Draxl, Caroline

Mayr, Georg J.

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Downloaded from orbit.dtu.dk on: Dec 17, 2017Meteorological wind energy potential of the Alps using ERA40 and wind measurementsof the Tyrolean AlpsDraxl, Caroline; Mayr, Georg J.Published in:EWEC 2009 Proceedings onlinePublication date:2009Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Draxl, C., & Mayr, G. J. (2009). Meteorological wind energy potential of the Alps using ERA40 and windmeasurements of the Tyrolean Alps. In EWEC 2009 Proceedings online EWEC.Meteorological Wind Energy Potential of the Alps Using ERA40 and Wind Measurements of the Tyrolean AlpsCaroline Draxl1,2                  Georg J. Mayr11University of Innsbruck, Department of Meteorology and Geophysics, Innrain 52, A-6030 Innsbruck, Austria2 Risø DTU¸ National Laboratory for Sustainable Energy, Technical University of Denmark, Frederiksborgvej 399, DK-4000 Roskilde, DenmarkCaroline.Draxl@risoe.dk        Georg.Mayr@uibk.ac.atAbstract:Wind  measurement  sites  in  the  Tyrolean  Alps were  evaluated  concerning  their  wind  energy potential. We used global reanalysis ERA40 data and data  from  wind measurement  stations  and analysed them statistically.Due to the decrease of density with height, alpine sites  suffer  from  a  nearly  linear  decrease  of harvestable  power  with  altitude,  which  is  more than  offset  by  the  increase  of  wind  speed  at altitudes above 1.5 km MSL. ERA40 data showed higher  potential  on  the  northern  than  on  the southern side of the Alps. Best locations are not isolated  peaks  but  on  ridges  within  wide orographic channels through which the frequently different  air  masses  on both  sides  of  the  Alps equilibrate. The best potential sites have median wind  speeds  of  up  to  7.1  m/s  and  extractable potentials between 2900 and 1600 kWh per year and square  meter  of  rotor  area.  The  profile  of horizontal wind speed at ridge sites is often not logarithmic but approximately constant within the height of a wind turbine due to an often (nearly) complete  absence  of  upwind  fetch.  Icing  can cause considerable downtimes.Keywords:  wind energy, complex terrain,  Alps, icing,  quality  control,  gap  flows,  wind measurement sites1 IntroductionAs wind energy usage increases,  wind turbines are moving into more challenging locations. Much of the knowledge about wind power meteorology cannot simply be transferred to the alpine setting. Therefore, within the scope of the EU-Interreg IIIb project  Alpine  Windharvest  (2001  to  2006)  we evaluated  wind  measurement  sites  in  the Tyrolean  Alps  from  a  meteorological  point  of view. 2 Meteorological  data  and their processingFour data sources were used: firstly, 45 years of data  from  the  global  reanalysis  ERA40  from ECMWF [1] to elucidate large scale differences between the Alps and similar altitudes above the lowlands and between the northern and southern side  of  the  Alps.  The  second  data  source constitutes several years of measurements from routine networks in the Tyrolean Alps to identify topographic features that boost wind energy yield. The third data are spatially dense but only short-term (70 days)  measurements  in the vicinity of the meteorologically most promising wind energy site in the Tyrolean Alps. They were used to study the  influence  of  small-scale  topographical variations,  of  valleys,  and  of  using  short measurement periods for evaluating wind energy potential.  The  fourth  data  source  was  Doppler sodar measurements at the most promising site to highlight  differences between vertical  profiles of horizontal wind over flat terrain and mountain ridges/gaps.  All  the  data  were  analysed statistically. The  wind  measurements  came  from  automatic weather  stations,  which  are  often  located  in exposed  mountainous  locations.  