Abstract
Abstract. Numerous studies have shown that atmospheric conditions affect wind turbine performance; however, some findings have exposed conflicting results for different locations and diverse analysis methodologies. In this study, we explore how the change in wind direction with height (direction wind shear), a site-differing factor between conflicting studies, and speed shear affect wind turbine performance. We utilized lidar and turbine data collected from the 2013 Crop Wind Energy eXperiment (CWEX) project between June and September in a wind farm in north-central Iowa. Wind direction and speed shear were found to follow a diurnal cycle; however, they evolved differently with increasing wind speeds. Using a combination of speed and direction shear values, we found large direction and small speed shear to result in underperformance. We further analyzed the effects of wind veering on turbine performance for specific values of speed shear and found detrimental conditions on the order of 10 % for wind speed regimes predominantly located in the middle of the power curve. Focusing on a time period of ramping electricity demand (06:00–09:00 LT – local time) exposed the fact that large direction shear occurred during this time and undermined turbine performance by more than 10 %. A predominance of clockwise direction shear (wind veering) cases compared to counterclockwise (wind backing) was also observed throughout the campaign. Moreover, large veering was found to have greater detrimental effects on turbine performance compared to small backing values. This study shows that changes in wind direction with height should be considered when analyzing turbine performance.
Highlights
Wind power generation directly depends on wind speed
Wind shear at the test site showed more veering cases than backing cases (Fig. 7a) and a predominance of wind speed increasing with height (Fig. 7b), as would be expected from the balance between Coriolis, pressure gradient, and frictional forces in the atmospheric boundary layer (Holton and Hakim, 2013)
The largest shear values occurred between 40 m and 60 m a.g.l. (Fig. 8), as would be expected given that turbulent fluxes increase near the surface, causing larger wind vector rotation and speed reduction
Summary
Wind power generation directly depends on wind speed. Power depends on atmospheric conditions like static stability, shear, and turbulence (e.g., Bardal et al, 2015; van den Berg, 2008; Kaiser et al, 2007; Rareshide et al, 2009; St. Martin et al, 2016; Sumner and Masson, 2006; Vanderwende and Lundquist, 2012; Wagner et al, 2010; Walter et al, 2009; Wharton and Lundquist, 2012b). Atmospheric turbine operating conditions diverge from simplified ones used for turbine design. Varying inflow speed and direction profiles, turbulence, transient conditions, and wake effects from upwind turbines alter power production
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