Wave Energy Converter Technologies Research Articles

Multi-freedom effects on a raft-type wave energy convertor- hydrodynamic response and energy absorption

This research delves into the effects of multiple degrees of freedom on a raft-type wave energy converter (WEC), using both experimental and computational methods. The WEC features two interconnected floating bodies linked by a specialized parallelogram joint, allowing for distinct heave, pitch, and surge motions. The power take-off (PTO) system, integrating Coulomb damping, is analysed for its impact on energy absorption. Despite varying damping amplitudes, peak energy absorption is consistent across different wave periods, as shown in scaled model tests. Computational simulations further investigate the multi-freedom effects by imposing constraints on various degrees of freedom, thus elucidating their individual and combined influences on energy absorption. Preliminary results indicate that the addition of degrees of freedom can enhance the WEC's efficiency, especially through coordinated release of the bodies' relative pitch and surge motions. The study also finds that the energy absorption efficiency from pitch motion depends on the release of surge motion. Moreover, the energy distribution among each degree of freedom is significantly affected by the device's multi-freedom characteristics. This research provides valuable insights and practical guidelines for the development of advanced multi-freedom WEC technologies.

Data-based modelling of arrays of wave energy systems: Experimental tests, models, and validation

One of the key steps towards economic feasibility of wave energy conversion technology concerns scaling up to farms of multiple devices, in the attempt to reduce installation costs by sharing infrastructure, and a consequent drop in levelised cost of energy. Moreover, whenever wave energy systems are deployed in proximity (in so-called arrays), the exploitation of the hydrodynamic interactions between single devices is fully enabled, potentially increasing the final energy outcome. To achieve this in real (operational) time, the employed energy-maximising control strategies require control-oriented array models, able to efficiently describe the dynamics of these interconnected systems in a representative fashion. This can be, nonetheless, a difficult task when considering first principles alone, under small motion assumptions, for modelling purposes. Recognising the uncertainty associated to array numerical models obtained from the linearisation of simplified system equations around their equilibria, this paper presents models of several array configurations identified following a frequency domain approach on the basis of experimental data. Tailored tests on laboratory-scale devices have been designed and conducted in the Aalborg University (Denmark) wave tank facility, with the purpose of performing representative system identification of the wave energy systems arrays. The obtained models are validated on different representative sea states configurations, in controlled and uncontrolled motion operational conditions. The validation results are fully discussed and analysed in terms of standard error measures and time lag, while the obtained models are made freely accessible via a linked repository (named OCEAN), in the attempt to openly provide validated models for different array configurations.

The Energy Conversion and Coupling Technologies of Hybrid Wind–Wave Power Generation Systems: A Technological Review

Based on the mutual compensation of offshore wind energy and wave energy, a hybrid wind–wave power generation system can provide a highly cost-effective solution to the increasing demands for offshore power. To provide comprehensive guidance for future research, this study reviews the energy conversion and coupling technologies of existing hybrid Wind–wave power generation systems which have not been reported in previous publications. The working principles of various wind and wave energy conversion technologies are summarised in detail. In addition, existing energy coupling technologies are specifically classified and described. All aforementioned technologies are comprehensively compared and discussed. Technological gaps are highlighted, and future development forecasts are proposed. It is found that the integration of hydraulic wind turbines and oscillating wave energy converters is the most promising choice for hybrid wind–wave power extraction. DC and hydraulic coupling are expected to become mainstream energy coupling schemes in the future. Currently, the main technological gaps include short their operating life, low energy production, limited economic viability, and the scarcity of theoretical research and experimental tests. The field offers significant opportunities for expansion and innovation.

