Intuitive LTI energy-maximising control for multi-degree of freedom wave energy converters: The PeWEC case

Energy-maximising wave energy conversion control strategies are commonly based upon direct optimal control theory, where the control problem is discretised and transcribed into a nonlinear programme, and a solution is found via numerical routines. Though appealing from an optimality viewpoint, the real-time application of such strategies to realistic (complex) wave energy systems, such as the PeWEC device, can become potentially challenging, due to its intrinsic multiple degree-of-freedom (DoF) nature. Furthermore, this pendulum-based system is not only multi-DoF in its nature, but also underactuated, i.e. only one mode, associated to the pendulum mechanism installed inside the wave-excited floating body, can be effectively actuated. We propose, in this paper, a set of four simple and intuitive energy-maximising controllers for the PeWEC system based, upon linear time-invariant (LTI) systems. We achieve this by deriving the so-called impedance-matching conditions for the PeWEC, and extending well-established LTI controllers, originally designed for fully actuated single-DoF systems, to this multi-DoF underactuated case. In particular, we explore, design, and synthesise both feedback, and feedforward configurations, making explicit emphasis in their main characteristics. Furthermore, we provide a performance assessment for each of the proposed controllers, showing their energy-maximising capabilities for the wave resource characterising the Mediterranean Sea

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

Muses

The Muses are goddesses and teachers of divine wisdom evoked in dance, music, and poetry. Late sources suggest that they invented the alphabet (Diod. Sic. 7.74.1) and the arts and sciences (Anth. Lat. 1.1.88; 1.2.664)

Optimal controller tuning for a nonlinear moored wave energy converter via non-parametric frequency-domain techniques

Wave energy production is a challenging problem, mainly due to the intrinsic complex nature of wave energy conversion (WEC) devices and their development environment. Although mooring systems are generally considered an essential part of a WECs, being able to solve the station-keeping problem, their influence in power-related problems can not be neglected. This study analyses the energy-maximizing control design procedure of a pendulum-based WEC, explicitly taking into account the influence of the mooring system. To achieve such an objective, a representative non-parametric model of the moored WEC is obtained via system identification procedures, and the PTO control parameter synthesis is achieved by exploiting the principle of impedance-matching. Simulations and computation of control parameters have been conducted with and without the mooring system, in order to quantify the importance of the mooring inclusion when addressing power generation problems

Input-Unknown Estimation for Arrays of Wave Energy Conversion Systems via LTI Synthesis

The incoming menace of global overheating and depletion of fossil fuels, highlight the need for alternative, renewable, energy sources. In this context, ocean wave energy has a massive potential to contribute towards global decarbonisation. In optimising wave energy converters (WEC) productivity, state-of-the-art, model-based optimal control techniques are fundamental to enhance energy absorption efficiency. However, the vast majority of these optimal approaches inherently require wave excitation force estimators. In particular, in array configurations, the interaction between WEC devices has to be taken into account to achieve a consistent excitation force estimation. In this paper, a linear time-invariant (LTI) estimation approach for a WEC farm is proposed. The technique proposed is based upon the so-called ‘simple and effective estimator’, recently presented in the WEC literature, which reformulates the wave excitation force estimation problem as a traditional tracking loop. The results show that the proposed approach provides accurate estimates of the exciting force for every device in the array, with almost no design effort, and mild computational requirements

On the influence of mooring systems in optimal predictive control for wave energy converters

Wave energy conversion systems have a massive potential in securing a reliable renewable energy mix. In their development, a crucial role is that of optimal control (OC) algorithms. Such systems are able to maximize the wave energy converter (WEC) power extraction, while respecting the corresponding of technological constraints. State-of-the-art OC techniques, such as Model Predictive Control (MPC), rely on mathematical models of the device to control, in a predictive fashion, the WEC system, thus maximizing the power production. Nonetheless, to date, control algorithms are usually developed and assessed on the basis of free floating WEC models, i.e. which neglect the mooring system influence. As a matter of fact, the anchorage introduces nonlinear dynamics in the device motion. Consequently, to test the idealized potential of a control strategy, such system is commonly neglected. Moorings are a fundamental WEC component, and have the potential to influence significantly the associated system dynamics. Neglecting this element can lead to deceptive results, either in terms of device theoretical productivity, and control strategy effectiveness. This paper proposes a systematic procedure to include, in the model used to synthesized such OC strategies, a linear representative model of the mooring system, presenting its benefits by discussing the consequent MPC loop development and corresponding performance assessment. Such procedure consists in retrieving an estimate of the frequency response of the moored system, using properly designed input conditions, and identifying the associated input–output linear system. A main objective of this study is, hence, to assess the difference between an MPC strategy designed and synthesized, with and without the proposed mooring control-oriented representation, always using as simulation system a high-fidelity numerical model for performance evaluation, which incorporates a full account of the mooring effects

Design considerations for a hybrid wind-wave platform under energy-maximising control

The environmental impact of emissions of fossil fuels and their rising prices, together with countries' commitment to mitigate the effect of the alarming and rapid climate changes, have been a crucial thrust to investigate new solutions to have an energy supply depending on renewable energies. In this scenario, offshore wind-wave hybrid platforms have been recently promoted: sharing facilities, infrastructure, and grid connections, give 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 movement 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 requirement for the wind turbine. The aim of this study is to investigate the effects of design changes on the dynamics of the hybrid wind-wave platform under energy-maximising control, which can be analysed in terms of the principle of impedance-matching. A semi-submersible platform with an incorporated flap-type WEC will be analysed both from a closed-loop and open-loop perspective, and the control system will be designed to maximise the energy produced by the WEC. Design changes on the wind-wave conversion platform will be in terms of flap dimensions, starting from a nominal geometry based on the so-called Oyster system. Analyses will be conducted by both increasing and decreasing the flap depth from the nominal case, to investigate the effect of the different geometries on the interactions between the WEC and the platform. A frequency-domain analysis of the overall input/output (velocity) system will be presented, highlighting the situations that can enhance the potential of both devices and exploit their synergies