Numerical Investigation of Bare and Ducted Horizontal Axis Marine Current Turbines

5 th International Conference on Ocean Energy ICOE 2014, November 4-6, Halifax, NS Canada Numerical Investigation of Bare and Ducted Horizontal Axis Marine Current Turbines Presented by: Mahrez Ait Mohammed PhD student Dept. of Ship Structure Mechanics Center, ENSTA Bretagne, France. Pr. Mostapha Tarfaoui & Dr. Jean Marc Laurens ENSTA Bretagne, France.

Context Various global studies have shown that marine currents have a large potential as a predictable sustainable resource for commercial scale generation of electrical power. There are many areas of the world in which extreme tidal currents are observed. The Celtic sea being the most dense in terms of renewable energy potential. (Tidal energy and offshore wind) In the region of the Cherbourg peninsula in France : The most attractive tidal site in France is the sea passage known as Le Raz Blanchard in French and the Race of Alderney in English. The water depth allows for the marine turbine systems to exceed 20 meters diameter without causing any perturbation to maritime traffic and the current velocity peaks above 3m/s. In order to present some realistic numerical results, the present study is using these data as input. 2

Context In the region of the Cherbourg peninsula in France : The most attractive tidal site in France is the sea passage known as Le Raz Blanchard in French and the Race of Alderney in English. The water depth allows for the marine turbine systems to exceed 20 meters diameter without causing any perturbation to maritime traffic and the current velocity peaks above 3m/s. In order to present some realistic numerical results, the present study is using these data as input. FLOW 10 m 20 m 10-15 m Rotor Hub Yaw system Column 2

Presentation Overview Objective Hydrodynamic design Blade Element Momentum Theory Boundary Element Method (Panel Method) Reynolds Averaged Navier-Stokes Solution Performance comparison between theory and data experiments Bare turbine design (without duct) Ducted turbine design Effect of the addition of duct Shape design of ducted turbine Conclusion and future perspectives 3

Objective Bare turbine design Ducted turbine design Is a duct a good idea? Before discussing the pros and the cons, let us examine the hydrodynamic performance of both systems. 4

Hydrodynamic design 3D effect section data Flow separation and stall Cost BEM Blade Element Momentum model N Limited No Required Required Semiempirical Very low High 6

Hydrodynamic design 3D effect section data Flow separation and stall Cost BEM Blade Element Momentum model Limited Required Semiempirical Very low Panel Method Boundary element Method Yes Not required No Medium High 6

Panel method The panel methods are based on the potential flow theory. We assume that the fluid is inviscid and non rotational ( ). The above assumption is equivalent to To mimic the behavior of viscous fluid flow around a lifting body we force the flow to be lined up with the trailing edge (Kutta-Joukowski condition). Concerning the mesh, the panel method only requires a surface mesh of the solid objects. Example of ducted marine current turbine mesh 7

Panel method The panel methods are based on the potential flow theory. We assume that the fluid is inviscid and non rotational ( ). The above assumption is equivalent to. To mimic the behavior of viscous fluid flow around a lifting body we force the flow to be lined up with the trailing edge (Kutta-Joukowski condition). Concerning the mesh, the panel method only requires a surface mesh of the solid objects. Propeller Terminology Turbine Terminology η 7

Performance comparison between theory and data experiments The experimental results of the bare turbine, tested and described by Bahaj et al 1. are used and compare with our numerical simulation. The turbine is three bladed with a Naca63-8xx hydrofoil. In the present work a 5 blade set angle is considered. 0.6 NACA 63-818 0.5 Power coefficient (Cp) 0.4 0.3 0.2 0.1 0 Panel Method 1.8m/s Tow. Tank 1.46m/s Bahaj et al. Cav. Tunnel 1.73m/s Bahaj et al. 4 5 6 7 8 9 TSR 1 A. S. Bahaj, W. M. J. Batten, G. McCann, Experimental Verifications of Numerical Predictions for the Hydrodynamic Performance of Horizontal Axis Marine Current Turbines, Renewable Energy. 2007, Vol. 32, pp. 2479-2490. 8

