Energy-maximising model predictive control for a multi degree-of-freedom pendulum-based wave energy system

Renewable energy sources can be a solution for the recent pollution increasing scenario and the need for diversification of the energy market. Among such alternative sources,wave energy represents a viable solution, due to the its high power density and accessibility.Nonetheless, wave energy is still in phase of development, and a key stepping stone towards commercialisation is strongly linked to the availability of optimal control strategies for maximum energy harvesting. With its ability to handle system constraints and optimise power absorption directly, model predictive control (MPC) has gained popularity within the WEC community as a potential solution for the corresponding energy-maximising problem. In this study, an MPC strategy is developed for real-time control of the so-called PeWEC energy harvesting system,providing also a solution for the wave excitation estimation and forecasting problem, inherently required by the MPC controller to achieve optimal performance. Improved computational requirements are obtained via definition of a reduced control-oriented model, describing the dynamics of the system in a compact form. The performance of the proposed strategy is illustrated via a comprehensive numerical appraisal

Combining pendulum and gyroscopic effects to step-up wave energy extraction in all degrees of freedom

The fight against the global threat of climate change requires, among other actions, to increase the penetration of renewable energy technologies and diversify the energy mix in order to support a resilient energy system that can reach net-zero greenhouse gas emissions. Offshore energy is expected to drive the energy transition, with wave energy having the major role to provide a reliable baseload and reduce the need for storage; however, its techno-economic feasibility requires reduction of costs and increase of energy conversion efficiency. This paper tackles a fundamental innovation of a device’s working principle which, jointly exploiting pendulum and gyroscopic effects, steps-up the overall conversion efficiency in real operational conditions. A recent patent proposes a technological solution that conveniently combines pendulum and gyroscopic effects in order to effectively exploit motion also outside the plane, namely in the three-dimensional space and from all degrees of freedom (DoFs). This paper tackles the endeavour of the analytical formulation of the electro-mechanical conversion system dynamics, considering at first the fully-nonlinear equation of motion, obtained through a Lagrangian approach. Consequently, incremental simplifications are applied to accommodate practical application, based on the study on the relative importance of each term in the equation of motion. Furthermore, preliminary results are produced and discussed, comparing the behaviour in response to 3-DoF to 6-DoF exploitation

Excitation forces estimation for non-linear wave energy converters: A neural network approach

Investigating optimal control algorithms is a continuing concern within the Wave Energy field. A considerable amount of literature has been published on optimal control architectures applied to Wave Energy Converter (WEC) devices. However, most of them requires the knowledge of the wave excitation forces acting on the WEC body. In practice such forces are unknown and an estimate must be used. In this work a methodology to estimate the wave excitation forces of a non-linear WEC along with the achievable accuracy, is discussed. A feedforward Neural Network (NN) is applied to address the estimation problem. Such a method aims to map the WEC dynamics to the wave excitation forces by training the network through a supervised learning algorithm. The most challenging aspects of these techniques are the ability of the network to estimate data not considered in the training process and their accuracy in presence of model uncertanities. Numerical simulations under different irregular sea conditions demonstrate accurate estimation results of the NN approach as well as a small sensitivity to changes in the plant parameters relative to the case study presented

Data-based control synthesis and performance assessment for moored wave energy conversion systems: the PeWEC case

With a model-based control strategy, the effectiveness of the associated control action depends on the availability of a representative control-oriented model. In the case of floating offshore wave energy converters (WECs), the device response depends upon the interaction between mooring system, any mechanical parts, and the hydrodynamics of the floating body. This study proposes an approach to synthesise WEC controllers under the effect of mooring forces building a representative data-based linear model able to include any relevant dynamics. Moreover, the procedure is tested on the moored pendulum wave energy converter (PeWEC) by means of a high-fidelity mooring solver, OrcaFlex (OF). In particular, the control action is computed with and without knowledge of the mooring influence, in order to analyse and elucidate the effect of the station-keeping system on the harvested energy. The performance assessment of the device is achieved by evaluating device power on the resource scatter characterising Pantelleria, Italy. The results show the relevance of the mooring dynamics on the device response and final set of control parameters and, hence, a significant influence of the station-keeping system on control synthesis and extracted mechanical power

