Investigation of the Potential Use of Tidal Current Turbines in the Ocean City, Maryland Inlet for Renewable Energy Generation

There is enormous potential and interest in renewable energy generation from marine tidal currents. Tidal currents have been recognized as a valuable resource for the sustainable generation of electrical power. Tidal currents are particularly attractive for power generation and advantageous when compared to other renewable energies due to their high predictability and fluid properties. The inlet between Ocean City, Maryland and Assateague Island has highly predictable tides and may have potential as a resource for renewable energy generation. In this paper, measurements of the tidal current velocity are made at various locations within the inlet. Measurements are made near the surface due to the energy flux of tidal channels being higher near the surface. The data show that the inlet is a potential candidate for deployment of vertical axis tidal turbines for small-scale renewable energy generation.

Lattice Boltzmann simulations for multiple tidal turbines using actuator line model

In numerical simulations of tidal current farms,large-scale computational fluid dynamic(CFD)simulations with a high-resolution grid are required to calculate the interactions between tidal turbines.In this study,we develop a numerical simulation method for tidal current turbines using the lattice Boltzmann method(LBM),which is suitable for large-scale CFD simulations.Tidal turbines are modeled by using the actuator line(ACL)model,which represents each blade as a group of actuator points in a line.In order to validate our LBM-ACL model,we perform simulations for two interacting tidal turbines,and results of turbine performance are compared with a water tank experiment.The proposed model successfully reproduces the variation of the torque due to wave effects and mean turbine performance.We have demonstrated a large-scale simulation for ten tidal turbines using 8.55×10^(8) grid points and 16 GPUs of Tesla P100 and the simulation has been completed within 9 hours with the LBM performance of 392 MLUPS per GPU.

Design methodology of permanent magnet generators for fixed-pitch tidal turbines with overspeed power limitation strategy

This paper deals with a design methodology of permanent magnets(PM)generators used for fixed-pitch tidal turbines in a marine renewable energy context.In the case of underwater turbines,fixed-pitch tidal turbines could be very attractive and interesting to reduce maintenance operation by avoiding using such a complex electromechanical system for blade-pitching.In this technological case,one of the main control challenges is to ensure power limitation at high tidal current velocities.This control mode can be achieved using the generator flux-weakening.In this context,this paper proposes an original and systemic design methodology to optimize the generator design taking into account the tidal turbine power limitation for high tidal currents velocities.

水流与海浪对HATST功率性能的CFD模拟

为确保涡轮机的长期可靠性,在设计潮流涡轮机时应考虑海浪条件。使用直径为0.9 m的三叶片水平轴潮流涡轮机(Horizontal Axis Tidal Stream Turbine,HATST)作为基准模型,采用计算流体动力学(Computational Fluid Dynamics,CFD)开源软件OpenFOAM预测HATST在海浪和水流条件下的性能。对其进行网格依赖性测试以选择最佳网格数,捕捉流动特征。将生成的波浪轮廓与三阶Stokes波浪理论结果进行比较,发现模拟结果与理论结果一致。研究不同波频和波幅对HATST功率性能的影响,结果表明,波幅对功率性能的影响较为明显,波频的影响相对较小。

The assessment of extactable tidal energy and the effect of tidal energy turbine deployment on the hydrodynamics in Zhoushan

In this study, we construct one 2-dimensional tidal simulation, using an unstructured Finite Volume Coastal Ocean Model （FVCOM）. In the 2-D model, we simulated the tidal turbines through adding additional bottom drag in the element where the tidal turbines reside. The additional bottom drag was calculated from the relationship of the bottom friction dissipation and the rated rotor efficiency of the tidal energy turbine. This study analyzed the effect of the tidal energy turbine to the hydrodynamic environment, and calculated the amount of the extractable tidal energy resource at the Guishan Hangmen Channel, considering the rotor wake effect.

