6.3 Overtopping Devices
4 min read•august 7, 2024
Overtopping devices are wave energy converters that use ramps to guide waves into elevated reservoirs. These systems capture water above sea level, creating a head difference that drives low-head turbines to generate electricity.
Key components include the , , and turbine. Designers optimize reservoir size, ramp angle, and turbine selection to maximize energy capture. Notable examples are the offshore and the shoreline-based .
Reservoir and Ramp
Reservoir Design and Function
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- Overtopping devices utilize a reservoir to capture and store water from incoming waves
- Reservoir is elevated above the mean water level to create a head difference
- Allows for a more consistent flow of water to the turbine, smoothing out
- Size and shape of the reservoir impact the device's efficiency and energy output
- Larger reservoirs can store more water, providing a steadier flow to the turbine (Wave Dragon)
- Reservoir geometry affects the water's and the device's overall performance
Ramp Configuration and Purpose
- Ramp is an inclined surface leading up to the reservoir
- Directs incoming waves upwards and into the reservoir
- Ramp angle and surface characteristics influence the overtopping efficiency
- Ramp design considerations include material, slope, and wave focusing elements
- Concrete or steel are common ramp materials, chosen for durability and resistance to wave forces
- Optimal ramp slope balances wave run-up height and overtopping volume (Tapchan)
- Wave focusing elements, such as reflectors or wings, can enhance overtopping by concentrating wave energy
Overtopping Rate and Optimization
- Overtopping rate is the volume of water entering the reservoir per unit time
- Depends on incoming wave height, period, and direction, as well as device geometry
- Higher overtopping rates generally lead to increased energy production
- Optimization of overtopping rate involves balancing reservoir size, ramp design, and wave conditions
- Numerical modeling and physical testing help determine the most efficient configuration
- Adaptive control systems can adjust ramp angle or reservoir level in response to changing wave conditions
Turbine and Energy Conversion
Low-Head Turbine Characteristics and Selection
- Overtopping devices typically employ low-head turbines due to the relatively small head differences
- Kaplan, Bulb, and are common choices for overtopping applications
- These turbines efficiently operate under low-head conditions (2-5 meters)
- Turbine selection depends on factors such as head range, flow rate, and generator type
- are well-suited for low-head, high-flow conditions and can have adjustable blades for efficiency
- are compact and submerged, making them suitable for integration into overtopping devices
- Crossflow turbines are simple, robust, and can handle varying flow rates without significant efficiency losses
Potential Energy Conversion Process
- Water stored in the elevated reservoir contains potential energy due to its position
- Potential energy is proportional to the water's mass and the head difference between the reservoir and turbine
- As water flows down from the reservoir, its potential energy is converted into kinetic energy
- Low-head turbine converts the kinetic energy of the flowing water into mechanical energy
- Turbine blades rotate as water passes through, driving a shaft connected to an electrical generator
- Generator converts the mechanical energy into electrical energy, which can be transmitted to the grid or stored for later use
Notable Overtopping Devices
Wave Dragon
- Wave Dragon is a large, floating overtopping device developed in Denmark
- Consists of a doubly-curved ramp, a reservoir, and several low-head turbines
- Ramp shape focuses incoming waves towards the central reservoir, enhancing overtopping
- Prototype tested at 1:4.5 scale in Nissum Bredning, Denmark, and a 7 MW pre-commercial demonstrator was planned for the Welsh coast
- Prototype achieved a wave-to-wire efficiency of 18% and an average power output of 140 kW
- Full-scale device is designed for offshore deployment in water depths of 25-40 meters
Tapchan (Tapered Channel)
- Tapchan (Tapered Channel) is a shore-based overtopping device concept
- Utilizes a tapered channel to focus incoming waves and guide them into an elevated reservoir
- Tapered shape of the channel amplifies the wave height, increasing overtopping rates
- A 350 kW prototype was built and tested on the Norwegian coast in the mid-1980s
- Prototype operated successfully for several years, demonstrating the feasibility of the Tapchan concept
- Achieved a wave-to-wire efficiency of around 20% and an average power output of 70 kW
- Tapchan design is well-suited for locations with a narrow wave direction distribution and a gently sloping seabed
- Shoreline integration minimizes the visual impact and simplifies maintenance access compared to offshore devices
Key Terms to Review (26)
A. M. B. Z. van der Meer: A. M. B. Z. van der Meer is a prominent figure in the field of coastal engineering, particularly known for his work on overtopping devices and wave impact on coastal structures. His contributions have provided valuable insights into the design and analysis of structures that interact with water, improving our understanding of how to protect coastlines against erosion and flooding. The principles developed by van der Meer are critical for optimizing the efficiency and safety of wave energy devices.
