Wave Energy Converter Power Research Articles

Design, dynamic modeling and wave basin verification of a Hybrid Wave–Current Energy Converter

Massive and high-density Marine and Hydrokinetic (MHK) energy is contained in the ocean, including waves, tidal streams, and ocean currents. Traditional MHK energy converters harvest energy from only a single MHK energy source, which does not fully exploit the energy potential that co-exists in multiple forms of MHK energy in the ocean. This paper presents the design and dynamics of a Hybrid Wave–Current Energy Converter (HWCEC) that can simultaneously convert both wave and current energy to electricity with a single Power Take-off (PTO) through the engagement and disengagement of the three one-way clutches. The critical design parameters are analyzed through modeling and simulation, including the transmission ratios, the electrical impedance, and the turbine-heave plate distance. Water basin tests in a wave–current tank were conducted, which shows the prototyped HWCEC can improve the electric power output by 38%–71% for regular waves and 79% for irregular waves, while providing a 70% reduction of the Peak to Average Ratio (PAR) of power compared to the baseline Wave Energy Converter (WEC). We further analyze the occurring percentages of the instant power of both WEC and HWCEC in irregular wave tests, shedding new insight on how HWCEC reduces the PAR.

Interrelationship between variables for wave direction-dependent WEC/site-configuration pairs using the CapEx method

Renewable energies are mostly compared using the Levelised Cost of Energy (LCoE). Two major components of LCoE are of interest in this study, the others being still experimental: the Annual Energy Production (AEP) and the Capital Expenditure (CapEx). Since many Wave Energy Converters (WECs), including Wavepiston, are wave direction-dependent, this research investigates the wave direction impact on the relationship between the site-configurations (site characteristics and wave climates), WEC-configurations and power, alongside AEP and CapEx. Wave climate and WEC power are expressed in 3-dimension by adding the wave direction to the common 2-dimensional space of wave height and period. Wavepiston's WEF costs are calculated using the CapEx method. Assessing the relationship between the diverse sources of AEP/CapEx variations, enabled the development of the Cost-AEP Threshold Criterion (CAEPTC) to determine the threshold where an increase in AEP becomes negligible compared to CapEx increase. CAEPTC is perhaps a better alternative to LCoE for enhanced WEC/site-configuration pairing. Based on this Criterion and the limited dataset (a challenge to most wave companies), it was apparent that the best WEC/site-configuration pairs came from configurations with the highest yearly energy production, which is not necessarily the most expensive, and for sites with lower wave energy.

Sea Wave Energy. A Review of the Current Technologies and Perspectives

The proposal of new technologies capable of producing electrical energy from renewable sources has driven research into seas and oceans. Research finds this field very promising in the future of renewable energies, especially in areas where there are specific climatic and morphological characteristics to exploit large amounts of energy from the sea. In general, this kind of energy is referred to as six energy resources: waves, tidal range, tidal current, ocean current, ocean thermal energy conversion, and saline gradient. This review has the aim to list several wave-energy converter power plants and to analyze their years of operation. In this way, a focus is created to understand how many wave-energy converter plants work on average and whether it is indeed an established technology.

Optimization of a three-dimensional hybrid system combining a floating breakwater and a wave energy converter array

Combining wave energy converters (WECs) with a floating breakwater provides a potential approach to help develop commercial-scale wave power operations. This paper aims to design and optimize a three-dimensional floating breakwater integrated with a WEC array by developing a numerical model of a multi-floating-body coupled system based on potential flow theory with viscous correction in the frequency domain. By analyzing the wave surface elevations around the breakwater, the size of the breakwater was optimized and the optimal installation locations of the WECs were determined. Under the condition of coupled constraints and six degrees-of-freedom motion, the interactions between the WEC array and the breakwater were analyzed. Subsequently, the number of WECs and the distance between the WECs and the breakwater were optimized to maximize the wave energy conversion performance of the hybrid system. Results show that the wave focusing areas appear more frequently near the breakwater. These focusing areas significantly improve WEC power, and thus better wave energy conversion performance can be achieved when the WECs are placed close to the breakwater. The vertical forces on the breakwater significantly increase due to the presence of the WECs, however the horizontal forces are decreased. The findings of this paper provide guidance to design and optimize a hybrid WEC-breakwater system in practical engineering applications.

