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Operating conditions leading to crack propagation in turbine blades of tidal barrages. Influence of head and operating mode
Abstract
Tidal energy systems and specifically tidal barrages are renewable energy systems, that use the potential energy of tides to produce electricity. Actually, there are few tidal power stations operating,
although recent studies show the potentiality of installing new units in a recent future due to the constant improvement of the technology related. As in other conventional hydropower systems, the runner is one of the critical components of the unit as it has to withstand an extreme wide range of relative head and two main operating modes (ebb and flood). Therefore, fatigue problems and undesirable failures are more likely to occur. Based on the analysis of a crack found
in one of the blades of a prototype, this paper analyzes and discusses under which operating conditions the runner is more likely to suffer from fatigue problems. CFD and FEM simulation models have been used to determine the stress hotspots, which approximate very well the point where the real crack was initiated. Based on a probabilistic approach of an existing initial defect or flaw, the reliability of the blades, or the probability of not growing a crack and therefore last
an infinite life, has been calculated for a wide range of operating conditions. While fatigue problems have been deeply discussed and analyzed in conventional turbines, the flow excitation characteristics in these units exhibit important differences and therefore the discussion and results provided in this paper could be useful for future designs and fatigue analyses of tidal turbines.
... 1. When the crack opens, the surface contact force of the breathing crack in the local coordinate system can be expressed as(Luo et al., 2019) ...
... For instance, rotating vortex ropes and vanless space vortex structures [1][2][3][4][5] threaten the safe operation of hydraulic turbines. And large relative variations in head [6], free surface vortices or upstream and downstream water level fluctuations [7,8] compromise the operation of marine and runof-river turbines. In order to minimize the risks of these phenomena, it is of paramount importance to determine the modal responses, i.e., the natural frequencies, mode shapes, and damping ratios, of the machines. ...
The operation of submerged rotating machines, such as marine current or tidal turbines, can present deleterious fluid phenomena that may provoke extreme structural vibrations. To predict their dynamic responses, it is necessary to know the added modal coefficients of their runners under a whirling motion. For that purpose, a bespoke test rig was designed to investigate the added modal coefficients of a submerged cylinder, which could rotate at different speeds both in air and completely submerged in water inside a cylindrical tank. First, the modes of vibration were experimentally measured by exciting the cylinder with a push-release method during steady tests or with ramps in rotating speed during transient tests. The calculated natural frequencies and damping ratios were then used in a mathematical model of the dynamic system to calculate the added modal coefficients. During steady tests, the natural frequencies and damping ratios of the whirling modes changed significantly as a function of the rotating speed. Additionally, a whirling mode was observed to change its direction at a given rotating speed. During transient tests, rotating speed ramps with high accelerations were found to present lower lock-in amplitude and frequencies. Moreover, fast downward ramps presented lock-in amplitudes four times higher than fast upward ramps. Consequently, the added modal coefficients changed accordingly as a function of the rotating speed, ramp acceleration, and ramp direction. For these reasons, it was confirmed that the modal responses of submerged rotating bodies must be calculated for each operational rotating speed, even at low velocities, and for each transient event in order to precisely predict their vibration behaviors.
... In recent decades, computational fluid dynamics (CFD) has become one of the mainstream effective means by scholars to study the flow of hydraulic turbomachinery such as propellers, pumps, and turbines. [1][2][3][4] CFD is more cost-effective, requires shorter preparation periods, and can capture more flow details than the experimental methods. 5,6 Nevertheless, there is still room for improvement in techniques to simulate the flow field of turbomachinery with CFD. 7 Hydraulic turbomachinery realizes the transformation of energy by utilizing the rotation of the impeller. ...
Traditional numerical simulation techniques, such as sliding mesh, dynamic mesh, and others, have many limitations in dealing with flow simulation with the large-scale movement of solid boundaries, which is the case for simulating the flow of complex-shaped hydraulic turbomachinery such as propellers, pumps, and turbines. The immersed boundary (IB) method provides a new approach to solve the above-mentioned limitations. Therefore, this study proposes a sharp-interface IB method based on the level-set function that is suitable for simulating the flow through turbomachinery with complex geometries. This method is applied to actual three-dimensional numerical simulations of high-Reynolds number propellers using an in-house computational fluid dynamics solver. The results show that the proposed method can provide comparatively accurate predictions of unsteady load coefficients within the propeller flow passage and capture the correct propeller wake characteristics as well as the interaction between the propeller wake and free surface. This study is aimed at providing a theoretical basis and engineering reference for the application of the IB method in engineering numerical simulations.
... However, the analysis on the dynamic behavior of the control mechanism needs to establish the more detailed model, i.e., the multibody dynamics [21,22] combined with the FSI method [22]. In addition, the dynamic behavior of the hydraulic turbine is closely related to the operating condition, and the dynamic stress will be significantly greater in some operating conditions than other operating conditions [23]. ...
