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(1)al. ay. a. NUMERICAL MODELLING OF HORIZONTAL AXIS TIDAL TURBINE WITH VARIABLE LENGTH BLADE. ve rs. ity. of. M. FARHANA ARZU. U. ni. DEPARTMENT OF CIVIL ENGINEERING FACULTY OF ENGINEERING UNIVERSITY OF MALAYA KUALA LUMPUR. 2018.

(2) of. M. al. FARHANA ARZU. ay. a. NUMERICAL MODELLING OF HORIZONTAL AXIS TIDAL TURBINE WITH VARIABLE LENGTH BLADE. U. ni. ve rs. ity. THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ENGINEERING SCIENCE. DEPARTMENT OF CIVIL ENGINEERING FACULTY OF ENGINEERING UNIVERSITY OF MALAYA KUALA LUMPUR. 2018.

(3) UNIVERSITY OF MALAYA ORIGINAL LITERARY WORK DECLARATION. Name of Candidate: Farhana Arzu Matric No:. KGA 140019. Name of Degree: Master of Engineering Science Title of Thesis: NUMERICAL MODELLING OF HORIZONTAL AXIS TIDAL TURBINE WITH VARIABLE LENGTH BLADE. ay. a. Field of Study: Water Resource Engineering. al. I do solemnly and sincerely declare that:. ni. ve rs. ity. of. M. (1) I am the sole author/writer of this Work; (2) This Work is original; (3) Any use of any work in which copyright exists was done by way of fair dealing and for permitted purposes and any excerpt or extract from, or reference to or reproduction of any copyright work has been disclosed expressly and sufficiently and the title of the Work and its authorship have been acknowledged in this Work; (4) I do not have any actual knowledge nor do I ought reasonably to know that the making of this work constitutes an infringement of any copyright work; (5) I hereby assign all and every rights in the copyright to this Work to the University of Malaya (“UM”), who henceforth shall be owner of the copyright in this Work and that any reproduction or use in any form or by any means whatsoever is prohibited without the written consent of UM having been first had and obtained; (6) I am fully aware that if in the course of making this Work I have infringed any copyright whether intentionally or otherwise, I may be subject to legal action or any other action as may be determined by UM. Date:. U. Candidate’s Signature. Subscribed and solemnly declared before,. Witness’s Signature. Date:. Name: Designation:. ii.

(4) NUMERICAL MODELLING OF HORIZONTAL AXIS TIDAL TURBINE WITH VARIABLE LENGTH BLADE ABSTRACT Marine renewable energy is one of the major alternative sources of energy to meet the current energy demand. Rotor blades have the main influence on the efficiency of tidal turbines. The variable length blade technology has already been used in designing wind. a. turbine blades for efficient energy extraction. The movable tip blade section of variable. ay. blade length turbine offers full control on performance characteristics and power capture,. al. but has limited application in marine field. In this study, a variable length blade horizontal axis tidal turbine (HATT) model is studied numerically to investigate the hydrodynamic. M. performance and power output. A new open source software package QBlade 0.8 and. of. ANSYS FLUENT 15.0 were used for two-dimensional BEMT (blade element momentum theory) and three-dimensional CFD (computational fluid dynamics). ity. simulations respectively. Both the simulation techniques have been validated against the. ve rs. available published data of the HATT models. The effect of different tip blade extensions on the non-dimensional performance parameters (power, thrust and moment coefficient) and power output of the rotor model were studied at rated and below-rated conditions of. ni. the model. The performance data then were compared with the standard fixed length blade. U. tidal turbine. Non-dimensional performance coefficients were observed to improve with the increment of rotor diameter at high TSRs. Peak power coefficient value was dropped by 9% when the blades extend from 10% to 40%. On the other hand, power extraction was enhanced up to 72% at below-rated tidal velocities without any loss in performance at rated condition. The model is found to be more efficient compared with the conventional tidal turbine models and thus recommended as a good candidate to replace the other conventional HATTs. Keywords: variable length blade, numerical simulation, performance, power.. iii.

(5) MODEL NUMERIKAL TURBIN SUDUT MELINTANG PASANG SURUT DENGAN BILAH LENGAN BELAS ABSTRAK. Tenaga boleh diperbaharui marin merupakan salah satu sumber utama tenaga alternatif untuk memenuhi permintaan tenaga semasa. Bilah pemutar mempunyai pengaruh utama ke atas kecekapan turbin pasang surut. Teknologi bilah panjang boleh. a. laras telah digunakan dalam merangka bilah turbin angin untuk pengekstrakan tenaga. ay. dengan lebih cekap. Bahagian bilah hujung bergerak dari turbin panjang bilah boleh laras menawarkan kawalan penuh ke atas prestasi dan penjanaan kuasa, tetapi mempunyai. al. aplikasi yang terhad dalam bidang marin. Dalam kajian ini, panjang paksi pemboleh ubah. M. model turbin pasang surut (HATT) dikaji secara berperingkat untuk mengkaji prestasi hidrodinamik dan penjanaan kuasa. Dengan menggunakan perisian dari sumber terbuka. of. iaitu QBlade 0.8 dan ANSYS FLUENT 15.0 digunakan untuk BEMT dua dimensi (teori. ity. momentum unsur bilah) dan simulasi CFD tiga dimensi. Kedua-dua teknik simulasi ini telah disahkan dengan data yang dipaparkan dari model HATT. Kesan pelanjutan bilah. ve rs. hujung yang berlainan pada parameter prestasi tidak berdimensi (kuasa, tujahan dan pekali momen) dan penjanan kuasa model pemutar telah dikaji pada syarat-syarat yang dinilai dan di bawah model ini. Data prestasi kemudian dibandingkan dengan turbin. ni. standard pasang surut yang mempunyai panjang yang tetap. Pekali prestasi bukan dimensi. U. diperhatikan dengan peningkatan diameter pemutar di TSR tinggi. Nilai pekali kuasa puncak dikurangkan sehingga 9% apabila bilah-bilah memanjang dari 10% hingga 40%. Sebaliknya, penjanaan kuasa dipertingkatkan sehingga 72% pada halaju pasang surut rendah tanpa sebarang kehilangan prestasi. Model ini didapati lebih cekap berbanding model turbin pasang konvensional dan dicadangkan untuk menggantikan HATT konvensional yang lain.. Kata kunci: panjang bilah boleh laras, simulasi numerikal, prestasi, kuasa. iv.

(6) ACKNOWLEDGEMENTS The author would like to take the opportunity to express her heartiest gratitude to her supervisor Dato’ Prof. Ir. Dr. Roslan Bin Hashim for his time, support, inspiration and expertise throughout this research. This report would not be possible without his critical comments, guidance and encouragement at various stages of research. The author is indebted to him forever.. a. The most sincere appreciation goes to University of Malaya (UM), Kuala Lumpur,. ay. Malaysia for supporting financially through High Impact Research Grant (H-16001-00D000047) and for excellent working environment for this research. The author also would. al. like to thank all the members in the department of Civil Engineering, University of. M. Malaya for their cooperation. She wishes them all to accomplish their goals successfully.. of. Finally, the author is thankful to her family and all those who cooperated and expressed. U. ni. ve rs. well-wishers.. ity. best wishes for her; appropriate words could not be found to express gratitude to all the. v.

(7) TABLE OF CONTENTS. ABSTRACT .....................................................................................................................iii ABSTRAK ...................................................................................................................... iiv Acknowledgements ........................................................................................................... v Table of Contents ............................................................................................................. vi List of Figures .................................................................................................................. ix. a. List of Tables.................................................................................................................... xi. al. ay. List of Symbols and Abbreviations ................................................................................. xii. CHAPTER 1: INTRODUCTION .................................................................................. 1 Research background ............................................................................................... 1. 1.2. Present status of renewable energy in Malaysia ...................................................... 3. 1.3. Problem statement ................................................................................................... 4. 1.4. Aim and objectives of the study .............................................................................. 6. 1.5. Scope of the study.................................................................................................... 7. ve rs. ity. of. M. 1.1. 1.6. Outline of the dissertation........................................................................................ 8. ni. CHAPTER 2: LITERATURE REVIEW ...................................................................... 9. U. Tidal energy extraction devices ............................................................................... 9 Horizontal axis tidal turbines (HATTs).................................................... 10 Vertical axis tidal turbines ........................................................................ 12 Alternative turbines .................................................................................. 13 Securing, installation and maintenance ................................................................. 15 Turbine blade design considerations ..................................................................... 16 Turbine blade performance .................................................................................... 18 Power and mechanical load control systems ......................................................... 19. vi.

