Preliminary performance assessment from towing tank testing of a horizontal-axis turbine

Abstract

Following the Norwegian government’s goal to be a ‘low emission society’ by 2050, there is a significant push to develop offshore renewable energy; principally offshore wind, but also wave and tidal stream. The MarinLab towing tank at the Western Norway University of Applied Sciences, Norway is 50 m long with a 3.0×2.2 m section and was inaugurated in 2016 as an educational and research facility, supporting local industry to achieve a low-carbon transition.

To advance knowledge of aero- or hydrodynamic interaction of future turbines for both wind and tidal stream energy, a model scale test-bed horizontal axis turbine has been developed. This turbine will enable testing of rotor diameters typically in the range of 500-600 mm. The turbine is instrumented with a torque-thrust sensor of 5 Nm/100 N capacity, custom-manufactured by Marin in the Netherlands. The turbine is speed regulated via a 4096 counts angular encoder connected to a 200 W Maxon EC-i brushless motor.

Before testing new rotor geometries, a benchmarking study is being undertaken with an existing known geometry. The benchmark rotor has diameter, D=700 mm and employs the same NACA 63418 airfoil as Mycek et al. (2014), allowing comparison. Unlike Mycek et al. (2014), the nacelle has a nominal diameter of 90 mm and length 760 mm. The size of the rotor risks exceeding the capacity of the torque sensor, such that maximum tow-speed is limited to 0.8 m/s. The benchmark blades are machined from solid aluminium in a four-axis CNC milling machine, following a method similar to Payne et al. (2017). Due to machining limitations, the minimum trailing edge thickness is specified as 0.2 mm. The final surface finish is achieved by manual polishing, and the final dimensions, quantified using a Hexagon ROMER Absolute laser scanner, are found to be within ±0.2 mm accuracy. Future blade sets will typically have a smaller diameter, allowing for 3D printing and consistency of surface finish.

A blade-element momentum model has been run with Prandtl tip and hub losses and 0° pitch (Fig. I). Lift and drag coefficients were calculated for a range of chord Reynolds numbers using XFOIL, for angles of attack between -5 to 16° and extrapolated to ±180° with Viterna’s method. Due to difficulties in estimating laminar-turbulent transition around critical Reynolds numbers, there is uncertainty in the XFOIL coefficients. At peak TSR, the chord-based Reynolds number is approximately 280k at 3/4 span. Curves for both Rec =250k (solid) and 500k (dashed) with Ncrit = 9 are presented, showing a significant drop in performance for lower Reynolds numbers, due to a collapse in Cl/Cd. Initial tests were conducted in near- zero ambient turbulence, though passive turbulence grids are available for future testing. Reasonable agreement on C_T is observed when corrected for blockage according to Bahaj (2007), and further assessment will provide results on C_P, tower loads and wake recovery.