Quality  control was  performed  to  ensure  that  the  10-minute average  data  were  in  physical  range  and  that periods  with  ice-covered  anemometers  were eliminated.2.1 Wind  situation  and  meteoro-logical aspects in the Alps Alpine wind energy sites lie in about 1500 m and 3500 m MSL. This leads to an approximately 15- 30% decrease of available wind power because of the decrease of air density with height (Fig. 1). However,  higher  speeds  that  occur  at  higher altitudes  due  to  the  poleward  temperature decrease  lead  to  a  resulting  increase  of extractable power above 1.5 km MSL. Since the strongest horizontal gradient of temperature lies to the north of  the Alps most  of  the time, wind speeds  there  are  considerably  higher. Consequently,   extractable  power  above  the northern  rim  of  the  Alps  is  much  higher  than above the southern rim. (Fig. 2)The Alps are not a homogeneous ridge but have many indentations through which the air can flow. These indentations can form more or less regular and wide channels. In general, air flowing through a narrowing of the terrain, a gap, accelerates so that  wind  speeds  may  become  substantially higher than the oncoming wind speeds upstream of the Alps [2].Since the Alps lie approximately perpendicular to the  average  north-south  temperature  gradient they frequently separate air  masses of  different temperatures on the northern and southern rims. This  temperature  difference  causes  a  pressure difference, which in turn accelerates the air. Since often the temperature difference between the two sides of the Alps is confined to below the alpine crest,  the  air  masses  can  only accelerate  and equilibrate through the gaps in the alpine crest. Frequent strong winds should therefore occur in these gaps [2]. Above these gaps, wind speeds on  isolated  mountain  peaks  are  reminiscent  of the free atmosphere above the lowlands.Since  the strong westerlies  and the polar  front zone move northwards in summer, winds in the colder seasons can reach values of about twice the strength of winds in summer.2.2  Computation  of  wind  energy potential parametersFrequently wind energy potential is stated using the  total wind  power  (i.e.  the  advection  of  the kinetic  energy).  However,  as  Betz  (1922-1925) showed  the  maximum  amount  is  about  40\% lower. 35.0 vACBP ⋅⋅⋅⋅= ρ     (Equ. 1)Equation 1: v is the wind speed upstream of the turbine, A is the cross-sectional area of the turbine's propeller, and ρ  is the density of air.We therefore consider it appropriate to only show the maximum  extractable  power,  i.e.  the upper limit.  Additionally  we  give  numbers  for  an exemplary turbine (Fig. 3) as could be installed in mountainous  environments.  As  turbine  design keeps improving, this will give a lower limit to the yield that can be expected in the future.The power computation requires measurements of wind speed and density. Density ρ itself is not measured  directly  but  computed  from temperature T [K] and pressure  measurements p [Pa] via the equation of state: (Equ. 2)where R is the specific gas constant for dry air (1004 J  (kg  K)-1).  For  the stations  that  did  not TRp⋅=ρFigure 1: Decrease of air density relative to the 1000 hPa level (approx. 150 m MSL) with altitude. The density decrease is directly proportional to a decrease of wind power. Vertical profiles of density are from ERA40 averaged over 45 years for grid points north (thin line, 48.375N, 10.125N, close to Ulm Germany) and south (bold: 45N, 10.125E, close to Parma, Italy) of the Alps, respectively.Figure 2: Vertical profile of average extractable power calculated from wind speeds over 45 years from the ECMWF reanalyses at two locations along the northern rim of the Alps (bold: 48.375 N, 10.125 E, close to Ulm) and one location along the southern rim of the Alps (thin: 45 N, 10.125 E, close to Parma). Locations marked with circles in Fig. 5measure  pressure  it  was  calculated  from neighbouring stations.         3 Results3.1 Potential  at  long-term stations in the Tyrol58 stations  were evaluated in  the Tyrol  region. Out  of  these,  21 had a  median wind  speed of below 2 m s-1, which is too low to be viable for wind  energy  use.  These  are  either  stations  in valleys  or  shielded  by  the  sourrounding topography. Valley floors in that part of the Alps are at  the most  a few kilometers wide and the valleys are often cut deeply into the surrounding topography. Of the remaining stations, results for the four with the highest extractable power and the best valley location, respectively, are shown in Figure  6 in  the Appendix.  Common to the four highest-potential stations is that they all are in an orographic channel and/or a gap.An indication of the difficulty of measuring winds at  exposed  locations  is  the  data  availability between only  41% and  99%.  