(8) Variable length Blade control system ...................................................... 21 Two-dimensional foil performance ....................................................................... 23 Blade element momentum theory .......................................................................... 25 Momentum theory .................................................................................... 26 Blade element theory ................................................................................ 30 Blade element momentum equations ....................................................... 33. a. Computational fluid dynamics (CFD) ................................................................... 35. ay. RANS viscous models .............................................................................. 38. al. Summary ................................................................................................................ 40. M. CHAPTER 3: METHODOLOGY ............................................................................... 41 Variable length blade HATT modelling ................................................................ 42. of. Two-dimensional (2D) BEMT simulation............................................................. 45. ity. Hydrofoil analysis .................................................................................... 46 Rotor model generation ............................................................................ 46. ve rs. Blade element momentum analysis .......................................................... 47 QBlade BEMT code validation ................................................................ 47. Three-dimensional CFD investigation method ...................................................... 48. U. ni. Geometry preparation ............................................................................... 48 Mesh generation ....................................................................................... 51 CFD solver setting .................................................................................... 52 Post processing (calculation of performance coefficients and power) ..... 54 Mesh selection .......................................................................................... 54 Time step selection ................................................................................... 55 CFD FLUENT model validation .............................................................. 56. vii.

(9) CHAPTER 4: RESULTS AND DISCUSSION .......................................................... 57. Validation results of QBlade simulation tool ........................................... 57. 4.1.2. Validation results of CFD simulation tool ............................................... 63. Non dimensional performance characteristics of HATT model ............................ 63 Performance coefficients prediction from BEMT study .......................... 64. 4.2.2. Performance coefficients prediction from CFD study.............................. 68. a. 4.2.1. Power extraction .................................................................................................... 70 4.3.1. Power prediction from BEMT analysis .................................................... 71. 4.3.2. Power prediction from CFD analysis ....................................................... 72. M. 4.3. 4.1.1. ay. 4.2. Validation of the simulation techniques ................................................................ 57. al. 4.1. CHAPTER 5: CONCLUSIONS & RECOMMENDATIONS FOR FUTURE. of. WORK…………. ........................................................................................................... 74 Summary of the work ............................................................................................ 74. 5.2. Conclusions ........................................................................................................... 75. 5.3. Recommendations for future work ........................................................................ 76. ve rs. ity. 5.1. References ....................................................................................................................... 77. U. ni. List of Publications and Papers Presented ...................................................................... 84. viii.

(10) LIST OF FIGURES. Figure 2.1: SeaGen device developed by Marine Current Turbine (MCT) .................... 10 Figure 2.2: Examples of horizontal axis tidal turbines ................................................... 11 Figure 2.3: Vertical axis tidal turbines ............................................................................ 13 Figure 2.4: Examples of major alternative turbines ........................................................ 15. a. Figure 2.5: Different control systems affecting blade performance ............................... 20. ay. Figure 2.6: Variable length blade turbine concept .......................................................... 23 Figure 2.7: Foil orientation ............................................................................................. 24. al. Figure 2.8: Blade element momentum analysis of HATT .............................................. 26. M. Figure 2.9: Single stream tube analysis........................................................................... 27. of. Figure 2.10: Rotating annular stream tube ...................................................................... 28 Figure 3.1: Flow chart of the research methodology ...................................................... 41. ity. Figure 3.2: Baseline model tidal turbine blade profile.................................................... 42. ve rs. Figure 3.3: Variable length blade HATT model ............................................................. 44 Figure 3.4: Blade profile of model HATT ...................................................................... 45 Figure 3.5: 3D turbine rotor geometry ............................................................................ 49. ni. Figure 3.6: Turbine geometry surrounded by sub-domain and main domain ................. 50. U. Figure 3.7: Meshing of blade surface, rotor and main domain ....................................... 51 Figure 4.1 Power coefficient data comparison for QBlade BEMT code validation (Case 1, set angle 0º) ................................................................................................................. 58 Figure 4.2 Power coefficient data comparison for QBlade BEMT code validation (Case 1, set angle 5º) ................................................................................................................. 58 Figure 4.3: Power coefficient data comparison for QBlade BEMT code validation (Case 1, set angle 10º) ............................................................................................................... 59 Figure 4.4: Thrust coefficient data comparison for QBlade BEMT code validation (Case 1, set angle 0º) ................................................................................................................. 59. ix.

(11) Figure 4.5: Thrust coefficient data comparison for QBlade BEMT code validation (Case 1, set angle 5º) ................................................................................................................. 60 Figure 4.6: Axial thrust coefficient data comparison for QBlade BEMT code validation (Case 1, set angle 10º) ..................................................................................................... 60 Figure 4.7: Power coefficient data comparison for QBlade BEMT code validation (Case 2) ..................................................................................................................................... 61 Figure 4.8: Power coefficient data comparison for QBlade BEMT code validation (Case 3) ..................................................................................................................................... 61. a. Figure 4.9: Power coefficient data comparison for CFD technique validation............... 63. ay. Figure 4.10: Lift coefficient vs angle of attack for NACA-63418 hydrofoil .................. 64. al. Figure 4.11: Drag coefficient vs angle of attack for NACA-63418 hydrofoil ................ 65. M. Figure 4.12: Power coefficient (CP) vs. TSR with different tip blade extensions .......... 66 Figure 4.13: Torque coefficient (CM) vs. TSR with different tip blade extensions ........ 67. of. Figure 4.14: Axial thrust coefficient (Ct) vs. TSR with different tip blade extensions .. 68. ity. Figure 4.15: Performance coefficient curves of 10% extended blades from CFD analysis ......................................................................................................................................... 69. ve rs. Figure 4.16: Performance coefficient curves of 40% extended blades from CFD analysis ......................................................................................................................................... 70 Figure 4.17: Effect of blade extensions on power output ............................................... 71. U. ni. Figure 4.18: Power output for minimum and maximum extended model ...................... 72. x.

(12) LIST OF TABLES. Table 3.1: Particulars of the tidal turbines for QBlade BEMT code verification ........... 47 Table 3.2: Cases considered in CFD simulation ............................................................. 48 Table 3.3: Important solver settings for CFD simulation ............................................... 53 Table 3.4: Mesh dependent peak CP checks for maximum extended model .................. 55. a. Table 3.5: Time dependent peak CP checks for maximum extended model ................... 56. ay. Table 4.1: Comparison of hydrodynamic performance data among different investigations ......................................................................................................................................... 62. U. ni. ve rs. ity. of. M. al. Table 4.2: Comparison of power capture at different tide speeds................................... 73. xi.

(13) LIST OF SYMBOLS AND ABBREVIATIONS. :. Rotor area, m2. a. :. Axial induction factor. a’. :. Tangential induction factor. B. :. Number of blades. D. :. Rotor diameter, m. :. Power coefficient. :. Thrust coefficient. :. Torque coefficient. :. Drag coefficient. :. Lift coefficient. :. Moment coefficient. :. Normal load coefficient. :. Tangential load coefficient. c. :. Chord length, m. dL. :. Sectional lift force, N. dD. :. Sectional drag force, N. dm. :. Sectional moment. dT. :. Sectional thrust force, N. dM. :. Sectional torque, N-m2. ni. ve rs. ity. of. M. al. ay. a. A. :. Total loss factor. FN. :. Normal load, N. FT. :. Tangential load, N. Ftip. :. Tip loss factor. Fhub. :. Hub loss factor. K. :. Turbulent kinetic energy, m2/s2. M. :. Torque, N-m2. N. :. Rotor rotational speed, rpm. U. F. xii.

