Wave Energy Extraction Research Articles

Sustainable solution to the critical challenges of an oceanic wave farm for maximizing power generation

In this paper, a general wave farm design procedure has been presented. For example, the design procedure is applied to the Bay of Bengal, Bangladesh. Firstly, the most effective site is selected to construct a wave farm considering various techno-economic factors. A coordinated drawing for the proposed site selection procedure is presented, which includes a coastal area map and sea, grid map, wave height, and different assorted factors. Transmission line cost is analyzed for nearly equipotential locations. Then wave height, time period, wave power, and vertical speed are observed for the selected location. In the next step, a wave farm layout has been designed considering point absorber type device and the oceanic wave dataset with statistical analysis. Further, a linear electrical generator (LEG) is designed and analyzed for the wave farm. The generator is simulated in the ANSYS/Maxwell environment. Since the wave parameters vary from time to time, the LEG is simulated for several vertical speeds. To find the highest output power, the load profiles are analyzed for each speed separately. In the next step, the LEG is optimized. Simulation result shows that the flux distribution of optimized LEG is better than its initially designed counterpart. Efficiency of the optimized LEG is 7 % greater than its initial design. The proposed method of site selection, statistical analysis of the wave dataset, and the optimized LEG design are combined in this paper for maximum wave energy extraction.

Propulsion and energy extraction performance of wave-powered mechanism considering wave and current

Wave-powered mechanism can extract wave energy as propulsion, enabling long-term ocean observation. The kinematic properties of wave propulsion mechanisms are susceptible to ocean current loads in addition to wave effects, presenting complex dynamic behaviors. In this paper, to understand the effects of ocean currents on the kinematic performances of wave propulsion mechanism, the dynamic performance of a wave propulsion mechanism in the combined condition of wave and current is analyzed based on numerical simulation method for fluid-rigid body coupling and experimental methods. The performance of the propulsion mechanism against the current under counter-current conditions and the contribution of flow to the propulsion performance under co-current conditions are discussed. The intrinsic relationship between wave height, current velocity and current direction is investigated, and the energy extraction performance of wave propulsion mechanism is analyzed. According to the results, the ability of propulsion mechanism to resist counter-current is obtained, and it is found that as the counter-current velocity increases, the hydrofoil will stall and the propulsion force and efficiency decreases significantly. Whilst, the co-current has a promoting effect on the forward of the mechanism, but within a certain range of co-current speed, the mechanism will be negatively affected by the shedding vortex. And the discrimination mechanisms for wave-dominated propulsion and current-dominated propulsion are obtained. This work reveals the combined effect of waves and currents on the motion performance of wave propulsion mechanism, providing a reference for the development of surface vehicles that utilize multiple ocean energies for synergistic propulsion.

Wave energy extraction from rigid rectangular compound floating plates

We present a theoretical model to analyse the hydrodynamics of wave energy converters (WECs) comprised of three-dimensional, rigid, floating, compound rectangular plates in the open sea. The hydrodynamic problem is solved by means of Green’s theorem and a free-surface Green’s function. Plate motion is predicted through decomposition into rigid natural modes. We first analyse the case of a single rectangular plate and validate our model against experimental results from physical model tests undertaken in the COAST laboratory at the University of Plymouth. Then we extend our theory to complex shapes and arrays of plates and examine how the geometry, incident wave direction and power take-off (PTO) coefficient affect the response of the platform and the consequent absorbed energy.

Performance of integrated FB and WEC for offshore floating solar photovoltaic farm considering the effect of hydroelasticity

The offshore floating solar photovoltaic (PV) farm deployed in the open sea could be subjected to large environmental loading due to waves or wakes from travelling ships. To reduce the motion of the solar farm, mitigation methods such as protection of the solar farm via floating breakwater (FB) could be deployed. The FB serves as a buffer between the floating farm and the incoming wave force, and as it attenuates the wave forces, this creates a sheltered zone on the leeward side of the FB. The energy generation from the solar farm could be further enhanced via the integration of attenuator-type wave energy converters (WEC) with the FB. This paper investigates the performance of the hybrid FB – WECs as a coastal protection structure cum wave energy extraction devices. The effect of FB on mitigating the hydroelastic response of the floating solar farm is investigated. Four types of FB, i.e., I-, L-, U- and BOX-shape FBs are considered and their effectiveness in wave attenuation under regular waves arriving from different directions are studied. In addition, due to the long structure length-to-wavelength ratio of the floating solar farm and FB, the hydroelastic response of the structures is taken into consideration.

