Effects of hydrodynamic interactions and control within a point absorber array on electrical output

Abstract A significant role is envisaged for ocean wave energy to meet the different renewable energy targets set by various governments and world bodies. To make use of this potential, the industry will soon be moving from sea testing of individual wave energy converters (WECs) to the deployment of arrays and farms of WECs. The total power extracted by an array of WECs is influenced by the hydrodynamic interactions between them, especially when the WECs are spaced very closely. By control of the power take-off (PTO) forces and moments acting on the WECs within the array, the hydrodynamic interactions between the WECs and the total power extracted by the array can be modified. In this paper, different resistive and reactive PTO control strategies, applied to a time-domain wave-to-wire model of a three-float Danish Wavestar device, are compared. The time-domain modelling approach, as opposed to the frequency-domain, allows the use of constraints on the maximum PTO moment to be applied in order to make the study realistic. In this paper, the effects that PTO control has on the hydrodynamic interactions between the floats and on the total power generated by the device, when placed in a range of irregular sea states, are studied. It was found that the performance of the three-float device improved as the sophistication of the PTO control strategy and the level of hydrodynamic interactions taken into account in the control problem increased. From among the different control strategies tested in this work, fully-coordinated global array control (matrix control) was found to maximise the time-averaged power generated by the array. Fully-coordinated control potentially enables wave farm developers and device designers to explore the opportunities of connecting and maximising energy yields from installations that will be necessary to contribute to meeting the 2020 and 2050 targets for offshore renewable energy.

[1]  J. Cruz,et al.  Estimating the loads and energy yield of arrays of wave energy converters under realistic seas , 2010 .

[2]  Peter Stansby,et al.  An Experimental Study of Closely Spaced Point Absorber Arrays , 2008 .

[3]  Enrique Vidal,et al.  Discrete Displacement Hydraulic Power Take-Off System for the Wavestar Wave Energy Converter , 2013 .

[4]  Gordon Lightbody,et al.  Maximisation of Energy Capture by a Wave-Energy Point Absorber using Model Predictive Control , 2011 .

[5]  R. H. Hansen.  Design and Control of the PowerTake-Off System for a Wave Energy Converter with Multiple Absorbers , 2013 .

[6]  John A. Nelder,et al.  A Simplex Method for Function Minimization , 1965, Comput. J..

[7]  M. J. D. Powell,et al.  An efficient method for finding the minimum of a function of several variables without calculating derivatives , 1964, Comput. J..

[8]  M. Tucker,et al.  Numerical simulation of a random sea: a common error and its effect upon wave group statistics , 1984 .

[9]  Tristan Perez,et al.  Time- vs. frequency-domain identification of parametric radiation force models for marine structures at zero speed , 2008 .

[10]  Peter Frigaard,et al.  Wave Generation Theory , 1993 .

[11]  Armin W. Troesch,et al.  The generation of digital random time histories , 1982 .

[12]  K. Budal.  Theory for Absorption of Wave Power by a System of Interacting Bodies , 1977 .

[13]  Ross Henderson,et al.  Design, simulation, and testing of a novel hydraulic power take-off system for the Pelamis wave energy converter , 2006 .

[14]  J. Falnes.  Radiation impedance matrix and optimum power absorption for interacting oscillators in surface waves , 1980 .

[15]  Rico Hjerm Hansen,et al.  Early Performance Assessment of the Electrical Output of Wavestar's prototype , 2012 .

[16]  T. Moan,et al.  Constrained Optimal Control of a Heaving Buoy Wave-Energy Converter , 2011 .

[17]  António F.O. Falcão,et al.  Wave energy utilization: A review of the technologies , 2010 .

[18]  Rico Hjerm Hansen,et al.  Modelling and Control of the Wavestar Prototype , 2011 .

[19]  D. Evans.  Maximum wave-power absorption under motion constraints , 1981 .

[20]  O. Sawodny,et al.  Nonlinear Model Predictive Control of a Point Absorber Wave Energy Converter , 2013, IEEE Transactions on Sustainable Energy.

[21]  R. Yemm,et al.  Pelamis: experience from concept to connection , 2012, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences.

[22]  J. Sneddon.  Sequential simplex optimization , 1992 .

[23]  Ketabdari Mohammad Javad,et al.  SIMULATION OF RANDOM IRREGULAR SEA WAVES FOR NUMERICAL AND PHYSICAL MODELS USING DIGITAL FILTERS , 2009 .

[24]  Jeffrey A. Oskamp,et al.  Power Calculations for a Passively Tuned Point Absorber Wave Energy Converter on the Oregon Coast , 2012 .

[25]  Aurélien Babarit,et al.  Comparison of latching control strategies for a heaving wave energy device in random sea , 2004 .

[26]  A. Clément,et al.  Wave energy in Europe: current status and perspectives , 2002 .

[27]  Markus Mueller,et al.  The UK Energy Research Centre (UKERC) Marine Renewable Energy Technology Roadmap , 2008 .

[28]  Charlotte Beels,et al.  Power absorption by closely spaced point absorbers in constrained conditions , 2010 .

[29]  J. Falnes,et al.  Wave-power absorption by parallel rows of interacting oscillating bodies , 1982 .

[30]  Jeffrey C. Lagarias,et al.  Convergence Properties of the Nelder-Mead Simplex Method in Low Dimensions , 1998, SIAM J. Optim..

[31]  Rico Hjerm Hansen,et al.  Control Performance Assessment and Design of Optimal Control to Harvest Ocean Energy , 2015, IEEE Journal of Oceanic Engineering.

[32]  A. Clément,et al.  Optimal Latching Control of a Wave Energy Device in Regular and Irregular Waves , 2006 .

[33]  J. Ringwood,et al.  Constrained control of arrays of wave energy devices , 2013 .

[34]  Maider Santos,et al.  Design, Construction and Testing of a Hydraulic Power Take-Off for Wave Energy Converters , 2012 .