Buoy Shape Research Articles

Impact of translator mass and buoy choice on a power absorption of point absorbing wave energy converter linear generator with linear generator power take off

AbstractOcean waves have the potential to contribute to future renewable electricity production. A wave energy converter (WEC) is a technology developed to absorb the energy of the wave and convert it to another form of energy. The Uppsala University WEC (UU WEC) is a point absorber with a direct drive permanent magnet synchronous linear generator power take off. Among other parameters affecting the value of absorbed power for UU WEC are the buoy size, mass of the system consisting of the buoy and translator, and available wave energy at the site of interest. This study reviews the earlier static model that considered only static forces as the buoyancy and gravity forces and neglected all dynamic forces. The static model was proposed to simplify the early‐stage design decision. Although the static model was applied to two UU WECs of different dimensions, the present study shows that the static model is not held for certain buoy and translator dimensions. As an alternative, the dynamic model which accounts for the impact of hydrodynamic forces and various translator masses is proposed. The dynamic model is based on Cummins' equation and the linear potential flow theory, and the damping force is approximated as a viscous damper with the constant damping coefficient optimizing the absorbed mechanical power under a particular sea condition. The dynamic model is applied to four fixed buoy geometries of two shapes (cylinder and cylinder with a moonpool), each of two different dimensions, but the method can be extended to other buoy shapes and dimensions. In addition, the impact of translator mass was assessed for two sites located on the west coast of Sweden and near Gran Canaria, Spain. A translator of 10–11 t promotes 16.8% higher annual average power absorption for a cylindrical buoy compared to a translator of 6 t for the same buoy. However, heavier translators up to 15 t provide only 1.1% increase in average annual absorbed power.

Theoretical evaluation of a hybrid buoyancy-compressed air energy storage system

Energy storage plays a pivotal role in the emerging green economy. This study, for the first time, presents the theoretical evaluation of a buoyancy power generator combining with the compressed air energy storage (CAES-BPG) system. A theoretical model that satisfies conservation laws and does not involve any adjustable parameters is developed for the calculation of system efficiency and power generation capacity. To accurately predict drag acting on the buoy, the low Reynolds number κ-ω SST model with a locally refined mesh (y+~1) is applied to fully resolve the viscosity affected regions including the viscous sublayers, whilst the surrounding flow is modelled by LES. The refined mesh moves passively with the buoy and does not change, whilst mesh in other regions undergoes smoothing and remeshing to accommodate the buoy's motion. Correlations of drag coefficients are then developed as a function of buoy shape, Reynolds number, and vessel-to-buoy size ratio. The aerofoil shape yielded a substantially smaller drag coefficient compared to the square, hex, and bullet shapes. System efficiency showed independency on buoy volume and shell material but decreased as the water level in the vessel increased, reaching 64 % for a water level of 1.5 m. Similarly, as the water level increased, the system output power decreased. The output power increased as the buoy volume increased, reaching ~22 kW for a buoy volume of 2 m3 at a water level of 1.5 m. This CAES-BPG system is highly automatic and fully green, and can operate continuously once compressed air is connected, showing great potential for implementation as an energy storage / utilisation system of small to medium capacities.

A Numerical Study of the Performance of Point Absorber Wave Energy Converters

Free-floating and submerged wave energy converters (SWECs) are regarded as promising technologies for renewable energy production. These converters rely on a heave-motion buoy to capture the kinetic energy of ocean waves and convert it into electrical energy through power conversion systems. To better understand the impact of various factors on power generation and efficiency, the effects of different buoy shapes (rectangular, circular cylinder, and trapezoidal fin), submergence depths (0, 0.1, and 0.2 m), wave heights (0.04, 0.06, and 0.1 m), and spring stiffness (50 and 100 N/m) were investigated. A 2D numerical wave tank with a buoy was simulated, and the results were validated against experimental data. Information on vorticity, vertical displacement, power absorption, and efficiency are provided. The findings indicate that the buoy shape and wave height significantly affect power absorption and efficiency. Additionally, this study reveals that increasing submergence leads to higher power absorption and lower conversion efficiency.

