Wave Energy Extraction Research Articles

Hydrodynamic performance of a novel WEC-breakwater integrated system consisting of triple dual-freedom pontoons

A breakwater consisting of three pontoon-type wave energy converters equipped with Power Take-Off (PTO) systems to extract wave energy from pitch and heave motions is proposed. The proposed integrated system has the potential to reduce the cost of the wave energy conversion system by sharing the essential infrastructure with the floating breakwater. In the model, the eigenfunction expansion matching method and technique of variables separation are used. The effects of the geometrical parameters (including pontoon width, draft and spacing) on the hydrodynamic performance characterized by the wave energy conversion efficiency, transmission and reflection coefficients are investigated, respectively. It is found that the effective bandwidth (transmission coefficient KT < 0.5 and hydrodynamic efficiency Cw > 0.3) and the peak efficiency are enhanced when the pitch and heave motions are considered simultaneously, compared with the single DoF motion system. The effective bandwidth increases with the decrease of width and draft of the front pontoon. And the variation of pontoon spacing affects significantly the distribution of the efficiency due to the Bragg-type reflection. Additionally, it is found that the variation of the geometrical parameters of the front pontoon on wave transmission is limited by changing the geometrical parameters of the front pontoon only.

Coupled CAD-CFD automated optimization for leading and trailing edge of an axial impulse turbine blade

A bidirectional impulse turbine with fixed guide vanes is used in an oscillating water column to extract wave energy. The turbine suffers from low aerodynamic efficiency, and it is required to improve performance. In the present study, coupled CAD-CFD automated optimization of the blade leading and trailing edge ratio was performed to maximize the efficiency. The design points were generated using a central composite design sampling technique and response surface optimization method estimated the optimum design. The optimization pipeline was automated using the Ansys Design Exploration module. Ansys Turbo Grid was used for generating structured hexahedral elements in the computational domain, and the problem was solved in Ansys CFX® v16.2. The results showed that the modified leading and trailing edge enhanced the performance compared to the reference blade by minimizing the pressure drop. The optimized design improved the relative peak efficiency and torque by 3.4% and 2% respectively. Additionally, a detailed flow analysis has been reported in this paper.

Heaving wave energy converter-type attachments to a pontoon-type very large floating structure

This study proposes heaving wave energy converter (WEC) - type attachments to a pontoon-type very large floating structure (VLFS) for extracting wave energy and reducing the hydroelastic response of the VLFS. Each heaving WEC-type attachment consists of a linear Power Take-Off (PTO) system connected to VLFS at one end and to the seabed at the other end. The performance of heaving WEC-type attachments is investigated with respect to PTO damping coefficient and stiffness, locations and the number of PTO systems, VLFS flexural rigidity, VLFS module connection design, wave period and incident wave angle. For the fluid–structure interaction analysis, the Mindlin plate theory is adopted for modelling VLFS, while the linear wave theory is adopted for modelling fluid motions. The hybrid finite element – higher order boundary element method (FE-HOBEM) is employed to solve the fluid–structure interaction problem. It is found that, for long waves and large oblique waves, the present WEC-VLFS system outperforms the previously proposed WEC-VLFS systems in terms of the power produced.

Optimal wave energy extraction for oscillating water columns using second‐order sliding mode control

Ocean waves are an important renewable energy resource and several fields of are concurrently working to improve technologies for harnessing their power. In that context, this study presents a control to optimise the performance of oscillating water column systems. As a first contribution, a novel criterion to attain maximum wave energy extraction is developed, resulting in an enhancement of the global power efficiency of the system. Then, taking advantage of the proposed criterion, a second-order sliding mode control set-up is designed, with power extraction maximisation the primary objective and reactive power regulation a secondary one. Simulation results confirm the highly satisfactory performance of the proposed controller and its robustness in the presence of the inherent uncertainties and disturbances in the non-linear system.

A multi-criteria approach for selection of wave energy converter/location

Wave energy plays a vital role in providing renewable energy in coastal areas. Several factors need to be considered for optimum site selection and type of the wave energy converter. Some factors such as impact of climate variation, exploitable storage of wave energy and its trend, design wave condition (as a representative of construction cost), accessibility and availability are related to the local sea state, while the characteristics of the Wave Energy Converter (WEC) need to be considered in the prediction power supply. Hence, this study focuses on the impact of different criteria on the selection of the appropriate location/WECs combinations and offers a Multi-Criteria Approach (MCA). The proposed multi-criteria approach combines the above-mentioned factors into one factor to consider both the efficiency of WECs as well as sea state to assist decision makers when planning for a pilot project. The approach was then applied to the wave condition of three locations previously suggested as hotspots for wave energy extraction. Comparison of MCA index in the stations with different types of WECs provided a more comprehensive view on the suitability of a location for a near-future pilot project and selection of the most efficient WEC.

