1,530 research outputs found
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
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
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
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