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Performance study on a counter-rotating tidal current turbine by CFD and model experimentation
Renewable Energy xxx (2014) 1e5
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Performance study on a counter-rotating tidal current turbine by CFD and model experimentation Nak Joong Lee a, In Chul Kim a, Chang Goo Kim b, Beom Soo Hyun c, Young Ho Lee a, b, * a
Dept. of Mechanical Engineering, Graduate School, Korea Maritime and Ocean University (KMOU), Busan 606-791, Republic of Korea Division of Mechanical and Energy System Engineering, KMOU, Busan, Republic of Korea c Division of Naval Architecture and Ocean System Engineering, KMOU, Busan, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 March 2014 Accepted 10 November 2014 Available online xxx
Among the various ocean energy resources in Korea, the tidal currents in the South western sea have a large potential for development tidal current power generation. The biggest advantage of tidal power is that it is not dependent on seasons or weather and is always constant. This makes power generation predictable and makes tidal power a more reliable energy source than other renewable energy sources. Marine current turbines convert the kinetic energy in tidal currents for power production. Single rotor turbines can obtain a theoretical maximum power coefficient of 59.3%, whereas dual rotor can obtain a maximum of 64%. Therefore by optimizing the counter rotating turbines, more power can be obtained than the single rotor turbines. In this study, we investigated the effect of varying the distance between the dual rotors on the performance and efficiency of a counter-rotating current turbine by using computational fluid dynamics (CFD) and experimental methods. It was found that the dual rotor produced more power than the single rotor. In addition, the blade gap distance affects the flow on the rear rotor blades as well as power output and performance of the turbine. The distance can be used a parameter for counter rotating turbine design. Finally, the numerical setup used for this study can be further used to evaluate the design of larger counter rotating blade designs. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Performance Counter-rotating Current turbine CFD Experiment
1. Introduction Global warming is one of the serious issues in the world mainly due to the burning of fossil fuels and emission of carbon dioxide to the atmosphere. The importance of alternative energy is steadily rising in the 21st century. In addition to hydropower, wind power and solar power, ocean energy is receiving attention from the industrial sector. Ocean energy conversion can be classified into two categories; Wave energy and tidal energy. These sectors have undergone notable growth in research and technologies in the past decade. Methods to produce power using wave energy range widely from conversion of wave motion, over-topping devices to the use of oscillating columns of air within enclosed structures [1]. However, the efficiency of most wave energy devices is subject to weather and seasonal variations. The main advantage of tidal sources is that
* Corresponding author. Division of Mechanical and Energy System Engineering, KMOU, Busan, Republic of Korea. Tel.: þ82 51 410 4293; fax: þ82 51 403 1214. E-mail address: [email protected] (Y.H. Lee).
it is constant throughout the year and makes energy production predictable and attractive for investment. Also, as tides are the product of the gravitational forces by the sun and moon, tidal energy can be seen as a renewable energy source that will exist for an indefinite time [2]. Tidal energy conversion devices can be divided into two main methods of operation, tidal range power and tidal current power. Tidal range turbines use the height difference in water levels that are caused by the high and low tides within an enclosed structure to produce power in a similar manner to hydroelectric dams. Tidal current turbines convert the kinetic energy within currents to produce power. The horizontal axis tidal current turbine is one of the machines used to harness tidal current energy, which appears to be the most technologically and economically viable one at this stage [3]. In Korea, the available land area is limited and so the maximum use of offshore energy resources can be a boon for the country. Among the various marine energy resources in Korea, there are places where large tidal range differences and fast tidal flows caused by geographical formations. Areas such as Incheon, the western and in the southern seas, the potential for tidal current power generation is large. Tidal range power has had success
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Fig. 1. 2D profile of the blade showing twist and chord lengths and the 3D rotor blade configuration.
