWh 25%
Annapolis Royal 20 MW 50 GWh 28%

For comparison: nuclear plants achieve 85-90%, wind 30-45%, solar PV 15-25%.

Theoretical Limits

Maximum Energy Extraction

The fundamental limit on tidal barrage energy is set by the available gravitational potential energy:

equation tidal_power $$P_{max} = \frac{1}{2} \rho g A R^2 \times \frac{2}{T}$$ ::: :::

Where T is the tidal period (~12.42 hours for semidiurnal tides).

For a 100 km² basin with 10 m tidal range:

In practice, extraction efficiency of 25-35% yields 290-400 MW average power from such a basin.

Tidal Resonance Constraints

Barrage construction can alter the tidal resonance of an estuary, potentially:

Studies of the Severn Estuary suggest a barrage would reduce the tidal range within the basin by approximately 25-30%, partially offsetting energy gains from the impoundment.

Global Resource Limit

The total global dissipation of tidal energy is approximately 3.7 TW, of which:

Only the coastal dissipation is potentially accessible, and only a fraction occurs at sites suitable for barrages. Various estimates suggest:

Source Global Tidal Barrage Potential
World Energy Council 500-1,000 TWh/year (60-115 GW)
Optimistic estimates 150-200 GW
Realistically developable 20-50 GW

For context, global electricity consumption is approximately 30,000 TWh/year. Even fully developed, tidal barrages could supply only 2-3% of global electricity demand.

Practical Limitations

Site Constraints

Economically viable tidal barrage sites must have:

  1. Large tidal range: >5 m mean range (ideally >7 m)
  2. Suitable geometry: Natural basin that can be enclosed with reasonable barrage length
  3. Low Gibrat ratio: Ratio of barrage length (m) to annual energy production (kWh)
    • La Rance: 0.36 (excellent)
    • Severn: 0.87 (good)
    • Passamaquoddy: 0.92 (marginal)

Only a handful of sites worldwide meet these criteria. Most are already identified and studied:

Sites with >1 GW potential:

Capital Costs

Tidal barrages require enormous upfront investment:

Project Capacity Cost Cost per kW
La Rance (1966) 240 MW €95M (original) ~€400/kW
Sihwa Lake (2011) 254 MW 298−560M∣298-560M 1,200-2,200/kW
Severn Barrage (proposed) 8.6 GW £20-30 billion £2,300-3,500/kW
Swansea Tidal Lagoon (proposed) 320 MW £1.3 billion £4,000/kW

For comparison:

The high capital cost is driven by:

Levelized Cost of Energy

Due to high capital costs and low capacity factors, tidal barrage LCOE is relatively high:

Plant/Project LCOE Estimate
La Rance (mature) €0.02-0.04/kWh (costs now recovered)
New-build barrage €0.15-0.30/kWh
Swansea Lagoon (proposed) £0.30/kWh initially
Severn Barrage (proposed) £0.15-0.25/kWh

However, tidal barrages have extremely long operational lifetimes (100+ years potential), and once capital is recovered, operating costs are very low. La Rance now produces electricity at €0.018/kWh, cheaper than nuclear.

Environmental Impacts

Tidal barrages cause significant environmental disruption:

Habitat loss:

- Intertidal mudflats reduced or eliminated - Critical feeding grounds for wading birds destroyed - La Rance: Initial ecosystem collapse during construction; 10+ years to establish new equilibrium - Severn Estuary: Potential loss of 3,000-14,000 hectares of intertidal habitat

Sediment transport:

Fish migration:

Water quality:

Ecological trade-offs:

The Severn Estuary is protected under multiple conservation designations (Ramsar, Special Protection Area, Special Area of Conservation), creating major regulatory obstacles to barrage development.

Construction Challenges

Building a barrage across a major estuary is among the most challenging civil engineering projects:

Scaling Characteristics

Economies of Scale

Tidal barrages exhibit strong economies of scale:

The Severn Barrage at 8.6 GW would have significantly lower per-MW costs than smaller projects, but the absolute cost (£20-30 billion) creates financing challenges.

Modular Alternatives: Tidal Lagoons

Tidal lagoons are artificial enclosures built offshore, avoiding the need to dam an entire estuary:

Advantages over barrages:

Proposed projects:

Lagoons remain unproven at commercial scale.

Learning and Cost Reduction

Unlike solar PV or wind, tidal barrages have seen minimal cost reduction:

Cost reduction pathways:

Current Status

Operational Plants

Only a handful of tidal barrage plants operate worldwide:

La Rance, France (1966)

La Rance demonstrated technical feasibility and long-term reliability. After 20 years to recover costs, it now produces among the cheapest electricity in France.

Sihwa Lake, South Korea (2011)

Sihwa was built by retrofitting an existing seawall, significantly reducing costs. It operates in flood-only mode to maximize water exchange for environmental remediation.

Annapolis Royal, Canada (1984)

Smaller installations:

Total global installed capacity: ~520 MW

Proposed Projects

Severn Barrage, UK:

Incheon Bay, South Korea:

Garolim Bay, South Korea:

Mersey Barrage, UK:

Gulf of Khambhat, India:

Market and Policy Context

Tidal barrage development has stalled for decades due to:

  1. High capital costs: Cannot compete with declining solar/wind costs
  2. Long payback periods: 20+ years to recover investment
  3. Environmental concerns: Habitat destruction, fish mortality
  4. Financing challenges: Private sector unwilling without government support
  5. Alternative technologies: Tidal stream turbines offer lower-impact option

Recent developments:

Outlook

Near-term (2025-2035):

Medium-term (2035-2050):

Long-term potential:

Tidal barrages will remain a niche technology, valuable at exceptional sites where large tidal ranges coincide with favorable geography, but unable to make major contribution to global electricity supply. Their primary advantages are:

The fundamental constraint is simple: there are very few places on Earth where the tides rise and fall enough to make barrage construction worthwhile, and most of those places have high ecological value that complicates development. Tidal barrages represent proven technology waiting for the right combination of site, finance, and policy alignment rather than a technology requiring fundamental breakthroughs.