SPH-based numerical modelling and performance analysis of a heaving point absorber type wave energy converter with a novel buoy geometry

Heaving point absorber (HPA) is one of the most promising technologies to absorb wave energy. Accordingly in this work, performance of a HPA in terms of capture-width-ratio (CWR) under the effect of hydrodynamics forces were analysed using the SPH-based open-source package: DualSPHysics. 395 cases of simulations, which include five different geometries with different densities of the buoy, different buoy height to wave height ratios, and damping coefficients of the WEC over a range of wave frequencies were studied and optimum operational parameters were found for the novel buoy geometry named: “Super cuboid”. Compared to other geometries, it produced up to 23 % of efficiency increment over a wide range of operational conditions. Furthermore, the flow patterns and pressure variation around different buoy geometries were studied and found that an increase in the projected area of the buoy in vertical direction tends to increase the pressure force which results in increase of CWR. The CWR is also slightly enhanced by the bottom shape of the buoy. Hence it is expected that the use of this novel buoy shape, will contribute to the design of more sustainable and reliable WECs in the future.

Optimization of Buoy Shape for Wave Energy Converter Based on Particle Swarm Algorithm

In order to improve the wave energy capture rate of the buoy of a wave energy generation device, this paper proposes a multi-degree of freedom method to optimize the shape of the buoy with maximum wave energy capture. Firstly, a multi-degree of freedom wave energy converter was designed, and the buoy shape was defined using a B-spline curve to generate the shape vector; then, a numerical model of the multi-degree of freedom wave energy converter was established and numerical calculations were carried out using AQWA/WEC-Sim software; on this basis, the particle swarm optimization algorithm was introduced to find the buoy shape corresponding to the maximum wave energy capture. Finally, the optimization of the buoy shape was in irregular waves. The results show that as the wave energy capture increased, the buoy shape tended to be flatter, with a smaller taper, and the optimal buoy shape had a better motion response than the conventional cone buoy. Eventually, the correctness of the buoy shape optimization method was verified through experimental testing.

NUMERICAL ANALYSIS OF WAVE ENERGY POINT ABSORBERS BUOY SHAPE

ABSTRACT: Energy demands have increased dramatically nowadays giving rise to the growing consumption of electricity, most of which is provided by fossil fuels, which has increased energy costs as well as the harmful emissions resulting from fuel combustion. This has resulted in advancements in the use of renewable energy sources to satisfy the current energy demands. These sources include the use of solar, wind, and wave power to generate energy in a green manner. This paper presents the use of wave energy point absorbers with various shapes to assess their performance when exposed to waves. Four buoy shapes are modelled and analyzed using Ansys Aqwa to observe their motion as well as their responsiveness to the out of balance forces subjected on them. The results show that the cylindrically shaped buoy can achieve around 52.8% more motion in the vertical direction when compared to the average motion of the different buoy designs. This shows that a cylinder buoy is the economic option for optimized wave energy absorption.

Spatial focussing of wave energy for improved power capture by an oscillating water column

