🌊Tidal and Wave Energy Engineering Unit 6 – Wave Energy Converter Tech

Fundamentals of Wave Energy

• Wave energy harnesses the kinetic and potential energy of ocean waves to generate electricity
• Waves are created by wind blowing over the surface of the ocean, transferring energy to the water
• Wave height, wavelength, and period are key parameters that determine the energy content of waves
• Wave power is proportional to the square of the wave height and the wave period
  • Doubling the wave height quadruples the available wave power
• Wave energy resources are most abundant in coastal regions with high wind speeds and long fetch (distance over which the wind blows)
• Global wave energy potential is estimated to be around 2 TW, with significant resources in the North Atlantic, North Pacific, and Southern Ocean
• Wave energy is a renewable and clean energy source that can contribute to reducing greenhouse gas emissions and dependence on fossil fuels

Types of Wave Energy Converters

• Wave energy converters (WECs) are devices designed to capture and convert wave energy into electricity
• Point absorbers are floating or submerged buoys that oscillate with the waves, driving a power take-off system
  • Examples include the PowerBuoy and the CETO system
• Oscillating water columns (OWCs) use the rise and fall of waves to compress and decompress air, driving a turbine
  • The Limpet and the Mutriku Wave Energy Plant are examples of OWC devices
• Overtopping devices allow waves to spill over into a reservoir, with the stored water flowing back out through a turbine
  • The Wave Dragon and the SSG (Seawave Slot-Cone Generator) are overtopping WECs
• Attenuators are long, snake-like devices that flex and bend with the waves, driving hydraulic pumps
  • The Pelamis Wave Energy Converter is a well-known attenuator design
• Oscillating wave surge converters (OWSCs) are hinged flaps attached to the seabed that oscillate back and forth with the waves
  • The Oyster and the WaveRoller are examples of OWSC devices
• Submerged pressure differential devices use the pressure difference between the wave crest and trough to pump fluid through a turbine
  • The Archimedes Wave Swing is a submerged pressure differential WEC

Wave Energy Converter Components

• WECs consist of several key components that work together to capture and convert wave energy
• The primary wave interface interacts directly with the waves, such as a buoy, flap, or air chamber
  • Designed to efficiently capture wave energy and withstand harsh marine conditions
• The power take-off (PTO) system converts the mechanical energy of the primary wave interface into electrical energy
  • Common PTO systems include linear generators, hydraulic systems, and air turbines
• Moorings and foundations secure the WEC in place and transmit loads to the seabed
  • Catenary, taut-leg, and pile foundations are used depending on the WEC design and site conditions
• Control systems optimize the performance of the WEC by adjusting its behavior to the prevailing wave conditions
  • Includes sensors, actuators, and control algorithms to maximize energy capture and minimize loads
• Electrical systems condition and transmit the generated power to shore or to an offshore substation
  • Includes transformers, switchgear, and subsea cables
• Monitoring and communication systems enable remote operation, maintenance, and performance assessment of the WEC
  • Utilizes sensors, data acquisition systems, and wireless communication technologies

Energy Capture and Conversion Processes

• Wave energy capture and conversion involve several key processes that transform wave motion into electrical energy
• Wave-structure interaction describes how the WEC responds to and extracts energy from the incident waves
  • Influenced by factors such as the WEC geometry, mass, stiffness, and damping characteristics
• Hydrodynamic modeling predicts the forces, motions, and power absorption of the WEC using numerical methods
  • Includes linear and nonlinear wave theories, boundary element methods, and computational fluid dynamics
• Mechanical power transmission transfers the captured wave energy from the primary wave interface to the PTO system
  • Achieved through direct mechanical linkages, hydraulic systems, or pneumatic systems
• Power take-off (PTO) systems convert the mechanical energy into electrical energy
  • Linear generators directly convert linear motion into electricity using permanent magnets and coils
  • Hydraulic PTOs use the motion of the WEC to pump high-pressure fluid through a hydraulic motor or turbine
  • Air turbines, such as Wells turbines or impulse turbines, are driven by the oscillating air flow in OWC devices
• Power conditioning and grid integration ensure that the generated electricity meets the required standards for grid connection
  • Includes rectification, voltage step-up, and power quality control using power electronic converters
• Control strategies optimize the energy capture and conversion processes by adapting the WEC behavior to the wave conditions
  • Includes passive control (e.g., tuning the natural frequency) and active control (e.g., latching, declutching, and reactive control)

Efficiency and Performance Metrics

• Efficiency and performance metrics quantify the effectiveness of WECs in capturing and converting wave energy
• Capture width is the width of the wavefront from which a WEC absorbs energy
  • Expressed in meters and influenced by the WEC dimensions and wave-structure interaction
• Capture width ratio (CWR) is the ratio of the capture width to the physical width of the WEC
  • A CWR greater than 1 indicates that the WEC absorbs energy from a wavefront wider than its physical width
• Power matrix represents the power output of a WEC for a range of wave heights and periods
  • Used to estimate the annual energy production (AEP) at a specific site
• Capacity factor is the ratio of the actual energy output to the theoretical maximum output over a given period
  • Typically ranges from 20-40% for WECs, depending on the site and technology
• Availability is the percentage of time that a WEC is operational and generating power
  • Affected by factors such as maintenance, repairs, and environmental conditions
• Levelized cost of energy (LCOE) is the average cost per unit of energy generated over the lifetime of a WEC
  • Accounts for capital costs, operating costs, and energy production
  • Used to compare the economic viability of different WEC technologies and projects

