Richard W. Kimball

Model Tests for a Floating Wind Turbine on Three Different Floaters

Wind energy is a promising alternate energy resource. However, the on-land wind farms are limited by space, noise, and visual pollution and, therefore, many countries build wind farms near the shore. Until now, most offshore wind farms have been built in relatively shallow water (less than 30 m) with fixed tower type wind turbines. Recently, several countries have planned to move wind farms to deep water offshore locations to find stronger and steadier wind fields as compared to near shore locations. For the wind farms in deeper water, floating platforms have been proposed to support the wind turbine. The model tests described in this paper were performed at MARIN (maritime research institute netherlands) with a model setup corresponding to a 1:50 Froude scaling. The wind turbine was a scaled model of the national renewable energy lab (NREL) 5 MW horizontal axis reference wind turbine supported by three different generic floating platforms: a spar, a semisubmersible, and a tension-leg platform (TLP). The wave environment used in the tests is representative of the offshore in the state of Maine. In order to capture coupling between the floating platform and the wind turbine, the 1st bending mode of the turbine tower was also modeled. The main purpose of the model tests was to generate data on coupled motions and loads between the three floating platforms and the same wind turbine for the operational, design, and survival seas states. The data are to be used for the calibration and improvement of the existing design analysis and performance numerical codes. An additional objective of the model tests was to establish the advantages and disadvantages among the three floating platform concepts on the basis of the test data. The paper gives details of the scaled model wind turbine and floating platforms, the setup configurations, and the instrumentation to measure motions, accelerations, and loads along with the wind turbine rpm, torque, and thrust for the three floating wind turbines. The data and data analysis results are discussed in the work of Goupee et al. (2012, “Experimental Comparison of Three Floating Wind Turbine Concepts,” OMAE 2012-83645).

Experimental Comparison of Three Floating Wind Turbine Concepts

Beyond many of the Earth’s coasts exist a vast deepwater wind resource that can be tapped to provide substantial amounts of clean, renewable energy. However, much of this resource resides in waters deeper than 60 m where current fixed bottom wind turbine technology is no longer economically viable. As a result, many are looking to floating wind turbines as a means of harnessing this deepwater offshore wind resource. The preferred floating platform technology for this application, however, is currently up for debate. To begin the process of assessing the relative advantages of various platform concepts for floating wind turbines, 1/50 th scale model tests in a wind/wave basin were performed at MARIN (Maritime Research Institute Netherlands) of three floating wind turbine concepts. The Froude scaled tests simulated the behavior of the 126 m rotor diameter NREL (National Renewable Energy Lab) 5 MW, horizontal axis Reference Wind Turbine attached via a flexible tower in turn to three distinct platforms, these being a tension leg-platform, a spar-buoy and a semi-submersible. A large number of tests were performed ranging from simple free-decay tests to complex operating conditions with irregular sea states and dynamic winds. The high-quality wind environments, unique to these tests, were realized in the offshore basin via a novel wind machine which exhibited low swirl and turbulence intensity in the flow field. Recorded data from the floating wind turbine models include rotor torque and position, tower top and base forces and moments, mooring line tensions, six-axis platform motions and accelerations at key locations on the nacelle, tower, and platform. A comprehensive overview of the test program, including basic system identification results, is covered in an associated paper in this conference. In this paper, the results of a comprehensive data analysis are presented which illuminate the unique coupled system behavior of the three floating wind turbines subjected to combined wind and wave environments. The relative performance of each of the three systems is discussed with an emphasis placed on global motions, flexible tower dynamics and mooring system response. The results demonstrate the unique advantages and disadvantages of each floating wind turbine platform.