The world’s oceans represent an abundant reserve of renewable energy. Wave power is a ubiquitous resource which is both temporarily and spatially consistent. This designates wave power as a particularly suitable energy resource for exploitation. Nevertheless, the development of wave energy technologies is lagging behind other renewable energy devices. Few grid-connected wave energy converters (WECs) have been constructed or deployed. The main barriers to the commercialisation of WECs is the relatively low energy conversion efficiencies that have been achieved, both with prototype devices, and with full scale installations.
 Many studies have been performed to augment the wave energy capture by WECs. Until now, investigations have predominantly focussed on modifying the device geometry (e.g. additional chambers in OWCs, varied buoy shapes for point absorbers etc.), or PTO systems to improve the energy yield. In this study, a novel approach is adopted, which demonstrates that significantly improved WEC performances can be achieved by manipulating the hydrodynamic wave field in the vicinity of the WEC to consolidate the wave energy. Few studies have been undertaken on methods to focus the linearly distributed wave energy at the WEC device. The concept of concentrating renewable energy resources for improved harvest is employed in other industries, for example, concentrated solar power plants utilize vast arrays of mirrors to consolidate the areally distributed solar energy to a central PTO tower. The present study demonstrates that wave energy can be focussed in an analogous manner.
 This research investigates the effectiveness of various geometry structures to reflect and focus the incident wave energy to point locations at which WEC devices can be positioned. To achieve this energy focussing effect, we install reflecting walls within a numerical wave tank to manipulate the hydrodynamic wave field. Three diverse wall configurations are examined (see Fig. 1). Initially, a straight wall is installed at an oblique angle to the incident waves. The waves are reflected from the wall and interact with propagating incident waves, forming a chequerboard type pattern on the free surface. Where the incident waves and reflected waves interact, a positive interference manifests which yields amplified free surface displacements, signifying energy concentration localisations. The second set of numerical experiments investigate wave focussing in the concave opening of a parabolic reflecting wall. Significantly augmented free surface displacement amplifications are observed at the parabolic wall focus. Finally, the study investigates wave focussing at the convex side of a double parabola wall. In this configuration, a complex reflecting wave field pattern emerges which may be particularly suitable for the installation of a WEC array. In each of the geometrical scenarios described, an oscillating water column (OWC) WEC is installed at the wave focussing position and its performance is analysed over a range of incident wave conditions. The performance of the OWC in each wave focussing case is compared with an equivalent OWC installed in open-seas conditions. In this manner, the effectiveness of the wall geometry in terms of the energy focussing and subsequent energy capture enhancement by the WEC is determined.

Numerical analysis of the buoy design to extract the effective kinetic wave energy

In many countries, sea wave energy is becoming more widely recognized as a significant and hopeful resource. The objective of this research is to evaluate the impact of alterations in buoy shape on the efficacy of the wave model by utilizing Computational Fluid Dynamics (CFD) to assess modifications in the buoy's dynamic behavior. The buoy's behavior is simulated using ANSYS Fluent, the volume of fluid approach, the open channel flow module, and 5th order Stokes wave models. To further investigate the wave sensitivity in challenging circumstances, Stokes waves were applied in the shallow domain at high wave steepness. The validation process consisted of a comparative analysis between the practical and mathematical results. The outcomes of a numerical fluid dynamical computation were compared, and laboratory observations were modified. For various configurations of underwater buoyancy, the drag and lift characteristics of numerical results were analyzed. The aim of this study was to identify the optimum buoy shape. The selection of this shape was based on several factors, of which the most significant were the least entropy, as well as the maximum lift and drag coefficient. Ideal shape was determined to be a spherical shape (shape B).

A double-objective prediction and optimization method for buoys performance based on the artificial neural network

Determining the optimal geometry of the buoy is an important step in the design of oscillating wave energy converters, its main problem of which is the complexity and computational cost of the buoy shape parameterization process. To solve the above problems, the Genetic Algorism Back Propagation (GABP) neural network is used to optimize the longitudinal section of the buoy, and the Bessel equation is used to simplify the coordinates. 280 buoys with regular shapes are used as the training set, and 5% of them are selected as the test set. The trained neural network can accurately predict both the maximum heave displacement and additional mass of the buoy under a uniform wave condition, with a prediction accuracy of 93%. An optimal buoy geometry shape is constructed by randomly varying Bessel control points which enables the trained neural network to perform double-objective optimization of the buoy longitudinal section. The method proposed in this study can provide a new idea for the optimization of buoy geometry shape and performance prediction.