The control mechanism of the Kaplan turbine is quite complex, and many of the failure cases of the control mechanism have been reported. The present work analyzed a failure case of the blade lever based on the multibody mechanics and fluid-structure interaction method. The flow characteristics in 49 operating conditions are simulated, and the pressure on the runner surface is applied on the FEM model. A dynamic stress isoline is proposed to carry out an in-depth analysis of the failure reason of the blade lever in full operating conditions. The fracture of the blade lever could be attributed to long time operation in high dynamic stress region, which leads to the accelerated fatigue damage. Results in this paper could be helpful for the design, operation and maintenance of the Kaplan turbine.
... Although people have been improving the hydraulic performance of the tidal turbine by the flow optimization, some engineering failure cases (Luo et al., 2020;Mom č ilovi ć et al., 2012;Urquiza et al., 2014;Zhang et al., 2020) implied that well designed structure can avoid crucial defects such as excessive stress concentration and resonance. The development of the FSI technology makes it more convenient and reliable for evaluating the reliability of the structure. ...
As a type of reliable renewable energy, tidal energy has great potential for development. But its technology is behind other renewable energy sources. One of the constraints is the efficiency and stability of tidal turbine. The objective of the present work is to optimize a rim-generator turbine runner by improving hydraulic performance and stress level. A multi-objective optimization based on the response surface methodology is adopted during the optimization process so that an optimal runner blade can be identified. Both the blade shape and the number of the blade are determined. The numerical method is verified by comparing with the experimental test, and the numerical results show good agreement with the experimental results. The hydraulic losses in the ebb generation mode and the flood generation mode are compared, and the mechanism of the difference of the hydraulic losses between the two modes are analyzed, especially the hydraulic losses in the guide vanes. The stress analysis based on the fluid-structure interaction theory shows the optimized rim-generator turbine runner has good structural strength. This work could contribute to the development of the rim-generator turbine, and further to the utilization of the tidal energy.
... When determining the dynamic behavior of bulb turbines, further complexities have to be considered. On one side, in these turbines the relative variation of the head is much more larger than in other turbines [7]. Also for these turbines, free surface vortices, fluctuations of the upstream and downstream level have a much larger effect on the unit stability [8,9]. ...
Bulb turbine units are one of the most installed turbines in run-of river projects with relatively low head. In order to enlarge the useful life of these turbines and avoid fatigue problems and cracks, it is of paramount importance to understand and determine the most relevant parameters and their influence on the dynamic response of the structure. In this paper, the modal characteristics of a bulb turbine unit in operation is numerically investigated, considering the rotation effect. A FEM model including alternator, shaft, runner and fluid is developed and the boundary conditions are determined. Firstly, the modal characteristic of the runner under different blade opening are analyzed. Then the influence of the rotation on the modal characteristic of the shaft and runner is discussed. The numerical method is verified by comparing with experimental results of a rotating and submerged disk. The results show that the runner modes are mainly blade-modes，which can be grouped according to the blade number, one jellyfish mode and four local modes in each group. A modified Campbell diagram of the global modes and a transformation matrix of natural frequency between dry modes and wet modes are proposed. Results of this study helps to understand the most influencing parameters, such as the added mass effect of water combined with the rotation. The proposed modified Campbell diagram could be used for an accurate calculation of natural frequencies and avoid possible resonance problems in future designs of bulb turbine units.
Detecting fatigue cracks in hydraulic turbine runners is costly, as it requires stopping the unit, emptying it of water and accessing the runner for inspection. Thus, an alternative way based on monitoring the changes of the structural modal response induced by the formation and growth of a crack was investigated. To do so, the crack propagation induced by a resonance was numerically predicted and experimentally machined on a disk-like structure that resembles a Kaplan turbine runner. The analysis of the results shows how the different stages of the fatigue crack growth can be monitored based on the change of the natural frequencies and mode shapes of several specific modes. Based on the obtained results, a structural health monitoring system is being designed to monitor the turbine runner modes of vibration without the need to stop and inspect the unit.
Five types of configurations of tidal turbine blade including validation model have been considered with different profile tidal turbine blade with twist angle of 9, 10, 10.5, 11, 11.5 and 12 degrees. An optimized model of tidal turbine blades is developed. The simulation of the optimized model i.e. 12 degree twist angle gives minimum value of stress and deformation at different loads i.e. 441, 271, 272 and 610N which has optimized and converged result compared to respected models of tidal turbine blade, it has also been observed that stress and deformation was reduced at static load of 610N in 12 degree twist angle tidal turbine blade with the Zylon, Kevlar and CFRP material, thus the observed fault is diagnosed in this present research work. The configuration of optimized model gives maximum convergence on all parameters amongst all the configurations used.