Control law design for the air-turbine-generator set of a fully submerged 1.5 MW mWave prototype. Part 1: Numerical modelling

A fully-submerged bottom-standing 1.5 MW wave energy converter (WEC) equipped with a membrane-pump system is under development for installation at a test site off the coast of Pembrokeshire, Wales, UK. The system comprises several flexible-membrane air cells that are compressed and sucked by the action of the waves, rectifying valves, low-pressure and high-pressure ducts, and a unidirectional air turbine that drives an electrical generator. This two-part paper reports the design and testing of an effective control law to be implemented into the programmable logic controller of the membrane-pump turbine-generator set, allowing efficient and safe operation for the wave climate at the Pembrokeshire test site. Part 1 of this paper concerns developing the design strategy of the control laws, the numerical model, and the performance evaluation of the WEC power take-off system. A genetic algorithm is used to design control laws to maximise each sea state's WEC time-average efficiency. An adaptive control algorithm is devised by cross-correlating the control laws parameters and the rotational speed. The final result is a control law that uses input the rotational speed signal and airflow density. It adapts to the sea state conditions while maximising the time-average total efficiency of the membrane-pump WEC for deployment off the Pembrokeshire coast.

Local Measurement-Based Protection Coordination System for a Standalone DC Microgrid

A reliable protection coordination system between the power electronic converters and solid-state device-based fast protection unit in a standalone dc microgrid supplied by a wave energy converter (WEC) and an energy storage unit is presented. A system model with solid-state circuit breakers is developed in Piecewise Linear Electrical Circuit Simulation (PLECS) to analyze the steady-state operation as well as faulted conditions. The effect of both resistive and constant power loads on a current-limited bus controlling converter behavior during an overload event is analyzed theoretically. Also, the variation in the WEC power output and its effect on bus voltage during overload are investigated. The analysis has led to a novel dual current-voltage feedback-based protection coordination system. The method does not require any communication between the solid-state circuit breakers and converters and eliminates the need for adaptive updating of the overcurrent threshold with varying renewable generation output. Bus capacitor discharge during a short-circuit is analyzed theoretically as well to formulate an instantaneous trip threshold selection utilizing device gate driver functionality. The proposed protection coordination system ensures rapid fault isolation and continued power delivery to the healthy segments. An SiC-based bidirectional, solid-state dc circuit breaker prototype is designed and fabricated, which has been used to implement the proposed protection method with a 380-V dc bus-based system.

Evaluation of long-term power capture performance of a bistable point absorber wave energy converter in South China Sea

Nowadays, extensive research has been conducted to generate electricity from ocean waves, but there are not too many studies on the capture of wave energy in a specific ocean area. As the largest sea area in China's offshore waters, the South China Sea contains a large amount of wave energy and is regarded as a natural wave energy power plant. A comparative study was conducted to investigate the effects of wave energy converter (WEC) parameters on the long-term power capture performance of a bistable point absorber WEC installed in the South China Sea in this paper. Combined with the joint probability distribution of significant wave height Hs and peak wave period Tp in South China Sea, a mathematical model was developed for the evaluation of long-term power performance of WECs. The governing equations of the point absorber WEC were established using the hybrid frequency-time domain method. The dynamic response and captured power can be obtained after numerically solving the equations of motion with the 4th order Runge-Kutta Method. The results show that the bistable mechanism has great advantages in the long-term captured power of WEC. In general, a circular truncated cone-shaped buoy WEC with a larger buoy size and smaller nonlinear parameter tend to result in better long-term power capture performance. The insights given in this study provide a motivation and foundation for developing a complete set of performance optimization methods of point absorber WECs installed in South China Sea.

