Empowering wave energy with control technology: Possibilities and pitfalls

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

Towards data-driven and data-based control of wave energy systems: Classification, overview, and critical assessment

Currently, a significant effort in the world research panorama is focused on finding efficient solutions to a carbon-free energy supply, wave energy being one of the most promising sources of untapped renewable energy. However, wave energy is not currently economic, though control technology has been shown to significantly increase the energy capture capabilities. Usually, the synthesis of a wave energy control strategy requires the adoption of control-oriented models, which are prone to error, particularly arising from unmodelled hydrodynamics, given the complexity of the hydrodynamic interactions between the device and the ocean. In this context, data-driven and data-based control strategies provide a potential solution to some of these issues, using real-time data to gather information about the system dynamics and performance. Thus motivated, this study provides a detailed analysis of different approaches to the exploitation of data in the design of control philosophies for wave energy systems, establishing clear definitions of data-driven and data-based control in this field, together with a classification highlighting the various roles of data in the control synthesis process. In particular, we investigate intrinsic opportunities and limitations behind the use of data in the process of control synthesis, providing a comprehensive review together with critical considerations aimed at directly contributing towards the development of efficient data-driven and data-based control systems for wave energy devices

Non-Linear Modeling of a Vibro-Impact Wave Energy Converter

This article proposes a non-linear vibro-impact mechanism, integrated inside a semi-submerged cylindrical buoy to form a self-contained and self-referenced vibro-impact wave energy converter (VIWEC), for performance enhancement. A non-linear mathematical model of the VIWEC is derived, considering linear wave-buoy interaction and non-linear vibro-impact mechanics. Numerical simulations are conducted to investigate the influence of the vibro-impact mechanism on the VIWEC's dynamics and performance. Numerical results conclude that the VIWEC is characterised by a band-pass frequency response and inherently decoupled from ocean waves of low frequencies, indicating high survivability under extreme sea states. The vibro-impact mechanism can also broaden the VIWEC's power capture bandwidth and limit the VIWEC's motion within its physical constraint. On the other hand, the VIWEC's dynamics are sensitive to design parameters, and an improper design may lead to rich and complex non-linear dynamics of the VIWEC, e.g. chaos and multi-stability. The proposed non-linear model can provide a platform for design optimisation and control development of the VIWEC

A multi-component model of the dynamics of salt-induced hypertension in Dahl-S rats

Background. In humans, salt intake has been suggested to influence blood pressure (BP) on a wide range of time scales ranging from several hours or days to many months or years. Detailed time course data collected in the Dahl salt-sensitive rat strain suggest that the development of salt-induced hypertension may consist of several distinct phases or components that differ in their timing and reversibility. To better understand these components, the present study sought to model the dynamics of salt-induced hypertension in the Dahl salt sensitive (Dahl-S) rat using 3 sets of time course data. Results. The first component of the model ("Acute-Reversible") consisted of a linear transfer function to account for the rapid and reversible effects of salt on BP (ie. acute salt sensitivity, corresponding with a depressed slope of the chronic pressure natriuresis relationship). For the second component ("Progressive-Irreversible"), an integrator function was used to represent the relatively slow, progressive, and irreversible effect of high salt intake on BP (corresponding with a progressive salt-induced shift of the chronic pressure natriuresis relationship to higher BP levels). A third component ("Progressive-Reversible") consisted of an effect of high salt intake to progressively increase the acute salt-sensitivity of BP (ie. reduce the slope of the chronic pressure natriuresis relationship), amounting to a slow and progressive, yet reversible, component of salt-induced hypertension. While the 3 component model was limited in its ability to follow the BP response to rapid and/or brief transitions in salt intake, it was able to accurately follow the slower steady state components of salt-induced BP changes. This model exhibited low values of mean absolute error (1.92 0.23, 2.13 0.37, 2.03 0.3 mmHg for data sets 1 - 3), and its overall performance was significantly improved over that of an initial model having only 2 components. The 3 component model performed well when applied to data from hybrids of Dahl salt sensitive and Dahl salt resistant rats in which salt sensitivity varied greatly in its extent and character (mean absolute error = 1.11 0.08 mmHg). Conclusion. Our results suggest that the slow process of development of salt-induced hypertension in Dahl-S rats over a period of many weeks can be well represented by a combination of three components that differ in their timing, reversibility, and their associated effect on the chronic pressure natriuresis relationship. These components are important to distinguish since each may represent a unique set of underlying mechanisms of salt-induced hypertension