Bare turbine design The design of tidal turbine is imposed by: The cavitation occurrence. Flow separation has to be avoided. NACA 63-415 where α: Local angle of attack ϕ: Local angle of the profile θ: Local angle of the twist a: Axial induction factor a : Tangential induction factor Pressure coefficient min (Cp min) -0.5-1 -1.5-2 -2.5-3 -3.5-4 NACA 63-415 -3-2 -1 0 1 2 3 4 5 6 7 Angle of attack in (Watt) Where: Z: Number of blades P/D: Average pitch (propeller pitch definition) c/d: Chord AER: Aspect ratio Kq: Torque coefficient Cp: Power coefficient 9

Bare turbine design (Watt) Where: Z: Number of blades P/D: Average pitch (propeller pitch definition) c/d: Chord AER: Aspect ratio Kq: Torque coefficient Cp: Power coefficient 2.24E+06 Power Coefficient 0.6 Bare turbine performance 0.5 0.4 0.3 0.2 0.1 0 1.9 2.9 3.9 4.9 5.9 6.9 TSR Pressure coefficient -1-0.5 0 0.5 1 0 0.2 0.4 0.6 0.8 1 x/c Local pressure coefficient (r/r=0.7) 10

Bare turbine design Power Coefficient 0.6 0.5 0.4 0.3 0.2 0.1 r/r c/d P/D t/c f/c 0.2 0.07142857 0.28865064 0.15-0.02 0.3 0.07142857 0.35359894 0.15-0.02 0.4 0.07142857 0.38177779 0.15-0.02 0.5 0.07142857 0.39604095 0.15-0.02 0.6 0.07142857 0.4041486 0.15-0.02 0.7 0.07142857 0.40916662 0.15-0.02 0.8 0.07142857 0.41247731 0.15-0.02 0.9 0.07142857 0.41477209 0.15-0.02 0.95 0.07142857 0.41566306 0.15-0.02 1 0.07142857 0.41642619 0.15-0.02 Bare turbine performance 0 1.9 2.9 3.9 4.9 5.9 6.9 TSR P/D 0.28865064 0.35359894 0.38177779 0.39604095 0.4041486 0.40916662 0.41247731 0.41477209 0.41566306 0.41642619 Pressure coefficient 1.5 0.5-0.5-1.5-2.5-3.5 2.24E+06 Pitch 24.67413332 20.56509281 16.89925935 14.15091209 12.10142785 10.53991319 9.320266606 8.345515542 7.92877136 7.550685995 (Watt) Where: Z: Number of blades P/D: Average pitch (propeller pitch definition) c/d: Chord AER: Aspect ratio Kq: Torque coefficient Cp: Power coefficient Pressure coefficient (Cp ITTC at TSR=4) (r/r) 0 0.2 0.4 0.6 0.8 1 Suction side α 7.065007473 5.135647948 4.076894047 3.37465628 2.87237177 2.496218928 2.204816355 1.972929877 1.873990238 1.784312468 Pressure side 10

Objective Hydrodynamic design Blade Element Momentum Theory Boundary Element Method (Panel Method) Reynolds Averaged Navier-Stokes Solution Performance comparison between theory and data experiments Bare turbine design (without duct) Ducted turbine design Effect of the addition of duct Shape design of ducted turbine Conclusion and future perspectives FLOW 10 m 20 m 10-15 m Rotor Hub Duct Yaw system Column 11

Theoretical performance of ducted turbine according to Werle et al.² Ducted turbine design Power coefficient Duct Drag coefficient Rotor thrust coefficient The power coefficient reaches 1.2, but it is defined according to the rotor area and not according to the overall area. ² M. J. Werle and W. M. Presz, Ducted Wind/Water Turbines and Propellers Revisited. Journal of Propulsion and Power.2008,Vol.24, PP.1146-1150. 12

Theoretical performance of ducted turbine according to Werle et al.² Power coefficient Duct Drag coefficient Effect of the addition of duct Rotor thrust coefficient Effect of the addition of duct to the rotor design 20 m Comparison between the rotor in the symmetrical duct section and the bare rotor with the same overall cross section of 20m. The length of the duct is equal to the radius of the overall cross section (10m). The power coefficient reaches 1.2 but it is defined according to the rotor are and not according to the overall area. ² M. J. Werle and W. M. Presz, Ducted Wind/Water Turbines and Propellers Revisited. Journal of Propulsion and Power.2008,Vol.24, PP.1146-1150. Power coefficient (Cp) Power coefficient (Cp*) 0.7 0.6 0.5 0.4 Cp : duct 0010 Cp: duct 0020 Cp: duct 0024 Evolution of the power coefficient (Cp) versus TSR for ducted turbine when Cp is computed from the rotor diameter. 0.3 1.9 2.9 TSR 3.9 4.9 5.9 0.36 0.35 Evolution of the power 0.34 coefficient (Cp*) versus TSR for 0.33 ducted turbine when Cp is 0.32 computed using the same overall 0.31 cross section. Cp*: duct 0010 0.3 Cp*: duct 0020 Cp*: duct 0024 0.29 1.9 2.9 3.9 TSR 4.9 5.9 6.9 12