Improving the Amount of Captured Energy of a Point-Absorber WEC on the Mexican Coast

Although there are constant improvements in wave energy converter (WEC) technology, it is crucial to investigate site-specific sea conditions for optimal power absorption and efficiency. This study compares the efficiency of a floating buoy-type WEC device, with three differently shaped floats: a semi-sphere, a cylinder considered suitable for a location near Ensenada, on the Baja California peninsula, and a novel, rounded, semi-rectangular float. A statistical analysis of the wave climate of the last 42 years was performed to define the conditions to which the device is subjected. The WEC location was chosen for shallow waters, using a computational model that solves the modified mild slope equation. The hydrodynamic response of the three float designs was then analyzed in the frequency and time domains, using the software ANSYS AQWA 19.2, to assess the dynamics of the floating body, the forces exerted, and the power absorbed, as well as the suitability of the proposed power take-off (PTO) system. The findings show that the proposed float design absorbs the most energy, with an annual power of 135.11 MW, and that the PTO mechanism is appropriate.

Standard log-capture differentials as performance metrics for deepwater wave power generation

As a support to performance assessment of deepwater wave energy converters, this work quantifies susceptibility of output power to short duration variations, independent of source wave statistics. Introduced as ς-metrics, the quantified measures may be a basis for technology comparisons, with possible convergence to a choice for implementation. The metrics further allow comparison between different versions of a specific converter technology, thereby helping in design revision and refinement.With power conversion represented in terms of a technology specific capture width function, the log-capture differential is defined relative to a hypothetical benchmark characterised by negligible power variability. Root-mean-square aggregation of the differential further leads to standard log-capture differentials as site independent ς-metrics for a capture-and-conversion mechanism.Study results are included in three phases. First, three established wave energy conversion technologies are analysed at respective power maximums, so as to illustrate applicability of log-capture differential and ς-metrics. Next, the metrics are analysed at fractional output power to realise converter performance at reduced variability. Finally for a given marine environment, ς-metrics are shown to be useful in projection of output power variability across technologies, together with the mean output power as fraction of rated maximum.

Repulsive magnetic levitation-based electromagnetic energy harvesting of a low-frequency ocean wave

In this study, an electromagnetic energy harvester for a low-frequency ocean wave was developed in a compact 3D-printed structure. Ocean wave energy conversion technologies exist, but maintaining them in the harsh marine environment is crucial for business. Friction increases maintenance costs. Therefore, magnetic levitation, being friction-free, is used for cost-effective, low-maintenance electromagnetic energy harvesting applications. Low-frequency oscillating energy is captured using repulsive magnetic levitation with a buoy and generating electricity using a permanent magnet and copper coil. A levitating magnet is repelled by a fixed one, inducing electricity as it passes through a coil. Experiments with a 0.1 Hz sine wave mimic the average frequency of ocean waves, showing successful voltage peaks at intervals. The output voltage and measured power from the harvester exhibit variations influenced by multiple parameters. The maximum output voltage observed was 3.4 V and an average of 99 mW of power was calculated. The experiment demonstrates the feasibility of using repulsive magnetic levitation for low-frequency wave energy harvesting and also encompasses various harvester configurations, including transfer magnet forces and top magnets.

Hardware in the Loop Wave Energy Converter Control Under Control Faults and Model Mismatch

Ocean wave energy conversion is operated in a challenging environment featuring massive and impulsive stresses and is subjected to large amounts of biofouling and marine growth. As most wave energy converter technologies are off-shore, operation, maintenance, and repair costs may be significant. Therefore, any cost-effective control and operation of Wave Energy Converters (WECs) must be robust to faults and changes in the operational environment. This paper presents a Bayesian policy gradient-based WEC control approach that is responsive and adaptive to faults in controller feedback, controller actuation, and changes in the plant model. The controller is tested via Hardware in the Loop real-time testbed. The results show that the proposed control is capable of adapting its policy to the loss of information from one or two sensors and recovering nearly full power operation. In the case where sensor loss occurs, after the loss of plate excitation force estimation and displacement sensors, the power decreases to 215.2 kW from 252.7 kW before recovering to 238.4 kW mean power over 20 minutes. Moreover, the Bayesian RL-based control model proves its adaptability by compensating a large portion of power loss due to PTO failures and rapid changes to the dynamic system because of non-destructive damage to the float structure.

