Wave Energy Device Research Articles

Numerical Investigation on Hydrodynamic Performance of an OWC Wave Energy Device in the Stepped Bottom

The efficiency response of an oscillating water column (OWC) wave energy converter is analysed by adopting a numerical approach and using various stepped bottom configurations for the seabed. A two-dimensional, fully non-linear higher-order boundary element method (HOBEM) model is developed to simulate the hydrodynamic characteristics and fluid structure interaction for a fixed OWC located in a numerical wave flume. A number of model tests are conducted for various regular incident wave conditions by modifying the wave amplitudes and wavelengths. The measured free surface elevation at the chamber centre and the oscillatory air pressure generated by the fluctuating free surface within the chamber are recorded. The hydrodynamic efficiency of the OWC is determined using these parameters. The simulation results are validated by comparing against previously published experimental and numerical data. Good agreement is observed in both cases. The geometric dimensions of the step are modified by altering the step height and step length. Furthermore, by altering the step length, the location of the front face of the step relative to the front wall of the OWC is adjusted. Therefore, the positional significance of the step can also be analysed in terms of its relative location to the OWC chamber and to the flow field development. It is shown that the geometry of the step and the position of the vertical face of the step relative to the OWC influences the hydrodynamic efficiency. The research demonstrates that by optimising the step geometry and position for a given wave condition a higher operational efficiency can be achieved.

Design of Mooring Solutions and Array Systems for Point Absorbing Wave Energy Devices—Methodology and Application

Abstract Most of the ocean energy technologies are considered to be in a pre-commercial phase and need technical development. This study focuses on the design of mooring solutions and compares array systems of a specific floating point-absorbing wave energy converter (WEC) developed by the company Waves4Power. A full-scale prototype of the WEC is installed in Runde (Norway) where it is moored with three polyester mooring lines, each having one floater and one gravity anchor. Based on this reference installation, the method of systems engineering was used to propose 22 conceptual mooring solutions for different array systems. They were compared and reduced to four top concepts in a systematic elimination procedure using Pugh and Kesselring matrices. The top concepts were assessed in detail by means of levelized cost of energy (LCOE), life cycle analysis (LCA), and risk analyses. The fatigue life of the mooring lines and the energy capture were calculated using results obtained from coupled hydrodynamic and structure response analyses in the dnv-gl deepc software. Two final concepts were proposed for the water depths 75 and 200 m.

Experimental investigation on the hydrodynamic performance of a cylindrical dual-chamber Oscillating Water Column device

The hydrodynamic performance of a stationary cylindrical dual-chamber Oscillating Water Column (OWC) wave energy device was experimentally studied to assess conversion efficiency in comparison with a single-chamber OWC. The contribution of the present work is to guide the design and optimization of the dual-chamber OWC device for efficiently capturing offshore wave energy. The effects of various parameters including wave steepness, the opening ratio, the inner- and outer-chamber drafts on the hydrodynamic efficiency of the proposed OWC device were considered. It was found that the hydrodynamic efficiency of the dual-chamber OWC device increases by comparison with the single-chamber one. A coupled resonant effect between the inner- and outer-chambers was observed for the dual-chamber OWC, which leads to the difference between the resonant frequencies and broadens the effective frequency bandwidth. The ratio of the orifice opening area to the area of the chamber columns has a significant influence on the hydrodynamic efficiency. The optimal opening ratio is founded to be between 1.5% and 2.0% in the present study. It was also observed that the hydrodynamic efficiency decreases with the increase of wave steepness and increases with the decrease of the outer-chamber draft.

