Experimental Study Of Tidal Turbines Loading And Performance In Oblique Waves And Currents

The worldwide potential of electric power generation from marine tidal currents, waves, or offshore winds is enormous. The high load factor resulting from the fluid properties and the predictable resource characteristics make tidal and wave energy resources attractive and advantageous for power generation and advantageous when compared to other renewable energies. The technologies are just beginning to reach technical and economic viability to make them potential commercial power sources in the near future. While only a few small projects currently exist, the technology is advancing rapidly and has huge potential for generating bulk power. Moreover, international treaties related to climate control and dwindling fossil fuel resources have encouraged us to harness energy sustainably from such marine renewable sources. Several demonstrative projects have been scheduled to capture tidal and wave energies. A number of these projects have now reached a relatively mature stage and are close to completion. However, very little is known to the academic world about these technologies beyond the basics of their energy conversion principles. While research emphasis is more towards hydrodynamics and turbine design, very limited activities are witnessed in power conversion interface, control, and power quality aspects. Regarding this emerging and promising area of research, this book aims to present recent results, serving to promote successful marine renewable energies integration to the grid or to standalone microgrids.

This research addresses the need for an improved characterisation of the onset flow turbulence and the unsteady hydrodynamic blade loads on tidal turbines for the purposes of predicting fatigue life. A new, extensive set of parameters which characterise the magnitudes of the turbulent fluctuations, the anisotropy and the scales of the turbulence at a tidal energy site have been presented. A novel application of rapid distortion theory estimated the velocity fluctuations to be amplified by 15% due to the presence of the turbine. The turbulence was also predicted to be well correlated over the outer span of a turbine blade at the frequencies of interest. Together, these results enabled a set of non-dimensional parameters describing the turbulence induced forcing on a turbine blade to be established. A model-scale horizontal-axis turbine was used to investigate the unsteady blade load response in a still-water towing tank. A set of wind tunnel tests of the S814 foil were also conducted and used to demonstrate that the lift on the blades could have been degraded by 10% at the relatively low Reynolds numbers at which the turbine was tested, relative to full-scale. This was owing to dominant laminar separation bubbles. Single frequency planar oscillations of the turbine were used to quantify the contribution of hydrodynamic unsteadiness to the blade-root bending moment. For attached flow, the unsteady bending moment was found to amplify the steady loads by up to 15 %. The total hydrodynamic added mass was up to 2.7 times larger than from non-circulatory forcing and decreased with frequency. Dynamic inflow theory and a returning wake model were able to provide qualitative predictions of these results at low frequencies. At low tip-speed ratios, phenomena consistent with delayed separation and dynamic stall were characterised and the unsteady loading was up to 25% larger than the steady load. Linear superposition of the single frequency responses was also demonstrated to offer a reliable technique to model the response to a multi-frequency forcing and to a large eddy.

The research in this thesis focuses on the investigation of tidal turbines using a small scale horizontal axis tidal turbine and a 2D hydrofoil testing rig, combining experiments with simulations to provide comprehensive results and to better understand some of the variables that affect their performance. The experimental campaigns were carried out at the University of Victoria fluids research lab and the Sustainable Systems Design Lab (SSDL). The experimental testing rigs were re-designed by the author and are now fully automated, including a friendly graphical user interface for easy implementation. Particle image velocimetry (PIV) technique was used as the quantitative flow visualization method to obtain the time-averaged flow fields. This thesis presents three investigations. The first study aims to quantify the impacts of channel blockage, free surface effects and foundations on hydrokinetic turbine performance, using porous discs and an axial flow rotor. The results were used to cross-validate computational fluid dynamics (CFD) simulations. It was found that as wall blockage increases, thrust and power are incremented with and without the inclusion of free surface deformation. Discrepancies between simulations and experimental results on free surface effects compared to a slip wall were obtained and hence further research is recommended and the author gives some advice on how to proceed in this investigation.The second study determines the performance of four hydrofoil candidates over a range of low Reynolds number (Re), delivering useful information that can be applied to low Re energy conversion systems and, specifically in this case, to improve the performance of the small scale tidal turbine at the SSDL lab. The study combines the 2D hydrofoil test rig along with PIV measurements in order to experimentally obtain lift and drag coefficients. The experiments were carried out in the recirculating flume tank over the range of low Re expected for the small scale rotor rig, in order to provide more accurate results to improve rotor blade design. In addition, numerical simulations using XFOIL, a viscid-inviscid coupled method, were introduced to the study. These results were analysed against experiments to find the most suitable parameters for reliable performance prediction. The final results suggested that adding a numerical trip at a certain chordwise distance produced more reliable results.Finally, an experimental study on turbine rotor performance and tip vortex behavior was performed using again the rotor rig and PIV. Blade design and rotor performance were assessed, showing good agreement with Blade Element Momentum (BEM) simulations, particularly at predicting the tip speed ratio corresponding to the maximum power coefficient point. Regarding the wake structure, tip vortex locations (shed from the blade tips) were captured using PIV in the near wake region, showing evidence of wake expansion. The velocity and vorticity fields are also provided to contribute to the development and validation of CFD and potential flow codes.

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.