Numerical analysis of the influence of air compressibility effects on an oscillating water column wave energy converter chamber

The most studied device used for extracting wave energy is the Oscillating Water Column (OWC). In general, numerical simulations of these cases by means of models based on Reynolds Averaged Navier-Stokes equations adopt the Volume of Fluid method to deal with the free surface flow which is considered incompressible in both water and air. The aim of this study is to investigate the influence of the compressibility effect on the air inside the OWC chamber by the FLUENT® numerical model. A methodology is implemented, taking into account both water and air flows incompressible, but, at every instant, a pressure condition is imposed on the top boundary of the chamber to consider the compressibility effect. This pressure condition is based on an analytical equation that considers the isentropic transformation of the air and effects of Wells and impulse turbines. Results of compressible and incompressible numerical models are compared. The amplification factor, the root mean square of air pressure inside the chamber and OWC efficiency in relation to incident wave period, wave height and turbine characteristic relation are analyzed. Results show that air compressibility effects can diminish the predicted OWC efficiency up to about 20% in both Wells and impulse turbines.

Experimental studies and time-domain simulation of a hinged-type wave energy converter in regular waves

This paper presents the design concept of a hinged-type wave energy converter, Sea Wave Energy Extraction Device (SeaWEED), and associated numerical and experimental studies of its performance in regular waves. The device is considered as an improved attenuator. A SeaWEED unit consists of four modules that are connected by rigid truss structures. The four-module array includes a non-energy producing nose module in the front, followed by two energy producing modules, and another non-energy producing module at the rear. A potential-flow-based time-domain program with the Lagrange multiplier approach was developed to simulate the dynamics of multiple constrained bodies. Model tests of a 1:35 scale model with and without the power take-off (PTO) units were carried out to validate the numerical method. Friction dampers were designed and manufactured to mimic the PTO units. The validated time-domain method was applied to predict the absorbed power by the device in regular waves.

Oscillating Wave Surge Converter-Type Attachment for Extracting Wave Energy While Reducing Hydroelastic Responses of Very Large Floating Structures

Abstract This paper presents an oscillating wave surge converter (OWSC)-type attachment, comprising a submerged vertical flap connected to the fore edge of a very large floating structure (VLFS) with hinges and linear power take-off (PTO) systems, for extracting wave energy while reducing hydroelastic responses of VLFS. In terms of reductions in hydroelastic responses of VLFS, the OWSC-type attachment is better than the recently proposed raft wave energy converter (WEC)-type attachment for relatively short waves (T &amp;lt; 7 s) and better than the conventional anti-motion device comprising a submerged vertical flap rigidly connected to the fore edge of VLFS for all wave periods. Importantly, the horizontal wave force acting on the submerged flap for the OWSC-type attachment is smaller than that for the conventional anti-motion device, leading to a more economical mooring system. In terms of wave energy extraction, the OWSC-type attachment is better than the raft WEC-type attachment for intermediate and long waves (T ≥ 7 s). In addition, for maximizing power production, the required flap length for the OWSC-type attachment is much smaller than the required pontoon length for the raft WEC-type attachment (about λ/10 as compared to about λ/3, where λ is the incident wavelength).

Experimental investigation of multiple Oscillating Water Column Wave Energy Converters integrated in a floating breakwater: Energy extraction performance