Theoretical performance investigation of a vertical cylindrical oscillating water column device in front of a vertical breakwater

The present paper deals with the investigation of the efficiency of an oscillating water column device placed in front of a reflecting vertical breakwater. At a first stage, the exciting forces and moments acting on the OWC along with the motion- and pressure-dependent hydrodynamic characteristics of the device, placed in front of a vertical wall, are derived by the solution of the diffraction, motion- and pressure-radiation problems supplemented by representative numerical results concerning the air pressure inside the oscillating chamber. Hydrodynamic interactions between the float and the adjacent breakwater are exactly taken into account using the methods of images. The second part of the paper is devoted to the geometric parametric analysis concerning the distance between the OWC device and the breakwater and the angle of wave propagation to examine how the presence of the vertical wall affects the efficiency of the wave energy device.

From Bore\u2013Soliton\u2013Splash to a New Wave-to-Wire Wave-Energy Model

We explore extreme nonlinear water-wave amplification in a contraction or, analogously, wave amplification in crossing seas. The latter case can lead to extreme or rogue-wave formation at sea. First, amplification of a solitary-water-wave compound running into a contraction is disseminated experimentally in a wave tank. Maximum amplification in our bore–soliton–splash observed is circa tenfold. Subsequently, we summarise some nonlinear and numerical modelling approaches, validated for amplifying, contracting waves. These amplification phenomena observed have led us to develop a novel wave-energy device with wave amplification in a contraction used to enhance wave-activated buoy motion and magnetically induced energy generation. An experimental proof-of-principle shows that our wave-energy device works. Most importantly, we develop a novel wave-to-wire mathematical model of the combined wave hydrodynamics, wave-activated buoy motion and electric power generation by magnetic induction, from first principles, satisfying one grand variational principle in its conservative limit. Wave and buoy dynamics are coupled via a Lagrange multiplier, which boundary value at the waterline is in a subtle way solved explicitly by imposing incompressibility in a weak sense. Dissipative features, such as electrical wire resistance and nonlinear LED loads, are added a posteriori. New is also the intricate and compatible finite-element space–time discretisation of the linearised dynamics, guaranteeing numerical stability and the correct energy transfer between the three subsystems. Preliminary simulations of our simplified and linearised wave-energy model are encouraging and involve a first study of the resonant behaviour and parameter dependence of the device.

Dynamic Response of Coupled Mooring Lines and Floating Wave Energy Devices in Waves / Currents

Various global studies have shown that ocean waves energy have large potential in renewable energy sector. Their role within renewable energy gets high priority in the future by the government of United Kingdom. The principle concept of wave energy is when wave energy is converted into potential energy by the wave energy devices to generate electricity. An understanding of the dynamic response of the devices and mooring lines is important for this paper. This paper deals with the analysis of the various effects that influence the different design of wave energy converter devices. The mooring design idea is also analyzed to show which mooring layout is suitable to fulfill the requirement. The design of mooring configuration also influence how wave power is extracted and how such system are operated and maintained. The effects investigated in this paper are regular and irregular waves, motion @ six degrees of freedom, maximum and minimum mooring tension, different waves direction, wave current, energy and power take off.

Numerical Simulation of the Breakwater-Ramp Design for Multistage-Overtopping Wave Energy Breakwater Hybrid Device

Multistage-overtopping wave energy device is an example of the overtopping concept in extracting energy from the ocean wave. The multistage opening allows the wave to overtop and stored inside the multistage reservoirs and later used for electricity generation. Previously, using concentrated V-shape-ramp on the conventional multi-reservoir Sea Slot-Cone Generator (SSG) has been proven to improve up to 67 percent in hydraulic power at the 3.5 meters of significant wave height and 9.3 meters of wave period. However, the performance begins to drop when operating at smaller wave height consisting of 2.5-meter significant wave height and 7.9 seconds of wave period. Thus, this follow-up paper presents a numerical study to improve device performance, especially at small wave height. Five breakwater-ramp designs were tested consisting of Basis, Design 1 to Design 5. The overtopping device performance was simulated using FLOW-3D CFD software. Results are presented in the form of mean overtopping discharge or potential energy stored and the total energy potential for each propose breakwater-ramp design. The numerical results show that the highest total energy potential represents as the potential power output is recorded by Design 4 and Design 5 with 63.26 and 63.05 kW respectively, compared to basis SSG device with only 29.14 kW at the wave height of 2.8 meters and a wave period of 9.3 seconds.

