Development of tidal stream generators

Overview

Tidal stream generators represent a distinct category of marine energy infrastructure designed to harness the kinetic energy of tidal currents flowing around coastlines. These systems are technically classified under several interchangeable terms within the energy sector, including tidal stream turbines (TST), tidal energy converters (TEC), and marine hydro-kinetic (MHK) generation. Unlike other forms of tidal energy, these devices do not rely on the gravitational potential energy of rising and falling water levels but rather capture the continuous movement of water masses driven by tidal forces.

Operating Principle and Fluid Dynamics

The fundamental operating principle of tidal stream generators is analogous to that of conventional wind turbines. Both technologies utilize rotating blades to convert the kinetic energy of a moving fluid into mechanical rotation, which is then transformed into electrical power. However, the engineering challenges and design parameters differ significantly due to the properties of the working fluid. Seawater is approximately 800 times more dense than air. This high density allows tidal turbines to generate substantial power even at slower flow velocities compared to wind speeds required for equivalent output.

The power available in a tidal stream is governed by the kinetic energy flux through the swept area of the turbine. The relationship can be expressed using the standard hydro-kinetic power formula, where power is proportional to the density of the fluid and the cube of the velocity. This cubic relationship means that small increases in tidal current velocity result in significant gains in energy capture, making site selection critical for efficiency. The higher density of water also imposes greater structural loads on the turbine components, requiring robust materials and precise aerodynamic or hydrodynamic blade designs to withstand continuous cyclic stress.

Distinction from Tidal Barrages

It is essential to distinguish tidal stream generators from tidal barrages and tidal lagoons, which operate on a fundamentally different mechanism. Tidal barrages function by impounding the rising and falling tide within an enclosed basin, effectively creating a reservoir. Power is generated as water flows through turbines mounted in the barrage structure, utilizing the head difference between the sea and the basin. In contrast, tidal stream generators are placed directly in the flow of tidal currents, similar to wind farms placed in prevailing wind corridors. They do not require large-scale civil engineering works to dam rivers or estuaries, which often reduces the environmental footprint related to sediment transport and intertidal habitat alteration. This distinction highlights the diversity of tidal energy technologies, each suited to specific geographical and hydrodynamic conditions.

What are the main types of tidal stream turbines?

Tidal stream generators, also known as tidal stream turbines (TST) or marine hydro-kinetic (MHK) devices, harness the kinetic energy of tidal currents. Unlike tidal barrages that impound water, these turbines operate similarly to wind turbines but function in a fluid approximately 800 times denser than air, allowing for power generation at slower velocities. Several technological variants have been developed to optimize this energy capture.

Horizontal-Axis Turbines

Horizontal-axis tidal turbines (HATT) are the most common configuration, resembling traditional wind turbines with blades rotating around a horizontal shaft. They can be mounted on fixed foundations or floating platforms, allowing for deployment in varying water depths. The power output is generally proportional to the cube of the current velocity.

Vertical-Axis Turbines

Vertical-axis tidal turbines (VATT) feature blades rotating around a vertical shaft. This design often allows the generator and gearbox to be mounted on the sea bed or a surface platform, simplifying maintenance. VATTs are typically omni-directional, meaning they can capture energy from currents flowing from either direction without needing to yaw.

Cross-Flow and Tidal Kites

Cross-flow turbines, such as the Darrieus type, utilize a vertical axis with curved blades that generate lift as the current passes through the rotor twice per revolution. Tidal kites are a more dynamic variant, using a tethered turbine that traces a figure-eight path through the current. This movement increases the relative velocity of the water passing through the rotor, significantly boosting power output compared to stationary devices.

The selection of turbine type depends on site-specific conditions, including current speed, water depth, and seabed topography. Each variant offers distinct advantages in terms of efficiency, maintenance accessibility, and adaptability to different marine environments.

History of tidal energy development

The development of tidal stream generators has evolved significantly since the early 2000s, transitioning from experimental prototypes to operational arrays. Tidal stream turbines (TST), also known as tidal energy converters (TEC) or marine hydro-kinetic (MHK) generation systems, operate on principles similar to wind turbines but are engineered to function in water, a fluid approximately 800 times denser than air. This density allows turbines to generate power at slower current velocities compared to wind speeds. It is distinct from tidal barrages or lagoons, which generate power by impounding the rising and falling tide rather than harnessing the kinetic energy of the current itself.

Early Milestones and SeaFlow

A significant milestone in this field occurred in 2003, marking the commissioning of early tidal stream generators. This period saw the deployment of the SeaFlow project, one of the first multi-turbine tidal arrays. SeaFlow utilized horizontal-axis turbines and was notable for its mooring systems and subsea cabling, demonstrating the feasibility of connecting multiple units to a single grid connection point. The project provided critical data on turbine performance, maintenance access, and the impact of tidal currents on foundation stability.