An experimental study of vortex evolution around an offshore stationary dual-chamber oscillating water column converter

The Oscillating Water Column (OWC) is a promising Wave Energy Converter (WEC) due to its practicality and efficiency. Conventional single-chamber OWCs exhibit constraints in extracting energy over diverse wave periods, while dual-chamber OWCs demonstrate enhanced adaptability, leading to improved energy capture rates. This study investigates an offshore stationary dual-chamber OWC with an underlying transverse channel designed to facilitate wave ingress and augment energy extraction. Experiments across various wave periods reveal that dual-chamber OWCs with transverse channels achieve superior wave energy extraction compared to single-chamber counterparts. Time-resolved particle image velocimetry (TR-PIV) is employed for detailed flow field measurement to elucidate the complex hydrodynamic processes. Analysis of the water column oscillation and flow dynamics reveals an intensified interaction between hydrodynamic processes in the dual chambers, especially with elongated transverse channels. The Ω method is used to examine vortex evolution, indicating heightened complexity in vortex formation within dual-chamber OWCs compared to single-chamber OWCs. Notably, large-scale vortices emerging in the forward chamber impede oscillation and diminish energy extraction efficiency. Optimizing the design of transverse channel entries, frontal walls, and internal chamber corners is crucial to mitigate flow separation and vortex generation, thereby enhancing overall energy extraction performance.

A Dual-Function Design of an Oscillating Water Column Integrated with a Slotted Breakwater: A Wave Flume Study

Wave energy conversion holds promise for renewable energy, but challenges like high initial costs hinder commercialization. Integrating wave-energy converters (WECs) into shore-protection structures creates dual-function structures for both electricity generation and coastal protection. Oscillating water columns (OWCs) have been well studied in the past with their simple generation mechanism and their out-of-water power take-off (PTO) system, which can minimize bio-fouling effects and maintenance costs compared to other submerged WECs. In addition, a slotted barrier allows for better circulation behind the breakwater while dissipating incoming wave energy through viscous damping. This study examines the performance of a new design which combines an OWC with a slotted breakwater. Small-scale (1:49) laboratory tests were performed with a piston-type wave generator. The performance is evaluated in terms of wave transmission, wave energy extraction, and wave loading under various wave conditions while focusing on the effects of the porosity of the slotted barrier and tide level changes. Results show that under larger waves, a decreasing wave transmission, increasing power extraction from the OWC, and energy dissipation from the slotted barrier are observed. On the other hand, under increasing wavelengths, wave transmission is observed to be constant; this is important for harbor design, which means that the breakwater is effective under a wider range of wavelengths. Porosity allows for more transmission while inducing less horizontal force on the structure.

Investigation of a Modified Wells Turbine for Wave Energy Extraction

The Oscillating Water Column (OWC) is the most promising self-rectifying device for power generation from ocean waves; over the past decade, its importance has been rekindled. The bidirectional airflow inside the OWC drives the Wells turbine connected to a generator to harness energy. This study evaluated the aerodynamic performance of two hybrid airfoil (NACA0015 and NACA0025) blade designs with variable chord distribution along the span of a Wells turbine. The present work examines the aerodynamic impact of the variable chord turbine and compares it with one with a constant chord. Ideally, Wells rotor blades with variable chords perform better since they have an even axial velocity distribution on their leading edge. The variable chord rotor blade configurations differ from hub to tip with taper ratios (Chord at Tip/Chord at Hub) of 1.58 and 0.63. The computation is performed in ANSYS™ CFX 2023 R2 by solving three-dimensional, steady-state, incompressible Reynolds Averaged Navier–Stokes (RANS) equations coupled with a k-ω Shear Stress Transport (SST) turbulence model in a non-inertial reference frame rotating with the turbine. The accuracy of the numerical results was achieved by performing a grid independence study. A refined mesh showed good agreement with the available experimental and numerical data in terms of efficiency, torque, and pressure drop at different flow coefficients. A variable chord Wells turbine with a taper ratio of 1.58 had a peak efficiency of 59.6%, as opposed to the one with a taper ratio of 0.63, which had a peak efficiency of 58.2%; the constant chord Wells turbine only had a peak efficiency of 58.5%. Furthermore, the variable chord rotor with the higher taper ratio had a larger operating range than others. There are significant improvements in the aerodynamic performance of the modified Wells turbine, compared to the conventional Wells turbine, which makes it suitable for wave energy harvesting. The flow field investigation around the turbine blades was conducted and analyzed.

