Wave Energy Converter Test Research Articles

The application of temporal gating in the measurement of response amplitude operators

Scale model testing of wave energy converters (WEC) in wave flumes and basins is essential for full scale development. One of the parameters commonly measured is the response amplitude operator (RAO), which represents the response of the WEC to wave excitation. These are typically measured at discrete frequencies using regular waves. This approach can be slow, depending on the fidelity and frequency resolution. An alternative is to use a broadband wave source, however reflections from walls can significantly contaminate the frequency response measurement. In this paper, a rapid method for measuring the RAO is presented using a chirp signal to generate waves in the frequency range of interest to measure the transfer function, and a temporal gating technique (commonly used in experimental acoustics and RF engineering) to remove reflections. The technique will be demonstrated on numerical data, as well as two scale experiments.

Power output estimation of a two-body hinged raft wave energy converter using HF radar measured representative sea states at Wave Hub in the UK

For the physical model testing of wave energy converters (WECs) in the wave basin, it is necessary to test the models in a small number of sea states. Previously, the H – T binning method was widely used to determine the sea states that are representative of an ocean area. However, it omitted much useful information such as the wave directionality. In this paper, a novel method, the K-means clustering technique is used in combination with High Frequency (HF) radar measured data from Wave Hub, UK. The results show that K-means clustering method better preserves the characteristics of the ocean area than the binning method. Furthermore, the impact of different regrouping methods on assessing the annual energy output of the model is investigated, by applying the K-means clustering method to a 1:25 two-body hinged raft WEC. It is found that although non-linear performance can be clearly observed in the model both physically and numerically. Due to the fact that most sea states from Wave Hub are out of the non-linearity range of the model, the non-linear effect on the overall performance of the WEC model in this ocean area is limited. It allows the annual energy output to be accurately predicted by using only a small number of representative sea states (defined as K) ≤15, based on K-means clustering method.

Field test study and quasi-static analysis of global characteristics and survivability of a wave energy converter test platform mooring system

This study presents a catenary spread mooring system design of a mobile ocean test berth, named the Ocean Sentinel (OS), and developed by the Northwest National Marine Renewable Energy Center to facilitate the ocean testing of wave energy converters (WECs). The original OS mooring design (which was similar to a conventional WEC point absorber mooring system) was evaluated through a field data analysis using a quasi-static approach. The field data analysis on the measurements of mooring tension and environmental conditions collected during an ocean test of the OS is based on a quasi-static analysis of the analytical catenary equations of mooring chains. Both global and survivability characteristics of the mooring system were evaluated. The global characteristics include the influence of the OS excursion on mooring tension, the directional control of the OS, and the environmental contributions of waves, current, and wind. The relationship between the dynamic mooring tension and the environmental conditions of waves and current was quantified through an empirical equation. It was found that the contribution of current to the mooring tension oscillation increased as the current velocity increased and was significant at high current velocity. The survivability characteristics include the anchor movability and strength capacities of mooring lines. Because anchor movement occurred near the end of the field test, a new systematic procedure of designing a mooring system with adequate anchor resistance was developed and applied to improve the OS mooring system design.

The impact of wave energy converter arrays on wave-induced forcing in the surf zone

An alternative metric for assessing nearshore hydrodynamic impact due to Wave Energy Converter (WEC) arrays is presented that is based on the modeled changes in alongshore radiation stress gradients in the lee of the array. The metric is developed using a previously observed relationship between measured radiation stresses and alongshore current magnitudes. Next, a parametric study is conducted using the spectral model SWAN to analyze the nearshore impact of different WEC array designs. A realistic range of array configurations, locations, and incident wave conditions are examined and conditions that generate alongshore radiation stress gradients exceeding a chosen impact threshold on a uniform beach are identified. Finally, the methodology is applied to two permitted WEC test sites to assess the applicability of the results to sites with more realistic bathymetries. For these sites, the overall trends seen in the changes in wave height, direction, and radiation stress gradients in the lee of the array are similar to those seen in the parametric study. However, interactions between the wave field and real bathymetry induce additional alongshore variability in wave-induced forcing. Results indicate that array-induced changes can exceed the natural variability up to 15% of the time with certain array designs and locations.

