Performance studies on a direct drive turbine for wave power generation in a numerical wave tank
- 1. Performance Studies on a Direct Drive Turbine for Wave Power Generation in a Numerical Wave Tank Professor Young-Ho LEE Division of Mechanical & Energy System Engineering, College of Engineering, Korea Maritime University, South Korea. Deepak Prasad Discipline of Mechanical Engineering, University of the South Pacific, Fiji Islands. Dr. M. R. Ahmed Discipline of Mechanical Engineering, University of the South Pacific, Fiji Islands.
- 2. Renewable Energy Sources 2 Wind Power Solar Power Biomass Hydro Power Ocean Energy Thermal Energy Mechanica l Energy WaveTide Fig. 1 Renewable energy sources
- 3. 3 Introduction Wave energy: Power flux – 15 to 20 times more than wind and solar. Estimated wave energy is in order of 1 to 10 TW. Most consistent of all the intermittent sources and truly renewable. Wave tanks have been used over the years to provide: helpful information on wave characteristics and employed to conduct prototype testing. However it is: Expensive, time consuming. Due to time constraint all design variables can not be tested.
- 4. 4 Introduction To overcome these problems much effort has been focused on the development of Numerical Wave Tank (NWT). NWT allows for rapid design changes and improvements in short time. With improving computer capabilities it is possible for these CFD packages to solve and give accurate solutions of real life problems.
- 5. 5 Introduction The current study employs a 3D NWT based on Reynolds Averaged Navier-Stokes Equation (RANSE) to generate waves using commercial CFD code ANSYS-CFX. A cross flow turbine is employed to generate power from incoming waves. Aim: To simulate waves using a Numerical Wave Tank. To validate the CFD code with the experimental data. To study flow characteristics.
- 6. 6 Base Model All Dimensions in mm Fig. 2 Schematic of the entire model Fig. 3 Schematic of the turbine and runner blade Parameter Value Blade entry angle, α 30º Blade exit angle, β 90º Outer Diameter, Do 260 mm Inner Diameter, Di 165 mm Number of Blades 30 Table 1 Turbine and runner parameters
- 7. Full model calculation. Numerical Wave Tank (NWT). Front Guide Nozzle. Augmentation Channel. Front Nozzle. Rear Nozzle. Turbine. Rear Chamber. 7 Test Section Fig. 4 Schematic of the test section
- 8. CFX-Pre Computational Domain 8 Simulation Type Transient Wave Maker Piston-Type (Specified Motion) Turbulence Model k-Epsilon Opening 1 atm Phase Water & Air @ 25°C Walls No-Slip Condition Surf. Tension Coeff. 0.075 N/m Density (kg/m3 ) Water = 997, Air 1.18 Opening Moving mesh section Wall Motion: Asin(ωt) Fig. 5 Computational domain. NWT Front Guide Nozzle Augmentation channel Rear chamber Table 2 ANSYS CFX-Pre conditions
- 9. 9 Wave Height & Velocity (NWT) Point Fig. 6 Water wave height in the numerical wave tank Fig. 8 Velocity contour in the numerical wave tank Fig. 7 Volume fraction showing the formation of waves in the wave tank
- 10. 10 Front Guide Nozzle Fig. 9 Velocity vector in front guide nozzle There is a recirculation region observed near the top left corner, denoted by A when water is flowing in. Due to this, the flow was directed towards the bottom and hence higher velocity recorded in region B. When water was retreating, higher velocity was observed in region A.
- 11. 11 Front Guide Nozzle Fig. 10 Average velocity in the front guide nozzle at different rpm It is observed that the velocity increases as the rpm increases, reaches a maximum and then decreases. This peak at 35 rpm is due to better flow characteristic because of small re-circulating region in the front guide nozzle. This also leads to better flow in the augmentation channel.