Evaluating Wave Potential and Assessing the Economic Viability of Wave Energy Converters in the South Java Seas

As an archipelagic nation, Indonesia holds substantial potential for wave energy as a renewable resource. Certain coastlines of islands facing the Indian Ocean, particularly in the western and southern regions, exhibit significant wave energy throughout the year. To identify suitable locations for Wave Energy Converter (WEC) installation, it is essential to assess wave hindcast data. This study utilizes NOAA and ERA5 reanalysis wave data to analyze wave characteristics in Indonesia from 2008 to 2018. Data processing with Ocean Data View is employed to estimate key wave parameters at various locations, including significant wave height, mean wave period, and mean wave direction. Two locations in the South Java seas were identified for WEC installation based on this research. The average values for the period 2008 to 2018 indicate a significant wave height of around 2m, with a maximum height of 5m, a wave period of 10–14s, and a wave direction of 195–210 degrees. Notably, NOAA data suggests a higher estimation of significant wave height compared to ERA5 data. The average annual wave power potential based on ERA5 and NOAA is 164.43 MW/m and 252.15 MW/m, respectively. Furthermore, this study incorporates an economic simulation for the construction of a multi-point absorber WEC. The objective is to offer insights into the Levelized Cost of Electricity (LCOE) and compare it with other WEC technologies. Assuming a WEC capacity of 130 kW, the total construction cost is estimated at $2,093,725, resulting in a Levelized Cost of Energy (LCOE) of $91/MWh.

Development and validation of the in-house hydrodynamics code and the DNV software for TALOS wave energy converter

The Lancaster in-house code is a time-domain analysis code, developed especially for the TALOS wave energy converter, a novel wave energy conversion technology. The TALOS wave energy converter is a point absorber-like wave energy converter, but with a unique power take-off (PTO) system. The whole PTO system is fully enclosed within the structure, hence no moving parts are exposed to the harsh marine environments.
 The PTO of the TALOS WEC consists of a mass ball within the structure, with a number of springs and dampers (for instance, direct drives or hydraulic systems) connecting between the mass ball and the structure. Such an arrangement of the PTOs would make the PTO essentially non-linear, regardless whether the actual PTO dampers are linear or nonlinear as well as the linear springs. Therefore, a time domain analysis must be established for the TALOS WEC. Towards the goal, an in-house time-domain model has been developed at Lancaster University, and now it is to be validated via the delicated numerical models built using well-established commercial software (and in the future via experimental data). Within this framework, the present paper presents an effort for validation using the DNV SESAM commercial hydrodynamic and structural analysis software. 
 To build a numerical model for TALOS WEC, the Lancaster in-house time domain code is based on the hybrid frequency-time domain approach. That is, the basic hydrodynamic parameters are analysed using the panel methods (WAMIT, HAMS, NEMOH), and then the relevant parameters are transformed for the Cummins’ time-domain equation, such as the added mass at infinite frequency, and the memory effect (the convolution terms). This work is an extension of the comparison between the in-house code and the DNV SESAM software, and the comparisons would include the relevant hydrodynamic analyses, the time-domain analyses with and without the TALOS PTO system, with the aims towards developing the validated tools of both the in-house code and the commercial software.

