• Energy Research
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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.

Modeling wave energy converters (WECs) to accurately predict their hydrodynamic behavior has been a challenge for the wave energy field. Often, this results in either low-fidelity linear models or high-fidelity numerical models that are too computationally expensive for operational use. To bridge this gap, we propose the use of dynamic mode decomposition (DMD) as a purely data-driven technique that can generate an accurate and computationally efficient model of WEC dynamics. Specifically, we model and predict the behavior of an oscillating surge wave energy converter (OSWEC) in mono- and polychromatic seas without an equation of motion or knowledge of the incident wave field. We generate data with the open-source code WEC-Sim, then evaluate how well DMD can describe past dynamics, and predict future behavior. We consider realistic challenges including noisy sensors, nonlinear dynamics, and irregular wave forcing. Specifically, by using an extension of DMD, we reduce the effect of noise on our system and significantly increase model accuracy outside the training region. Additionally, by introducing time delays, we accurately describe weakly nonlinear dynamics, even though DMD is a linear algorithm. Finally, we use Optimized DMD (optDMD) to model OSWEC behavior in response to irregular waves. While optDMD accurately models training data, future prediction is inaccurate, demonstrating the limits of modeling efforts without access to information about the incident wave field. These findings provide insight into the use of DMD, and its extensions, on systems with limited time-resolved data and present a framework for applying similar analysis to lab- or field-scale experiments.

For economic reasons, above a certain water speed, it is desirable for current turbines to maintain a constant power output. This requires a control strategy to shed power. Here, such a strategy is evaluated through simulation and laboratory experiment for a helically bladed turbine with four blades and a straight-bladed turbine with two blades. These are contrasting cases because hydrodynamic torque produced by the straight-bladed turbine has a substantially more azimuthal phase variability than the helically bladed turbine. For practical implementation, a control algorithm is desired that requires only a time-average characteristic performance curve and estimates of angular velocity and control torque. Additionally, the transition between power maximizing and power tracking regimes should be smooth and automatic. The controller can be further constrained to only apply a resistive torque to the turbine. A control strategy satisfying these constraints is shown experimentally to achieve these objectives for both types of turbines at a variety of power set points. The power-tracking error (<3%) is primarily at the blade passage frequency for the straight-bladed turbine and at the rate of turbine rotation for the helical turbine. While partially a consequence of how the generator drive is tuned, comparison to simulation indicates that perfect power-tracking is not generally possible even under ideal conditions for a fixed-pitch, cross-flow turbine using purely resistive torque control.

Acoustic emissions from current energy converters remain an environmental concern for regulators because of their potential effects on marine life and uncertainties about their effects stemming from a lack of sufficient observational data. Several recent opportunities to characterize tidal turbine sound emissions have begun to fill knowledge gaps and provide a context for future device deployments. In July 2021, a commercial-off-the-shelf hydrophone was deployed in a free-drifting configuration to measure underwater acoustic emissions and characterize a 25 kW-rated tidal turbine at the University of New Hampshire’s Living Bridge Project in Portsmouth, New Hampshire. Sampling methods and analysis were performed in alignment with the recently published IEC 62600-40 Technical Specification for acoustic characterization of marine energy converters. Results from this study indicate acoustic emissions from the turbine were below ambient sound levels and therefore did not have a significant impact on the underwater noise levels of the project site. As a component of Pacific Northwest National Laboratory’s Triton Field Trials (TFiT) described in this Special Issue, this effort provides a valuable use case for the IEC 62600-40 Technical Specification framework and further recommendations for cost-effective technologies and methods for measuring underwater noise at future current energy converter project sites.

Integrated instrumentation packages are an attractive option for environmental and ecological monitoring at marine energy sites, as they can support a range of sensors in a form factor compact enough for the operational constraints posed by energetic waves and currents. Here we present details of the architecture and performance for one such system—the Adaptable Monitoring Package—which supports active acoustic, passive acoustic, and optical sensing to quantify the physical environment and animal presence at marine energy sites. we describe cabled and autonomous deployments and contrast the relatively limited system capabilities in an autonomous operating mode with more expansive capabilities, including real-time data processing, afforded by shore power or in situ power harvesting from waves. Across these deployments, we describe sensor performance, outcomes for biological target classification algorithms using data from multibeam sonars and optical cameras, and the effectiveness of measures to limit biofouling and corrosion. On the basis of these experiences, we discuss the demonstrated requirements for integrated instrumentation, possible operational concepts for monitoring the environmental and ecological effects of marine energy converters using such systems, and the engineering trade-offs inherent in their development. Overall, we find that integrated instrumentation can provide powerful capabilities for observing rare events, managing the volume of data collected, and mitigating potential bias to marine animal behavior. These capabilities may be as relevant to the broader oceanographic community as they are to the emerging marine energy sector.

