Category Archives: Instrumentation

Capacitive sensors for offshore scour monitoring


Panagiotis Michalis, Mohamed Saafi, and Martin Judd – Proceedings of the ICE – Energy, November 2013

Abstract

One of the main challenges in the design and operation of offshore wind turbines arises from the uncertainty about maximum scour depth around their foundations. Scour action can lead to excessive excavation of the surrounding seabed and is being considered as a major risk for offshore wind farm developments. An ability to gather information concerning the evolution of scouring will enable the validation of models derived from laboratory-based studies, the assessment of different engineering designs and the development of improved scour countermeasure techniques. However, real-time scour data are not being collected due to a lack of available instrumentation techniques. This paper proposes a new scour monitoring technology for offshore wind turbine installations. The monitoring system consists of arrays of small capacitive scour probes installed around the foundation structure and linked to a wireless network to enable remote data acquisition. Based on this research, it is concluded that the sensor is capable of exhibiting high sensitivity to scour and sediment deposition processes for common sea floor mediums under different temperature conditions in saline water. The proposed monitoring system has considerable potential for field applications that will contribute to improving the resilience and sustainability of offshore structures.

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Numerical Modeling to Aid in the Structural Health Monitoring of Wave Energy Converters


William Finnegan and Jamie Goggins – Key Engineering Materials, July 2013

Abstract

A vital aspect of ensuring the cost effectiveness of wave energy converters (WECs) is being able to monitor their performance remotely through structural health monitoring, as these devices are deployed in very harsh environments in terms of both accessibility and potential damage to the devices. The WECs are monitored through the use of measuring equipment, which is strategically placed on the device. This measured data is then compared to the output from a numerical model of the WEC under the same ocean wave conditions. Any deviations would suggest that there are problems or issues with the WEC. The development of accurate and effective numerical models is necessary to minimise the number of times the visual, or physical, inspection of a deployed WEC is required. In this paper, a numerical wave tank model is, first, validated by comparing the waves generated to those generated experimentally using the wave flume located at the National University of Ireland, Galway. This model is then extended so it is suitable for generating real ocean waves. A wave record observed at the Atlantic marine energy test site has been replicated in the model to a high level of accuracy. A rectangular floating prism is then introduced into the model in order to explore wave-structure interaction. The dynamic response of the structure is compared to a simple analytical solution and found to be in good agreement.

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Experimental Evaluation of a Mixer-Ejector Marine Hydrokinetic Turbine at Two Open-Water Tidal Energy Test Sites in NH and MA


Matthew Rowell, Martin Wosnik, Jason Barnes, and Jeffrey P. King – Marine Technology Society Journal, August 2013

Abstract

For marine hydrokinetic energy to become viable, it is essential to develop energy conversion devices that are able to extract energy with high efficiency from a wide range of flow conditions and to field test them in an environment similar to the one they are designed to eventually operate in. FloDesign Inc. developed and built a mixer-ejector hydrokinetic turbine (MEHT) that encloses the turbine in a specially designed shroud that promotes wake mixing to enable increased mass flow through the turbine rotor. A scaled version of this turbine was evaluated experimentally, deployed below a purpose-built floating test platform at two open-water tidal energy test sites in New Hampshire and Massachusetts and also in a large cross-section tow tank. State-of-the-art instrumentation was used to measure the tidal energy resource and turbine wake flow velocities, turbine power extraction, test platform loadings, and platform motion induced by sea state. The MEHT was able to generate power from tidal currents over a wide range of conditions, with low-velocity start-up. The mean velocity deficit in the wake downstream of the turbine was found to recover more quickly with increasing levels of free stream turbulence, which has implications for turbine spacing in arrays.

