Experimental and Analytical Study of Helical Cross-Flow Turbines for a Tidal Micropower Generation System
Niblick, Adam L
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This study investigates the feasibility of a micro-scale tidal hydrokinetic generator to power autonomous oceanographic instrumentation, with emphasis on turbine design and performance. This type of "micropower" system is intended to provide continuous power on the order of 20 Watts. System components are reviewed and include turbine, electrical generator, gearbox, controller, converter, and battery bank. A steady-state model predicts system energy storage and power output in a mixed, mainly semidiurnal tidal regime with peak currents of 1.5 m/s. Among several turbine designs reviewed, a helical cross-flow turbine is selected, due to its self-start capability, ability to accept inflow from any direction, and power performance. Parameters impacting helical turbine design include radius, blade profile and pitch, aspect ratio, helical pitch, number of blades, solidity ratio, blade wrap ratio, strut design, and shaft diameter. The performance trade-offs of each are compared. A set of three prototype-scale turbines (two three-bladed designs, with 15% and 30% solidity, and a four-bladed design with 30% solidity and higher helical pitch) and several strut and shaft configurations were fabricated and tested in a water flume capable of flow rates up to 0.8 m/s. Tests included performance characterization of the rotating turbines from freewheel to stall, static torque characterization as a function of azimuthal angle, performance degradation associated with inclination angles up to 10° from vertical, and stream-wise wake velocity profiles. A four-bladed turbine with 60° helical pitch, 30% solidity, and circular plate "end cap" provided the best performance; this design attained efficiency of 24% in 0.8 m/s flow and experienced smaller performance reductions for tilted orientations relative to other variants. Maximum turbine efficiency increased with increased flume velocity. A free-vortex model was modified to simulate the helical turbine performance. Model results were compared to experimental data for various strut design and inflow velocities, and performance was extrapolated to higher flume velocities and a full-scale turbine (0.7 m<super>2</super> relative to 0.04 m<super>2</super> in flume tests). The model predicts experimental trends correctly but deviates from experimental values for some conditions, indicating the need for further study of secondary effects for a high chord-to-radius ratio turbine.
- Mechanical engineering