Hashin Failure Theory Based Damage Assessment Methodology of Composite Tidal Turbine Blades and Implications for the Blade Design

A damage assessment methodology based on the Hashin failure theory for glass fiber reinforced polymer(GFRP)composite blade is proposed. The typical failure mechanisms including the fiber tension/compression and matrix tension/compression are considered to describe the damage behaviors. To give the flapwise and edgewise loading along the blade span, the Blade Element Momentum Theory(BEMT) is adopted. In conjunction with the hydrodynamic analysis, the structural analysis of the composite blade is cooperatively performed with the Hashin damage model. The damage characteristics of the composite blade, under normal and extreme operational conditions,are comparatively analyzed. Numerical results demonstrate that the matrix tension damage is the most significant failure mode which occurs in the mid-span of the blade. The blade internal configurations including the box-beam, Ibeam, left-C beam and right-C beam are compared and analyzed. The GFRP and carbon fiber reinforced polymer(CFRP) are considered and combined. Numerical results show that the I-beam is the best structural type. The structural performance of composite tidal turbine blades could be improved by combining the GFRP and CFRP structure considering the damage and cost-effectiveness synthetically.

Research on Pitch Control Strategies of Horizontal Axis Tidal Current Turbine

Based on blade element momentum theory and generator characteristic test,a dynamic simulation model of 150 kW horizontal-axis tidal current turbine was established.The matching of the dynamic characteristics between the turbine and generator under various current velocities is studied,and the influence of the pitch angle on the matching is analyzed.For the problem of maximum power output in case of low current speed and limiting power in high current speed,the relation between optimal pitch angle and output power is analyzed.On the basis of dynamic characteristic analysis,the variable pitch control strategy is developed.The performance of the turbine under various tidal conditions is simulated.The research results show that the designed controller enables the turbine to operate efficiently under the condition of low current speed,and achieve the goal of limited power at high current speed.

Numerical Investigation of Flow Motion and Performance of A Horizontal Axis Tidal Turbine Subjected to A Steady Current

Horizontal axis tidal turbines have attracted more and more attentions nowadays, because of their convenience and low expense in construction and high efficiency in extracting tidal energy. The present study numerically investigates the flow motion and performance of a horizontal axis tidal turbine with a supporting vertical cylinder under steady current. In the numerical model, the continuous equation and incompressible Reynolds-averaged Navier-Stokes equations are solved, and the volume of fluid method is employed to track free surface motion. The RNG k-ε model is adopted to calculate turbulence transport while the fractional area/volume obstacle representation method is used to describe turbine characteristics and movement. The effects of installation elevation of tidal turbine and inlet velocity on the water elevation, and current velocity, rotating speed and resultant force on turbine are discussed. Based on the comparison of the numerical results, a better understanding of flow structure around horizontal axis tidal turbine and turbine performance is achieved.

Performance Analysis of a Vertical Axis Tidal Turbine With Flexible Blades

In this study, a vertical axis tidal turbine with flexible blades is investigated. The focus is on analyzing the effect of flexible airfoils types and blade flexibility on turbine net output power. To this end, five different flexible airfoils （Symmetric and Non-symmetric） are employed. The results show that the use of a thick flexible symmetric airfoil can effectively increase output power compared to that achievable with a conventional rigid blade. Moreover, the use of highly flexible blades, as opposed to less flexible or rigid blades, is not recommended.