In the present study, a 40 W horizontal axis three bladed tidal current turbine was designed using the Blade Element Momentum
Theory (BEMT). BEMT refers to the combination of the momentum theory equations with the blade element theory equations [6]. Using these equations and known airfoil data, a blade profile can be calculated. The rotor has a diameter (D) of the 500 mm, a design water speed of 1 m/s and design rotational speed is 190 rpm. Fig. 1 shows the single airfoil of NACA-63421 with various twist angles and chord length (left) and 3D modeling of a blade (right). Table 1 summarizes the specifications of blade and shows the local radial position; twist angle (q), chord length (C), and the radial length in mm. The 3D model was meshed using ICEM CFD with a total of 6.2 million nodes in a hexa grid. The mesh near the blade was refined to ensure that the y plus value was less than 5. This ensures that a sufficient number of nodes are near the wall to resolve the boundary layer flow and capture changes in variables in the near wall region during calculation [7]. Only a single blade was modeled for calculation with the other blades accounted for by specifying a periodic condition. Fig. 2a shows the hexahedral mesh of the dual rotor setup. The distance from the blade to the inlet, top and outlet was 3, 5 and 7 times the diameter of blade respectively. The model is solved via commercial CFD finite volume solver, ANSYS CFX, ver13. A frozen rotor model was specified and the turbulence model used was keu Shear Stress Transport (SST) model. The SST model is able to model the transport of turbulent shear stress and gives accurate predictions on the onset and amount of flow separation under adverse pressure gradients [8]. The blade was modeled as a wall with no slip condition, while at the inlet an inflow condition was specified. An opening condition was specified at the top and at the outlet, an outflow boundary was assigned as shown in Fig. 2b. A rotational speed of 190 RPM was specified for the blade in the numerical simulation. The steady state simulation was solved with a high resolution advection scheme until the simulation reached the required RMS residual target of 1E-05.
Table 1 Specification of blade.
3. Experimental procedure
commercially in South Korea and now commercialization of wave and tidal current power expected in the near future [4]. Developing and utilizing these tidal energy sources will allow Korea to be less reliant on energy imports, reduce adverse environmental impact and help in international environmental efforts to reduce carbon dioxide emissions. Therefore, developing tidal turbines for power generation can provide many benefits to society. Currently, single rotor horizontal axis tidal turbines are used for tidal current power production. These operate on the principle of creating lift forces from the tidal stream similar to wind turbines and have the benefit of the technological advancements made in wind energy. The present paper looks at a design of a counterrotating dual rotor horizontal axis turbine for tidal current power. By the Betz theorem, single rotor turbines can obtain a maximum power coefficient of 59.3% whereas a dual rotor can obtain a maximum of 64% [5]. The counter rotating dual rotor turbine has been designed with the front rotor and rear rotor connected by bevel gear. This connection allows for both rotors to rotate at identical rotational speeds and allows torque from both rotors to be transferred through one shaft. The performance of the counter rotating turbine system is investigated experimentally and numerically. First, the performance of the turbine was studied by varying the water speed and analyzing the performance characteristics. Second, the distance between the front and rear rotor was varied, this was done to determine if the distance between the rotors would have a beneficial effect to the performance. The experimental and CFD results were then compared to validate the accuracy of the numerical tests. 2. Numerical procedure
Local position
q [ ]
Position [mm]
C [mm]
Airfoil distribution
0.05 0.1 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1
23 23 18.529 15.524 13.055 11.028 9.35 7.948 6.764 5.756 4.893 7.948 6.764 5.756 4.893 4.153 3.523 2.998 2.593 2.353 2.113 1.873
12.5 25 62.5 75 87.5 100 112.5 125 137.5 150 162.5 125 137.5 150 162.5 175 187.5 200 212.5 225 237.5 250
30 30 52.365 50.158 47.951 45.744 43.537 41.33 39.123 36.916 34.709 41.33 39.123 36.916 34.709 32.502 30.295 28.088 25.881 23.674 21.467 19.261
Cylinder Cylinder NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421
The experiment was conducted in the water tank with a vertically circulating water channel located within Korea Maritime and Ocean University. The height of the tank is 1200 mm, the width is 1800 mm and is 4000 mm long. The height of the water level used for this experiment was 900 mm. The measurement points for water velocity were taken at 15 different equidistant points in the water tank. The pitot tube was kept 1 m in front of the turbine and measured the water velocity. 5 measurements of 1 min duration was taken at a point and averaged. The average difference in the measurements was 2%. Fig. 3 shows a schematic of the experimental setup. The torque meter (model SBB) had a measurement range from 0e2 kgf-m. The torque meter contained a RPM sensor (model MP-981) with a measurement range of up to 10,000 RPM. A forced air cooled type powder brake (model PRB-5Y3F) was used to control the RPM of the turbine. The distance between the dual rotors was varied at 0.5D, 0.75D and 1D. Due to physical limitations of the current model, further reduction was not possible but experiments with a larger model are planned. For each case, the water velocity was
Please cite this article in press as: Lee NJ, et al., Performance study on a counter-rotating tidal current turbine by CFD and model experimentation, Renewable Energy (2014), http://dx.doi.org/10.1016/j.renene.2014.11.022
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Fig. 2. a) Hexahedral mesh of blade domain in ICEM CFD (Left). The mesh is refined much near the walls. b) Preprocessing setup of the dual rotor turbine with specified boundary conditions (Right).
