Performance analysis of a HAT tidal current turbine and wake flow characteristics

Having very strong current on the west coast with up to 10 m tidal range, there are many suitable sites for the application of tidal current power (TCP) in Korea. The turbine, which initially converts the tidal energy, is an important component because it affects the efficiency of the entire system. To design a turbine that can extract the maximum power on the site, the depth and duration of current velocity with respect to direction should be considered. To extract a significant quantity of power, a tidal current farm with a multi-arrangement is necessary in the ocean. The interactions between devices contribute significantly to the total power capacity. Thus, the study of wake propagation is necessary to understand the evolution of the wake behind a turbine. This paper introduces configuration design of horizontal axis tidal current turbine based on the blade element theory, and evaluating its performance with CFD. The maximum efficiency of the designed turbine was calculated as 40% at a tip speed ratio (TSR) of 5. The target capacity of 300 kW was generated at the design velocity, and the performance was stable over a wide range of rotating speeds. To investigate the wakes behind the turbine, unsteady simulation was carried out. The wake velocity distribution was obtained, and velocity deficit was calculated. A large and rapid recovery was observed from 2D to 8D downstream, followed by a much slower recovery beyond. The velocity was recovered up to 86% at 18D downstream.     

Influence of blade deformation and yawed inflow on performance of a horizontal axis tidal stream turbine

For a better design of tidal stream turbines operated in off-design conditions, analyses considering the effects of blade deformation and yawed inflow conditions are necessary. The flow load causes deformation of the blade, and the deformation affects the turbine performance in return. Also, a yawed inflow influences the performance of the turbine. As a validation study, a computational fluid dynamics (CFD) simulation was carried out to predict the performance of a horizontal axis tidal stream turbine (HATST) with rigid blades. The numerical uncertainty for the turbine performance with blade deformation and a yawed inflow was evaluated using the concept of the grid convergence index (GCI). A fluid–structure interaction (FSI) analysis was carried out to estimate the performance of a turbine with flexible composite blades, with the results then compared to those of an analysis with rigid blades. The influence of yawed inflow conditions on the turbine performance was investigated and found to be important in relation to power predictions in the design stages.     

Blade sections for wind turbine and tidal current turbine applications–current status and future challenges

The designers of horizontal axis wind turbines and tidal current turbines are increasingly focusing their attention on the design of blade sections appropriate for specific applications. In modern large wind turbines, the blade tip is designed using a thin airfoil for high lift : drag ratio, and the root region is designed using a thick version of the same airfoil for structural support. A high lift to drag ratio is a generally accepted requirement; however, although a reduction in the drag coefficient directly contributes to a higher aerodynamic efficiency, an increase in the lift coefficient does not have a significant contribution to the torque, as it is only a small component of lift that increases the tangential force while the larger component increases the thrust, necessitating an optimization. An airfoil with a curvature close to the leading edge that contributes more to the rotation will be a good choice; however, it is still a challenge to design such an airfoil. The design of special purpose airfoils started with LS and SERI airfoils, which are followed by many series of airfoils, including the new CAS airfoils. After nearly two decades of extensive research, a number of airfoils are available; however, majority of them are thick airfoils as the strength is still a major concern. Many of these still show deterioration in performance with leading edge contamination. Similarly, a change in the freestream turbulence level affects the performance of the blade. A number of active and passive flow control devices have been proposed and tested to improve the performance of blades/turbines. The structural requirements for tidal current turbines tend to lead to thicker sections, particularly near the root, which will cause a higher drag coefficient. A bigger challenge in the design of blades for these turbines is to avoid cavitation (which also leads to thicker sections) and still obtain an acceptably high lift coefficient. Another challenge for the designers is to design blades that give consistent output at varying flow conditions with a simple control system. The performance of a rotating blade may be significantly different from a non‐rotating blade, which requires that the design process should continue till the blade is tested under different operating conditions. Copyright © 2012 John Wiley & Sons, Ltd.     

