Toward understanding the physical link between turbines and microclimate impacts from in situ measurements in a large wind farm

Abstract

Recent wind farm studies have revealed elevated nighttime surface temperatures but have not validated physical mechanisms that create the observed effects. We report measurements of concurrent differences in surface wind speed, temperature, fluxes, and turbulence upwind and downwind of two turbine lines at the windward edge of a utility‐scale wind farm. On the basis of these measurements, we offer a conceptual model based on physical mechanisms of how wind farms affect their own microclimate. Periods of documented curtailment and zero‐power production of the wind farm offer useful opportunities to rigorously evaluate the microclimate impact of both stationary and operating turbines. During an 80 min nighttime wind farm curtailment, we measured abrupt and large changes in turbulent fluxes of momentum and heat leeward of the turbines. At night, wind speed decreases in the near wake when turbines are off but abruptly increases when turbine operation is resumed. Our measurements are compared with Moderate Resolution Imaging Spectroradiometer Terra and Aqua satellite measurements reporting wind farms to have higher nighttime surface temperatures. We demonstrate that turbine wakes modify surface fluxes continuously through the night, with similar magnitudes during the Terra and Aqua transit periods. Cooling occurs in the near wake and warming in the far wake when turbines are on, but cooling is negligible when turbines are off. Wind speed and surface stratification have a regulating effect of enhancing or decreasing the impact on surface microclimate due to turbine wake effects.

Simulated wind farm wake sensitivity to configuration choices in the Weather Research and Forecasting model version 3.8.1

“…The aggregate impact of these individual turbine wakes can extend over 50 km downwind of a wind farm, particularly during stable conditions when little atmospheric 10 convection is present to erode the wake (Christiansen and Hasager, 2005;Platis et al, 2018). Consequences of these wake effects include local changes to surface fluxes that can raise surface temperatures at night due to turbine-induced mixing of the nocturnal inversion (Baidya Roy, 2004;Baidya Roy and Traiteur, 2010;Zhou et al, 2012;Rajewski et al, 2013;Smith et al, 2013;Rajewski et al, 2016;Siedersleben et al, 2018a) and loss of power and revenue for downwind wind farms operating in the wind speed deficit (Nygaard, 2014;Nygaard and Hansen, 2016;Nygaard and Newcombe, 2018;Lundquist et al, 2018).…”

Evaluation of the wind farm parameterization in the Weather Research and Forecasting model (version 3.8.1) with meteorological and turbine power data

“…This campaign was a component of the larger CWEX project, which explored the interactions of wind turbines with crops, surface fluxes, and near-surface flows in different atmospheric stability regimes in flat terrain (Rajewski et al, 2013). Research facilitated by the CWEX projects include diurnal changes in observed turbine wakes (Rhodes and Lundquist, 2013), turbine interactions with moisture and carbon dioxide fluxes , LES modelling of turbine wakes in changing stability regimes , nocturnal low-level jet (LLJ) occurrences (Vanderwende et al, 2015), diurnal changes of the microclimate near wind turbines (Rajewski et al, 2016), multiple-wake interactions (Bodini et al, 2017), the evolution of turbine wakes during the evening transition (Lee and Lundquist, 2017), and coupled mesoscalemicroscale modelling (Muñoz-Esparza et al, 2017).…”

Incorporation of the Rotor-Equivalent Wind Speed into the Weather Research and Forecasting Model’s Wind Farm Parameterization

“…Wind farms extract kinetic energy from the wind, resulting in wakes (Lissaman 1979;Högström et al 1988;Wang and Prinn 2010;Iungo et al 2013;Smith et al 2013). Several studies have found a nighttime warming trend downwind of wind farms (Baidya Roy and Traiteur 2010;Zhou et al 2012;Smith et al 2013;Rajewski et al 2013;Armstrong et al 2016;Rajewski et al 2016), which is likely due to vertical mixing induced by turbine operation. Daytime increases in upward latent heat flux and downward CO 2 flux also occur (Rajewski et al 2013).…”

“…Several studies have found a nighttime warming trend downwind of wind farms (Baidya Roy and Traiteur 2010;Zhou et al 2012;Smith et al 2013;Rajewski et al 2013;Armstrong et al 2016;Rajewski et al 2016), which is likely due to vertical mixing induced by turbine operation. Daytime increases in upward latent heat flux and downward CO 2 flux also occur (Rajewski et al 2013). As these effects are not negligible, they should be considered in weather models.…”