Fig. 3. Schematic of the experimental setup.
varied from 0.6 m/s to 1.3 m/s and the performance of the turbine was recorded. The results from the experiments were recorded into a data logger. The blockage ratio of the system was calculated to be approximately 12% and a blockage correction factor was applied to the results [9,10]. 4. Results Streamlines from the numerical simulations are shown in Fig. 4 and Fig. 5. From the numerous streamlines of the different cases, the cases that clearly showed the effect of blade gap distance are presented. Streamlines on the front and rear blades at a water
velocity of 1 m/s is shown in Fig. 4. These show how the blade gap distance affects the flow on the suction surface on both of the blades. The arrow indicates the direction of the flow and 3 different blade gap distances, 100 mm, 150 mm and 250 mm are shown. It was observed that the flow on the rear blade stayed attached to the majority of the blade in the chord-wise direction. Flow separation mainly occurs at the root and close to the trailing edge of the blade. As the distance increased, the points where flow separation occurred moved toward the trailing edge. This is also seen in Fig. 5. This figure shows the streamlines of the suction side of blades at 1.2 m/s for 3 different cases. The flow is shown to be stable from the leading edge to the middle on the rear blades while the separated
Fig. 4. Suction side surface streamlines at 1 m/s at 3 different blade gap distances.
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Fig. 5. Surface streamlines on the suction side of the front and rear blades at 1.2 m/s.
flow from the root moves towards the tip. Also, the flow on the front blade remains unaffected by any changes in distance. The current design of this turbine does not have any active methods of power control such as blade pitching and therefore relies on a passive method such as stall or flow separation on the blades to control power output if needed. As blade gap distance affects the flow the rear blade, this parameter could be taken into consideration when designing future turbines that might use stall as a method of power control. The power output versus the water velocity is plotted for the different numerical cases are shown in Fig. 6. The figure contains the comparison between single rotor case and the dual rotor counter rotating (CR) turbine cases with different blade gap distances from 50 to 500 mm. Both single and dual rotor cases show that at the design speed of 1 m/s, the power produced was almost 50 W except for the case with 50 mm separation between the two rotors. The lower power predicted is thought to be due to the small space between the two blade domains and boundary conditions causing CFD calculations to incorrectly predict the flow and performance. This case is currently under further review and experiments that reduce the separation distance to this point will be done in the future to clarify if the error is due to the numerical conditions. The results show from 1 m/s and onward, the output power for all cases increases linearly and reaches a peak at the water velocity of 1.8 m/s before reducing at 2 m/s. This reduction is caused when the critical stall angle has been reached by the flow on the blade and thus significant flow separation/stall occurring on the blades reduces the lift forces. The counter rotating turbine is able to produce higher power output than a single turbine after 1 m/s. The highest power output was produced by the dual rotor case with a
gap distance of 200 mm at 1.8 m/s with output of approximately 200 W. Increasing the water velocity increases the power output of the turbine but in order to see how efficient the turbine is producing power, a measure of efficiency can used. Namely, the power coefficient (Cp) variation at different water velocities can be studied. The power coefficient is defined as the ratio of mechanical power produced by the turbine to the power available at a water velocity. The Cp calculated in CFD simulations is plotted against the water velocity in Fig. 7. For the single rotor turbine, the highest efficiency is seen at the design water velocity of 1 m/s with a Cp value of 0.44 whereas the dual rotor turbine is seen to be the most efficient at 1.2 m/s with the maximum Cp value increasing by about 3%. The single rotor Cp reduces sharply after the 1 m/s while the dual rotor cases show a lower decrease from the peak Cp values. Also, with exception of the 50 mm dual rotor case, the Cp values for the 100 mme250 mm dual rotor cases are slightly higher than the other dual rotor cases suggesting that optimal power production can be obtained by reducing the distance between the rotors. In future experiments on larger designs, the distance can be reduced further to study the effect. Fig. 8 shows both the experimental and CFD results for power output as the water speed is varied. The distance between the blades is a function of the diameter of the rotor (D). As the water speed increases, the power output also increases. The CFD and experiment results show good agreement. The experiments could not go past 1.4 m/s as the water tank began to have stability problems at that water velocity. In the future, it is planned to continue experiments proper.