Linear elastic fracture mechanics (LEFM) approaches are seldom used to analyze fatigue problems in hydraulic turbomachinery, where extremely simplified fatigue models are normally considered. In one of the largest Kaplan hydraulic turbines in the world, the piston rod, which is an important load transfer component, was found broken twice at the same location. This event, which leads to extremely high direct reparation costs and indirect costs, is a very useful case for research of fatigue problems in hydraulic turbines by means of LEFM. For this purpose, the crack growth progress in the piston rod is simulated with numerical LEFM techniques. A probabilistic model for the initial flaw size is considered. A reliability criterion, consisting in the probability of not having a critical crack or a fracture in the piston rod after a specific service time, is defined. Based on this model, the influence of initial crack models and operating services of the machine on the reliability of the unit is discussed. The calculated crack path and predicted number of loading cycles as a function of the initial crack size agree with the actual fracture section and operating time to fracture.
At partial load and overload conditions, Francis turbines are subjected to hydraulic instabilities that can potentially result in high dynamic solicitations of the turbine components and significantly reduce their lifetime. This study presents both experimental data and numerical simulations that were used as complementary approaches to study these dynamic solicitations. Measurements performed on a reduced scale physical model, including a special runner instrumented with on-board strain gauges and pressure sensors, were used to investigate the dynamic phenomena experienced by the runner. They were also taken as reference to validate the numerical simulation results. After validation, advantage was taken from the numerical simulations to highlight the mechanical response of the structure to the unsteady hydraulic phenomena, as well as their impact on the fatigue damage of the runner.
In the operation of hydraulic turbines, no-load and very low load conditions are among the most damaging. Even though there is no power generation, there is still a significant amount of energy which has to be entirely dissipated, mainly in the runner, where the flow is quite complex, with large scale unsteady and chaotic vortices resulting from partial pumping. This paper presents different approaches to perform stress analyses at low load conditions on a Francis turbine, taking into account the pressure fluctuations on the runner blades due to the large stochastic flow structures inherent in no-load operating regimes. With appropriate mesh density and time step, unsteady computational fluid dynamics (CFD) simulations using the SAS-SST turbulence model can be used on a Francis runner to predict the pressure fluctuations with reasonable accuracy when compared to measurements. These calculated pressure loads can then be used to predict the dynamic stresses with finite-element analyses (FEA). Different approaches are discussed ranging from quasi-static single-blade models to full runner time- dependent one-way fluid-structure interaction (FSI). Pros and cons of the different modelling strategies will be discussed in a detailed analysis of the structural results with comparisons to experimental data. Once the time signal of the stochastic stress at no-load conditions is obtained, the runner fatigue damage related to this operating condition can be estimated using different tools such as time signal extrapolation and rainflow counting.
Traditionally, hydro power plants have been operated close to best efficiency point, the more stable operating condition for which they have been designed. However, because of changes in the electricity market, many hydro power plants operators wish to operate their machines differently to fulfil those new market needs. New operating conditions can include whole range operation, many start/stops, extensive low load operation, synchronous condenser mode and power/frequency regulation. Many of these new operating conditions may impose more severe fatigue damage than the traditional base load operation close to best efficiency point. Under these conditions, the fatigue life of the runner may be significantly reduced and reparation or replacement cost might occur sooner than expected.
In order to design reliable Francis runners for those new challenging operating scenarios, Andritz Hydro has developed various proprietary tools and design rules. These are used within Andritz Hydro to design mechanically robust Francis runners for the operating scenarios fulfilling customer's specifications. To estimate residual life under different operating scenarios of an existing runner designed years ago for best efficiency base load operation, Andritz Hydro's design rules and tools would necessarily lead to conservative results.
While the geometry of a new runner can be modified to fulfil all conservative mechanical design rules, the predicted fatigue life of an existing runner under off-design operating conditions may appear rather short because of the conservative safety factor included in the calculations. The most precise and reliable way to calculate residual life of an existing runner under different operating scenarios is to perform a strain gauge measurement campaign on the runner. This paper presents the runner strain gage measurement campaign of a mid-head Francis turbine over all the operating conditions available during the test, the analysis of the measurement signals and the runner residual life assessment under different operating scenarios. With these results, the maintenance cost of the change in operating mode can then be calculated and foreseen by the power plant owner.
