Static and Fatigue Analysis of Composite Turbine Blades Under Random Ocean Current Loading

Fang Zhou, H. Mahfuz, G. Alsenas, H. Hanson – Marine Technology Society Journal, April, 2013

Abstract

The objective of this paper is to investigate how U.S. National Renewable Energy Laboratory (NREL)-designed modeling tools commonly used for wind turbine blade design and analysis can be applied to the design of ocean current turbines (OCT). Design, static analysis, and fatigue life predictions of a horizontal-axis, ocean current turbine composite blade were investigated using NREL’s PreCom, BModes, AeroDyn, FAST with seawater conditions. PreComp was used to compute section properties of this OCT blade. BModes calculated mode shapes and frequencies of the blade. Loading on a turbine blade in the Gulf Stream at a South Florida location (26^o4.3’N 79^o50.5’W, 25-m depth) was calculated with AeroDyn. FAST was then used to obtain the dynamic response of the blade, including flap and edge bending moment distribution with respect to blade rotation. Static analysis was performed by using a combination of Sandia’s NuMAD and ANSYS. Palmgren-Miner’s cumulative fatigue damage model was employed with damage estimation based on the material fatigue property data in DOE/MSU Composite Material Fatigue Database. During service life, OCT blades are subjected to cyclic loads and random ocean current loading. Hence, the blades experience repeated and alternating stresses, which can lead to fatigue failure. These loads were weighted by rate of occurrence from a histogram analysis of in situ measurements conducted by the Southeast National Marine Renewable Energy Center (SNMREC).

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Towards a data assimilation system for morphodynamic modeling: bathymetric data assimilation for wave property estimation

I.D. Garcia, G.E. Serafy, A. Heemink, and H. Schuttelaars – Ocean Dynamics, April, 2013

Abstract

Data assimilation is mainly concerned with the proper management of uncertainties. The main objective of the present work is to implement and analyze a data assimilation technique capable of assimilating bathymetric data into a coupled flow, wave, and morphodynamic model. For the case presented here, wave significant height, wave direction of incidence, and wave peak period are being optimized based on bathymetric data taken from a twin experiment. An adjoint-free variational scheme is used. In this approach, a linear reduced order model (ROM) is constructed as an approximation of the full model. The ROM is an autoregressive model of order 1 (AR1) that preserves the parametrization. Since the ROM is linear, the construction of its adjoint is straightforward, making the implementation of 4D variational data assimilation effortless. The scheme is able to update the morphodynamic model satisfactorily despite the fact that the model shows nonlinear behavior even for very small perturbations of all three parameters. The size and direction of the perturbations necessary for constructing the ROM have a significant impact on the performance of the technique.

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Optimal Constant DC Link Voltage Operation of a Wave Energy Converter

V. Kurupath, R. Ekströmemail, and M. Leijonemail – Energies, April, 2013

Abstract

This article proposes a simple and reliable damping strategy for wave power farm operation of small-scale point-absorber converters. The strategy is based on passive rectification onto a constant DC-link, making it very suitable for grid integration of the farm. A complete model of the system has been developed in Matlab Simulink, and uses real site data as input. The optimal constant DC-voltage is evaluated as a function of the significant wave height and energy period of the waves. The total energy output of the WEC is derived for one year of experimental site data. The energy output is compared for two cases, one where the optimal DC-voltage is determined and held constant at half-hour basis throughout the year, and one where a selected value of the DC-voltage is kept constant throughout the year regardless of sea state.