Floating wave energy harvester: a new perspective

Sustainable development principles have brought forth the imperative to harness renewable energy sources to conserve our fragile ecosystems. Among the various renewable energy options, ocean wave energy has garnered significant global interest and is now poised as a leading contender. However, current wave energy collection methods suffer from limitations and technical immaturity, thereby necessitating the need for a more advanced and innovative solution. This paper presents a concept for the development of a floating wave harvester, which aims to convert the energy of ocean waves into usable electrical energy. The focus of this document is to outline the methodology behind the design and construction of the harvester’s buoys, which play a crucial role in determining the overall effectiveness of the system. The buoy shape and material choice are critical to the harvester’s dynamic behavior, energy absorption capacity, and the ability to transform wave kinetic energy into direct and indirect electrical energy. The primary objectives of this ongoing research project are to define the system requirements and specifications and to implement these through practical application. The development of this floating wave harvester holds the potential to significantly contribute to the renewable energy landscape and make significant strides in sustainable development.

Design of an S-shaped point-absorber wave energy converter with a non-linear PTO to power the satellite-respondent buoys in the East China Sea

An S-shaped wave energy converter (SB-WEC) was designed to assist near-zero-energy oceanography. The SB-WEC comprises a non-linear mechanical power take-off (PTO) and a novel S-shaped buoy (SB) for operation in the wave climate of the National Shallow Sea Comprehensive Test Site in China. The PTO used a ball screw-based transmission system to harvest wave energy during upward and downward strokes, and its operational characteristics were validated through simulations and experimental testing. A comparative analysis was performed to select an optimal buoy shape, and tests were conducted in a lab-scale wave flume on the 3D-printed scaled buoys. Using an ultramodern algorithm, buoy scaling was performed to identify the pure shape-change effects on the heave response irrespective of their size. The optimal buoy size was evaluated, and an algorithm was developed in MATLAB Simscape to study its feasibility of running the non-linear PTO in realistic waves. A study on the tunability of the PTO using a rotary motion rectifier (RMR) suggested that the application of the proposed RMR is not suitable in waves below 0.3 Hz. During experimental tests, the PTO generated 13.5W at 0.3 Hz (realistic wave frequency) with an energy conversion efficiency above 50%. The experiments showed that under identical design and test conditions, the S-shaped buoy (SB) could achieve a 34% greater RMS heave response than the cylindrical-hemispherical buoy. A feasibility study based on MATLAB-AQWA showed that the RMS power absorption of the full-scale SB buoy was 17% higher than the input power of the PTO in irregular waves, validating the proposed buoy shape and size. The SB-WEC can reasonably power the sensors in the satellite-respondent buoys.

Design and analysis of the bulbous-bottomed oscillating resonant buoys for an optimal point absorber wave energy converter

This study proposes innovative bulbous-bottomed buoy designs for an optimal oscillating-body wave energy converter (WEC). The effects of buoy shapes and the shape-influenced hydrodynamic coefficients on the motion and power absorption efficiency of a single-body heaving WEC are investigated. The frequency-domain analyses and spectral modeling were performed in ANSYS AQWA considering regular and irregular waves for optimal and sub-optimal power take-off (PTO). The wave climate data was obtained for ShiDao station China, and the middle frequency of the wave range with the highest probability of occurrence was selected as standard natural frequency, which was achieved by equalizing the diameter, mass, and volume of all the buoys. The bulbous-bottomed buoys could achieve 28.1% higher absorption efficiencies than the non-bulbous shapes. The buoy with the most expansive bulbous bottom of 1.6m height (SB-3) is the most optimal buoy with the highest power absorption and efficiency in both optimal and suboptimal PTO modes, with a maximum of 16.8% and 21.5% increase in response amplitude operator (RAO) and power absorption, correspondingly, relative to non-bulbous cylindrical-hemispherical (C-HS) buoy. The experimental validation of the simulated RAO and power of the 1:12.5 scale model of the SB-3 revealed 28% and 27.18% greater values, respectively, compared to C-HS, justifying that the SB-3 buoy could be a better choice for designing an oscillating-body WEC in all waves. By increasing the bulbous bottom width by 90% and fillet height by 100%, the performance of the bulbous buoys was enhanced by 12.2%.

