🌊Tidal and Wave Energy Engineering Unit 4 – Wave Energy Conversion Fundamentals

Introduction to Wave Energy

• Wave energy harnesses the power of ocean waves to generate electricity
• Waves are created by wind blowing over the surface of the ocean, transferring energy to the water
• Wave energy is a renewable and clean energy source with the potential to contribute significantly to global energy production
• The amount of energy available in waves depends on factors such as wave height, wavelength, and water density
• Wave energy is most abundant in coastal regions with consistent wind patterns and long fetches (uninterrupted distances over which the wind blows)
• Countries actively researching and developing wave energy technologies include the United Kingdom, United States, Australia, and Portugal
• Wave energy has the advantage of being more predictable and consistent compared to other renewable sources like wind and solar

Wave Mechanics and Characteristics

• Waves are oscillations that propagate through a medium, such as water, transferring energy from one point to another
• The most common types of waves in the ocean are surface gravity waves, which are driven by the gravitational pull of the Earth and the wind
• Wave height is the vertical distance between the crest (highest point) and the trough (lowest point) of a wave
• Wavelength is the horizontal distance between two consecutive crests or troughs
• Wave period is the time it takes for two consecutive crests or troughs to pass a fixed point
• Wave frequency is the number of waves passing a fixed point per unit time, measured in Hertz (Hz)
• The energy in a wave is proportional to the square of its amplitude (half the wave height)
• As waves propagate, they undergo various phenomena such as refraction (bending due to changes in water depth), diffraction (spreading around obstacles), and interference (superposition of multiple waves)

Types of Wave Energy Converters

• Wave energy converters (WECs) are devices designed to capture the energy from ocean waves and convert it into electricity
• Point absorbers are floating or submerged buoys that absorb energy from waves in all directions
  • Examples include the PowerBuoy (Ocean Power Technologies) and the CETO system (Carnegie Clean Energy)
• Attenuators are long, snake-like devices that are oriented parallel to the direction of wave propagation, flexing as waves pass by
  • The Pelamis Wave Energy Converter is a well-known example of an attenuator
• Oscillating water columns (OWCs) are partially submerged structures with an air chamber that uses the rise and fall of waves to compress and decompress air, driving a turbine
  • The Limpet (Land Installed Marine Power Energy Transformer) is an example of an OWC
• Overtopping devices capture water from incoming waves into a reservoir, which is then released through a turbine to generate electricity
  • The Wave Dragon is an example of an overtopping device
• Submerged pressure differential devices are located below the water surface and use the pressure difference created by passing waves to pump fluid through a turbine
  • The Archimedes Wave Swing is an example of a submerged pressure differential device

Energy Conversion Principles

• Wave energy converters transform the kinetic and potential energy of waves into electrical energy
• The energy conversion process typically involves two stages: primary conversion (from wave energy to mechanical energy) and secondary conversion (from mechanical energy to electrical energy)
• Primary conversion mechanisms include:
  • Heave (vertical motion): Used by point absorbers and attenuators
  • Surge (horizontal motion): Used by oscillating wave surge converters
  • Pitch (rotational motion): Used by some attenuators and submerged pressure differential devices
• Secondary conversion is typically achieved using linear generators, hydraulic systems, or air turbines
  • Linear generators convert the linear motion of a buoy or float directly into electricity
  • Hydraulic systems use the motion of a WEC to pump a fluid through a turbine, which drives a generator
  • Air turbines, such as Wells turbines, are used in oscillating water column devices to convert the bi-directional airflow into unidirectional rotation
• The efficiency of the energy conversion process depends on factors such as the WEC design, the wave conditions, and the power take-off system