Simulating Extreme Directional Wave Conditions

Wave tank tests often involve simulating extreme wave conditions as they enable the maximum expected loads to be inferred: a vital parameter for structural design. The definition, and simulation of, extreme conditions are often fairly simplistic, which can result in conditions and associated loads that are not representative of those that would be observed at the deployment location. Here we present a method of defining, simulating at scale, and validating realistic site-specific extreme wave conditions for survival testing of wave energy converters. Bivariate inverse-first order reliability method (I-FORM) environmental contours define extreme pairs of significant wave height and energy period ( H m 0 – T E ), while observed extreme conditions are used to define realistic frequency and directional distributions. These sea states are scaled, simulated and validated at the FloWave Ocean Energy Research Facility to demonstrate that the site-specific extreme wave conditions can be re-created with accuracy. The presented approach enables greater realism to be incorporated into tank testing with survival sea states. The techniques outlined and explored here can provide further and more realistic insight into the response of offshore structures and devices, and can help make important design decisions prior to full-scale deployment.

Physical scale model testing of a flexible membrane wave energy converter: Videogrammetric analysis of membrane operation

In small-scale testing of wave energy converters (WECs), a key focus is on characterising the interdependent relationship between the primary converter and simulated power take-off system. If primary conversion is via the deformation of a flexible material, this task often requires non-contact measurement. In this paper, we introduce the development of an underwater non-contact measurement technique called videogrammetry, and its novel application to characterise the primary converter operation of a flexible membrane WEC. The work was part of Bombora Wave Power’s concept validation wave tank tests at 1:15 scale. Details of the WEC and how it works is followed by an in depth description on applying underwater videogrammetry. A qualitative and quantitative analysis of membrane operation in a regular wave case is provided and discussed in terms of absorbed energy and power production. Two data sets are compared in this analysis. One data set is from videogrammetry and the other is airflow measurement data (airflow induced in the system due to membrane deformation converts wave energy to mechanical energy). This comparison quantifies the accuracy of videogrammetry, and also serves to verify airflow measurements that were used to determine performance indicators of the WEC throughout the entire test campaign. The results compare reasonably well. Sources of uncertainty for videogrammetry are discussed and improvements suggested. Preliminary best practices for applying videogrammetry in wave energy experiments are provided.

Hydrodynamic analysis of a heaving wave energy converter

In the present paper, the development of a model scale Wave Energy Converter (WEC) and an experimental WEC test rig are presented, and results of numerical simulations and experimental measurements are shown. The presented point absorber WEC is coupled to a generic power take-off (PTO) and is restricted to pure heave motion in regular waves. Experiments were carried out at the Universidad Austral de Chile (UACh) Wave Tank and results from responses and efficiencies were compared with data from the BEM (boundary element method) code WAMIT. Numerical and experimental results showed good agreement. Finally, results were extrapolated and superposed with typical wave energy spectra found in different Chilean regions, providing a first performance estimation for a wave energy converter in Chile. Results are discussed and compared with an existing technology and give an insight of the potential for wave energy technologies in Chile. Further investigation is proposed for an analysis in irregular waves and the use of a more advanced PTO (power take-off) in the future.

Power Take-Off Simulation for Scale Model Testing of Wave Energy Converters

Small scale testing in controlled environments is a key stage in the development of potential wave energy conversion technology. Furthermore, it is well known that the physical design and operational quality of the power-take off (PTO) used on the small scale model can have vast effects on the tank testing results. Passive mechanical elements such as friction brakes and air dampers or oil filled dashpots are fraught with nonlinear behaviors such as static friction, temperature dependency, and backlash, the effects of which propagate into the wave energy converter (WEC) power production data, causing very high uncertainty in the extrapolation of the tank test results to the meaningful full ocean scale. The lack of quality in PTO simulators is an identified barrier to the development of WECs worldwide. A solution to this problem is to use actively controlled actuators for PTO simulation on small scale model wave energy converters. This can be done using force (or torque)-controlled feedback systems with suitable instrumentation, enabling the PTO to exert any desired time and/or state dependent reaction force. In this paper, two working experimental PTO simulators on two different wave energy converters are described. The first implementation is on a 1:25 scale self-reacting point absorber wave energy converter with optimum reactive control. The real-time control system, described in detail, is implemented in LabVIEW. The second implementation is on a 1:20 scale single body point absorber under model-predictive control, implemented with a real-time controller in MATLAB/Simulink. Details on the physical hardware, software, and feedback control methods, as well as results, are described for each PTO. Lastly, both sets of real-time control code are to be web-hosted, free for download, modified and used by other researchers and WEC developers.