Investigations into Balancing Peak-to-Average Power Ratio and Mean Power Extraction for a Two-Body Point-Absorber Wave Energy Converter

The power harnessed by wave energy converters (WECs) in oceans is highly variable and, therefore, has a high peak-to-average power (PTAP) ratio. To minimize the cost of a WEC power take off (PTO) system, it is desirable to reduce the PTAP ratio while maximizing the mean power extracted by WECs. The important issue of how PTAP ratio reduction measures (such as adding an inertia element) can affect the mean power extracted in a reference model has not been thoroughly addressed in the literature. To investigate this correlation, this study focuses on the integration of the U.S. Department of Energy’s Reference Model 3, a two-body point absorber, with a slider-crank WEC for linear-to-rotational conversion. In the first phase of this study, a full-scale numerical model was developed that predicts how PTO system parameters, along with an advanced control algorithm, can potentially affect the proposed WEC’s PTAP ratio as well as the mean power extracted. In the second phase, an appropriate scaled-down model was developed, and extracted power results were successfully validated against the full-scale model. Finally, numerical and hardware-in-the-loop (HIL) simulations based on the scaled-down model were designed and conducted to optimize or make trade-offs between the operational performance and PTAP ratio. The initial results with numerical and HIL simulations reveal that gear ratio, crank radius, and generator parameters substantially impact the PTAP ratio and mean power extracted.

Numerical investigation of adaptive damping control for raft-type wave energy converters

This paper aims to study the hydrodynamic characteristics and power generation capacity of raft-type WEC (Wave Energy Converter), and to investigate the feasibility of adaptive damping control for this kind of WEC. The numerical model of Pelamis WEC is built in software ANSYS-AQWA. By calling external force and power routines at each time step to calculate the torque and power generated by the generation system and back to the software, the time domain numerical simulation of Pelamis WEC's hydrodynamic response and power generation capacity is realized. Then the numerical simulation of adaptive damping is realized by defining the damping as a function of wave parameters, where the wave parameters are required to be forecasted and is computed in MathWorks Simulink based on the auto regression and Fourier transform methods. Then the hydrodynamic response and power generation performance of the Pelamis WEC under different stiffness and damping (constant) and adaptive damping conditions are calculated. Results show that the power generation capacity of the Pelamis WEC can be promoted by more than 15.9% with the adaptive damping control strategy based on the wave parameters. However, it is still limited to some factors, especially the wave forecasting method.

A Comparative Study of Metaheuristic Algorithms for Wave Energy Converter Power Take-Off Optimisation: A Case Study for Eastern Australia

One of the most encouraging sorts of renewable energy is ocean wave energy. In spite of a large number of investigations in this field during the last decade, wave energy technologies are recognised as neither mature nor broadly commercialised compared to other renewable energy technologies. In this paper, we develop and optimise Power Take-off (PTO) configurations of a well-known wave energy converter (WEC) called a point absorber. This WEC is a fully submerged buoy with three tethers, which was proposed and developed by Carnegie Clean Energy Company in Australia. Optimising the WEC’s PTO parameters is a challenging engineering problem due to the high dimensionality and complexity of the search space. This research compares the performance of five state-of-the-art metaheuristics (including Covariance Matrix Adaptation Evolution Strategy, Gray Wolf optimiser, Harris Hawks optimisation, and Grasshopper Optimisation Algorithm) based on the real wave scenario in Sydney sea state. The experimental achievements show that the Multiverse optimisation (MVO) algorithm performs better than the other metaheuristics applied in this work.

