Experimental study of active and passive blade pitch control strategies for axial-flow marine current turbines

Abstract

Cost and reliability remain among the main barriers limiting widespread adoption ofriverine, estuarine, or ocean current turbine power generation. In particular, structural loads are significantly greater than for wind turbines with equivalent power output, which contributes to higher costs. Compounded with uncertainties about hydrodynamic loads, this can contribute to structural failure or excessive and expensive safety factors. Consequently, control strategies to mitigate structural loads and reduce cost are of considerable importance. Load reduction is of particular interest when currents exceed a certain threshold (i.e., theturbine-specific “rated speed”), and a control strategy is implemented to maintain a constant power output. Most fixed-pitch turbines will use a speed control strategy, increasing or decreasing the rotation rate to achieve the efficiency required for power regulation. However, these “overspeed” and “underspeed” control strategies correspond to large increases in thrust or torque, respectively, that require overdesigning the turbine blades or generator. Blade pitch control circumvents this trade-off, as decreased angles of attack simultaneously reduce thrust and torque. This does, however, require actuators to change blade pitch. While active pitch control is the conventional standard for wind turbines in these above-rated conditions, similar variable blade pitch mechanisms have not yet been uniformly adopted by marine current technology developers due to the higher cost of inspection, maintenance, and repairs relative to wind turbines. For this reason, passive adaptive blade pitch control, in which blades are designed to elastically deform under load without an actuator, sensor, or control logic, is conceptually attractive. Improved understanding of the loading associated with both speed and pitch control strategies is critical to optimizing a design for minimal cost and maximal reliability. Therefore, the overarching goal of this work is to experimentally investigate active and passive pitch control methods, characterize their potential for load reduction, and establish appropriate scaling relations for passive adaptive blades. The three underlying objectives supporting this goal are outlined below. The first objective is to demonstrate active blade pitch control in above-rated flow conditionsand compare the measured turbine loads to those observed with overspeed and underspeed control in order to develop our understanding of the trade-offs associated with each. To this end, we experimentally characterized power performance and turbine loading over a range of blade pitch settings and tip-speed ratios for a three-bladed axial-flow turbine. We then implemented a control strategy to maintain power output in time-varying currents using blade pitch control and compared the turbine performance under this control strategy to overspeed and underspeed control strategies for a fixed pitch turbine. The experiments were conducted with a laboratory-scale 0.45-m diameter turbine in an open channel flume with a 35% blockage ratio. During pitch characterization experiments, inflow velocity was maintained at 0.8 m/s with 4% turbulence intensity. During time-varying inflow experiments, currents varied from 0.7-0.8 m/s over a 20-minute period, while a proportional controller regulated either blade pitch or rotor speed, and we recorded turbine power output and turbine loads. In this velocity range, where turbine performance is independent of Reynolds number, we demonstrate that pitch control substantially reduces torque requirements relative to underspeed control and streamwise turbine loads relative to overspeed control. Additional tests were conducted for underspeed control and pitch control in a Reynolds-dependent regime with time-varying inflow between 0.4-0.5 m/s and 0.5-0.6 m/s. These cases suggest that blade pitch control could provide even greater benefits relative to speed control in small-scale applications. The second objective is to develop our understanding of passive adaptive blade fabricationand the effect of fiber orientation to inform a passive pitch control design. By tailoring the ply angle in a unidirectional carbon fiber blade, a desired twist can be induced in response to bending of the blade under load. In developing this form of passive adaptive control, a fundamental question is how to non-dimensionalize the fluid-structure interaction to make laboratory-scale experiments relevant to full-scale applications. To address these questions, we first conducted an experimental investigation into the effect of fiber angle on blade performance and blade deformation during turbine operation. The composite blades were fabricated with 0◦, 2.5◦, 5◦, and 10◦ fiber orientations, where a positive fiber orientation results in a reduced angle of attack as load increases (i.e., a “pitch-to-feather” control strategy). Blades were tested in a recirculating flume at 0.7 m/s (Rec = 5.3 · 104 − 2.0 · 105) while measuring force and torque on the rotor. Simultaneously, a high-speed camera observed in-situ deflection and twist at the blade tip. Results show a greater reduction in CP and CT for blades with larger fiber orientations relative to the neutral blade set, while even small fiber orientations were observed to limit thrust at high tip-speed ratios. To explore the correct non-dimensional scaling for this physical process, we performed a set of Cauchyscaled experiments using blades with identical bend-twist couplings but different bending stiffness. These results demonstrate that the Cauchy number is a meaningful parameter for scaling passive adaptive current turbine blades and to model steady-state hydrodynamic and hydroelastic behavior. The third and final objective is to implement passive pitch control to develop our understandingof the trade-offs between speed, active pitch, and passive pitch control methods. Two passive blade pitch control strategies for the same lab-scale turbine were developed and tested experimentally in a recirculating flume. The goal of the control is to regulate mechanical power, while minimizing rotor loads, when flow conditions exceed the rated condition. Both strategies used the 5◦ fiber blade set from the aforementioned study. One control strategy combined passive adaptive blades with overspeed control (actuating rotational speed above the tip-speed ratio corresponding to peak efficiency) while the other combined passive adaptive blades with active pitch control (actuating blade pitch using motors at the blade root). Both strategies were implemented in linearly increasing inflow from 0.7 m/s to 0.8 m/s and compared to control strategies using rigid, aluminum blades under the same flow conditions. The passive adaptive blades combined with active pitch control show no improvement in steady-state load reductions relative to rigid blades used with active pitch control. However, the passive adaptive blades combined with overspeed control show reduced torque and only a 12% increase in thrust relative to the rated flow condition. This indicates that passive adaptive blades combined with overspeed control can be an effective strategy in currents above the rated flow speed, removing the need for an active pitch mechanism in some applications. In addition to measuring turbine loads, deflection and twist of the passive adaptive blades during experimental testing were observed using a high-speed camera to support our understanding of the bend-twist behavior during turbine operation over a range of flow speeds, rotation rates, and preset pitch angles. Overall, active and passive pitch control strategies for Region III are shown to offer significantload reductions in thrust and torque relative to rigid blade speed control strategies. While controller selection is discussed primarily relative to their associated loads, we discuss additional considerations including blade design, channel blockage, range and frequency of flow variation, and Reynolds-number. These discussions underline the value of future investigations into active and passive pitch control for smoothing high-frequency loads and scaling between lab- and full-scale passive adaptive rotors, among other work.