Different control strategies lead to similar performance for crossflow turbines

In a crossflow turbine, aka vertical-axis turbine, water or wind moves across blades arranged like an eggbeater, producing torque that moves the rotor to generate power. This type of turbine has seen less commercial success than its horizontal-axis turbine counterpart for several reasons, including the prevalence of more unsteady flow dynamics near the rotor. Understanding the relationship between these unsteady dynamics and turbine control is critical for developing crossflow turbines capable of generating renewable energy on a larger scale.

During operation in the field, an electrical generator converts the turbine’s rotational motion into electricity. The generator and turbine produce torque that is, on average, equal and opposing. In the lab, the same conditions must be met. Polagye et al. experimentally compared the performance of a crossflow turbine using two common control strategies that emulate electrical generation: speed-regulated control, which holds the turbine rotation rate constant, and torque regulated control, which imposes a constant resistive torque on the turbine and causes the turbine periodically accelerate and decelerate.

They found the two control strategies yield very similar turbine performance values but exhibit small differences in power output and structural loads, which can be explained by differences in how the blades move through the water. Though tested in water, most aspects of the author’s results apply to wind turbines.

Author Brian Polagye said he hopes their work makes it easier for researchers conducting turbine-related experiments to make informed choices between various options to emulate electrical generation in the lab. Next, the authors will test how these results scale-up using larger turbines in the field.

Source: “Comparison of cross-flow turbine performance under torque-regulated and speed-regulated control,” by Brian Polagye, Ben Strom, Hannah Ross, Dominic Forbush, and Robert J. Cavagnaro, Journal of Renewable and Sustainable Energy (2019). The article can be accessed at https://doi.org/10.1063/1.5087476 .