Design and Testing of Scale Model Wind Turbines for Use in Wind/Wave Basin Model Tests of Floating Offshore Wind Turbines

Model basin testing is a standard practice in the design process for offshore floating structures and has recently been applied to floating offshore wind turbines. 1/50th scale model tests performed by the DeepCwind Consortium at Maritime Research Institute Netherlands (MARIN) in 2011 on various platform types were able to capture the global dynamic behavior of commercial scale model floating wind turbine systems; however, due to the severe mismatch in Reynolds number between full scale and model scale, the strictly Froude-scaled, geometrically similar wind turbine underperformed greatly. This required significant modification of test wind speeds to match key wind turbine aerodynamic loads, such as thrust. To execute more representative floating wind turbine model tests, it is desirable to have a model wind turbine that more closely matches the performance of the full scale design.This work compares the wind tunnel performance, under Reynolds numbers corresponding to model test Froude-scale conditions, of an alternative wind turbine designed to emulate the performance of the National Renewable Energy Laboratory (NREL) 5 MW turbine. Along with the test data, the design methodology for creating this wind turbine is presented including the blade element momentum theory design of the performance-matched turbine using the open-source tools WT_Perf and XFoil. In addition, a strictly Froude-scale NREL 5 MW wind turbine design is also tested to provide a basis of comparison for the improved designs. While the improved, performance-matched turbine was designed to more closely match the NREL 5 MW design in performance under low model test Reynolds numbers, it did not maintain geometric similitude in the blade chord and thickness orientations. Other key Froude scaling parameters, such as blade lengths and rotor operational speed, were maintained for the improved designs. The results of this work support the development of protocols for properly designing scale model wind turbines that emulate the full scale design for Froude-scale wind/wave basin tests of floating offshore wind turbines.Copyright

Wind/Wave Basin Verification of a Performance-Matched Scale-Model Wind Turbine on a Floating Offshore Wind Turbine Platform

In 2011, the DeepCwind Consortium performed 1/50th-scale model tests on three offshore floating wind platforms at the Maritime Research Institute Netherlands (MARIN) using a geometrically scaled model of the National Renewable Energy Laboratory (NREL) 5 MW reference turbine. However, due to the severe mismatch in Reynolds number between full scale and model scale, the strictly Froude-scaled, geometrically-similar (geo-sim) wind turbine underperformed greatly, which required significant modification of test wind speeds to match key wind turbine aerodynamic loads, such as thrust. The conclusion from these prior efforts was to abandon a geometrically similar model turbine and use a performance-matched turbine model in its place, keeping mass and inertia properties properly scaled, but utilizing modified blade geometries to achieve required performance at the lower Reynolds numbers of the Froude scaled model. To this end, the University of Maine and MARIN worked in parallel to develop performance-matched turbines designed to emulate the full scale performance of the NREL 5 MW reference turbine at model scale conditions. An overview of this performance-matched wind turbine design methodology is presented and examples of performance-matched turbines are provided.The DeepCwind semi-submersible platform was retested at MARIN in 2013 using the MARIN Stock Wind Turbine (MSWT), which was designed to closely emulate the performance of the original NREL 5 MW turbine. This work compares the wind turbine performance of the MSWT to the previously used geometrically scaled NREL 5 MW turbine. Additionally, turbine performance testing of the 1/50th-scale MSWT was completed at MARIN and a 1/130th-scale model was tested at the University of Maine under Reynolds numbers corresponding to the Froude-scaled model test conditions. Results from these tests are provided to demonstrate effects on model test fidelity. Comparisons of the performance response of the geometrically matched turbine to the performance-matched turbines are also presented to illustrate the performance-matched turbine methodology. Lastly, examples of the fully dynamic floating system performance using the original geometrically scaled NREL 5 MW turbine and the MSWT are investigated to illustrate the implementation of the model test procedure as well as the effects of turbine performance on floater response. Using the procedures employed for the MARIN tests as a guide, the results of this work support the development of protocols for properly designing scale model wind turbines that emulate the full scale design for Froude-scale wind/wave basin tests of floating offshore wind turbines.© 2014 ASME

Unified Rotor Lifting Line Theory

A unified lifting line method for the design and analysis of axial flow propellers and turbines is presented. The method incorporates significant improvements to the classical lifting line methods for propeller design to extend the method to the design of turbines. In addition, lifting line analysis methods are developed to extend the usefulness of the lifting line model to allow generation of performance curves for off-design analysis. The result is a fast computational methodology for the design and analysis of propellers or turbines that can be used in preliminary design and parametric studies. Design and analysis validation cases are presented and compared with experimental data.