Numerical Research and Open Sea Tests of a 100 kW Modified Edinburgh Duck Wave Energy Convertor

This paper presents a modified wave energy device. The wave energy device retains the primitive capture shape of the famous Edinburgh Duck, but uses a new type of underwater-stabilized substrate. The hydrodynamic calculation with frequency-domain models was performed to find out the optimum wave period for the wave energy converter. The results to be presented were obtained by using the Boundary Element Method (BEM), based on linear wave theory. The characteristic function expands of velocity potential in the cylindrical coordinate system was applied to limit computational domain and improve computational speed. The hydrodynamic performance of the device was assessed. The influence of hydraulic damping coefficient was evaluated. Focus was given to the capture width ratio, motion of the device and optimal hydraulic damping coefficient. After two years of construction in the shipyard, the device was constructed and an open sea test had been finished near WanShan Island, South China Sea. During the test, the daily power generation and power curve of the device were measured. In this test, amounts of test data, experience and lessons had been achieved and will be summed up and presented in the paper.

Wave-to-Wire Model Development and Validation for Two OWC Type Wave Energy Converters

The Tupperwave device is a closed-circuit oscillating water column (OWC) wave energy converter that uses non-return valves and two large fixed-volume accumulator chambers to create a smooth unidirectional air flow, harnessed by a unidirectional turbine. In this paper, the relevance of the Tupperwave concept against the conventional OWC concept, that uses a self-rectifying turbine, is investigated. For this purpose, wave-to-wire numerical models of the Tupperwave device and a corresponding conventional OWC device are developed and validated against experimental tests. Both devices have the same floating spar buoy structure and a similar turbine technology. The models include wave-structure hydrodynamic interaction, air turbines and generators, along with their control laws in order to encompass all power conversion stages from wave to electrical power. Hardware-in-the-loop is used to physically emulate the last power conversion stage from mechanic to electrical power and hence validate the control law and the generator numerical model. The dimensioning methodology for turbines and generators for power optimisation is explained. Eventually, the validated wave-to-wire numerical models of the conventional OWC and the Tupperwave device are used to assess and compare the performances of these two OWC type wave energy device concepts in the same wave climate. The benefits of pneumatic power smoothing by the Tupperwave device are discussed and the required efficiency of the non-return valves is investigated.

Three Dimensional Radiated Water Wave with a Submerged Cylinder in Presence of Circular Plate

This work deals with the problem of radiated by wave interaction with a couple of submerged cylinders in water which can be considered as a wave energy device and the problem arising from the rotational motion of submerged upper cylinder which one contains in the device. In this work, we approach theoretically to solve the problem based on the method of separation of variables and we derive the radiated velocity potentials numerically based on linear wave theory and eigenfunctions are introduced for each region by using free surface condition. Then we calculate the hydrodynamic coefficients due to rotational of the upper cylinder by using Bernoulli’s equation of pressure by neglecting the atmospheric pressure and unknown constants are calculate by using matched conditions between the regions Finally, we present all numerical results graphically for different radii of the cylinders.

