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With an increasing focus on climate action and energy security, an appropriate mix of renewable energy technologies is imperative. Despite having considerable global potential, wave energy has still not reached a state of maturity or economic competitiveness to have made an impact. Challenges include the high capital and operational costs associated with deployment in the harsh ocean environment, so it is imperative that the full energy harnessing capacity of wave energy devices, and arrays of devices in farms, is realised. To this end, control technology has an important role to play in maximising power capture, while ensuring that physical system constraints are respected, and control actions do not adversely affect device lifetime. Within the gamut of control technology, a variety of tools can be brought to bear on the wave energy control problem, including various control strategies (optimal, robust, nonlinear, etc.), data-based model identification, estimation, and forecasting. However, the wave energy problem displays a number of unique features which challenge the traditional application of these techniques, while also presenting a number of control 'paradoxes'. This review articulates the important control-related characteristics of the wave energy control problem, provides a survey of currently applied control and control-related techniques, and gives some perspectives on the outstanding challenges and future possibilities. The emerging area of control co-design, which is especially relevant to the relatively immature area of wave energy system design, is also covered. [ABSTRACT FROM AUTHOR]

This paper presents a systematic methodology to optimise the geometry of a three-body hinge-barge wave energy converter, to maximise the energy extraction of the device in given sea states and in site-specific wave climates. To that end, a 5-degree-of-freedom mathematical model is proposed to describe the system dynamics in two-dimensional space and a two-layer optimisation is designed to find the optimal design and control variables. The inner-layer optimisation is used to find the optimal control parameters of the power take-off system and the outer-layer optimisation, which uses a genetic algorithm, is employed to find the optimal design parameters of barge lengths and of optimal ballast positioning. In the case study, this methodology is applied to a 1:20 scale prototype of the McCabe Wave Pump device. Numerical results indicate that the optimal dimensions of the device, under given sea states, can be found efficiently and accurately, and that there appears to be no obvious benefit in the use of three barges, over a two-barge system. • Geometric optimisation of a 3-barge hinge barge device (no comparable work to date). •Combined optimisation of geometry AND optimal positions for ballast masses. •2-level optimisation, updating the control for each geometry/ballast specification. •The analysis points to an optimal 2-barge configuration. [ABSTRACT FROM AUTHOR]

Wave energy is a significant source of renewable energy harnessed by wave energy converters (WECs). However, due to the relatively high levelised cost of energy (LCoE), wave energy has not attained a commercial stage yet. To minimise the LCoE, since the optimum (uncontrolled) WEC design typically differs from the optimum controlled WEC design, control co-design (CCD) techniques are essential. With CCD, the WEC control-related aspects are taken into account from the start of the WEC design phase and, ideally, the best control-informed WEC design is then achieved. This paper specifically focuses on CCD for an oscillating-water-column (OWC) WEC, equipped with a Wells turbine and a bypass valve. In essence, a parametric CCD approach is devised to find the optimum (control-informed) turbine rotor diameter, and bypass valve diameter, for the considered OWC WEC. In particular, the optimum design parameters minimise a 'simplified' LCoE, which is chosen as a suitable performance function. Despite the LCoE is primarily sensitive to the power take-off size, rather than to the bypass valve size, peak-shaving control with a bypass valve potentially increases the capacity factor and, consequently, can minimise the LCoE for small-to-medium sized turbines. • A parametric control co-design approach for an OWC with a bypass valve is devised. • A novel sea state based control approach for a bypass valve of an OWC is designed. • Peak-shaving control with a bypass valve potentially increases the capacity factor. • Peak-shaving control can minimise the LCoE for small-to-medium sized turbines. • LCoE is primarily sensitive to the PTO size, rather than to the bypass valve size. [ABSTRACT FROM AUTHOR]

Abstract Mathematical models incorporating all the necessary components of wave energy converters (WECs) from ocean waves to the electricity grid, known as wave-to-wire (W2W) models, are vital in the development of wave energy technologies. Ideally, precise W2W models should include all the relevant nonlinear dynamics, constraints and energy losses. This paper presents a balanced W2W model that incorporates high-fidelity models for each conversion system, and can accommodate different types of WECs, hydraulic power take-off (PTO) topologies, electric generators and grid connections. The models of the different conversion stages presented herein are efficiently implemented in the W2W model using a multi-rate integration scheme that reduces the computational requirements by a factor of 10. Two W2W models, i.e. one with the constant-pressure hydraulic PTO configuration and one with the variable-pressure configuration, are compared in this paper. Results show that a higher PTO efficiency (30% higher for the constant-pressure configuration) does not necessarily imply a higher electricity generation (2% higher for the variable-pressure configuration), which reinforces the need for high-fidelity W2W models for the design of successful WECs. Highlights • The paper presents a novel high-fidelity wave-to-wire model. • Multirate integration scheme reduces computational requirements by a factor of 10. • The impact of different hydraulic system configurations is evaluated. • Higher PTO system efficiency does not necessarily mean higher generated power. • Wave-to-wire models are necessary to evaluate the holistic performance of WECs. [ABSTRACT FROM AUTHOR]

