Clean and environment-friendly energy are of key interest in the present day as it enables in achieving the Sustainable Development Goals (SDGs) as well as enhances living standards. The Ministry of New and Renewable Energy (MNRE) is working to achieve the Indian Government's target of attaining 175 GW through renewable energy resources. The major resource dominating the present renewable energy mix in India is the solar, followed by wind, biomass, hydro and the least of all, ocean energy despite of having a long coastline of about 7500 km which is estimated to have a total power of 50&nbsp;GW. In comparison with its origin (wind energy), ocean waves are more persistent and highly concentrated as they have a higher energy flux.Among several technologies to harvest wave energy, the point absorber wave energy converter (PAWEC) have attained the stage of technology demonstration and full-scale testing. A PAWEC is a wave-activated body that makes use of the relative motion between a buoyant floater excited by wave forces and a rigid structure which is connected to a generator for power extraction. Their characteristic dimensions being smaller than the incoming wavelength make them highly suitable for energy absorption from multidirectional waves. Nevertheless, PAWECs suffer a narrow energy capture bandwidth in stochastic wave conditions with maximum power absorption centred around the resonant frequency. This makes them a non-profitable investment in the global energy market against other renewable energy devices.To address this challenge and make point absorbers equally competent, an external control technology called the latching is introduced. Latching is a sub-optimal control strategy that aims to tune the system to resonance irrespective of the excitation wave frequency. A direct consequence of latching is the magnification of buoy motion which results in the onset of vortices that manifests as cyclic viscous drag. These viscous forces are highly nonlinear which when neglected results in an unrealistic optimization of output power.In the present work latching is applied numerically to a down-scaled point absorber system which produces a maximum power of 7.12 W when tested under regular wave conditions. The objective is to improve the power as well as the efficiency of the prototype for a range of deterministic wave-states. A hydrodynamic model is developed for the same which is classified as inviscid and viscous models. The hydrodynamic coefficients along with the excitation forces are obtained from Boundary Element Method (BEM) using a potential flow solver to model the linear part. Additionally, for the nonlinear framework the viscous damping coefficient ‘Cd’ is evaluated from the experimental free-decay test. The resultant hydrodynamic model is numerically solved using the fourth-order Runge-Kutta method and the time-domain response is plotted as the output from which power is evaluated.The results are compared with experimental data for an uncontrolled system to emphasize the improvement in wave energy conversion using controls. Results show that the application of latching control can approximately double the energy harvest. Further, it is seen that the viscous dissipations play a great role in altering the optimization criterion which results in a new latching strategy.&nbsp;

Power Optimization of a Point Absorber Wave Energy Converter

Paper Proceedings: https://asmedigitalcollection.asme.org/GTINDIA/GTINDIA2021/volume/85536

ASME 2021 Gas Turbine India Conference (GTINDIA2021)

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