Francis turbines are increasingly required to operate from 0-100% of power output. For the design of these turbines, a sound understanding of formerly "off-design" operating points, e.g. speed-no-load, is necessary. One way to assess loads at these operating points is to apply scale resolving CFD methods in order to account for the broad-band turbulence spectrum. This results in stochastic load patterns, which are characteristic for these off-design points. In this paper, two CFD approaches for scale resolving simulations are applied to a Francis model turbine to resolve the larger anisotropic scales in turbine flow at speed-no-load operation. The first uses a hybrid RANS-LES model presented by Menter & Egorov [2]. Here the resolved scales are adapted continuously by RANS-LES blending based on the actual flow condition. The second approach is a LES with a dynamic model based on Germano et al. [3]. For both approaches, spectra of resulting pressure fluctuations at the draft tube cone wall are presented. Additionally, pressure loads from LES are applied to calculate mean and dynamic stresses. The dynamic stresses are finally compared with measurements in the corresponding prototype turbine.
According to the recorded operation data in Jiangxia experimental tidal power station, the relation between the tidal level and the reservoir area water head was analyzed, and the change pattern of the tidal level with the operating parameters was then concluded. The hydraulic characteristics of the bidirectional bulb turbine under multiple operating conditions were investigated by 3D numerical simulation. Based on this, optimization design of a new bidirectional bulb turbine with the fixed flow passage size, including parameters (diameter ratio of runner to bulb, guide vane angle and guide vane/runner blade shape) redesign, was conducted. Key achievement of model units after model acceptance test was also available. The new type unit in Jiangxia tidal power station, unit 6#, in which the research outcome has been directly utilized, achieved stable operating for both forward and reversed generation, pumping and draw off. Moreover, comparing with other units equipped, there is an over 6% efficiency increase for both forward and reversed generation. Besides, turbine general performance has also been improved.
With the increasing development of solar power and wind power which give an unstable output to the electrical grid, hydropower is required to give a rapid and flexible compensation, and the hydraulic turbines have to operate at off-design conditions frequently. Prototype Francis runners suffer from strong vibrations induced by high pressure pulsations at part load, low part load, speed-no-load and during start-stops and load rejections. Fatigue and damage may be caused by the alternating stress on the runner blades. Therefore, it becomes increasingly important to carry out fatigue analysis and life time assessment of the prototype Francis runners, especially at off-design conditions. This paper presents the fatigue analyses of the prototype Francis runners based on the strain gauge site measurements and numerical simulations. In the case of low part load, speed-no-load and transient events, since the Francis runners are subjected to complex hydraulic loading, which shows a stochastic characteristic, the rainflow counting method is used to obtain the number of cycles for various dynamic amplitude ranges. From middle load to full load, pressure pulsations caused by Rotor-stator- Interaction become the dominant hydraulic excitation of the runners. Forced response analysis is performed to calculate the maximum dynamic stress. The agreement between numerical and experimental stresses is evaluated using linear regression method. Taking into account the effect of the static stress on the S-N curve, the Miner's rule, a linear cumulative fatigue damage theory, is employed to calculate the damage factors of the prototype Francis runners at various operating conditions. The relative damage factors of the runners at different operating points are compared and discussed in detail.
Reliable fatigue life assessment of Francis runners combines two parts: At first, the load universe describing how the plant will be operated. And secondly, for all essential operating conditions, component stresses due to static and dynamic loading have to be predicted and considered in the design process by the manufacturer. Therefore, dynamic loading conditions and the resulting impact on the fatigue life of hydroelectric components are an integral part of research activities. Especially off-design conditions and transient operations have been addressed in the last years. Based on strain gauge measurements in prototype runners, model test experiences, and advanced numerical simulations, the understanding of dynamic loads has been highly improved. From correlations of measurement and simulation, standard procedures have been developed to enhance the fatigue life. The present paper summarizes findings of recent investigations enabling Francis runners which combine high efficiency and a robust mechanical design.
More efforts are put on hydro-power to balance voltage and frequency within seconds for primary control in modern smart grids. This requires hydraulic turbines to run at off-design conditions. especially at low load or speed-no load. Besides. the tendency of increasing power output and decreasing weight of the turbine runners has also led to the high level vibration problem of the runners. especially high head Francis runners. Therefore. it is important to carry out the static and dynamic stress analyses of prototype high head Francis runners. This paper investigates the static and dynamic stresses on the prototype high head Francis runner based on site measurements and numerical simulations. The site measurements are performed with pressure transducers and strain gauges. Based on the measured results. computational fluid dynamics (CFD) simulations for the flow channel from stay vane to draft tube cone are performed. Static pressure distributions and dynamic pressure pulsations caused by rotor-stator interaction (RSI) are obtained under various operating conditions. With the CFD results. static and dynamic stresses on the runner at different operating points are calculated by means of the finite element method (FEM). The agreement between simulation and measurement is analysed with linear regression method. which indicates that the numerical result agrees well with that of measurement. Furthermore. the maximum static and dynamic stresses on the runner blade are obtained at various operating points. The relations of the maximum stresses and the power output are discussed in detail. The influences of the boundary conditions on the structural behaviour of the runner are also discussed.