Efficiency and Power Output

• The efficiency of a wave energy converter is the ratio of the electrical power output to the available wave power
• Wave power is the rate at which energy is transferred by a wave, measured in watts per meter (W/m) of wave crest length
• The available wave power ($P$) can be calculated using the following equation: $P = \frac{1}{8}\rho g^2 H^2_s T_e$
  • $\rho$ is the water density (approximately 1,025 kg/m³ for seawater)
  • $g$ is the acceleration due to gravity (9.81 m/s²)
  • $H_s$ is the significant wave height (average height of the highest one-third of waves)
  • $T_e$ is the energy period (the period of a sinusoidal wave with the same energy as the real sea state)
• The power output of a WEC depends on its efficiency and the available wave power
• Factors affecting WEC efficiency include:
  • The ability to capture energy from a wide range of wave heights and periods
  • The efficiency of the power take-off system
  • Losses due to friction, turbulence, and electrical resistance
• Typical efficiencies of WECs range from 25% to 50%, depending on the technology and wave conditions
• The capacity factor of a WEC is the ratio of its actual power output over time to its maximum possible output
  • Capacity factors for wave energy are typically higher than those for wind and solar, due to the more consistent nature of waves

Environmental Considerations

• Wave energy converters have the potential to impact marine ecosystems and coastal environments
• The installation of WECs may disturb the seabed and alter local hydrodynamics, affecting sediment transport and coastal morphology
• The presence of WECs can create artificial reefs, attracting marine life and potentially altering local biodiversity
• Noise generated by WECs during operation may affect marine mammals and other sensitive species
  • However, the noise levels are generally lower than those produced by ships and other human activities
• The risk of collisions between marine animals and WECs is a concern, particularly for large, slow-moving species such as whales
  • Mitigation measures include the use of acoustic deterrents and the careful selection of WEC locations
• The visual impact of WECs on coastal landscapes can be a concern for local communities and tourism
  • Submerged or low-profile designs can help minimize visual impact
• The use of non-toxic, biodegradable materials and lubricants can help reduce the risk of pollution in the event of a leak or accident
• Environmental impact assessments and monitoring programs are essential to identify and mitigate potential negative effects of wave energy development

Challenges and Limitations

• The harsh marine environment poses significant challenges for the design, installation, and maintenance of wave energy converters
  • WECs must withstand corrosion, biofouling, and extreme wave conditions
  • Regular maintenance and repairs can be difficult and costly due to the remote locations and challenging access
• The variability of wave conditions across different time scales (hourly, daily, seasonal) can affect the consistency and reliability of power output
  • Energy storage systems or grid integration with other renewable sources may be necessary to balance supply and demand
• The cost of wave energy is currently higher than that of other renewable technologies like wind and solar
  • The high capital costs and limited operational experience of WECs contribute to the higher levelized cost of energy (LCOE)
• The lack of standardization in WEC designs and components can hinder the development of a robust supply chain and economies of scale
• Regulatory and permitting processes for wave energy projects can be complex and time-consuming, varying across different countries and jurisdictions
• The potential environmental impacts of wave energy development, as well as conflicts with other ocean users (e.g., fishing, shipping, recreation), can create challenges for site selection and public acceptance
• Limited grid infrastructure in remote coastal areas can make it difficult to transmit the generated electricity to end-users

Future Developments and Research

• Continued research and development efforts aim to improve the efficiency, reliability, and cost-effectiveness of wave energy converters
• Advanced materials, such as composites and nanomaterials, are being explored to enhance the durability and performance of WECs in harsh marine environments
• Numerical modeling and simulation tools are being developed to optimize WEC designs and predict their performance in various wave conditions
  • These tools can help reduce the need for expensive physical prototypes and field tests
• Control strategies, such as real-time wave forecasting and adaptive control, are being investigated to maximize power output and minimize structural loads on WECs
• Novel power take-off systems, such as dielectric elastomer generators and magnetostrictive materials, are being researched to improve the efficiency and reliability of energy conversion
• Hybrid systems that combine wave energy with other renewable sources, such as wind or solar, are being explored to increase the overall capacity and stability of power generation
  • Co-located wind-wave farms can share infrastructure and reduce costs
• The development of wave energy arrays, consisting of multiple WECs working together, can help mitigate the variability of power output and increase the total capacity
  • Research is ongoing to optimize the layout and spacing of WECs in arrays to maximize energy capture and minimize interactions
• Advancements in energy storage technologies, such as batteries and hydrogen production, can help address the intermittency of wave energy and facilitate grid integration
• Collaborative efforts between industry, academia, and government agencies are crucial to address the technical, economic, and environmental challenges facing wave energy development