安装于跨海桥梁下部结构的竖轴潮流能水轮机水动力性能研究

文章采用物理模型实验和基于Fluent的二维数值模拟方法研究了竖轴水轮机安装在典型桥梁下部结构(单圆桩、圆端形墩台、四圆桩)且相对尺寸发生变化时,水轮机水动力性能的变化情况.研究结果表明:与桥梁下部结构组合后,竖轴水轮机的能量输出显著提升,单圆桩提升幅度最大,圆端形墩台次之,四圆桩最小;单圆桩和圆端形墩台相对水轮机尺寸...     

Assessing the influence of inflow turbulence on noise and performance of a tidal turbine using large eddy simulations

Large eddy simulations of a model scale tidal turbine encountering inflow turbulence have been performed. This has allowed both unsteady blade loading and hydrodynamic noise radiation to be predicted. The study is motivated by the need to assess environmental impact of tidal devices, in terms of their acoustic impact on marine species.Inflow turbulence was accounted for using a synthetic turbulence generator, with statistics chosen to represent the gross features of a typical tidal flow. The turbine is resolved in a fully unsteady manner using a sliding interface technique within the OpenFOAM^® libraries. Acoustic radiation is estimated using a compact source approximation of the Ffowcs Williams–Hawkings equation.It is observed that the long streamwise length scale of the inflow turbulence results in characteristic ‘humps’ in the turbine thrust and torque spectra. This effect is also evident in the far-field noise spectra. The acoustic sources on the blades are visualised in terms of sound pressure level and “Powell's source term”. These measures show that the dominant sources are concentrated at the blade leading edges towards the tip. This results from the high loading of the turbine blades, and causes the sound to radiate more akin to a monopole than a dipole.The full scale source level, obtained from scaling of the simulation results, is found to be lower than comparable measured data reported in the literature; this is attributed to additional sources not included in the present study. Based on the predicted source level, no physical impact on fish is expected.     

固定偏角垂直轴潮流能水轮机叶片安装位置试验研究

叶片安装位置是影响固定偏角垂直轴潮流能水轮机水动力性能的关键参数之一,为了研究其对水轮机性能影响的基本规律,建立了垂直轴水轮机水槽模型试验系统,设计了垂直轴水轮机性能和载荷测试方法。通过试验结果的分析,得到了叶片安装位置对水轮机能量利用率、推力系数、侧向力系数和合力系数的影响规律,为水轮机设计提供了试验依据。     

Marine current energy resource assessment and design of a marine current turbine for Fiji

Pacific Island Countries (PICs) have a huge potential for renewable energy to cater for their energy needs. Marine current energy is a reliable and clean energy source. Many marine current streams are available in Fiji's waters and large amount of marine current energy can be extracted using turbines. Horizontal axis marine current turbine (HAMCT) can be used to extract marine current energy to electrical energy for commercial use. For designing a HAMCT, marine current resource assessment needs to done. A potential site was identified and resource assessment was done for 3 months. The coordinates for the location are 18°12′1.78″S and 177°38′58.21″E; this location is called Gun-barrel passage. The average depth is 17.5 m and the width is nearly 20 m – the distance from land to the location is about 500 m. A multi cell aquadopp current profiler (ADCP) was deployed at the site to record marine currents. Strong marine currents are recorded at this location, as a combination of both tidal and rip currents. The maximum current velocity exceeds 2.5 m/s, for days with large waves. The average velocity was 0.85 m/s and power density for the site was 525 W/m². This site has good potential for marine current and HAMCT can be installed to extract power. A turbine with diameter between 5 and 8 m would be suitable for this site. Therefore, a 5 m HAMCT is designed for this location. The HF10XX hydrofoils were used from blade root (r/R = 0.2) to tip (r/R = 1.0). HF10XX series hydrofoil sections were designed to operate at varying turbine operating conditions; these hydrofoils have good hydrodynamic characteristics at the operating Reynolds number. The turbine is designed to operate at rated marine current speed of 1.5 m/s, cut in speed of 0.5 m/s and cut off speed of 3 m/s at a tip speed ratio (TSR) of 4.2.     