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Experimental Update of the Overtopping Model Used for the Wave Dragon Wave Energy Converter

S. Parmeggiani, J.P. Kofoed, and E. Friis-Madsen – Energies, April, 2013

Abstract

An overtopping model specifically suited for Wave Dragon is needed in order to improve the reliability of its performance estimates. The model shall be comprehensive of all relevant physical processes that affect overtopping and flexible to adapt to any local conditions and device configuration. An experimental investigation is carried out to update an existing formulation suited for 2D draft-limited, low-crested structures, in order to include the effects on the overtopping flow of the wave steepness, the 3D geometry of Wave Dragon, the wing reflectors, the device motions and the non-rigid connection between platform and reflectors. The study is carried out in four phases, each of them specifically targeted at quantifying one of these effects through a sensitivity analysis and at modeling it through custom-made parameters. These are depending on features of the wave or the device configuration, all of which can be measured in real-time. Instead of using new fitting coefficients, this approach allows a broader applicability of the model beyond the Wave Dragon case, to any overtopping WEC or structure within the range of tested conditions. Predictions reliability of overtopping over Wave Dragon increased, as the updated model allows improved accuracy and precision respect to the former version.

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Experimental Study Related to the Mooring Design for the 1.5 MW Wave Dragon WEC Demonstrator at DanWEC

S. Parmeggiani, J.P. Kofoed, and E. Friis-Madsen – Energies, April, 2013

Abstract

The paper presents the results of an experimental study identifying the response of a 1.5 MW Wave Dragon to extreme conditions typical of the DanWEC test center. The best strategies allowing for a reduction in the extreme mooring tension have also been investigated, showing that this is possible by increasing the surge natural period of the system. The most efficient strategy in doing this is to provide the mooring system with a large horizontal compliance (typically in the order of 100 s), which shall be therefore assumed as design configuration. If this is not possible, it can also be partly achieved by lowering the floating level to a minimum (survivability mode) and by adopting a negative trim position. The adoption of the design configuration would determine in a 100-year storm extreme mooring tensions in the order of 0.9 MN, 65% lower than the worst case experienced in the worst case configuration. At the same time it would lead to a reduction in the extreme motion response, resulting in heave and pitch oscillation heights of 7 m and 19° and surge excursion of 12 m. Future work will numerically identify mooring configurations that could provide the desired compliance.

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Assessing the China Sea wind energy and wave energy resources from 1988 to 2009

C-W Zheng, J. Pan, J-X Li – Ocean Engineering, June, 2013

Abstract

In this study, the wave field in the China Sea was simulated over the period from 1988 to 2009 using the third-generation wave model WAVEWATCH-III (WW3), with Cross-Calibrated, Multi-Platform (CCMP) wind field as the driving field. The China Sea wind energy density and wave energy density were calculated using the CCMP wind and WW3 model simulation results. The China Sea wind energy and wave energy resource were analyzed, synthetically considering the value of energy density, probability of exceedance of energy density level, exploitable wind speed and exploitable significant wave height (SWH), the stability of energy density, total storage and exploitable storage of energy resources, thus providing the guidance for the location of wind and wave power plants. Our results show that most of the China Sea contains abundant wave energy and offshore wind energy resources, with wind energy density above 150 W/m², wave energy density above 2 kW/m, high occurrence of exploitable wind and wave energy in large scale waters, wind energy storage above 2×10³ kW h m⁻², wave energy storage above 4×10⁴ kW h m⁻¹. The richest area is in the northern South China Sea (wind energy density 350–600 W/m², wave energy density 10–16 kW/m, wind energy storage 3×10³–5×10³ kW h m⁻², wave energy storage 8×10⁴–16×10⁴ kW h m⁻¹), followed by southern South China Sea and the East China Sea (wind energy density 150–450 W/m², wave energy density 4–12 kW/m, wind energy storage 2×10³–4×10³ kW h m⁻², wave energy storage 4×10⁴–12×10⁴ kW h m⁻¹). The Yellow Sea and Bohai Sea resources are relatively poorer (wind energy density below 300 W/m², wave energy density below 4 kW/m, wind energy storage below 2.5×10³ kW h m⁻², wave energy storage below 6×10⁴ kW h m⁻¹).