Cross-flow turbines harness kinetic energy in wind or moving water. Due to their unsteady fluid dynamics, it can be difficult to predict the interplay between aspects of rotor geometry and turbine performance. This study considers the effects of three geometric parameters: the number of blades, the preset pitch angle, and the chord-to-radius ratio. The relevant fluid dynamics of cross-flow turbines are reviewed, as are prior experimental studies that have investigated these parameters in a more limited manner. Here, 223 unique experiments are conducted across an order of magnitude of diameter-based Reynolds numbers ($\approx 8\!\times\!10^4 - 8\!\times\!10^5$) in which the performance implications of these three geometric parameters are evaluated. In agreement with prior work, maximum performance is generally observed to increase with Reynolds number and decrease with blade count. The broader experimental space clarifies parametric interdependencies; for example, the optimal preset pitch angle is increasingly negative as the chord-to-radius ratio increases. As these experiments vary both the chord-to-radius ratio and blade count, the performance of different rotor geometries with the same solidity (the ratio of total blade chord to rotor circumference) can also be evaluated. Results demonstrate that while solidity can be a poor predictor of maximum performance, across all scales and tested geometries it is an excellent predictor of the tip-speed ratio corresponding to maximum performance. Overall, these results present a uniquely holistic view of relevant geometric considerations for cross-flow turbine rotor design and provide a rich dataset for validation of numerical simulations and reduced-order models. This article appeared in Hunt et al., Renewable and Sustainable Energy Reviews 206, 114848 (2024), and may be found at https://doi.org/10.1016/j.rser.2024.114848. Data supporting this manuscript may be found in the Dryad digital repository at https://doi.org/10.5061/dryad.mpg4f4r8p

Cross-flow or vertical-axis turbines are flow energy conversion devices in which lift forces cause blades to rotate around an axis perpendicular to the flow. In marine currents, rivers, and some wind energy applications, cross-flow turbines are a promising alternative to more conventional axial-flow turbines. The performance implications of the choice of structure used to mount turbine blades to the central shaft are examined experimentally in a recirculating water flume. Turbine performance is found to be strongly dependent on the choice of the mounting structure. Power loss due to rotational drag on these structures is estimated experimentally by rotating the mounting structure without blades. Through a perturbation-theory approach, interactions between turbine blades and mounting structures are examined. Analytical models for the power loss due to mounting structure drag are introduced and shown to be consistent with experiments. To provide guidance for cross-flow turbine design, the models are re-formulated in terms of non-dimensional turbine geometric and operational parameters. Mounting blades solely at their mid-span is shown to decrease performance through multiple fluid effects. Using foil cross-section struts located at the turbine blade tips is found to result in the highest turbine performance.

Cross-flow turbines convert kinetic power in wind or water currents to mechanical power. Unlike axial-flow turbines, the influence of geometric parameters on turbine performance is not well-understood, in part because there are neither generalized analytical formulations nor inexpensive, accurate numerical models that describe their fluid dynamics. Here, we experimentally investigate the effect of aspect ratio—the ratio of the blade span to rotor diameter—on the performance of a straight-bladed cross-flow turbine in a water channel. To isolate the effect of aspect ratio, all other non-dimensional parameters are held constant, including the relative confinement, Froude number, and Reynolds number. The coefficient of performance is found to be invariant for the range of aspect ratios tested (0.95–1.63), which we ascribe to minimal blade–support interactions for this turbine design. Finally, a subset of experiments is repeated without controlling for the Froude number, and the coefficient of performance is found to increase, a consequence of the Froude number variation that could mistakenly be ascribed to aspect ratio. This highlights the importance of a rigorous experimental design when exploring the effect of geometric parameters on cross-flow turbine performance.