Control Of Wave Energy Devices Research Articles

Effect of radar measurement uncertainties on time-domain impedance matching control of wave energy devices

Effect of radar measurement uncertainties on time-domain impedance matching control of wave energy devices

Optimal Control of Wave Energy Devices with Nonlinear Reactive Power Constraints

This paper proposes an optimal control strategy for wave energy devices subject to physical and nonlinear reactive power constraints. Using a pseudospectral Legendre method, optimization is carried out with energy maximization as a target. Physical limitations are applied to the WEC position, velocity, and controller, while a reactive power constraint is added to reduce the negative peaks in the system output power. The optimized velocity of the WEC is used as a reference input to a velocity-following control loop, which tracks it. Since wave energy converters are subjected to parametric uncertainties in stiffness, radiation force coefficients, and physical disturbances, robust backstepping control is employed in the lower-loop to handle these. The proposed control scheme is validated numerically on a cylindrical buoy wave energy converter system.

Deterministic incident-wave elevation prediction in intermediate water depth

Potential performance gains from optimal (non-causal) impedance-matching control of wave energy devices in irregular ocean waves are dependent on deterministic wave elevation prediction techniques that work well in practical applications. Although a number of devices are designed for operation in intermediate water depths, little work has been reported on deterministic wave prediction in such depths. Investigated in this paper is a deterministic wave-prediction technique based on an approximate propagation model that leads to an analytical formulation, which may be convenient to implement in practice. To improve accuracy, an approach to combine predictions based on multiple up-wave measurement points is evaluated. The overall method is tested using experimental time-series measurements recorded in the U.S. Navy MASK basin in Carderock, MD, USA. For comparison, an alternative prediction approach based on Fourier coefficients is also tested with the same data. Comparison of prediction approaches with direct measurements suggest room for improvement. Possible sources of error including tank reflections are estimated, and potential mitigation approaches are discussed.

Long-term wave prediction and analysis based on time-series analysis

The efficiency of the oscillating float-type wave energy devices is closely related to the wave conditions in its working area. Seasonal changes in actual sea conditions are obvious, and they vary greatly throughout the year. Therefore, it is necessary to analyse and predict the wave, and adjust the oscillating float-type wave energy device’s parameters according to the wave conditions to improve the efficiency. Based on the analysis of the long-term wave variation and the short-term stationary characteristics of the wave, the moving regression algorithm is used to predict the wave. Adjust the device parameters according to the wave condition prediction data, and match the wave power and load power to achieve a certain efficiency in the wide power range of the device. The research results can be used for variable load control of wave energy devices.

Wave energy conversion under constrained wave-by-wave impedance matching with amplitude and phase-match limits

This paper investigates an approach to limit the fullness of ‘tuning’ provided by wave-by-wave impedance matching control of wave energy devices in irregular waves. A single analytical formulation based on the Lagrange multiplier approach of Evans [1] is used to limit the velocity amplitude while also limiting the closeness of the phase match between velocity and exciting force. The paper studies the effect of the present technique in concurrently limiting the device velocity and the required control/actuation force. Time domain application requires wave-profile prediction, which here is based on a deterministic propagation model. Also examined in the time domain is the effect of possible violation of the displacement constraint, which for many designs implies impacts at hard stops within the power take-off mechanism. Time domain simulations are carried out for a 2-body axisymmetric converter (with physical end-stops) in sea states reported for a site off the US east coast. It is found that the approach leads to effective power conversion in the less energetic sea states, while as desired, considerable muting of the optimal response is found in the larger sea states. Under the assumptions of this work, the end-stop collisions are found to have a minor effect on the power conversion. The present approach could be used to guide the design of power take-off systems so that their displacement stroke, maximum force, and resistive and reactive power limits are well-matched to the achievable performance of a given controlled primary energy converter.

Hydrodynamic Control of Wave Energy Devices [Bookshelf

A control system can be optimized for various fundamentally distinct performance objectives, leading to various canonical design problems such as disturbance rejection and tracking. Most of these can be recast as design problems, where it is desired to make some aspect of the closed-loop system (such as the norm of an input/output channel or mean-square tracking error) as close to zero as possible. However, not all control design objectives fit neatly into this paradigm, and one of the notable exceptions is the problem of harvesting maximal energy from dynamic phenomena. For certain classes of problems (characterized by the passivity of the mapping u v and the boundedness of the disturbance), such objectives can be shown to have a finite lower bound, resulting in a finite maximum available energy. This fact has been known to control and system theorists for decades and is of particular importance in robustness analysis. However, beyond the obvious connections to the classical concept of “impedance matching,” the idea that this objective is useful as the primary measure of a control system’s performance—a thing to be optimized in closed loop and balanced against other objectives—is a concept that has received considerably less attention. It may, therefore, be surprising to learn that this problem has a rich and decades-long history in the area of ocean wave energy conversion. This book begins with a brief but fascinating overview of this still-evolving history, from its early beginnings in Japan in the 1950s, through its surge of activity in Europe and America during the oil crisis of the 1970s, the rally of global industrial activity in the 1990s, and into the present. The text succeeds at illustrating these various aspects, while also giving the reader a strong appreciation for the history and evolution of the field. One is left with an accurate and compelling snapshot of where the research community is headed and an appreciation for the central role that control theory will play in its future. Although this is not the first text written on wave energy technology, it does focuses primarily on advanced control techniques.