(14) :. Ensemble average of experiments. n. :. Rotor rotational speed, rev/s. P. :. Power, W. R. :. Rotor radius, m. Rhub. :. Hub radius, m. r. :. Local radius, m. T. :. Thrust, N. TSR. :. Tip speed ratio. TSR. :. Local tip speed ratio. U. :. Inflow velocity, m/s. Utip. :. Blade tip velocity, m/s. Urated. :. Rated velocity, m/s. u´. :. Velocity fluctuation, m/s. ua. :. Velocity average, m/s. α. :. Angle of attack, degrees. ε. :. Turbulence dissipation rate, m/s. μt. :. Turbulent viscosity. φ. :. Inflow angle, degrees. ve rs. ity. of. M. al. ay. a. Ne. :. Fluid density, kg/m3. ω. :. Rotor rotational speed, rad/s. ωs. :. Specific dissipation rate, 1/s. ni. ρ. :. Twist angle, degrees. σ. :. Local solidity. ν. :. Kinematic viscosity, m2/s. U. θ. xiii.

(15) CHAPTER 1: INTRODUCTION. 1.1. Research background. With the rapid increase in population all over the world, existing non renewable energy sources are depleting at an alarming rate. The unsustainable usage of fossil fuels, coal, oil, natural gas is leading towards adverse climate change by constant emission of greenhouse gases (Jaber, Badran, & Abu-Shikhah, 2004). Understanding the facts has led. a. the researchers to start finding ways to address this problem, such as inspiring the. ay. development of new renewable energy technologies (Herring, 2006). Most of the. al. countries worldwide has paid much attention towards the production of “green”. M. electricity from renewable energy sources to meet the ever-increasing energy demand (Larcher & Tarascon, 2015) and lessen carbon emission.. of. In the ASEAN region, a small portion of energy produced comes from renewable. ity. energy (4%) while energy from fossil fuels occupies 74%, combustible biomass and waste occupies 22% (Low, 2012). In 2015, a collated data regarding the dependence of South. ve rs. East Asian countries on oil and gas as the primary energy source was published by (Quirapas et. al., 2015). The study showed that the countries like Brunai, Singapore, Malaysia, Combodia are highly dependent on oil and gas as the primary energy source. ni. (50% or more). Although considerable variation in natural energy resources is observed. U. among South East Asian countries, the significant amount of renewable energy available in this region is not much exploited yet (Ölz & Beerepoot, 2010). Government policies throughout this region are progressively supporting renewable energy and the green environment. Several private organizations have their intension to invest in renewable energy, which could mitigate climate change by means of the reduction in greenhouse gas emissions. Being surrounded by water, ocean renewable energy is considered to be one of the most relatively significant renewable energy source in the region. A recent study. 1.

(16) has claimed about the prospect of harnessing ocean renewable energy (ORE) in the region; however, there are numerous challenges to successfully exploit this potential (Quirapas et al., 2015).. Ocean is the most power dense unexploited renewable energy source. Oceans are considered to be capable of making a major contribution on future energy requirement without risking serious damage to the environment. This is expected to have the potential. a. equal to or more than wind energy to fulfill our future electricity demands. Several forms. ay. of energy including thermal, offshore wind, wave, tidal and ocean current energy exist in. al. ocean and the energy types are being inspected as major sources for power generation.. M. Because of the technical limitations and economic considerations, developments in thermal energy is limited (Elghali et al., 2007). Tidal energy has the benefit of being less. of. responsive to climate change; while solar, wave, wind and HydroElectric Power (HEP) are sensitive to the random changes in renewable fluxes brought due to the shifts of. ity. climate regimes (Nicholls-Lee, 2011).. ve rs. Most of the technology related to the popular tidal energy extraction device namely. horizontal axis tidal turbines (HATTs) is derived from the wind industry. However, the working fluid in which they operate produces higher structural loading with additional. ni. biological fouling, probable contact with the free surface, amplified material corrosion. U. and the probability of cavitation on rotor blade. Therefore, the design principle for a HATT needs a high degree of robustness with minimum maintenance schedule to cut down both installation and operational cost.. There are several design aspects that could be improved to maximize the energy capture which is the most apparent aim of the device. The major turbine components that have the key impact on energy extraction from the tidal flow are the rotor blades. The existing devices are being suffered greatly from the low energy extraction capacity both 2.

(17) above and below rated condition. The efficiency, and hence, annual power extraction, of a turbine could be amplified by altering and optimizing the blade design, while maintaining minimum load on rotor. In this study, hydrodynamic performance investigation for a novel horizontal axis tidal turbine with variable length blade has been carried out.. 1.2. Present status of renewable energy in Malaysia. a. Malaysian energy sector is basically dependent on oil (49.7%) and gas (20.1%) sources. al. from natural gas was 39,973 ktoe (STEC, 2015).. ay. as the primary energy source (STEC, 2013). In 2013, Malaysia's major energy supply. M. In Malaysia, the target is to achieve 5% of renewable energy contribution for the nation's electricity, which was about 1% in last decade (Yaakob, Ab Rashid, & Mukti,. of. 2006). Intensive study on different ocean renewable energy sources are carried out in. ity. Malaysia. Initially, an extensive study has been carried out by Yaakob et al. (2006) based on the available oceanographic data of Malaysia to determine the prospect and suitability. ve rs. of different ocean renewable energy sources (ocean thermal energy, tidal energy, wave energy, salinity gradient and marine current) and concluded that the Malaysia is comparatively less potent location for ocean-derived renewable energy. Mirzaei,. ni. Tangang, and Juneng (2014) has studied the wave energy potential along the east coast. U. of Peninsular Malaysia and the findings showed that the annual average wave power for selected sites are between 0.5 kW/m and 4.6 kW/m in the northern and southern section of the coast.. However, Lim and Koh (2010) has identified potential tidal energy extraction sites which were predicted to produce 14.5 GW h/yr. In Sabah, tidal energy potential was predicted at 8188 GWh/yr from Kota Belud and 386 GWh/yr from Sibu. Although this is a minor portion of the total energy demand but it still postures a probable solution to the 3.

(18) energy demand (Koh & Lim, 2010). Universiti Teknologi Malaysia (UTM) has started an OTEC Centre which is founded in Kuala Lumpur but it would be observing Sabah as its field of study and research. 1200 m depth is observed within 125 km distance from the shore and the temperature at the bottom is about 41º C at 1200 m water depth (Yaakob, 2013).. The Straits of Malacca has identified to have a great potential for marine renewable. a. energy extraction because of its average tidal current speed of 2 ms-1 (4 knots) (Chong &. ay. Lam, 2013). Marine Renewable Energy Research Group (MREUM), University of. al. Malaya has been conducting extensive research work on the tidal energy extraction. M. devices that are suitable to install this location since 2013. One of the major focus of the studies is the hydrodynamic performance improvement of both horizontal and vertical. of. axis tidal turbine. As a part of the research work, a model horizontal axis tidal turbine. 1.3. ity. with variable length blade has been selected for present study.. Problem statement. ve rs. Tidal stream energy extraction devices are mainly categorized into vertical axis and. horizontal axis turbines depending on the rotor’s axis of rotation. A typical vertical axis tidal turbine contains multiple hydrofoil-shaped blades that are attached vertically. ni. between a bottom and top support and the blades rotate perpendicular to the direction of. U. flow (Kiho, Shiono, & Suzuki, 1996). Horizontal axis tidal turbine (HATT) consists of multiple blades which rotate parallel to the direction of flow. The majority of the tidal turbines designed for energy extraction are the horizontal axis tidal turbines (Batten, Bahaj, Molland, & Chaplin, 2008) with the advantages such as self-starting mechanism, reduced gear coupling requirement, and pitch control system to avoid excess mechanical load in high tidal stream velocities that has already been proven in similar fields like wind. 4.

