Horizontal Axis Tidal Stream Turbine Research Articles

Experimental and numerical investigations on the performance and wake characteristics of a tidal turbine under yaw

This paper presents an extensive examination of the wake characteristics and hydrodynamic performance of a 1:60 scale horizontal axis tidal stream turbine under yawed inflow conditions. Experimental investigations were conducted in a laboratory flume, while numerical simulations utilized the actuator line method and large-eddy simulation technique. The turbine was subjected to an inflow turbulence intensity of 6% and was tested at five different yaw angles (5°, 10°, 15°, 20°, and 30°). The findings demonstrate a substantial reduction in the power coefficient as the yaw angle increases, with a notable drop of 47.1% recorded at a yaw angle of 30°. Furthermore, numerical simulations of wake characteristics, including mean velocity and turbulence intensity, were examined and compared with measurements, yielding satisfactory agreement. The yawed wake flow exhibited asymmetrical distributions of mean velocity and deviations from the centerline. Additionally, distinct deformations of the yawed wake in the y-z cross sections were observed, indicating a discernible curled kidney shape that intensified with larger yaw angles. Moreover, interactions between the turbine wake and support structures affected the width and turbulence intensity of the near wake, introducing additional unsteadiness that accelerated wake recovery.

An adaptive yaw method of horizontal-axis tidal stream turbines for bidirectional energy capture

Tracking the flow direction is one of the effective ways to harvest more energy for a horizontal-axis tidal stream turbine (HATST). This paper presents a novel HATST design that incorporates a yaw method with a tail wing, inspired by horizontal-axis wind turbines. The design enables the HATST to adaptively yaw toward the bidirectional flow with a reciprocating rotation. To ensure stability, a small yaw angle was reserved and permanent magnets were employed to hold the yaw mechanism steady. The kinetic equations of the yaw mechanism were established, and the influence of the yaw shaft's position and the tail wing's mass on the yaw performance was analyzed. A test model was constructed to validate the design and analysis, and the optimum parameters were discussed. Experimental results demonstrated that the HATST yawed well toward the inflow and maintained the designed yaw angle of approximately 5° at different velocities. Compared with the device without magnetic-assisted yaw, the HATST achieves more than 4.47% increase in output power in the velocity range of 0.15 m/s to 0.31 m/s, and exhibits minimal swing amplitude. This research provides a novel and practical solution for the effective utilization of bidirectional tidal steam energy.

Research on Wake Field Characteristics and Support Structure Interference of Horizontal Axis Tidal Stream Turbine

The harnessing and utilization of tidal current energy have emerged as prominent topics in scientific inquiry, due to their vast untapped resource potential, leading to numerous investigations into the efficacy of hydrokinetic turbines under various operational conditions. This paper delineates the wake field characteristics and performance of horizontal axis tidal stream turbines under the influence of support structures, using a comprehensively blade-resolved computational fluid dynamics (CFDs) model that employs Reynolds-averaged Navier–Stokes (RANS) equations in combination with the RNG k-ε turbulence model. To achieve this, the study utilized experimental tank tests and numerical simulations to investigate the distribution characteristics and recuperative principles of the turbine’s wake field. The velocity distribution and energy augmentation coefficient of the wake field showed strong agreement with the experimental results. To further assess the effect of support structures on the flow field downstream of the unit and its performance, the hydrodynamic attributes of the turbine wake field were analyzed with and without support structures. The interference elicited by the support structure modified the velocity distribution of the near-wake flow field, resulting in a 4.41% decrease in the turbine’s power coefficient (Cp), significantly impacting the turbine’s instantaneous performance.

