The wave energy shadow cast by an offshore wave energy converter array

Plymouth University funded PhD project

Student: Kieran Monk

Director of studies: Dr Daniel Conley

The extraction of energy by an offshore wave energy converter (WEC) array will create a wave energy shadow down wave. The ability to predict the evolution of the wave energy shadow from the re-distribution of wave energy between the offshore array and the coast is of topical interest due to the significant stake-holder concerns about the potential impacts of offshore WEC installations. Waves play an important role in mass transport, assist in mixing and force sediment transport as well as providing a recreational medium for water users.The extraction of wave energy by a WEC array will result in a wave energy deficit down wave. Wave directional spreading will play an important role in re-distributing the remaining forward propagating wave energy so that the maximum reduction in wave energy at any one point, compared to the undisturbed wave field, will reduce with increasing distance from the device. An overtopping type WEC device extracts energy from the system by capturing a volume of water with a positive potential energy. This will create a sharp gradient in the surface elevation along the geometrical shadow line of the device, thus inducing the diffraction effect. A point absorber extracts energy by generating an out of phase radiation wave and its physical presences will also result in diffraction. Both wave diffraction and radiation will cause a portion of the incident planar wave to become radial. This will also act to redistribute wave energy and the net effect of radiation and diffraction on the wave energy shadow is less obvious and has received less attention than the effect of wave directional spreading.Many types of wave propagation models exist (Phase averaged, Mild-slope and Boussinesq) which can be used to predict the wave energy shadow from an offshore WEC array. However, only phase resolving models are capable of accounting for diffraction and radiation correctly. Phase resolving models are computationally expensive and can surfer from instabilities and boundary limitations. It can be hard to find a balance between a domain resolution that is sufficiently small to avoid instabilities but sufficiently large to model a large spatial area in an acceptable time frame and within the memory limitations of the computer. This makes it difficult to assess wave energy installations like the Wavehub project which is situated 25km offshore.

Figure 1 (a-f). The wave shadow down wave of an Overtopping type WEC array for (a) monochromatic unidirectional incident waves (b) spectral unidirectional waves and (c-f) directional spectral waves.

The aim of this research is to develop a computationally efficient, highly stable, analytical solution for predicting the evolution of the wave energy shadow cast by arrays of overtopping type devices, point absorber type devices and for mixed arrays. Ultimately the solution will resolve diffraction, radiation, interference, directional spreading and mild shallow water process.

The solution will be used to assess specific cases studies such as the wave hub project using real buoy data to assess the predicted change in wave climate at the coast and to look specifically at the importance of correctly accounting for wave scattering.

Publications approved and pending

Monk, K., Zou, Q. and Conley, D., Numerical and analytical simulations of wave interference about a single row array of wave energy converters. Proceedings of the 9th European Wave & Tidal Energy

Monk, K. Zou, Q. Conley, D. 2012a. An approximate solution for the wave energy shadow in the lee of an array of overtopping type wave energy converters. Costal engineering

Monk, K., Zou, Q.-P., and D. C. Conley 2012b. Numerical and Analytical Simulations of Wave Interference about a Single Row Array of Wave Energy Converters, Estuary and Coast, under review

Monk, K., Zou, Q-P., and Conley. D. 2012c. The effect of diffraction on the redistribution of wave energy from an overtopping type wave energy converter array, Proceedings of the 33rd International Conference of Coastal Engineerin