Fig. 6. Power output obtained by CFD for single rotor and several dual rotor cases with varying distances between the rotors at varying water speeds.
Fig. 7. The power coefficient (Cp) against water speeds for single and dual rotor cases calculated by CFD.
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model and predict the performance of different designs as well model larger turbines for performance estimation. 5. Conclusions
Fig. 8. Comparison of the power output between CFD and experiment. The distance between the blades is a function of the rotor diameter (D).
The streamlines showed the flow conditions over the front and rear blades at two different velocities. It was observed that as the distance increased, the flow separation occurred closer to the trailing edge. This could be a parameter to consider for passive power control on dual rotor turbines. The CFD results showed that the dual rotor setup produced more power than the single rotor setup from 1 m/s and higher. The numerical results also predicted a 3% higher maximum efficiency for the dual counter rotating turbine. Increasing water velocities led to an increase in power output but for optimal power production, distances between the rotors should be reduced with the exception of the 50 mm case. The 50 mm case is currently under review and future experiments are planned to reduce the distance to this point and clarify any numerical errors. The maximum Cp from CFD and the experiment was 0.457 and 0.456. Experiments are planned to be fully completed in the future. The graphs of power characteristics and power coefficient show good agreement for CFD and experiment results. Therefore using this numerical setup, it will be possible estimate the performance of different turbine blade designs and model larger counter-rotating turbines. Acknowledgment This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (Grant No. 20133030000260). References
Fig. 9. The power coefficient (Cp) for the CFD cases and experiments are compared.
In Fig. 9, the Cp versus the water speed for experiments and CFD is shown. The maximum Cp for CFD results is 0.457 at 1.2 m/s and the highest Cp from the experiment results was 0.456 at 1.16 m/s. The experiments and CFD results for the Cp comparison also show good agreement. Using this numerical setup, it may be possible to
[1] Falcao A FdeO. Wave energy utilization: a review of technologies. Renew Sustain Energy Rev 2010;14:899e918. [2] Kim BG, Kim CJ, Choi MS. A study on the performance of an 100 kW class tidal current turbine. J Korean Soc Mar Environ Saf 2012;18:145e52. [3] Ng KW, Lam WH, Ng KC. 2002e2012: 10 years of research progress in horizontal-axis marine current turbines 2013;6:1497e526. [4] Hyun BS, Jo CH, Kim MC, Moon CJ. Technology status and prospect of tidal current power generation. Bull Soc Nav Archit Korea 2011;48:12e9. [5] Newman BG. Actuator-disc theory for vertical-axis wind turbines. J Wind Eng Indus Aerodyn 1983;15:347e55. [6] Manwell JF, McGowan JG, Rogers AL. Wind energy explained: theory, design and application. England: Wiley and Sons; 2002pp111e113. [7] ANSYS CFX. Solver modeling guide. ANSYS Inc.; 2009. p. 117e9. Release 12. [8] ANSYS CFX. Solver theory guide. ANSYS Inc.; 2009. p. 61e3. Release 12. [9] Bahaj AS, Molland AF, Chaplin JR, Batten WMJ. Power and thrust measurements of marine current turbines under various hydro dynamic flow conditions in a cavitation tunnel and a towing tank. J Renew Energy 2007;32: 407e26. [10] Chen TY, Liou LR. Blockage corrections in wind tunnel tests of small horizontal-axis wind turbines. J Exp Therm Fluid Sci 2011;35:565e9.