The reliability assessment of large rotating structures like hydroelectric Francis runners is often limited by our capacity to define a proper limit state combined with a relevant degradation model. In this paper, we propose that the proper limit state for fatigue reliability of such structures is the onset of high cycle fatigue (HCF). Based on this premise, a prior interval for our limit state based on available literature is presented. The prior assumptions are believed to be the first step toward validation of the applicability and suitability of the proposed model. The paper includes an overview of the theoretical background for reliability assessment of Francis turbine runners, the methodology used, and the results obtained from the information gathered from the available literature. (C) 2013 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of CETIM
High Cycle Fatigue (HCF) plays an important role in Francis runner
reliability. This paper presents a model in which reliability is defined
as the probability of not exceeding a threshold above which HCF
contributes to crack propagation. In the context of combined Low Cycle
Fatigue (LCF) and HCF loading, the Kitagawa diagram is used as the limit
state threshold for reliability. The reliability problem is solved using
First-Order Reliability Methods (FORM). A study case is proposed using
in situ measured strains and operational data. All the parameters of the
reliability problem are based either on observed data or on typical
design specifications. From the results obtained, we observed that the
uncertainty around the defect size and the HCF stress range play an
important role in reliability. At the same time, we observed that
expected values for the LCF stress range and the number of LCF cycles
have a significant influence on life assessment, but the uncertainty
around these values could be neglected in the reliability assessment.
Purpose
Some comparison of unsteady flow calculation and the measured stress showed that the dynamic stresses in blades are closely related to hydraulic instability. However, few studies have been conducted for the hydraulic machinery to calculate dynamic stresses caused by the unsteady hydraulic load. The present paper aims to analyse the stresses in blades of a Kaplan turbine.
Design/methodology/approach
By employing a partially coupled solution of 3D unsteady flow through its flow passage, the dynamic interaction problem of the blades was analyzed. The unsteady Reynolds‐averaged Navier‐Stokes equations with the SST κ ‐ ω turbulence model were solved to model the flow within the entire flow path of the Kaplan turbine. The time‐dependent hydraulic forces on the blades were used as the boundary condition for the dynamics problem for blades.
Findings
The results showed that the dynamic stress in the blade is low under approximately optimum operating conditions and is high under low‐output conditions with a small guide vane opening, a small blade angle and a high head.
Research limitations/implications
It is assumed that there is no feedback of blade motion on the flow. Self‐excited oscillations are beyond the scope of the present paper.
Originality/value
The authors developed a code to transfer the pressure on blades as a boundary condition for structure analysis without any interpolation. The study indicates that the prediction of dynamic stress during the design stage is possible. To ensure the safety of the blades it is recommended to check the safety coefficient during the design stage for at least two conditions: the 100 percent output with lower head and the 50 percent output with the highest head.
Pressure oscillations caused by vortex rope were measured in the draft tube of a prototype Francis turbine. The three-dimensional, unsteady Reynolds-averaged Navier-Stokes equations with the RNG K-e turbulence model were solved to model the flow within the entire flow path of the prototype hydraulic unit including the guide vanes, the runner, and the draft tube. The model was able to predict the pressure fluctuations that occur when operating at 67-83% of the optimum opening. The calculated frequencies and amplitudes of the oscillation show reasonable agreement with the experiment data. However, the results at 50% opening were not satisfactory. Pressure oscillations on the runner blades were found to be related to the precession of vortex ropes which caused pressure on the blades to fluctuate with frequencies of -f n+fd (fn is the rotational frequency and f d is vortex procession frequency). The peak-to-peak amplitudes of the pressure oscillations on the blades at the lower load conditions (67% opening) were higher than at higher load conditions (83% opening). Fluctuations on the suction side tended to be stronger than on the pressure side.
The objective of this work is to evaluate the effects of mean stress on the fatigue behavior of ASTM A743 CA6NM alloy steel. It is used in several hydrogenator turbine components. In order to achieve it, 33 specimens were experimentally evaluated under axial loads with stress ratio of - 1 and more 60 specimens were tested under stress ratio 0, 1/3 and 2/3. Based on the obtained results it was possible to determine parameters that describe the fatigue behavior of the evaluated material, obtain its S-N curves, its endurance limit and its scatter bands. In the assessment of the mean stress effects of fatigue life, Goodman, Gerber, Walker and Kwofie’s relations were tested in order to evaluate the validity of the use of such rules for the tested material. According to the obtained results it was possible to verify that Goodman and Gerber’s relations do not model correctly the reduction effect fatigue life and presented high scatter. The predictions of Walker and Kwofie’s relation are consistent and the Walker’s relation presented smaller scatter than Kwofie’s relation. Walker’s relation makes it possible to evaluate in a consistent way the effect of the presence of mean stresses on fatigue strength.