(19) engineering and propeller. World’s first 1.2 MW commercial scale tidal turbine SeaGen is a twin rotor pitch controlled horizontal axis tidal turbine (Fraenkel, 2010).. The energy extraction by the tidal stream devices can be improved in some straight forward ways. First one is the employment of improved blade design, which leads to better hydrodynamic performance or higher efficiency, can be employed to maximize output power of a tidal turbine. The maximum theoretical efficiency of an ideal kinetic. a. energy extraction device in a free stream is 59.3% (Yuce & Muratoglu, 2015; Fraenkel,. ay. 2014; Guney, 2011). In practice, after considering actual hydrodynamic performance and. al. efficiency losses associated in the generator and the gearbox, maximum power coefficient. M. achieved by most modern wind turbine is much lower than theoretical limit (52%) (Pasupulati, Wallace, & Dawson, 2005). SeaGen has reported to achieve maximum 48%. of. rotor efficiency (Fraenkel, 2010). Therefore, there is a little scope of improvement while considering associated cost involved in manufacturing that negates any benefits. Another. ity. option of increasing energy output is to increase swept area of rotor using larger blades,. ve rs. however, both the structure and the components must be built with appropriate ultimate strength to survive in extreme weather condition involves extensive additional cost.. A few investigations in wind and ocean turbines have claimed that the use of ducts or. ni. diffuser which increases the velocity of flow at the rotor plane, as a successful technology. U. to increase the power extraction efficiency. However, Fraenkel (2010) stated that the device efficiency when referenced to the cross-section of entry is no better than for a turbine of a similar swept-area to the cross-section of the entry flow to the duct. These improvements in device performance are achieved through the application of different controlling approaches (which maximize energy capture as well as cut down system loads) such as adjustable speed, pitch, tethered, flexible blades and so on. All these control systems existing in tidal turbine industry uses turbine blades with fixed length.. 5.

(20) Variable length blade turbine is a comparatively new non-conventional power and load control system which is until to date implemented particularly in horizontal axis turbines in wind industry. This system is capable to improve energy extraction at the low wind speed by extending the blade. Thus, the cost per unit electricity production will reduce greatly and its extendibility/ fold ability of blade also offers an inexpensive way to mitigate the site specific turbine design (Pasupulati et al., 2005). A similar concept was. a. proposed in 2010 for tidal turbine by the name of folding tidal turbine (FTT) was. ay. mentioned by Lam and Bhatia (2013).. al. The cost per unit electricity production is relatively higher in marine industry than the. M. other sources of renewable energy since the fixed blade length turbines can extract energy efficiently only at the rated tidal velocity for a specific site. Thus, implementation of this. of. simple mechanism can be a major solution to lessen this problem. However, limited performance data is available for such type of wind turbine rotor and no performance. ity. investigation data is found for tidal turbine. The present study aims to numerically. ve rs. investigate hydrodynamic performance (thrust, moment and power coefficients) and power capture at various tide speeds of a variable length blade HATT and compare the obtained results with existing standard tidal turbine.. Aim and objectives of the study. ni. 1.4. U. The aim of the research is to propose an effective way to increase energy extraction. using variable length blades of horizontal axis tidal turbine (HATT). The following objectives were set to meet the aim:. 1. To validate blade element momentum theory (BEMT) based simulation software QBlade and computational fluid dynamics (CFD) simulation software ANSYS FLUENT used for the study.. 6.

(21) 2. To predict the non-dimensional performance characteristics of a variable length blade horizontal axis tidal turbine (HATT) model at extended blade conditions using the numerical simulation. 3. To evaluate the power extraction of the HATT model through BEMT and CFD numerical modelling for different blade extensions.. 1.5. Scope of the study. a. The focus of this research work is to investigate the hydrodynamic performance. ay. of a model variable length blade HATT through numerical computation with the intention. al. of proposing an effective way to increase energy extraction by tidal turbine. Only the. M. rotor part (blades and hub) of the turbine is modelled and selected for performance study. The other parts of the tidal turbine are considered to have negligible impact on the. of. performance.. ity. BEMT is used to predict hydrodynamic performance parameters i.e. power, thrust and torque coefficients and the associated power output of the full scale variable length blade. ve rs. HATT for different blade extensions varying the tide speed. CFD is a time expensive method of performance analysis, so, the performance is predicted for the rotor model with maximum and minimum blade extension only to study the effect of blade extension. Mesh. ni. density (coarse, medium and fine) and time step (0.1s~0.001s) dependence is inspected. U. for constant tip speed ratio (TSR) to find an optimum time step and mesh density for faster and accurate solution. Power, thrust and torque coefficients and the power output are evaluated for various tip speed ratio (TSR) altering the tide speed from 0.5 ms-1 to 2.5 ms-1.. 7.

(22) 1.6. Outline of the dissertation. The complete study is presented in the five different chapters given as:. Chapter one illustrates the general background, drives for the research on harnessing energy from tidal stream. Objectives and scope of the research are also provided and then concluded with the outline of the thesis through the study.. a. Chapter two outlines the devices under development and considerations of the blade. ay. design. A review of previous research works related to tidal turbine hydrodynamics, different power and load control system is provided. Theory behind two-dimensional. al. (2D) and three-dimensional (3D) computational methods (CFD and BEMT) are also. M. explained.. of. Chapter three consists of the description of the model HATT with variable length blade that is used for the performance study. the step by step detail of simulation techniques. ity. (CFD and BEMT) associated for the variable length blade HATT study.. ve rs. Chapter four contains the result and discussion of the work. Validation results of the. BEMT code and CFD modelling used in the study is also provided in the section. Essential graphs are given sequentially for discussion and resultant data has been. ni. analyzed. The results obtained from computation have been evaluated towards a. U. comprehensive investigation of the capability in enhancing power capture.. Chapter five enumerates the summary of the whole work including the conclusions. Finally, the thesis finishes with some suggestion on probable future works.. 8.

(23) CHAPTER 2: LITERATURE REVIEW. Harnessing energy by Marine Current Energy Devices (MCEDs) offers a sustainable and predictable alternative to other renewable energy technologies (Frost, 2016; Rourke, Boyle, & Reynolds, 2010). Tidal stream technology has seen a rapid expansion in recent years with over 50 devices now in development, several devices at the commercial deployment stage and arrays of devices in the planning stage. This chapter outlines the. a. different types of device under development and identifies some of the design. ay. considerations. An overview of the relevant theory used to assess the performance of a. al. HATT is discussed. Details of the underlying theory of the numerical modelling are also. M. given.. Tidal energy extraction devices. of. Tidal energy is one of the most used renewable energy for many decades, however the. ity. requirements of power are significantly excess compared to the output of the preliminary devices. A modern tidal power plant was built at La Rance, France in 1967 and was the. ve rs. first successfully used commercial purpose device (Nicholls-Lee & Turnock, 2008). Overshot waterwheel and paddlewheel are the primary hydro-mechanical devices which have efficient bulb type hydroelectric turbine generator sets whereas, French barrage have. ni. twenty-four, low head, 10MW, bulb type turbine generator sets and it has been working. U. for 40 years producing around 600 GWh/year (Perier, 2007).. In 1990, the first tidal turbine was introduced in Corran Narrows, Loch Linnhe, Scotland, as a “proof of concept” model. The diameter of this turbine was 3.5 m and this was designed for achieving a maximum 10 kW shaft power but recorded highest consistent power was more than 15 kW (Macnaughton, Fraenkel, Paish, Hunter, & Derrick, 1993). In September, 2003 world’s first tidal stream turbine grid connected was. 9.

(24) installed in Norway coast near Kvalsund, which was an improved model of this turbine with 3 MW designed capacity at 2.5 ms-1 current (Roach, 2003).. Most of the extraction devices of tidal energy can be categorized on the basis of fluid motion type (linear or rotational) produced by them, the direction of the rotor axis or linear motion and the insertion of flow acceleration mechanism. Devices can be horizontal axis, vertical axis, hydrofoil, variable pitch, fixed pitch, zero head, lagoon, tethered,. a. barrage, ducted, water column, surface piercing, azimuthal, submerged, bi-directional.. ay. All these devices can be divided into three basic categories which are vertical axis tidal. al. turbines (VATTs), horizontal axis tidal turbines (HATTs) and alternative devices.. ni. ve rs. ity. of. M. Horizontal axis tidal turbines (HATTs). b) Artistic impression of operation (Taylor,2007). U. a) Raised above condition (Fraenkel, 2010). Figure 2.1: SeaGen device developed by Marine Current Turbine (MCT). The operation of this type of tidal turbine is similar to horizontal axis or axial flow wind turbines as the rotational axis is parallel to tidal flow and the device contains hydrofoils radially structured around the hub. As the relative fluid flows over the airfoil. 10.