On the definition of the power coefficient of tidal current turbines and efficiency of tidal current turbine farms

During the last decade, the development of tidal current industries has experienced a rapid growth. Many devices are being prototyped. For various purposes, investors, industries, government and academics are looking to identify the best device in terms of of cost of energy and performance. However, it is difficult to compare the cost of energy of new devices directly because of uncertainties in the operational and capital costs. It may however be possible to compare the power output of different devices by standardizing the definition of power coefficients. In this paper, we derive a formula to quantify the power coefficient of different devices. Specifically, this formula covers ducted devices, and it suggests that the duct shape should be considered. We also propose a procedure to quantify the efficiency of a tidal current turbine farm by using the power output of the farm where no hydrodynamic interaction exists between turbines, which normalizes a given farm's power output. We also show that the maximum efficiency of a farm can be obtained when the hydrodynamic interaction exists.     

Aeroservoelastic design definition of a 20 MW common research wind turbine model

Wind turbine upscaling is motivated by the fact that larger machines can achieve lower levelized cost of energy. However, there are several fundamental issues with the design of such turbines, and there is little public data available for large wind turbine studies. To address this need, we develop a 20 MW common research wind turbine design that is available to the public. Multidisciplinary design optimization is used to define the aeroservoelastic design of the rotor and tower subject to the following constraints: blade‐tower clearance, structural stresses, modal frequencies, tip‐speed and fatigue damage at several sections of the tower and blade. For the blade, the design variables include blade length, twist and chord distribution, structural thicknesses distribution and rotor speed at the rated. The tower design variables are the height, and the diameter distribution in the vertical direction. For the other components, mass models are employed to capture their dynamic interactions. The associated cost of these components is obtained by using cost models. The design objective is to minimize the levelized cost of energy. The results of this research show the feasibility of a 20 MW wind turbine and provide a model with the corresponding data for wind energy researchers to use in the investigation of different aspects of wind turbine design and upscaling. Copyright © 2016 John Wiley & Sons, Ltd.     

Experimental and numerical studies of blade roughness and fouling on marine current turbine performance

The impact of blade roughness and biofouling on the performance of a two-bladed horizontal axis marine current turbine was investigated experimentally and numerically. A 0.8 m diameter rotor (1/25th scale) with a NACA 63-618 cross section was tested in a towing tank. The torque, thrust and rotational speed were measured in the range 5 < λ < 11 (λ = tip speed ratio). Three different cases were tested: clean blades, artificially fouled blades and roughened blades. The performance of the turbine was predicted using blade element momentum theory and validated using the experimental results. The lift and drag curves necessary for the numerical model were obtained by testing a 2D NACA 63-618 aerofoil in a wind tunnel under clean and roughened conditions. The numerical model predicts the trends that were observed in the experimental data for roughened blades. The artificially fouled blades did not adversely affect turbine performance, as the vast majority of the fouling sheared off. The remaining material improved the performance by delaying stall to higher angles of attack and allowing measurements at lower λ than were attainable using the clean blades. The turbine performance was adversely affected in the case of roughened blades, with the power coefficient (C_P) versus λ curve significantly offset below that for the clean case. The maximum C_P for this condition was 0.34, compared to 0.42 for the clean condition.     

Study on the rotor design method for a small propeller-type wind turbine

Small propeller-type wind turbines have a low Reynolds number, limiting the number of usable airfoil materials. Thus, their design method is not sufficiently established, and their performance is often low. The ultimate goal of this research is to establish high-performance design guidelines and design methods for small propeller-type wind turbines. To that end, we designed two rotors: Rotor A, based on the rotor optimum design method from the blade element momentum theory, and Rotor B, in which the chord length of the tip is extended and the chord length distribution is linearized. We examined performance characteristics and flow fields of the two rotors through wind tunnel experiments and numerical analysis. Our results revealed that the maximum output tip speed ratio of Rotor B shifted lower than that of Rotor A, but the maximum output coefficient increased by approximately 38.7%. Rotors A and B experienced a large-scale separation on the hub side, which extended to the mean in Rotor A. This difference in separation had an impact on the significant decrease in Rotor A’s output compared to the design value and the increase in Rotor B’s output compared to Rotor A.     