- 12. 12 Turbine Power Fig. 11 Comparison between experimental data and CFD results Variable Unit Experiment CFD H m 0.2 0.195 T s 2.0 2.0 ΔH m 0.071 0.065 Q m3 /s 0.03 0.032 PWave W/m 86.74 82.46 PWP W 20.85 20.36 Table 3 Comparing experimental and CFD results For CFD, the peak power is 6.71 W compared to 6.8 W obtained experimentally. The efficiency at 35 rpm is 44.73% and 45.33% respectively from CFD and experiments. The difference is within 3%.
- 13. 13 Flow Characteristics Fig. 12 Velocity vector in the augmentation channel at 35 rpm The flow accelerates approaching stage 1 as expected. (Nozzle effect) The water passes through the turbine passage at stage 1 while imparting energy to the runner. From the exit at stage 1 to the entry of blades at stage 2, the flow again accelerates a little. At stage 2, the passing water imparts energy to the runner once more before flowing into the rear nozzle.
- 15. 15 Conclusion Commercial CFD code ANSYS-CFX was successfully used to generate waves in a NWT using a piston type wave-maker. The results of CFD simulation showed good agreement with the experimental data. The difference in result was within 3%. The maximum turbine power was obtained at 35 rpm. For CFD, the maximum power was 6.71 W compared to 6.8 W obtained experimentally. The efficiency at 35 rpm was 44.73% and 45.33% respectively for CFD and experiment.
Editor's Notes
- #3: RENEWABLE ENERGY IS THE ENERGY OBTAINED FROM CURRENTS OF ENERGY OCCURRING IN THE NATURAL ENVIRONMENT. THE MAJORITY OF THE RENEWABLE ENERGY TECHNOLOGIES IS DIRECTLY OR INDIRECTLY POWERED BY THE SUN. APART FROM WIND AND SOLAR ENERGY, THE OCEAN CONTAINS VAST AMOUNT OF ENERGY.IN FORM OF THERMAL AND MECHANICAL ENERGY. MECHANICAL ENERGY : TIDES AND WAVES.
- #4: The primary objective is to maximise the energy conversion.
- #7: NWT LENGTH = 15M HEIGHT = 1.5M WIDTH = 1M REAR CHAMBER WIDTH = 0.7M EVERY COMPONENT IN THE AUGMENTATION CHANNEL HAS A WIDTH OF 0.7M TURBINE SECTION INCLUDES = FRONT GUIDE NOZZLE, FRONT AND REAR NOZZLE, REAR CHAMBER AND INTERNAL FLUID REGION
- #8: A NUMERICAL WAVE TANK WAS USED TO SIMULATE THE SEA CONDITIONS THE AUGMENTATION CHANNEL CONSISTS OF FRONT AND REAR NOZZLE AND TURBINE.
- #9: The computational domain was divided into 5 parts. Moving mesh section – wave maker NWT Front guide nozzle Augmentation channel – consists of front nozzle, rear nozzle and turbine Rear Chamber
- #10: As expected, kinetic energy at the surface is more as shown in Fig. 8.
- #11: Front guide nozzle guides the flow into the channel. Performance is really important to the flow downstream.
- #12: The front guide nozzle inlet is denoted by x/Lo = 0 and the exit as x/Lo= 1. Lo is the length of the front guide nozzle which is 700 mm. The results show a gradual increase in velocity in the guide nozzle as desired.
- #13: The peak in efficiency basically indicates that the interaction between the turbine and flow is maximized at this optimum rotational speed. At this speed maximum energy is extracted hence higher turbine power and efficiency. For T = 2s, highest efficiency of 44.73% is obtained at rotational speed of 35 rpm. At 35 rpm the velocity recorded in the front guide nozzle was the highest as previously highlighted in Fig. 10. This lead to better flow in the augmentation channel which ultimately meant higher energy extraction from the turbine.
- #14: The rear spiral wall ensures that the flow accelerates uniformly.
- #15: SIMULATION FOR T = 2s at 35 rpm