Numerical Study of A Generic Tidal Turbine Using BEM Optimization Methods

Three blade-geometry optimization models derived along with assumptions from the blade element momentum(BEM)approach are studied by using a steady BEM code to improve a small horizontal-axis rotor of three blades that has been previously used in experiments.The base rotor blade has linear-radially varying chord length and pitch angle,while the other three models noted as Burton,Implicit and Hansen due to their references and characteristics yield blades of non-linearly varying chord length and pitch angle.The aim is to compare these rapid models and study how assumptions embedded in them affect performance and induction factors.It is found that the model that has the least assumptions(Hansen)and which considers the blade-profile drag in its optimization procedure yields the highest power coefficient,C_(P),at the optimal tip speed ratio(TSR),about 7%higher than the base one and also higher C_(P) at high TSR.It produces an axial induction factor distribution along the blade that is closest to the 1 D optimal value of 1/3.All optimized tangential induction-factor distributions along the blade closely vary as inverse to the square of the radial distance,while being mildly higher than the base distribution.It shows that sufficient swirl is necessary to increase power but at a level causing not too much energy loss in unnecessary swirl of the wake.At high TSR,all optimized rotors adversely produce higher thrust than the base one,but the one with most embedded assumptions(Burton)produces the highest thrust.Details of all three optimization models are given along with the distributions of the power,thrust,blade hydrodynamic efficiency and induction factors.

Numerical study of power production from tidal energy in the Khuran Channel and its feedback on background hydrodynamics

This study focuses on the development of a farm of tidal turbines in the Khuran Channel.The important factors include the location of turbines and their hydrodynamic effects on the environment.A three-dimensional circulation model for the Persian Gulf is employed for the comprehensive evaluation of the tidal energy potential throughout the study area.The model is validated by using in situ observations of water level and current data.The appropriate potential points for extracting the tidal energy were identified in the Persian Gulf using the model results.The Khuran Channel,located in the north of Qeshm Island,was found to be the best place to extract tidal energy inside the Persian Gulf.By adding the term of momentum losses to the governing equations,the feedback of extracting energy on the hydrodynamic around Qeshm Island was studied.The simulation results show that the average daily power production of a tidal farm with 99 turbines for one month is approximately 1.3 MW.This tidal farm also has a significant impact on the water level inside the Khuran Channel,especially near the tidal farm where these fluctuations exceed 4 cm.The change in the current speed caused by wake reaches 0.4 m/s.Wake effects were active up to 7 km downstream of the turbines.The current velocity was also estimated to be 1.6 m/s and 2.1 m/s during the spring and ebb tides within the channel,respectively.

Numerical and Experimental Study of the 3D Effect on Connecting Arm of Vertical Axis Tidal Current Turbine

Vertical axis tidal current turbine is a promising device to extract energy from ocean current. One of the important components of the turbine is the connecting arm, which can bring about a significant effect on the pressure distribution along the span of the turbine blade, herein we call it 3D effect. However, so far the effect is rarely reported in the research, moreover, in numerical simulation. In the present study, a 3D numerical model of the turbine with the connecting arm was developed by using FLUENT software compiling the UDF（User Defined Function） command. The simulation results show that the pressure distribution along the span of blade with the connecting arm model is significantly different from those without the connecting arm. To facilitate the validation of numerical model, the laboratory experiment has been carried out by using three different types of NACA aerofoil connecting arm and circle section connecting arm. And results show that the turbine with NACA0012 connecting arm has the best start-up performance which is 0.346 m/s and the peak point of power conversion coefficient is around 0.33. A further study has been performed and a conclusion is drawn that the aerofoil and thickness of connecting arm are the most important factors on the power conversion coefficient of the vertical axis tidal current turbine.

Counter-Rotating Type Tidal Stream Power Unit Mounted on a Mono-pile

The authors have invented a unique counter-rotating type tidal stream power unit, which is composed of tandem propellers and a double rotational armature type generator without a stator. The front and the rear propellers drive, as for an upstream type, the inner and the outer rotational armatures in the counter-rotating directions respectively, which keep the rotational torques counter-balanced between both propellers and armatures. This paper investigates experimentally the output and forces acting on a pile in a water channel, to get design materials of the mono-pile type tidal stream power unit. The output is maximal at the moderate rotational speed, as the same as a wind turbine. The force acting on the pile is affected by the drag, the Karman vortex and the dynamic balances of the tandem propellers, and has dominant frequencies due to not only the individual but also the interacting rotation of the front and the rear propellers.

