International Journal of Marine Energy最新文献

A model-free algorithm is developed for the reactive control of a wave energy converter. Artificial neural networks are used to map the significant wave height, wave energy period, and the power take-off damping and stiffness coefficients to the mean absorbed power and maximum displacement. These values are computed during a time horizon spanning multiple wave cycles, with data being collected throughout the lifetime of the device so as to train the networks off-line every 20 time horizons. Initially, random values are selected for the controller coefficients to achieve sufficient exploration. Afterwards, a Multistart optimization is employed, which uses the neural networks within the cost function. The aim of the optimization is to maximise energy absorption, whilst limiting the displacement to prevent failures. Numerical simulations of a heaving point absorber are used to analyse the behaviour of the algorithm in regular and irregular waves. Once training has occurred, the algorithm presents a similar power absorption to state-of-the-art reactive control. Furthermore, not only does dispensing with the model of the point-absorber dynamics remove its associated inaccuracies, but it also enables the controller to adapt to variations in the machine response caused by ageing.

For a heave-pitch-surge three-degrees-of-freedom wave energy converter, the heave mode is usually decoupled from the pitch-surge modes for small motions. The pitch-surge modes are usually coupled and are parametrically excited by the heave mode, depending on the buoy geometry. In this paper, a Model Predictive Control is applied to the parametric excited pitch-surge motion, while the heave motion is optimized independently. The optimality conditions are derived, and a gradient-based numerical optimization algorithm is used to search for the optimal control. Numerical tests are conducted for regular and Bretschneider waves. The results demonstrate that the proposed control can be implemented to harvest more than three times the energy that can be harvested using a heave-only wave energy converter. The energy harvested using a parametrically excited model is higher than that is harvested when using a linear model.

For Marine current turbine (MCT), low speed Surface-mounted Permanent Magnet generator is a solution to satisfy the efficiency and fault tolerant requirements. This is supposed to be true if the winding is made with several non-shifted three-phase stars that could be supplied with standard modular voltage source inverters. This paper investigates the impact of the star number on the MCT energy yield if the system is conceived to operate with disconnected inverters. For this purpose, a method to calculate the extracted power according to the tidal speed for a given star number and a given activated star number is detailed. A rainflow counting method is used to account the stress due to the tidal speed change on the star converter: the impact of the star number on the resilience capability of the MCT is then quantified. By assuming a ten-year period without converter repair, according to the introduced probabilistic approach, the star number increase improves the reliability and three-star configuration appears as a trade-off.

In this numerical study a tidal turbine in straight and yawed flows is simulated using the actuator line (AL) method coupled with Large Eddy Simulation (LES) of turbulence for the turbine previously studied experimentally by Bahaj et al. (2007). Importantly, the AL model is fully coupled to an existing GPU based computational fluid dynamic solver, enabling high resolution simulations in reasonable time frames using desktop size server systems. Simulation results using the blade element actuator disk (BEAD) method are also presented to support the results from the AL method and highlight its advantages over the BEAD method. Results obtained from this study show that the AL method is capable of capturing wake unsteadiness and the tip and root vortices resulting from the turbine blades. Predicted power and thrust coefficients agree well with experimental data, being within 0.77% and 1.91%, respectively, at the design tip speed ratio. However, the absence of hub geometry in this method affects the downstream wake pattern along its centerline.

Tidal energy represents a promising resource for the future energy mix. For harnessing tidal currents free stream horizontal axis turbines have been investigated for some years. The acting physics is very similar to the one of horizontal axis wind turbines, with the additional phenomenon of cavitation, which causes performance reduction, flow induced noise and severe damages to the turbine blade and downstream structures.

The paper presents an enhanced semi-analytical model that allows the prediction of the performance characteristics including cavitation inception of horizontal axis tidal turbines. A central component is the well-known blade element momentum theory which is refined by various submodels for hydrofoil section lift and drag as a function Reynolds number and angle of attack, turbine thrust coefficient, blade hub and tip losses and cavitation. Moreover, the model is validated by comparison with comprehensive experimental data from two different turbines.

Predicted power and thrust coefficient characteristics were found to agree well with the experimental results for a wide operational range and different inflow velocities. Discrepancies were observed only at low tip speed ratios where major parts of the blades operate under stall conditions. The predicted critical cavitation number is somewhat larger than the measured, i.e. the prediction is conservative. As an overall conclusion the semi-analytical model developed seems to be so fast, accurate and robust that it can be integrated in a future workflow for optimizing tidal turbines.

This paper presents a method to compute the total tidal energy resource available in French Atlantic and Channel waters. An analysis of outputs from MARS2D hydrodynamic model allowed us to identify 20 potential sites for extraction of tidal energy. These sites are presented in the form of an atlas. Four promising sites with annual average currents exceeding 2 m/s, were identified in the English Channel (Raz Blanchard, Raz Barfleur, Paimpol-Bréhat) and at the westernmost part of Brittany (Fromveur Passage).