Performance enhancement and load reduction of a 5 MW wind turbine blade

A wind turbine rotor blade, based on the U.S. National Renewable Energy Laboratory (NREL) 5 MW reference turbine, is optimized for minimum cost of energy through simultaneous consideration of aerodynamics and bend-twist coupling. Eighty-three total design variables are considered, encompassing airfoil shapes, chord and twist distributions, and the degree of bend-twist coupling in the blade. A recently developed method requiring significantly less computation than finite element analysis is used for planning and predicting the bend-twist coupling behavior of the rotor. Airfoil performance is computed using XFOIL, while the wind turbine loads and performance are computed using the NREL FAST code. The objective function is annual cost of energy (COE), where reductions in flapwise bending loads and blade surface area are assumed to decrease rotor cost through reduced material requirements. The developed optimization process projects decreased blade loads while maintaining wind turbine performance.     

新型竖轴潮流发电支撑结构的力学及模态分析

针对新型竖轴潮流发电支撑结构进行精细化建模,在3种不同工况下对结构进行线性静力分析,并在极端工况下对结构进行几何非线性静力分析,最后对结构整体进行模态分析。研究表明,几何非线性静力分析更接近实际情况,分析得到的最大应力比线性静力分析得到的最大应力略大;需注意杆件交点处或结构不同部位组合连接处,该处结构应力最大;水轮机正常运转时的频率不在结构自振频率范围之内,因而不会发生共振。     

Development and validation of a computational model for design analysis of a novel marine turbine

The vast tidal and wave energy resources represent a potential to use marine energy systems to supply part of the global energy demand. However, there are many advances required to develop economic and reliable marine energy systems, which some of these advances can be achieved by using the existing knowledge and experience from offshore and wind energy industry. This research presents a novel marine energy system that integrates the concept of a vertical and horizontal axis wind turbines by combining a Darrieus and Wells type rotor. However, many other different concepts have been proposed, but models that account for hydrodynamic, structure and control are needed to determine their technical and economical feasibility. With the use of the double‐multiple streamtube theory, a hydrodynamic model is developed to predict the performance and the loads on the turbine blades coupled with a finite element model to compute strains and stresses. To validate the model, we used strain data from field measurement of the demo prototype. The validated model was used to compute extreme stresses and calculate the fatigue life. The model gives reliable estimates of stresses and fatigue life. With this result, the design analysis of the turbine blades can be optimized for any site condition and expected life time. Copyright © 2012 John Wiley & Sons, Ltd.     

Validation of four LES and a vortex model against stereo-PIV measurements in the near wake of an actuator disc and a wind turbine

In this paper we report the results of a workshop organised by the Delft University of Technology in 2014, aiming at the comparison between different state-of-the-art numerical models for the simulation of wind turbine wakes. The chosen benchmark case is a wind tunnel measurement, where stereoscopic Particle Image Velocimetry was employed to obtain the velocity field and turbulence statistics in the near wake of a two-bladed wind turbine model and of a porous disc, which mimics the numerical actuator used in the simulations. Researchers have been invited to simulate the experimental case based on the disc drag coefficient and the inflow characteristics. Four large eddy simulation (LES) codes from different institutions and a vortex model are part of the comparison. The purpose of this benchmark is to validate the numerical predictions of the flow field statistics in the near wake of an actuator disc, a case that is highly relevant for full wind farm applications. The comparison has shown that, despite its extreme simplicity, the vortex model is capable of reproducing the wake expansion and the centreline velocity with very high accuracy. Also all tested LES models are able to predict the velocity deficit in the very near wake well, contrary to what was expected from previous literature. However, the resolved velocity fluctuations in the LES are below the experimentally measured values.     

