Wave Energy Conversion Efficiency Research Articles

An artificial neural network model of coastal erosion mitigation through wave farms

In this work, a novel approach based on artificial intelligence (AI) to assess the efficiency of wave energy converter (WEC) farms in coastal protection isdeveloped. We consider as a case study a beach subjected to severe erosion: Playa Granada (S Spain). More specifically, we analyse the changes in the dry beach area (quantified through the Pelnard-Considère equation) with and without wave farm protection by means of an Artificial Neural Network (ANN) model. The model is selected after a thorough comparative study involving forty ANN architectures, with one and two hidden layers, and two training algorithms (Levenberg-Marquadt and Bayesian regression). The best results are obtained with a [5-10-1] architecture trained with the Bayesian regression algorithm. Once validated, this ANN model is applied to optimize the design and position of the wave farm. The results confirm that ANN models are a useful design tool for hybrid wave farms.

Economic Feedback Model Predictive Control of Wave Energy Converters

In this paper, we propose an economic feedback model predictive control (MPC) scheme to improve energy conversion efficiency of wave energy converters (WECs) and guarantee their safe operation over a wide range of sea conditions. The proposed MPC control law consists of two terms: one state feedback gain designed offline to maximize operating range and one term calculated online to maximize the energy output. Compared with the existing MPC strategies developed for the WEC control problem, the proposed feedback economic MPC strategy has the following distinguishing advantages: First, the satisfaction of safety constraints and the recursive feasibility can be guaranteed to ensure WEC's safe operation in a large range of sea states. Second, the novel MPC can notably improve energy production efficiency. Third, the controller design procedure is more convenient and straightforward compared with the existing MPC strategies. The efficacy of the proposed MPC strategy is demonstrated by numerical simulations with a point absorber as a case study. By comparison with a representative existing MPC strategy, the proposed economic MPC can significantly improve energy output.

Potential application of submerged horizontal plate as a wave energy breakwater: A 2D study using the WCSPH method

The submerged horizontal plate (SHP) has been extensively studied as an open-type breakwater, and some research explored its potential as a wave energy converter. However, these two functionalities were seldom combined in the literature. In this study, a 2D numerical model solving the wave-SHP interaction was built by using the weakly compressible smoothed particle hydrodynamics (WCSPH) method. The thin-walled boundary condition was verified through a dam-break impact case. The accuracy of wave profile and flow velocity calculations was validated by comparing with available experiments of solitary and regular waves interactions with SHPs. Comparison of the wave dynamic and flow dynamic between SHP and submerged step were made, which showed the advantages and disadvantages of the SHP. Then a parametric study of regular waves-fixed SHP interaction was conducted. Empirical formulas for the wave transmission coefficient and energy conversion efficiency were obtained by regressing numerical results using the Levenberg-Marquardt algorithm. Finally, a mathematical programming problem concerning the combined functionalities of SHP was established. Using the complex method, the optimal length and submergence of SHP regarding different emphases taken on the regular wave attenuation and energy utilization were solved. The results preliminarily demonstrated the potential application of SHP as a wave energy breakwater and provided reference on its parameter design, although this work was carried out in 2D and ignored the effects of energy convertor and piles on wave and flow patterns.

Wave energy conversion and hydrodynamics modelling technologies: A review

It has been well accepted that wave energy may have the potential to significantly contribute to the future renewable energy mix but so far it remains as the largest untapped renewable resources since the current wave energy technologies are technically immature for reliable and economic energy production. The greatest challenges would be how the performance of wave energy converters can be reliably assessed and how the wave energy conversion efficiency can be improved. These two challenges are strongly linked, with the former producing the required tools for the latter, which is a critical part for reducing the overall cost of wave energy production. For understanding the issues involved in wave energy conversion, the relevant energy conversion technologies are discussed, with a focus on hydrodynamics modelling for the wave energy converters.To achieve the goal, the review presents the fundamental understandings to wave energy conversions and the descriptions and discussions are made for what are the challenges in wave energy development, how the reliable numerical and physical modelling techniques for wave energy converters can be carried out, and how to optimise the power take-off and wave energy devices for improving wave energy conversion. Particularly, the issues with the hydrodynamics modelling are discussed in details, including the important issues with the control technologies and the end-stop problems.