Hammerfest Strøm and Arctic Deployment

Following these early successes, development expanded to diverse geographical locations. The Hammerfest Strøm project in Norway represented a key advancement in deploying tidal turbines in harsh, cold-water environments. This project utilized vertical-axis turbines, offering a different design approach compared to the horizontal-axis models used in SeaFlow. The deployment in the Arctic conditions tested the durability of turbine blades, gearboxes, and mooring systems under extreme temperatures and salinity levels. These trials were crucial for understanding the operational challenges of tidal energy in northern latitudes.

MeyGen Phase 1 and Commercial Scaling

By 2017, the sector had progressed to larger-scale commercial deployments, exemplified by the MeyGen Phase 1 project in Scotland. MeyGen utilized horizontal-axis turbines and aimed to establish one of the world’s largest tidal stream arrays. The project involved multiple turbine units connected to a substation on a nearby island, highlighting advancements in grid integration and power transmission. This phase demonstrated the potential for tidal energy to contribute significantly to local power grids, providing predictable and renewable electricity. The success of MeyGen Phase 1 laid the groundwork for future expansions and the continued refinement of tidal stream technology.

Global testing sites and commercial arrays

Global testing sites have been critical for validating tidal stream generator designs. The European Marine Energy Centre (EMEC) in Orkney and the Fundy Ocean Research Center for Energy (FORCE) in Canada serve as primary hubs for deploying tidal energy converters. These facilities allow developers to test marine hydro-kinetic turbines in high-velocity currents before commercial scaling. The dense nature of water, approximately 800 times that of air, requires robust structural testing at these sites to handle the slower but more powerful tidal flows compared to wind.

Commercial Arrays

The MeyGen project in Scotland represents a leading commercial tidal stream array. It utilizes tidal stream turbines to harness the strong currents of the Pentland Firth. The Morlais project in Wales has also advanced commercial deployment, focusing on integrating tidal energy converters into the local grid. In Asia, the Zhoushan tidal energy project in China has emerged as a significant commercial array. These sites demonstrate the operational status of tidal stream generators as a viable renewable energy source.

The power available in a tidal stream is proportional to the cube of the velocity and the density of the fluid. This relationship is expressed as P=21​ρAv3, where ρ is the water density, A is the swept area, and v is the velocity. This formula highlights why high-density water currents are attractive for energy generation, even at lower velocities than wind. The operational status of these arrays confirms the practical application of this principle.

Challenges and market evolution

How do tidal stream generators compare to wind turbines?

Fluid Dynamics and Power Density

Tidal stream generators operate on principles analogous to wind turbines but function within a significantly denser medium. Water is approximately 800 times more dense than air, which fundamentally alters the energy capture dynamics. This high density allows tidal turbines to generate substantial power even at slower flow velocities compared to wind speeds required for equivalent output. The power available in a tidal current is governed by the kinetic energy of the moving fluid, often expressed as P=21​ρAv3, where ρ represents fluid density, A is the swept area, and v is the velocity. Because ρ for water is so much higher, the cubic relationship with velocity means that even moderate tidal flows can yield high energy densities.

Structural and Foundation Differences

The operational environment dictates distinct structural requirements for tidal stream turbines compared to their wind counterparts. While wind turbines must withstand dynamic aerodynamic loads and often utilize monopile or jacket foundations to handle wave action at the surface, tidal turbines are fully submerged. This submersion protects the mechanical components from harsh atmospheric conditions and salt spray corrosion, though it introduces challenges related to biofouling and hydrodynamic drag. The foundations for tidal stream generators must anchor the turbine to the seabed, often utilizing gravity bases, monopiles, or tension-leg platforms, depending on water depth and seabed geology. Unlike wind turbines, which are exposed to the elements, tidal turbines benefit from the stabilizing effect of the water column, reducing some structural fatigue but increasing the complexity of maintenance access.

Technological Classification

It is important to distinguish tidal stream generators from other tidal energy technologies. Tidal stream turbines, also known as tidal energy converters (TEC) or marine hydro-kinetic (MHK) generation systems, harness the kinetic energy of moving water. This is different from tidal barrages or lagoons, which operate on a potential energy principle by impounding water during high tide and releasing it through turbines as the tide falls. Tidal stream generators are thus more similar to wind farms in their direct conversion of kinetic flow into electricity, whereas barrages function more like conventional hydroelectric dams. This distinction is critical for understanding the site selection and environmental impact of each technology.

See also

• Power plants in Estonia
• Foundations for offshore wind turbines
• Blue hydrogen production: A case study on CO2 emission reduction in steam methane reforming
• Voerde Powerplant: Technical Profile and Operational Context
• Landfill gas power