How to build a better vertical-axis turbine

The iconic picture of a wind farm comprises several towers with long-bladed propellers facing into the wind. An alternative geometry, a cross-flow or vertical-axis turbine, also shows promise. Optimized arrays of vertical-axis wind turbines might harvest more power for a given array footprint than their horizontal counterparts (see the Quick Study by John Dabiri, Physics Today, October 2014, page 66 ). In river and ocean settings, the turbines’ typically slower blade speed poses fewer risks to fauna, and their rectangular profile (see the figure) are well suited to the shape of shallow water channels. The blades run parallel to the axis of rotation, so there are many options for mounting them. They can be attached at either end to solid disks, for example, or struts of various cross sections can be used at the ends or at intermediate positions. Similar to the way the winglets at the tips of airplane wings can reduce the induced drag, the mounting of the turbine blades can affect the overall performance. Benjamin Strom, Noah Johnson, and Brian Polagye of the University of Washington’s Pacific Marine Energy Center have now characterized the impact of 10 different mounting structures. Using a recirculating water flume, the researchers measured how the mechanical efficiency of a two-bladed turbine varied with a given mounting strategy. Analytical models for the drag on the various mounting geometries agreed well with the data; the blade-induced flow also appeared to have a noticeable effect on the mount drag. Overall, the best-performing system featured thin, airfoil-shaped mounting struts attached to the ends of the blades; unexpectedly, mid-span struts fared the worst. As the number of blades increases, disk-shaped mounts may become preferable. (B. Strom, N. Johnson, B. Polagye, J. Renew. Sustain. Energy 10, 034504, 2018 .)