Abstract It is important to consider nonlinear effects when designing controllers to maximise generated energy in wave energy converters (WECs). Due to the substantial extra computation and complexity added when considering nonlinearities in the controller calculations, quantifying the extent of nonlinearity in WECs' behaviour is crucial to avoid designing overcomplicated control strategies. This paper suggests two nonlinearity measures to quantify the nonlinearity degree of wave-to-wire (W2W) models in steady-state, using the best linear approximation identified through a minimisation problem as a benchmark. The first measure, referred to as the original nonlinearity measure, evaluates the nonlinear effects of the wave-absorber hydrodynamic interaction. The second measure, referred to as the power nonlinearity measure, quantifies the nonlinear effects in power take-off (PTO) systems, considering the quadratic response of the power signal. The degree of nonlinearity of two WEC models, a partially-nonlinear hydrodynamic model with an ideal PTO model and a complete nonlinear W2W model, is evaluated using monochromatic and polychromatic waves over a wide range of wave periods and heights, covering the whole operational space of a WEC. Highlights • Linearisation-based nonlinearity measures, original and power nonlinearity measures , are evaluated for wave-to-wire models. • Both original and power nonlinearity measures increase when the wave energy converter operates close to the resonant period. • Both original and power nonlinearity measures increase under reactive control, compared to the resistive control case. • Hydrodynamic nonlinearities are the primary nonlinearity in wave energy converter systems. [ABSTRACT FROM AUTHOR]

Abstract Non-linear, power-maximising control of wave energy converters (WECs) can be achieved within a receding-horizon control framework, whereby an upper loop calculates a reference trajectory in real-time, ensuring maximal power absorption under operational constraints, while a tracking loop drives the device along the generated trajectory. This paper articulates the four fundamental components of such a control strategy: reference generation calculations, tracking loop , and wave excitation estimation and forecasting. The upper-loop optimisation problem is efficiently solved through a Fourier spectral method, taking into account non-linear dynamics and constraints. Tracking is achieved through a linear state feedback, combined with a non-linear feed-forward term. An extended Kalman filter is used for excitation force estimation, based on noisy WEC position and acceleration measurements. Finally, wave excitation forecasts are based on a linear predictor, whose coefficients are derived from the wave spectrum (on a sea-state-by-sea-state basis). The practical issues and trade-offs, which arise when the four components listed above are combined within a practical implementation, are investigated by means of realistic numerical simulations, using a WEC model comprising a combination of static and velocity-dependent non-linear forces. Highlights • Receding-horizon optimal control of wave energy converters (WECs) is studied. • The reference trajectory is updated in real time using Fourier spectral control. • Trajectory tracking is ensured using combined feed-forward and feedback terms. • The only measurements are the WEC position and acceleration, with noisy sensors. • Wave estimation and forecasting are solely based on available measurements. [ABSTRACT FROM AUTHOR]

The wave energy control competition established a benchmark problem which was offered as an open challenge to the wave energy system control community. The competition had two stages: In the first stage, competitors used a standard wave energy simulation platform (WEC-Sim) to evaluate their controllers while, in the second stage, competitors were invited to test their controllers in a real-time implementation on a prototype system in a wave tank. The performance function used was based on converted energy across a range of standard sea states, but also included aspects related to economic performance, such as peak/average power, peak force, etc. This paper compares simulated and experimental results and, in particular, examines if the results obtained in a linear system simulation are borne out in reality. Overall, within the scope of the device tested, the range of sea states employed, and the performance metric used, the conclusion is that high-performance WEC controllers work well in practice, with good carry-over from simulation to experimentation. However, the availability of a good WEC mathematical model is deemed to be crucial. • Comprehensive documentation of the first international wave energy control competition (WECCCOMP). • Description of the various control algorithms used and experimental procedure employed. • Comparison of both simulation and experimental wave tank testing results, with critical analysis. • Proposal of the WECCCOAP system as a benchmark problem for the appraisal of WEC control systems. [ABSTRACT FROM AUTHOR]

Accurate and computationally efficient mathematical models are fundamental for designing, optimizing, and controlling wave energy converters. Many wave energy devices exhibit significant nonlinear behaviour over their full operational envelope, so nonlinear models may become indispensable. Froude-Krylov nonlinearities are of great importance in point absorbers but, in general, their calculation requires an often unacceptable increase in model complexity/computational time. However, for axisymmetric bodies, it is possible to describe the whole geometry analytically, thereby allowing faster calculation of nonlinear Froude-Krylov forces. In this paper, a convenient parametrization of axisymmetric body geometries is proposed, applicable to devices moving in surge, heave, and pitch. While, in general, Froude-Krylov integrals must be solved numerically, by assuming small pitch angles, it is possible to simplify the problem, and achieve a considerably faster algebraic solution. However, both nonlinear models compute in real-time. The framework presented in the paper offers flexibility in terms of computational and fidelity levels, while still representing important nonlinear phenomena such as parametric pitch instability. Models with lower computational requirements may be more suitable for repetitive calculations, such as real-time control, or long-term power production assessment, while higher fidelity models may be more appropriate for maximum load estimation, or short-term power production capability assessment. [ABSTRACT FROM AUTHOR]