By defining characteristics of a turbine array (spacing and power coefficient), we compute the total extractable power for three cases: high, medium and low theoretical performance. Considering the sites with average currents exceeding 1.5 m/s, the total extractable power ranges from 1.46 to 9.71 GW for the low and high performance cases. For all 20 sites, where average currents exceed 0.5 m/s, the total extractable power ranges from 2.49 to 16.58 GW for the low and high performance cases.

Finally, we focus on the most promising site: the Raz Blanchard (Alderney Race) located in the Manche department (English Channel). The variability of the current intensities and directions are presented. Within this high tidal potential site, we define a zone (12 km²) wherein the current speeds are consistently high (exceeds 2.5 m/s for at least 40% of the year studied) and most often following the NNE/SSW axis.

Compact arrays of small wave absorbers have been proposed as an advantageous solution for the extraction of wave energy when compared to a big isolated point absorber. Numerous challenges are associated with the numerical modeling of such devices, notably the computation of the hydrodynamic interactions among the large number of floats of which they are composed. Efficient calculation of the first-order linear hydrodynamic coefficients requires dedicated numerical tools, as their direct computation using standard boundary element method (BEM) solvers is precluded. In this paper, the Direct Matrix Method interaction theory by Kagemoto and Yue (1986) is used as an acceleration technique to evaluate the performance of a generic wave energy converter (WEC) inspired by the Wavestar SC-concept and to perform layout optimization. We show that there exists an optimum number of floats for a given device footprint. Exceeding this number results in a “saturation” of the power increase, which is undesirable for the economic viability of the device. As in previous studies on multiple absorber WECs, significant differences were observed in energy production among floats, due to hydrodynamic interactions.

The new and costly nature of tidal stream energy extraction technologies can lead to narrow margins of success for a project. The design process is thus a delicate balancing act – to maximise the energy extracted, while minimising cost and risk. Scenario specific factors, such as site characteristics, technological constraints and practical engineering considerations greatly impact upon both the appropriate number of turbines to include within a tidal current turbine array (array size), and the individual locations of those turbines (turbine micro-siting). Both have been shown to significantly impact upon the energy yield and profitability of an array.

The micro-siting arrangement for a given number of turbines can significantly influence the power extraction of a tidal farm. Until the layout has been optimised (a process which may incorporate turbine parameters, local bathymetry and a host of other practical, physical, legal, financial or environmental constraints) an accurate forecast of the yield of that array cannot be determined. This process can be thought of as ‘tuning’ an array to the proposed site to maximise desirable outcomes and mitigate undesirable effects.

The influence of micro-siting on the farm performance means that determining the optimal array size needs to be coupled to the micro-siting process. In particular, the micro-siting needs to be repeated for any new trial array size in order to be able to compare the performance of the different farm sizes. Considering the large number of design variables in the micro-siting problem (which includes at least the positions of each turbine) it becomes clear that algorithmic optimisation is a key tool to rigorously determine the optimal array size and layout.

This paper proposes a nested optimisation approach for solving the array size and layout problem. The core of this approach consists of two nested optimisation procedures. The ‘outer’ optimisation determines the array size. At each ‘outer’ iteration the power extracted by $N$ turbines is found via a separate optimisation of their micro-siting on the proposed site. The ‘inner’ optimisation is treated as a computationally expensive black-box solver, mapping array size to power (and additionally returning the optimal micro-siting design). This forms the basis of a practical approach to the array sizing problem based on Bayesian optimisation, in which a surrogate model is built and used to maximise the utility of each evaluation made by the solver.

This paper introduces and reports on the implementation of this novel surrogate-assisted array design approach which, coupled with a simple financial model is demonstrated through optimisation of array size for two test scenarios to maximise the financial return on the array for the developer.

In the present paper, the development of a model scale Wave Energy Converter (WEC) and an experimental WEC test rig are presented, and results of numerical simulations and experimental measurements are shown. The presented point absorber WEC is coupled to a generic power take-off (PTO) and is restricted to pure heave motion in regular waves. Experiments were carried out at the Universidad Austral de Chile (UACh) Wave Tank and results from responses and efficiencies were compared with data from the BEM (boundary element method) code WAMIT. Numerical and experimental results showed good agreement. Finally, results were extrapolated and superposed with typical wave energy spectra found in different Chilean regions, providing a first performance estimation for a wave energy converter in Chile. Results are discussed and compared with an existing technology and give an insight of the potential for wave energy technologies in Chile. Further investigation is proposed for an analysis in irregular waves and the use of a more advanced PTO (power take-off) in the future.

Progress toward commercial deployment of in-stream tidal energy devices and commercial arrays has frequently met with delays, particularly in the UK and Canada. While some delays are due to the manifestation of the uncertainties inherent in new technology development, this study seeks to better understand the strategic timing decisions of tidal energy conversion companies in developing a globally-distributed renewable resource. The study consists of semi-structured interviews with executives and senior managers of organizations in the international tidal energy industry. Themes emerging from the interviews find companies manage risk by timing their investments across an international portfolio of seabed leases, consistent with the exercise of real options.