6.1 Point Absorber Systems
4 min read•august 7, 2024
Point absorber systems are wave energy converters that use buoys to capture energy from ocean waves. These devices harness the up-and-down motion of waves to generate electricity, with designs ranging from single-body to multi-body absorbers.
The effectiveness of point absorbers depends on their design, , and mooring configuration. Optimizing these elements is crucial for maximizing energy capture and ensuring the device's survival in harsh marine environments.
Buoy Types and Motion
Buoy Designs and Characteristics
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- Point absorber systems utilize a buoy, a floating structure that moves in response to wave action
- Buoys are designed to capture energy from the heave motion, the up and down movement of the waves
- Single-body absorbers consist of a single floating buoy that directly drives the power take-off system
- Rely on the relative motion between the buoy and a fixed reference (seabed or a submerged plate)
- Examples include the PowerBuoy by Ocean Power Technologies and the Wavebob
- Two-body absorbers, also known as multi-body absorbers, utilize the relative motion between two floating bodies to drive the power take-off system
- Typically consists of a buoy and a submerged reaction plate or a second buoy
- The relative motion between the two bodies is used to generate power
- Examples include the OPT PB3 PowerBuoy and the Wavebob Mk2
Optimizing Buoy Motion for Energy Capture
- Buoy shape and size are critical factors in optimizing energy capture from waves
- Spherical, cylindrical, and disk-shaped buoys are common designs
- The buoy's natural frequency should match the dominant wave frequency for efficient energy absorption
- Heave motion, the vertical of the buoy, is the primary motion harnessed for
- Other motions, such as pitch and surge, can also contribute to energy capture in some designs
- Single-body absorbers rely solely on the heave motion of the buoy relative to a fixed reference
- Simplifies the design but may limit energy capture in certain wave conditions
- Two-body absorbers exploit the relative motion between two floating bodies, enhancing energy capture
- Allows for a wider range of motion and adaptability to different wave conditions
- Introduces additional complexity in design and mooring requirements
Power Take-Off Systems
Power Take-Off (PTO) System Functions and Types
- The power take-off (PTO) system converts the mechanical energy captured by the buoy into electrical energy
- PTO systems for point absorbers can be classified into three main categories: hydraulic, linear generator, and others (pneumatic, mechanical, etc.)
- Hydraulic PTO systems use the motion of the buoy to drive hydraulic cylinders, which then power a hydraulic motor coupled to an electric generator
- Offer high force density, enabling compact designs
- Require additional components such as accumulators, valves, and reservoirs
- Examples include the PTO systems used in the Wavebob and Searev devices
- Linear generators directly convert the linear motion of the buoy into electrical energy without intermediate mechanical systems
- Consist of a stator and a translator, with either the stator or translator attached to the buoy
- Provide direct energy conversion, reducing complexity and improving efficiency
- Examples include the Archimedes Wave Swing and the Seabased linear generator
PTO System Design Considerations and Challenges
- PTO system efficiency is crucial for overall wave energy converter performance
- Losses in the PTO system can significantly reduce the net power output
- Efficient power conditioning and control strategies are essential to maximize energy extraction
- Reliability and durability of PTO components in the harsh marine environment are critical challenges
- Exposure to saltwater, biofouling, and extreme weather conditions can degrade performance over time
- Regular maintenance and robust design are necessary to ensure long-term operation
- Scalability and cost-effectiveness of PTO systems are important considerations for commercial viability
- Designing PTO systems that can be easily scaled up for larger power output
- Reducing the cost of PTO components and simplifying manufacturing processes
Mooring and Resonance
Mooring System Design and Functions
- Mooring systems are essential for keeping point absorbers in place and withstanding various environmental loads
- The mooring system must balance the need for restraint with the allowance for sufficient buoy motion to capture energy
- Mooring configurations for point absorbers can be categorized as single-point, multi-point, or submerged
- Single-point mooring uses a single tether to connect the buoy to the seabed, allowing the buoy to weathervane and align with the waves
- Multi-point mooring utilizes multiple tethers to provide greater stability and resistance to drift
- Submerged mooring systems use a combination of buoyancy and gravity to maintain the buoy's position below the water surface
- Mooring lines can be made of various materials, such as synthetic ropes (polyester, nylon), steel cables, or elastomeric tethers
- Material selection depends on factors such as load capacity, elasticity, and durability in the marine environment
Resonance and Its Role in Energy Capture
- occurs when the natural frequency of the buoy matches the dominant frequency of the incoming waves
- Operating at or near resonance allows for maximum energy absorption, as the buoy's motion is amplified
- The natural frequency of the buoy is determined by its mass, geometry, and the stiffness of the mooring system
- Adjusting these parameters can help tune the buoy to the prevailing wave conditions at a specific site
- Resonance can be achieved through careful design of the buoy and mooring system
- Adding supplementary mass or changing the buoy's shape can lower its natural frequency
- Incorporating elastomeric tethers or pneumatic springs can provide the necessary stiffness for resonance
- Challenges in maintaining resonance include the variability of wave conditions and the need for adaptable systems
- Wave frequency and height can change over time, requiring active control or passive adaptability
- Designing systems that can maintain resonance across a range of wave conditions is an ongoing area of research
Key Terms to Review (18)
Buoy: A buoy is a floating device that is anchored in water, used for navigation, marking positions, or supporting instruments. In the context of energy systems, buoys can serve as platforms for devices that capture wave or tidal energy, playing a critical role in energy conversion processes. Their design and function are closely tied to the efficiency and stability of energy generation from marine environments.
Capture width: Capture width is a measure of the effective area over which a wave energy converter can absorb energy from incoming waves. This concept is crucial for understanding how efficiently a device can convert wave energy into usable power, and it plays a significant role in evaluating the performance of different wave energy systems, including point absorbers and oscillating water columns.
