Improved Understanding of Vertical Mixing in the Lower Atmospheric Boundary Layer in the Presence of Wind Turbines via Numerical Simulations and Measurements

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Wind energy has been growing steadily in the U.S. and worldwide in the past decades and it is projected to continue its growth in the near future. Wind energy has countless benefits: it is renewable and abundant, does not produce any harmful pollutant or greenhouse gas emissions, and is so technologically advanced that it is often economically competitive with other traditional energy sources. However, concerns are rising about possible undesirable effects of wind turbines in the lower boundary layer, especially on near-surface temperature. The literature is highly divided about what these effects could be. Numerical simulations have shown: warming during day and night; warming at night and cooling during the day; warming or cooling depending on atmospheric stability; cooling; and no significant effects. These contradictory findings highlight the inherent complexity of the feedbacks between wind turbines and atmosphere.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. Wakes are plume-like volumes downwind of wind turbines that are characterized by lower wind speeds and higher turbulence than the undisturbed upwind flow. Enhanced vertical mixing would manifest as increased vertical momentum fluxes (thus increased wind shear) and increased turbulent kinetic energy (TKE) near the ground. The few observational campaigns that have measured changes in near-surface properties by wind turbines have been flawed or inconclusive and have not measured directly vertical momentum flux changes. Therefore, no direct evidence of the enhanced vertical mixing mechanism is available today. In addition, enhanced vertical mixing does not explain all the observed effects.Intellectual Merit:The research hypothesis put forward in this project is that vertical mixing is generally not enhanced, but, more likely, reduced near the surface downwind of wind turbines. The vertical wind profile is greatly altered due to the extraction of kinetic energy by wind turbines in the region around the rotor disk of diameter D spanned by the turbine blades, but the resulting wind shear near the ground is generally unaltered or reduced, not enhanced, below the rotor. Near-surface heat, moisture, and momentum fluxes, and eventually temperature, are therefore generally not directly affected by wake turbulence, but rather by a combination of the reduced, not enhanced, wind shear, atmospheric stability, and undisturbed flow properties.To test this hypothesis, a combined observational and numerical approach will be carried out. The measurement campaign, named VERTEX for VERTical Enhanced MiXing, will be conducted near the University of Delaware Gamesa 2-MW wind turbine in Lewes, using 12-15 surface flux towers, a 60-m meteorological towers, and 2 scanning lidars. VERTEX will quantify changes in momentum and heat fluxes, air temperature, and TKE near the ground due to the wind turbine. The interactions between the wind turbine and the atmospheric flow will be simulated with at least two state-of-the-art numerical codes that can resolve the details of the turbulent wake down to a few meter resolution. Data from VERTEX will be used to validate the simulations. The sensitivity to a variety of inflow and atmospheric stability conditions will also be assessed.Broader Impacts:This research will provide an understanding if and how wind turbines can alter near-surface properties in any significant way. This understanding is important because it will either enhance wind energy deployment or slow it if such changes are considered too dramatic. Research findings will be communicated to: 1) the scientific community via participation to conferences and peer-reviewed papers; 2) the general public via targeted outreach efforts, including an exhibit at the annual University of Delaware's Coast Day and press releases; and 3) university students via a newly developed graduate and undergraduate course on "Energy in the atmosphere".