Currently, ocean wave energy technology is in its infancy relative to the mature renewable energy technologies such as wind and solar. Due to its early stage of development, ocean wave energy has high associated Levelised Cost of Electricity (LCOE), a measure of lifetime costs relative to lifetime energy production. Several solutions have been derived in an attempt to reduce this high LCOE, of which breakwater integration of wave energy converters presents a viable option. Current full-scale commercial and demonstration devices indicate that OWC device integrated breakwaters are typically limited to nearshore and onshore operational regions. However, industries such as aquaculture and offshore wind are exploring the viability of placing these structures in deeper waters, where these traditional concepts would not be applicable, providing opportunity for the development of floating offshore multi-purpose structures. This article describes a proof-of-concept for a floating breakwater integrated with Oscillating Water Column (OWC) Wave Energy Converters (WEC). For an integrated device of this type there are multiple key aspects that are inter-related and each must be understood: energy extraction performance, wave attenuation and quantifying platform motions. In order to adequately report on each aspect in a logical manner the study is presented in two parts. This paper covers the energy extraction aspects, while the second part deals with wave attenuation and motion characteristics [1]. The wave energy extraction characteristics of the installed devices are explored across parameters including device configurations, breakwater width, power take-off damping, wave height and motion constraints, all of which was achieved through model scale hydrodynamic experimentation. The major findings indicate that OWC device spacing is a key parameter in the design of multi-device structures, as device-device interaction can have constructive or destructive interferences on the energy extraction. Additionally, the results of the OWC device performance, under the influence of the aforementioned parameters, provides new insights to the development of floating offshore multi-purpose structures and their feasibility.

Spatio-Temporal Assessment of Climate Change Impact on Wave Energy Resources Using Various Time Dependent Criteria

The wave energy resources in the Indian Ocean can be considered as a potential alternative to fossil fuels. However, the wave energy resources are subject to short-term fluctuations and long-term changes due to climate change. Hence, considering sustainable development goals, it is necessary to assess both short-term (intra-annual) variation and long-term change. For this purpose, the simulated wave characteristics were utilized, and the wave power and its variation and change were analyzed in the whole domain and nearshore areas. The short-term fluctuation was investigated in terms of monthly and seasonal variations and the future change was discussed based on absolute and relative changes. Both analyses show that the Southern Indian Ocean, despite experiencing extreme events and having higher wave energy potential, is more stable in terms of both short and long-term variation and change. The assessment of the total and exploitable storages of wave energy and their future change revealed the higher potential and higher stability of the nearshores of the Southern Indian Ocean. It can be concluded that based on various factors, the south of Sri Lanka, Horn of Africa, southeast Africa, south of Madagascar and Reunion and Mauritius islands are the most suitable areas for wave energy extraction.

Two-mode WEC-type attachment for wave energy extraction and reduction of hydroelastic response of pontoon-type VLFS

Proposed herein is a two-mode wave energy converter-type attachment to the fore edge of pontoon-type very large floating structures (VLFSs) for wave energy extraction and reduction of hydroelastic response. This attachment consists of floating horizontal and submerged vertical plates that are connected to the VLFS with hinges and linear power take-off systems. The use of both floating horizontal and submerged vertical plates combines the superior performances of floating plates in extracting wave energy from short waves (T < 5 s) and of submerged vertical plates in extracting wave energy from intermediate and long waves (T > 7 s). When deployed in sites where 5 s ≤ T ≤ 7 s, the two-mode attachment yields a larger power production than the sole usage of floating plates or the sole usage of submerged vertical plates. Interestingly, the two-mode attachment also combines the superior performance of submerged vertical plates in reducing hydroelastic response for relatively short waves (T < 7 s) and the superior performance of floating plates in reducing hydroelastic response for relatively long waves (T ≥ 9 s). Moreover, this two-mode attachment furnishes a good balance between extracting wave energy and reducing hydroelastic response of VLFS for the same wave period.

Recent Progress in Electrical Generators for Oceanic Wave Energy Conversion

Oceanic wave energy extraction through electrical generator is one of the most interesting topics in the field of power engineering. Almost all the existing relevant review paper focus on electrical generator with the working principle of electromagnetic induction or piezoelectric or triboelectric effect. In this paper, all the existing types (based on principle of operation) of electrical generator used for wave power harvesting are discussed. This paper not only covers recent progress in electrical power generation by electro-magnetic induction, piezoelectric generator, and electrostatic induction, but also presents critical comparative review as well where suitable use and weakness of each type of generators are discussed. Moreover, the application of advanced magnetic core, winding, and permanent magnets are discussed with extensive explanation which are not focused in the existing reviews. Various new constructional features of the electrical generators such as split translator flux switching, two-point absorber, triangular coil, dual port linear generator, piezoelectric, triboelectric nanogenerator, etc. are highlighted with principles of operation. It also includes emerging human intervened optimization method for determining optimum shape of generator and cooling system which is necessary to prevent demagnetization of the permanent magnet. Finally, the way of supply the generated electrical power form the generator to load/grid is thoroughly described in a separate section that would be obvious for successful operation. The comparison among all types of generators in terms of output voltage, current, scale of power production, power-frequency characteristics, power density, cascading, and approaches are tabulated in this paper.