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Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Performance study on a counter-rotating tidal current turbine by CFD and model experimentation Nak Joong Lee a, In Chul Kim a, Chang Goo Kim b, Beom Soo Hyun c, Young Ho Lee a, b, * a
Dept. of Mechanical Engineering, Graduate School, Korea Maritime and Ocean University (KMOU), Busan 606-791, Republic of Korea Division of Mechanical and Energy System Engineering, KMOU, Busan, Republic of Korea c Division of Naval Architecture and Ocean System Engineering, KMOU, Busan, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 March 2014 Accepted 10 November 2014 Available online xxx
Among the various ocean energy resources in Korea, the tidal currents in the South western sea have a large potential for development tidal current power generation. The biggest advantage of tidal power is that it is not dependent on seasons or weather and is always constant. This makes power generation predictable and makes tidal power a more reliable energy source than other renewable energy sources. Marine current turbines convert the kinetic energy in tidal currents for power production. Single rotor turbines can obtain a theoretical maximum power coefficient of 59.3%, whereas dual rotor can obtain a maximum of 64%. Therefore by optimizing the counter rotating turbines, more power can be obtained than the single rotor turbines. In this study, we investigated the effect of varying the distance between the dual rotors on the performance and efficiency of a counter-rotating current turbine by using computational fluid dynamics (CFD) and experimental methods. It was found that the dual rotor produced more power than the single rotor. In addition, the blade gap distance affects the flow on the rear rotor blades as well as power output and performance of the turbine. The distance can be used a parameter for counter rotating turbine design. Finally, the numerical setup used for this study can be further used to evaluate the design of larger counter rotating blade designs. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Performance Counter-rotating Current turbine CFD Experiment
1. Introduction Global warming is one of the serious issues in the world mainly due to the burning of fossil fuels and emission of carbon dioxide to the atmosphere. The importance of alternative energy is steadily rising in the 21st century. In addition to hydropower, wind power and solar power, ocean energy is receiving attention from the industrial sector. Ocean energy conversion can be classified into two categories; Wave energy and tidal energy. These sectors have undergone notable growth in research and technologies in the past decade. Methods to produce power using wave energy range widely from conversion of wave motion, over-topping devices to the use of oscillating columns of air within enclosed structures [1]. However, the efficiency of most wave energy devices is subject to weather and seasonal variations. The main advantage of tidal sources is that
* Corresponding author. Division of Mechanical and Energy System Engineering, KMOU, Busan, Republic of Korea. Tel.: þ82 51 410 4293; fax: þ82 51 403 1214. E-mail address: [email protected] (Y.H. Lee).
it is constant throughout the year and makes energy production predictable and attractive for investment. Also, as tides are the product of the gravitational forces by the sun and moon, tidal energy can be seen as a renewable energy source that will exist for an indefinite time [2]. Tidal energy conversion devices can be divided into two main methods of operation, tidal range power and tidal current power. Tidal range turbines use the height difference in water levels that are caused by the high and low tides within an enclosed structure to produce power in a similar manner to hydroelectric dams. Tidal current turbines convert the kinetic energy within currents to produce power. The horizontal axis tidal current turbine is one of the machines used to harness tidal current energy, which appears to be the most technologically and economically viable one at this stage [3]. In Korea, the available land area is limited and so the maximum use of offshore energy resources can be a boon for the country. Among the various marine energy resources in Korea, there are places where large tidal range differences and fast tidal flows caused by geographical formations. Areas such as Incheon, the western and in the southern seas, the potential for tidal current power generation is large. Tidal range power has had success
http://dx.doi.org/10.1016/j.renene.2014.11.022 0960-1481/© 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Lee NJ, et al., Performance study on a counter-rotating tidal current turbine by CFD and model experimentation, Renewable Energy (2014), http://dx.doi.org/10.1016/j.renene.2014.11.022
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Fig. 1. 2D profile of the blade showing twist and chord lengths and the 3D rotor blade configuration.