Numerical simulation of an oscillating water column device installed over a submerged breakwater

In this paper, an example of a symbiotic combination of marine structures is studied. The base hydrodynamic performance of a traditional offshore oscillating water column (OWC) device is compared against the case in which it is constructed over a submerged breakwater. Numerical simulations are performed with the open source package OpenFOAM and the toolbox waves2Foam. The classical free surface capturing method volume of fluid (VOF) is employed to model first-order Stokes waves. The effects of the dimensions (height and length) of the submerged breakwater on the hydrodynamic characteristics, such as wave energy conversion efficiency (ξ), reflection coefficient (Cr), transmission coefficient (Ct), and energy dissipation ratio (ED) are explored thoroughly. Moreover, the effect of wave nonlinearity induced by increasing incident wave heights on the OWC device is examined. The results show that a proper configuration of the submerged breakwater is significantly helpful to optimize the energy conversion curve. Varying breakwater length will lead to a periodic variation in energy absorption efficiency. In addition, a higher incident wave height will lead to a lower energy absorption efficiency but a stronger dissipation ratio.

Hydrodynamic Performance Study of Wave Energy–Type Floating Breakwaters

The integration of wave energy converters (WECs) with floating breakwaters has become common recently due to the benefits of both cost-sharing and providing offshore power supply. In this study, based on viscous computational fluid dynamics (CFD) theory, we investigated the hydrodynamic performances of the floating box and Berkeley Wedge breakwaters, both of which can also serve as WECs. A numerical wave flume model is constructed using Star-CCM+ software and applied to investigate the interaction between waves and wave energy converters while completing the verification of the convergence study of time and space steps. The effects of wave length on motion response and transmission coefficient of the floating box breakwater model are studied. Comparisons of our numerical results and published experimental data indicate that Star-CCM+ is very capable of accurately modeling the nonlinear wave interaction of floating structures, while the analytical potential theory overrates the results especially around the resonant frequency. Optimal damping can be readily predicted using potential flow theory and can then be verified by CFD numerical results. Next, we investigated the relationship between wave frequencies and various coefficients using the CFD model under optimal damping, including the motion response, transmission coefficient, reflection coefficient, dissipation coefficient, and wave energy conversion efficiency. We then compared the power generation efficiencies and wave dissipation performances of the floating box and Berkeley Wedge breakwaters. The results show that the power generation efficiency of the Berkeley Wedge breakwater is always much higher than that of the floating box breakwater. Besides, the wave dissipation performance of the Berkeley Wedge breakwater is much better than that of the floating box breakwater at lower frequency.

Experimental study on the performance of a floating array-point-raft wave energy converter under random wave conditions

An array-point-raft wave energy converter (ARR WEC) integrating multiple-point absorption and raft type wave energy capturing technologies is proposed and experimentally investigated in this study. A 10 kW pilot device was developed, and a three-month real sea test was carried out in the Taiwan Strait, China. The experimental results confirmed the feasibility and effectiveness of the new system. The overall performance, heaving performance, power output and wave energy conversion efficiency of the pilot APR WEC running under random waves are reported and analyzed in detail. The heaving motions of the oscillating buoys and the instantaneous and average power output of the permanent magnet generator (PMG) are affected significantly by the number of oscillating buoys used to collect wave energy. More oscillating buoys used increase the production of electricity and improve the power quality, but lead to the reduction of energy conversion efficiency in long wave periods. The increase of electrical resistance of PMG results in an increased wave conversion efficiency. The experimental results obtained are valuable in the optimal design and operation of the ARR WEC system proposed.

Study on force and wave energy conversion efficiency of buoys in low wave energy density seas