Technologising the wave

Despite the recent shift from renewable energy to a low carbon policy, the UK policy discourse still recognises marine energy as part of the country’s future energy mix. Production of what we call an “assemblage” of technology and ocean waves triggers complex sets of initiatives that provide the basis for the economic viability and credibility of wave energy extraction. However, questions are rarely asked about how the natural phenomenon being part of this assemblage is construed as a resource to become a key element of promises and assessments of potential of renewable energy. This study sheds light on under-explored aspects of the credibility–economy and valuation practices formed around renewable energy that have not yet been problematised in social studies of energy. Arguing that ocean waves become an energy resource largely through resource assessment practices, we examine such practices in the context of the production of scientific and policy discourses around wave energy. Considering waves as an object of expertise, we examine how “wave data” constituted through measurements, statistical analysis, modelling and visualisation, contribute to the assessment and legitimisation of wave energy developments. We also evaluate the prospects for wave energy to be a “good” in future economic exchange.

Optimization of Artificial Neural Networks Based Models for Wave Height Prediction

For an efficient wave energy extraction, the evolution of some specific parameters must be known. These parameters, like significant wave height and period, are mainly determined by the wind speed and influenced by some sea environment characteristics. Their evolution in time is one of the basic information necessary for designing of an accurate energy conversion system. In many scientific works the benefits of artificial neural networks based modeling are presented. These models allow the prediction and optimization of the wave parameters starting from experimentally acquired data. Due to specific calculus method of the artificial neural networks, in order to obtain accurate results, a very important step is the appropriate neural model design. If the model is optimal correlated with the data processed, the results obtained can be more significant than those coming from the mathematical formulas. The main neural models parameters that must be taken into account for an optimal design are model structure, transfer function and training algorithm. This paper presents an investigation of the results obtained with different models, proving that for a specific dataset a specific neural model offers the best results. Several models are analyzed, for a dataset corresponding to specific point in Black Sea and a comparison of results is presented.

Increase in Volumetric Electrical Power Density of a Linear Generator by Winding Optimization for Wave Energy Extraction

In this article, a method of winding optimization of the linear generator is proposed to increase the electrical power generation from the oceanic wave. A linear generator is designed in the ANSYS/Maxwell environment to analyze the proposed winding optimization method. The electrical and magnetic properties of the generator are extensively analyzed. The cross-sectional area and the number of turns of the copper conductor are optimized to maximize the output power. Load characteristics are also considered for different conductor sizes and turn numbers to determine a suitable operating point. A cooling system is also incorporated in the optimized linear generator to extract the thermal energy produced during the operation, which eventually enhances the electrical power generation. It is found that the optimized linear generator finally generates around 42% more electrical power compared to that of the conventional one. Therefore, the volumetric power density of the linear generator is noticeably increased with the proposed optimization method that also results in the minimization of material cost of the generator. A downscale prototype of the linear generator is constructed in the laboratory. It is expected that the proposed winding optimization method can be used in designing other linear generators.

Understanding coastal impacts by nearshore wave farms using a phase-resolving wave model

When extracting wave energy, arrays of wave energy converters (or wave farms) may alter surrounding wave and flow fields. This paper studies the modification of hydrodynamic processes at the coastline induced by nearshore wave farms using a recently developed phase-resolving wave-flow model. Changes to nearshore hydrodynamics were assessed for various farm configurations of submerged point-absorbers positioned 1–3 km offshore that were subject to realistic sea-states. In the lee of the farms, wave heights were attenuated and onshore directed flows were generated that extended several hundred meters shoreward but did not impinge the coast. For scenarios in which the wave shadow extended to the coast, the nearshore wave height and setup were reduced resulting in longshore pressure gradients driving longshore flows that converged in the lee of the farms. Changes were largest for compact farms at smaller offshore distances, and conversely, were significantly smaller for wider spaced arrays at greater offshore distances. Based on a bulk longshore sediment transport formulation, the converging flow patterns indicate conditions favourable for the accumulation of sediment in the direct lee and divergence of sediments at locations up/down coast from the farm, suggesting a reorientation of the shoreline in response to the wave farm configurations considered.