CFD Simulations of the Effects of Wave and Current on Power Performance of a Horizontal Axis Tidal Stream Turbine

To ensure the long-term reliability of tidal stream turbines, waves and currents must be considered during the design phase. In this paper, a three-bladed horizontal axis turbine with a diameter of 0.9 m was used as the baseline model. OpenFOAM, an open-source computational fluid dynamics (CFD) library platform, was used to predict the performance of a horizontal axis tidal stream turbine (HATST) under waves and currents. A mesh dependency test was carried out to select the optimal mesh to capture the flow’s features. As a validation study, the power of the turbine under only the current was predicted and was found to be consistent with the experimental results. The generated wave profile under a current was compared with the results obtained using the third-order Stokes wave theory. The performance of the HATST was predicted for various wave frequencies and heights and compared with experimental data. The effect of the wave height on the power performance was greater than the wave frequency. Vortical flow structures behind the turbine were investigated for various wave conditions. The generated tip vortices propagated upward and downward at wave crest and trough conditions, respectively.

Experimental investigation on wake and thrust characteristics of a twin-rotor horizontal axis tidal stream turbine

Study on the performance of the tidal stream turbines could greatly improve tidal stream energy development. Up to date, few studies about the twin-rotor horizontal axis tidal turbines (TRHATTs) are available. In this study, laboratory experiments with a range of approaching flow velocity and yaw angles are conducted to investigate the wake structures in the lee side of the turbine. The experimental results show that there are three ‘rod-liked’ wakes behind the turbine, which merge into one wake further downstream. Increase of the approaching flow velocity can accelerate wake velocity recovery and enhances turbulence intensity. Correspondingly, the thrust and the fluctuation of thrust also increase. When the turbine is subjected to the yawed inflow, one side of the near wake mixing accelerates and the turbulence intensity distribution becomes asymmetric. The study shows that the thrust is slightly reduced with an increase of the yaw angle, and reaches the maximum when the yaw angle is 40°. Fluctuation range of the thrust increases with the increase of the yaw angle, and starts to decrease when the yaw angle exceeds 30°.

Power production of the counter-rotating tidal current turbine, including duct effect

ABSTRACT In this paper, a counter-rotating tidal current turbine (CRTCT) with and without duct is investigated by Reynolds-Averaged Navier-Stokes (RANS) equation method. The (Shear Stress Transport) turbulence model is adopted in the steady-state simulation. To validate the accuracy of the numerical simulation, a single horizontal axis tidal stream turbine (HATST) is analysed and the numerical results of the thrust and power coefficients are compared with the experimental data at various tip speed ratios (TSR). It was found that numerical results agree well with the experimental data. The simulation method is extended to the CRTCT with and without duct. It is indicated that the total power coefficient of ducted CRTCT is 33% bigger than that of without duct. Moreover, different numerical results of the power, torque, and thrust coefficients of the rear and front turbines, pressure and velocity contours, and streamlines are presented and discussed.

Updatable Probabilistic Evaluation of Failure Rates of Mechanical Components in Power Take-Off Systems of Tidal Stream Turbines

This paper presents a method for the probabilistic evaluation of the failure rates of mechanical components in a typical power take-off (PTO) system of a horizontal-axis tidal stream turbine (HATT). The method is based on a modification of the method of the influence factors, when base failure rates, relevant influence factors and, subsequently, resulting failure rates are treated as random variables. The prior (i.e., initial) probabilistic distribution of the failure rates of a HATT component is generated using data for similar components from other industries, while taking into account actual characteristics of the component and site-specific operating and environmental conditions of the HATT. A posterior distribution of the failure rate is estimated numerically based on a Bayesian approach as new information about the component performance in an operating HATT becomes available. The posterior distribution is then employed to obtain the updated mean and lower and upper confidence limits of the failure rate. The proposed method is illustrated by applying it to the evaluation of the failure rates of two key components of the PTO system of a typical HATT—main seal and main bearing. In particular, it is shown that uncertainty associated with the method itself has a major influence on the failure rate evaluation. The proposed method is useful for the reliability assessment of both PTO designs of new HATTs and PTO systems of operating HATTs.

