Pukha Lenee-Bluhm

Analyses of Wave Scattering and Absorption Produced by WEC Arrays: Physical/Numerical Experiments and Model Assessment

Knowledge of the effects of wave energy converters (WECs) on the near and far wave fields is critical to the efficient and low-risk design of waveforms. Several computational wave models enable the evaluation of WEC array effects, but model validation has been limited. In this chapter, we validate two popular models with very different formulations: the phase-resolving model WAMIT and the phase-averaged Simulating WAves Nearshore (SWAN) model. The models are validated against wave data from an extensive set of WEC array laboratory experiments conducted by Oregon State University and Columbia Power Technologies, Inc (CPT). The experimental WECs were 1:33 scale versions of a commercial device (CPT “Manta”), and several different WEC array configurations were subjected to a range of regular waves and random sea states. The wave field in the lee of the WEC arrays was mapped, and the wave shadow was quantified for all sea states. In addition, the WEC power capture performance was measured independently via a motion-tracking system and compared to the observed wave energy deficit (i.e., the wave shadow). Overall, WAMIT displays skill in predicting the wave field both in offshore and in the lee of the WEC arrays. WAMIT simulations demonstrate partial standing wave patterns that are consistent with the observations. These patterns are related to wave scattering processes, and their presence increases the magnitude of the wave shadow in the lee of WECs . The pattern is less pronounced at longer wave periods where WECs behave more like wave followers. In these situations, the wave shadow is primarily controlled by the WEC energy capture and less so by scattering. The SWAN model accounts for the frequency-dependent energy capture of the devices and performs well for cases when the wave shadow is primarily controlled by the WEC energy capture. For regular wave cases, inclusion of the wave diffraction process is necessary, but SWAN simulations for wave fields with frequency and directional spreading capture the general character of the wave shadow even without diffraction. Finally, we suggest that WECs designed to operate such that the expected significant wave energy lies at periods near, or larger than, the period of peak energy extraction will minimize the wave shadow effect for a given gross extraction of wave energy, which leads to more efficient arrays with respect to environmental impact.

Towards a Definition and Metric for the Survivability of Ocean Wave Energy Converters

Survivability is a term that is widely used in the ocean wave energy industry, but the term has never been defined in that context. The word itself seems to have an intrinsic meaning that people understand; this fact often leads to the term’s misuse and its confusion with “reliability”. In order to design systems that are capable of long term survival in the ocean environment, it must be clear what “survivability” means and how it affects the design process and ultimately the device being deployed. Ocean energy is relatively predictable over the span of months, days, and even hours, which makes it very promising as a form of renewable energy. However, the variation of the energy content of ocean waves in a given location is likely high due to the effect of storms and the seasons. Wave energy converters must be built to be reliable while operating and survivable during severe conditions. Therefore, probabilistic design practices must be used to insure reliability and survivability in conditions that are constantly changing. Reliability is used to numerically express the failures of a device that occur while the system is operational, and it is usually expressed in terms of the mean time between failure (MTBF). However, in the context of ocean wave energy converters, the devices are likely to be continuously deployed in conditions that push them beyond their operating limits. During these times it is likely that wave energy converters will be placed in some sort of “survival mode” where the device sheds excess power, reducing system loading. Survivability is focused specifically on failures that occur during these times, when the device is experiencing conditions that surpass its operational limits. Developing a highly survivable wave energy converter is an outstanding goal, but without a standard definition of the term survivability, progress towards that goal cannot be measured. The purpose of this paper is to provide an initial definition for survivability, and to introduce a simple metric that provides an objective comparison of the survivability of varying wave energy converter technologies.Copyright

Adaptive damping power take-off control for a three-body wave energy converter

The performance of the power take-off (PTO) system for a wave energy converter (WEC) depends largely on its control algorithm. This paper presents an adaptive damping control algorithm which improves power capture across a range of sea states. The comparison between this control algorithm and other active control approaches such as linear damping is presented. Short term wave elevation forecasting methods and wave period determination methods are also discussed as pre-requirements for this method. This research is conducted for a novel WEC, developed by Columbia Power Technologies (COLUMBIA POWER). All hydrodynamic models are validated with their 1:7 and 1:33 scale tests.

Direct Drive Wave Energy Buoy

Presentation from the 2011 Water Peer Review in which principal investigator discusses project progress and results for this project which will be used to inform the utility-scale design process, improve cost estimates, accurately forecast energy production and to observe system operation and survivability.

Direct Drive Wave Energy Buoy – 33rd scale experiment

Columbia Power Technologies (ColPwr) and Oregon State University (OSU) jointly conducted a series of tests in the Tsunami Wave Basin (TWB) at the O.H. Hinsdale Wave Research Laboratory (HWRL). These tests were run between November 2010 and February 2011. Models at 33rd scale representing Columbia Power’s Manta series Wave Energy Converter (WEC) were moored in configurations of one, three and five WEC arrays, with both regular waves and irregular seas generated. The primary research interest of ColPwr is the characterization of WEC response. The WEC response will be investigated with respect to power performance, range of motion and generator torque/speed statistics. The experimental results will be used to validate a numerical model. The primary research interests of OSU include an investigation into the effects of the WEC arrays on the near- and far-field wave propagation. This report focuses on the characterization of the response of a single WEC in isolation. To facilitate understanding of the commercial scale WEC, results will be presented as full scale equivalents.