Understanding the interplay between baroreflex gain, low frequency oscillations, and pulsatility in the neural baroreflex

The neural baroreflex, which regulates mean arterial pressure (MAP) via the action of the brain, consists of baroreceptors which measure MAP, and actuators that can produce a change in MAP, such as the heart and parts of the peripheral resistance containing innervated smooth muscle. The brain is the controlling unit, maintaining an appropriate MAP in spite of various disturbances. Under certain circumstances, including haemorrhage and other states of distress, the gain of the neural baroreflex can change, causing low frequency (LF) oscillations (sometimes termed Mayer waves) in blood pressure (BP). Though their purpose is unclear, the origins of these LF oscillations has previously been explained via a nonlinear feedback model, though focusing on the peripheral resistance as an MAP actuator only. The present paper now includes analytical and simulation results explaining the LF oscillation phenomenon for the full neural baroreflex, containing both peripheral resistance (PR) and cardiac branches. However, the main contribution of the paper is to examine the effect of blood pulsatility, or a lack of pulsatility, on the neural baroreflex, and how it's effect can manifest in the presence of LF oscillations. This may have importance in cases where pulsatility is reduced (for example where left-ventricular assist devices are present), or completely absent (for example in turbine-based artificial hearts)

Simple Controllers forWave Energy Devices Compared

The design of controllers for wave energy devices has evolved from early monochromatic impedance-matching methods to complex numerical algorithms that can handle panchromatic seas, constraints, and nonlinearity. However, the potential high performance of such numerical controller comes at a computational cost, with some algorithms struggling to implement in real-time, and issues surround convergence of numerical optimisers. Within the broader area of control engineering, practitioners have always displayed a fondness for simple and intuitive controllers, as evidenced by the continued popularity of the ubiquitous PID controller.Recently, a number of energy-maximising wave energy controllers have been developed based on relatively simple strategies, stemming from the fundamentals behind impedance-matching. This paper documents this set of (5) controllers, which have been developed over the period 2010?2020, and compares and contrasts their characteristics, in terms of energy-maximising performance,the handling of physical constraints, and computational complexity. The comparison is carried out both analytically and numerically, including a detailed case study, when considering a state-of-the-art CorPower-like device.Fil: García Violini, Diego Demián. Universidad Nacional de Quilmes. Departamento de Ciencia y Tecnología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Faedo, Nicolás Ezequiel. Politecnico di Torino; ItaliaFil: Jaramillo Lopez, Fernando. Maynooth University; IrlandaFil: Ringwood, John V.. Maynooth University; Irland

Parametric representation of arrays of wave energy converters for motion simulation and unknown input estimation: A moment-based approach

When it comes to parameterisation of dynamical models for arrays of Wave Energy Converters (WECs), the most used approach, within the wave energy literature, provides a state-space representation whose order (dimension) increases quadratically with the number of devices composing the WEC array. This represents a major drawback for key WEC design elements, such as motion simulation and unknown input estimation, being the latter essential to effectively maximise energy extraction from ocean waves. We present herein a multi-input, multi-output (MIMO) parameterisation strategy based on a system-theoretic interpretation of moments. The state-space representations computed with this moment-based approach exactly match the steady-state behaviour of the target WEC system at specific (user-selected) interpolation points, providing efficient low dimensional models that can accurately represent the input-output dynamics of WEC arrays. Moreover, we show that there exists an intrinsic connection between wave excitation force estimation strategies and the moment-based parameterisation method proposed in this paper. We exploit this mathematical correlation to provide low order models that deliver the same degree of wave excitation force estimation accuracy to that obtained by implementing the currently-used parameterisation methods, with mild computational requirements. The performance of the strategy is analysed in terms of a case study, considering a WEC array composed of state-of-the-art CorPower-like devices, for both WEC motion simulation and wave excitation force estimation scenarios

A Broadband Time-Varying Energy Maximising Control for Wave Energy Systems (LiTe-Con+): Framework and Experimental Assessment

Motion 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.Fil: García Violini, Diego Demián. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes. Departamento de Ciencia y Tecnología; Argentina. Maynooth University; IrlandaFil: Pena Sanchez, Yerai. Universidad del País Vasco; EspañaFil: Faedo, Nicolás Ezequiel. Politecnico di Torino; ItaliaFil: Ferri, Francesco. Aalborg University; DinamarcaFil: Ringwood, John V.. Maynooth University; Irland