Analysis of Turbulent Flow on Tidal Stream Turbine by RANS and BEM

Nowadays,concerns arise because of the depletion of fossil fuel resources that forced scientists to develop new energy extraction methods.One of these renewable resources is tidal energy,where Iran has this potential significantly.There are many ways to obtain the kinetic energy of the fluid flow caused by the moon’s gravitational effect on seas.Using horizontal axis tidal turbines is one of the ways to achieve the kinetic energy of the fluid.Since this type of turbine has similar technology to horizontal axis wind turbines,they may be an appropriate choice for constructing a tidal power plant in Iran.This paper presents the numerical simulation and momentum method of a three-bladed horizontal axis tidal turbine.To validate the thrust and power coefficients for a fixed pitch angle at the blade tip speed ratio of 4 to 10 are compared with experimental results.In this modelling,the rotating geometry simulation has been used.Results show that using a numerical method and blade element momentum,we can predict the horizontal axis tidal turbine’s thrust with an error of less than 10%.The numerical method has better accuracy in higher speed ratios,and it is appropriate to predict the behaviour of fluid in collision with turbines and its wake effects.

Counter-Rotating Type Tidal Stream Power Unit Moored by Only One Cable

The authors have proposed that a counter-rotating type tidal stream power unit mounted rigidly on a pile, and outputs of the power unit and forces acting on the pile were investigated experimentally at a previous paper, A single propeller makes the pile undertake a reaction force orthogonal to the stream direction. On the contrary, proposed counter-rotating propellers do not require undertaking the reaction force of the pile, because the rotational torque is counter-balanced in the unit. This advantage means that the unit can be moored by only one cable. Continuously, this paper proposes such a power unit with tandem propellers, and experimentally investigates a behavior of the unit floating in a water channel. The vibrations of the power unit are induced from not only the individual but also the interacting rotations of the front and the rear propellers.

Actuator line method flow structures and morphology interaction around a monopile-supported tidal stream turbine using the actuator line–Sediment transport coupling simulation

Marine turbines have been extensively utilized to harness tidal stream energy from free-flowing tides and currents. However, the assessment of the influences of these marine structures on the surrounding environment is still in its early stage. In this study, a numerical model that couples hydrodynamics and sediment transport is developed to simulate the scour processes around a monopile-supported horizontal axial tidal stream turbine under steady currents. The flow characteristics are calculated by solving the 3-D Navier-Stokes equations with the k -ω shear stress transport (SST) turbulence model for closure. The simulation of sediment bed elevation is achieved by solving the Exner equation. The turbine rotor is parameterized using the actuator line method. The developed model is validated against wake velocity and scour depth measurement obtained from previous literature, showing a good agreement. Subsequently, the effects of tip clearance on the flow characteristics around the turbine model on a rigid flatbed are examined. Finally, the scour processes of the turbine model are presented, along with the vortex system within the scour hole. The numerical model proposed in this study has the potential to contribute to the understanding of the scour mechanism of the tidal stream turbines.

Comparison of computational fluid dynamics and fluid structure interaction models for the performance prediction of tidal current turbines

CFD models perform rigid body simulations and ignore the hydroelastic behavior of turbine blades.In reality,the tidal turbine blades deform due to the onset flow.Deformation of the turbine blade alters the angle of attack and pressure difference across the low pressure and high pressure surface of the blade.Therefore,the performance of a Tidal Current Turbine(TCT)is modelled in this study using Computational Fluid Dynamic(CFD)and coupled Fluid Structure Interaction(FSI)simulations to compare the predictions of both models.Results of the performance parameters predicted from both the models are also compared with experimental data.The difference between experimental value of C P and predicted value from the rigid blade CFD and FSI models is less than 10%.The FSI model accounted for the blade deformation and a maximum blade tip deflection of 0.12 mm is observed representing a case of small deformation.The extent of deformation is not enough to alter the angle of attack and flow separation behavior at the blade.The variation in predicted pressure difference across the blade surfaces between the two models resulted in different C P prediction.Almost similar wake predictions are obtained from both the models.

