Flow-driven rotor simulation of vertical axis tidal turbines: A comparison of helical and straight blades

International Journal of Naval Architecture and Ocean Engineering.
2014.
Jun,
6(2):
257-268

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

- Published : June 30, 2014

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Darrieus turbine
;
Helical-bladed
;
Flow-driven rotor simulation
;
Self-starting capability
;
Torque fluctuation
;
Tidal steam generation

INTRODUCTION

The increasing global economy and limitations of fossil fuel availability have encouraged research on renewable energy. Tidal energy is regular, predictable, and available at higher power densities as compared to other weather-dependent renewable resources. Just as in England or Canada, Korea, a leading country of tidal energy generation, has large the resources of tidal energy and has attempted to extract energy with tidal barrages as well as tidal stream generators. An in-situ experiment involving a tidal stream power plant with a helical-bladed Darrieus turbine was carried out at the Uldolmok narrow channel between Jindo islands and Haenam in Korea (
Han et al., 2009
). However, due to economic and social challenges, the commercialization of hydrokinetic tidal power extraction is not yet realized.
There are several types of tidal stream generators, including drag- or lift-type devices as well as horizontal or vertical axis turbines. A Savonius vertical axis turbine is a typical example of a drag-type device, which usually operates at low speeds. The optimal power coefficient normally occurs when the TSR is lower than 1. There have been many attempts to optimize the power coefficient through parameter studies; however, the maximum recorded power coefficient of the Savonius turbine was found to be nearly 20% (
Akwa et al., 2012
;
Menet and Bourabaa, 2004
). Meanwhile, the horizontal-axis turbine (HAT) is known as the most efficient tidal stream generator. In a lab-scale experimental study, the power coefficient achieved was as high as 48% (
Batten et al., 2007
). In order to achieve high efficiency, the HAT needs to be aligned properly with variable stream lines. In comparison, a Darrieus vertical-axis turbine (VAT) can operate in all flow directions, though it tends to exhibit somewhat lower efficiency than HAT. In an experimental study with a free stream velocity of 1.2m/s, the maximum efficiency was 33% with a straight-bladed Darrieus turbine (
Shiono et al., 2002
). A helical-bladed Darrieus turbine achieved a power coefficient of 41.2% in an optimal design study (
Yang and Shu, 2012
).
The computational fluid dynamics (CFD) simulation is frequently used as a numerical approach as an alternate to more expensive experimental studies in order to validate the performances of turbines. Additionally, the blade element momentum theory (BEMT) is a theoretical method that is used for the analysis and design of HATs (
Batten et al., 2008
;
Clarke et al., 2007
), with the results showing good agreement with those of lab-scale experiments in terms of the power coefficients. On the other hand, even if the computational cost is high, research using CFD simulations is conducted through three-dimensional analyses of HATs (
Lee et al., 2012
), as this is a viable means of investigating vortex activities over the surfaces or near the tips of the blades in detail. Meanwhile, for the Darrieus VAT, no current theoretical method perfectly captures its actual performance as compared to detailed CFD simulations (
Dai et al., 2011
;
Islam et al., 2008
;
Jung et al., 2009
). BEMT methods with single- or multiple-stream tube, vortex, and cascades models show improvements in how well they predict the performance of a Darrieus VAT; however, they still exhibits drawbacks. Thus, a CFD simulation becomes a popular tool when used to analyze the performance of a Darrieus VAT (
Carrigan et al., 2012
;
Ghatage and Joshi, 2011
;
Sabaeifard et al., 2012
). Two-dimensional CFD with less computation than three-dimensional CFD is used in the design of sections of Darrieus VATs instead of a theoretical method. For instance, with help of CFD tools, the cambered airfoil was found to improve the self-starting capability of a Darrieus VAT (
Beri and Yao, 2011
). An increase in the number of blades was also proposed to reduce both the torque and RPM fluctuations (
Castelli et al., 2012
). Among these approaches for performance improvements, a helical-bladed Darrieus turbine is considered to be a strong candidate for overcoming the disadvantages of a straight-bladed Darrieus turbine, such as the fluctuation of the torque and the RPM as well as the low self-starting capability (
Shiono et al., 2002
). In order to design the helical-bladed turbine and explore three-dimensional effects such as tip loss, three-dimensional CFD with a high computational cost is mandatory.
The approaches described above involving the use of CFD simulations and the BEMT are typically utilized when the TSR is determined. However, in the actual operating conditions of an experiment, tidal stream turbines begin to rotate from zero angular velocity when the flow speed reaches a sufficient value to rotate them. Afterwards, the TSR of a turbine is determined when a certain load is applied to a turbine in the direction opposite the rotation direction (
Bahaj et al., 2007
). Although the flow speed is stable under real operating conditions, the TSR as well as the torque are known to fluctuate in Darrieus turbines. Therefore, to capture realistic operational characteristics in an experimental study, a flow-driven rotor simulation, in which the body is driven by the flow, is more appropriate than a simulation with a given TSR. In this work, we introduce a flow-driven rotor simulation using FLUENT with a six-DOF solver to estimate the performance of a Darrieus VAT. First, a three-bladed turbine with the NACA 0020 section, as used in a previous study, is studied in order to investigate its basic performance characteristics in 2D CFD simulations. The performance of a helical-bladed turbine is then investigated as compared to a straightbladed turbine by 3D CFD flow-driven rotor simulations to assess the fluctuation and self-starting capability as well as the power coefficient.
NUMERICAL METHODS