Umesh A. Korde and John V. Ringwood: Hydrodynamic control of wave energy devices

Umesh A. Korde and John V. Ringwood: Hydrodynamic control of wave energy devices

Preliminary consideration of energy storage requirements for sub-optimal reactive control of axisymmetric wave energy devices

This paper investigates sub-optimal time-domain reactive control of wave energy devices based on real-time up-wave surface elevation information. The paper first presents the overall approach required for such control, and recalls the need for future force and response information due to the causality of the radiation force and the non-causality of the exciting force (in relation to the wave profile at device centroid). The present approach constitutes a long-wave approximation, and is based on linearized wave propagation and device dynamics models. The long-wave approximation allows direct use of the instantaneous up-wave surface-elevation measurement. For predominantly heaving axisymmetric primary energy converters in an approximately uni-directional irregular incident wave field, the amount of cumulative reactive energy is compared with the cumulative absorbed energy. Calculations are carried out for a heaving axisymmetric buoy, when floating and when fully submerged. Oscillations are assumed to be unconstrained, so as to obtain a basic understanding of the best performance improvements and the implementation challenges associated with the present control approach. Although the net reactive energy is zero in the absence of actuator losses, it is concluded, that the reactive force magnitudes and reactive energy requirements should be an important design-level consideration. Time-domain calculations show that the reactive energy requirements for the submerged buoy may be significantly greater (compared with the floating buoy) in spectra with energy periods ≥ 13 s.

Renewable Energy and Optimization-Based Control [About This Issue

The first feature article, “Energy-Maximizing Control of Wave-Energy Converters,” by John V. Ringwood, Giorgio Bacelli, and Francesco Fusco provides an overview of the motivation, background to, and state of the art in energy-maximizing control of wave energy devices. The underpinning mathematical modeling is described and the control fundamentals established. Two example control schemes are presented, which include an optimization-based control algorithm. The article also presents some algorithms for wave forecasting, which can be a necessary requirement, due to the acausal nature of some optimal control strategies. One of the control schemes is extended to show how cooperative control of devices in a wave farm can be beneficial. The article also includes perspectives on the interaction between control and the broader objectives of optimal wave-energy device geometry and full technoeconomic optimization of wave-energy converters.

Energy Storage Requirements for Near-Optimal Reactive Control of a Wave Energy Device

This paper investigates an approach to near-optimal real-time reactive control of wave energy devices based on time-domain up-wave surface elevation information. The paper first presents the overall approach required for such control, and recalls the need for future force and response information due to the causality of the radiation force and the non-causality of the exciting force (in relation to the wave profile at device centroid). The present approach is a realistic approximation based on linearized wave propagation and device dynamics models. For a predominantly heaving submerged point absorber in an approximately uni-directional incident wave field, the amount of instantaneous reactive power is compared with the instantaneous absorbed power. Although the net reactive energy is zero in the absence of actuator losses, the instantaneous reactive energy requirement for small devices in swell-dominated spectra may be significant. The time-domain calculations here confirm this and show that substantial amounts of energy may need to be exchanged with an external energy storage unit or the grid until sufficient energy has been absorbed.

Control, forecasting and optimisation for wave energy conversion

This paper presents an overview of the motivation, background to and state-of-the-art in energy maximising control of wave energy devices. The underpinning mathematical modelling is described and the control fundamentals established. Two example control schemes are presented, along with some algorithms for wave forecasting, which can be a necessary requirement, due to the non-causal nature of some optimal control strategies. One of the control schemes is extended to show how cooperative control of devices in a wave farm can be beneficial. The paper also includes perspectives on the interaction between control and the broader objectives of optimal wave energy device geometry and full techno-economic optimisation of wave energy converters.

On the control of wave energy devices in multi-frequency waves

This paper presents a method for controlling a wave energy device to ensure high performance in random seas. The method is developed theoretically and a design proposed and analysed for an oscillating water column device. A PID (proportional integral differential) circuit and a digital filter are used in the feedback to drive a DC linear motor, maintaining zero phase shift between the wave excitation force and the velocity of the moving part of the device for nonharmonic motions. A practical design is worked out for a model oscillating water column and it is shown that the energy spent in achieving control can be made considerably less than that gained through control.