In the present study, a 40 W horizontal axis three bladed tidal current turbine was designed using the Blade Element Momentum
Theory (BEMT). BEMT refers to the combination of the momentum theory equations with the blade element theory equations [6]. Using these equations and known airfoil data, a blade profile can be calculated. The rotor has a diameter (D) of the 500 mm, a design water speed of 1 m/s and design rotational speed is 190 rpm. Fig. 1 shows the single airfoil of NACA-63421 with various twist angles and chord length (left) and 3D modeling of a blade (right). Table 1 summarizes the specifications of blade and shows the local radial position; twist angle (q), chord length (C), and the radial length in mm. The 3D model was meshed using ICEM CFD with a total of 6.2 million nodes in a hexa grid. The mesh near the blade was refined to ensure that the y plus value was less than 5. This ensures that a sufficient number of nodes are near the wall to resolve the boundary layer flow and capture changes in variables in the near wall region during calculation [7]. Only a single blade was modeled for calculation with the other blades accounted for by specifying a periodic condition. Fig. 2a shows the hexahedral mesh of the dual rotor setup. The distance from the blade to the inlet, top and outlet was 3, 5 and 7 times the diameter of blade respectively. The model is solved via commercial CFD finite volume solver, ANSYS CFX, ver13. A frozen rotor model was specified and the turbulence model used was keu Shear Stress Transport (SST) model. The SST model is able to model the transport of turbulent shear stress and gives accurate predictions on the onset and amount of flow separation under adverse pressure gradients [8]. The blade was modeled as a wall with no slip condition, while at the inlet an inflow condition was specified. An opening condition was specified at the top and at the outlet, an outflow boundary was assigned as shown in Fig. 2b. A rotational speed of 190 RPM was specified for the blade in the numerical simulation. The steady state simulation was solved with a high resolution advection scheme until the simulation reached the required RMS residual target of 1E-05.
Table 1 Specification of blade.
3. Experimental procedure
commercially in South Korea and now commercialization of wave and tidal current power expected in the near future [4]. Developing and utilizing these tidal energy sources will allow Korea to be less reliant on energy imports, reduce adverse environmental impact and help in international environmental efforts to reduce carbon dioxide emissions. Therefore, developing tidal turbines for power generation can provide many benefits to society. Currently, single rotor horizontal axis tidal turbines are used for tidal current power production. These operate on the principle of creating lift forces from the tidal stream similar to wind turbines and have the benefit of the technological advancements made in wind energy. The present paper looks at a design of a counterrotating dual rotor horizontal axis turbine for tidal current power. By the Betz theorem, single rotor turbines can obtain a maximum power coefficient of 59.3% whereas a dual rotor can obtain a maximum of 64% [5]. The counter rotating dual rotor turbine has been designed with the front rotor and rear rotor connected by bevel gear. This connection allows for both rotors to rotate at identical rotational speeds and allows torque from both rotors to be transferred through one shaft. The performance of the counter rotating turbine system is investigated experimentally and numerically. First, the performance of the turbine was studied by varying the water speed and analyzing the performance characteristics. Second, the distance between the front and rear rotor was varied, this was done to determine if the distance between the rotors would have a beneficial effect to the performance. The experimental and CFD results were then compared to validate the accuracy of the numerical tests. 2. Numerical procedure
Local position
q [ ]
Position [mm]
C [mm]
Airfoil distribution
0.05 0.1 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1
23 23 18.529 15.524 13.055 11.028 9.35 7.948 6.764 5.756 4.893 7.948 6.764 5.756 4.893 4.153 3.523 2.998 2.593 2.353 2.113 1.873
12.5 25 62.5 75 87.5 100 112.5 125 137.5 150 162.5 125 137.5 150 162.5 175 187.5 200 212.5 225 237.5 250
30 30 52.365 50.158 47.951 45.744 43.537 41.33 39.123 36.916 34.709 41.33 39.123 36.916 34.709 32.502 30.295 28.088 25.881 23.674 21.467 19.261
Cylinder Cylinder NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421 NACA-63421
The experiment was conducted in the water tank with a vertically circulating water channel located within Korea Maritime and Ocean University. The height of the tank is 1200 mm, the width is 1800 mm and is 4000 mm long. The height of the water level used for this experiment was 900 mm. The measurement points for water velocity were taken at 15 different equidistant points in the water tank. The pitot tube was kept 1 m in front of the turbine and measured the water velocity. 5 measurements of 1 min duration was taken at a point and averaged. The average difference in the measurements was 2%. Fig. 3 shows a schematic of the experimental setup. The torque meter (model SBB) had a measurement range from 0e2 kgf-m. The torque meter contained a RPM sensor (model MP-981) with a measurement range of up to 10,000 RPM. A forced air cooled type powder brake (model PRB-5Y3F) was used to control the RPM of the turbine. The distance between the dual rotors was varied at 0.5D, 0.75D and 1D. Due to physical limitations of the current model, further reduction was not possible but experiments with a larger model are planned. For each case, the water velocity was
Please cite this article in press as: Lee NJ, et al., Performance study on a counter-rotating tidal current turbine by CFD and model experimentation, Renewable Energy (2014), http://dx.doi.org/10.1016/j.renene.2014.11.022
N.J. Lee et al. / Renewable Energy xxx (2014) 1e5
3
Fig. 2. a) Hexahedral mesh of blade domain in ICEM CFD (Left). The mesh is refined much near the walls. b) Preprocessing setup of the dual rotor turbine with specified boundary conditions (Right).