On data-based control-oriented modelling applications in wave energy systems

The development of effective energy-maximising control strategies has a crucial role in the empowerment of wave energy technology, and in its improvement towards economic viability. Within the state-of-the-art, most of the strategies adopted to maximise the absorbed energy exploit a model of the wave energy converter (WEC) to be controlled, i.e. they are model-based. These models attempt to replicate the WEC dynamics with a sufficient degree of fidelity, trying, at the same time, to minimise their associated computational burden. However, due to the presence of the hydrodynamic effects , which inherently characterise wave energy systems, simultaneously achieving high-fidelity and computational efficiency is not trivial. Oversimplification of the problem through, for example, linearity assumptions, could lead to non-representative models and/or large uncertainty levels. To overcome these issues, in the last decade, several approaches based on data have been proposed in the wave energy field. These approaches, falling under the umbrella of system identification techniques, exploit data coming from experimental tests or high fidelity simulations, and build control -oriented models with a pre-defined level of complexity. In this paper, we analyse the different strategies that have been adopted in the literature to build data-based control-oriented models for WECs, highlighting the characteristics of each approach, together with their opportunities and inherent drawbacks. An analysis of eventual “partial” data-based modelling of WEC subsystems (e.g. moorings, PTO, or hydrodynamics only) is also reported. Moreover, considerations on the choice of inputs and outputs depending on the WEC type are reported, in an attempt to highlight the different issues that characterise the system identification problem depending on the WEC technology. Finally, conclusions are drawn regarding the capabilities that this type of approach has in (at least partially) solving the modelling issues that affect WEC control system design, and the pitfalls that pure adoption of these strategies has when applied on larger scales, or in the operational stage.

Optimisation of Air turbines for OWC Wave Energy Converters: Sensitivity of Realistic Wave Climates

Wave Energy, Oscillating Water Column, Air turbines, Optimisation, Genetic Algorithms, Wave climates.
 Among all the Wave Energy Converter (WEC) technologies suggested in the last decades, the Oscillating Water Column (OWC) technology seems to be the most robust and reliable technology. Different are currently in operation, such as the Mutriku Wave Power Plant installed in a harbour, or are being developed, such as the MARMOK floating OWC device developed by IDOM and tested for over a year in the Biscay Marine Energy Platform (BIMEP). One of the key elements of the OWC technologies is the power take-off (PTO) system that converts the pneumatic energy trapped in the chamber into electrical energy. Such PTO system consists of an air turbine coupled to an electric generator, and has been the object of several studies, including numerical and experimental works that cover a wide range of different air turbine configurations, and some of the proposed research lines even reaching to combine both approaches. The most common turbine, mainly due to its relative simplicity both on the conceptual andmechanical aspects, is the Wells monoplane turbine, including variations such as the biplane and the counter-rotating configurations. However, other configurations such as the impulse turbine or the more recent bi-radial turbine have also been analysed.
 The preliminary design of these turbines usually relies on analytical models based on the blade element method, using dimensionless parameters for representing the behavioural charts of the different configurations. In fact, in order to better represent the behaviour of air turbines in realistic conditions with polychromatic waves, it is usual to consider the stochastic version of these dimensionless parameters so that they provide an overall indicator of their sea-state-related behaviour. However, the air turbines, regardless of their configuration, include a large number of different geometrical parameters, which complicates the optimisation procedure and leads to a decision-making process that relies on an expertise-based intuition. In this sense, suggests an optimisation method based on a Genetic Algorithm (GA) that enables the articulation of all the relevant parameters. This GA-based optimisation method articulates the information about the hydrodynamic behaviour of the WEC and the pneumatic conversion within the chamber. Hence, the optimisation is sensitive to the characteristics of the wave climate and, thus, the behaviour of the WEC in that specific wave climate.
 However, in order to make wave energy economically viable, mass production of the WECs, including their PTO systems, is a crucial point. As a consequence, standard WEC floaters and PTO system elements may need to be used in the different locations under different resource conditions. In order to evaluate the sensitivity of the optimal air turbine designs to the characteristics of specific wave climates, the present study will define optimal air turbines for different locations worldwide, comparing the characteristics of the different designs.