This work investigates the unsteady pressure fluctuations and inception of vortical flow in a hydraulic turbine during no-load-speed conditions. The focus of the present study is to experimentally measure and numerically characterize time-dependent pressure amplitudes in the vaneless space, runner and draft tube of a model Francis turbine. The numerical model consists of the entire turbine including Labyrinth seals. Compressible flow was considered for the numerical study to account for the effect of flow compressibility and the reflection of pressure waves. The results clearly showed that the vortical flow in the blade passages induces high-amplitude stochastic fluctuations. A distinct flow pattern in the turbine runner was found. The flow near the blade suction side close to the crown was more chaotic and reversible (pumping), whereas the flow on the blade pressure side close to the band was accelerating (turbine) and directed toward the outlet. Flow separation from the blade leading edge created a vortical flow, which broke up into four parts as it traveled further downstream and created high-energy turbulent eddies. The source of reversible flow was found at the draft tube elbow, where the flow in the center core region moves toward the runner cone. The vortical region located at the inner radius of the elbow gives momentum to the wall-attached flow and is pushed toward the outlet, whereas the flow at the outer radius is pushed toward the runner.
As interest in tidal-electric power generation continues to grow in response to demands for renewable sources of energy, readers can now turn to Elements of Tidal-Electric Engineering for the first comprehensive treatment of the subject. The author, Robert H. Clark, a leader in the field for almost fifty years, has spearheaded several important research projects and consulted with governments and private industries around the world on tidal-electric issues. The focus of this text is the feasibility study. Power engineers gain both the knowledge and the skills needed to accurately determine the feasibility of a proposed tidal power development plan, including: • Major factors to consider in selecting a site for preliminary assessment • Tidal power schemes and mode • Hydraulic and mathematical models of estuaries to predict the estuary’s response to physical changes and the effects caused by operation of the proposed plant • Civil works required for tidal power development and the associated tidal generating equipment • Procedures to optimize plant output • Economic evaluation and risk assessment • Environmental impact of proposed construction and operation The book ends with an examination of commercially operating plants and a brief review of sites that have been the subject of investigation in the last half century. References and bibliographies direct readers to primary source material for further study. Until publication of this text, power engineers have had to rely on random journal articles and anecdotal information to perform a feasibility investigation. With the publication of Elements of Tidal-Electric Engineering these engineers have a single, integrated source that methodically covers all the issues. © 2007 by the Institute of Electrical and Electronics Engineers, Inc. All rights reserved.
While larger and larger turbines are being developed, hydraulic stability has become one of the key issues for their performance assessments. An accurate prediction of their pressure fluctuations is vital to the success of new model development. In this paper, we briefly introduced the method, i.e., the three-dimensional unsteady turbulent flow simulation of the complete flow passage, which we used for predicting the pressure fluctuations of a model Kaplan turbine. In order to verify the prediction, the model turbine was tested on the test rig at the Harbin Electric Machinery Co., Ltd. (HEC), China, which meets all the international standards. Our main findings from this numerical prediction of pressure fluctuations for a model Kaplan turbine are as follows. (1) The approach by using 3D unsteady turbulent flow including rotor-stator interaction for the whole flow passage is a feasible way for predicting model turbine hydraulic instability. The predicted values at different points along its flow passage all agree well with the test data in terms of their frequencies and amplitudes. (2) The low-frequency pressure fluctuation originating from the draft tube is maximal and influences the stability of the turbine operation mostly. The whole flow passage analysis shows that the swirling vortex rope in the draft tube is the major source generating the pressure fluctuations in this model turbine. (3) The second harmonic of the rotational frequency 2f n is more dominant than the blade passing frequency Zfn in the draft tube. This prediction, including the turbulence model, computational methods, and the boundary conditions, is valid either for performance prediction at design stage and/or for operation optimization after commissioning.
A lack of observed data can lead to significant discrepancy between estimated and actual fatigue reliability. In this study, the load spectrum derived from the strain measurements and operation history of a hydroelectric turbine are used to identify the information necessary to avoid major bias in the estimated fatigue reliability of such structure. Our results demonstrate that a limited number of parameters need to be considered. Any further simplifications may lead to major reliability estimate discrepancies. Furthermore, we observe that the parameters which influence initial reliability hide the influence of other parameters on the reliability failure rate associated with the structure’s life expectancy. We conclude that a typical sensitivity analysis made on parameter values needs to be complemented with a sensitivity study on the chosen assumptions in order to properly evaluate the risks associated with the operation of hydroelectric turbines.