(25) shaped rotating blades, it produces a pressure variance, and hereafter lift and drag forces. The lift force completely dominates over the drag force which resulting turbine movement around the rotational axis. This type of rotors are self-starting.. Horizontal axis tidal turbines (HATTs) tend to have higher efficiencies in comparison with VATTs but are more complex in design. The typical blade design includes twist and taper to achieve higher efficiency (Khan, Bhuyan, Iqbal, & Quaicoe, 2009). Generally,. a. peak efficiencies of HATTs range from around 39% to 48% (Jo, Lee, Kim, & Lee, 2013;. ve rs. ity. of. M. al. ay. Mason-Jones, 2010).. (Tidal Energy, 2012). U. ni. (SMD, 2016). (Power, 2015). d) Deep Gen (Generation, 2010). Open Hydro, (2012). Figure 2.2: Examples of horizontal axis tidal turbines. 11.

(26) Existing commercial turbine SeaGen is a horizontal axis tidal turbine with 1.2MW capacity, shown in Figure 2.1. It was developed by Marine Current Turbines (MCT) and installed in UK waters in 2008. Horizontal axis tidal turbines (HATTs) are the mature and most promising tidal turbine technologies. There are many forms of HATT, depending on the basis of blades number and supporting structure type of the device to fix in position.. Other HATTs at various degrees of development (Figure 2.2) are the 1 MW TidEL. a. from SMD (SMD, 2016), OpenHydro (OpenHydro, 2012), Kinetic Hydropower System. ay. (KHPS) by Verdant Power ( Verdant Power, 2015), the 500 kW Deep Gen from Tidal. al. Generation Ltd (Tidal Generation, 2010) (now Alstom) and the 1.2 MW Delta Stream. the near future ( Tidal Energy, 2012).. M. from Tidal Energy Ltd scheduled to be deployed at Ramsey Sound in Pembrokeshire in. of. Vertical axis tidal turbines. ity. In vertical axis tidal turbines (VATTs), the rotation axis is perpendicular toward fluid flow and these devices are either drag based or lift based. Lift based rotors operate in. ve rs. similar manner of the horizontal axis turbines. On the other hand, drag based rotors operate like water wheel. As the fluid hits the blade, it rotates the turbine. Savonius turbines (Figure 2.3a) are drag based design and Darrieus turbines (Figure 2.3b) are lift. ni. based design. A relatively new type of turbine is cycloidal vertical axis tidal turbine. The. U. operation concept of this turbine is much similar to a typical Darrieus vertical axis turbine. Rotor blades of this turbine have an adjustable angle of attack and, each blade have the ability to rotate upon their individual axis for process optimization.. The main advantage of a VATT is that it’s operational efficiency is independent of the direction of tidal flow and can rotate the blades of rotor without any pitch or yaw mechanism (Eriksson, Bernhoff, & Leijon, 2008). In addition, the simple straight-blade design of VATTs requires less design and manufacturing costs while compared with 12.

(27) HATT blade (Khan et al., 2009). VATTs also produce less noise due to the lower rotational speed.. One major disadvantage of VATTs is lower peak efficiency (Khan et al., 2009) which is around 37% to 40% (Eriksson et al., 2008; Han, Park, Lee, Park, & Yi, 2013). Other disadvantages of VATTs include the low starting torque and for this reason VATTs may require starting mechanism (Khan et al., 2009); and the main reason for torque ripple is. ve rs. ity. of. M. al. ay. a. to the change in attack angle with the rotation cycle (Eriksson et al., 2008).. b) Darrieus turbine (Boyer, 2013). Figure 2.3: Vertical axis tidal turbines. U. ni. a) Savonius turbine (Flowers, 2011). Alternative turbines Working principle of some turbines is quite different from that of the horizontal and vertical axis turbines. Oscillating hydrofoils and venturi effect devices (Figure 2.4a to 2.4b) are such two major types of the alternative devices. Oscillating hydrofoils capture energy from tide using oscillatory motion apart from rotary motion. The arm of such device is lifted as the hydroplane being lifted by currents. Hydraulic cylinders are actuated 13.

(28) at the arm or frame junction with this arm lift and the resulting high-pressure oil revolves a hydraulic motor which, in turn, drives generator. Once the arm and hydroplane touches their higher limit, the angle of hydroplane is upturned and the cycle is repeated (Goldin, 2001). The seabed mounted Stingray (Fraenkel, 2006), Pulse Stream (UK, 2011) and bioSTREAM (Systems, 2013) are the examples of such type of device. At certain instance, it only uses small percentage of the available energy and thus has an unlikely. a. high efficiency while compared with the vertical and horizontal axis turbines.. ay. Venturi effect devices are two types. The first one is basically a HATT or VATT with. al. a duct around the device which improves the velocity of flow, for example the Lunar. M. Tidal Turbine (Energy, 2012) and the Neptune Proteus. The second type utilizes a venturi and utilizes the drop in fluid pressure at the throat to draw a secondary fluid through a. of. distinct turbine, for example the Spectral Marine Energy Converter (VerdErg, 2013). This design is advantageous as it can generate constant power and has no immersed moving. ity. parts; however, pure venturi effect devices do not possess high relative energy extraction. ve rs. efficiency that of other less complex turbines.. The advanced turbine designs include the HATTs which are used to power hydraulic. energy converters based onshore (Jones & Chao, 2009), Minesto connect the turbine into. ni. a kite that is ‘‘flown’’ in the tidal stream (Minesto, 2015), Flumill uses two counter-. U. rotating helical screws mounted parallel to each other (Flumill, 2016), and Tidalsail uses sails to convert current to electricity (Tidalsails, 2013).. 14.

(29) b) Lunar Tidal Turbine (Energy, 2012). ay. a. a) bioStream (Systems, 2013). al. Figure 2.4: Examples of major alternative turbines. M. Securing, installation and maintenance. of. It is difficult and challenging to install and recover tools within a fast-flowing tidal stream. The thrust on a marine turbine is considerably higher than that experienced by a. ity. same rated power wind turbine, although the first one is much smaller. Therefore, holding the rotor consistently and securely in place is the most important structural difficulties in. ve rs. the severe and corrosive subsea environment (Kirke, 2005; Orme, Masters, & Mima, 2006).. ni. To allow the operation of the HATT, there must be means of fixing the turbine at. U. some depth over the water column. The means by which the turbine is attached will greatly depend on the depth of the water and proximity to the nearest onshore service location (Snodin, 2001). Several concepts for the tidal turbines fixing are under consideration that range from pile-mounted turbines to moored turbines, those subjected to the sea bed and semi-submersible designs.. 1. gravity base, in this case the device is tied to a weighted structure, as used in OpenHydro (Figure 2.2e);. 15.

