H. Tuba Özkan-Haller, Merrick C. Haller, J. Cameron McNatt, Aaron Porter and Pukha Lenee-Bluhm
Introduction
The deployment of wave energy converters (WECs) on a commercial scale will necessitate the grouping of devices into arrays to minimize the costs of installation, mooring, maintenance, and electrical cabling for power delivery. The fundamental purpose of WECs is to remove energy from the waves, so they necessarily decrease the wave height in their lee, i.e., they cast a wave shadow. In general, WECs not only capture energy but also redistribute it through the processes of radiation and scattering. The near-field effects of the shadowing and redistribution can have significant implications for the design and performance of WEC arrays (e.g., Beels et al. 2010; Borgarino et al.2012; Nihous 2012; Babarit 2013; de Andrés et al.
2014; Kara2016; Sinha et al.2016). In addition, the far-field effects may extend to the nearshore region (e.g., Millar et al.2007; Beels et al.2010; Palha et al.2010;
Smith et al. 2012), where wave-driven currents and sediment transport are the dominant physical processes and are potentially affected by the offshore WEC array
H. Tuba Özkan-Haller (✉)
College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, USA
e-mail: ozkan@coas.oregonstate.edu M.C. Haller
School of Civil and Construction Engineering, Oregon State University, Corvallis, USA
J. Cameron McNatt
Mocean Energy, Edinburgh, UK A. Porter
Coast and Harbor Engineering, Edmonds, WA, USA P. Lenee-Bluhm
Columbia Power Technologies, Corvallis, OR, USA
© Springer International Publishing AG 2017 71
(Rusu and Guedes Soares2013; Gonzalez-Santamaria et al.2013; Abanades et al.
2014aand 2014b; Mendoza et al.2014; O’Dea et al.2015). Far-field effects may also impact commercial and recreational activities, or ecological processes.
The wave energy industry is still in its nascent stage. Diverse proposed WEC technologies exist, and severalfield testing sites for wave energy technology have been developed around the world. Comprehensive reviews of WEC technologies can be found in Falcão (2010) and Babarit et al. (2012). Point absorber WECs extract wave energy when wave momentum is transferred to the mechanical motions of the device, which is subsequently converted to other forms of energy.
However, the process is not simple—some wave energy is reflected off of the device and additional radiated waves are generated by WEC motion (for reviews of numerical simulation methods, see Li and Yu2012and Day et al.2015). This can lead to a very complex wavefield with short-scale variability in the region of the WEC array (e.g., Chatjigeorgiou2011; Borgarino et al.2012; McNatt et al.2013).
However, the farfield is generally smoothed by the process of wave diffraction and the decay of scattered/radiated waves, and the length scales of variability increase in the farfield.
A large number of WEC array studies are performed analytically or computa-tionally. Computational methods are necessary because they underlie the predictive tools used to design WEC arrays and to estimate the nearshore impacts of wave farms atfield scales. To gain confidence in these computational tools, they need to be validated with experimental data. To date, only a handful of WEC array experiments have been performed in the laboratory, and WEC array data at thefield scale do not yet exist.
Different WEC technologies—including the Salter Duck (Payne et al.2008), the Manchester Bobber (Alexandre et al.2009; Weller et al.2010), the Savonius rotor (Tutar and Veci 2016), oscillating water columns (OWCs; Ashton et al. 2009;
Folley and Whittaker2013; Iturrioz et al.2014), bottom-pitching WECs (Flocard and Finnigan2010), and the wave overtopping device WaveCat (Fernandez et al.
2012)—have recently been tested in the laboratory. The study reported here con-cerns arrays of point absorber WECs (Columbia Power “Manta”); preliminary results were reported by Haller et al. (2011) and Porter et al. (2012).
Much of the previous experimental work was concerned primarily with the energy capture performance of individual WECs and WEC arrays, as opposed to analysis of the wavefield changes induced by WEC arrays. Alexandre et al. (2009) presented observations of induced changes in the wave spectrum in the lee of WEC arrays using a set of three wave gages. Their WEC array experiments were at a small scale (1:67) but involved a substantial number of WECs (5 × 1 and 5 × 2 arrays) and a single-input Bretschneider wave spectrum. Weller et al. (2010) pro-vided additional data from the same facility with an additional WEC array con-figuration (3 × 4). That work focused on WEC power capture and WEC array interaction factors but did not analyze the wavefield changes.
72 H. Tuba Özkan-Haller et al.
Ashton et al. (2009) performed WEC array experiments at a larger scale (1:20) with three different array configurations (1 WEC, 2 × 1 and 3 × 2 WEC arrays).
They used six wave gages spaced within and around the WEC arrays, and they noted differences between the measured WEC power capture and the wave power deficit measured in the WEC array wave shadow. In fact, in the case of the 3 × 2 WEC array, a wave power surplus was observed in the downstream wave gage.
However, this result is likely an effect of their wave gages being in the nearfield of the WEC array where the wavefield is highly variable at short scales; hence, the wave shadow is much harder to resolve and is not representative of the far-field wave shadow. Ashton et al. (2009) also noted difficulties with analyzing monochromatic wave conditions, again due to high spatial variability. Very recently, Stratigaki et al. (2015) presented a database of laboratory observations of the wavefield modification induced by a large 5 × 5 rectilinear array of heaving WECs (“WECwakes” project).
Finally,field data derived from a single WEC field deployment were published by Eriksson et al. (2007) and Waters et al. (2007,2011). The deployment involved a floating buoy of 3 m diameter attached to a linear generator and installed at the 25-m depth near Lysekil, Sweden. These data concern device performance as a function of wave conditions.
Here, we report on a comprehensive set of laboratory tests that analyze the near-and far-field modifications due to the presence of an array of five-point absorber WECs. The set of experiments described here is most similar to the set used by Ashton et al. (2009), in that they were performed with WEC arrays of one/three/five devices (here at the 1:33 scale) under both regular monochromatic and fully directional random wave conditions. The key differences in the present work are a significant increase in the available instrumentation for wave observations and a significantly larger suite of tested wave conditions. Further, this work describes two prediction strategies, the phase-resolving model WAMIT (see WAMIT, Inc.2000) and the phase-averaged Simulating WAves Nearshore (SWAN) model (Booij et al.
1999), and compares them to the wave observations. Results reveal thefidelity of each model and help to frame their appropriate future implementation and uses for WEC array modeling. Additionally, the experimental and model data contribute to the description of the WEC-induced wave field and inform effective WEC array design.
This chapter begins with a review of the laboratory experiments (section“WEC Array Laboratory Experiments”), including details about the wave conditions, WEC devices, instrumentation, and observational strategies for the determination of absorbed wave power. In section“Numerical Modeling,” we discuss the two wave modeling strategies, giving special attention to the wave power extraction formu-lations. Model results and comparisons to wave observations are given in section
“Results.” In section “Discussion,” we discuss further model simulations and their implications for the SWAN model WEC formulation with regard to the capability of simulating wave shadows induced by WEC arrays.
Analyses of Wave Scattering and Absorption Produced by WEC… 73