Experimental investigation for suction slots of wells turbine and shapes of point absorber

When it comes to reducing carbon emissions, A dependable source of renewable energy is wave energy. The endless energy crisis offers a multitude of potential solutions, including wave energy. Since there are no emissions, it is energy that is good for the environment. As a renewable energy source, it is frequently used at all times. To test the effectiveness of the Wells turbine, suction slits have been used on the turbine blades in this work. The aerofoil utilized in the Wells turbine blade was NACA015. An arrangement for comparing various body types is made in the experimental buoy setup. To engage with wave motion, the setup includes a buoy that floats in the lab. Results indicated that the three slots had good pressure drop coefficient behavior, good turbine coefficient, and maximum turbine efficiency. The buoy shapes (5) and (3) for the point absorber exhibited the best properties according to the relevant parameters.

Absorbed power density approach for optimal design of heaving point absorber wave energy converter: A case study of Durban sea characteristics

This work proposes an approach for the optimal sizing of a cylindrical heaving wave energy converter (WEC). The approach is based on maximising the absorbed power density (APD) of the buoy, with the diameter being the decision variable. Furthermore, two types of buoy shapes were compared to get the best option. The two buoy shapes are the cone cylinder buoy (CCB) and the hemisphere cylinder buoy (HCB). The aim was therefore to determine the best shape and as well as the optimal size of the cylindrical point absorber. To validate the approach, the simulation was performed under Durban (South Africa) sea characteristics of 3.6 m wave significant height and 8.5 s peak period, using the openWEC simulator. The buoy diameter range considered was from 0.5 m to 10 m for both shapes. Simulation results revealed that a diameter of 1 m was the optimal solution for both buoy shapes. Furthermore, the APD method revealed that the HCB was more efficient than the CCB. The power density of the HCB was 1070 W/m2, which was almost double the power density of the CCB, while the two shapes present almost the same absorbed power.

Multi-objective Optimization of the Buoy Shape of an Ocean Wave Energy Converter using Neural Network and Genetic Algorithm

The point absorber is one of the most popular types of ocean wave energy converter (WEC) that harvests energy from the ocean. Often such a WEC is deployed in an ocean location with tidal currents or ocean streams, or serves as a mobile platform to power the blue economy. The shape of the floating body, or buoy, of the point absorber type WEC is important for the wave energy capture ratio and for the current drag force. In this paper, a new approach to optimize the shape of the point absorber buoy is developed to reduce the ocean current drag force on the buoy while capturing more energy from ocean waves. A specific parametric modelling is constructed to define the shape of the buoy with 12 parameters. The implantation of neural networks significantly reduces the computational time compared with solving hydrodynamics equations for each iteration. And the optimal shape of the buoy is solved using a genetic algorithm with multiple self-defined functions. The final optimal shape of the buoy in a case study reduces 68.7% of current drag force compared to a cylinder-shaped buoy, while maintaining the same level of energy capture ratio from ocean waves. The method presented in this paper has the capability to define and optimize a complex buoy shape, and solve for a multi-objective optimization problem.

Design and Analysis of Buoy Geometries for Wave Energy Generator

Objectives/Methods: The main objective here is to make an optimum design of buoy. Wave power generation is one of the extensive research fields all over the world. The ocean waves are usually converted into electrical energy by a device called a wave energy converter (WEC). The electricity generated depends upon the intensity of the waves. In a traditional way, the motion of a buoy that occurred in a vertical direction due to waves in water, can be converted into current generation. Here, the numerical simulation using the Ansys tool is done to find an optimum design of buoy shape to get maximum energy conversion. Findings: The simulations have shown that the drag force for the Oval shape is minimum 0.063 N and the first mode of frequency for the Conical shape is minimum 165 Hz for Point-Absorber type Wave Energy Converter. Keywords Buoy, wave energy converter, buoy shapes

Numerical Study on the Optimization of Hydrodynamic Performance of Oscillating Buoy Wave Energy Converter