Bulb turbines: Bulb turbines are a type of water turbine used in hydroelectric power generation, designed to operate efficiently in low-head sites. These turbines feature a bulbous casing that houses the turbine runner, which is directly submerged in water flow, allowing for optimal energy extraction from tidal and wave movements. This design makes them particularly suited for overtopping devices, where they can harness the energy from elevated water levels effectively.
Computational Fluid Dynamics: Computational Fluid Dynamics (CFD) is a branch of fluid mechanics that uses numerical analysis and algorithms to solve and analyze problems involving fluid flows. It plays a crucial role in simulating the behavior of fluids in various engineering applications, helping optimize designs and predict performance under different conditions, especially for energy systems harnessing tidal and wave energy.
Concrete Armor Units: Concrete armor units are large, precast concrete blocks designed to protect coastal structures and shorelines from erosion and wave action. These units are often used in overtopping devices to dissipate the energy of waves, preventing damage to infrastructure and enhancing stability in marine environments. Their design allows for efficient stacking and interlocking, which helps them withstand extreme weather conditions and the forces exerted by water.
Crossflow Turbines: Crossflow turbines are a type of hydro turbine that harness energy from water flowing perpendicular to the turbine's axis. These turbines are designed to operate efficiently at low heads and variable flow conditions, making them suitable for small-scale hydroelectric power generation. Their unique design allows for water to flow in two passes through the turbine, which can enhance energy capture and efficiency compared to traditional designs.
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 Impact Assessment (EIA): An Environmental Impact Assessment (EIA) is a systematic process used to evaluate the potential environmental effects of a proposed project or development before it is carried out. This process ensures that decision-makers consider environmental impacts along with economic and social factors, aiming to minimize negative consequences and promote sustainable development. EIA is crucial in identifying potential impacts, such as habitat disruption or water quality issues, particularly for projects involving energy generation technologies and their interactions with ecosystems.
Fluid Dynamics: Fluid dynamics is the branch of physics that studies the behavior of fluids (liquids and gases) in motion. This field is crucial for understanding how energy can be harnessed from ocean movements, such as waves and tides, as it provides insights into the forces and flow patterns that can impact energy conversion systems, efficiencies, and designs. Fluid dynamics principles help engineers predict how water interacts with structures and devices that capture ocean energy, enabling them to optimize performance and reliability.
Hydraulic jump devices: Hydraulic jump devices are structures used to control and dissipate energy in flowing water, particularly in the context of overtopping scenarios. These devices are designed to create a sudden change in flow regime, transitioning from supercritical to subcritical flow, which results in a turbulent flow that dissipates kinetic energy. This energy dissipation is crucial for protecting structures and ensuring stable water levels in tidal and wave energy applications.
Hydraulic loading: Hydraulic loading refers to the amount of water or fluid that is directed onto a surface or through a system over a specific period of time. This concept is crucial in the design and performance evaluation of overtopping devices, as it influences their efficiency and effectiveness in capturing and utilizing wave energy. Understanding hydraulic loading helps engineers determine how much water can be expected to flow over or through these structures, ensuring they are built to withstand the pressures and forces exerted by the waves.
K. s. k. b. n. m. van der Meer: k. s. k. b. n. m. van der Meer refers to a theoretical framework and empirical analysis used to evaluate the performance of overtopping devices in wave energy conversion systems. This framework helps in understanding how energy is extracted from waves, particularly focusing on the efficiency of various overtopping mechanisms and their design parameters.
Kaplan turbines: Kaplan turbines are a type of water turbine designed to generate electricity in hydropower plants, particularly in low-head sites with high flow rates. These turbines feature adjustable blades that allow them to efficiently harness kinetic energy from flowing water, making them suitable for variable flow conditions. Their design enables optimal performance under a range of operational scenarios, ensuring maximum energy extraction.
Marine ecology: Marine ecology is the study of the relationships and interactions between marine organisms and their environment, including both biotic and abiotic factors. Understanding these relationships is crucial for managing marine ecosystems, especially as they relate to human activities and the development of marine energy technologies. The health of marine ecosystems directly affects energy resources, such as tidal and wave energy systems, highlighting the need for sustainable design and implementation practices.