Modern design of hydraulic turbines aims to achieve very high levels of
efficiency and structural integrity in the environment of highly
variable loading conditions. To combine those requirements, a profound
knowledge of static and dynamic loads acting on hydraulic components is
necessary. Dynamic loadings of Francis runners strongly depend on head
range and required operating conditions. For higher head (low specific
speed) Francis runners, the main dynamic loading is caused by
Rotor-Stator Interaction (RSI). The knowledge of dynamic loads and
stresses in Francis runners is often derived from strain gauge
measurements and/or simulation of flow interacting with structural
components. Reliable simulations of Fluid-Structure Interaction (FSI)
are necessary to predict the dynamic behavior, especially in terms of
hydrodynamic mass and damping properties. This contribution describes
the equipment and procedure of strain gauge measurements in Francis
prototype runners, including the challenges to overcome during such
tests. It provides an overview of strain gauge test results for Francis
runners in different head ranges and describes the characteristic
differences depending on specific speed. Measurement data is compared
with state of the art simulations of Francis runner dynamics to predict
dynamic stresses caused by RSI. The contribution underlines the fact
that a calibrated analysis procedure is the key to optimize the design
of Francis runners.
The western and southern coasts of Korea are ideal locations for developing tidal current energy due to geographical features, such as ria coasts and strong tides. In order to develop techniques for harnessing tidal energy, field measurements, numerical modeling, and estimations of potential tidal energy were conducted in Lake Sihwa Bay prior to the construction of a tidal power plant. The construction of a 1 MW pilot plant was initiated in April 2006 and completed in 2007. An ongoing study is focused on the commercialization and industrialization of tidal current energy in Korea. Construction of additional power plants is planned at several proposed sites after this technology has been established.
A Tidal Power Plant (TPP) is being constructed in the middle section of the existing Lake Sihwa dike located near the southern Incheon Port in Korea. The project, which will be completed in 2010, is to harness the largest tidal energy in the Kyeonggi Bay in the eastern Yellow Sea. While noting the current progress in terms of plant construction, this paper outlines the overall project in the tidal regime and uses predictive local flow modeling. The results of two-dimensional finite element method simulations that predict the real-time tidal characteristics during the construction and after the completion of the tidal power plant are presented, including a method to estimate the electricity output from the plant in the future.
The Brazilian North and Northeast regions have possibilities on harnessing tidal power, where the greatest tidal amplitudes are to be found. In the 1970s, a barrage constructed in the Bacanga estuary, a civil construction project for reducing the distance between the city of São Luís do Maranhão and the port of Itaqui, was planned to be the first Brazilian tidal power plant. The favorable conditions associated with the considerable tidal heights, which can be as much as 6.5m, made this an ideal place for building a pilot plant. Although, a scheme had been proposed to harness tidal energy in a conventional way with the use of the reservoir, the tidal plant in its original form was not implemented due economic and technical restrictions originated from the turbo generators chosen at that time. This paper presents an alternative concept for tidal power plant when the reservoir presents limitations caused by urban occupation, silting up and deterioration of the original structures. These limitations have been verified following the construction of the Bacanga barrage which led to an alternative proposal for the future installation of a pilot tidal power plant. The power generation possible to be harnessed was simulated through a model for each hour from the water head data, outflow, levels, areas and volumes of the reservoir and estuary. The development of the conceptual project, with the utilization of low-head turbines, a floating platform for the turbines and, particularly, taking into consideration those restrictions to be found at the present time, it can be concluded that Bacanga tidal power plant of is technically viable.
The stresses in Kaplan turbine piston rod were analyzed to predict failure conditions based on the 3D unsteady flow through the Kaplan turbine. The results show that the predicted stress concentration position in the rod agrees well with the actual fracture position. The mean stress and the dynamic stress in the rod varied greatly with operating conditions. The dynamic stress reached 46.0MPa at the high head low output condition, which caused the final rod failure. The mean of the dynamic stresses in the rod increase with increasing blade angle error, but the amplitudes decrease.
The stresses in piston rods, a very important part of the blade-control system in Kaplan turbines, were analyzed to predict failure conditions. The turbine blades suffer from pressure oscillations whose forces are transferred to the piston rods. The 3D unsteady flow through the Kaplan turbine was simulated to predict the forces on the piston rod. The unsteady Reynolds-averaged Navier–Stokes (RANS) equations with the SST κ–ω turbulence model were solved to model the flow within the entire flow path of the Kaplan turbine. The unsteady hydraulic forces on the blades were then used as the boundary condition for dynamic analysis of piston rods either fixed with a nut or fixed with a retainer ring for various operating conditions. The results show that the mean stress and the dynamic stress for the nut structure were less than for the retainer ring structure because the nut structure had a pretightening force. The dynamic stress with the retainer ring structure reached 23.7 MPa at the high head low output condition with a very low safety coefficient, which caused the final breakage of the rod. The predicted stress concentration position in the rod agreed well with the fracture position.