Abstract The oscillating buoy wave energy converter (OBWEC) captures wave energy through the undulating movement of the buoy in the waves. In the process of capturing wave energy, the hydrodynamic performance of the buoy plays an important role. This paper designed the “Haida No. 1” OBWEC, in which the buoy adopts a form of swinging motion. In order to further improve the hydrodynamic performance of the buoy, a 2D numerical wave tank (NWT) model is established using ADINA software based on the working principle of the device. According to the motion equation of the buoy in the waves, the influence of the buoy shape, arm length, tilt angle, buoy draft, buoy width, wave height and Power Take-off (PTO) damping on the hydrodynamic performance of the buoy is studied. Finally, a series of physical experiments are performed on the device in a laboratory pool. The experimental results verify the consistency of the numerical results. The research results indicate that the energy conversion efficiency of the device can be improved by optimizing the hydrodynamic performance of the buoy. However, the absorption efficiency of a single buoy for wave energy is limited, so it is very difficult to achieve full absorption of wave energy.

The effects of geometrical buoy shape with nonlinear Froude-Krylov force on a heaving buoy point absorber

This study examined the effects of buoy shape and Nonlinear Froude-Krylov force (NFK) on a heaving-buoy-type Wave Energy Converter (WEC). Based on the Maclaurin expansion, the theoretical solutions of the NFK were derived for three different buoy shapes; hemispheric buoy, circular vertical cylinder, and truncated conical cylinder. A hydraulic power take-off system was adopted, and the latching control strategy was applied to maximize the extracted power from the WEC. The nonlinear effects of the Froude-Krylov force and restoring force on the heaving point absorber were investigated by comparing the heave Response Amplitude Operator (RAO) and time-averaged power extraction. The results showed that the conventional linear analyses were overestimated by up to 50% under the high amplitude wave condition. The latching control strategy was the most effective when peak wave period of regular or irregular wave was 0.4–0.45 times the heave natural period of the buoy.

Shape Optimization Design of a Heaving Buoy of Wave Energy Converter Based on Fully Parametric Modeling and CFD Method

Hydrodynamic shape of the heaving buoy is an important factor of the motion response in waves and thus energy conversion efficiency for point absorbers (PAs). Experience-based design considerations are currently limited by design ideas and time consumptions, hence the need for faster and smarter methods. An automated optimization method based on a fully parametric modeling method and Computational Fluid Dynamics (CFD), is proposed in the present paper. Using this method, a benchmark buoy is screen designed and then optimized for maximizing heave motion response. The geometry is described parametrically and deformed by means of the free-form deformation (FFD) method. During the optimization process, the expansion factor of control points is the basis for variations. A combination of Sobol and non-dominated sorting genetic algorithm II (NSGA-II) is used to search for the solutions. After several iterations, the heaving buoy shape with optimal heave motion response is obtained. The analyses show that the heave motion response has increased 55.3% after optimization. The developed methodology is valid and seems to be a promising way to design a novel buoy that can significantly improve the wave energy conversion efficiency of PAs in future works.

Influence of the shape of a buoy on the efficiency of its dual-motion wave energy conversion

A procedure is proposed for the geometric design of the buoy of a single-body wave energy converter (WEC) based on the characteristics of incident waves. The effect of the buoy shape on its hydrodynamic performance and the absorption efficiency of WEC model is considered. The model absorbs the wave energy through the dual motion (heave and surge) of the buoy. In order to compare the dynamics of various buoys in waves, a set of requirements is proposed to create identical conditions. Three groups of buoy geometry, including conical, spherical and unusual geometry are considered. The changes in the hydrodynamic parameters of the buoy due to the buoy shape variations are studied to clarify the effect of these changes on the efficiency of the WEC model. The obtained results show that increasing a ratio of hydrodynamic coefficients of the buoy, increases the produced power regardless of the wave frequency. Also, an appropriate shape for the buoy can raise the WEC model performance from suboptimal mode to optimal mode and would also lead to a significant increase in the absorbed power and efficiency.