(30) 2. piled devices, like SeaGen these devices installed to either single or multiple piles (Figure 2.1); 3. flexible moorings that are made up of of a tether with cables, chains or ropes and 4. anchor which is used for securing the device to the seabed allowing alignment with approaching tidal flows or waves. Some of these devices. a. involve contra-rotating rotors. Either the rotors have separate but parallel. ay. axes of rotation, like TidEL (Figure 2.2a) or have the same axes of rotation, as investigated by Clarke, Connor, Grant, Johnstone, & Ordonez-Sanchez. M. remains aligned with the flow.. al. (2008). Consequently, zero net torque is produced and hence the device. of. Other securing methods that have been proposed involve several hydrofoils mounted to the structure which grip the device in position through down forces. ity. resulted from tidal flows (EMEC, 2012). Whichever scheme is selected, all. ve rs. reliability and safety related fes must be considered, and should be cost effective (Harris, Johanning, & Wolfram, 2004). Pile based structures seems to be costly and its application is limited up to depth 40 m by the recent technology ( Clarke,. U. ni. Connor, Grant, Johnstone, & Ordonez-Sanchez, 2010).. Turbine blade design considerations. Turbine rotor blades being the key components of the energy conversion process, one. of the most major design aspect that have potential for improvement. As the horizontal axis tidal turbine (HATT) design has to face different problems while functioning the same structure in air; hence, the geometry and twist of blade vary from those used on the HAWT. As the fluid density is different, the thrust experienced by horizontal axis tidal turbine (HATT) is considerably more than that produced by a horizontal axis water. 16.

(31) turbine (HAWT) of the same power at rated condition (Nicholls-Lee & Turnock, 2007), although the swept area of HATT have been considerably small. Some other loads that exist on a HATT but not experienced by HAWT involve cavitation, wave loading and increased cyclic loads. The changes in static pressure and velocity over the vertical water column also impose cyclic dynamic effects on the rotor blades.. Tidal turbine design is a balance between energy yield and unit energy production. ay. device including (Clarke, Connor, Grant, & Johnstone, 2007):. a. cost. Several points are needed to consider while designing a tidal energy extraction. al. 1. requirement of strong anchorages as extreme downstream drag forces are. M. produced because of strong tidal streams. 2. corrosion or dependability of submerged components,. of. 3. the turbine must be tied with anchor in such a way which permits periodic. ity. maintenance and equipment repair, whereas this is expensive in the plain sea condition and the expense should be reduced,. ve rs. 4. turbine efficiency reduction because of the marine growth on the rotor blades,. 5. impact on other wild life and marine traffic in the area where the device is. U. ni. installed,. 6. damage of the turbine and superstructure due to storm, 7. efficient energy harness from reverse current flows which might not be completely rectilinear, and design for reliability and lifetime performance, together with decommissioning as a HATT is expected to experience around 1x108 rotational cycles over a 20 years life. 8. device matching with generator, as the generator needs to run at nearly constant rotor revolution speed or within an operational RPM range. In the. 17.

(32) first case, a mechanism is required to adjust the pitch of the blade to control output power, while in the second case a relatively simpler fixed pitch blades can be used.. Turbine blade performance Turbine performance assessment is dependent on three different characteristics measures, these are:. (2.1). ay. a. Power coefficient, C. al. where, P is the power produced from a rotor revolution speed of n revs/sec and M is. M. the generator torque. Considering the Reynolds number effects negligible, the actual power harnessed by a geometrically similar turbine is proportional to the cube of the free. C ρAU !. ity. P. of. stream velocity and the rotor cross sectional area, as illustrated below:. #$. ve rs. Thrust coefficient, C". (2.2). (2.3). Torque coefficient, C. (2.4). U. ni. Here, T is thrust loading which should be repelled by the turbine supporting structures.. C. %&. $'(. (2.5). Power coefficient, CP is boosted by controlling the pitch of the blades which may be described by considering the tip speed ratio, TSR. This is the ratio of blade tip velocity, Utip, to tide speed, U. With respect to performance, CP may be optimized for a particular value of TSR. Similar to the wind industry, Utip should be varied through the pitch control. 18.

(33) of the turbine blades to maintain a constant TSR as the tidal stream velocity is not constant. Modelling studies suggest that for low velocity sites, a 23% increase in annual energy capture is expected to achieve by the variable pitch devices in comparison with the fixed pitch devices (Molland, Bahaj, Chaplin, & Batten, 2004).. The effective onset speed experienced by a local section will depend on the relative involvement of the uninterrupted free-stream liquid velocity, U, and that, owing to the. ay. 2+,. -. !.. (2.6). al. ). a. speed of rotation,. M. where, N is rotor blade revolutions per minute. For a turbine of radius, R, the tip speed efficiently controls the relative velocity, and is defined as:. of. /(. (2.7). ity. TSR. ve rs. Thus this ratio controls the overall performance of the turbine.. Power and mechanical load control systems. Power and blade load control can be implemented either by the mechanism that. ni. entirely affecting the rotor, or via devices mounted on blades (or blade itself). Wind and. U. marine turbines use power and load control systems mainly to. i.. improve power extraction at low wind/tide speeds and. ii.. control the rated power of rotor at high wind/tide speeds to avoid overloading of the generator.. 19.

(34) a ay. M. al. Figure 2.5: Different control systems affecting blade performance. Figure 2.5 (adopted from Wiratama, 2012) shows a number of nonconventional and. of. conventional power and mechanisms of load control that affect the rotor performance.. ity. Some control systems react only to the differences of fluid flow with extended time scales, whereas others have shorter time scales and hence can be utilized for regulating the. ve rs. impact of flow variations by the smaller time-scales. As shown in the figure, control systems can be categorized depending on the controlling factors affecting the blade cross-. ni. section (airfoil), blade span and twist.. U. All control systems mentioned in Figure 2.5, apart from the telescopic or variable. length blades, modify the performance of turbine by imposing a variation in the angle of attack. The angle of attack is linked to the blade twist angle, blade pitch angle and inflow angle.. The control methods also classified as active and passive. In the active control system, regulating parameter have to be adjusted through commands from controller and is independent of the turbine operational condition and this offers a whole control on power. 20.

(35) and/or rotor blade loading. The flow kinematics nearby a blade part is guided by regulating parameters (i.e., morphing airfoil, microtab, telescopic blades, flap), the total blade (i.e., individual and conventional pitch control systems), or the whole rotor (i.e. yaw, tilt and rotor speed).. In passive control system, the regulating parameter is influenced by the turbine operational condition. Actually, no distinct regulator remains in place. The rotor blade. a. also acts as a regulator. Flow kinematics around the entire blade have been affected by. ay. the variation of the turbine operational condition (e.g., tide speed) either by altering. al. inflow angle (i.e., stall-regulated blades), or by altering both blade elastic twist and inflow. M. angle (i.e., blades of adaptive). Turbine operational condition variation has been leads to limited control on rotor power and/or blade loading.. of. In case of telescopic and adaptive blades as well as blades using morphing airfoils,. ity. microtabs and flaps, modifications are essential to apply on the baseline rotor blade. ve rs. topology and/or geometry and/or aerodynamic/hydrodynamic characteristics.. The following section describes the scope of using one of the simple but effective. nonconventional control mechanisms namely telescopic or variable length blade which is. ni. mainly focused in the study. An overview of the performance analysis from previous. U. research works on wind turbine and tidal turbine will be discussed to analyze feasibility of the performance study of telescopic/foldable or variable length blade HATT.. Variable length Blade control system The concept of turbine with variable rotor diameter at various operation speeds is one of the most recently revealed power and load control system in wind industry which was patented to Dawson & Wallace (2009). This novel concept is shown in Figure 2.6. It is shown that the wind turbine can increase the energy extraction by extending a tip blade. 21.

(36) out of a root blade to change the diameter in low wind speeds and also reducing loads on the rotor in high wind speed conditions by retracting the same (Dawson, 2006; GE Wind Energy, 2006; Pasupulati et al., 2005).. A prototype was manufactured and field tested by a collaboration of DOE, Energy Unlimited Inc. and Dawson (2006). A number of research and development areas including aerodynamics, control, and manufacture optimization of the variable length. a. blades were identified after the field tests. Field tests have been showed that the energy. ay. taken by the variable length bladed turbine (blade length varies from 7.5 m to 10.8 m). al. was increased by 25% with the increase of blade length up to 44% (Pasupulati et al.,. M. 2005).. An analytical investigation (McCoy & Griffin, 2006) showed agreement with field. of. data and stated that blade length increase by 28% could increase the energy capture by. ity. 21%. In addition, another analytical study by Sharma and Madawala (2012) showed that, blade length increase by 50% can double the energy harnessed by the fixed bladed wind. ve rs. turbine. Recently an analysis on a 10 kW telescopic blade horizontal axis wind turbine (TBHAWT) indicated an increase in 18% energy output (Imraan, Sharma, & Flay, 2013). Apart from improved energy yield and mechanical load control strategies, the variable. ni. length blade rotor concept offers advantages including reduction in shipping and. U. installation cost and requirement of site specific rotor design. This concept is still at development stage in wind industry and information about the aerodynamic performance are limited.. In marine industry, a recent study investigated a similar concept of extendable bladed vertical axis tidal turbine (VATT) namely, folding tidal turbine (FTT) and identified significant reduction in transportation and installation cost (Lam & Bhatia, 2013). However, hydrodynamic performance information of the tidal turbine to represent energy 22.