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Optimal causal control of a wave energy converter in a random sea

J.T. Scruggs, S.M. Lattanzio, A.A. Taflanidis, and I.L. Cassidy – Applied Ocean Research, August, 2013

Abstract

This paper concerns the design of feedback control systems to maximize power generation of a wave energy converter (WEC) in a random sea. In the literature on WEC control, most of the proposed feedback controllers fall into three categories. Many are static; i.e., they extract power by imposing an equivalent damping or resistive load on the power take-off (PTO) devices. Others are dynamic and are designed to maximize power generation at all frequencies, which results in an anticausal feedback law. Other dynamic control design methods are causal, and are tuned to achieve the anticausal performance at only a single frequency. By contrast, this paper illustrates that the determination of the true optimal causal dynamic controller for a WEC can be found as the solution to a nonstandard linear quadratic Gaussian (LQG) optimal control problem. The theory assumes that the control system must make power generation decisions based only on present and past measurements of the generator voltages and/or velocities. It is shown that unlike optimal anticausal control, optimal causal control requires knowledge of the stationary spectral characteristics of the random sea state. Additionally, it is shown that the efficiency of the generator factors into the feedback synthesis. The theory is illustrated on a linear dynamical model for a buoy-type WEC with significant resonant modes in surge and pitch, and equipped with three spatially-distributed generators.

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Power collection from wave energy farms

J. Sjolte, G. Tjensvoll, and M. Molinas – Applied Sciences, April, 2013

Abstract

Most Wave Energy Converters (WECs) produce highly distorted power due to the reciprocal motion induced by ocean waves. Some WEC systems have integrated energy storage that overcomes this limitation, but add significant expenses to an already costly system. As an alternative approach, this article investigates the direct export option that relies on aggregate smoothing among several WECs. By optimizing the positioning of the WEC devices with respect to the incoming waves, fluctuations may be mutually canceled out between the devices. This work is based on Fred. Olsen’s WEC system Lifesaver, and a WEC farm consisting of 48 devices is designed in detail and simulated. The major cost driver for the electrical export system is the required oversize factor necessary for transfer of the average power output. Due to the low power quality, this number can be as high as 20 at the entry point of the electrical system, and it is thus crucial to quickly improve the power quality so that the downstream power system is efficiently utilized. The simulations undertaken in this work indicate that a high quality power output can be achieved at the farm level, but that a significant oversize factor will be required in the intermediate power system within the farm.

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The effect of surface waves on the performance characteristics of a model tidal turbine

L. Luznik, K.A. Flack, E.E. Lust, and K. Taylor – Renewable Energy, November, 2013

Abstract

Scale model tests were conducted on a three bladed horizontal axis tidal turbine in a large tow tank facility at the United States Naval Academy. Performance characteristics are presented for a turbine towed at a constant carriage speed for cases with and without surface waves. Intermediate waves were modeled that are representative of swells typically found on the continental shelf of the United States eastern seaboard. The oscillatory wave velocity present in the water column results in significant variations in measured turbine torque and rotational speed as a function of wave phase. This in turn produces cyclic variations in tip speed ratio and power coefficient. The power coefficient over the entire wave phase did not show a difference from the experiments without waves for a range of tip speed ratios, as reported in previous studies. The waves limited the lower range of tip speed ratios at which the turbine could operate. These results highlight the impact of surface waves on turbine design and performance, and the importance of understanding the site-specific wave conditions.

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Flume tank characterization of marine current turbine blade behaviour under current and wave loading

B. Gaurier, P. Davies, A. Deuff, G. Germain – Renewable Energy, November, 2013

Abstract

The long term reliability of tidal turbines is critical if these structures are to be cost-effective. Optimized design requires a combination of material durability models and structural analyses which must be based on realistic loading conditions.

This paper presents results from a series of flume tank measurements on strain gauged scaled turbine blades, aimed at studying these conditions. A detailed series of tests on a 3-blade horizontal axis turbine with 400 mm long blades is presented. The influence of both current and wave-current interactions on measured strains is studied. These tests show that wave-current interactions can cause large additional loading amplitudes compared to currents alone, which must be considered in the fatigue analysis of these systems.

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