Spectral control co-design of wave energy converter array layout

Wave energy systems are designed to maximise energy absorption from the oscillatory motion of waves, which can make a crucial contribution in a new carbon free generation matrix [1]. Thus, the global ocean energy market (including wave and tidal) is expected to grow by more than 700\% by 2028 [2]. However, the harsh conditions faced in the ocean, the strong variability of the resource, and the high force/velocity ratios pose significant challenges to the development of the different ocean energy technologies. Therefore, to date, wave energy converter (WEC) technology is still under development and still needs a significant progress to enhance the energy conversion efficiency and, consequently, to become commercial viable. Crucially, to meet these existing challenges, there are a number of key considerations to progress the effectiveness and efficiency of WEC technologies. In particular, energy maximising control systems, to maximise energy absorption, and the optimisation of WEC array schemes, to take advantage of constructive interaction effects, are currently considered as some of the key drivers for the development of efficient WECs [1]. Recent studies have shown that total design of WECs, analysing the impact of each individual component on the rest of the system, is a key methodology to achieve effective designs. For example, the levelised cost of energy (LCoE) is considered in the objective function in [3], analysing the interplay between the control system based on a spectral controller and the specifications of the power take-off (PTO) system defined as the maximum stroke and force. This methodology has been generally labelled as `co-design', or, when the development is carried out in a control-aware manner, `control co-design'. Similarly, in [4], the interaction between control and optimal WEC geometry is studied. In addition, as discussed before, another key driver in achieving effective wave energy system is the operation of WEC arrays. In particular, in [5] and [6] different array layouts are analysed in terms of computational efficiency and control, respectively. Considering the results in [5], where methods based on harmonic balance techniques are used to analyse the computational demand related to different array layouts, an assessment of the impact of different array layouts on the LCoE, is performed in this study. Figure 1 illustrates different array layouts (a), and the impact of different separation and number of WECs on the resulting LCoE (b). (Figure in PDF abstract)For the implementation of this study, a spectral control methodology is considered to virtually achieve optimal control solutions, even in constrained scenarios. In tandem, different layout templates, composed of multiple point absorber WECs, are considered. To achieve a clear global indicator based on the LCoE, each layout is analysed, in terms of the separation distance and the number of WECs, as well as capital and operation expenditure (CapEx and OpEx, respectively). Following the methodological guidelines considered in [3], a general co-design scheme is designed, essentially based on an exhaustive search method, indicating the interplay between different array layouts and LCoE.

Effects of projected wave climate changes on the sizing and performance of OWCs: a focus on the Mediterranean and Atlantic European coastal waters

Reliable estimations of the Annual Energy Production (AEP) achievable with a certain Wave Energy Converter (WEC) are among the fundamental elements for a sound evaluation of the related Levelized Cost of Energy (LCOE) that plays a crucial role in the investment decision-making process. The lack of reliability in estimates of the device productivity can be a result of the uncertainty in the assessment of the available wave energy resource since the geometry of the device which maximizes the AEP is dependent on the specific wave climate at the foreseen installation site.
 The Climate Data Store of the Copernicus Climate Change Service delivers projections of the wave climate along the 20 m bathymetric contours of the whole European coastlines, covering the period 2040-2100, under two Representative Concentration Pathway (RCP) scenarios (RCP4.5 and RCP8.5) [1].
 This work addresses the effect of such long-term wave climate changes on the optimal sizing and performances of Oscillating Water Column (OWC) WECs to be installed along the Atlantic European coastline, expanding a previously published work in which the Mediterranean coastline was investigated [2]. The capture width of the OWC WEC under different wave conditions is computed using an empirical model [3] capable of predicting the device performance with acceptable accuracy and limited computational time.
 The results show that the optimal geometry of the OWC WEC varies significantly in the different geographical locations, and that the long-term changes in the wave energy resource could cause a slight modification of the optimal geometry in each potential installation site. The AEP of the OWC WEC in the projected wave climate scenario can be significantly different compared to the present one, potentially contributing to reducing the LCOE of this wave energy conversion technology in some geographical locations.
 REFERENCES:
 [1] Caires, S., Yan, K., 2022. Ocean surface wave time series for the European coast from 1976 to 2100 derived from climate projections. (Accessed on Feb. 2022) DOI: 10.24381/cds.572bf382. Copernicus Climate Change Service (C3S) Climate Data Store (CDS)
 [2] Simonetti, I., Cappietti, L., 2023, Mediterranean coastal wave-climate long-term trend in climate change scenarios and effects on the optimal sizing of OWC wave energy converters, Coastal Engineering, 173, 104247.
 [3] Simonetti, I., Cappietti, L., Oumeraci, H., 2018. An empirical model as a supporting tool to optimize the main design parameters of a stationary oscillating water column wave energy converter. Appl. Energy 231, 1205–1215.