Designing, optimizing and controlling a wave energy converter requires the construction of a mathematical model in order to simulate the behaviour of the device. Given the nonlinear nature of fluid-structure interactions, the definition of the model is not straightforward and should take into account the specific application it is intended for. Two of the most important characteristics of a model are the computational time and the expected accuracy, which usually are mutually conflicting. The inclusion of nonlinearities potentially increases the model accuracy, but at a higher computational price. Considering a heaving wave energy converter with and without the application of latching control, this paper studies and compares nine different modelling options, eight of which are based on potential theory and consider nonlinear Froude-Krylov and viscous drag forces, while one is based on fully-nonlinear computational fluid dynamics. The value of including nonlinearities in the hydrodynamic model is discussed in relation to the computational cost of the eventual accuracy benefits, under a range of scenarios. [ABSTRACT FROM AUTHOR]

With an increasing requirement to lower the costs of delivered renewable energy, the maintenance costs for marine renewable energy (MRE), due to accessibility issues, are an obvious focal point. In particular, condition-based maintenance and prognostics can help to optimise maintenance activities and forewarn of impending maintenance requirements, mindful of the constrained access to MRE systems due to limited weather windows of sufficient duration. This paper focusses on offshore wind, tidal flow and wave energy as target MRE domains and provides a comprehensive review of condition-based maintenance methodologies currently employed in MRE systems. While, of the three energy domains, offshore wind is the more mature, giving the opportunity to propagate such methods to the less mature areas of tidal and wave, there are also many components and challenges which are common to all three domains, e.g. generator systems and grid interface. [ABSTRACT FROM AUTHOR]

Over the next decades, tidal stream energy aims to become a fully commercially viable energy source. For this purpose, complete knowledge regarding tidal stream resource assessment is essential. In this context, the International Electrotechnical Commission has developed a technical standard for the assessment of the tidal stream resource, “IEC 62600-201 TS: Marine energy - Wave, tidal and other water current converters - Part 201: Tidal energy resource assessment and characterisation” , offering a vast set of recommendations in the fields of data collection, numerical modelling, data analysis and reporting of the results with the purpose of standardising tidal stream resource characterisation. The standard divides resource assessments into two different classes: feasibility and layout design. The model setup procedure (mesh resolution, boundary conditions) and the computational effort required vary significantly from one class to another. For these reasons, the objective of the present work is to explore the proposed standard using the Orkney Region (N Scotland) as a case study. Overall, it was found that the standard works well, offering a detailed characterisation of the tidal resource; however, in order to improve its manageability, some aspects related to the grid resolution requirements and the approach to model a tidal energy converter could be revisited for future editions. [ABSTRACT FROM AUTHOR]

For wave energy to become a commercially viable source of energy, a complete understanding of the wave resource characterisation is needed. In this context, the IEC (International Electrotechnical Commission) has developed a technical specification for the assessment of the wave resource, IEC-TS 62600-101: Marine energy-Wave, tidal and other water current converters-Part 101: Wave energy resource assessment and characterisation (IEC-62600-101), which presents a series of recommendations for standardising wave resource characterisation. The IEC-62600-101 classifies resource assessment studies into three different classes: reconnaissance, feasibility and design. The model setup requirements (mesh resolution, boundary conditions) and the effort (validation process, computational times) vary considerably from one class to the other. On these grounds, the objective of this work is to explore this methodology using the Irish West Coast as a case study. Overall, it was found that the methodology proposed performs well, offering a detailed characterisation of the resource; however, with the aim of making the technical specification more manageable, some aspects related to the seasonality of the wave resource and the validation and model setup procedures may be revisited for future editions. [ABSTRACT FROM AUTHOR]

Increasingly, there is a focus on utilising renewable energy resources in a bid to fulfil increasing energy requirements and mitigate the climate change impacts of fossil fuels. While most renewable resources are free, the technology used to usefully convert such resources is not and there is an increasing focus on improving the conversion economy and efficiency. To this end, advanced control technology can have a significant impact and is already a relatively mature technology for wind turbines. Though wave energy systems are still in their infancy, significant benefits have been shown to accrue from the appropriate use of control technology. To date, the application communities connected with wind and wave energy have had little communication, resulting in little cross fertilisation of control ideas and experience, particularly from the more mature wind area to wave. This paper examines the application of control technology across both domains, both from a comparative and contrasting point of view, with the aim of identifying commonalities in control objectives and potential solutions. Key comparative reference points include the articulation of the stochastic resource models, specification of control objectives, development of realistic device models, and development of solution concepts. Not least, in terms of realistic system requirements are the set of physical and legislative constraints under which such renewable energy systems must operate, and the need to provide reliable and fault-tolerant control solutions, which respect the often remote and relatively inaccessible location of many offshore deployments. [ABSTRACT FROM AUTHOR]