Influence of Power Take-Off Modelling on the Far-Field Effects of Wave Energy Converter Farms

The study of the potential impact of wave energy converter (WEC) farms on the surrounding wave field at long distances from the WEC farm location (also know as “far field” effects) has been a topic of great interest in the past decade. Typically, “far-field” effects have been studied using phase average or phase resolving numerical models using a parametrization of the WEC power absorption using wave transmission coefficients. Most recent studies have focused on using coupled models between a wave-structure interaction solver and a wave-propagation model, which offer a more complex and accurate representation of the WEC hydrodynamics and PTO behaviour. The difference in the results between the two aforementioned approaches has not been studied yet, nor how different ways of modelling the PTO system can affect wave propagation in the lee of the WEC farm. The Coastal Engineering Research Group of Ghent University has developed both a parameterized model using the sponge layer technique in the mild slope wave propagation model MILDwave and a coupled model MILDwave-NEMOH (NEMOH is a boundary element method-based wave-structure interaction solver), for studying the “far-field” effects of WEC farms. The objective of the present study is to perform a comparison between both numerical approaches in terms of performance for obtaining the “far-field” effects of two WEC farms. Results are given for a series of regular wave conditions, demonstrating a better accuracy of the MILDwave-NEMOH coupled model in obtaining the wave disturbance coefficient (Kd) values around the considered WEC farms. Subsequently, the analysis is extended to study the influence of the PTO system modelling technique on the “far-field” effects by considering: (i) a linear optimal, (ii) a linear sub-optimal and (iii) a non-linear hydraulic PTO system. It is shown that modelling a linear optimal PTO system can lead to an unrealistic overestimation of the WEC motions than can heavily affect the wave height at a large distance in the lee of the WEC farm. On the contrary, modelling of a sub-optimal PTO system and of a hydraulic PTO system leads to a similar, yet reduced impact on the “far-field” effects on wave height. The comparison of the PTO systems’ modelling technique shows that when using coupled models, it is necessary to carefully model the WEC hydrodynamics and PTO behaviour as they can introduce substantial inaccuracies into the WECs’ motions and the WEC farm “far-field” effects.

A New Bi-Level Optimisation Framework for Optimising a Multi-Mode Wave Energy Converter Design: A Case Study for the Marettimo Island, Mediterranean Sea

To advance commercialisation of ocean wave energy and for the technology to become competitive with other sources of renewable energy, the cost of wave energy harvesting should be significantly reduced. The Mediterranean Sea is a region with a relatively low wave energy potential, but due to the absence of extreme waves, can be considered at the initial stage of the prototype development as a proof of concept. In this study, we focus on the optimisation of a multi-mode wave energy converter inspired by the CETO system to be tested in the west of Sicily, Italy. We develop a computationally efficient spectral-domain model that fully captures the nonlinear dynamics of a wave energy converter (WEC). We consider two different objective functions for the purpose of optimising a WEC: (1) maximise the annual average power output (with no concern for WEC cost), and (2) minimise the levelised cost of energy (LCoE). We develop a new bi-level optimisation framework to simultaneously optimise the WEC geometry, tether angles and power take-off (PTO) parameters. In the upper-level of this bi-level process, all WEC parameters are optimised using a state-of-the-art self-adaptive differential evolution method as a global optimisation technique. At the lower-level, we apply a local downhill search method to optimise the geometry and tether angles settings in two independent steps. We evaluate and compare the performance of the new bi-level optimisation framework with seven well-known evolutionary and swarm optimisation methods using the same computational budget. The simulation results demonstrate that the bi-level method converges faster than other methods to a better configuration in terms of both absorbed power and the levelised cost of energy. The optimisation results confirm that if we focus on minimising the produced energy cost at the given location, the best-found WEC dimension is that of a small WEC with a radius of 5 m and height of 2 m.