Experimental evaluation of phase and velocity control for a cyclorotor wave energy converter

The research presented in the paper is dedicated to the analysis of the 3D experimental testing results of a 1:20 scale prototype LiftWEC cyclorotor wave energy converter (WEC). The scaled prototype was built and tested in the Hydraulic and Offshore Engineering wave Tank (HOET) by Ecole Centrale Nantes (ECN) in 2022. The analysis is conducted using the analytical control-oriented point-vortex model. The presented research covers a range of tests, with particular focus on cases where positive mechanical power generation has been recorded. The analysis of such cases is important, in highlighting the conditions needed for optimum energy conversion, for future development of cyclorotor WEC technology. The study also reviews the results of tests where the rotor rotational speed is varied within each period of monochromatic waves. This is the first experimental test of such a control strategy for cyclorotor WECs.

Wave Energy Power Take-off Validation with a Hydraulicly Actuated Rotary Dynamometer and a Bi-directional High-power DC Supply

There are many organizations working towards the commercialization of wave energy converter technologies and are advancing their designs through the technology readiness levels (TRLs). A critical step before the field deployment of prototype wave energy converters is the validation of the subsystems and components that are contained in the wave energy converter through laboratory testing and performance characterization. In 2021, the National Renewable Energy Laboratory (NREL) developed and demonstrated a system for testing power takeoffs (PTO) with a low-speed, high-torque dynamometer and a grid-tied high-power DC power source and sink before field deployment. The hydraulic dynamometer allows for the simulation of PTO actuation from wave motion and is capable of a wide range of wave periods and heights which are represented as various speeds and torques from the dynamometer. The high-power bidirectional power supply allows for hardware in the loop and controller in the loop testing to be conducted on WEC power electronics. This presentation was made to describe the methods used by NREL research staff to test all components and sub-systems in the PTO of a novel wave energy converter before field deployment.

A robotized 6-DOF dry test rig for wave power

Wave power has the potential to contribute significantly to sustainability by reducing our global dependence on fossil fuels. Due to harsh conditions and high costs associated with offshore testing, lab experiments are favourable for resource-efficient validation and optimization in developing Wave Energy Converter (WEC) technologies. The limited scale and availability of existing wave tanks, and the limited flexibility of existing dry test rigs does however put significant restraints on such experiments. In this paper we introduce an alternative novel robotized dry test rig concept for wave power, evaluate its performance and discuss its potential. A full-scale robotized dry test rig demonstrator is constructed and used for experiments with a WEC prototype device. High motion flexibility and accuracy is thereby validated, also for repeating recorded wave and buoy motions. Compared to other dry test rigs, no special components were used and the motion trajectories were defined in full 6-Degrees-Of-Freedom. Two real-time hydrodynamic motion response methods are also demonstrated in the test rig, enabling emulation of actual offshore operation as well as development of advanced WEC control strategies. With a larger industrial robot manipulator, the introduced test rig concept could achieve realistic scaled force and power experiments with most point absorber WECs.

Experimental study of high performance mercury-based triboelectric nanogenerator for low-frequency wave energy harvesting

Wave energy is acknowledged as one of the most promising renewable energy sources, effectively developing wave energy conversion technologies might contribute to solving the power shortage dilemma. However, the optimum performance frequency range of current wave energy capture devices does not match the low-frequency ocean kinetic energy. Here, a triboelectric nanogenerator based on liquid mercury (M-TENG) is presented for low-cost and high-power harvesting. The start-up performance of the M-TENG unit is analyzed at low operating frequencies, and discuss the effect of kinematic inputs, structural parameters and materials aspects on output performance. Meanwhile, the sloshing characteristics of mercury in a rectangular unit were calculated by the SST k-ω model, deriving the matching law between mercury volume and electrical output. Furthermore, 18-unit integrated M-TENG produce a peak power output of 4.57 mW, the mercury with its unique physical properties generates higher transferred charge density up to 35.84 mC/m3, sufficient to light 270 commercial LEDs at 1.00 Hz. The relatively high volumetric power density provides a realistic self-supply technological solution for renewable energy harvesting and guarantees the widespread use of milliwatt power equipment in the marine industry.