Wake structure and mechanical energy transformation induced by a horizontal axis tidal stream turbine

The horizontal axis turbine is one of the most promising energy extractors for tidal stream energy utilization. However, there is no widely accepted model for describing the wake structure of a tidal stream turbine. Besides, the flow velocity variation in wakes is accompanied by energy transformation and has hardly been investigated to date. In this paper, a brief review of existing laboratory tidal stream turbine experiments is first presented, followed by a discussion about the controlling variables that includes cross-comparisons between different results from physical laboratory tests. Theoretical derivation and experimental data analysis are conducted to enable a better understanding of the energy transformation and wake development processes, and tidal stream energy extraction. It has been found that part of the flow kinetic energy has been converted into potential energy before reaching the rotor, commonly used power coefficients are not equivalent when quantifying energy extraction, and it is possible to reconstruct turbine wakes using 3 independent data-based parameters.

Experimental investigation into downstream field of a horizontal axis tidal stream turbine supported by a mono pile

A deep insight into wake of a tidal stream turbine can be essential to make tidal stream energy commercially viable. This study is to experimentally investigate downstream field of a horizontal axis tidal stream turbine supported by a mono pile. Two scenarios with and without a rotor are considered and compared with each other in this study. An advanced Acoustic Doppler Velocimeteris employed to measure the time-varying velocities up to eight rotor diameter downstream. Analyses of results show that the flow field within three rotor diameter downstream is strongly influenced by the supporting structures, which lead to asymmetric recovery trend for velocity, turbulence and Reynolds shear stress. Besides, flow characteristics within various lateral distances away from the rotating axis tend to recover in different rates. Both the rotor rotation and supporting structures result in highly anisotropic turbulence in the downstream field. Bed shear stress in the downstream field nearly doubles due to the rotating rotor, although its distribution pattern does not change.

Characterisation of underwater operational sound of a tidal stream turbine.

The underwater sound emitted during the operation of the Atlantis AR1500 turbine, a 1.5 MW three bladed horizontal axis tidal-stream turbine, was measured in the Pentland Firth, Scotland. Most sound was concentrated in the lower frequencies, ranging from 50 to 1000 Hz. Within 20 m of the turbine, third-octave band sound pressure levels were elevated by up to 40 dB relative to ambient conditions. In comparison, ambient noise at these frequencies fluctuated by about 5-10 dB between different tidal states. At the maximum recording distance of 2300 m from the turbine, median sound pressure levels when the turbine was operational were still over 5 dB higher than ambient noise levels alone. A higher frequency, tonal signal was observed at 20 000 Hz. This signal component appears at a constant level whenever the turbine is operational and did not change with turbine rotation rate. It is most likely produced by the turbine's generator. This study highlights the importance of empirical measurements of turbine underwater sound. It illustrates the utility and challenges of using drifting hydrophone systems to spatially map operational turbine signal levels with reduced flow noise artefacts when recording in high flow environments.

Scour around a mono-pile foundation of a horizontal axis tidal stream turbine under steady current

Seabed scour induced by a pile-supported horizontal axis tidal stream turbine is likely to cause structural instability. Therefore, it's significant to gain an insight into scour process around a tidal stream turbine to make energy harness from tidal stream commercially viable. This study experimentally investigates seabed scour around a horizontal axis tidal stream turbine with a mono-pile foundation. Results of the analyses indicate that the existence of a spinning rotor amplifies scour process around the pile due to flow acceleration between blade tip and seabed. Flow intensity and tip clearance play an important role in effecting the scour process around the turbine model. The scour depth generally increases with increasing of flow intensity and decreasing of tip clearance. When the flow intensity increases or the tip clearance decreases to a certain value, the depth or size of scour hole could increase greatly if the velocity of accelerated flow exceeds the critical velocity of sediment materials. Compared with flow intensity and tip clearance, the impact of distance between rotor and pile seems to be minor. Based on experimental results, empirical equations are proposed for predicting the size of scour holes around the pile foundation of the turbine model.