Fig. 3. Schematic of the experimental setup.
varied from 0.6 m/s to 1.3 m/s and the performance of the turbine was recorded. The results from the experiments were recorded into a data logger. The blockage ratio of the system was calculated to be approximately 12% and a blockage correction factor was applied to the results [9,10]. 4. Results Streamlines from the numerical simulations are shown in Fig. 4 and Fig. 5. From the numerous streamlines of the different cases, the cases that clearly showed the effect of blade gap distance are presented. Streamlines on the front and rear blades at a water
velocity of 1 m/s is shown in Fig. 4. These show how the blade gap distance affects the flow on the suction surface on both of the blades. The arrow indicates the direction of the flow and 3 different blade gap distances, 100 mm, 150 mm and 250 mm are shown. It was observed that the flow on the rear blade stayed attached to the majority of the blade in the chord-wise direction. Flow separation mainly occurs at the root and close to the trailing edge of the blade. As the distance increased, the points where flow separation occurred moved toward the trailing edge. This is also seen in Fig. 5. This figure shows the streamlines of the suction side of blades at 1.2 m/s for 3 different cases. The flow is shown to be stable from the leading edge to the middle on the rear blades while the separated
Fig. 4. Suction side surface streamlines at 1 m/s at 3 different blade gap distances.
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Fig. 5. Surface streamlines on the suction side of the front and rear blades at 1.2 m/s.
flow from the root moves towards the tip. Also, the flow on the front blade remains unaffected by any changes in distance. The current design of this turbine does not have any active methods of power control such as blade pitching and therefore relies on a passive method such as stall or flow separation on the blades to control power output if needed. As blade gap distance affects the flow the rear blade, this parameter could be taken into consideration when designing future turbines that might use stall as a method of power control. The power output versus the water velocity is plotted for the different numerical cases are shown in Fig. 6. The figure contains the comparison between single rotor case and the dual rotor counter rotating (CR) turbine cases with different blade gap distances from 50 to 500 mm. Both single and dual rotor cases show that at the design speed of 1 m/s, the power produced was almost 50 W except for the case with 50 mm separation between the two rotors. The lower power predicted is thought to be due to the small space between the two blade domains and boundary conditions causing CFD calculations to incorrectly predict the flow and performance. This case is currently under further review and experiments that reduce the separation distance to this point will be done in the future to clarify if the error is due to the numerical conditions. The results show from 1 m/s and onward, the output power for all cases increases linearly and reaches a peak at the water velocity of 1.8 m/s before reducing at 2 m/s. This reduction is caused when the critical stall angle has been reached by the flow on the blade and thus significant flow separation/stall occurring on the blades reduces the lift forces. The counter rotating turbine is able to produce higher power output than a single turbine after 1 m/s. The highest power output was produced by the dual rotor case with a
gap distance of 200 mm at 1.8 m/s with output of approximately 200 W. Increasing the water velocity increases the power output of the turbine but in order to see how efficient the turbine is producing power, a measure of efficiency can used. Namely, the power coefficient (Cp) variation at different water velocities can be studied. The power coefficient is defined as the ratio of mechanical power produced by the turbine to the power available at a water velocity. The Cp calculated in CFD simulations is plotted against the water velocity in Fig. 7. For the single rotor turbine, the highest efficiency is seen at the design water velocity of 1 m/s with a Cp value of 0.44 whereas the dual rotor turbine is seen to be the most efficient at 1.2 m/s with the maximum Cp value increasing by about 3%. The single rotor Cp reduces sharply after the 1 m/s while the dual rotor cases show a lower decrease from the peak Cp values. Also, with exception of the 50 mm dual rotor case, the Cp values for the 100 mme250 mm dual rotor cases are slightly higher than the other dual rotor cases suggesting that optimal power production can be obtained by reducing the distance between the rotors. In future experiments on larger designs, the distance can be reduced further to study the effect. Fig. 8 shows both the experimental and CFD results for power output as the water speed is varied. The distance between the blades is a function of the diameter of the rotor (D). As the water speed increases, the power output also increases. The CFD and experiment results show good agreement. The experiments could not go past 1.4 m/s as the water tank began to have stability problems at that water velocity. In the future, it is planned to continue experiments proper.
