7.3 Tidal Fence and Tidal Lagoon Concepts

Tidal fences and lagoons offer innovative ways to harness tidal energy. Fences use turbines in a barrier across bays, while lagoons create enclosed areas to capture and release tidal water. Both systems aim to generate electricity from the natural ebb and flow of tides.

These concepts build on traditional tidal barrage technology but with less environmental impact. Fences allow water and marine life to pass through, while lagoons can be designed to work with existing coastal features. They represent exciting developments in tidal energy extraction.

Tidal Fence and Lagoon Concepts

Tidal Fence Characteristics

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• Consists of a series of vertical axis turbines mounted within a fence-like structure
• Designed to be deployed across the mouth of a bay or estuary to capture tidal flow
• Allows water to pass through the turbines, generating electricity from the tidal currents
• Typically requires water depths of at least 20-30 meters for optimal operation
• Can be designed with a central gap to allow for navigation and marine life passage

Tidal Lagoon and Artificial Lagoon Principles

• Tidal lagoons are partially or fully enclosed bodies of water that experience tidal fluctuations
• Artificial lagoons are man-made structures designed to capture and store tidal energy
• Lagoons are created by constructing a barrier or embankment to enclose a portion of the coastline
• Water enters the lagoon during high tide and is retained as the tide recedes, creating a head difference
• Electricity is generated as water is released back to the sea through turbines mounted in the embankment

Impoundment and Energy Storage

• Impoundment refers to the process of enclosing and storing water within a lagoon or reservoir
• Tidal lagoons act as energy storage systems, holding water at high tide and releasing it at low tide
• The stored potential energy is converted into kinetic energy as water flows through the turbines
• Impoundment allows for a more consistent and predictable energy output compared to tidal stream systems
• The size and depth of the impoundment determine the amount of energy that can be stored and generated

Tidal Lagoon Components

Caisson Structure and Turbine Housing

• Caisson structures are large, watertight chambers used in the construction of tidal lagoons
• Caissons are prefabricated offsite and floated into position before being sunk and filled with concrete
• Turbine housings are integrated into the caisson structure to accommodate the hydroelectric turbines
• The number and size of turbines depend on the lagoon's dimensions and tidal range
• Caisson structures provide a stable foundation for the turbines and help maintain the lagoon's integrity

Sluice Gates and Water Flow Control

• Sluice gates are adjustable barriers used to control the flow of water in and out of the lagoon
• Gates are opened during high tide to allow water to enter the lagoon and closed as the tide recedes
• During low tide, the sluice gates are opened to release water back to the sea through the turbines
• The control of water flow through the sluice gates is critical for optimizing energy generation
• Advanced control systems monitor tidal conditions and adjust the gates accordingly

Embankment Design and Construction

• The embankment is the barrier that encloses the lagoon and separates it from the open sea
• Embankments are typically constructed using rock-fill or sand-fill material, with a clay or geomembrane core for impermeability
• The height and width of the embankment depend on the tidal range, wave conditions, and desired lagoon size
• Embankment design must consider factors such as stability, erosion resistance, and seismic resilience
• Construction techniques, such as vibro-compaction and deep soil mixing, are used to ensure the embankment's integrity

Advanced Tidal Lagoon Designs

Multiple Basin Systems for Optimized Energy Generation

• Advanced tidal lagoon designs incorporate multiple basins to optimize energy generation
• A two-basin system consists of a high-level basin and a low-level basin separated by an internal embankment
• Water is transferred between the basins through turbines, generating electricity during both ebb and flood tides
• Multiple basin systems can generate energy more consistently and efficiently than single basin designs
• The arrangement and sizing of the basins can be optimized based on the local tidal characteristics and energy demand

Pumped Storage and Ancillary Services

• Tidal lagoons can be designed to incorporate pumped storage capabilities
• During periods of low electricity demand, excess energy is used to pump water from the low-level basin to the high-level basin
• The stored water is released back through the turbines during peak demand periods, providing additional energy generation
• Pumped storage allows tidal lagoons to offer ancillary services, such as grid balancing and frequency regulation
• By varying the pumping and generation cycles, tidal lagoons can help stabilize the grid and support the integration of other renewable energy sources