The Alaiz Experiment (ALEX17): revealing mountain-valley large scale flow patterns
ALEX17 aims to provide a vast range of atmospheric measurements within a mountain-valley area upstream from CENER’s wind turbine test site
ALEX17 is the acronym for ALaiz EXperiment 2017, the last full-scale experiment within the NEWA (New European Wind Atlas) project, whose primary objective is to create a wind atlas of Europe that includes the state-of-the-art in modelling the wind resource, as well as the creation of a comprehensive database. ALEX17 aims to present a utility-scale measurement campaign to characterize the wind flow in complex terrain, through a combination of measurement technologies. The measurement campaign data set will be able to validate the reliability of numerical simulation models of wind flow in complex terrain, in order to reduce uncertainties when evaluating the wind resource.
The unique characteristic of this measurement campaign is the instrumentation of a site with large dimensions and elevation changes, where a strong coupling between mesoscale phenomena and microscale site effects are expected.
Research Team & Data Accessibility
ALEX17 is a joint effort between research groups of Technical University of Denmark (DTU), National Renewable Energy Centre of Spain (CENER) and University of Balearic Islands (UIB). The experiment was conducted as part of the EU ERANET+ NEWA (New European Wind Atlas) project.
The experimental data set is open access and can be accessed with with metadata and citation guidelines at https://doi.org/10.11583/DTU.c.4508597.v1. A direct download link will be available soon.
Site description and Instrumentation
The experimental domain is focused around the Alaiz mountain range with a height of approximately 1100 m a.s.l, where the largest wind power density is located (Figure 1). The prevailing wind directions at the mountain top are aligned to NNW and SSE. Most of the instruments were installed inside the Elorz valley to the north, which is 10km wide and 6km long, with the valley floor at 500 m a.s.l. and bounded by the North Ridge (Tajonar) with it’s peak at 850 m a.s.l.
The dimensions, on the order of 20x20 km, and elevation differences of up to 600 m make this site a challenge both for numerical models and remote sensing instruments. Five Long-Range WindScanners (WS1,…,WS5 on the map, Figure 2) where commissioned and programmed to synchronously measure a symbolic 10 km Z-shaped transect, represented by the red and blue lines. Along the transect lines line-of-sight measurements can be combined to reconstruct a 2D (red lines) or 3D (blue line) wind vector.
The multi scanning lidar scans also give the opportunity to visualize the wind flow and characterize relavant large scale flow patterns.
Complementary measurements of wind profiles are present with six 80 m met masts distributed along the valley, having one 118 m met mast (MP5) on top of the mountain as a reference. The MP5 along with four 80 m masts (orange dots) are instrumented with 3D sonic anemometers, hence are able to measure turbulent fluxes.
To complete the instrumentation, a commercial doppler wind lidar profiler (WLS70) was placed near the city of Pamplona, in the boundary of the experimental domain, measuring wind profiles up to 2000 m a.g.l. At the center of the domain, next to WS2 and M7, a RASS-sodar profiler and an energy budget station are also present for micrometeorological studies. ALEX17 also has 9 surface layer stations with 2D sonics and temperature measurements distributed along the valley and slopes, which can provide a measure of spatial variability of momentum and heat fluxes.
The experimental campaign & data report provides more details of each instrument along with data availability.
Experiment Timeline
The measurement campaign consists of an extensive observational period with two years of data from the 118m MP5 met mast from July/2017 to July/2019. Inside this timeline all other instruments were installed.
Within this period, met masts distributed along the valley measured for 11 months, whereas the WindScanner systems operated with synchronous measurements for 9 months, which is considered a long-term measurement period for this type of equipment. The data availability of each instrument can be seen in detail in the experimental campaign & data report.
The Intensive Observational Period (IOP) is considered when all instruments were measuring concurrently for a 4-months period between August/2018 to December/2018.
Wind turbine test site
The experimental campaign will also be useful to better characterize the inflow on the wind turbine test site located on top of the Alaiz mountain range (Figure 3). The elevation map below shows the test stands along with four 118m met masts along with the southern part of the Z-shaped transect.
The mountain range is also populated by commercial wind farms (black dots), showing the data set potential for better characterization of wind turbine siting parameters in complex terrain. Wind turbine data isn’t part of the ALEX17 data set, however the remaining met masts on site can be accessed in collaboration with CENER.
With such a measurement layout it is possible for siting engineers to test their models for resource assessment or numerical site calibration.
Flow patterns
The multi lidar measurements were able to quantify common atmospheric flow patterns in the site and expose the interaction between the Alaiz mountain with the north ridge 6km away. Figure 4 shows snapshots of radial wind speed plots measured by the lidars.
For southerly winds, the lidar measurements were able to capture the wind turbine wake deflected by downstream valley winds. Contrastingly when northerly winds are present, the measurements show the impact of the north ridge and valley flow at the mountain top, where gravity waves can be formed on the lee side of the ridge and propagate with large wavelengths up to the mountain.
The multi scanning lidar measurements are able to expose common flow patterns affecting siting parameters and evidence the influence between the two ridges.
Preliminary results will be used for the selection of flow cases for upcoming benchmarks. The data set is also open for further studies on wind resource in complex terrain and other micrometeorology topics.
References
Jakob Mann. (2019, April). Overview of the New European Wind Atlas Project. Zenodo. http://doi.org/10.5281/zenodo.2684818
Santos, Pedro; Mann, Jakob; Vasiljevic, Nikola; Courtney, Michael; Sanz Rodrigo, Javier; Cantero, Elena; et al. (2019): The Alaiz Experiment (ALEX17): wind field and turbulent fluxes in a large-scale and complex topography with synoptic forcing. figshare. Collection. https://doi.org/10.11583/DTU.c.4508597.v1
Santos, Pedro. (2019, April). The Alaiz Experiment (ALEX17): visualizing the flow in complex terrain with multi scanning lidars (Version v1). Zenodo. http://doi.org/10.5281/zenodo.3108565
Santos, Pedro, Borbón, Fernando, Mann, Jakob, Cantero, Elena, Vasiljević, Nikola, Sanz Rodrigo, Javier, … Cuxart, Joan. (2019, June). Multi scanning lidar measurements for resource assessment: a case study in complex terrain. Zenodo. http://doi.org/10.5281/zenodo.3358598
Vasiljević, N.; Lea, G.; Courtney, M.; Cariou, J.-P.; Mann, J.; Mikkelsen, T. Long-Range WindScanner System. Remote Sens. 2016, 8, 896. https://doi.org/10.3390/rs8110896
Acknowledgements
This measurement campaign was performed with support of the Danish Energy Agency and Ministry of Science, Innovation and Universities of Spain through the project NEWA (FP7-ENERGY.2013.10.1.2, European Commission’s grant agreement number 618122).