Conversion efficiency: Conversion efficiency refers to the ratio of useful energy output from a system to the total energy input into that system, expressed as a percentage. High conversion efficiency indicates that a larger portion of the input energy is being transformed into usable output energy, which is particularly crucial in optimizing energy systems like those harnessing tidal and wave energy. Understanding conversion efficiency helps in evaluating the effectiveness of different technologies and systems designed for energy capture and utilization.
David Mackay: David Mackay was a prominent physicist and engineer known for his work in energy and sustainability, particularly in the context of renewable energy technologies. He is best known for his book 'Sustainable Energy – Without the Hot Air,' which presents a comprehensive analysis of energy consumption and the feasibility of various renewable energy sources, including tidal and wave energy. His insights have influenced the development and optimization of energy systems, addressing both technical challenges and the need for effective control strategies.
Energy Conversion: Energy conversion refers to the process of changing energy from one form to another, enabling its utilization for various applications. This is crucial in harnessing energy from natural resources, particularly in ocean energy systems, where kinetic and potential energy from waves and tides are transformed into mechanical and then electrical energy. Understanding energy conversion is essential for optimizing performance and efficiency in different ocean energy technologies.
Environmental Sustainability: Environmental sustainability refers to the responsible management of resources to meet current needs without compromising the ability of future generations to meet their own needs. This concept emphasizes the importance of maintaining ecological balance, reducing pollution, and conserving biodiversity, ensuring that natural systems remain healthy and resilient over time.
European Marine Energy Centre Guidelines: The European Marine Energy Centre (EMEC) Guidelines are a set of best practices and technical standards designed to support the development, testing, and deployment of marine energy technologies, particularly focusing on wave and tidal energy systems. These guidelines promote safety, environmental sustainability, and efficiency in marine energy projects, ensuring that they are carried out with minimal impact on the marine environment while maximizing their potential for energy generation.
Heave-type point absorbers: Heave-type point absorbers are a specific kind of wave energy converter that captures energy from the vertical movement of waves, primarily utilizing the heaving motion of the device. These devices are designed to float on the water's surface and convert the up-and-down motion caused by wave action into mechanical or electrical energy. This makes them effective in harnessing energy from ocean waves, contributing to renewable energy solutions.
Hydraulic actuation: Hydraulic actuation refers to the use of pressurized fluid to generate mechanical motion and force. In wave energy systems, this method is often employed to convert the kinetic energy of waves into usable energy, making it a key component in various energy conversion technologies.
Hydrodynamic efficiency: Hydrodynamic efficiency refers to the effectiveness with which a system converts the energy from moving water into usable mechanical energy. It reflects how well a device, like a wave or tidal energy converter, harnesses kinetic and potential energy from water movement while minimizing losses due to drag, turbulence, and other factors. High hydrodynamic efficiency is crucial for optimizing energy capture and ensuring that the maximum possible energy is converted into usable forms.
International Electrotechnical Commission (IEC) Standards: IEC standards are globally recognized specifications and guidelines developed by the International Electrotechnical Commission to ensure the safety, performance, and interoperability of electrical and electronic devices and systems. These standards play a crucial role in various sectors, including renewable energy technologies, as they help set the benchmarks for design, testing, and implementation, fostering international trade and enhancing consumer safety.
Marine Impact Assessment: Marine impact assessment is a systematic process used to evaluate the potential environmental effects of proposed marine projects, particularly in the context of ocean energy development. This evaluation aims to ensure that marine ecosystems are protected and that any negative impacts are minimized or mitigated. Such assessments are crucial in understanding how various ocean energy types and technologies, like wave energy converters and point absorber systems, interact with marine life and habitats.
Mooring system design: Mooring system design refers to the engineering process of creating a reliable anchoring system that secures floating structures, such as point absorber devices, in place against environmental forces like waves, wind, and currents. Effective mooring systems are essential for ensuring the stability and operational efficiency of these structures, particularly in offshore energy applications where the dynamic marine environment can exert significant loads on the moored device.
Oscillation: Oscillation refers to the repetitive variation, typically in time, of some measure about a central value or between two or more different states. In the context of energy systems, oscillation is crucial as it represents the periodic motion of wave energy devices and is central to their functionality and efficiency.
Power take-off system: A power take-off system is a crucial component in energy conversion devices that captures and converts mechanical energy from a moving medium, such as waves or tidal currents, into electrical energy. This system is essential for efficiently transferring the harvested energy to the grid or storage systems, optimizing the performance of various energy converters, including wave energy converters and tidal stream systems.
Resonance: Resonance is a phenomenon that occurs when a system is driven at its natural frequency, resulting in an amplification of the response. This concept is crucial in understanding how energy transfer can be maximized, particularly in wave energy systems where optimal energy extraction from ocean waves is desired. It highlights the importance of matching the frequency of wave oscillations with the natural frequencies of converters to achieve efficient energy conversion.
Sergio F. G. Silva: Sergio F. G. Silva is a notable researcher in the field of wave energy, particularly recognized for his contributions to the understanding and development of point absorber systems. His work focuses on optimizing the efficiency and performance of these systems, which convert wave energy into usable power. Silva's research not only emphasizes the technical aspects but also addresses environmental impacts and sustainability within the context of marine renewable energy.
Surge-type point absorbers: Surge-type point absorbers are a type of wave energy converter designed to capture the energy of ocean waves by utilizing the relative motion between a buoy and the submerged components. These systems primarily convert vertical and horizontal wave movements into usable energy, effectively harnessing the kinetic and potential energy of waves. The efficiency of surge-type point absorbers is heavily influenced by their design, orientation, and the characteristics of the wave environment in which they operate.