Predicted ecological consequences of wave energy extraction and climate-related changes in wave exposure on rocky shore communities

Abstract Wave energy has the potential to contribute in the transition to decarbonized electricity generation. Extracting wave energy might be expected to have ecological impacts on rocky shore intertidal communities where exposure is one of the most important factors determining species structure and composition. With global climatic change, coastal exposure is predicted to increase with greater significant wave height. The wave-exposed west coast of Orkney, Scotland, UK, is the site of pre-commercial wave device testing. Surveys of 39 rocky shore sites along this coast identified key species and abundances, and quantified exposure-modifying topographic variables. A spectral wave model was constructed to compare baseline, wave extraction, climate change, and combined scenarios. Generalized additive modelling was used to describe the relationship between species, topography, and exposure. Results show that individual species differentially respond to exposure changes with ‘winners’ and ‘losers’ at site level. Overall, community responses are expected to be far greater following predicted climatic change than to industrial-scale wave energy extraction, depending on spatial scale. In combination, energy extraction may reduce the effects of climate-change-related increases in wave exposure of rocky shores. Predicting how location-specific biotic assemblages respond to changes in wave energy as a result of long-term forcing agents provides a valuable marine resource management tool.

Computational Fluid Dynamics Simulation on Blade Geometry of Novel Axial FlowTurbine for Wave Energy Extraction

Wave energy converters (WECs) utilizing the Oscillating Water Column (OWC) principle have gained prominence for harnessing kinetic energy from ocean waves. This study explores an innovative approach by transforming the pivoting Denniss–Auld turbine blades into a fixed configuration, offering a simplified alternative. The fixed-blade design emulates the maximum pivot points during the OWC’s exhalation and inhalation phases. Traditional Denniss–Auld turbines rely on complex hub systems to enable controllable blade rotation for performance optimization. This research examines the turbine’s efficiency without mechanical actuation. The simulations were conducted using ANSYS™ CFX 2023 R2 to solve the three-dimensional, incompressible, steady-state Reynolds-Averaged Navier–Stokes (RANS) equations, employing the k-ω SST turbulence model to close the system of equations. A grid convergence study was performed, and the numerical results were validated against available experimental and numerical data. An in-depth analysis of the intricate flow field around the turbine blades was also conducted. The modified Denniss–Auld turbine demonstrated a broad operating range, avoiding stalling at high flow coefficients and exhibiting performance characteristics like an impulse turbine. However, the peak efficiency was 12%, significantly lower than that of conventional Denniss–Auld and impulse turbines. Future research should focus on expanding the design space through parametric studies to enhance turbine efficiency and power output.

Optimization of latching control for duck wave energy converter based on deep reinforcement learning

In the field of wave energy extraction, employing active control strategies amplifies the Wave Energy Converter's (WEC) response to wave motion. In this regard, a numerical simulation study was conducted on the interaction between Duck WEC and waves under discrete latching control, proposing an optimized latching duration strategy based on Deep Reinforcement Learning (DRL). The study utilized an improved Edinburgh Duck model to establish a numerical wave flume (NWF), verifying the motion of waves and the device. A DRL and Computational Fluid Dynamics (CFD) coupled framework was developed to implement the latching control strategy. In the limited trichromatic wave testing conditions provided in this paper, the latching control time is optimized using a DRL agent, and its capture efficiency is compared with two non-predictive latching control strategies. The DRL agent control showed a 13.4% improvement relative to reference threshold control, with less than 6.3% differences compared to optimal threshold latching results. Improvement of approximately 5.6% was observed compared to optimal constant latching control. For the first time in nonlinear fluid dynamics numerical simulation, this study demonstrates the efficacy of model-free reinforcement learning with discrete latching actions, establishing an environment-driven control strategy, and having the potential to become a general strategy for WEC latching control.

