This review focuses on the most suitable form of hydrodynamic modeling for the next generation wave energy converter (WEC) design tools. To design and optimize a WEC, it is estimated that several million hours of operation must be simulated, perhaps one million hours of WEC simulation per year of the R&D program. This level of coverage is possible with linear potential flow (LPF) models, but the fidelity of the physics included is not adequate. Conversely, while Reynolds averaged Navier–Stokes (RANS) type computational fluid dynamics (CFD) solvers provide a high fidelity representation of the physics, the increased computational burden of these models renders the required amount of simulations infeasible. To scope the fast, high fidelity options, the present literature review aims to focus on what CFD theories exist intermediate to LPF and RANS as well as other modeling options that are computationally fast while retaining higher fidelity than LPF.

https://www.mdpi.com/2077-1312/8/1/35/pdf

Efficient Nonlinear Hydrodynamic Models for Wave Energy Converter Design—A Scoping Study

Model testing of an oscillating water column spar-buoy wave energy converter isolated and in array: Motions and mooring forces

CFD modelling of a small–scale fixed multi–chamber OWC device

Multi–chamber oscillating water column wave energy converters and air turbines: A review

Ocean wave energy is a renewable energy which remains too costly for large-scale electricity generation. The oscillating water column (OWC) wave energy converter (WEC) is a promising device-type with a rectifying air turbine and generator which convert alternating airflow induced by the water motion into kinetic energy then into electric energy. Applying control at each stage of energy conversion could increase the electric energy output of the device. As researchers overcome the modeling challenges of OWC, such as the nonlinearities due to air compressibility and power take-off (PTO) dynamics, we can integrate specific control algorithms to test their ability to improve the efficiency of the OWC. Herein, we present a state-space model of an array of OWC WECs restricted to heave motion with nonlinear PTO dynamics. We apply second-order sliding mode control (SMC) which commands a smooth torque signal to a direct-drive generator to maintain a reference turbine angular velocity. Because the algorithm can yield high turbine torques, we investigate a simple feed-forward relation for the control of a valve to limit the turbine airflow and discard mechanical power. We find that implementing the SMC algorithm and valve control can improve electric energy conversion most effectively in less energetic sea states.

Second Order Sliding Mode Control of Oscillating Water Column Wave Energy Converters for Power Improvement

Theoretical, numerical and experimental studies on arrays of ocean wave energy converter are reviewed. The importance of extracting wave power via an array as opposed to individual wave-power machines has long been established. There is ongoing interest in implementing key technologies at commercial scale owing to the recent acceleration in demand for renewable energy. To date, several reviews have been published on the science and technology of harnessing ocean-wave power. However, there have been few reviews of the extensive literature on ocean wave-power arrays. Research into the hydrodynamic modelling of ocean wave-power arrays is analysed. Where ever possible, comparisons are drawn with physical scaled experiments. Some critical knowledge gaps have been found. Specific emphasis has been paid on understanding how the modelling and scaled experiments are likely to be complementary to each other.

A review of hydrodynamic investigations into arrays of ocean wave energy converters

This article presents a method for obtaining the spatial free surface elevation and velocity field for the water surface in a wave flume over a relatively large measurement area for this type of application (approximately 1.5 m × 1.5 m). The technique employs proprietary videogrammetry software to post-process stereo images captured by multiple synchronised machine vision cameras. Dimensional resolution and other limitations are similar to that experienced for particle imaging velocimetry systems (x, y resolution of 2 mm). Imaging of the free surface was enabled by the use of millions of bespoke slightly positively buoyant fluorescent flakes. Ultraviolet light was used as the primary light source to excite the fluorescent flakes. Reflected ultraviolet light was attenuated by a high-pass filter fitted to the cameras so that only the emitted light from the fluorescent flakes was visible. The software was validated using a simple linear translation experiment. An application is demonstrated for the radiated wave field generated from a submerged sinusoidal heaving sphere for two cases: one single and five consecutive oscillations. Results agree with linear wave theory which indicates that the floating flakes had minimal impact on the water surface particle motion at the scale tested. It is, therefore, concluded that spatial measurement of the free surface elevation and velocity using the method presented has good resolution over a large measurement field. The flakes were found to follow the free surface well, but the measurement area is constrained to where the pattern of flakes exists in the image. Hence, application of floating markers is not suitable for experiments with significant outflow/upwelling which would wash away the floating markers from the intended measurement area.

/pdf/application-of-photogrammetry-for-spatial-free-surface-11qir1ey7x.pdf

Application of photogrammetry for spatial free surface elevation and velocity measurement in wave flumes

Preliminary investigation on the use of tank wall reflections to model WEC array effects

The power from array of wave energy converters can be calculated theoretically from semi-analytic hydrodynamic added mass and damping coefficients of various devices (fixed or floating). However, the semi-analytic approach requires the body to be an axi- symmetric in shape. The newly released open source solver NEMOH is based on the Boundary Element Method (BEM); it solves problems of the interactions of bodies with water waves [1]. NEMOH can be used to calculate hydrodynamic coefficients as an alternative to the semi-analytic method. In contrast to the semi analytic technique, one can model a body with arbitrary shape in BEM. In this paper, we compare the performance of NEMOH with the solution obtained by solving the boundary value problem exactly. Similarly to the formulation leading to a BEM, in the semi-analytic method we formulate the problem of water-wave interaction with bodies using linear inviscid wave theory. The unknown velocity potential is then expanded in terms of eigenfunctions in various zones incorporating different boundary conditions in the problem domain. The unknown coefficients from the eigenfunction expansion are then expressed in a matrix by matching the hydrodynamic pressure and velocity across various zones. At this stage, the systems of equations are truncated to a finite number of terms. The number of terms required is obtained by examining the convergence of the solution. We apply NEMOH to an array of two heaving devices. In this case we compare the solution from NEMOH with the formulation of based on multiple scattering. This formulation of multiple scattering is comprised of the wave diffraction and radiation problems separately where formulations have been adopted from Mavrakos and Koumoutsakos [2] and Mavrakos [6] respectively. We also compare the elapsed CPU time with increasing resolution in NEMOH with increasing number of terms in eigenfunction expansion.

https://researchbank.swinburne.edu.au/file/48a974d7-4ea4-4e3b-bb8f-fa6734436e14/1/2016_chowdhury_computation of power from.pdf

Brian Winship

Papers

A review of hydrodynamic investigations into arrays of ocean wave energy converters

Application of photogrammetry for spatial free surface elevation and velocity measurement in wave flumes

Preliminary investigation on the use of tank wall reflections to model WEC array effects

Computation of power from an array of wave energy converters using a boundary element method