Short-term Wave Forecasting as a Univariate Time Series Problem. (EE/2009/JVR/03)

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Real-time Forecasting and Control for Oscillating Wave Energy Devices

Ocean wave energy represents a signicant resource of renewable energy and can make an important contribution to the development of a more sustainable solution in support of the contemporary society, which is becoming more and more energy intensive. A perspective is given on the benefits that wave energy can introduce, in terms of variability of the power supply, when combined with oshore wind. Despite its potential, however, the technology for the generation of electricity from ocean waves is not mature yet. In order to raise the economic performance of Wave energy converters (WECs), still far from being competitive, a large scope exists for the improvement of their capacity factor through more intelligent control systems. Most control solutions proposed in the literature, for the enhancement of the power absorption of WECs, are not implemented in practise because they require future knowledge of the wave elevation or wave excitation force. The non-causality of the unconstrained optimal conditions, termed complex-conjugate control, for the maximum wave energy absorption of WECs consisting of oscillating systems, is analysed. A link between fundamental properties of the radiation of the floating body and the prediction horizon required for an effective implementation of complex-conjugate control is identified. An extensive investigation of the problem of wave elevation and wave excitation force forecasting is then presented. The prediction is treated as a purely stochastic problem, where future values of the wave elevation or wave excitation force are estimated from past measurements at the device location only. The correlation of ocean waves, in fact, allows the achievement of accurate predictions for 1 or 2 wave periods into the future, with linear Autoregressive (AR) models. A relationship between predictability of the excitation force and excitation properties of the floating body is also identified. Finally, a controller for an oscillating wave energy device is developed. Based on the assumption that the excitation force is a narrow-banded harmonic process, the controller is effectively tuned through a single parameter of immediate physical meaning, for performance and motion constraint handling. The non-causality is removed by the parametrisation, the only input of the controller being an on-line estimate of the frequency and amplitude of the excitation force. Simulations in (synthetic and real) irregular waves demonstrate that the solution allows the achievement of levels of power capture that are very close to non-causal complex-conjugate control, in the unconstrained case, and Model predictive control (MPC), in the constrained case. In addition, the hierarchical structure of the proposed controller allows the treatment of the issue of robustness to model uncertainties in quite a straightforward and effective way

Robust cross-country analysis of inequality of opportunity

International rankings of countries based on inequality of opportunity indices may not be robust vis-vis the specific metric adopted to measure opportunities. Indices often aggregate relevant information and neglect to control for normatively irrelevant distributional factors. This paper shows that gap curves can be estimated from cross-sectional data and adopted to test hypotheses about robust cross-country comparisons of (in)equality of opportunity. (C) 2019 The Authors. Published by Elsevier B.V

Suboptimal Causal Reactive Control of Wave Energy Converters Using a Second Order System Model

Wave Energy Converters (WECs) based on oscillating bodies can achieve optimal energy absorption under certain conditions associated with reactive control. These conditions, in general, are not realisable in practice because non-causal and future values of the excitation force need to be known. In this paper, an alternative approach is presented, where the relationship between the optimal velocity and the excitation force is realised through a simple coefficient of proportionality, thus removing the problem of non-causality. From theoretical considerations and numerical simulations over a range of heaving WECs in different sea conditions, it is shown that such suboptimal and causal approximation, while significantly reducing the complexity and improving the robustness of reactive control, allows the achievement of values of energy capture very close to the ideal optimum

On state and inertial parameter estimation of free-falling planar rigid bodies subject to unsche dule d frictional impacts

This paper addresses the problem of simultaneous state estimation and inertial and frictional parameter identification for planar rigid-bodies subject to unscheduled frictional impacts. The aim is to evaluate to what level of accuracy, given noisy captured poses of an object free-falling under gravity and impacting the surrounding environment, it is conceivable to reconstruct its states, the sequence of normal and tangential impulses and, concurrently, estimate its inertial properties along with Coulomb’s coefficient of friction at contacts. To this aim we set up a constrained nonlinear optimization problem, where the unscheduled impacts are handled via a complementarity formulation. To assess the validity of the proposed approach we test the identification results both (i) with respect to ground truth values produced with a simulator, and (ii) with respect to real experimental data. In both cases, we are able to provide accurate/realistic estimates of the inertia-to-mass ratio and friction coefficient along with a satisfactory reconstruction of systems states and contact impulses

Robust control of wave energy converters

Energy-maximising controllers for wave energy devices are normally based on linear hydrodynamic device models. Such models ignore nonlinear effects which typically manifest themselves for large device motion (typical in this application) and may also include other modelling errors. In this paper, we present a methodology for reducing the sensitivity to modelling errors and nonlinear effects by the use of a hierarchical robust controller, which also allows good energy maximisation to be recovered through a passivity-based control approach

Quantification of the Prediction Requirements in Reactive Control of Wave Energy Converters

Optimal reactive control for maximum ocean wave power absorption from Wave Energy Converters (WECs) consisting of oscillating systems, is based on the principle of tuning their oscillation so that it is in resonance with the excitation force produced by the incident waves. Reactive control, however, is non-causal and cannot be implemented in real time. This paper analyses the prediction requirements of one possible solution, where predictions of the excitation force are utilised to resolve the non-causality. The study is focused on the analysis of the required forecasting horizon against the achievable prediction. Also, through the aid of numerical simulations of a number of specific systems over several wave conditions, a link is found between some fundamental properties of the system and the prediction requirements