Flow structures in wake of a pile-supported horizontal axis tidal stream turbine

This study presents results from laboratory experiments to investigate the wake structure in the lee side of a scaled three-bladed horizontal axis tidal stream turbine with a mono-pile support structure. Experiments are conducted for a range of approaching flow velocity and installation height of rotor. Analysis of the results shows that bed shear stress increases with the increase of approaching velocity and decrease of installation height within 2D (D is the diameter of the rotor) downstream of the rotor. The flow field within 2D downstream of the rotor is greatly influenced by the presence of nacelle and mono-pile. Low stream-wise flow velocity and large turbulence intensity level is detected along the flume center right behind the nacelle and mono-pile from 1D to 2D downstream of the rotor. Stream-wise velocity at the blade tip height lower than the nacelle increases sharply from 1D to 2D and gradually grows afterwards. Correspondingly, the turbulence intensity decreases quickly from 1D to 2D and slowly afterwards. Large bed shear stress is measured from 1D to 2D, which is closely related to turbulence induced by the mono-pile. It is also found that the presence of the mono-pile might make the flow field more ‘disc-shaped’.

Numerical models to predict the performance of tidal stream turbines working under off-design conditions

As previously experienced by the wind industry, it is envisaged that tidal stream turbine blades will present misalignments or blade deformations over time as they are constantly working under harsh and highly unsteady environments. Blade misalignment will affect the power capture of a tidal stream turbine and if not detected in time could affect other components of the drive train. Therefore, the aim of this paper is to compare the use of two numerical modelling techniques to predict the performance of a tidal stream turbine working under off-design conditions, in this case, the misalignment of one or more blades. The techniques used in this study are Blade Element Momentum Theory and Computational Fluid Dynamics. The numerical models simulate the performance of a three-bladed horizontal axis tidal stream turbine with one or two blades offset from the optimum pitch setting. The simulations were undertaken at 1.0 m/s flow speeds. The results demonstrated that both unsteady BEMT and steady or transient CFD are able to predict power coefficients when there is a certain level of misalignment in one or even two blades. However, both techniques failed to accurately predict a loss of power performance at high rotational speeds.

Comparison of Actuator Line Method and Full Rotor Geometry Simulations of the Wake Field of a Tidal Stream Turbine

This study aims to investigate the wake characteristics of a horizontal axis tidal stream turbine supported by a monopile using a numerical approach. Computational fluid dynamics (CFD) simulations based on the open source software OpenFOAM have been performed to enhance understanding of a turbine’s wake. The numerical simulations adopt both the actuator line method and the full rotor geometry method. The numerical results are found to be consistent with experimental data, although some discrepancies are observed at a distance of one rotor diameter downstream. Comparison of numerical results from both methods is performed. The results show that both methods can obtain important flow features and provide similar simulation in the wake of the turbine model. The actuator line method is able to give a better prediction in stream-wise velocity distribution, although it underestimates the turbulence intensity, circumferential velocity and vorticity magnitude slightly, compared with the full rotor geometry method. It is also found that the wake of the monopile and the rotor interact strongly in the downstream field, especially in the region immediately behind the structure. A strong interaction occurs within approximately two rotor diameters downstream.

Multi-fidelity optimization of blade thickness parameters for a horizontal axis tidal stream turbine

Cross-sectional geometry of a horizontal axis tidal stream turbine (HATST) blade was optimized using surrogate models and computational fluid dynamics (CFD) analysis. The blade thickness parameters of a 100 kW class HATST model, i.e., relative thickness and maximum relative thickness location, were varied to examine change of turbine performance in terms of power coefficient. Multiple surrogates such as response surface approximation, radial basis function, Kriging and weighted average surrogates were implemented to the CFD analysis results with design parameter variation to search the optimal design. It was found that the Kriging model was suitable for this HATST optimization problem as it produced the smallest cross-validation error and high accuracy. The optimized design enhanced the power coefficient by 17.9%, which shows a way to implement the present approach to tidal stream turbine design and optimization.