IMPACT OF HYDROPOWER PLANT ON CLIMATE CHANGE

Ocean waves are an immense, unused source of energy. Since for climate change, global attention is nowadays constantly being drawn to the fact that the level of CO2 is rising, creating the so called “greenhouse effect”, and influencing further climate changes, the focus on generating electricity from renewable sources forces itself as an important area of future research. The possibility to exploit the energy potential created by sea waves indisputably exists. Various researches conducted in the field of unused energy sources prove the necessity of seriously considering this potential of sea waves. The illustration of this point in noted in a recently performed estimation that up to 15 per cent of current UK electricity demand could be met by wave energy, and when this is combined with tidal stream generation, even up to 20 per cent of the UK demand for electrical power could be met. This paper introduces the readers to the topic of water power, as a natural source of renewable energy, investigating the possibility of using power of waves as one of the technically and ecologically appropriate solutions of exploiting this renewable natural source. It is a fact widely acknowledged that climate change and global warming are the number-one challenge of the contemporary world; practically every available research predicts that with the current rate of development the world is heading for at least 2, and even more likely 3-4, degrees warmer climate by the end of this century. As energy production and use account for two-thirds of global greenhouse-gas emissions, the energy industry is in a key role to cut emissions – while powering economic growth, boosting energy security and increasing energy access. In view of this, hydropower can make a major contribution to climate change mitigation. Not only is hydropower practically CO₂-free, thus eliminating its possible contribution to the greenhouse effect, but it is also both more efficient and flexible compared with existing fossil alternatives for base load and balancing power. Even though the world is aware of the potential of hydropower, and despite the fact that many wave energy devices are nowadays being investigated, most of these are currently at the research and development stage, with only a small range of devices having been tested at large scale, deployed in the oceans. Thus, one of the goals of this paper would be to present the technology used to turn the power of waves unto electric power, as well as the various types of turbines and devices which represent the latest technological achievements in this area. The goal of this paper is also to present the existing devices, their manner of functioning, and their efficiency and safety from the environmental aspect. It is also important to note that hydropower’s own overall environmental impact is limited, thus making hydropower more benevolent to the environment than other energy sources that have been exploited so far.

Wave energy conversion under constrained wave-by-wave impedance matching with amplitude and phase-match limits

This paper investigates an approach to limit the fullness of ‘tuning’ provided by wave-by-wave impedance matching control of wave energy devices in irregular waves. A single analytical formulation based on the Lagrange multiplier approach of Evans [1] is used to limit the velocity amplitude while also limiting the closeness of the phase match between velocity and exciting force. The paper studies the effect of the present technique in concurrently limiting the device velocity and the required control/actuation force. Time domain application requires wave-profile prediction, which here is based on a deterministic propagation model. Also examined in the time domain is the effect of possible violation of the displacement constraint, which for many designs implies impacts at hard stops within the power take-off mechanism. Time domain simulations are carried out for a 2-body axisymmetric converter (with physical end-stops) in sea states reported for a site off the US east coast. It is found that the approach leads to effective power conversion in the less energetic sea states, while as desired, considerable muting of the optimal response is found in the larger sea states. Under the assumptions of this work, the end-stop collisions are found to have a minor effect on the power conversion. The present approach could be used to guide the design of power take-off systems so that their displacement stroke, maximum force, and resistive and reactive power limits are well-matched to the achievable performance of a given controlled primary energy converter.

Identification of dynamic models for a wave energy converter from experimental data

This paper addresses the issue of hydrodynamic model identification from recorded tank test data, for a prototype wave energy device. The study focusses on nonlinear Kolmogorov–Gabor polynomial models, with linear models also used as a baseline reference. Six different experimental data sets are employed for model identification and validation, all derived from a JONSWAP input sea state. Compared to identification on numerical data, this study shows that the determination of model structure and orders is not so straightforward, but that consistent and useful computationally efficient models can be obtained. For the particular tests undertaken, in which the prototype device generally behaves as a wave follower, the nonlinear models only show very marginal performance improvement over the linear ones.