Theoretical investigation on a heaving wave energy converter-dual-arc breakwater integration system array

The hydrodynamic performance of an array of heaving wave energy converter (WEC)-dual-arc breakwater integration systems was investigated using a theoretical model in the present paper based on the potential flow theory, with each dual-arc breakwater consisting of a bottom-mounted inner solid and outer perforated arc-shaped cylinder. The multiple scattering problem was solved using a hybrid iterative method. The mean transmission coefficient was introduced to quantify the wave attenuation performance. After successfully validating the theoretical model, the array effect and the hydrodynamic characteristics of the proposed integration system array are investigated. Focusing on the layout of a linear equidistant column, the calculation results indicate that the WEC and dual-arc breakwater array have mutual benefits in enhancing the wave energy extraction and wave attenuation performance. Compared to an array of bottom-mounted solid and porous cylinders, the proposed integration system array provides much better protection to the leeward region except at a very low wave frequency. In the considered frequency region, a closely arranged integration system array under the perpendicular incident waves has a very constructive array effect on the overall performance. For two staggered columns of integration systems, the wave energy absorption of the leeward column is improved and the sheltering effect to the leeward region is better at a high frequency as compared to the one-column layout; in contrast, the overall performance for two aligned columns is better at a low frequency.

Finding the best shape of floating wave energy converters for different primary geometries: Experimental and numerical investigations

This research aimed to identify the optimal geometry for maximizing wave energy extraction from wave energy converters (WECs) by employing different primary geometries of floating bodies (specifically, cube, cylinder, and sphere). Laboratory experiments were conducted using a wave tank to generate waves with varying characteristics. The floats were subjected to waves of different wave conditions, including height (4 cm, 6 cm, and 9 cm) and frequency (0.9 Hz and 0.83 Hz). Multiple probes and sensors were used to collect experimental data. The results revealed that when hit by a wave with a height of 9 cm and a frequency of 0.9 Hz, the rectangular cube experienced an average displacement of 12 cm and 14 cm along its length, with the same weight and draft, respectively. Furthermore, comparisons of the experimental data across different scenarios indicated that the rectangular cube, elongated perpendicular to the incident waves, performed well in most of the test cases. The efficiency of primary wave energy conversion was evaluated for all floated bodies and wave conditions, encompassing a range of values from 0.02 to 0.34. Additionally, numerical simulations of wave and float displacement demonstrated good agreement with the experimental observations.

Generalized machine learning models to predict significant wave height utilizing wind and atmospheric parameters

Significant wave height (SWH) is a key parameter for wave energy extraction, ship navigation, oil and gas extraction, coastal structure construction, etc. Direct measurements of SWH using buoys are expensive and accurate over a limited area while numerical models are computationally expensive, inaccurate, with limited generalizability. Thus, this work focuses on developing generalizable machine learning models for predicting SWH from wind parameters (speed, direction, and gust) and atmospheric parameters (temperature and pressure). Two deep learning models (Artificial Neural Network (ANN) and Self Normalizing Neural Network (SNN) and two gradient boosting tree-based models (XGBoost and LightGBM) have been used in this study. Three different data sets were collected from the National Data Buoy Center: Data Set-1 (DS1): 12 years of data from 47 stations; Data Set-2 (DS2): 14 months of additional data from 6 stations randomly selected from DS1; Data Set-3 (DS3): 13 years data from completely 6 new stations. DS1 was split into training, testing, and validation datasets. Training and hyper-parameter tuning was done on the train and validation dataset, while the performance of the models was evaluated on the DS1 test dataset, DS2, and DS3, employing three error metrics: Mean Squared Error (MSE), Mean Absolute Error (MAE), and R-square score (R2). The collected data underwent data cleaning, preprocessing, and exploratory data analysis before modeling. The deep learning models have demonstrated superior fitting capacity to the tree-based models, achieving the lowest MSE (0.047), MAE (0.153), and highest R2 score (0.953) on test data. However, the gradient boosting models demonstrate better generalizing capacity than the deep learning models on DS2 and DS3. This study helps further Sustainable Development Goal 7 (SDG 7) by allowing fast and cheap assessment of wave height for ocean energy site development.