Experimental study of the turbulence intensity effects on marine current turbines behaviour. Part I: One single turbine

The ambient turbulence intensity in the upstream flow plays a decisive role in the behaviour of horizontal axis marine current turbines.Experimental trials, run in the IFREMER flume tank in Boulogne-Sur-Mer (France) for two different turbulence intensity rates, namely 3% and 15%, are presented. They show, for the studied turbine configuration, that while the wake of the turbine is deeply influenced by the ambient turbulence conditions, its mean performances turn out to be slightly modified.The presented conclusions are crucial in the view of implanting second generation turbines arrays. In addition, complete and detailed data sets (wake profiles and performance graphs) are made available to the scientific community in order to encourage further comparisons.     

Demonstrating that power and instantaneous loads are decoupled in a vertical‐axis wind turbine

The flow in the meridian plane of a high aspect ratio vertical‐axis wind turbine (VAWT) can be described as two dimensional. The wake that is generated by the VAWT in a two‐dimensional flow consists of shed vorticity and is a result of the temporal variation of bound circulation on the blades, following Kelvin's theorem. The strength and location of the vorticity that is produced by the VAWT in a two‐dimensional flow are thus independent of the average bound circulation on the blade. Two independent computational models—a potential flow panel model and a method that is based on the vorticity–velocity formulation of the Navier–Stokes equations—have been used to show that the VAWT can produce the same power for different azimuthal distributions of the blade aerodynamic loading. It is thus demonstrated that the instantaneous blade aerodynamic loading and the power conversion of a VAWT are decoupled. This observation has, potentially, significant impact on the design of the VAWT and reopens the research on asymmetric blade shapes in order to optimize the performance of this turbine configuration. Copyright © 2013 John Wiley & Sons, Ltd.     

Assessment of a comprehensive aeroelastic tool for horizontal‐axis wind turbine rotor analysis

This paper presents the development of a computational aeroelastic tool for the analysis of performance, response and stability of horizontal‐axis wind turbines. A nonlinear beam model for blades structural dynamics is coupled with a state‐space model for unsteady sectional aerodynamic loads, including dynamic stall effects. Several computational fluid dynamics structural dynamics coupling approaches are investigated to take into account rotor wake inflow influence on downwash, all based on a Boundary Element Method for the solution of incompressible, potential, attached flows. Sectional steady aerodynamic coefficients are extended to high angles of attack in order to characterize wind turbine operations in deep stall regimes. The Galerkin method is applied to the resulting aeroelastic differential system. In this context, a novel approach for the spatial integration of additional aerodynamic states, related to wake vorticity and dynamic stall, is introduced and assessed. Steady‐periodic blade responses are evaluated by a harmonic balance approach, whilst a standard eigenproblem is solved for aeroelastic stability analyses. Drawbacks and potentialities of the proposed model are investigated through numerical and experimental comparisons, with particular attention to rotor blades unsteady aerodynamic modelling issues. Copyright © 2016 John Wiley & Sons, Ltd.     

Wind resource assessment and comparative economic analysis using AMOS data on a 30 MW wind farm at Yulchon district in Korea

Detailed feasibility studies are necessary for wind farm development projects because the profitability changes greatly according to wind resource, wind turbine, CAPEX (capital expenduture), OPEX (operation expenditure), SMP (system marginal price), and REC (renewable energy certificate) price. Although measuring wind data over one year in the proposed site is essential, it is a cost-intensive and time-consuming process; hence, in the early stages of development, pre-feasibility studies are conducted using reference wind data from the neighboring areas. In the present study, a pre-feasibility study was conducted in Yulchon district of South Korea to develop a 30-MW wind farm. A wind resource map of Yulchon district was predicted using the AMOS (Aerodome Meteorological Observation System) wind data measured at Yeosu Airport. Three cases of wind farms each with different wind turbines were designed, and comparative economic analysis was carried out. The wind farm designed with SL3000/113 wind turbine recorded the highest profitability with project NPV of 33.62 billion KRW(33.29 million USD, 24.33 million EUR) and project IRR of 9.81%.