Numerical Analysis of the Hydrodynamic Performance of HATST with Different Blade Geometries

This paper is presented to the hydrodynamic performance of horizontal axis tidal stream turbine (HATST) with different blade geometries. The Reynolds-averaged Navier-Stokes (RANS) equations solver of the Star-CCM+ software is employed with a realizable k-e turbulent model. The turbine has three blades and different blade NACA sections. The numerical results of pressure distribution, power and thrust coefficients (CP and CT) are presented and discussed at various tip speed ratios (TSR). It is found that the maximum power coefficients are obtained around 0.37 to 0.41 at TSR between 4 and 5. The results of thrust coefficients are found that 6418 is more than other sections.

Impact of Environmental Turbulence on the Performance and Loadings of a Tidal Stream Turbine

A large-eddy simulation (LES) of a laboratory-scale horizontal axis tidal stream turbine operating over an irregular bathymetry in the form of dunes is performed. The Reynolds number based on the approach velocity and the chord length of the turbine blades is approximately 60,000. The simulated turbine is a 1:30 scale model of a full-scale prototype and both turbines operate at very similar tip-speed ratio of λ ≈ 3. The simulations provide quantitative evidence of the effect of seabed-induced turbulence on the instantaneous performance and structural loadings of the turbine revealing how large-scale, energetic turbulence structures affect turbine performance and bending moments of the rotor blades. The data analysis shows that wake recovery is notably enhanced in comparison to the same turbine operating above a flat-bed and this is due to the higher turbulence levels generated by the dune. The results demonstrate the need for studying in detail the flow and turbulence characteristics at potential tidal turbine deployment sites and to incorporate observed large-scale velocity and pressure fluctuations into the structural design of the turbines.

Influence of upstream turbulence on the wake characteristics of a tidal stream turbine

Abstract The influence of the upstream turbulence intensity on the flow characteristics downstream of a laboratory-scale horizontal axis tidal stream turbine is investigated in this study. Three test cases with the same mean velocity and different turbulence intensities are simulated numerically using the hybrid large eddy simulation/actuator line modelling technique. The mean velocity components, mean turbulent fluctuations, velocity deficit and wake extension are compared along the streamwise direction to examine the upstream turbulence effects. The inflow conditions are generated by the mapping method using the mean velocity and turbulent profiles experimentally obtained for a turbulent open channel flow. Comparing results for the mean velocity and turbulent fluctuations shows that the upstream turbulence level strongly affects the flow characteristics downstream of the turbine by influencing the tip vortices breakdown process and in turn wake recovery. The comparison also reveals that the ambient turbulence level strongly influences the velocity deficit but it does not significantly affect the streamwise velocity and the radial location of tip vortices in the flow.

Hydrodynamic performance of the horizontal axis tidal stream turbine using RANS solver

Hydrodynamic performance of the horizontal axis tidal stream turbine using RANS solver

Design and hydrodynamic analysis of horizontal axis tidal stream turbines with winglets

Three horizontal axis tidal stream turbines (HATST) with winglets are proposed and investigated to enhance the energy conversion efficiency. Based on the conventional tidal turbine, three different winglets including trapezoid, triangle and blended types are equipped on the tip of the blade. The power coefficient, the thrust coefficient, the pressure distribution and the tip vortex are analyzed and compared to investigate the effect of the winglets on the hydrodynamic performance of HATST. Numerical simulation results show that the hydrodynamic model (Song et al., 2012) agrees well with the experimental test. At the optimal tip speed ratio (TSR = 5), the proposed tidal turbines could improve the energy conversion efficiency. The best hydrodynamic performance is demonstrated for the HATST with the triangle winglet, which reduces the intensity of the tip vortex and improves the energy conversion efficiency at all TSRs. The triangle tip turbine performs 4.34% increment in power coefficient and 3.97% increment in thrust coefficient at the optimal TSR.