Effects of wind turbine wakes on microclimate properties near the ground
| Author(s) | Wu, Sicheng | |
| Date Accessioned | 2022-01-28T15:12:45Z | |
| Date Available | 2022-01-28T15:12:45Z | |
| Publication Date | 2021 | |
| SWORD Update | 2021-09-30T19:14:40Z | |
| 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, in particular on surface temperature. 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 characterized by lower wind speeds (i.e., a wind speed deficit) and higher turbulent kinetic energy (TKE) than the undisturbed upwind flow. ☐ This dissertation attempts to quantify and explain the effects of wind turbine wakes on microclimate properties near the ground with both observations and simulations. The observational component is the VERTEX (VERTical Enhanced miXing) measurement campaign, which was conducted in August-October 2016 near a large wind turbine in coastal Delaware. In VERTEX, near-surface TKE, turbulent fluxes, wind speed, temperature and moisture are measured and compared under and outside of the wake of an operational wind turbine in Lewes, Delaware. To further understand wakes of single versus multiple turbines (i.e., overlapping wakes), large-eddy simulations (LES) with the Weather Research and Forecasting Model (WRF-LES) were performed with the turbine modeled as a generalized actuator disk. ☐ The first finding is that atmospheric stability strongly controls the temperature response of the ground to the wake, with warming observed in stable conditions, slight cooling under unstable conditions, and slight warming or no change under neutral conditions. With overlapping wakes, the stability effects on temperature are of the same sign as those of a single wake, but generally stronger in magnitude. The best measure of stability in the field is found to be the temperature lapse rate between hub height and the ground. Using only local measurements at the ground would not capture the effects of temperature inversions intersecting the rotor and using a simple day/night classification will miss stable events during the days and all neutral cases. It is important to note that the observed near-ground temperature changes during VERTEX were <0.4°C in magnitude. ☐ The second finding of this dissertation is that vertical mixing near the ground is not enhanced by wind turbine wakes. In VERTEX, friction velocity, TKE, and wind speed are found to be reduced near the ground under the wake, while sensible heat flux and moisture are not significantly affected by the wake. The LES results also indicate a reduction of vertical momentum flux below single and overlapping wakes. As such, enhanced vertical mixing cannot be the mechanism responsible for the temperature changes. ☐ The third finding is that the mechanism that causes the temperature changes near the ground in the presence of wind turbine wakes is the vertical convergence (or divergence) of turbulent heat fluxes, as confirmed with both observations and simulations. Under stable conditions, the enhancement of the downward turbulent heat flux in the rotor area and the lack of change in surface heat flux cause convergence below the rotor, which induces warming. Vice versa, under unstable conditions, the enhancement of the (weak) upward turbulent heat flux in the rotor area and the lack of change in surface heat flux cause a weak divergence below the rotor, which induces a slight cooling. Overlapping wakes enhance the vertical convergence under stable conditions and the vertical divergence under unstable conditions, thus causing a slightly stronger warming and cooling, respectively. ☐ Lastly, a wind turbine wake cannot be fully characterized just by the wind speed deficit, because added TKE does not behave like the wind speed deficit. In other words, the wake has a "dual nature", with both wind speed deficit and added TKE being necessary to fully characterize a wake. The dual nature of wind turbine wakes is found in the vertical profiles (maximum near hub height for wind speed deficit but near the top-tip for added TKE), expansion rate (larger for added TKE than for wind speed deficit), and touch-down properties (wind speed deficit reaches the ground for both a single wake and overlapping wakes, whereas the added TKE only reaches the ground for overlapping wakes). But then, to properly understand the wind turbine wake effects on the environment, a third property is necessary: atmospheric stability. Atmospheric stability controls the shape of the wake, its expansion (in terms of both wind speed deficit and added TKE), and ultimately its effects on surface temperature (generally warming under stable, slight or no cooling under unstable, and slight or no warming under neutral conditions). The temperature effects are driven by the vertical convergence and divergence of the turbulent heat fluxes below the rotor, not by enhanced vertical mixing. | en_US |
| Advisor | Archer, Cristina L. | |
| Degree | Ph.D. | |
| Department | University of Delaware, School of Marine Science and Policy | |
| DOI | https://doi.org/10.58088/k2a3-m735 | |
| Unique Identifier | 1293984287 | |
| URL | https://udspace.udel.edu/handle/19716/30166 | |
| Language | en | |
| Publisher | University of Delaware | en_US |
| URI | https://login.udel.idm.oclc.org/login?url=https://www.proquest.com/dissertations-theses/effects-wind-turbine-wakes-on-microclimate/docview/2587010763/se-2?accountid=10457 | |
| Keywords | Turbulence | |
| Keywords | Vertical mixing | |
| Keywords | Wake | |
| Keywords | Wind energy | |
| Title | Effects of wind turbine wakes on microclimate properties near the ground | en_US |
| Type | Thesis | en_US |