Marine Spatial Planning (MSP): Marine Spatial Planning (MSP) is a systematic process that guides where and when human activities occur in marine environments to reduce conflicts and enhance ecological, social, and economic benefits. By creating a framework for managing the ocean space, MSP helps balance competing uses like fishing, shipping, and energy development while ensuring the protection of marine ecosystems. This approach is essential for the sustainable use of marine resources, particularly in areas where development pressures are increasing.
Model testing: Model testing is the process of evaluating the performance, efficiency, and behavior of a physical or numerical model to ensure it accurately simulates real-world conditions. This is crucial for designs like overtopping devices, as it helps engineers predict how these systems will perform under various environmental scenarios, enabling them to make necessary adjustments before full-scale implementation.
Overtopping Turbines: Overtopping turbines are specialized devices designed to harness the energy generated from waves overtopping a structure, typically a breakwater or similar coastal installation. They utilize the kinetic energy of incoming waves, which flow over the top of the structure and then are directed through turbines to generate electricity. This technology captures energy in a unique way by taking advantage of the natural movement of water, making it an innovative solution in the realm of wave energy conversion.
Potential Energy: Potential energy is the stored energy in an object due to its position or state. In the context of ocean energy, potential energy plays a crucial role in how water levels and gravitational forces can be harnessed for energy generation, particularly in tidal and wave energy systems. The variations in sea level and tidal range are essential factors that determine the amount of potential energy available for conversion into usable power.
Power output: Power output refers to the amount of energy produced by a system over a specified time period, usually measured in watts (W) or kilowatts (kW). In the context of renewable energy systems, like those harnessing tidal and wave energy, understanding power output is crucial for evaluating their efficiency and feasibility. It directly impacts the design and operation of energy generation devices and systems, influencing how effectively they can convert natural forces into usable electricity.
Ramp: In the context of overtopping devices, a ramp is a structure that is designed to direct water flow and create hydraulic conditions favorable for energy extraction. Ramps are typically angled or sloped surfaces that facilitate the movement of water, allowing it to flow over the device efficiently. These structures play a crucial role in capturing wave energy and converting it into usable power.
Reservoir: A reservoir is a natural or artificial lake or storage area designed to hold water for various purposes, such as hydropower generation, irrigation, flood control, or water supply. In the context of overtopping devices, reservoirs play a crucial role in managing water flow and energy capture, allowing for controlled release of water to maximize energy production and minimize environmental impacts.
Steel structures: Steel structures are frameworks made primarily of steel that provide support and stability to buildings, bridges, and other constructions. These structures are known for their strength, durability, and resistance to various environmental factors, making them a popular choice in engineering applications, especially for overtopping devices that harness tidal and wave energy.
Sustainable Design: Sustainable design is an approach to creating products, systems, and environments that minimizes negative impacts on the environment while promoting the well-being of current and future generations. This concept emphasizes the use of renewable resources, energy efficiency, and waste reduction, ensuring that designs are environmentally friendly and socially responsible. In engineering contexts, sustainable design seeks to integrate environmental considerations into every stage of development, from conception to decommissioning.
Tapchan: A tapchan is a type of energy converter used in overtopping wave energy devices, designed to collect and convert the energy from waves into usable power. It functions by allowing water to flow into a reservoir through a channel, where it then drives turbines to generate electricity. The design and efficiency of a tapchan are crucial in maximizing energy extraction from ocean waves while minimizing environmental impact.
Turbine efficiency: Turbine efficiency is a measure of how effectively a turbine converts the energy from a fluid flow, such as water or air, into mechanical energy. This efficiency is crucial for assessing the performance of energy conversion devices, including those designed to harness tidal and wave energy, as it directly impacts the overall energy output and operational effectiveness of the system.
Wave Dragon: Wave Dragon is a type of wave energy converter designed to harness the energy of ocean waves and convert it into electricity. It operates by using an overtopping design that allows waves to flow over a ramp, filling a reservoir that drives turbines to generate power. This innovative technology combines features of both wave energy converters and overtopping devices, making it a significant player in renewable energy production.
Wave energy capture: Wave energy capture refers to the process of harnessing the kinetic and potential energy generated by ocean waves and converting it into usable electrical power. This technology utilizes various devices and systems that can absorb, convert, and store the energy produced by waves, contributing to renewable energy solutions and reducing reliance on fossil fuels.