The Severn Estuary has a spring tidal range approaching 14Â m and is regarded as having one of the highest tidal ranges in the world. Various proposals have been made regarding the construction of a tidal barrage across the estuary to enable tidal energy to be extracted. The barrage scheme originally proposed by the Severn Tidal Power Group (STPG) would be the largest project for tidal power generation in the world if built as proposed. Therefore, it is important to study the impact of different operating modes for this barrage on the tidal power output and flood inundation extent in the estuary. In this paper, an existing two-dimensional hydrodynamic model based on an unstructured triangular mesh has been integrated with a new algorithm developed for the estimation of tidal power output, which can account for three barrage operating modes, including ebb generation, flood generation, and two-way generation. The refined model was then used to investigate the impact of different barrage operating modes on the tidal power output and the associated extent of flood inundation along the Severn Estuary. Predicted results indicate that the mode of flood generation would produce the least electrical energy and cause a larger reduction in the maximum water levels upstream of the barrage. Two-way generation would provide an improvement to these conditions, and produce an equivalent amount of electricity to that from ebb generation, with a low installed capacity and a small loss of intertidal zones. Therefore, the mode of ebb generation or two-way generation would appear to be a preferred option for power generation, because both would offer benefits of acceptable electrical energy and reduced flood risk.
Tidal energy has the potential to play a valuable part in a sustainable energy future. It is an extremely predictable energy source, depending only on the gravitational pull of the moon and the sun and the centrifugal forces created by the rotation of the earth-moon system. Tidal energy has been exploited on a significant scale since the construction of the La Rance tidal barrage in France in 1967. A tidal barrage utilises the potential energy of the tide and has proven to be very successful, despite opposition from environmental groups. Kinetic energy can also be harnessed from tidal currents to generate electricity and involves the use of a tidal current turbine. This is the more desired method of capturing the energy in the tides. However, tidal current turbine technology is currently not economically viable on a large scale, as it is still in an early stage of development. This paper provides an up-to-date review of the status of tidal energy technology and identifies some of the key barriers challenging the development of tidal energy. The future development of tidal current devices and tidal barrage systems is discussed as well as examining the importance of a supportive policy to assist development.
The change of water discharge capability of the sluice caisson of tidal power plant according to the change of geometrical shape of the sluice caisson was investigated by performing laboratory experiments. The major design parameters that constitute general shape of the sluice caisson were deduced and a total of 32 different shapes of sluice caisson models were subjected to the hydraulic experiments. For every sluice caisson model, the water discharge capability was estimated with five different flow rates and three different water level conditions. The experiments were carried out in an open channel flume with a great care to measure flow rate and water level accurately, which are key physical quantities in estimating the water discharge capability of the sluice caisson models. By analyzing the experimental results, influence of the respective design parameters on the performance of the sluice caisson was examined and the general guidelines to enhance the water discharge capability were suggested. The discharge coefficient of the best sluice caisson model ranged from 2.3 to 3.1 depending on the experimental conditions, which is far higher than the values that were adopted in the past feasibility studies in Korea.
Facing great pressure of economic growth and energy crisis, China pays much attention to the renewable energy. An overview of policy and legislation of renewable energy as well as status of development of renewable energy in China was given in this article. By analysis, the authors believe that ocean energy is a necessary addition to existent renewable energy to meet the energy demand of the areas and islands where traditional forms of energy are not applicable and it is of great importance in adjusting energy structure of China. In the article, resources distribution and technology status of tidal energy, wave energy, marine current energy, ocean thermal energy and salinity gradient energy in China was reviewed, and assessment and advices were given for each category. Some suggestions for future development of ocean energy were also given.
Fatigue and cracks have occurred in many large hydraulic turbines after they were put into production. The cracks are thought to be due to dynamic stresses in the runner caused by hydraulic forces. Computational fluid dynamics (CFD) simulations that included the spiral case, stay vane, guide vane, runner vane, and draft tube were run at various operating points to analyze the pressure distribution on the runner surface and the stress characteristics in the runner due to the fluid-structure interactions (FSI). The dynamic stresses in the Francis turbine runner at the most dangerous operating point were then analyzed. The results show that the dynamic stresses caused by the hydraulic forces during off-design operating points are one of the main reasons for the fatigue and cracks in the runner blade. The results can be used to optimize the runner and to analyze other critical components in the hydraulic turbine.
Prices of oil and other fossil fuels have proven a powerful incentive for the alternative energy hunters. The alternatives include the various forms of ocean energy that, often considered uneconomical for electricity generation, have become attractive and competitive. Many sites throughout the world have been considered, at one time or another, suitable for implantation of a tidal power station, but very few have witnessed implementation of often ambitious plans. The Rance River tidal power plant, near St Malo in Brittany (France) is an exception. It is celebrating in 2006, 40 years of durable, loyal and productive service.
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