For wind turbines in complex terrain, the devil is in the detail

Abstract

The cost of energy produced by onshore wind turbines is among the lowest available; however, onshore wind turbines are often positioned in a complex terrain, where the wind resources and wind conditions are quite uncertain due to the surrounding topography and/or vegetation. In this study, we use a scale model in a three-dimensional wind-testing chamber to show how minor changes in the terrain can result in significant differences in the flow at turbine height. These differences affect not only the power performance but also the life-time and maintenance costs of wind turbines, and hence, the economy and feasibility of wind turbine projects. We find that the mean wind, wind shear and turbulence level are extremely sensitive to the exact details of the terrain: a small modification of the edge of our scale model, results in a reduction of the estimated annual energy production by at least 50% and an increase in the turbulence level by a factor of five in the worst-case scenario with the most unfavorable wind direction. Wind farm developers should be aware that near escarpments destructive flows can occur and their extent is uncertain thus warranting on-site field measurements.

Effect of Reynolds number and inflow parameters on mean and turbulent flow over complex topography

“…The resolution of the model and the ability to measure closer to the surface level thus appears to be one contributing factor. A separate study by Lange et al (2016b) recently submitted for publication investigates the effect of sharpening the escarpment edge. Preliminary analysis shows that this has a significant effect on the flow behaviour in the near-surface region.…”

Characterization of flow recirculation zones at the Perdigão site using multi-lidar measurements

“…Recirculation occurs behind abrupt drops in terrain 2714 R. Menke et al: Characterization of flow recirculation zones elevation such as cliffs but also in the lee of gentle slopes of a certain steepness (Wood, 1995;Xu and Yi, 2013;Kutter et al, 2017). In addition, small terrain features (Lange et al, 2017) and the presence of forest canopy (Sogachev et al, 2004;Finnigan and Belcher, 2004) can modify the occurrence of recirculation drastically. Furthermore, the stratification of the atmosphere influences the occurrence of recirculation; for instance, recirculation was prevalent during neutral and unstable atmospheric conditions during the observational study of Kutter et al (2017).…”

“…Here, we investigate measurements from the Perdigão 2017 campaign where, in addition to sonic anemometers on 100 m high masts, pulsed Doppler lidars observed the flow over the two parallel ridges. To surpass the capabilities of point measurements from networks of masts, Doppler lidars have, not only in the Bolund campaign, proved capable of measuring flow phenomena such as wind turbine and wind farm wakes (Käsler et al, 2010;Iungo et al, 2013;Aitken et al, 2014;Menke et al, 2018a) and the effect of roughness changes on wind fields in different heights and nearshore flows (Mann et al, 2017). In addition to the advancements in the measurement technology, the orography of the Perdigão site can be approximated by two two-dimensional ridges as the Askervein's orography with a three-dimensional hill of about half the height of the Perdigão ridges.…”

Neural Networks for Predicting the Output of wind flow Simulations Over Complex Topographies

“…Previous simplifications that ignored topography changes or reduced them to isolated features are often now not enough. For example, Lange et al [25] found minor changes in the terrain could have a significant effect on flow parameters for wind turbines and wind variables were sensitive to topography details.…”

“…Typical methods to do this may include scale wind tunnel testing, field experiments and numerical simulation [21], [25]- [27]. Each have associated issues such as scale, resources or limits to data being measured.…”