The performance of wave energy converters is significantly affected by buoys. This research studied the force and conversion efficiency of buoys in low wave energy density seas and involved the selection of a cylindrical buoy and a hemispherical buoy, which were applied in a sealed-buoy wave energy converter (SBWEC). The underwater equivalent area was utilized as the variable to investigate the force, pitch response amplitude operator (RAO), and conversion efficiency of the buoys in irregular waves. The results show that the common responses occurred both in the cylindrical and hemispherical buoys. The force of the buoys in a resonance state was distributed in a concentric ring shape and gradually decreased from the inner to the outer areas. The force, pitch RAOs, and conversion efficiency increased with a corresponding increase in the underwater equivalent area. Results determined that owing to the difference in the underwater equivalent area and shape, the force of the cylindrical buoy with a diameter 2 m and 4 m was 194–1455% larger than that of the hemispherical buoy with the same diameter. The pitch RAOs of the cylindrical buoy were 0.6–52.59% larger than that of the hemispherical buoy because of the difference in force. In turn, the average conversion efficiency of the cylindrical buoy was 1.31% greater than that of the hemispherical buoy. The novel SBWEC made the wave energy conversion efficiency of the buoys in low wave energy density seas 0.6–1.4% less than that in high wave energy density seas. The utilization of the underwater equivalent area provides a new method for researching the force and conversion efficiency of buoys.

Wave tank experiments on the power capture of a float-type wave energy device with a breakwater

The objective of this study is to discuss the configuration and hydrodynamic performance of a float-type shoreline wave energy converter coupled with a breakwater. The energy converter is designed to extract wave power by making use of the wave-induced pitch motion of the float which is hinged on a breakwater. Meanwhile, the float is connected with hydraulic pistons, generating the high pressure water stream to drive alternators for electricity generation. A scale model was built in the laboratory at Hohai University, and the friction force was employed as the power take-off damping source. During the experiments, variations in several key parameters, including the wave elevation, wave energy converter (WEC) response (decay motion, free response, or damped motion of the WEC with varying damping levels), and wave-induced tensile forces on the steel wires were instantaneously measured. With these data, the wave reflection coefficient, instantaneous output power, and wave energy conversion efficiency of the model were estimated. The experimental results show that the developed WEC is effective in dissipating the incident wave energy, especially for longer waves, and is able to extract wave energy at a meaningful rate from regular waves. The average output power and wave energy conversion efficiency of the model under the resonance state can be up to 0.13 W and 32.6%, respectively. Although the WEC model harnesses more energy under longer waves, further examination reveals that the device is more efficient when installed in less energetic seas. Meanwhile, the water depth and the spacing between the breakwater and the float are significant factors for the posture and motion characteristics of the float during the process of interaction with water waves.

Dynamic Properties and Energy Conversion Efficiency of A Floating Multi-Body Wave Energy Converter

The present study proposed a floating multi-body wave energy converter composed of a floating central platform, multiple oscillating bodies and multiple actuating arms. The relative motions between the oscillating bodies and the floating central platform capture multi-point wave energy simultaneously. The converter was simplified as a forced vibration system with three degrees of freedom, namely two heave motions and one rotational motion. The expressions of the amplitude-frequency response and the wave energy capture width were deduced from the motion equations of the converter. Based on the built mathematical model, the effects of the PTO damping coefficient, the PTO elastic coefficient, the connection length between the oscillating body and central platform, and the total number of oscillating bodies on the performance of the wave energy converter were investigated. Numerical results indicate that the dynamical properties and the energy conversion efficiency are related not only to the incident wave circle frequency but also to the converter's physical parameters and interior PTO coefficients. By adjusting the connection length, higher wave energy absorption efficiencies can be obtained. More oscillating bodies installed result in more stable floating central platform and higher wave energy conversion efficiency.

Numerical study of hydrodynamic behavior and conversion efficiency of a two-buoy wave energy converter

In this paper we propose a two-buoy wave energy converter composed of a heaving semi-submerged cylindrical buoy, a fixed submerged cylindrical buoy and a power take-off (PTO) system, and investigate the effect of the fixed submerged buoy on the hydrodynamics of the heaving semi-submerged buoy based on the three-dimensional potential theory. And the dynamic response of the semi-submerged buoy and the wave energy conversion efficiency of the converter are analyzed. The difference of the hydrodynamics and the wave energy conversion efficiency of a semi-submerged buoy converter with and without a fixed submerged buoy is discussed. It is revealed that the influence of the fixed submerged buoy on the exciting wave force, the added mass, the radiation damping coefficient and the wave energy conversion efficiency can be significant with a considerable variation, depending on the vertical distance between the heaving semi-submerged buoy and the fixed submerged buoy, the diameter ratio of the fixed submerged buoy to the heaving semi-submerged buoy and the water depth.

