Beneath the waves, a quiet energy revolution has begun.

Japan has deployed its first megawatt-scale underwater turbine, tapping into powerful ocean currents to generate clean, renewable electricity. Unlike wind or solar, ocean currents flow steadily day and night, making this technology highly reliable.

Anchored deep underwater, the turbine spins continuously as currents pass through, converting natural movement into usable power without emissions. This breakthrough highlights how the oceans can become a major source of sustainable energy, pushing Japan—and the world—closer to a future powered by nature itself.

Underwater Turbines: Harnessing the Power of Flowing Water

Underwater turbines, often called marine hydrokinetic turbines or tidal/current turbines, are devices that convert the kinetic energy of moving water—from tides, ocean currents, or rivers—into electricity. They are the submerged equivalents of wind turbines, operating in a much denser medium.

Core Technology & How They Work

The fundamental principle is simple: flowing water turns a rotor, which drives a generator to produce electricity.

1. Rotor/Blades: Designed to capture water flow. They are shorter, thicker, and stronger than wind turbine blades to withstand higher density forces and potential debris.
2. Nacelle: The watertight housing containing the gearbox, generator, and control systems.
3. Support Structure: This varies by installation:
   • Seabed-Mounted: Fixed directly to the seafloor in shallow, high-velocity areas (common for tidal stream farms).
   • Floating/Moored: Tethered to the seabed but floating in the water column, used in deeper waters or for ocean current projects.
   • Piled or Gravity-Based: Fixed structures driven or placed into the seabed.
4. Power Cable: Transmits electricity to a subsea junction, then to shore via an export cable.

Key Types & Designs

• Horizontal-Axis Turbines (HATs): The most common design, resembling underwater windmills. The axis of rotation is parallel to the water flow. (e.g., SIMEC Atlantis’s MeyGen project turbines).
• Vertical-Axis Turbines (VATs): The axis of rotation is perpendicular to the flow. They can capture flow from any direction without yawing, useful in bi-directional tidal sites.
• Crossflow Turbines (or Kinetic Helical Turbines): Often shaped like an eggbeater or helical screw, they can operate in lower velocity flows.
• Oscillating Hydrofoils: Use a lifting surface (hydrofoil) that moves up and down in the current, rather than rotating, to drive a hydraulic system or generator.

Primary Energy Sources

1. Tidal Stream Energy: Harnesses the predictable, regular flow of water caused by rising and falling tides through channels and straits. This is the most developed sector, with several commercial arrays operating globally.
2. Ocean Current Energy: Taps into persistent, large-scale currents like the Gulf Stream. This is a more constant but technically challenging deep-water resource.
3. Riverine (In-Stream) Energy: Uses the flow of rivers, typically for smaller-scale, off-grid applications for remote communities.

Major Advantages

• Predictable & Reliable: Tides are governed by lunar cycles, predictable centuries in advance, unlike intermittent wind and solar.
• High Energy Density: Water is ~832 times denser than air, so a smaller, slower turbine can produce the same power as a much larger wind turbine.
• Low Visual Impact: Entirely submerged, with only minimal surface infrastructure.
• High Capacity Factor: Can generate power for 18-22 hours per day during predictable tidal cycles, leading to high utilization rates.

Significant Challenges & Environmental Considerations

• Extreme Marine Environment: Corrosion, biofouling, and powerful storm waves require robust, expensive materials and coatings.
• Installation & Maintenance: Difficult, weather-dependent, and costly operations requiring specialized vessels and divers/ROVs.
• Environmental Interactions: Potential impacts must be carefully monitored:
  • Marine Life: Risk of collision with marine mammals, fish, and diving birds (though slow-moving blades are less risky than ship propellers).
  • Underwater Noise: During construction and operation.
  • Habitat & Sediment Change: Alteration of local flow patterns and seabed.
• Grid Connection: High cost of subsea cables and grid integration for remote sites.
• High Capital Cost (CAPEX): The technology is still in early commercial stages, making upfront costs high, though operational costs are low.

Global Leaders & Notable Projects

• Scotland, UK: A world leader. The MeyGen project in the Pentland Firth is the largest planned tidal stream array, with phases already operational.
• France: Home to the Paimpol-Bréhat tidal project by EDF.
• Canada: The Fundy Ocean Research Center for Energy (FORCE) in Nova Scotia’s Bay of Fundy, home to the world’s highest tides, is a major test site.
• South Korea: The Sihwa Lake Tidal Power Station is the world’s largest tidal barrage (a different, dam-like technology), but the country is also investing in tidal stream.
• China, Japan, and the Netherlands also have active demonstration projects.

The Future Outlook

The sector is transitioning from demonstration to pre-commercial and early commercial arrays. Key trends include:

• Cost Reduction: Learning from wind energy’s path, aiming for significant LCOE (Levelized Cost of Energy) reductions through scaled manufacturing and installation.
• Floating Turbine Designs: To access deeper, more powerful resources and simplify installation.
• Advanced Materials: Use of composites and anti-fouling technologies.
• Hybrid Systems: Combining tidal with offshore wind or solar on shared infrastructure and grid connections.
• Improved Environmental Monitoring: Using AI and sonar to understand and mitigate ecological impacts.

Conclusion

Underwater turbines represent a potent, predictable form of renewable energy with the potential to provide baseload-like power from a largely untapped resource. While significant technological, economic, and environmental hurdles remain, the industry is making steady progress. As technology scales and costs fall, tidal and current energy are poised to become a meaningful part of the future clean energy mix, especially for coastal nations and communities.