Additional Wind/Wave Basin Testing of the DeepCwind Semi-Submersible With a Performance-Matched Wind Turbine

In 2011 the DeepCwind Consortium, led by the University of Maine (UMaine), performed an extensive series of floating wind turbine model tests at the Maritime Research Institute Netherlands (MARIN) offshore basin. These tests, which were conducted at 1/50 scale, investigated the response of three floating wind turbine concepts subjected to simultaneous wind and wave environments. The wind turbine blades utilized for the tests were geometrically-similar models of those found on the National Renewable Energy Laboratory (NREL) 5 MW reference wind turbine and performed poorly in the Froudescaled, low-Reynolds number wind environment. As such, the primary aerodynamic load produced by the wind turbine, thrust, was drastically lower than expected for a given Froude-scaled wind speed. In order to obtain appropriate mean thrust forces for conducting the global performance testing of the floating wind turbines, the winds speeds were substantially raised beyond the target Froude-scale values. While this correction yielded the desired mean thrust load, the sensitivities of the thrust force due to changes in the turbine inflow wind speed, whether due to wind gusts or platform motion, were not necessarily representative of the full-scale system. In hopes of rectifying the wind turbine performance issue for Froude-scale wind/wave basin testing, efforts have been made by UMaine, Maine Maritime Academy and MARIN to design performance-matched wind turbines that produce the correct thrust forces when subjected to Froude-scale wind environments. In this paper, an improved, performancematched wind turbine is mounted to the DeepCwind semisubmersible platform investigated in 2011 (also studied in the International Energy Association’s OC4 Phase II Project) and retested in MARIN’s offshore basin with two major objectives: 1) To demonstrate that the corrective wind speed adjustments made in the earlier DeepCwind tests produced realistic global performance behaviors and 2) To illustrate the increased capability for simulating full-scale floating wind turbine responses that a performance-matched turbine has over the earlier, geometrically-similar design tested. As an example of this last point, this paper presents select results for coupled wind/wave tests with active blade pitch control made possible with the use of a performance-matched wind turbine. The results of this paper show that the earlier DeepCwind tests produced meaningful data; however, this paper also illustrates the immense potential of using a performance-matched wind turbine in wind/wave basin model tests for floating wind turbines. INTRODUCTION In recent years, significant effort has been put forth researching, designing and testing floating wind turbine concepts in hopes of building an industry that can harness the strong, persistent winds off many of the world’s coasts [1-5]. An important part of this effort has been the development of coupled-aero-hydro-elastic floating wind turbine simulation tools [6, 7] and their subsequent validation with wind/wave basin model test data [8, 9]. Model testing of floating wind turbines in a wind/wave basin provides an opportunity to create valuable data in a controlled environment with minimal time, cost and risk. However, while many floating wind turbine model tests have been performed [4, 5, 10-12], it has proven difficult to properly model the aerodynamic loading of the wind turbine in the low-Reynolds number, Froude-scaled environment of a model wind/wave basin [13]. Even in the comprehensive 2011 DeepCwind model tests led by the University of Maine (UMaine) and performed at the Maritime Research Institute Netherlands (MARIN) of tension-leg platform, spar-buoy and semi-submersible floating wind turbines, poor wind turbine performance from the geometrically-similar blade design required increased wind speeds to create the correct mean thrust values [14]. This data has been used extensively for numerical model validation and it is important to understand the extent to which these test shortcomings distort the test data from the true full-scale response. Proceedings of the ASME 2014 33rd International Conference on Ocean, Offshore and Arctic Engineering OMAE2014 June 8-13, 2014, San Francisco, California, USA