Analytical solutions for hydrodynamic responses of arrays of floating truncated cylinders using multi-term Galerkin method and its application to a new wave energy converter device

In the context of linear water wave theory, the analytical solutions for the diffraction and radiation of a truncated cylinder array are developed in the presence of ambient incident waves. Each cylinder in the array can oscillate with five degrees of freedom (DOFs), i.e., surge, sway, heave, roll, and pitch. This paper adopts the multi-term Galerkin method to expand the fluid velocity at the interface of different regions into a set of basis functions containing Gegenbauer polynomials, which accurately and efficiently characterizes the cube root singularity of the fluid velocity near the edges of the truncated cylinders. Using the dynamic equilibrium equations, the amplitudes of each DOF of the cylinders in the array are solved. The analytical solution presented in this paper converges rapidly, and high-precision hydrodynamic response results can be obtained using just a few truncated terms (e.g., the upper bounds of m0 = 5 and p0 = 22 can yield results of five-figure accuracy). For the 4-cylinder array, under the same accuracy conditions (the error less than 1%), the computation time of the conventional method developed by Zeng et al. [“Hydrodynamic interactions between waves and cylinder arrays of relative motions composed of truncated floating cylinders with five degrees of freedom,” J. Fluids Struct. 115, 103785 (2022d)] based on the exact algebraic method [Kagemoto and Yue, “Interactions among multiple three-dimensional bodies in water waves: An exact algebraic method,” J. Fluid Mech. 166, 189–209 (1986)] is 3.9 times longer than that of the present method. As the number of cylinders increases, the advantage of the present method in terms of convergence speed becomes more apparent, e.g., for the 16-cylinder array, the conventional solution takes 6.3 times longer than the present solution. To extract wave energy more efficiently, a new 5DOF wave energy converter (WEC) device that can extract energy in 5DOFs is proposed. The present method is adopted to investigate the hydrodynamic performance of the 5DOF WEC arrays. Compared with the traditional 1DOF (heave) WEC, the 5DOF WEC can significantly improve the energy capture performance of arrays, especially in the high-frequency wave region.

Energy harvest on TSUSUCA DOLPHIN under irregular waves: Experimental studies

Finding sustainable and infinite green energy options in a world that relies primarily on fossil fuels is crucial as traditional sources decline. Wave energy is an excellent example of an environmentally friendly power source. A multi-utility bio-inspired floating device consisting of two TSUSUCA (TSUnami Protection Solar Utilization and other Coastal Applications) Dolphin-shaped bodies integrated with a linear motion electricity generator (LMEG) connected to a rigid deck by a hinged connection is tested experimentally under irregular waves. This novel and innovative multi-utility floating offshore device can function as a tsunami barrier and a wave energy extraction device. Its performance and operating mechanism are investigated under 8 different sets of irregular waves. The examined sea states are represented by JONSWAP and PM wave spectra with a range of significant wave heights (0.05–0.20 m) and peak periods (1.5 and 2.0 s). The experimental results show that the overall electrical power efficiency of LMEG coupled to each Dolphin obtained for set 6 is 32.04 % and 30.81 % for both JONSWAP and PM spectrums, respectively. This bio-inspired device and its performance confirms the proof-of-concept while the presented study shall become a prime factor towards the development of more similar devices to harness wave energy.

Wave Energy Conversion through Oscillating Water Columns: A Review

An oscillating water column (OWC) is designed for the extraction and conversion of wave energy into usable electrical power, rather than being a standalone renewable energy source. This review paper presents a comprehensive analysis of the mathematical modeling approaches employed in OWC systems, aiming to provide an in-depth understanding of the underlying principles and challenges associated with this innovative technology. A prominent classification within the realm of wave energy devices comprises OWC systems, which exhibit either fixed or floating configurations. OWC devices constitute a significant proportion of the wave energy converter prototypes currently operational offshore. Within an OWC system, a hollow structure, either permanently fixed or floating, extends below the water’s surface, creating an enclosed chamber where air is captured over the submerged inner free surface. This comprehensive study offers a thorough assessment of OWC technology in conjunction with air turbines. Additionally, the investigation delves into theoretical, computational, and experimental modeling techniques employed for analyzing OWC converters. Moreover, this review scrutinizes theoretical, computational, and experimental modeling methodologies, providing a holistic understanding of OWC converters. Ultimately, this work contributes a thorough assessment of OWC technology’s current state, accentuating its potential for efficient wave energy extraction and suggesting future research avenues.

