The VERTEX field campaign: observations of near-ground effects of wind turbine wakes

ABSTRACT

Wind energy has been growing steadily in the U.S. and worldwide in the past decades. As wind farms are projected to increase in size and number, however, concerns are rising about possible undesirable effects of wind turbines near the Earth's surface. The literature is highly divided about what these effects could be, including warming, cooling, both, or neither. Only one mechanism, however, has been widely accepted (but never tested) to explain how wind turbines affect the lower boundary layer, namely that turbulence generated in wind turbine wakes enhances vertical mixing near the ground. Wakes are plume-like volumes downwind of wind turbines that are characterised by lower wind speeds and higher turbulent kinetic energy (TKE) than the undisturbed upwind flow. The few observational campaigns that have measured changes in near-surface properties by wind turbines have not provided an answer with respect to vertical mixing near the ground. To fill this knowledge gap, the VERTEX (VERTical Enhanced miXing) measurement campaign was conducted in August-October 2016 near a large wind turbine in coastal Delaware, using 15 surface flux towers, a 50-m meteorological tower, a radiometer, and two scanning lidars. During VERTEX, lidar scans and a wake detection algorithm were used to detect wake events and identify which sites were affected by the wake of the wind turbine. TKE, momentum and heat fluxes near the ground were compared between the sites below the wake and those outside of it. Preliminary findings based on two case studies (30 August and 20 September 2016) suggest a lack of enhancement of vertical mixing near the ground.

KEYWORDS:

• Wind power
• wind turbine
• turbulent kinetic energy
• wind turbine wake
• vertical mixing

Acknowledgments

We would like to thank: the Townsend family for allowing us to access and install instrumentation on their land; Siemens Gamesa Renewable Energy, especially Jairo Arias, for allowing us to shut down and restart the G90 turbine a few times during VERTEX and for sharing their data; Brian J. Carroll and Meredith Sperling of the University of Maryland for their help with post-processing the lidar PPI and RHI scans; the many personnel of the Earth Observing Laboratory (EOL) of the National Center for Atmospheric Research (NCAR) that helped us during VERTEX, including Brigitte Baeuerle, Kurt Knudson, Kate Young, Gordon Mclean, Dan Buonome, Jeff Bobka, Gary Granger, John Militzer, Andy Tuthill, Clayton Arendt, and Matt Paulus; Neil Kelley of Wind Energy Technology Solutions for sharing information on the San Gorgonio campaign; and Kevin Beam and Jon Swallow from UD Marine Operations for their extensive help with the logistics of boat access to the sites. We will always remember and be grateful for the help of Tom Horst of EOL/NCAR, who helped us design VERTEX and recently passed away.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Cristina L. Archer http://orcid.org/0000-0002-7837-7575

Additional information

Funding

Funding for this research was provided by the U.S. National Science Foundation's Division of Atmospheric and Geospace Sciences (Award n. 1564565).