Variability of wecs’ performance according to the wave directional spreading variation

The exploitation of Renewable Energy Sources (RES) is critical to move forward and to aim for global sustainability. In this scenario, the development of devices able to take advantage of wave energy resources is continuously advancing. The basic methodology to acquire energy from waves is based on the identification of sites having high energy and the optimization of Wave Energy Converters (WECs), according to the sea states’ characteristics. The principal parameters which describe wave conditions are the significant wave height and the wave energy period, which allow estimating the respective average wave power, and the peak direction, which indicates the main incoming wave direction. However, these parameters give only limited information since the directional distribution of the different wave components is missing. In the wave spectrum analysis, the directional spread is a measure of the range of wave directions and states if wave components are coming from similar directions or various ones. Considering the spreading parameter is especially significant in directional WEC devices. The main scope of this paper is to highlight the relevance of directional analysis about wave and WEC device interaction. Therefore, considering the direction of every wave component, an accurate and detailed analysis can be conducted. Assuming Pendulum Wave Energy Converter (PeWEC) as a case study, the correlation between the performance and the omni- and uni-directional sea states is shown. Especially, the investigation results are focus on the influence that the variation of directional spreading value has on the PeWEC performing features

Pilot-aided trellis-based demodulation

In optical transmission systems based on high order modulations the impact on system performance of Wiener phase noise affecting the received carrier phase can be relevant. To make less severe phase noise effects, solutions based on the use of known pilot symbols, that aid carrier phase recovery, have been recently introduced. This letter proposes a pilot-aided demodulation scheme where the memory of phase noise is dealt with by a trellis-based demodulation algorithm. The benefits in terms of achievable information rate and bit error rate compared to adversary schemes are demonstrated by computer simulations for strong phase noise with 4-ary quadrature amplitude modulation (4-QAM) and 16-QAM

LMI-based passivation of LTI systems with application to marine structures

Due to the inherent relevance of passive (physically representative) models for control, state-estimation, and motion simulation in the field of marine systems, in this paper, an optimisation-based approach to passivation of linear time-invariant (LTI) systems is proposed with application to physically consistent dynamical modelling of marine structures. In particular, the presented strategy is based upon the introduction of a suitably designed perturbation, computed via minimisation of a linear objective subject to a specific set of linear matrix inequalities (LMIs). The performance of the passivation technique is showcased in terms of two case studies: An offshore platform, with a frequency-domain response computed by means of hydrodynamic codes, and a 1:20 scale wave energy converter (WEC), is characterised in terms of real experimental data

Energy-maximising experimental control synthesis via impedance-matching for a multi degree-of-freedom wave energy converter

We present, in this paper, an experimental framework for design and synthesis of impedance-matching-based (IM) controllers capable of maximising energy extraction in inherently multi degree-of-freedom wave energy converter (WEC) systems, and its subsequent application to the Intertial Sea Wave Energy Converter (ISWEC) device, by incorporating recent advances in IM-based theory. In particular, we consider a 1/20th scale prototype of the ISWEC system, tested as part of a larger experimental campaign conducted within the tank facilities available at University degli Studi di Napoli Federico II, subject to a variety of wave conditions. We adopt two different control structures to realise an approximation of the TM principle, fully tuned based upon interpolation of a particular (experimentally obtained) non-parametric empirical transfer function estimate, which defines the optimal frequency-domain input-output response for energy-maximising behaviour. Furthermore, a performance comparison between controller tuning based upon traditional linear boundary element method models, and the presented experimental approach, is also offered, showing that the latter can consistently outperform the foriiter in realistic scenarios, for the set of analysed sea-states. Copyright (C) 2022 The Authors