Achieving optimum power extraction of wave energy converters through tunable mechanical components

Wave energy converters (WECs) play a significant role in harnessing the vast potential of marine renewable energy. To enhance the power extraction of WECs, it is necessary but challenging to design the parameters of the WECs for the time-varying wave conditions. This research explores the utilization of tunable mechanical components within both the power take-offs (PTO) and wave capturing structures. We first briefly review the tunable mechanisms and then conduct a theoretical analysis of their applications for maximizing power extraction under regular and irregular wave conditions. The study investigates the impact of PTO stiffness and inertance on power extraction through an analytical approach. The results reveal that both parameters exhibit a monotonic effect within specific frequency ranges, and showed the optimum power extraction can be achieved through the tunable mechanical components. Furthermore, the research highlights the importance of designing the frequency tuning limit for two-body WECs to coincide with the desired wave conditions, as it enables all PTO parameters to achieve an optimal power output. Based on the analysis, the paper gives insightful suggestions on the design of PTO and WEC systems, including mass ratio between the two bodies, mooring stiffness, and other relevant parameters. It is found that the tuning through the mechanical component can be significant for the single-body WEC yet less effective for the two-body WEC especially at irregular wave conditions.

The Application of Short-Term Deterministic Wave Prediction to Offshore Electricity Generation.

The field of wave energy extraction from the oceans is moving from the realm of drawing board dreams to the reality of device farms in the open ocean. Several devices are now at the stage of full scale testing and methods of increasing their economic potential are required. Maximum energy capture from a given sea state is one such way of increasing production and for devices to achieve this an accurate prediction of the sea surface elevation in the vicinity of these devices is required. The method proposed is based on the work of Belmont [1] and Zhang [2], extending from unidirectional modelling to directional. Preliminary simulations for a unidirectional buoy excited by a Pierson-Moskowitz spectrum are given and the proposed method for directional modelling is then set forth.

Effects of small marine energy deployments on oceanographic systems

The placement and operation of marine energy deployments in the ocean have the potential to change flow patterns, decrease wave heights, and/or remove energy from the oceanographic system. Changes in oceanographic systems resulting from harvesting marine energy, particularly tidal and wave energy, may be of concern. These changes include alterations in nearfield and farfield physical processes, as well as potential secondary environmental effects such as changes in sediment transport patterns, biological processes, or coastal erosion. Knowledge of changes in oceanographic systems associated with marine energy is primarily available from numerical modeling studies, informed by some laboratory tests and very few field measurements. A literature review was conducted using the Tethys knowledge base and other online sources, building on conclusions from the Ocean Energy Systems-Environmental State of the Science report. Potential changes in oceanographic systems that may be caused by marine energy differ between tidal and wave devices because of different extraction mechanisms and siting locations. Numerical models show that tidal extraction on the order of hundreds of megawatts or with significant channel blockage is required to create changes in oceanographic processes that exceed natural variability. Effects from wave energy extraction in arrays are localized and dependent on array spacing and proximity to the shore. Available evidence supports the conclusion that the risk of significant environmental effects from such changes could be retired (i.e., less investigation required for every project) for small deployments—those representative of the state of the industry in 2021. Determining changes in oceanographic systems to be low risk for small deployments can thereby streamline environmental consenting by reducing monitoring needs at this early stage in the industry.

Two-body dynamic simulations of a combined semi-submersible floating offshore wind turbine and torus wave energy converter

Dynamic responses of a combined wind and wave energy system which consist of a 5MW semi-submersible floating offshore wind turbine (FOWT) and a torus-shaped wave energy converter (WEC) are investigated. In the investigated configuration, the torus WEC is constrained to move only in the heave direction with respect to the FOWT. This results in a two-body dynamic system that allows for the extraction of wave energy through the relative heave motion. A Modelica-based multi-body system (MBS) code is used to perform fully coupled dynamic analyses of the combined system. The wind turbine loads are calculated using the state-of-the-art Blade Element Momentum (BEM) method, while the wave power take-off (PTO) system is modelled as a linear spring and a viscous damper. A case study is presented where the mass of the torus is varied to investigate the impact on the two-body dynamics of the combined system. The system performance is assessed in terms of the maximum wave power absorption and the quality of wind power production.