- Flow solver

We exploit ANSYS FLUENT, which uses the finite volume method to solve the Navier-Stokes equation as a flow solver. A pressure-based Reynolds-averaged Navier-Stokes (RANS) model is used to compute the flow properties in the unsteady condition. The sliding mesh method is used to transfer fluid media from the inner rotating domain, which contains the turbine, to the outer domain. The shear stress transport (SST)
- Computational turbine model and problem definition

Two-dimensional (2D) computational fluid dynamics (CFD) simulations are frequently used for designing sections of a Darrieus vertical-axis turbine (VAT). Previous work involving 2D CFD simulations (KORDI, 2011) conducted a parametric study of the hydrofoil profile, the number of blades, the solidity, and the diameter. Through this parametric study, an optimal design with three blades was obtained with respect to the power coefficient, fluctuation, and other parameters. The detailed information pertaining to the turbine model is summarized in
Table 1.
The diameter of the turbine was 3m, the sections used were the NACA 0020 with a chord length of 0.4415
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- Flow-driven rotor simulation with a given load

In contrast to a simulation with the TSR given, the rotational speed of the blade (
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RESULTS AND DISCUSSION

- Horizontal-axis turbine (HAT) benchmarking test

First, to benchmark the flow-driven rotor simulation, the experimental data of a HAT with an 80cm diameter in a cavitation tunnel (
Batten et al., 2007
) are used. These are known as some of the best data measured from experiments with a three-blade HAT. A free stream velocity of 1.73
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- 2D simulation

First, a 2D flow-driven rotor simulation of the Darrieus turbine is conducted to ascertain its performances quickly and compare them to those of a simulation with a given TSR. The simulation conditions are summarized in
Table 1.
The mass and moment of inertia of the turbine which made of aluminum, are utilized in the UDF. The torques and RPMs of the Darrieus turbine are shown in
Fig. 4.
Briefly, the figure presents the performance of the turbine under three different load conditions: a free load condition and two applied load conditions with counter torques
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- 3D simulation

- 3D tip effect on power coefficient

Here, the performance of the Darrieus turbine is investigated through two configurations with the same height: a straightbladed and a helical-bladed turbine. Additional information is also shown in
Table 1.
The height of the turbine is 7.2
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Detail information of the targeted turbine and flow condition.

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- Self-starting capability and fluctuation

The helical-bladed turbine is known to have advantages over the straight-bladed turbine in terms of its self-starting capability and the reduced fluctuation of its torque curve, as visually exhibited by the 3D flow-driven rotor simulation shown in
Fig. 9.
Fig. 9(A)
shows the hydrodynamic torque on the rotational axis of the turbines with the RPM in the free load condition. In the beginning, both of the turbines were forced to rotate under high hydrodynamic torque, 32
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CONCLUSION

In this study, flow-driven rotor simulations are conducted in an effort to investigate the operational characteristics of a Darrieus turbine which can be captured in an actual experiment instead of a simulation with a given tip speed ratio (TSR). Specifically, the self-starting capability, fluctuation of the torque and the RPM characteristics while also considering an over-loading condition are clearly demonstrated in a flow-driven rotor simulation. Two-dimensional (2D) computational fluid dynamics (CFD) simulations initially show that the power coefficient predicted from a flow-driven rotor simulation is in very good agreement with the prediction from a simulation with a given TSR. Next, the three-dimensional (3D) effect on the power coefficient of Darrieus turbines is explored in detail by comparing the results from 2D and 3D simulations. Finally, through 3D CFD simulations for an optimal design, the helical-bladed turbine shows prominent advantages over a straight-bladed turbine of the same size, including an improvement in its self-starting capability and the minimization of the fluctuation of the torque levels and RPM while extracting power as well as an increase of its power coefficient from 33% to 42% under the given operating conditions. Eventually, in the design stage of a Darrieus turbine, it is certain that a flow-driven rotor simulation can provide more information than a simulation with a given TSR before expensive experimental work.
Acknowledgements

This research is supported by the project titled “development of active-controlled tidal stream generation” funded from the Ministry of Oceans and Fisheries, Korea (20110171) and by the project titled “development of techniques for improving performance of tidal current power generation system” funded from Korea Institute of Ocean Science and Technology (PE99222).

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Citing 'Flow-driven rotor simulation of vertical axis tidal turbines: A comparison of helical and straight blades
'

@article{ E1JSE6_2014_v6n2_257}
,title={Flow-driven rotor simulation of vertical axis tidal turbines: A comparison of helical and straight blades}
,volume={2}
, url={http://dx.doi.org/10.2478/IJNAOE-2013-0177}, DOI={10.2478/IJNAOE-2013-0177}
, number= {2}
, journal={International Journal of Naval Architecture and Ocean Engineering}
, publisher={The Society of Naval Architects of Korea}
, author={Le, Tuyen Quang
and
Lee, Kwang-Soo
and
Park, Jin-Soon
and
Ko, Jin Hwan}
, year={2014}
, month={Jun}