Shape design of ducted turbine Effect of the addition of symmetrical duct profile to the rotor design Duct profile NACA 0010 NACA 0020 NACA0024 Rotor diameter (m) 18 16 15.2 Cp max 0.4362 0.5507 0.6174 Cp* max 0.3525 0.3515 0.3556 TSR 4 4.5 5 Power output (MWatts) 1.5311 1.5267 1.5446 Loss relative to bare turbine -31.76% -31.95% -31.16% The addition of symmetrical duct at the same overall cross section is not advantageous because we get a loss in energy close to 32%. Effect of the addition of camber duct profile to the rotor design 0.2 Y/c 0-0.2-0.4 0 0.2 0.4 0.6 0.8 1 X/c naca 0020 EMB20 & 12% camber EMB20 camber% 0% 4% 8% 10% 12% Rotor diameter (m) 16.00 15.80 15.55 15.15 14.75 Cp max 0.5507 0.6501 0.7577 0.9019 0.9826 Cp* max 0.3515 0.4047 0.4568 0.5174 0.5344 TSR 4.5 4.0 4.0 5.0 5.0 Power output (MWatts) 1.5267 1.7576 1.9841 2.2474 2.3222 Loss or gain relative to bare turbine -31.95% -21.67% -11.57% +0.16% +3.5% Power (Watts) 2.4E+06 2.2E+06 2.0E+06 1.8E+06 1.6E+06 1.4E+06 EMB20 &12% camber 1.2E+06 EMB20 & 10% camber 1.0E+06 Bare turbine 0.1 0.2 RPS 0.3 0.4 0.5 Power produced by the bare turbine and the camber ducted turbine versus RPS at the same overall cross section of 20m 13

Shape design of ducted turbine The effect of the chord length has been tested for the EMB 20 and 12% camber L/D 0.5 0.375 0.250 0.125 Rotor diameter (m) 14.75 16.3 17.38 18.53 Cp max 0.9826 0.8152 0.5966 0.5126 Cp* max 0.5344 0.5414 0.4505 0.4400 TSR 5 5 4 4 Power output (MW) 2.3210 2.3516 1.9567 1.9111 Gain or Loss relative to bare turbine Where: L: The length of the duct D: The overall cross section +3.5% +4.80% -12.80% -14.83% Torque coefficient (Kq) 1.8E-02 1.6E-02 1.4E-02 1.2E-02 1.0E-02 8.0E-03 6.0E-03 4.0E-03 TSR=5 1 3 5 7 9 11 13 15 Iterations 1.2 Power coefficient max for duct profile EMB20 & 12% camber 1 0.8 0.6 0.4 0.2 Cp max Cp* max 0 0.1 0.2 L/D 0.3 0.4 0.5 0.6 Ducted turbine design: 2.35Mega Watts (naca63-415 rotor, 12% max camber duct, L/D=0.375 and 20m overall diameter) 14

Conclusion and future perspectives A design method involving BEM and Panel method code was developed and used to obtain the geometry of an horizontal axis marine current turbine for the Race of Alderney. A bare turbine was designed which can attain up to 90% of the Betz limit. The addition of an accelerating duct at the same overall diameter with bare turbine was investigated. It was found that, to obtain a sufficient acceleration capable to compensate the loss of blade diameter, the section has to be seriously cambered when there is no flaring of the duct profile at the exit. The effect of the flaring of the duct exit is currently being studied. The obtained ducted water turbine has a potential power output of 2.35MW, 5% more than the bare turbine with the same overall diameter. The device is designed for uni-directional flow. Adding a duct will not only increase the total weight but also the manufacturing cost. In this study only the hydrodynamics aspects have been taken into account. Our future line of investigation will concern the composite materials and the structural behavior of the ducted water turbine. Materials and layup optimization. 15

Questions 16