(37) capture phenomena is still unavailable. Such type of power control concept has not yet been investigated for HATT to inspect viability of increasing the power extraction. ve rs. ity. of. M. al. ay. a. particularly at low tide speed.. Figure 2.6: Variable length blade turbine concept. ni. Two-dimensional foil performance. U. Two-dimensional foil performance is analyzed by performance per unit span i.e. in. terms of the drag, local lift and moment coefficients.. 1. 0. 23 4. 1. 0. 23 4. 1. 0. 23 4. (2.8). (2.9). (2.10) 23.

(38) where, dL and dD are the lift and drag forces in the perpendicular and parallel. M. al. ay. a. directions to W, and dm is the moment about z axis (Figure 2.7).. of. Figure 2.7: Foil orientation. ity. Typical techniques of representing 2D performance data are in terms of change in angle of attack or drag as a lift function. Lift data varies almost linearly with angle of. ve rs. attack, until stall (where substantial areas of flow separation occur). Flow separation changes the drag regime from one dominated by viscous shear (caused by the shearing of. ni. a viscous fluid over the surface of a body due friction), to one where pressure drag (the form drag created as a body is moved/moves through a viscous fluid) predominates. U. (Molland & Turnock, 2011).. At the stall condition, considerable drop in lift occurs because of the movement in the effort centre of the developed force, and rapid increase in drag force occurs. In case of stall, the speed of the process development is the vital factor. One of the turbine speed control process employs stall regulation where the decrease in lift and increase in drag controls the attainable driving torque. In this process, 3D effects are vital to the efficiency with which stall regulation can be used. 24.

(39) Avoiding cavitation, maximum lift to drag ratio is the preferred operational condition of foil. As the turbine rotates and the tip speed ratio modifies the effective angle of attack experienced by the section is effected, thus the section behavior away from the optimum is of significant importance. One efficient method for defining shape of section utilizes a camber line to describe the mid-thickness position height, around which a deviation in thickness is set as a function of thickness/chord ratio. A foil with zero camber is. a. symmetric in shape and will produce zero lift at zero angle of attack. A foil with camber. ay. will produce positive lift at zero angle of attack with the lift value dependent on camber.. al. Blade element momentum theory. M. One of the most common and oldest computational method for wind and ocean turbine performance analysis is BEMT, which combines the blade element theory and momentum. of. theory to inspect turbine performance. The momentum theory is based on the momentum balance along the rotating annular stream tube passing through a turbine plane. It is. ity. assumed that work done by the moving fluid passing through the turbine causes a pressure. ve rs. loss at turbine plane. The induced velocity in axial and tangential direction due to the loss of momentum can be determined by this theory.. In the blade element theory, each of the rotor blades are assumed to be divided into. ni. infinite number of independent elements, as shown in Figure 2.8. The hydrodynamic. U. forces can be determined based on the condition of local inflow from the local airfoil’s hydrodynamic characteristics. The lift and drag forces are calculated for each blade section and then integrated along the blade span to get the moment and forces acting on the turbine.. 25.

(40) a ay. M. al. Figure 2.8: Blade element momentum analysis of HATT. of. In blade element momentum theory, these two approaches are coupled to provide an iterative procedure that inspect induced axial and tangential velocity and then calculate. ity. hydrodynamic forces. The detail of the BEMT based numerical code used for this study. ve rs. is described by Hansen (2008).. Momentum theory. In momentum theory, turbine rotor is initially considered as an infinitely thin actuator. ni. disk. The actuator disk represents a rotating device with infinite number of blades. As an. U. extractor of energy, the impact of the rotating mechanism depends on the step change in static pressure and thus varies the total pressure along a streamline, whereas retaining steadiness of flow speed. The outer stream tube of the upstream area (less than the disk area) enlarges once passing the disk. The static pressure is initially below atmospheric and the speed of flow less than free stream at this expansion region or wake. Since the static pressure through the wake matches to the atmospheric pressure, additional expansion happens and further reduction in the flow speed occurs.. 26.

(41) Figure 2.9 demonstrates four stations along the stream tube: 1 – some way upstream of the turbine rotor, 2 –just before the rotor blades, 3 – just after the rotor blades and 4 – some way downstream of the turbine rotor. Between stations 2 and 3 energy is extracted from the tide that results in a pressure change. Assume P = P5 and U2 = U3. Between stations 1 and 2; and between stations 3 and 4 also assume that the flow is frictionless. So Bernoulli’s equation can be applied which. 6 +. 89. and. 6! +. ay. 89. 89!. 65 +. 895. (2.11). U. ni. ve rs. ity. of. M. al. 6 +. a. yields,. Figure 2.9: Single stream tube analysis. Hence, 6 − 6!. 8(9 − 95 ). (2.12). For flow down an annulus at position r and thickness dr, and as force equals to the pressure divided by area: 27.

(42) @A. >6 − 6! ?@B. (2.13). 8(9 − 95 )@B. Then, @A. (2.14). Considering the drop in axial flow speed through the turbine, the axial induction factor, a, is:. (2.15). D1. 9 (1 − C). and 95. 9 (1 − 2C). M. 89 G4C(1 − C)I2+J@J. (2.17). U. ni. ve rs. ity. of. @A. (2.16). al. Substitution yields:. a. 9. D1 ED. ay. C. Figure 2.10: Rotating annular stream tube. Now considering the rotating annular stream tube, Figure 2.10, with the same four stations as described earlier; due to the turbine rotation, as the water passes between stations 2 and 3 the blade wake also rotates. Consider angular momentum is conserved in. 28.

(43) this annular stream tube. The blade wake is considered to rotate with an angular velocity, C and the angular velocity of the rotating blades is ). Reminding from basic physics that: Moment of Inertia of an annulus, K. 0. Torque, N. 0. (2.18). K. (2.19). 0>OP?. 0> Q P?. 0. 0. 0. 0. J. (2.20). 82+J@J9. 8B9. Thus,. 82+J@J9. J. ve rs. @N. ity. @LR. of. For the rotating annular element. al. @LR J. (2.21). M. @N. ay. So for a small element the corresponding torque will be:. a. Angular Moment, M. LJ. (2.22). (2.23). According to the definition of angular induction factor, a´ P. T. (2.24). U. ni. C´. Combination of equations (2.16), (2.23) and (2.24) provides the torque equation of the. rotating annular element of fluid as: @N. 4C´(1 − C)+89 J ! @J. (2.25). Therefore, momentum theory has yielded equations for the axial force (Equation 2.17) and momentum (Equation 2.25) on an annular element of fluid.. 29.

(44) Blade element theory Two-dimensional (2D) foil characteristics can be used to calculate the lift and drag forces acting on a turbine blade by means of blade element theory. Each blade is divided into a number of 2D sections which are the blade elements. Blade element theory is established based on two key assumptions:. There are no hydrodynamic interactions between the elements of blade.. ii.. The forces acting on the elements of blade are only dependent on the drag and lift. a. i.. ay. coefficients.. al. The blade is divided into a number of small parts, and later the drag and lift forces. M. acting on each elements of blade are determined. The forces can be integrated along the blade, and over one revolution of rotor (if the inflow is non-uniform) to find the forces. of. and moments generated by the whole turbine rotor. Figure 2.8 demonstrations forces and. ity. velocities acting on a single blade element.. ve rs. Each elements of blade experiences a little change in flow, as they have a variation in rotational speed, twist angle and chord length. The relative velocity, W of the blade section is a combination of the axial (1 – a)U1 and the tangential (1 + a´)ωr velocity at. ni. the turbine rotor plane (see Figure 2.8). θ is the local blade pitch angle is the local angle. U. between the axis of chord and the rotational plane. The local angle of pitch is a. combination of the blade pitch angle, θp and the blade twist, β as θ = θp + β, where the pitch angle is the angle between the tip chord axis and the rotor plane and the twist is the angle measured relative to the tip chord. U is the angle between the rotational plane and the relative velocity, W and Figure 2.8 illustrates the local angle of attack which is given. as: V. U−W. (2.26). 30.