O&M Models for Ocean Energy Converters: Calibrating through Real Sea Data

Of the cost centres that combine to result in Levelised Cost of Energy (LCOE), O&M costs play a significant part. Several developers have calculated component costs, demonstrating how they can become commercially competitive with other forms of renewable energy. However, there are uncertainties relating to the O&M figures that can only be reduced through lessons learned at sea. This work presents an O&M model calibrated with data from real sea experience of a wave energy device deployed at the Biscay Marine energy Platform (BiMEP): the OPERA O&M Model. Two additional case studies, utilising two other O&M calculation methodologies, are presented for comparison with the OPERA O&M Model. The second case study assumes the inexistence of an O&M model, utilising a Simplified Approach. The third case study applies DTOcean’s (a design tool for ocean energy arrays) O&M module. The results illustrate the potential advantages of utilising real sea data for the calibration and development of an O&M model. The Simplified Approach was observed to overestimate LCOE when compared to the OPERA O&M Model. This work also shows that O&M models can be used for the definition of optimal maintenance plans to assist with OPEX reduction.

Parametric modelling of a reconfigurable wave energy device

To date, one of the difficulties with wave energy devices is modulating the power transfer from the waves to the generator. In wind turbines, this is achieved by turbine pitch control, leading to the ubiquitous flat power curve. Recently, a wave energy converter, having moveable vanes, has been developed by the National Renewable Energy Laboratory (NREL) in the US. This paper described the development of a compact hydrodynamic model for the NREL device, based on multiple linear hydrodynamic modelling (Murray-Smith and Johansen, 1997), which can be used to model the hydrodynamic behaviour of the device across its full operational spectrum of vane angles. To show the utility of the model, we demonstrate a flat power curve for the WEC, using the vane angles as a control input.

A fundamental coupling methodology for modeling near-field and far-field wave effects of floating structures and wave energy devices

This research focuses on the numerical modelling of wave fields around (oscillating) structures such as wave energy converters (WECs), to study both near and far field WEC effects. As a result of the interaction between oscillating WECs and the incident wave field, additional wave fields are generated: the radiated and the diffracted wave field around each WEC. These additional wave fields, together with the incident wave field, make up the perturbed wave field. Several numerical methods are employed to analyse these wave fields around WECs. For example, for investigating wave-structure (wave-WEC) interactions, wave energy absorption and near field effects, the commonly used and most suitable models are based on Boundary Element Methods for solving the potential flow formulation, or models based on the Navier-Stokes equations. These models are here referred to as ‘wave-structure interaction solvers’. On the other hand, for investigating far field effects of WEC farms in large areas, wave propagation models are most suitable and commonly employed. However, all these models suffer from a common problem; they cannot be used to model simultaneously both near and far field effects due to limitations.In this paper, a generic coupling methodology is presented, developed to combine the advantages of the above two approaches; (a) the approach of wave-structure interaction solvers, which are used to investigate near field effects because they can more correctly model wave energy absorption and the resulting wave fields induced by oscillating WECs or WEC farms. These solvers suffer from high computational cost and thus are mainly used for limited: (i) areas around WECs; (ii) number of WECs, and (b) the approach of wave propagation models, which are used for predicting far field effects and which can model the effect of WEC farms on the wave field and the shoreline in a cost-effective manner, but usually cannot deliver high-fidelity results on wave energy absorption by the WECs.In addition, a novel wave generation technique is presented, for generating the perturbed wave field induced by an oscillating WEC, in a wave propagation model. The results obtained from the proposed coupling methodology and wave generation technique along a circle are validated and show very good agreement. Finally, the benefits of the proposed coupling methodology to model floating bodies in a phase resolving wave propagation model are discussed.

Effect of the PTO Damping Force on the 2D Oscillating Buoy Wave Energy Converter Integrated into a Pile-Restrained Floating Breakwater

The aim of the study is to evaluate the parametric effect of a power take-off (PTO) system on an integration system of an oscillating buoy wave energy device with a pile-restrained floating breakwater. To consider the viscous effect, the study is carried out with three conditions using OpenFOAM, i.e., the linear optimal PTO system, the fixed PTO damping coefficient, and the Coulomb PTO system. The results show that the maximum wave energy capture ratio (CWR) of the device can merely reach 0.35 in the present work, when considering the viscous effect. This result is smaller than that obtained from potential flow. In addition, the linear optimal PTO coefficient under viscous flow should be larger than the theoretical optimal coefficient in terms of the potential flow. A comparison of the Coulomb damping model and the linear damping model also shows that the Coulomb damping model has a good performance in terms of both wave energy capture efficiency and coastal protection. Hence, the effect of the PTO system on such an integration system can be an essential factor to achieve an optimal design.

