Session R24: Energy: Water Power II

Effect of free-stream turbulence on the hydrodynamic performance and wake structure of an H-Darrieus tidal turbine

The effect of free-stream turbulence on a four-bladed H-Darrieus tidal turbine was investigated through a series of experiments conducted in a water tunnel at a diameter-based Reynolds number (Re_D) of 0.5 x 10⁶ and a tip-speed ratio (TSR) of 2.65.  Particle image velocimetry (PIV) and continuous measurements of the torque applied to the shaft of the turbine were performed in conjunction with modal decomposition methods to obtain phase- and ensemble-averaged quantitative flow patterns in the wake of the turbine and to quantify its power extraction performance. The inflow turbulence levels were varied between < 1%, 5%, and 10% by installing fractal grids upstream of the test section. The wake flow characteristics and the direct torque measurements indicate that an increase in the free-stream turbulence intensity leads to a higher frequency of non-periodic vortex shedding, which in turn results in an earlier collapse of the outer shear layer in the near-wake of the turbine. These results offer two practical implications in the context of tidal turbines. The improved wake recovery distance in tidal sites with higher turbulence levels allows for closer turbine spacing, and the added non-periodic loading on the turbine blades may require a more robust turbine design in tidal environments with increased turbulence levels.   

One-way fluid-structure coupling of a cross-flow turbine in high confinement

Cross-flow turbines differ from conventional axial-flow turbines in that their axis of rotation is perpendicular to the direction of the flow. The geometry gives rise to a rectangular swept area that is more easily stacked in arrays to exploit the effects of confinement. This research explores the effect of confinement on turbines specifically in terms of the increased load on blades. Due to the high forces exerted on confinement-exploiting turbines, the blades undergo deformations in pitch and heave. These deformations can then interfere with the dynamic stall process, resulting in a coupled fluid-structure interaction. These dynamics are explored through a one-way fluid-structure interaction simulation. The deformations are computed through a non-deforming fluids simulation. These deformations are mapped to two-dimensional pitch and heave trajectories, which are then input back into the fluids solver to deduce the final changes in loads due to deformation.  Simulations are computed for a pair of two-bladed turbines, and changes in performance and flow fields are reported. This research will help to determine allowable loads and deformations under high confinement configurations.        

A parametric evaluation of the interplay between geometry and scale on cross-flow turbines

The effects of geometry on cross-flow (vertical-axis) turbine performance can be challenging to predict due to the unsteady interactions the rotor experiences during each rotation. We experimentally characterize the interplay between scale and geometry by varying the ratio of the chord length to turbine radius, the preset pitch angle between the chord line and rotational tangent, and the blade count. In total, 223 unique experiments were conducted, evaluating geometric trends in cases spanning diameter-based Reynolds numbers (Re) from 8x10⁴ - 8x10⁵. We observe that maximum performance generally increases with Re and decreases with blade count. In addition, for a given Re and blade count, the optimal preset pitch angle increases with chord-to-radius ratio. Because these experiments parametrically vary the chord-to-radius ratio and blade count, we can evaluate the performance of multiple turbines with the same solidity (the ratio of total blade chord to rotor circumference). We find that while solidity can be a poor predictor of maximum performance, across all scales and geometries investigated, it is an excellent predictor of the tip-speed ratio corresponding to maximum performance.   

Influence of rotor geometry on the performance of high-blockage cross-flow turbine arrays

Cross-flow turbines show great promise for extracting power from river and tidal currents due to their ability to achieve high blockage. In these flows, confinement from the channel boundaries and thrust on the turbine cause accelerated flow through the rotor, which increases the turbine's efficiency. Although the effects of blockage on turbine performance are well documented, the design of cross-flow turbine rotors to best harness these effects has received little attention. Here, we experimentally investigate how rotor geometry interacts with confinement to influence the performance of a cross-flow turbine array. We consider 45 unique rotor geometries by varying three parameters: the chord-to-radius ratio, the number of blades, and the preset pitch angle. The performance of each rotor geometry is evaluated at blockage ratios of 35%, 45%, and 55% using a laboratory-scale array consisting of two identical, counter-rotating turbines. Across the tested blockages, the chord-to-radius ratio and preset pitch angle are found to affect performance in similar ways to that observed in prior studies of single turbines at low blockage. However, as the blockage ratio increases, geometries with more blades outperform their counterparts with fewer blades, indicating that turbines with higher thrust coefficients may be preferable for high blockage settings.   

