  Cross-flow turbines, also known as vertical-axis turbines, have numerous
features that make them attractive for wind and marine renewable energy. To
maximize power output, the turbine blade kinematics may be controlled during
the course of the blade revolution, thus optimizing the unsteady fluid dynamic
forces. Dynamically pitching the blades, similar to blade control in a
helicopter, is an established method. However, this technique adds undesirable
mechanical complexity to the turbine, increasing cost and reducing durability.
Here we introduce a novel alternative requiring no additional moving parts: we
optimize the turbine rotation rate as a function of blade position resulting in
motion (including changes in the effective angle of attack) that is precisely
timed to exploit unsteady fluid effects. We demonstrate experimentally that
this approach results in a 79% increase in power output over industry standard
control methods. Analysis of the fluid forcing and blade kinematics show that
maximal power is achieved through alignment of fluid force and rotation rate
extrema. In addition, the optimized controller excites a well-timed dynamic
stall vortex, as is found in many examples of biological propulsion. This
control strategy allows a structurally robust turbine operating at relatively
low angular velocity to achieve high efficiency and could enable a new
generation of environmentally-benign turbines for wind and water current power
generation.