Fig. 6. Power output obtained by CFD for single rotor and several dual rotor cases with varying distances between the rotors at varying water speeds.
Fig. 7. The power coefficient (Cp) against water speeds for single and dual rotor cases calculated by CFD.
Please cite this article in press as: Lee NJ, et al., Performance study on a counter-rotating tidal current turbine by CFD and model experimentation, Renewable Energy (2014), http://dx.doi.org/10.1016/j.renene.2014.11.022
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model and predict the performance of different designs as well model larger turbines for performance estimation. 5. Conclusions
Fig. 8. Comparison of the power output between CFD and experiment. The distance between the blades is a function of the rotor diameter (D).
The streamlines showed the flow conditions over the front and rear blades at two different velocities. It was observed that as the distance increased, the flow separation occurred closer to the trailing edge. This could be a parameter to consider for passive power control on dual rotor turbines. The CFD results showed that the dual rotor setup produced more power than the single rotor setup from 1 m/s and higher. The numerical results also predicted a 3% higher maximum efficiency for the dual counter rotating turbine. Increasing water velocities led to an increase in power output but for optimal power production, distances between the rotors should be reduced with the exception of the 50 mm case. The 50 mm case is currently under review and future experiments are planned to reduce the distance to this point and clarify any numerical errors. The maximum Cp from CFD and the experiment was 0.457 and 0.456. Experiments are planned to be fully completed in the future. The graphs of power characteristics and power coefficient show good agreement for CFD and experiment results. Therefore using this numerical setup, it will be possible estimate the performance of different turbine blade designs and model larger counter-rotating turbines. Acknowledgment This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (Grant No. 20133030000260). References
Fig. 9. The power coefficient (Cp) for the CFD cases and experiments are compared.
In Fig. 9, the Cp versus the water speed for experiments and CFD is shown. The maximum Cp for CFD results is 0.457 at 1.2 m/s and the highest Cp from the experiment results was 0.456 at 1.16 m/s. The experiments and CFD results for the Cp comparison also show good agreement. Using this numerical setup, it may be possible to
[1] Falcao A FdeO. Wave energy utilization: a review of technologies. Renew Sustain Energy Rev 2010;14:899e918. [2] Kim BG, Kim CJ, Choi MS. A study on the performance of an 100 kW class tidal current turbine. J Korean Soc Mar Environ Saf 2012;18:145e52. [3] Ng KW, Lam WH, Ng KC. 2002e2012: 10 years of research progress in horizontal-axis marine current turbines 2013;6:1497e526. [4] Hyun BS, Jo CH, Kim MC, Moon CJ. Technology status and prospect of tidal current power generation. Bull Soc Nav Archit Korea 2011;48:12e9. [5] Newman BG. Actuator-disc theory for vertical-axis wind turbines. J Wind Eng Indus Aerodyn 1983;15:347e55. [6] Manwell JF, McGowan JG, Rogers AL. Wind energy explained: theory, design and application. England: Wiley and Sons; 2002pp111e113. [7] ANSYS CFX. Solver modeling guide. ANSYS Inc.; 2009. p. 117e9. Release 12. [8] ANSYS CFX. Solver theory guide. ANSYS Inc.; 2009. p. 61e3. Release 12. [9] Bahaj AS, Molland AF, Chaplin JR, Batten WMJ. Power and thrust measurements of marine current turbines under various hydro dynamic flow conditions in a cavitation tunnel and a towing tank. J Renew Energy 2007;32: 407e26. [10] Chen TY, Liou LR. Blockage corrections in wind tunnel tests of small horizontal-axis wind turbines. J Exp Therm Fluid Sci 2011;35:565e9.
Please cite this article in press as: Lee NJ, et al., Performance study on a counter-rotating tidal current turbine by CFD and model experimentation, Renewable Energy (2014), http://dx.doi.org/10.1016/j.renene.2014.11.022