Experimental study on primary efficiency of a new pentagonal backward bent duct buoy and assessment of prototypes

To improve the wave energy conversion efficiency of backward bent duct buoy (BBDB), a pentagonal BBDB model was designed and fabricated for experiments in a wave flume (2D) as well as in a wave basin (3D). Incident wave height, wave period, water elevation in the air chamber and pressure difference between indoor and outdoor of the air chamber were measured. The test results show that the maximum value of capture width ratio (CWR) of the model in the wave flume is about 119.1%, which is much better than the highest value 79.1% obtained under the same experimental conditions in historical documents. Under regular wave, the maximum value of CWR tested in the wave basin is about 146.8%, which is equivalent to the highest value in historical documents and has relatively wide bandwidth. Under irregular waves, the maximum average value of CWR tested in the wave basin is about 87.2%, which is superior to the highest value 52% in historical documents. The combination of experimental results of the model and the latest research achievements of air turbine can help to break the thoughts that wave energy utilization technology can't balance low cost and high efficient conversion.

Wave protection effect of periodic row of bottom-hinged flap-type wave energy converters

A flap-type wave energy converter (WEC) is combined with a nearshore breakwater to expand the application of WECs both economically and environmentally. Based on the linear potential theory, an eigenfunction expansion solution is developed for a periodic row of bottom-hinged flap-type WECs exposed to normal waves. Additionally, the viscous effect is considered using the ship rolling solution method with a viscous damping term included in the equation of motion, and the viscous damping expression is also described. The proposed solution is verified by comparison with published literatures. The results including the wave energy conversion efficiency, the reflected and transmitted proportion of the incident wave energy are presented for a range of wave periods and geometric ratios. It is demonstrated that better wave protection effects can be attained with smaller gaps between the WECs, where the transmitted proportion of the incident wave energy is lower. An optimal geometric ratio thus exists for a given wave power absorption and a specific wave period.

Impact of climate change on wave energy resource: The case of Menorca (Spain)

The aim of this paper was to analyse how changes in wave patterns, due to the effect of climate change, can affect wave energy power and yield around Menorca (NW Mediterranean Sea). The present and future wave energy conditions were derived from recently developed high-resolution wave projections in the NW Mediterranean. These wave projections were forced by surface wind fields obtained, respectively, by 5 different combinations of global and regional circulation models (GCMs and RCMs) for the A1B scenario. The results showed that the projected future spatial and directional distributions of wave energy are very similar to those of the present conditions. The multi-model ensemble average illustrated a slight general decrease in the annual and seasonal wave power (except for summer). However, the inter-model variability is large since two models showed opposite trends to the other 3 in most cases. Such inter-model variability is lower(higher) for winter(autumn). Another result is the reduction of the temporal variability in the future, considering both the multi-model mean and each single model projection. Such a decrease is consistent with the future seasonal redistribution of energy throughout the year. This would entail an increase in the efficiency of wave energy converters deployed in this area due to the more regular temporal distribution of the energy.

A maximum power point tracking algorithm for buoy-rope-drum wave energy converters

The maximum power point tracking control is the key link to improve the energy conversion efficiency of wave energy converters (WEC). This paper presents a novel variable step size Perturb and Observe maximum power point tracking algorithm with a power classification standard for control of a buoy-rope-drum WEC. The algorithm and simulation model of the buoy-rope-drum WEC are presented in details, as well as simulation experiment results. The results show that the algorithm tracks the maximum power point of the WEC fast and accurately.

Latching control of a floating oscillating-water-column wave energy converter

The OWC spar-buoy is an axisymmetric floating version of an oscillating-water-column (OWC) based device whose power take-off (PTO) system is an air turbine/generator set. Latching has been regarded as one of the most promising techniques to improve the efficiency of wave energy converters. In the case of the OWC spar-buoy, latching control is performed by opening/closing a high-speed stop valve installed in series with the turbine. The present paper has three main objectives. Firstly, to assess the performance improvements that can be achieved with a latching control strategy within a receding horizon framework. Secondly, to establish the practical requirements of this type of control by evaluating the sensitivity of the turbine power output to several receding horizon time intervals. Finally, to test and validate experimentally the proposed algorithms in a small-scale PTO test rig. All the experimental tests were performed considering irregular wave conditions.

