The Alaiz mountain range is located in Navarre (Spain), around 15 km SSE from Pamplona. The prevailing wind directions are from the North and from the South. To the North a large valley is found at around 700 m lower altitude. To the South, complex terrain is found with the presence of some wind farms, the closest one situated 2 km behind the row of six wind turbine stands of the test site. Five reference met masts, 118 m tall, are located in front of the turbine positions at a distance of around 250 m.
The site is characterized by two roughness levels. The western part of the test site, limiting with the MP3, position is covered with a dense canopy composed of bushes and beech trees 10-15 m high. The eastern part is covered by low bushes not higher than 0.5 m. A uniform roughness length of 0.4m and 0.05m can be assumed for the two roughness levels.
The test site has been operational since end of 2009 with the first wind turbines installed in the summer of 2011.
Figure 1: Topography of the Alaiz site and layout of the test site.
Five 118-m tall met masts are operational on the site to monitor the wind conditions upwind from the turbine sites. The standard configuration of each mast includes cup anemometers and wind vanes at [78, 90, 102, 118] m and temperature/humidity measurements at [81, 97, 113] m. Replicated cup anemometers are situated 2 m below the reference ones. The MP5 position, reference met mast of the site, is equipped with an extra cup anemometer at 40 m, two vertical propellers at [78, 118] m, three sonic anemometers at [40, 80, 120] m (installed since November 2012) and two temperature/humidity sensors at [2, 38] m (installed since January 2011).
Alaiz instrumentation set ups are compliant with IEC 61400-12-1 with MEASNET cup anemometer calibration.
A site calibration phase was initially performed to obtain the speed-up factors between the reference masts and the turbine positions. Two campaigns were conducted: A4-A5-A6 vs MP0-MP5-MP6 (6 months) and A1-A2-A3 vs MP1-MP3-MP5 (4 months). After the site calibration, the met masts were installed in their operational positions. In the summer of 2011 the first wind turbine prototypes were installed.
Figure 2: Availability of wind data during the first two years of operation of the Alaiz Test Site.The duration of the site calibration campaigns (SC1 and SC2) and the start-up of wind turbine prototypes A3-A5 (WT) are indicated
The Alaiz test site is situated in the centre of the Navarre region, whose topography is characterized by the Pyrenees mountain range to the North and the Ebro valley to the South. These large topographic features combined by synoptic activity of opposite sign in the Cantabric and Mediterranean Seas are responsible of a characteristic channeled wind along the Ebro valley which is calledCierzo. This distinct wind climate regime produces a significant wind power share of the total installed capacity in Spain.
The flow field in Alaiz observed at MP5 is highly dependent on the incoming atmospheric stability. The Froude number, based on the mast velocity U0 at 118 m, the Brunt-Väisälä frequency obtained from the temperature gradient in the 2-38 m layer and an obstacle length of WT= 1000 m, is used to classify the wind conditions at the site (Figure 3; Sanz Rodrigo et al., 2013). The characteristic length is chosen such that, for northerly winds, a resonance peak in the wind shear is observed at Fr-1 ~ 1, separating the very stable regime, where the flow rather goes around the mountain than above it (Fr-1 >> 1), and the neutral regime (Fr-1 ~ 0), where the wake behind the mountain ridge dominates the flow field (Stull, 1988).
Figure 3: Distribution of wind shear, turbulence intensity and vertical flow tilt with atmospheric stability for northenly (left) and southernly (right) winds (Sanz Rodrigo et al., 2012).
Northerly winds are characterized by the important speed-ups generated by the sloping terrain, which results in rather low turbulence intensities of the order of 8%. The wind shear in the rotor area 40-118 m is normally very low and often negative (power-law exponent α). This is due to the presence of a velocity maximum at lower heights closer to the ground. At Fr-1 ~ 1 the mean wind shear reaches a maximum although the variability is very high ranging from negative to very extreme positive shear exponents. The ratio of the vertical and the horizontal velocity, obtained from vertical propeller and cup anemometer measurements, represents the tilt angle of the flow which is around 0.1 at 80 m and rather uniform for a wide range of stabilities. The turbulence intensity is also quite uniform although it increases rapidly in unstable conditions beyond Fr-1< -0.5.
Southerly winds are characterized by the presence of roughed terrain and a wider range of stabilities. Before the summer of 2011, there were no wind turbines in the test site and the wakes from the neighboring wind farm, situated ~40 rotor diameters upstream, can be considered well mixed with the boundary layer flow. A monotonic increase of turbulence intensity is observed from very low values (~6%) in the very stable regime to very high values in the very unstable regime (~25%). In unstable conditions, flow separation occurs, manifested by decreasing values of the tilt angle that eventually change sign and produce a maximum in the wind shear.
Sanz Rodrigo J., Borbón Guillén F., Gómez P., Courtney M. S., Wagner R., Dupont E., 2013, Multi-Site Testing and Evaluation of Remote Sensing Instruments for Wind Energy Applications. Renewable Energy 53: 200-210
Stull R.B., 1988, An Introduction to Boundary Layer Meteorology, Kluwer Academic Publishers, 666 pages