Bivariate joint distribution modeling of wave climate data using a copula method

The bivariate joint distribution of the significant wave height and the wave period is of great importance in characterizing the wave climate at a wave energy converter test site. In this paper, we ...

Effects of non-vertical linear motions of a hemispherical-float wave energy converter

The hydraulic power-take-off mechanism (HPTO) is one of the most popular methods in wave energy converter (WECs). However, the conventional HPTO with only one direction motion has some drawbacks which limit its power capture capability. This paper proposes an adjustable slope angle wave energy converter (ASAWEC) and investigates the effect of slope angle on the performance of the proposed wave energy converter to find the optimal slope angle with the purpose to increase the power capture capability as well as energy efficiency. A mathematical model of components from a floating buoy to a hydraulic motor was modeled. A small scale WEC test rig was fabricated to verify the power capture capability and efficiency of the proposed system throughout experiments.

Model-prototype similarity of oscillating-water-column wave energy converters

Model testing in wave tanks or under sheltered sea conditions is an essential step in the development of wave energy converters. The paper focuses on the rules for geometric, hydrodynamic, thermodynamic and aerodynamic similarity in model testing of wave energy converters of oscillating-water-column (OWC) type, with emphasis on air compressibility effects in the air chamber and on air turbine aerodynamics. It is shown that the correct volume scale ratio for the air chamber is far from identical to the volume scale ratio for the submerged part of the converter, and should take into account the thermodynamics of the compressible flow through the air turbine or through the turbine simulator (orifice or other). For those cases when the model is large enough to be fitted with a scaled air turbine, dimensional analysis is applied to obtain ratios for turbine size and rotational speed, and also to establish relationships between rotational speed control algorithms. A numerical example is presented to illustrate the importance of appropriately simulating the air compressibility effects when testing at model scale.

Basin Testing of Wave Energy Converters in Trondheim: Investigation of Mooring Loads and Implications for Wider Research

This paper describes the physical model testing of an array of wave energy devices undertaken in the NTNU (Norwegian University of Science and Technology) Trondheim basin between 8 and 20 October 2008 funded under the EU Hydralabs III initiative, and provides an analysis of the extreme mooring loads. Tests were completed at 1/20 scale on a single oscillating water column device and on close-packed arrays of three and five devices following calibration of instrumentation and the wave and current test environment. One wave energy converter (WEC) was fully instrumented with mooring line load cells, optical motion tracker and accelerometers and tested in regular waves, short- and long-crested irregular waves and current. The wave and current test regimes were measured by six wave probes and a current meter. Arrays of three and five similar WECs, with identical mooring systems, were tested under similar environmental loading with partial monitoring of mooring forces and motions. The majority of loads on the mooring lines appeared to be broadly consistent with both logistic and normal distribution; whilst the right tail appeared to conform to the extreme value distribution. Comparison of the loads at different configurations of WEC arrays suggests that the results are broadly consistent with the hypothesis that the mooring loads should differ. In particular; the results from the tests in short crested seas conditions give an indication that peak loads in a multi WEC array may be considerably higher than in 1-WEC configuration. The test campaign has contributed essential data to the development of Simulink™ and Orcaflex™ models of devices, which include mooring system interactions, and data have also been obtained for inter-tank comparisons, studies of scale effects and validation of mooring system numerical models. It is hoped that this paper will help to draw the attention of a wider scientific community to the dataset freely available from the Marintek website.

A Novel Ocean Sentinel Instrumentation Buoy for Wave Energy Testing

AbstractThis paper presents a novel Ocean Sentinel instrumentation buoy that the Northwest National Marine Renewable Energy Center (NNMREC) has developed with AXYS Technologies for the testing of wave energy converters (WECs). NNMREC is a Department of Energy-sponsored partnership among Oregon State University (OSU), the University of Washington (UW), and the National Renewable Energy Laboratory (NREL). The Ocean Sentinel instrumentation buoy is a surface buoy based on the 6-m NOMAD (Navy Oceanographic Meteorological Automatic Device) design. The Ocean Sentinel provides power analysis, data acquisition, and environmental monitoring, as well as an active converter interface to control power dissipation to an onboard electrical load. The WEC being tested and the instrumentation buoy are moored with approximately 125 meters separation; connected by a power and communication umbilical cable. The Ocean Sentinel was completed in 2012 and was deployed for the testing of a WEC at the NNMREC open-ocean test site, north of Newport, OR, during August and September of 2012.