Model Tests for a Floating Windturbine on Three Different Floaters

Wind energy is a promising alternate energy resource. However, the on-land wind farms are limited by space, noise, and visual pollution, and therefore many countries build wind farms near shore. Up to now, most of offshore wind farms have been built in relatively shallow water (less than 30m) with fixed tower type wind turbines. Recently, several countries plan to move wind farms to deep water offshore locations to find stronger and steadier wind fields as compared to near shore locations. For the wind farms in deeper water, floating platforms have been proposed to support the wind turbine.The model tests described in this paper were performed at MARIN (Maritime Research Institute Netherlands) with a model set-up corresponding to a 1:50 Froude scaling. The wind turbine was a scaled model of the National Renewable Energy Lab (NREL) 5MW, horizontal axis reference wind turbine supported by three different generic floating platforms: a spar, a semi-submersible and a tension-leg platform (TLP). The wave environment used in the tests is representative of the offshore in the state of Maine. In order to capture coupling between the floating platform and the wind turbine, the 1st bending mode of the turbine tower was also modeled.The main purpose of the model tests was to generate data on coupled motions and loads between the three floating platforms and the same wind turbine for the operational, design, and survival seas states. The data are to be used for calibration and improvement of existing design analysis and performance numerical codes. An additional objective of the model tests was to establish advantages and disadvantages among the three floating platform concepts on the basis of test data.The paper gives details of the scaled model wind turbine and floating platforms, the set-up configurations, and the instrumentation to measure motions, accelerations and loads as well as wind turbine rpm, torque and thrust for the three floating wind turbines. The data and data analysis results are the subject of another paper in this conference [1].Copyright

Modeling and validation of a cross flow turbine using free vortex models and an improved 2D lift model

A number of numerical methods have been developed to predict the performance and aerodynamic loads of the Darrieus turbine. Prior work by Reference [1] using blade element methods (BEM) and free vortex methods (FVM) [2] has produced reasonable models that predict the hydrodynamic performance of the Darrieus turbine. The validated models reasonably estimate the performance at low solidities (Nc/R≪1), but lose accuracy at higher solidity ratios. Dynamic stall and flow curvature has been recognized by [2] [3] and [4] to be significant modeling parameters which have limited the accuracy of prior models. The current numerical model extends the predictions of the FVM model to a higher solidity ratio range. An improved model is presented for the condition of high angles of attack and for dynamic stall,. Experimental data on a series of two (Nc/R≈.9) and four (Nc/R≈1.8) blade configurations are presented as validation of the modified analytical vortex model.

Combustion and emissions of a glycerol-biodiesel emulsion fuel in a medium-speed engine

ABSTRACT Emulsion fuels are one option currently being explored to reduce powerplant and maritime emissions. Emulsification enables hydrophilic, typically low-value, molecules to be incorporated into traditional hydrocarbon fuels. Energetic oxygenated molecules, such as glycerol, are biorenewable and have the potential to reduce refueling costs and carbon emissions. When properly formulated, emulsions improve diesel combustion characteristics and reduce particulate matter (PM) and oxides of nitrogen (NOx). This paper explores the utilisation of glycerol-biodiesel emulsion (23 wt% glycerol) in a one-megawatt, six-cylinder, medium-speed diesel engine at constant speed to determine impacts on combustion dynamics, emissions and overall suitability. Fuel performance is compared to ultra-low-sulphur diesel (ULSD) and 380 cSt. heavy fuel oil (RMG 380). Idle emissions are shown to be comparatively poor due to low combustion stability and longer combustion delay. As load is increased to 25% of peak output, reductions in carbon monoxide (CO) and PM are observed. The lower energy density, however, restricts peak engine power which achieved 90% of full load at maximum fuel consumption. Despite engine maximum power derating, glycerol emulsion fuels require no engine modification and show promise as a powerplant fuel with a low-carbon footprint.

Characterization of a Wind Tunnel for Use in Offshore Wind Turbine Development

In this work a wind tunnel with an open jet configuration is investigated for use in offshore wind turbine testing. This study characterizes the open-jet wind-tunnel using measurements of the velocity field detailing mean velocities and turbulence intensities with and without a scaled wind turbine. Measurements have been taken downstream to evaluate the expected area of turbine operation and the shear zone. The effects on the flow due to the wake and turbine blockage have also been identified. Additionally, the combination of honeycomb and screens necessary to produce a homogeneous flow at the desired velocity with low turbulence intensity has been identified.This work provides a useful data set that will be used as a benchmark to evaluate the benefits of recirculating wind tunnels. The data set has resulted in identifying conditions that would prevent producing the desired flows. The data set has also resulted in recommendations concerning the shape of the wind tunnel sections at the University of Maine’s wind-wave (W2) facility to minimize its interactions with the turbine wake expansion, turbine blockage, and the turbine associated wake shear zone.Copyright