Influence of the downstream blade sweep on cross-flow turbine performance

Cross-flow turbine blades encounter a relatively homogenous inflow for half their rotation (the upstream sweep) and then pass through their own wake for the other half (the downstream sweep). Most research on cross-flow turbine optimization has focused on the power-generating phases of the upstream sweep, but the downstream sweep significantly affects net turbine performance. Here, a single-bladed cross-flow turbine is used to experimentally isolate power contributions from the upstream and downstream sweeps.  Two-component, phase-locked, PIV data is obtained inside the turbine swept area to investigate the hydrodynamic mechanisms underlying the observed degradation in downstream performance at high tip-speed ratios. We find that power generation from the upstream sweep continues to increase beyond the optimal tip-speed ratio. In contrast, power generation from the downstream sweep is net neutral until just before the optimal tip-speed ratio, after which it decreases faster than the upstream power generation increases. This indicates that the optimal tip-speed ratio is driven by the point at which the downstream sweep consumes appreciable power. At high tip-speed ratios, lift production in the downstream sweep becomes increasingly normal to the rotation direction while drag increases with rotation rate. These results highlight the need to consider the role of the downstream sweep on overall turbine performance and the projection of lift and drag into the direction of blade rotation.   

Scaling up from the laboratory to open-water: Performance and Load Measurements for a Grid-Connected Tidal Energy Conversion System

Cross-flow turbines (CFTs) are inherently unsteady fluid energy conversion devices, even in a uniform, turbulence-free inflow, such as a towing tank. When CFTs are deployed in real tidal flows additional unsteady phenomena come into play, e.g., blade or rotor scale turbulence, or “tidal gusts” of longer time scales.  Originating with the NSF-funded Living Bridge Project, a floating tidal energy conversion (TEC) system has been deployed since 2018 at the Atlantic Marine Energy Center (AMEC) Tidal Energy Test Site at Memorial Bridge in Portsmouth, NH, where tidal currents can reach speeds greater than 2.5 m/s during spring ebb tides. The TEC system consists of 3.2m diameter x 1.7m vertical axis CFT coupled to a direct drive PM generator, connected to the bridge grid via a rectifier and grid-following inverter. During a recent collaboration with the National Renewable Energy Laboratory (NREL) we made concurrent power performance, thrust load and tidal current resource measurements (via acoustic Doppler profilers and velocimeters), while the turbine operated in grid-synchronous mode.  Results from these data sets will be presented, and operational characteristics analyzed. Spectral analysis highlights the importance of tidal gusts and turbulence on annual power production estimates.   

Dynamics of cross-flow turbines with varying blade materials and unsupported blade span

Cross-flow turbines (CFTs) can be used to harness energy from tidal, river, or ocean currents. This work aims to reduce the levelized cost of energy of CFTs by using less rigid, less expensive materials and reducing the number of supports. To achieve this, turbine performance was investigated in a towing tank with a modular 1-meter diameter CFT with three blades and two struts. Blade material, strut separation distance, and tip speed ratio were varied while maintaining a sufficiently high towing speed for Reynolds number independence. For each material, the blades were instrumented with high-resolution fiber optic sensors to measure blade strain, providing insight on the impact of blade deformation on turbine performance. The reliability and survivability of CFTs in part depends on the fatigue life of the blade materials, which can be analyzed using a fluid-structure interaction (FSI) model that considers blade hydroelasticity. The collected high-quality, high-resolution experimental dataset is being used to validate computationally intensive FSI models, providing crucial groundwork to improve the efficiency and cost-competitiveness of CFTs.   

Tidal Energy Resource and Turbulence Characterization at the Atlantic Marine Energy Center Tidal Energy Test Site at Memorial Bridge in Portsmouth, NH

A Tidal Energy Test Site has been developed by the Atlantic Marine Energy Center (AMEC) in an energetic tidal estuary on the East Coast to conduct research and help progress tidal energy conversion towards commercial feasibility. The aim of this work is to accurately assess the tidal energy resource at the test site, including the effect of turbulence on the total available energy. The mean energy density will be derived from the ensembled-averaged velocity measurements from two Acoustic Doppler Current Profilers (ADCPs) located about 3 turbine diameters up- and downstream of the Energy Extraction Plane (EEP). The turbulence characteristics of the flow will be measured using three Acoustic Doppler Velocimeters (ADVs). The ADVs will be deployed evenly spaced in the cross-stream direction near the EEP and be used to assess non-uniformities across the EEP. ADVs can measure instantaneous velocity with sufficiently high sampling rates to resolve the relevant timescales of the turbulent flow that contribute to tidal energy conversion. The measurements are being conducted in preparation for the deployment of an instrumented 2.5 m diameter reference model turbine.