Collocating offshore wind and wave generators to reduce power output variability: A Multi-site analysis

Offshore wind and wave power are renewable energy sources that share the marine environment. Collocating these systems can reduce power variability; however, the extent of this differs between sites. Regional differences in combined system variability are investigated. Potential power output of wind and wave systems is simulated for sites throughout the US East and West Coasts and UK North Sea using field observations of met-ocean conditions. The power production potential is approximated using an idealized power curve of the Siemens Gamesa SWT-6.0-154 6 MW wind turbine and the Pelamis 750 kW wave energy converter power matrix. The regional comparative approach provides novel insight into collocated system power variability on the US East Coast, and how it contrasts with the previously studied US West Coast and UK North Sea. Hourly and diurnal variability are found to decrease on the US West Coast and UK North Sea, with minimal effect on the US East Coast. The benefits for the West Coast extend to seasonal time scales (but as expected not to annual scales), but benefits for all regions depend on the specific wind-wave capacity mix. Reduced variability can benefit collocated wind and wave power, but these effects differ regionally and must be assessed locally.

Evaluating the Future Efficiency of Wave Energy Converters along the NW Coast of the Iberian Peninsula

The efficiency of wave energy converters (WECs) is generally evaluated in terms of historical wave conditions that do not necessarily represent the conditions that those devices will encounter when put into operation. The main objective of the study is to assess the historical and near future efficiency and energy cost of two WECs (Aqua Buoy and Pelamis). A SWAN model was used to downscale the wave parameters along the NW coast of the Iberian Peninsula both for a historical period (1979–2005) and the near future (2026–2045) under the RCP 8.5 greenhouse scenario. The past and future efficiency of both WECs were computed in terms of two parameters that capture the relationship between sea states and the WEC power matrices: the load factor and the capture width. The wave power resource and the electric power capacity of both the WECs will decrease in the near future. The load factor for Aqua Buoy will decrease in the entire area, while it will remain unchanged for Pelamis in most of the area, except north of 43.5° N. The capture width and cost of energy will increase for both devices. The methodology here applied can be easily applied to any device and coastal domain under different climate change scenarios.

Applicability analysis of the wave energy converter in low wave energy density sea areas

The capture power of wave energy converter is affected by the interaction between the buoy and wave. This study aims to investigate the interaction between the buoy and wave motion in low wave energy density seas using the hydrodynamic calculation software AQWA and to achieve a reasonable optimal configuration for the greater capture power based on the orthogonal experiment. Results show that the wavelength in the resonance state increases with the increasing of buoy mass and increases first and then decreases with the increasing of buoy diameter. Taking capture power as the optimization goal, applicability analysis denotes that the wave energy converter with the mass of 4.2 t and the buoy diameter of 4 m is suitable for the Bohai Sea and the north of the Yellow Sea. The maximum capture power of 150 W is obtained in the Bohai Strait owing to the wavelength of 14 m. This study provides a new idea for the mass control, diameter selection and arrangement of the wave energy converter.

A simplified model for expedient computational assessment of the novel REEFS wave energy converter power output

REEFS is a new multipurpose wave energy converter. It produces electric energy and contributes to shore protection. It is a large nearshore submerged device that can originate high waves breaking like the natural reefs do. The device makes use of the wave’s pressure and velocity spatial differentials, to produce electric energy. A laboratorial concept proof based in a small scale physical model (1.5:100) was already successfully performed. However, to further develop the device, it is necessary to analyze the impact of its geometry and deployment depth in power output. For such purposes, computational models have proved to be more affordable than experimental approaches. However, the novelty of the concept inhibits the use of already existing computational models. In this article, a mathematical model specially developed to assess REEFS power output is presented. It is a simplified model to enable expedient use required by the exploratory nature of the initial development stages. This mathematical model is solved numerically by a dedicated computational program. The results were compared with a laboratorial concept proof. After adequate calibration, the numerical model exhibited a good agreement with the laboratorial results, indicating it can be used for expedient assessment of the REEFS power output.