Environmental Considerations

• Environmental considerations are crucial in the development and deployment of wave energy projects
• WECs can potentially impact marine habitats and biodiversity through changes in hydrodynamics, noise, and physical presence
  • Careful site selection and environmental impact assessments are necessary to minimize negative effects
• Collision risks exist for marine mammals, fish, and seabirds that may interact with WECs or associated infrastructure
  • Mitigation measures include device design modifications, acoustic deterrents, and monitoring programs
• Underwater noise generated by WECs during operation and installation can affect marine animal behavior and communication
  • Noise reduction strategies, such as using quieter components and sound insulation, can help minimize impacts
• Changes in sediment transport and coastal morphology may occur due to the presence of WECs and their effect on wave patterns
  • Numerical modeling and long-term monitoring are essential to assess and manage any potential changes
• Entanglement risks are associated with mooring lines and cables that may pose a hazard to marine life
  • Using stiffer, shorter, or more visible lines and implementing entanglement prevention measures can reduce risks
• Visual impact and public acceptance are important considerations for nearshore and visible offshore WEC installations
  • Engaging stakeholders, conducting visual impact assessments, and designing aesthetically pleasing WECs can improve public perception
• Decommissioning and recycling plans should be developed to ensure the sustainable and responsible end-of-life management of WECs
  • Includes the safe removal of devices, recycling of materials, and restoration of the site to its original condition

Challenges and Limitations

• Wave energy technologies face several challenges and limitations that hinder their widespread adoption
• High capital costs associated with the design, fabrication, and installation of WECs are a significant barrier to commercialization
  • Reducing costs through economies of scale, standardization, and technological advancements is crucial
• Survivability in extreme wave conditions is a critical challenge for WECs deployed in harsh offshore environments
  • Designing robust and reliable devices that can withstand storms, corrosion, and biofouling is essential
• Grid integration and power quality issues arise from the variable and intermittent nature of wave energy
  • Developing efficient power conditioning systems and energy storage solutions is necessary for smooth grid integration
• Maintenance and accessibility can be difficult and costly for offshore WECs, especially in remote locations
  • Designing for maintainability, remote monitoring, and predictive maintenance strategies can help minimize downtime
• Limited operational experience and performance data from real-world deployments hinder the validation and optimization of WEC designs
  • Establishing test sites, conducting long-term trials, and sharing knowledge among developers is essential
• Regulatory and permitting processes can be complex and time-consuming, varying across different jurisdictions
  • Streamlining approval processes and establishing clear guidelines for wave energy projects can facilitate development
• Competition with other marine users, such as fishing, shipping, and recreation, can lead to spatial conflicts
  • Marine spatial planning and stakeholder engagement are necessary to find compatible solutions and co-existence strategies

Future Developments and Research

• Future developments and research in wave energy aim to address current challenges and improve the viability of the technology
• Advanced materials, such as composite materials and high-strength polymers, are being explored to enhance the durability and performance of WECs
  • Lightweight, corrosion-resistant, and cost-effective materials can improve the survivability and economic feasibility of devices
• Novel WEC designs and concepts are being developed to increase energy capture efficiency and reduce costs
  • Includes multi-mode devices that combine different energy capture principles, such as point absorbers with OWCs
  • Modular and scalable designs allow for flexibility in deployment and capacity
• Control strategies and algorithms are being refined to optimize energy capture and minimize structural loads on WECs
  • Advanced control techniques, such as model predictive control and reinforcement learning, can adapt to changing wave conditions in real-time
• Numerical modeling and simulation tools are being improved to better predict WEC performance, loads, and environmental impacts
  • High-fidelity models that incorporate nonlinear effects, fluid-structure interaction, and array interactions are being developed
  • Machine learning techniques are being applied to optimize WEC design parameters and control strategies
• Hybrid systems that combine wave energy with other renewable energy sources, such as offshore wind or solar, are being investigated
  • Co-located or integrated systems can share infrastructure, reduce costs, and provide a more stable power output
• Improved power take-off and electrical systems are being developed to increase efficiency and reliability
  • Direct-drive generators, efficient hydraulic systems, and advanced power electronic converters are being researched
• Environmental monitoring and mitigation strategies are being advanced to minimize the potential impacts of wave energy projects on marine ecosystems
  • Includes the development of low-impact installation methods, real-time monitoring systems, and adaptive management approaches
• Collaborative research and development efforts, involving academia, industry, and government agencies, are crucial for advancing wave energy technology
  • International cooperation, knowledge sharing, and joint research projects can accelerate progress and support the growth of the wave energy sector