Assessment of Primary Energy Conversion of a Closed-Circuit OWC Wave Energy Converter

Tupperwave is a wave energy device based on the Oscillating-Water-Column (OWC) concept. Unlike a conventional OWC, which creates a bidirectional air flow across the self-rectifying turbine, the Tupperwave device uses rectifying valves to create a smooth unidirectional air flow, which is harnessed by a unidirectional turbine. This paper deals with the development and validation of time-domain numerical models from wave to pneumatic power for the Tupperwave device and the conventional OWC device using the same floating spar buoy structure. The numerical models are built using coupled hydrodynamic and thermodynamic equations. The isentropic assumption is used to describe the thermodynamic processes. A tank testing campaign of the two devices at 1/24th scale is described, and the results are used to validate the numerical models. The capacity of the innovative Tupperwave OWC concept to convert wave energy into useful pneumatic energy to the turbine is assessed and compared to the corresponding conventional OWC.

Analytical Research on Energy Conversion Characteristic of a Kind of Point Absorber Wave Energy Device

ABSTRACT Zheng, X.; Zhou, S.; Ma, Y., and Wang, L., 2018. Analytical research on energy conversion characteristic of a kind of point absorber wave energy device. In: Liu, Z.L. and Mi, C. (eds.), Advances in Sustainable Port and Ocean Engineering. Journal of Coastal Research, Special Issue No. 83, pp. 991–999. Coconut Creek (Florida), ISSN 0749-0208. The coaxial floater and buoy on the surface of fluid compose a point absorber wave device. The hydrodynamic fluid/solid interaction becomes more complex due to relative heave motion. In order to investigate influencing factors of wave energy device efficiency, This paper aims at presenting the heaving mode hydrodynamic coefficients and exciting force of the wave energy device by using the coupling hydrodynamic analysis method and establishing heave coupled motion equation of the device by considering spring factors of the Power Take Off (PTO) system. The hydrodynamic coefficient and capture width ratio (CWR) of the wave energy device for various geometrical co...

Enhancing the resilience of energy systems: Optimal deployment of wave energy devices following coastal storms

Isolated coastal areas and remote islands may be particularly vulnerable to damage from powerful storms, storm surges, and flooding. Because wave energy converters are designed to survive rough seas and can be transported by sea, they could potentially play a role in poststorm operations and contribute to power-grid recovery. To that end, this paper addresses questions such as how many devices would need to be deployed and in what sequence, in order to optimize some performance index that includes, as functions of time, both, the energy needed and the energy converted by the device units. In this work, the wave-by-wave dynamics of the devices are controlled to optimize mean power conversion over 20 min, assuming sea-state stationarity over that period. Sea-state variations between 20 min and 13 h are found to be small (relative to variations between 13 and 60 h) for a candidate deployment site near a Caribbean island. Targeting deployment over 5–7 days, two optimization schemes are considered: (i) maximization of the power conversion capacity over a specified time interval and (ii) minimization of the time taken to deploy the desired conversion capacity. With the wave energy devices controlled for optimal conversion over successive 20-min periods, the optimization is carried out over the number of converter units added as a function of time. The results indicate that optimal deployment sequences can be evaluated for given conversion capacity/recovery-time targets and the type of recovery desired [strategies (i) or (ii) above]. However, depending on the energy richness of the “normal” wave climate at the deployment site, cost-effective recovery may require closer consideration of the trade-offs among device configurations, size, and number of units needed for desired capacity. The potential for wave energy devices to power early recovery operations and to support power-grid black-start could be worth considering further as a means to enhance the resilience of coastal and island energy systems.