Abstract: Most wave energy devices are subject to finite constraints on both the power take-off (PTO) stroke length and the maximum force that the PTO can tolerate. It is also often the case that greater stroke lengths can reduce the maximum force in the PTO and vice versa. Ultimately, some informed choice of PTO constraints must be made in order to ensure that PTO constraints are not violated and that the trade-off between position and force constraints is made in such as way that maximum energy is captured by the converter. This paper presents a tool to allow device developers to check the satisfaction of constraints for a given hydrodynamic model and set of sea conditions and, where constraints are not satisfied, shows how to relax the constraints to maximize energy capture. The tool is algebraic, requiring no simulation and the results are presented through intuitive geometrical constructs. Sample application results are presented for single- and two-body wave energy systems. [Copyright &y& Elsevier]

Abstract: Guyton’s original integrative physiology model was a milestone in integrative physiology, combining significant physiological knowledge with an engineering perspective to develop a computational diagrammatic model. It is still used in research and teaching, with a small number of variants on the model also in circulation. However, though new research has added significantly to the knowledge represented by Guyton’s model, and significant advances have been made in computing and simulation software, an accepted common platform to integrate this new knowledge has not emerged. This paper discusses the issues in the selection of a suitable platform, together with a number of current possibilities, and suggests a graphical computing environment for modelling and simulation. By way of example, a validated version of Guyton’s 1992 model, implemented in the ubiquitous Simulink environment, is presented which provides a hierarchical representation amenable to extension and suitable for teaching and research uses. It is designed to appeal to the biomedical engineer and physiologist alike. [Copyright &y& Elsevier]

Abstract: Salt-induced hypertension has been demonstrated in a variety of species including rats, monkeys, chimpanzees and humans. Until recently, the multiple phases of this blood pressure increase due to high salt intake had not been closely studied. This work builds upon a recent study, which developed a grey-box multi-component model of salt-induced hypertension in the Dahl-S rat. The previous 3-component model has been extended here to include additional model dynamics to improve the model fit and add new important elements to the model response. The model was optimised using numerical techniques with experimental data from 4 different protocols with Dahl-S, Dahl-R and FF2 hybrid rats. Results show a marked improvement over the previous model and confirm the merit of the 5-component model structure. A comparison between the model dynamics for different rat strains has also been included. [Copyright &y& Elsevier]

This paper presents a method, implemented as a freely available computer programme, which is used to estimate the economics of renewable microgeneration of electricity from wind and solar energy sources. A variety of commercial small wind turbines and photovoltaic (PV) panels are considered and combined with raw energy data gathered from a variety of locations. Both residential and holiday home user profiles are available and options are selectable concerning feed-in tariffs (if available), government incentive schemes and the cost of capital borrowing. The configuration of the generation setup, which can consist of wind, PV and combination of wind/PV, is fully selectable by the user, with a range of appropriate default data provided. A numerical example, based on Irish data, is presented, which suggests that payback periods for solar and wind microgeneration systems can vary greatly (2.5–500 years), depending on the location, installation and economic variables. [Copyright &y& Elsevier]

Abstract: The control of blood pressure is a complex mixture of neural, hormonal and intrinsic interactions at the level of the heart, kidney and blood vessels. While experimental approaches to understanding these interactions are useful, it remains difficult to conduct experiments to quantify these interactions as the number of parameters increases. Thus, modelling of such physiological systems can offer considerable assistance. Typical mathematical models which describe the ability of the blood vessels to change their diameter (vasoconstriction) assume linearity of operation. However, due to the interaction of multiple vasocontrictive and vasodilative effectors, there is a significant nonlinear response to the influence of neural factors, particularly at higher levels of nerve activity (often seen in subjects with high blood pressure) which leads to low blood flow rates. This paper proposes a number of nonlinear mathematical models for the relationship between neural influences (sympathetic nerve activity (SNA)) and renal blood flow, using a feedback path to model the predominantly nonlinear effect of local vasoactive modulators such as nitric oxide, which oppose the action of SNA. The model structures are motivated by basic physiological principles, while the model parameters are determined using numerical optimisation techniques using open-loop data collected from rabbits. The models were verified by demonstrating correlation between experimental results and model outputs. [Copyright &y& Elsevier]

