Comparing Torque-regulated and Speed-regulated Cross-flow Turbine Performance

Project Motivation:

Unlike better-studied axial-flow, or horizontal-axis, turbines, cross-flow, or vertical-axis, turbines have inherently unsteady fluid dynamics. The way this hydrodynamic variation interacts with power generation schemes can be confusing. For example, during power generation, cross-flow turbines can be “torque-regulated,” controlled by a steady braking torque which causes the rotation rate to vary, versus “speed-regulated,” controlled by a steady rotation rate which causes the braking torque to vary. Previous studies have shown both versions can provide similar net energy generation, but the systematic forces and dynamics can be counter-intuitive and haven’t been described in detail.

 
A photo showing a cross-flow turbine experimental setup like the one used in this study in the Alice C. Tyler flume at UW’s    Harris Hydraulics Lab   .

A photo showing a cross-flow turbine experimental setup like the one used in this study in the Alice C. Tyler flume at UW’s Harris Hydraulics Lab.

 

Summary:

A series of experiments in the Alice C. Tyler flume at the University of Washington assessed performance of a cross-flow turbine under speed- or torque-regulated flow. The experiments characterized the power, torque and force coefficients of a cross-flow turbine operated by holding the rotation rate constant (speed-regulated control) as well as by holding the opposing torque constant (torque-regulated control).

Annotated images of the experimental apparatus shown schematically and as implemented (minus water).

Annotated images of the experimental apparatus shown schematically and as implemented (minus water).

Current Results:

The project showed that there are small, but measurable, differences in turbine power output and structural loads depending on speed- or torque-regulated control and systematically lays out the relevant forces and dynamics. The main differences can be explained by differences in the blade kinematics (i.e., the angle of attack as a function of location within a revolution).

A plot comparing speed-regulated (blue) versus torque-regulated (red) turbine control and the resulting power output (y-axis). While torque-regulated control produces slightly higher net power, speed-regulated control provides a larger range of operational speeds (x-axis) at only a limited net power cost.

A plot comparing speed-regulated (blue) versus torque-regulated (red) turbine control and the resulting power output (y-axis). While torque-regulated control produces slightly higher net power, speed-regulated control provides a larger range of operational speeds (x-axis) at only a limited net power cost.

Understanding the role that control strategies can play on turbine hydrodynamics is an essential piece of maturing cross-flow turbines to the point that they can be applied to renewable power generation at larger scales.

Research Team:

More information:

Different control strategies lead to similar performance for crossflow turbines
AIP Scilight, 16 August 2019, doi: 10.1063/1.5123155

Comparison of Cross-Flow Turbine Performance under Torque-Regulated and Speed-Regulated Control
Polagye, B., Strom, B., Ross, H., Forbush, D., and Cavagnaro, R.
Journal of Renewable and Sustainable Energy, 13 August 2019, doi: 10.1063/1.5087476

Sponsor:

Funding for this project was provided by the U.S. Department of Defense Naval Facilities Engineering Command. Hannah Ross is supported by the National Science Foundation Graduate Research Fellowship Program under Grant DGE-1256082.

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Last updated: August 13, 2019