Experimental study on an isolated oscillating water column wave energy converting device in oblique waves

The oscillating water column is the most widely used wave energy conversion technology for long-term operations, which has the potential to play a critical role in massive applications of wave energy. In this study, an isolated OWC model employing a generic cubic air chamber and an impulse turbine model was tested in a wave tank using regular and irregular wave scenarios. The wave distribution characteristics surrounding the isolated OWC model in oblique waves with various incident angles and other testing conditions were reported. As the incident waves were forward, the symmetrical wave-height distributions could be observed with enhancement and reduction zones in front of the skirt wall and in the sheltered area of the model. Furthermore, the wave distributions were asymmetrical and more complicated in oblique waves. In addition, the effects of the incident wave angle, height, period, and rotation speed of the turbine model on the staged and overall energy-harvesting performances were analyzed. The OWC model obtained the peak energy-harvesting performance, which reduced as the incident angle decreased. As the incident angle decreased to 60° and zero, the overall capture width ratio reduced to approximately two-thirds and a quarter of the value for the incident angle of 90°. Based on experimental results, the annual energy production of a prototype isolated OWC plant in a typical sea state in North China was predicted. The annual energy production of the plant in the actual sea condition was approximately two-thirds of that only in the forward wave incidence.

Acoustic noise emission of air turbines for wave energy conversion: Assessment and analysis

The oscillating-water-column (OWC) system is one of the most developed wave energy conversion technologies, and several prototypes have been deployed and tested on the sea. Noise emission is essential to this technology’s long-term environmental impact assessment. The power take-off system of an OWC comprises a self-rectifying air turbine that directly drives an electrical generator. The paper compares the noise emission of a biradial turbine with that of a Wells turbine. The noise emission of the biradial turbine was measured at the Instituto Superior Técnico variable-flow test rig, while the Wells turbine data were collected in the literature. Acoustic measurement procedures followed the guidelines of the ISO 3746 Standard. A definition of specific noise level is used to compare the noise emission from geometrically similar air turbines or other air turbines performing the same duty. The results show that the noise levels of the biradial turbine are significantly lower than those of the Wells turbine at off-design conditions for the same pneumatic power.

Peak shaving control in OWC wave energy converters: From concept to implementation in the Mutriku wave power plant

Control algorithms for wave energy conversion technologies with air turbines require contingency tools to prevent the rated power and maximum rotational speed from being exceeded. Survival problems limit energy conversion when the available power is too large compared to the turbo-generator-rated power. Short-term predictions require information from local wave-measuring buoys, which raises reliability concerns. Wave groups in the ocean result in extremely high wave power peaks that cannot be smoothed by low-inertia power-take-off (PTO) systems, leading to survival problems. The most common strategy to cope with this situation is to set the rated power of the PTO several times more than the annual average produced electrical power. The PTO is shut-down when the available power is excessive. These two engineering decisions have a detrimental impact on the capacity factor and the energy cost. The paper shows a new control algorithm for wave energy oscillating-water-column (OWC) devices that can control the shutter position of a fast-acting valve to dissipate the pneumatic energy excess. Hence, the PTO can operate even when the available power is significantly larger than the generator’s rated power. The algorithm was validated with the 30 kW biradial turbine prototype from the OPERA H2020 European project under real sea conditions at Mutriku’s wave power plant. Results show the algorithm’s effectiveness in real situations and during a generator failure simulated during the experimental campaign. This new concept may reduce the energy cost from wave energy and open a new field of design possibilities for OWC wave energy converters.