Abstract: Cadmium distribution was studied in different subcellular fractions of gill and hepatopancreas tissues of eastern oysters Crassostrea virginica. Oysters were exposed for up to 21 days to low sublethal Cd concentrations (25μgL−1). Gill and hepatopancreas tissues were sampled and divided into organelle fractions and cytosol by differential centrifugation. Organelle content of different fractions was verified by activities of marker enzymes, citrate synthase and acid phosphatase for mitochondria and lysosomes, respectively. In both tissue types, there was a significant accumulation of cadmium in cytosol reaching 230–350ngmg−1 protein. Among organelles, mitochondria were the main target for Cd bioaccumulation in gills (250–300ngmg−1 protein), whereas in hepatopancreas tissues, the highest cadmium accumulation occurred in lysosomes (90–94ngmg−1 protein). Although 75–83% of total cadmium burden was associated with the cytosol reflecting high volume fraction of this compartment, Cd concentrations in organelle fractions reached levels that could cause dysfunction of mitochondria and lysosomes. Organ- and organelle-specific patterns of cadmium bioaccumulation support our previous in vivo studies, which showed adverse effects of cadmium exposures on mitochondrial oxidation in gills and on the lysosomal system of hepatopancreas. This may have important implications for the development of biomarkers of effect for heavy metals and for understanding the mechanisms of toxic effects of metals. [Copyright &y& Elsevier]

The purpose of these studies was to evaluate if there were relationships between lysosomal destabilization or glutathione concentrations and gamete viability of oysters, Crassostrea virginica. Oysters were collected from field sites during the peak spawning period (May–June) during 2001 and 2002. Lysosomal destabilization rates and glutathione concentrations of hepatopancreas tissues (e.g. digestive gland) were determined. Eggs and sperm from the same adults were also used to conduct embryo development assays with reference seawater collected from a clean site, site water, and also a range of Cd concentrations (the Cd exposures were used to determine if there were differences in susceptibility to pollutants). Baseline embryo development success (e.g. percent normal development when the assays were conducted with reference seawater or site water) was related to lysosomal destabilization, but not glutathione status. However, the susceptibility of embryos to metal toxicity was related to glutathione status, i.e., sensitivity to Cd exposures increased with decreasing glutathione levels. These studies support the hypotheses that there are linkages between embryo development success and susceptibility to pollutant stress and cellular biomarker responses. These kinds of effects on reproductive success could lead to subtle but significant long-term effects on recruitment and viability of oyster populations. [Copyright &y& Elsevier]

Coastal ecosystems are increasingly impacted by anthropogenic activities and contaminant inputs, including heavy metals. This paper investigates the responses to laboratory cadmium exposures in the North American Pacific coast temperate sea anemone Anthopleura elegantissima. Anemones were exposed to cadmium chloride (control, 20 and 100 μg l−1 Cd) for 14 days (sampled at days 0, 2, 7 and 14) and analyzed for Cd content using inductively coupled plasma mass spectroscopy (ICP-MS). Higher levels of Cd were demonstrated in symbiotic anemones compared to symbiont-free (aposymbiotic) anemones (e.g. 100 μg l−1 Cd exposure at day 14; 12.4±2.4 and 4.5±1.9 ng Cd mg−1 dry weight, respectively; t-test; p<0.001). These higher levels in symbiotic anemones were not due to increasing uptake of Cd by the algal symbionts (levels in algal-containing pellet fractions were constant over dose and time; t-test; p>0.05) but rather to increased accumulation by both partners. No changes in algal cell density were observed in symbiotic anemone tentacle clips at any dose or time point (ANOVA; p>0.05). Changes in glutathione (GSH) levels have been proposed as a useful biomarker in other marine invertebrates in response to heavy metal stress, but GSH studies have not been reported for this species. We demonstrate the presence of GSH in A. elegantissima and its depletion (58.2% of control values) following 1-week exposure to buthionine sulfoximine (BSO), a nontoxic chemical known to deplete GSH. Levels of GSH were higher in symbiotic compared to aposymbiotic anemones (control averages, 563±81 and 485±58 nmol GSH mg−1 wet weight, respectively; t-test; p<0.001). Following Cd exposure, significant reductions (t-test; p<0.05) in GSH levels were observed only in the aposymbiotic anemones (exposure to 100 μg l−1 Cd resulted in a 27% reduction from control levels at day 14). Our results demonstrate that the accumulation of Cd depends upon symbiotic state and provides the first identification of a GSH response in anemones and indeed in any cnidarian in response to heavy metals. [Copyright &y& Elsevier]

Lysosomal destabilization assays have been used as valuable biomarkers of pollutant exposures in a variety of bivalve and fish species. The responses of oysters, Crassostrea virginica, deployed at and native to various reference and degraded sites were evaluated for lysosomal destabilization during both summer and winter seasons. In both native and deployed oysters, lysosomal destabilization rates tended to be higher during the winter at both reference and polluted sites. There are at least two hypothetical explanations. Greater lysosomal destabilization rates may be related to physiological changes associated with mobilization of nutrient reserves during the winter and gametogenesis. However, lysosomal destabilization in deployed oysters was correlated with tissue metal concentrations. These data also support a second hypothesis that seasonal differences in physico-chemical factors (such as reduced levels of acid volatile sulfides) may increase the bioavailability of metals during the winter so that adverse effects are more pronounced. [Copyright &y& Elsevier]