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Near and far field flow disturbances induced by model hydrokinetic turbine: ADV and ADP comparison


V.S. Neary, B. Gunawan, C. Hill, and L.P. Chamorro – Renewable Energy, December, 2013

Abstract

Wake flows downstream of hydrokinetic turbines are characterized by hub and tip vortices, a velocity deficit and an increase in turbulence intensity. Velocity and turbulence recovery in the wakes of individual turbines constrains the density of turbines in an array and limits the amount of energy that can be produced by a turbine farm. However, few hydrokinetic turbine flow recovery studies have been conducted, especially on the far-field flow characteristics. Nor have studies evaluated the accuracy of acoustic Doppler profiler measurements in the wakes of turbines. The present study examines vertical profiles of mean velocity and turbulence, as well as longitudinal profiles of velocity deficit and turbulence levels measured at the symmetry plane of a model three-blade axial flow turbine in a large open channel flow. Mean velocity and turbulence statistics are measured using an acoustic Doppler velocimeter (ADV) and a pulse coherent acoustic Doppler profiler (ADP). ADV and corrected-ADP derived values of mean velocity, turbulence intensity and root-mean-square velocity constitute a well-documented data set that can be used to validate numerical models simulating the effects of hydrokinetic turbine arrays. We found that 80% of the flow recovery occurred about ten diameters downstream from the rotor plane, which suggests that practical values for longitudinal spacing of turbines should be between ten and fifteen diameters. Significant errors observed in mean velocity and turbulence statistics derived from ADP measurements in the near wake region raise concerns on the use of these instruments for such measurements in lab and field studies. Although the cause of some of the errors requires further investigation, we show that errors in turbulence intensity can be successfully corrected with supplemental ADV measurements.

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Marine and Hydrokinetic Technology (MHK) Instrumentation, Measurement, and Computer Modeling Workshop


W. Musial, M. Lawson, and S. Rooney – National Renewable Energy Laboratory Technical Report, February, 2013

Abstract

The Marine and Hydrokinetic Technology (MHK) Instrumentation, Measurement, and Computer Modeling Workshop was hosted by the National Renewable Energy Laboratory (NREL) in Broomfield, Colorado, July 9−10, 2012. The workshop brought together over 60 experts in marine energy technologies to disseminate technical information to the marine energy community, and to collect information to help identify ways in which the development of a commercially viable marine energy industry can be accelerated.
The workshop was comprised of plenary sessions that reviewed the state of the marine energy industry and technical sessions that covered specific topics of relevance. Each session consisted of presentations, followed by facilitated discussions. During the facilitated discussions, the session chairs posed several prepared questions to the presenters and audience to encourage communication and the exchange of ideas between technical experts. Following the workshop, attendees were asked to provide written feedback on their takeaways from the workshop and their best ideas on how to accelerate the pace of marine energy technology development.
The first four sections of this document give a general overview of the workshop format, provide presentation abstracts, supply discussion session notes, and list responses to the post-workshop questions. The final section presents key findings and conclusions from the workshop that suggest what the most pressing MHK technology needs are and how the U.S. Department of Energy (DOE) and national laboratory resources can be utilized to assist the marine energy industry in the most effective manner.

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Synchronized control of cross-flow-water-turbine-based twin towers


M Vallet, I Munteanu, AI Bratcu, S Bacha, D Roye – Renewable Energy, June 2012

Highlights
► A system composed of cross-flow-water-turbine-based twin vertical towers is studied. ► The angular position synchronization of the two towers is ensured by control action. ► This approach is similar to the phase-locked loop techniques used in electronics. ► Real-time experiments on a power hardware-in-the-loop simulator have been carried out. ► The results sustain the reliability and effectiveness of the approach.

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Power calculations for a passively tuned point absorber wave energy converter on the Oregon coast


JA Oskamp, HT Özkan-Haller-Renewable Energy, September 2012

Highlights
► Power calculations were performed for one year of hourly spectral wave data for a non-resonating wave energy converter. ► Passive tuning was considered over time scales of hours to a year. Optimal tuning coefficients varied greatly. ► The annual power output declined only 3% when tuning was performed annually rather than hourly. ► An array of four interacting WECs placed 10 diameters apart produced 5% less annual power than equivalent individual WECs.

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