Three-dimensional numerical simulation of a vertical axis tidal turbine using the two-way fluid structure interaction approach

The objective of this study was to develop, as well as validate the strongly coupled method (two-way fluid structural interaction (FSI)) used to simulate the transient FSI response of the vertical axis tidal turbine (VATT) rotor, subjected to spatially varying inflow. Moreover, this study examined strategies on improving techniques used for mesh deformation that account for large displacement or deformation calculations. The blade's deformation for each new time step is considered in transient two-way FSI analysis, to make the design more reliable. Usually this is not considered in routine one-way FSI simulations. A rotor with four blades and 4-m diameter was modeled and numerically analyzed. We observed that two-way FSI, utilizing the strongly coupled method, was impossible for a complex model; and thereby using ANSYS-CFX and ANSYS-MECHANICAL in work bench, as given in ANSYS-WORKBENCH, helped case examples 22 and 23, by giving an error when the solution was run. To make the method possible and reduce the computational power, a novel technique was used to transfer the file in ANSYS-APDL to obtain the solution and results. Consequently, the results indicating a two-way transient FSI analysis is a time- and resource-consuming job, but with our proposed technique we can reduce the computational time. The ANSYS STRUCTURAL results also uncover that stresses and deformations have higher values for two-way FSI as compared to one-way FSI. Similarly, fluid flow CFX results for two-way FSI are closer to experimental results as compared to one-way simulation results. Additionally, this study shows that, using the proposed method we can perform coupled simulation with simple multi-node PCs (core i5).

Numerical investigation of modal and fatigue performance of a horizontal axis tidal current turbine using fluid-structure interaction

The tidal power has the potential to play a vital role in a sustainable energy future.The main objective of this paper is to investigate the performance and fatigue life of tidal current turbine(TCT)using fluid structure interaction(FSI)modeling.The performance of TCT was predicted using Ansys CFX.The performance curve,pressure distribution on the blade,and velocity streamline were visualized for eight repetitive analyses at different tip speed ratio.The hydrodynamic load calculated from CFD analysis was transferred to FEA model for investigation of the structural response of TCT.Modal analysis was performed to examine the mode shapes and natural frequencies of TCT.The fatigue analysis were performed and number of cycles and safety factor at different equivalent alternating stresses were investigated.The results of the simulation confirm that the turbine has a maximum value of the coefficient of performance atλ=5,the turbine operating frequency is not close to its natural frequency,and it is safe under the applied fatigue loads with a high factor of safety.

Performance and Internal Flow of a Counter-rotating Type Tidal Stream Turbine

In the past decade, the tidal energies have caused worldwide concern as it can provide regular and predictable re- newable energy resource for power generation. The majority of technologies for exploiting the tidal stream energy are based on the concept of the horizontal axis propellers, which can be derived from the design and operation of wind turbines. However, there are some peculiar features such as the propeller working in the seawater with free surface and the possible occurrence of cavitation as compared with wind turbines. Especially, for a coun- ter-rotating type tidal stream power turbine, it is difficult to accurately predict the interaction between the front and rear blades at the design stage by blade element momentum theory. As a result, CFD shows its advantage to predict the performance of counter-rotating type propellers of the tidal stream turbi^le. In order to improve the accuracy of CFD predictions, the predicted results must be verified with experimental values. In this paper, a CFD model using block-structured grid was set up and experimental test was performed in a water tunnel for a tidal stream turbine with counter-rotating type propellers. The comparison between CFD predictions and experimental data shows quite good agreement on the power coefficients, which provides an evidence of validation of the CFD model. Such results offer the necessary confidence in the accuracy of the set up CFD model for the coun- ter-rotating type tidal stream turbine.