Unlike more established renewable energy conversion technologies, such as wind turbines, wave power systems have reached neither commercial maturity, nor technological convergence. The significant variation in device geometries and operating principles has resulted in a diversification of effort, with little coordination or true comparative analysis. The situation is compounded by the relative lack of systematic optimisation applied to the sector, partly explained by the complexity and uncertainty associated with wave energy system models, as well as difficulties in the evaluation of appropriate target function metrics. This review provides a critical overview of the state-of-the-art in wave energy device geometry optimisation, comparing and contrasting various optimisation approaches, and attempting to detail the current limitations preventing further progress, and convergence, in the development of optimal wave energy technology. [Display omitted] • First survey on geometric optimisation of wave energy converters (WECs). • Wave conditions, modelling methods and control strategies mainly influence WEC hydrodynamics. • Optimised geometry depends heavily on optimisation criteria, rather than algorithms. • It is important to build a co-design framework for WEC optimisation. • Technology performance level should be prioritised over technology readiness level. [ABSTRACT FROM AUTHOR]

Wave-to-wire models are valuable tools for a variety of applications in the development of successful wave energy converters. However, computational requirements of these wave-to-wire models are often prohibitive for certain applications that require fast mathematical models, such as power assessment or control design. The need for computationally fast models is traditionally achieved by assuming linear hydrodynamics and simplifying power take-off (PTO) dynamics with a linear damper in the mathematical model, though these assumptions can be relatively unjustified. However, these computationally appealing mathematical models can have a fidelity level which compromises their use in particular applications. Therefore, this paper suggests an application-sensitive systematic complexity reduction approach that reduces computational requirements of a high-fidelity simulation platform (HiFiWEC), i.e. a CFD-based numerical wave tank coupled to a high-fidelity PTO model, while retaining a level of fidelity in a sense specific to particular applications. The illustrative case study analysed here includes a point absorber with a hydraulic PTO system. Results show that reduced wave-to-wire models designed via the systematic complexity reduction approach retain the application-relevant fidelity (up to 95% fidelity compared to the HiFiWEC) for similar computational requirements shown by the traditionally used linear mathematical models. • A systematic complexity-reduction approach is presented for W2W models. • A W2W model is systematically reduced to application-dependent acceptable levels. • The HiFiWEC simulation platform is used as the basis for complexity reduction. • The approach is analysed for seven potential applications of W2W models. • Computational cost is reduced by an order of magnitude, while retaining 95% fidelity. [ABSTRACT FROM AUTHOR]

In pursuing environmental sustainability and reducing carbon footprints, wave energy converters (WECs) will play an important role in harnessing the immense energy embedded in ocean waves. While WECs have promising potential to meet global energy needs, their technology performance level is lower than that of solar and wind devices, leading to a higher levelised cost of energy (LCoE). Over the last two decades, researchers have primarily focused on developing control technology to create more effective controllers, designed to manipulate the motion of WECs aggressively, aiming to maximise their energy-harvesting capacity in an effort to minimise the LCoE. However, exaggerated WEC motion can, in the harsh ocean environment, lead to significant decreases in maintenance intervals and system reliability, leading to increases in operational costs (OpEx). There may also be an adverse effect on device lifetime, as well as the inevitable LCoE increases associated with increased OpEx. This paper aims to define the lifespan control problem for WECs by reviewing current advancements in longevity analysis within the wave energy application area, as well as other pertinent areas. The obstacles and opportunities for future research will also be covered. • Providing a general novel basis for future lifetime-aware control studies of WECs. • Investigating candidate degradation evaluation metrics for lifetime-aware control of WECs. • Specifying possible challenges in the longevity control of WECs for future studies. [ABSTRACT FROM AUTHOR]

• Wind and wave renewable energy combination more efficient in the European Atlantic. • Coastal sites. • Satellite altimetry can provide wind and wave energy resource estimates. • The wave energy of swell is less variable than that of the wind sea. In this study, the combined wind and wave energy potential assessment is presented for various locations in European coastal waters. The objective is to investigate the feasibility of satellite altimetry-based assessments of combined wind/wave renewable energy potential on the European shelf. The study is motivated by the potential reduction in energy supply variability by combining wind and wave. The method consists of using a homogenized multi-mission altimeter database available by the European Space Agency Sea State Climate Change Initiative (Sea_State_cci) that comprises 26 years of data, from January 1991 to December 2018, that allows extending the time range and spatial coverage to estimate site wind and wave power densities. An empirical model is used to estimate the wave energy period from the altimeter Ku band significant wave height and radar backscatter coefficient required to compute the wave power density. The results show that wind/wave energy is relatively correlated in the Mediterranean but not in the North Atlantic sites studied. Thus, the Western North Atlantic sites are the most adequate places for wind and wave farms, from the point of view of combined exploitation. The different characteristics of the studied sites show some correspondence between variability and mean wave power, which is an essential input to a marine renewable energy strategy in any jurisdiction. The level of overall variability decreases with an increase in mean wave power, related to the higher power swell waves are not highly correlated with the local wind. [ABSTRACT FROM AUTHOR]