(45) Further, it is seen that:. tan U. > EY?D1. (2.27). > ZY´?TQ. By definition, the drag is parallel to the relative velocity, W and the lift is perpendicular to the same velocity experienced by the airfoil due to the vortex system of a tidal turbine. In addition, if the drag and lift coefficients CD and CL are known, the drag, D and lift, L. 8[ \. ]. 8[ \. (2.28). al. M. ay. a. force per unit length can be obtained from the following equations:. (2.29). of. M. and:. ity. Where, c is the sectional blade chord length.. ve rs. Thus, the normal and tangential forces acting on the rotor plane are given by: ^. M cos U + ] sin U. (2.30). U. ni. And:. ^. M sin U − ] cos U. (2.31). The equations (2.30) and (2.31) are normalized with respect to 8[ \ yielding, cos U +. sin U. (2.32). sin U −. cos U. (2.32). and:. 31.

(46) where,. 1. cd. (2.33). ce. (2.34). 23 4. and:. 23 4. ay. From Figure 2.8, it is clear from the geometry that:. a. 1. 9 >1 − C?. [ cos U. )J>1 − C´?. (2.35). al. [ sin U. (2.36). of. M. and:. ity. In the control volume, the portion of the annular area enclosed by blades is defined as. ve rs. solidity σ:. f>J?. 4>Q?g -Q. (2.37). ni. B defines the number of blades, r is the radial position of the control volume and c(r). U. is the local chord. Since Fi and F$ are the normal and tangential forces per length, the normal or thrust. force and the torque on the control volume of thickness dr are: @A. j^ @J. (2.38). @N. Jj^ @J. (2.39). and:. 32.

(47) Combining equations 2.33, 2.35 and 2.38 yields:. @A. 8j. D1 > EY? klm n. \. @J. (2.40). Similarly, equations 2.34, 2.35, 2.36 and 2.39 provide the torque equation which can be written as:. op n qro n. \. @J. (2.41). Blade element momentum equations. a. D1 > EY?TQ> ZY´?. ay. 8j. @N. al. Momentum theory provides two equations for axial thrust and torque which express. M. the values by flow parameters, and blade element theory provides two other equations for the same parameters that express them by the foil’s lift and drag coefficients. These four. of. equations from momentum and blade element theory are solved to achieve final set of. ity. equation.. Equating equations 2.17 and 2.40 and applying the definition of solidity, an expression. ve rs. for the axial induction factor a is obtained:. stuv ∅ Z xyd. (2.42). ni. C. U. Equating equations 2.25 and 2.41 an expression for the tangential induction factor a´. is obtained: Cz. stuv∅{|t∅ Z xye. (2.43). Pure BEMT possess limitations, to overcome which correction factors are to be introduced in the induction factor equations 2.42 to 2.43 obtained from basic momentum and axial thrust equations. Main objective of the BEMT mainly determines a and Cz using. 33.

(48) iterative method with the initial assumption of axial and tangential induction factor values. The relative velocity W and inflow angle ϕ faced by each blade section can be determined from the assumed induction factors. From inflow angle, the angle of attack is determined and the associated lift and drag coefficients,. and. are obtained. Then. using the equations 2.44 and 2.45 new value of a and C z can be calculated as follows:. Cz m}~. (2.44). Z. Cz + s•tuv∅{|t∅. Z. (2.45). al. xye. a. xyd. ∅. ay. C + s•tuv. Cm}~. M. The original BEMT does not account three-dimensional effects like the tip vortices and hub vortices into the wake on the induced velocity field. To compensate the. of. deficiency, the following corrections have been considered to obtain more robust. i.. ity. performance data.. Prandtl tip and hub loss factor is the most commonly used method for the. ve rs. correction of two-dimensional profile data. However, a recent tip and hub loss correction is proposed by Shen, Mikkelsen, Sørensen, and Bak (2005) which was. ni. compared with Prandtl correction and found to provide more realistic performance. U. data along the blade and also shows better agreement with experimental data in a study by Masters, Chapman, Willis, and Orme (2011). Therefore, for the turbine simulation this method is used in the study. -. ^ l€. cosE [‚. ƒE„.. †>‡ˆ‰? • ‡ Š‹Œ ∅. ]. (2.46). where, •. ‚ E.. •(g. ‘’‰ E. ). 34.

(49) where, R is the radius of the rotor, r is the radius of the sectional element and TSRr is the local speed ratio. Similarly, the hub loss factor is defined as in equation 2.47:. ^“”„. -. cosE •‚. •E„.. †G‰ˆ‡–—˜ I ™ ‡–—˜ Š‹Œ ∅. Ž. (2.47). Therefore, the total loss factor can be determined combining equations 2.46. ay. (2.48). Glauert proposed a correction for thrust coefficient,. is described in detail by. M. ii.. ^ l€ . ^“”„. al. ^. a. and 2.47 as,. Hansen (2008). Buhl (2005) proposed an update of the correction that is used in. of. this study for the simulations of turbine. The correction in axial coefficient value. #cE .E!šPe >•.E! c?Z ! cE•.. c>!cE5?. (2.49). ve rs. C. ity. is given by,. Computational fluid dynamics (CFD). A powerful tool that can be used to numerically analyze the HATTs performance is. ni. through the use of discretization methods such as CFD, where the theory surrounding the. U. methodology is very well-established enabling, if required, the development of customized code. However, ample commercial codes are available that have gone through rigorous empirical testing and evaluation from both academic and industrial application. This has the benefit to save time and the cost involved in the personalized software codes development. The CFD software package ANSYS FLUENT and ANSYS Inc. another software package GAMBIT 2.4.6 was used in a part of the work presented within this document.. 35.

(50) GAMBIT software package is designed to develop and mesh model for ANSYS post processing. The output of the software is the input of ANSYS FLUENT simulation. It has user friendly graphical user interface (GUI) that makes designing and meshing of any model simple and intuitive. Its easy module helps user to design model from scratch, can assign boundary and continuum type and has wide range of curve, face and volume meshing options. The grid size can easily be handled by size function and quality can be. a. checked in terms of skewness.. ay. ANSYS FLUENT applies the finite-volume approach to solve the governing. al. equations for a fluid flow field with predefined or user defined material properties for 2D. M. and 3D domains. It also permits the combined use of several physical models such as those relating to cell motion, turbulence and interaction between free surface (FLUENT,. of. 2006). Several turbulence models are available ranging between One-Equation Models (OEM) and Large-Eddy Simulation (LES). With the increase in complexity of the viscous. ity. models used for analysis computational cost also increases.. ve rs. When utilizing the actual geometry of a HATT blade, the capacity of cell motion. application is vital to calculate the energy extracted from the moving fluid as the apparent flow angle is dependent on the rotational velocity at a specific radius. However, the. ni. computational time can be increased considerably due to the cell density toward the. U. structural surface boundaries when physical geometry is considered (FLUENT, 2006). With the use of Reynolds Averaged Navier-Stokes (RANS) equations, turbulence models can be applied to close the governing equations within significantly reduced solution convergence time when compared with the extreme approaches like DNS. FLUENTTM offers a range of viscous models that fall under the RANS category. These include the one equation Spalart-Allmaras (SA) model and the two equation models such as the Standard k-ε, Realizable k- ε, RNG k- ε, k-ω Shear Stress Transport (SST) and 36.

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