1,721,006 research outputs found
Set-membership identification of an inertial wave energy system
No full textWithin the process of designing effective control strategies able to maximise the energy absorbed by wave energy systems, one of the crucial tasks to be achieved is the development of representative control-oriented models. Usually, these models are developed following a physics-based approach based on first principles. However, with this approach, the simplifications needed to derive models compatible with real-time control applications inherently add a certain degree of uncertainty, which could result in suboptimal energy-maximising performances. For this reason, the applications of system identification techniques are becoming popular in the field of wave energy. The consequent modelling process is data-based, and, in this way, the uncertainty associated with the device model depends on the data fidelity and the assumed model structure. However, the quantification of uncertainty is not trivial, and is done most of the time a posteriori, i.e after the system model has been identified. Within this paper, we propose a data-based modelling procedure based on the concept of set-membership, which provides dynamical models consistent with a given level of uncertainty. To assess the capabilities of this technique, we detail its application to the problem of identifying the dynamics of an inertial wave energy converter from data obtained through system-identification-oriented tests in simulation. The identified model is then validated on a dataset generated by different simulations, to assess the capabilities of this methodology
Time-domain parametric models for floating structures: A Loewner-based approach
Full text linkDesign, control, and optimisation of offshore floating structures have undergone significant evolution in recent years, driven by cutting-edge technology, including novel marine renewable energy sources and autonomous underwater vehicles. A key cornerstone is the availability of mathematical models capable of providing an accurate (yet computationally tractable) prediction of their behaviour, under different ocean conditions. The most widely adopted approach for capturing fluid–structure interactions is based on linear potential flow theory, where the system’s hydrodynamic behaviour is described through a finite set of frequency-dependent linear coefficients. A well-known limitation of this frequency-domain approach is its inherently non-parametric nature: if not parameterised accordingly, effective time-domain simulation necessitates the numerical solution of a convolution operator, which describes memory effects due to the surrounding fluid, an approach inconvenient for both simulation (computational) and control design (representational compatibility). Not only is a closed-form expression fundamental, but any candidate parametric model also needs to comply with the physical properties characterising a floating structure, including input/output stability, minimum-phase behaviour, and passivity. This paper presents a novel approach to producing physically consistent parametric structures for time-domain modelling of floating systems, utilising a Loewner-based method. The models, capable of providing approximate interpolation of raw frequency-domain data computed with off-the-shelf hydrodynamic solvers, accurately capture the complex behaviour of multi-mode and multi-body offshore structures, while respecting the dynamical properties associated with the system’s physics. The technique is illustrated in detail, using four different offshore structures from various fields of ocean engineering, highlighting the benefits of the proposed time-domain modelling framework
Frequency-domain-based regularisation of energy-maximising control for wave energy conversion
No full textMaximising energy through optimal control strategies is crucial in developing ocean wave energy harvesting technologies. Among these strategies, direct optimal control techniques, including model predictive control, are widely employed for energy maximisation. The specific energy-based objective function might result in non-convex formulations according to the discretisation procedure used, directly affecting real-time feasibility. To address this, modifications are commonly made to the overall objective function to ensure convexity. These modifications are virtually always based on numerical computation of a set of eigenvalues, and depend on the designer's ability to balance numerical accuracy with the objective of energy maximisation. In this study, leveraging the duality between time-domain and frequency-domain control formulations, a condition based on the spectral properties of the model, characterising the energy harvester, is proposed, to determine an accurate regularisation condition for the problem. The developed methodology is demonstrated with numerical examples
Towards modelling and control strategies for hybrid wind-wave energy converters: Challenges and opportunities
Full text linkHybrid wind-wave energy converters (HWWECs) offer a promising renewable energy solution by harnessing both wind and wave resources, supporting the diversification of the global energy mix. Despite their potential, HWWEC development is at an early stage, with key challenges hindering commercial availability. To reach optimal performance, HWWECs must effectively balance wind and wave energy absorption, an inherently complex task that requires accurate (yet tractable) mathematical modelling and the subsequent design of tailored control strategies, capable of handling this trade-off systematically. Aiming to shed light on the state-of-the-art of this crucial problem, this study provides a critical review of current research on modelling and control of HWWECs, focusing on the development of control-oriented dynamical models and their role in control design. Modelling is divided into key macroareas, including, e.g., aerodynamics, hydrodynamics and mooring systems, common to a large class of HWWEC concepts. Given their non-traditional nature, controllers are prioritised on the wave energy converter side and categorised according to the defined performance objective. Among the key findings, this review highlights significant gaps, including a lack of a unified/generalised modelling framework, inconsistencies among similar devices when deriving control-oriented models, and a lack of development and detailed understanding of the parameterisation and solution of the associated control problem. By identifying these gaps and highlighting clear future directions, this study provides a primer for researchers seeking to enter this emerging field, contributing to accelerating the development of HWWECs and helping establish their role as a key technology in the energy transition
A Broadband Time-Varying Energy Maximising Control for Wave Energy Systems (LiTe-Con+): Framework and Experimental Assessment
Full text linkMotion of wave energy converters (WECs) is usually exaggerated as a consequence of the application of control strategies for energy absorption maximisation. With the aim of preserving the physical integrity of the devices, constraint handling mechanisms, as part of the underlying control strategies, are considered a key component. Recent developments in wave energy control include a linear time-invariant-based controller presented in the literature as LiTe-Con, which provides a simple constraint handling mechanism. However, this handling method can lead to
conservative performance in certain scenarios. To overcome such
limitations, this study presents a time-varying methodology for an online adaptation of the constraint handling mechanism in LiTe-Con, while preserving its original simplicity and efficiency.
Experimental assessment of the presented control methodology is provided in this study, using a broad range of operating conditions. Results show that the presented control strategy (LiTe-Con+)
exceeds the performance achievable with the original LiTe-Con. Additionally, the benefits of LiTe-Con+, such as low computational demand, technical versatility, and impressive performance level are
highlighted
On the behaviour of a combined wind-wave energy conversion platform under energy-maximising control conditions
Full text linkClimate changes are increasingly impacting human welfare and, together with population growth, are rising the energy demand. To mitigate their negative effects, the need to harvest energy from renewable sources, while reducing the dependency on fossil fuels, has become pressing. This has led to the pursuit of new concepts that can exploit natural resources efficiently. In this scenario, offshore wind-wave hybrid platforms have been recently promoted: sharing facilities, infrastructure, and grid connections, gives these systems the potential to increase energy production at a lower cost. However, an efficient realisation of these two combined technologies requires two potentially conflicting control objectives: On the one hand, for the wind turbine, a reduced motion of the platform is required, which essentially translates to enhanced stability of the structure, so that its behaviour resembles standard onshore wind technologies. On the other hand, to maximise the energy produced, wave energy converters (WECs) require optimal control technology which often leads to large amplitude motion, potentially conflicting with the stability required for the wind turbine. The aim of this study is to provide a better understanding of how the energy-maximising control problem for WEC systems interacts with both conversion systems, and to elucidate their corresponding synergies. A semi-submersible platform with an incorporated flap-type WEC is analysed from a closed-loop perspective, with the control system designed to maximise the energy produced by the WEC
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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