Comparison of cross-flow turbine performance under torque-regulated and speed-regulated control

Abstract

When experimentally evaluating the performance of a wind or water current turbine, one must impose a regulating torque on the turbine rotor by electrical or mechanical means. Some options limit this controlling torque to a purely resistive quantity, while servomotors and stepper motors allow torque to be applied in the direction of turbine rotation. Any control mode that results in net positive power for a turbine may be of interest for energy harvesting, and all of these are net “fluid-driven.” Here, we present experiments that characterize the power, torque, and force coefficients of a cross-flow turbine operated at a constant rotational speed or under a constant imposed control torque. Time- and phase-average performance coefficients are largely equivalent for the two strategies although torque-regulated control is restricted to a narrower range of rotational speeds and the two strategies result in slightly different blade kinematics.

Implications of biofouling on cross-flow turbine performance

“…Barnacle fouling does not appear to cause significant phase shifts in power or thrust. This may be a consequence of speed-regulated control, which maintains blade kinematics across the cases [33]. Phase shifts might occur under other control strategies (e.g., torqueregulated control).…”

“…For the upper load cell, a 6-axis Mini45 (ATI Industrial Automation) was used, and for the lower load cell a 6-axis Nano25 (ATI Industrial automation) was used. A Yaskawa servomotor (SGMCS-05B3C41) equipped with a 2 18 count encoder was used to control the turbine's rotation at a constant angular velocity [33]. Turbine performance data was acquired using a National Instruments 6351 DAQ at a sampling rate of 1000 Hz with each tipspeed ratio maintained for 60 s. Inflow velocity was measured five turbine diameters upstream of the turbine at the vertical and cross-stream midpoint of the flow using an Acoustic Doppler Velocimeter (Nortek Vector) sampling at 64 Hz.…”

“…From the instantaneous data, cycle averages were calculated for each rotation, time-averages were calculated over all rotations, and phase-averages were calculated as a function of azimuthal blade position in 6° azimuthal bins. Because it is difficult to temporally correlate free stream turbulence and its interaction with the turbine rotor, the denominator in all the performance metrics is a time average of velocity over [33] an integer number of turbine rotations. The variability in time-average and phase-average performance coefficients was quantified by the inter-quartile range (IQR) (i.e., interquartile range of cycle averages and values underpinning the phase average in each azimuthal bin, respectively).…”

“…Further, the boundary layer thickness varies with azimuthal position. Order of magnitude approximations were obtained using the software Xfoil 2 for static angles of attack and nominal velocity (speed encountered by the blade) estimated as a function of azimuthal position assuming negligible induction [33]. Nominal angle of attack (α n ) and velocity (U n ) are calculated as functions of azimuthal position, θ (defined in Fig.…”

Design and implementation of a power smoothing system for cross-flow current turbines

“…Cross-flow turbines ("vertical-axis") have distinct, potentially advantageous properties compared to axial-flow turbines ("horizontal-axis"), including lower maximum blade speed, bi-directional functionality in reversing tidal flows, and potential to increase system efficiency in tightlypacked, high-blockage arrays [8,9]. Typical control strategies involve maintaining an optimal tip-speed ratio for a given inflow velocity through regulation of rotor speed or torque [10]. Turbines with a high mechanical conversion efficiency (i.e., power in flow to power on shaft) typically have a small number of straight blades [11].…”