Abstract Accurate knowledge of tidal turbine impacts on the far-field hydrodynamic conditions, which extend from 3 to 20 diameters downstream the turbine, is essential for the estimation of tidal resource, farm layout design and environmental impact. For this purpose, tidal turbine operation is modelled within coastal models, as enhanced bottom friction, or momentum sinks. In Delft3D-FLOW, a state-of-the-art coastal model, turbine operation is usually represented via momentum losses, using the Porous Plate tool. However, the Porous Plate tool presents significant limitations to accurately represent energy extraction and geometry of tidal turbines. Recently, a new tool (Actuator Disc) based on the Momentum Actuator Disc Theory (MADT) was developed in Delft3D-FLOW, overcoming the aforementioned limitations and showing excellent results against laboratory data. The aim of this work is to compare the behaviour of the Actuator Disc and Porous Plate on the far-field hydrodynamics. Overall, significant differences were found, with the Porous Plate significantly underestimating the impact on instantaneous and residual flow velocities and turbulence conditions, when the turbine operates at its rated power. Consequently, MADT appears as the best alternative to investigate the far-field hydrodynamic impacts of tidal turbine operation and previous research based on the Porous Plate tool should be revisited. Highlights • Misrepresentations on the far-field hydrodynamics may jeopardise the long-term viability of tidal farms. • Tidal turbine operation was traditionally modelled within coastal models as enhanced bottom drag or momentum sinks. • MADT overcomes most of the limitations of the enhanced bottom drag or momentum sink approaches. • A comparison between the momentum sink approach and MADT was carried out, using Delft3D-FLOW as benchmark. • The momentum sink approach misrepresents considerably the impacts on velocity and turbulence conditions. [ABSTRACT FROM AUTHOR]

For the research and development (R&D) of wave energy converters (WECs), numerical wave tanks (NWTs) provide an excellent numerical tool, enabling a cost-effective testbed for WEC experimentation, analysis and optimisation. Different methods for simulating the fluid dynamics and fluid structure interaction (FSI) within the NWT have been developed over the years, with increasing levels of fidelity, and associated computational expense. In the past, the high computational requirements largely precluded Computational Fluid Dynamics (CFD) from being applied to WEC analysis. However, the continual improvement and availability of high performance computing has led to the steady increase of CFD-based NWTs (CNWT) for WEC experiments. No attempt has yet been undertaken to comprehensively review CNWT approaches for WECs. This paper fills this gap and presents a thorough review of high-fidelity numerical modelling of WECs using CNWTs. In addition to collating the published literature, this review tries to make a step towards a best practice guideline for the applications of CFD in the field of wave energy. [ABSTRACT FROM AUTHOR]

The motion of a Wave Energy Converter (WEC) can be described in terms of an integro-differential equation, which involves a convolution product. The convolution term, which accounts for the radiation forces, represents a computational and representational drawback both for simulation, and analysis/design of control strategies. Several studies attempt to find a suitable finite parametric form that approximates the radiation impulse response, to express the equation of motion in the time-domain by a state-space representation. Ideally, this approximated parametric model should behave as closely as possible to the system under analysis, particularly at key frequencies, such as the resonant frequency of the device. This study presents a method to obtain a parametric model of both the force-to-motion dynamics and/or the radiation force convolution term, based on moment-matching. Recent advances in moment-matching, allow the computation of a model that exactly matches the frequency response of the original system at the chosen frequencies, while enforcing specific physical properties of the device, depicting a robust and efficient method to compute a state-space representation for the dynamics of a WEC. The potential of the algorithm is illustrated by numerical examples, and the approximation error is shown to be monotonically decreasing with increasing model order. [ABSTRACT FROM AUTHOR]

The wave energy sector has made and is still doing a great effort in order to open up a niche in the energy market, working on several and diverse concepts and making advances in all aspects towards more efficient technologies. However, economic viability has not been achieved yet, for which maximisation of power production over the full range of sea conditions is crucial. Precise mathematical models are essential to accurately reproduce the behaviour, including nonlinear dynamics, and understand the performance of wave energy converters. Therefore, nonlinear models must be considered, which are required for power absorption assessment, simulation of devices motion and model-based control systems. Main sources of nonlinear dynamics within the entire chain of a wave energy converter - incoming wave trains, wave-structure interaction, power take-off systems or mooring lines- are identified, with especial attention to the wave-device hydrodynamic interaction, and their influence is studied in the present paper for different types of converters. In addition, different approaches to model nonlinear wave-device interaction are presented, highlighting their advantages and drawbacks. Besides the traditional Navier-Stokes equations or potential flow methods, ‘new’ methods such as system-identification models, smoothed particle hydrodynamics or nonlinear potential flow methods are analysed. [ABSTRACT FROM AUTHOR]