Time-domain simulation of a slack-moored floating oscillating water column and validation with physical model tests

The development of devices for extracting wave energy from the ocean is largely supported by numerical models, as they allow the simulation of different configurations without the large costs of tank testing. From the different available options, time-domain models offer a very good combination between accuracy, flexibility and computational time. They allow the incorporation of non-linearities from power take-off systems, mooring lines, sophisticated control techniques and other relevant hydrodynamic effects. In this paper, we present a time-domain model to simulate the dynamics and power performance of a slack-moored Spar-buoy OWC (Oscillating Water Column) wave energy converter. The model considers linear hydrodynamics, mean drift forces, viscous drag effects and air compressibility inside the OWC chamber. The mooring system is simulated using a quasi-static approach. The floating structure is defined as a rigid body with six degrees of freedom, whereas the OWC free surface is assumed flat. The converter motion and power extraction from regular and irregular wave simulations are compared with experimental results from small-scale model tests in a wave channel. Numerical results show good agreement with experimental data except when parametric resonance is observed and near the channel cut-off frequencies.

A Planar Generator for a Wave Energy Converter

This article presents a permanent magnet planar translational generator which is able to exploit multiple modes of sea wave energy extraction. Linear electrical generators have recently been studied for the exploitation of sea wave energy, but, to the best of our knowledge, no synchronous planar translational generator has been proposed. In this article, to maximize the energy extraction, we have considered all the potential modes of motion due to wave excitation and included them within the mathematical model of the proposed system. The principle of operation of the generator can be summarized as follows: the moving part (translator) of the generator is driven from the sea waves and induces and electromotive force (EMF) on the windings mounted to the armature. The movement of the translator is 2-D and, therefore, all the movement modes of the wave, except heave, can be exploited. The proposed mathematical model includes the dynamic equations of the translator and the electric equations of the windings. The coupling parameters (inductances and fluxes) have been determined by finite element method analysis. Optimization of the device has been performed by considering both, the parameters of the electromagnetic circuit, and, the parameters associated with the stochastic features of the wave.

Hydrodynamic performance of a dual-floater hybrid system combining a floating breakwater and an oscillating-buoy type wave energy converter

The high cost of power generation impedes commercial-scale wave power operations. The objective of this work is to provide a cost-sharing solution by combining wave energy extraction and coastal protection. A two-dimensional numerical wave tank was developed using Star-CCM+ Computational Fluid Dynamics software to investigate the hydrodynamic performance of a dual-floater hybrid system consisting of a floating breakwater and an oscillating-buoy type wave energy converter (WEC), and was compared with published experimental results. The differences between the hydrodynamic performance of the hybrid system, a single WEC and a single breakwater were compared. Wave resonance in the WEC-breakwater gap has a significant impact on system performance, with the hybrid system demonstrating both better wave attenuation and wave energy extraction capabilities at low wave frequencies, i.e., wider effective frequency. Forces on the breakwater were generally reduced due to the WEC. Wave resonance in the narrow gap has an adverse effect on the energy efficiency of the hybrid system with an asymmetric WEC, while a beneficial effect with a symmetric WEC. The wave energy conversion efficiency of hybrid system can be improved by increasing the draft and width of the WEC and decreasing the distance between the WEC and the breakwater. The findings of this paper make wave energy economically competitive and commercial-scale wave power operations possible.

수로형 공명구조물내 배열된 다수 파력발전장치의 파랑에너지 추출

본 논문에서는 ‘Y’자 수로형 공명구조물내에 다수의 파력발전장치를 배열하였을 때, 원기둥의 수직 운동응답과 에너지 추출효율에 관해 살펴보았다. 긴 채널과 유도판으로 구성되는 수로형 공명구조물은 수로내 유체의 공진으로 파랑에너지를 증폭시킨다. 그리고 원기둥과 선형발전기로 구성되는 파력발전장치는 수로내 최대파고가 발생하는 파복(anti-node) 위치에 설치하였다. 원기둥에 작용하는 동유체력계수(부가질량, 파랑감쇠력)와 파기진력은 WAMIT 상용 수치해석코드를 이용하여 계산하였고, 수직 운동응답은 제주대학교 2차원 조파수조에서 실시한 모형실험 결과와 비교하였다. RAO 곡선은 여러 개의 피크값을 보였고, 수치계산 결과는 실험결과와 잘 일치함을 확인하였다. 그리고 원기둥은 흘수가 짧은 모델과 긴 모델을 선정하여 운동성능을 비교하였다. 파력발전장치로부터 추출되는 시간평균파워와 취득폭 비(Capture Width Ratio, CWR)를 계산한 결과, 수로내에 배열된 파력발전장치는 수로형 공명구조물 없이 단독으로 운용하는 파력발전장치보다 넓은 주기대역에서 높은 효율을 보여주었다.