🌊Tidal and Wave Energy Engineering Unit 3 – Tidal Hydrodynamics & Resource Analysis

Fundamentals of Tidal Systems

• Tidal systems are driven by the gravitational interactions between the Earth, Moon, and Sun
• Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth
• Tidal forces create bulges in the ocean on the sides of the Earth closest to and farthest from the Moon
• Tidal range is the vertical difference between high tide and low tide
  • Varies depending on location and can be influenced by local bathymetry and coastline shape
• Tidal currents are the horizontal flow of water accompanying the rising and falling of the tide
• Tidal cycles can be diurnal (one high and one low tide per day), semidiurnal (two high and two low tides per day), or mixed (a combination of diurnal and semidiurnal)
• Tidal patterns are influenced by the relative positions of the Earth, Moon, and Sun, leading to spring tides (higher high tides and lower low tides) and neap tides (lower high tides and higher low tides)

Tidal Forces and Patterns

• Tidal forces are primarily caused by the gravitational pull of the Moon on the Earth's oceans
• The Sun also exerts a gravitational force on the Earth's oceans, although it is less than half as strong as the Moon's force
• The combined gravitational forces of the Moon and Sun create two tidal bulges on opposite sides of the Earth
• Spring tides occur when the Earth, Moon, and Sun are aligned (during new and full moons), resulting in higher high tides and lower low tides
  • Gravitational forces of the Moon and Sun are additive during spring tides
• Neap tides occur when the Moon and Sun are at right angles to each other relative to the Earth (during first and third quarter moons), resulting in lower high tides and higher low tides
  • Gravitational forces of the Moon and Sun partially cancel each other out during neap tides
• Tidal patterns vary depending on the location and can be influenced by factors such as the shape of the coastline, ocean floor topography, and local wind patterns
• The rotation of the Earth causes the tidal bulges to move around the planet, creating the observed tidal cycles

Hydrodynamic Principles in Tidal Environments

• Tidal hydrodynamics involves the study of fluid motion in tidal environments
• Tidal currents are driven by the pressure gradient force resulting from the difference in water levels between high and low tides
• Coriolis force, caused by the Earth's rotation, deflects tidal currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere
• Bottom friction influences tidal current velocities, with higher friction leading to reduced current speeds
• Tidal currents can be affected by bathymetry, with currents often being stronger in shallow areas and constricted channels
• Turbulence plays a significant role in tidal environments, affecting mixing, sediment transport, and energy dissipation
  • Turbulent eddies can be generated by bottom friction, flow separation, and shear instabilities
• Stratification can occur in tidal environments due to differences in temperature, salinity, or suspended sediment concentration, affecting the vertical structure of tidal currents
• Resonance can amplify tidal ranges and currents in certain locations, such as estuaries and bays, where the natural frequency of the basin matches the tidal forcing frequency

Tidal Energy Resource Assessment

• Tidal energy resource assessment involves quantifying the available tidal energy at a specific location
• Tidal stream resource assessment focuses on the kinetic energy of tidal currents
  • Tidal current velocity is the primary factor determining the available kinetic energy
• Tidal range resource assessment focuses on the potential energy available due to the difference in water levels between high and low tides
• Tidal resource assessments typically involve a combination of field measurements, numerical modeling, and data analysis
• Field measurements can include deploying acoustic Doppler current profilers (ADCPs) to measure tidal current velocities and water levels
• Numerical models, such as depth-averaged or three-dimensional hydrodynamic models, simulate tidal flows and help predict tidal currents and water levels
  • Models are validated using field measurements to ensure accuracy
• Tidal energy resource assessments consider factors such as tidal current velocity, water depth, tidal range, and site accessibility
• Resource assessments also evaluate the temporal variability of tidal energy, including variations over tidal cycles, spring-neap cycles, and seasonal changes
• Site-specific constraints, such as environmental sensitivities, competing uses of the marine space, and grid connection, are considered in tidal energy resource assessments