In guiding the progression, development, and operation of wave energy converters (WECs) in a more efficient way, mathematical analysis and understanding of the dynamic process is essential. Mathematical WEC models, obtained either by numerical analysis or physical modelling, form the basis of most (model-based) energy maximising control strategies available in the literature, where experimental design and system identification methodology directly impact the resulting model. This study, using an experimental-based WEC model (which can be used for linear control design), investigates the dynamic behaviour of a WEC by analysing the dominant poles of the system, generated using fully nonlinear computational fluid dynamics (CFD)-based numerical wave tank (NWT) experiments. The aim is to effectively track the dominant dynamics of the WEC, using different force-input amplitude levels in the NWT setup, and perform a comparison with the classical linear boundary-element-methods (BEM) equivalent methodology. Thus, the presented case studies are shown to agree with previously proposed model assessment of linear WEC models, based on a free-decay NWT setup. In addition, the representative WEC models determined as part of this study can be used for WEC controller design, either singly, or using a form of model/controller gain scheduling. • Representative linear wave energy converter models for model-based control design. • Assessment of wave energy device dominant dynamics. • Dominant pole trends for wave energy converters using force-input experiments. • Model identification of wave energy converters from numerical wave tank data. [ABSTRACT FROM AUTHOR]

Under control action, wave energy devices typically display nonlinear hydrodynamic behaviour, making the design of energy maximising control somewhat onerous. One solution to approach the optimal performance for nonlinear control problem under model mismatches is to employ a linear control strategy, which can be robust to linear model mismatches. However, accurate characterisation of the uncertainty in the linear model is vital, if the controller is to adequately capture the full extent of the uncertainty, while not being overly conservative due to overestimation of the uncertainty. This paper describes a procedure, employing CFD-based numerical tank experiments, to accurately produce a nominal linear empirical transfer function model, along with an accurate estimate of the uncertainty bounds in that linear model, due to hydrodynamic uncertainty. A robust control case study is provided, illustrating the nominal model estimation process, and its corresponding uncertainty set, including the complete procedure, required to generate the robust controller. Robust control results, on the fully nonlinear CFD model, are provided to demonstrate the efficacy of the modelling and control philosophy. • Input signal synthesis and post-processing in numerical wave tank experiments. • Reducing conservatism in robust control of wave energy devices. • High-fidelity nominal and uncertainty model from computation fluid dynamics setup. • Quantifying hydrodynamic uncertainty description of the wave energy converters. • Empirical transfer function refinement based on physics of wave energy converters. [ABSTRACT FROM AUTHOR]

Abstract: Nanoparticles may be introduced into aquatic environments during production processes and also as a result of release following their use in various commercial formulations and biologic applications. Filter-feeding bivalve mollusks such as oysters are valuable model species for characterizing nanoparticle bioavailability and interactions with basic cellular processes. The adults release their gametes into the environment, so their embryos and larvae are also likely targets of nanoparticles. The purpose of these studies was to characterize the toxicity of metal nanoparticles on embryonic development of oysters, Crassostrea virginica and to compare the relative sensitivity of embryos to adults. Newly-fertilized oyster embryos were exposed to silver nanoparticles (AgNP) and then the percent normal development after 48h was assessed. Studies were conducted with adult oysters in which they were also exposed to AgNP for 48h, and the effects on lysosomal destabilization were determined. The expression of metallothionein (MT) gene expression was also assessed in both embryos and adults. Adverse effects on embryonic development were observed at concentrations similar to those that caused both statistically and biologically significant effects on lysosomal destabilization of adults. Significant increases in MT mRNA levels were observed in both embryos and adult oysters, and MT levels were highly induced in embryos. While we do not know whether the toxicity and gene expression responses observed in this study were due to the nanoparticles themselves or the Ag ions that dissociated from the nanoparticles, these kinds of basic studies are essential for addressing the potential impacts of nanoengineered particles on fundamental cellular processes as well as aquatic organisms. [ABSTRACT FROM AUTHOR]

Wave energy converters (WECs) require active control to maximise energy capture over a wide range of sea conditions, which is generally achieved by making the device resonate. The exaggerated device motion arising at resonance, however, may result in nonlinear effects that are ignored by the linear models that are typically employed. In particular, nonlinear viscous forces are significant for particular device types, such as hinged flaps, which we take as a case study in this paper. The paper develops a general nonlinear WEC control methodology based on pseudospectral methods. The continuous time energy maximisation problem is fully discretised (both state and control), and the optimal solution is obtained by solving the resulting finite dimensional optimisation problem. By way of example, the nonlinear viscous damping for a hinged flap WEC is incorporated into the control model which also considers non-ideal power take-off efficiency. It is shown that the ratio of energy captured to energy dissipated is significantly increased with the nonlinear controller, compared to the linear case. [ABSTRACT FROM AUTHOR]