Establishment of Motion Model for Wave Capture Buoy and Research on Hydrodynamic Performance of Floating-Type Wave Energy Converter

Abstract Floating-type wave energy converter has the advantages of high wave energy conversion efficiency, strong shock resistance ability in rough sea and stable output power. So it is regarded as a promising energy utilization facility. The research on hydrodynamic performance of wave capture buoys is the precondition and key to the wave energy device design and optimization. A simplified motion model of the buoys in the waves is established. Based on linear wave theory, the equations of motion of buoys are derived according to Newton’s second law. The factors of wave and buoys structural parameters on wave energy absorption efficiency are discussed in the China’s Bohai Sea with short wave period and small wave height. The results show that the main factor which affects the dynamic responses of wave capture buoys is the proximity of the natural frequency of buoys to the wave period. And the incoming wave power takes a backseat role to it at constant wave height. The buoys structural parameters such as length, radius and immersed depth, influence the wave energy absorption efficiency, which play significant factors in device design. The effectiveness of this model is validated by the sea tests with small-sized wave energy devices. The establishment methods of motion model and analysis results are expected to be helpful for designing and manufacturing of floating-type wave energy converter.

Estimation of the wave energy conversion efficiency in the Atlantic Ocean close to the European islands

This paper presents an evaluation of the efficiency of twelve state of the art wave energy converters in the Atlantic Ocean, in the vicinity of the most important European islands and archipelagos (Iceland, Archipelago of Azores, Madeira Archipelago and Canary Islands). An analysis of the wave conditions in the target areas was first performed by considering a 10-year interval (2004–2013) of wave data provided by the European Centre for Medium-Range Weather Forecasts. For this reason, twenty reference points, all located in water depths of about 50 m, were defined. In order to provide a general picture of the wave potential and also to highlight the presence of some hot spots, several wave parameters, such as significant wave height, mean wave direction and wave power, were evaluated. Then, for every nearshore area, based on the bivariate distributions of the sea states occurrences and also on the power matrix of each device, the performances of each wave energy converter were estimated in terms of the expected electrical power. The results of the present work provide valuable information for the future wave farm projects, which could become in the near future a reliable and effective way to produce energy in island environments.

Design and modeling of an innovative wave energy converter using dielectric electro-active polymers generator

This paper proposes a design and modeling of an innovative wave energy converter using Dielectric electro active polymer (DEAP). Firstly, an accurate model of conventional DEAP generator is investigated and validated under a specific range of ocean waves. Then, a structure design of an antagonistic DEAP generator so-called energy capture unit (ECU), which consists of two DEAPs in antagonistic connection mode to increase harvested energy efficiency, is modeled and validated by experimental data. A new design of WECs is then developed with array of ECUs to increase the output energy. In addition, by using the linear potential wave theory, the hydrodynamic forces are calculated under regular wave conditions. Consequently, a complete analytical model of the proposed WECs using multiple ECUs under hydrodynamic behavior is then obtained to investigate the performance of energy conversion. Finally, based on the developed analytical model, the stretch ratio known as an important factor to efficiency and output power is investigated under the influence of the floating buoy’s mass. Then, the resonance behavior of the WECs with a typical wave frequ ncy can be tuned by optimizing the floating’s mass to increase the degree of utilization of the device. The simulation results indicate hat the efficiency of wave energy converter can be up to 25% thanks to resonance behavior.

An experimental study on a trapezoidal pendulum wave energy converter in regular waves

Experimental studies were conducted on a trapezoidal pendulum wave energy converter in regular waves. To obtain the incident wave height, the analytical method (AM) was used to separate the incident and reflected waves propagating in a wave flume by analysing wave records measured at two locations. The response amplitude operator (RAO), primary conversion efficiency and the total conversion efficiency of the wave energy converter were studied; furthermore, the power take-off damping coefficients corresponding to the load resistances in the experiment were also obtained. The findings demonstrate that the natural period for a pendulum wave energy converter is relatively large. A lower load resistance gives rise to a larger damping coefficient. The model shows relatively higher wave energy conversion efficiency in the range of 1.0–1.2 s for the incident wave period. The maximum primary conversion efficiency achieved was 55.5%, and the maximum overall conversion efficiency was 39.4%.