For tidal current energy to support the Blue Economy and increase access to clean, renewable electricity, further technological development is needed to decrease costs and increase reliability of current turbines at relevant scales. Blade pitch control strategies can significantly reduce structural loads in above-rated flow conditions by shedding power at decreased angles of attack. This can be accomplished through an active strategy (using motorized blades) or a passive strategy (using flexible, self-twisting blades). Meanwhile, fixed-pitch turbines typically rely on speed control alone to regulate power, which increases component cost in response to increased thrust or torque from “overspeed” or “underspeed” control, respectively. In this study, we experimentally implemented two passive pitch control strategies designed to maintain constant power output and two strategies using rigid, aluminum blades. We compare measured turbine loads across the cases, as well as first-of-its-kind, in-situ observations of blade deformation during turbine operation.
We tested the following four control strategies with a 0.45-meter diameter turbine in a recirculating flume, starting from a rated flow speed of 0.7 m/s and increasing, over time, to 0.8 m/s:
- Passive adaptive blades combined with overspeed control
- Passive adaptive blades combined with active pitch control
- Rigid blades combined with underspeed control
- Rigid blades combined with active pitch control
For both passive pitch strategies, we fabricated blades in-house with unidirectional carbon fiber oriented off-axis, such that the blades twist passively as they deflect in response to loading. We recorded deflection and twisting of the blade tip using a high-speed camera to develop our understanding of the bend-twist behavior during turbine operation over a range of flow speeds, rotation rates, and preset pitch angles.
For passive adaptive blades combined with active pitch control, we found no improvement in load reductions over the rigid blades combined with active pitch control. This is a consequence of relatively low thrust loads associated with this strategy, which leads to minimal blade deformation. Because this approach increases turbine cost without a corresponding reliability improvement, passive adaptive blades combined with active pitch control is an ineffective strategy for smoothing low-frequency load fluctuations in above-rated flow conditions.
For passive adaptive blades combined with overspeed control, the measurements of blade deformation revealed a significant increase in deflection and twist with increases in inflow velocity, allowing power shedding at lower tip-speed ratios than required for overspeed control with rigid blades. We also measured a 12% increase in thrust relative to the rated flow condition, as the currents increased by 14% (from 0.7 to 0.8 m/s). This is a promising result compared to the 127% increase in thrust observed during prior experiments of overspeed control with rigid blades. This indicates that passive adaptive blades combined with overspeed control can be an effective strategy for both power regulation and load reduction in above-rated flow conditions. This strategy obviates the need for an active pitch mechanism, thereby increasing reliability and decreasing maintenance costs. We anticipate that this work will prompt future studies of passive pitch control for current turbines, including at larger scales and in response to turbulent and cyclic loads.