Tidal Stream and Range Analysis

• Tidal stream analysis focuses on the assessment and characterization of tidal currents for energy extraction
• Tidal stream power density is proportional to the cube of the tidal current velocity ($P = \frac{1}{2} \rho v^3$, where $P$ is power density, $\rho$ is water density, and $v$ is current velocity)
• Cut-in speed is the minimum tidal current velocity required for a tidal stream turbine to start generating power
• Rated speed is the tidal current velocity at which a tidal stream turbine reaches its maximum power output
• Capacity factor is the ratio of the actual energy output of a tidal stream turbine to its theoretical maximum output over a given period
• Tidal range analysis focuses on the assessment and characterization of the difference in water levels between high and low tides for energy extraction
• Tidal range power potential is proportional to the square of the tidal range ($P \propto R^2$, where $P$ is power potential and $R$ is tidal range)
• Tidal barrage systems utilize the potential energy of the tidal range by capturing water at high tide and releasing it through turbines at low tide
• Tidal lagoon systems are similar to tidal barrages but are constructed as self-contained structures off the coast

Environmental Impacts and Considerations

• Tidal energy development can have various environmental impacts that must be considered and mitigated
• Changes in tidal currents and hydrodynamics due to tidal energy extraction can affect sediment transport, erosion patterns, and coastal morphology
• Tidal energy devices can pose collision risks to marine animals, such as marine mammals, fish, and seabirds
  • Collision risk can be mitigated through device design, operational strategies, and monitoring
• Underwater noise generated by tidal energy devices during construction and operation can impact marine animal behavior and communication
• Electromagnetic fields (EMF) generated by subsea cables can potentially affect the navigation and behavior of sensitive marine species
• Habitat alteration or loss can occur due to the installation of tidal energy devices and associated infrastructure
  • Careful site selection and design can minimize habitat disturbance
• Changes in water quality, such as increased turbidity or decreased dissolved oxygen, can result from tidal energy development
• Cumulative impacts of multiple tidal energy projects in a region must be assessed and managed
• Environmental monitoring and adaptive management strategies are crucial for identifying and mitigating any unforeseen impacts of tidal energy development

Tidal Energy Conversion Technologies

• Tidal stream turbines extract kinetic energy from tidal currents, similar to wind turbines
  • Horizontal axis turbines have a rotor mounted on a horizontal shaft, with blades rotating perpendicular to the tidal current
  • Vertical axis turbines have a rotor mounted on a vertical shaft, with blades rotating parallel to the tidal current
• Tidal kite systems use underwater kites tethered to the seabed to extract energy from tidal currents
• Tidal range technologies harness the potential energy from the difference in water levels between high and low tides
  • Tidal barrages are dam-like structures built across the mouth of an estuary or bay, with turbines integrated into the barrage to generate power
  • Tidal lagoons are artificial structures built offshore to capture and release seawater through turbines, generating power
• Oscillating hydrofoils use the lift force generated by tidal currents flowing over a hydrofoil to drive a hydraulic system and generate power
• Tidal energy technologies face challenges related to survivability in harsh marine environments, reliability, and cost-effectiveness
• Advancements in materials, manufacturing, and control systems are crucial for improving the performance and reducing the costs of tidal energy technologies

Case Studies and Real-World Applications

• The MeyGen Tidal Energy Project in Scotland is one of the world's largest tidal stream projects, with a planned capacity of 398 MW
  • The project uses horizontal axis turbines developed by SIMEC Atlantis Energy
• The Sihwa Lake Tidal Power Station in South Korea is the world's largest tidal range power facility, with a capacity of 254 MW
  • The project utilizes a tidal barrage with 10 submerged bulb turbines
• The Jiangxia Tidal Power Station in China, operational since 1980, has a capacity of 3.9 MW and demonstrates the long-term feasibility of tidal range technology
• The Rance Tidal Power Station in France, operational since 1966, has a capacity of 240 MW and is an example of a successful tidal barrage project
• The Roosevelt Island Tidal Energy (RITE) Project in New York, USA, is a pilot project testing tidal stream turbines in the East River
• The Pentland Firth and Orkney Waters in Scotland are a key site for tidal energy development, with several projects in various stages of planning and operation
  • The region has some of the strongest tidal currents in the world, with velocities exceeding 5 m/s
• The Bay of Fundy in Canada has one of the highest tidal ranges in the world, with a maximum range of over 16 meters, making it a promising location for tidal range energy development
• Tidal energy projects are being developed in various other countries, including France, Australia, Japan, and India, demonstrating the global interest in this renewable energy resource