Ocean wave energy potential along the west coast of the Sumatra island, Indonesia

The past few decades have seen considerable interest in exploration and research of ocean wave energy as a potential energy substitute for fossil-based fuel. In this study, a Wavewatch III spectrum wave model was driven to simulate significant wave height spanning for a period of 25 years, from 1991 to 2015 on the west coast of the island of Sumatra. The 25-year-average of wave energy shows some noticeable hot spots in certain areas that have a value of significant wave height up to 2.33 m and a wave energy 67.29 kW/m. These hotspot occurrences have a similar pattern as statistics collected for the seasonal characteristics that are associated with tropical monsoons with the average value of wave energy reaching its peak in an easterly monsoon season up to 98.21 kW/m, and the lowest average value occurring in the westerly monsoon season, lasting from December to February, with a prevalent value of 10 kW/m. Additional statistical parameters of possible wave energy site selections were considered, such as Coefficient of Variance, Monthly Variability Index, Optimum Hotspot Identifier, Wave Development Index, and accessibility to find the ideal location for wave energy converter deployment. These statistics give insight into potential prospective points for ocean-wave energy harvesting. Eight hotspots were finally selected based on the afore-mentioned statistical considerations and were further analyzed through wave energy characterization and obtained energy calculation through Pelamis, Archimedes Wave Swing, and Wave Dragon Wave Energy Converter power matrices. Finally, inter-annual variability and particular extreme events are discussed.

WEC fault modelling and condition monitoring: A graph‐theoretic approach

The nature of wave resources usually requires wave energy converter (WEC) components to handle peak loads (i.e., torques, forces, and powers) that are many times greater than their average loads, accelerating equipment degradation. Moreover, due to their isolated nature and harsh operating environment, WEC systems are projected to possess high operations and maintenance (O&M) cost, i.e., around 27% of their leveled cost of energy. As such, developing techniques to mitigate these costs through the application of condition monitoring and fault tolerant control will significantly impact the economic feasibility of grid connected WEC power. Toward this goal, models of faulty components are developed in the open source modeling platform, WEC-Sim, to estimate the performance and measurable states of a WEC operating with likely device and sensor failures. Two types of faulty component models are then applied to a point absorber WEC model with basic controller damping and spring forces. Resulting changes in device behavior are recorded as a benchmark, and a graph-theoretic approach is proposed for fault detection and identification utilizing multivariate time series. Simulation results demonstrate that these faults can greatly affect the WEC performance, and that the proposed method can effectively detect and classify different types of faults.

Predicting absorbed power of a wave energy converter in a nonlinear mixed sea

In this paper the power performances of a point absorber wave energy converter (WEC) operating in a nonlinear multi-directional mixed sea have been rigorously investigated. The absorbed power of the WEC Power Take Off (PTO) system has been calculated by incorporating a second order irregular wave model into a nonlinear dynamic filter. This is a new methodology that is uniquely proposed to the ocean wave energy research community. The predicted results of the WEC power performances have been rigorously analyzed and systematically compared, and the advantages of using our proposed new methodology under extreme wave states have been convincingly substantiated. The research findings in this paper highlight that it is important to employ a nonlinear wave theory under extreme wave states for studying the power performances of wave energy converters, but that linear wave theory gives good results under normal wave states.

On establishing generalized analytical phase control conditions in two body self-reacting point absorber wave energy converters

It is widely suggested that step gains in wave energy converter power capture performance must be realized to achieve economic viability. One method of fulfilling power capture gains is to invoke resonant conditions between the device and the incoming ocean wave. However, a general method that can establish the prerequisites for achieving resonant conditions in an arbitrarily complex wave energy converter architecture is nonexistent.In this work, we present an analytical procedure, built on the mechanical circuit framework, for identifying the resonant conditions of an arbitrarily complex wave energy converter architecture. To demonstrate the procedure, we select three complex two body point absorber devices as a case study, each with a geometry controllable feature set. Through invoking resonant conditions in each architecture, we illustrate how the choice of topology has significant influence on the power capture characteristics of the WEC device. Selecting the highest performing architecture, we then reveal how the analytical equations can be applied to promote technology innovation by supplying design criterion prior to locking down the WEC design. Finally, we apply the analytics within a numerical case study and present a hierarchy describing the incremental performance improvements realized through implementing steps in control complexity for this device.