Improving mesoscale wind speed forecasts using lidar-based observation nudging for airborne wind energy systems

Abstract

Abstract. Airborne wind energy systems (AWESs) aim to operate at altitudes above conventional wind turbines where reliable high-resolution wind data are scarce. Wind light detection and ranging (lidar) measurements and mesoscale models both have their advantages and disadvantages when assessing the wind resource at such heights. This study investigates whether assimilating measurements into the mesoscale Weather Research and Forecasting (WRF) model using observation nudging generates a more accurate, complete data set. The impact of continuous observation nudging at multiple altitudes on simulated wind conditions is compared to an unnudged reference run and to the lidar measurements themselves. We compare the impact on wind speed and direction for individual days, average diurnal variability and long-term statistics. Finally, wind speed data are used to estimate the optimal traction power and operating altitudes of AWES. Observation nudging improves the WRF accuracy at the measurement location. Close to the surface the impact of nudging is limited as effects of the air–surface interaction dominate but becomes more prominent at mid-altitudes and decreases towards high altitudes. The wind speed frequency distribution shows a multi-modality caused by changing atmospheric stability conditions. Therefore, wind speed profiles are categorized into various stability conditions. Based on a simplified AWES model, the most probable optimal altitude is between 200 and 600 m. This wide range of heights emphasizes the benefit of such systems to dynamically adjust their operating altitude.

Offshore and onshore ground-generation airborne wind energy power curve characterization

“…A common proxy for atmospheric stability is the Obukhov length (Obukhov, 1971;Sempreviva and Gryning, 1996), a metric that exclusively uses surface data (see section 2.2 and equation 1). Previous studies (Sommerfeld et al, 2019b) showed that Obukhov-length-classified wind speed profiles diverge with height, especially during neutral and stable conditions. This indicates vertically heterogeneous atmospheric stability and suggests that surfacebased stability categorization is insufficient for higher altitudes.…”

“…"Onshore" wind data at the Pritzwalk Sommersberg airport (lat: 53 • 10 47.00 N, lon: 12 • 11 20.98 E) in northern Germany and comprises 12 months of WRF simulation between September 2015 and September 2016. The area surrounding the airport mostly consists of flat agricultural land with the town of Pritzwalk to the south and is therefore a fitting location for wind energy generation (See (Sommerfeld et al, 2019a) and (Sommerfeld et al, 2019b) for details). The FINO3 research platform in the North Sea (lat: 55 • 11, 7 N, lon: 7 • 9, 5 E) was chosen as a representative "offshore" location due to the proximity to several offshore wind farms and the amount of comprehensive reference measurements (Peña et al, 2015).…”

“…The onshore simulation was performed with version 3.6.1 before the 2018 release of WRF version 4.0.2 1 in which the offshore simulations were computed. The setup of the model has been adapted and constantly optimized for wind energy applications by the authors in the framework of various projects and applications in recent years (Dörenkämper et al, 2015(Dörenkämper et al, , 2017Dörenkämper et al, 2020;Hahmann et al, 2020;Sommerfeld et al, 2019c).…”

“…We chose the same classification as in (Sommerfeld et al, 2019b) for consistency. Neutral stratification occurs approximately 20% of the year at both locations.…”

Ground-generation airborne wind energy design space exploration

“…Results in this study are exclusively based on WRF mesoscale simulations, since measuring wind conditions at mid-altitudes is difficult due to reduced data availability [137] and measurements are hard to find, proprietary or confidential. We compare AWES performance for an onshore location in northern Germany near the city of Pritzwalk [136] and an offshore location at the FINO3 research platform in the North Sea. WT and AWES performance using logarithmic wind profiles are compared as reference.…”