Reasons  for  low availability are manifold, e.g. icing, sensor failure, logger  failure,  prolonged interruption of  wireless (usually GSM networks) data transmission.3.2  Effects  of  local  topography  and short-term measurementsThe Wipp Valley (black rectangle in Fig. 5) is well known for gap flows and related foehn events and thus  promises  to  include  potential  wind  energy sites. Since the valley was densely instrumented during a 70day period, small-scale topographical effects  on  wind  power  potential  could  be examined.Since  the  Wipp  Valley  lies  downstream  of  a narrow  pass  (Brenner  Pass)  that  runs perpendicular to the main alpine crest, downslope windstorms  (Foehn)  exist  along  both  valley directions north and south [2]. The valley consists of two gaps: the narrow Brenner corrugation and an upper wide gap at approximately 2100 m MSL with a width of roughly 20 km.  The main alpine crest continues on both sides of the pass with an average  elevation  of  3000 m  MSL.  The  cross-sectional area of the lower Brenner gap is 1/15 of the  upper  one.  Most  of  the  air  reaching  the downstream  (northern)  side  of  the  Brenner  will therefore have to come through the upper gap. The topography of the valley has influence on its wind power potential.  The highest  potential can be  found  in  the  upper  gap  where  the  stations benefit  from  the  gap  flow,  which  attempts  to equilibrate  between  the  frequently  different  air masses  on  both  sides  of  the  Alps.  However, stations  situated  at  the  narrow  bottom  of  the valley as well as stations above the alpine crest do not show high wind energy potentials. At the valley floor,  additional turbulent friction from the side  walls  lowers  the  wind  speeds.  Summits situated above the alpine crest experience winds from  the  free  atmoshpere,  though,  but  lack accelerating gap flow.3.3  Vertical  wind  profile  at  a ridge locationThe lack of upwind fetch for sites on ridges and peaks invalidates the assumption of a logarithmic like  wind  profile  as  over  homogenous  terrain. Doppler  sodar  measurements  at  Sattelberg  in combination with a weather station showed that highest wind speeds were in the lowest 3-50 m. Above  that  to  the  maximum  range  of  150  m speeds  decreased.  The  site  lacks  an  upwind fetch in its main wind directions. Further Doppler sodar measurements within the EU project Alpine Windharvest in  locations  outside  of  the  Tyrol confirmed that wind speeds in the height range relevant  for  wind  turbines  do  not  increase  with height when there is no fetch from that direction.Figure 3: Power curve of an exemplary wind turbine used to compute the wind energy that could have been harvested if the turbine had been situated on a wind energy site during the given wind conditions.4 Discussion4.1  Comparing these results to sites already used for wind energyThe best potential sites from the semi-operational network have median wind speeds of up to 9.4 m/s  and extractable  potentials  between 2900 and 1600 kWh/(am²). This translates to between 2200 and 3500 full load hours. An  already  installed  alpine  wind  park  in  the eastern Alps, the Tauerwindpark, produces less. Eleven turbines harvest a combined 45.5 GWh/a (http://www.tauernwind.com),  i.e.  approximately 4.1  GWh/a,  which  can  be  normalized  with  the area swept by the turbine blades to 1198.4 kWh (am)-1.  One  wind  turbine  in  Petronell-Carnuntum  in Lower Austria produced 1223 kWh/(am)-1 . This is  the  highest  value  ever  achieved by a  single wind  turbine  in  Austria.  The  computed corresponding  value  of  the  Sattelberg  is  more than  twice  as  high.4.2 IcingA  dentrimental  effect  of  the  high  altitude  of possible alpine wind parks are low temperatures and  an  accompanying  icing  problem.  The investigations  showed  that  some  stations  had gaps in the measurement data caused by icing which  reached  up  to  almost  50%  at  some locations.  Icing  of  turbine  blades  and  icing  of anemometers  need  not  be  concomittant,  even though they are closely related. To maximize the yield  of  alpine  wind  parks,  special  technical solutions to prevent icing might be necessary.5 ConclusionThe wind energy potential evaluated from routine weather station locations in the Tyrol showed that the best locations would outperform flatland sites in  eastern  Austria  and even offshore  locations. The area, on which turbines could be erected, is much  more  limited.  Depending  on  region  and altitude, icing could reduce annual yield and call for  special  technical  counter-measures.  The highest yields were not found at isolated peaks (which would have extremely limited or no space for turbines anyways) but in ridges and peaks in wide orographic channels, i.e. not on the valley floor along the deepest incisions.References[1]. Uppala, S. M., Kållberg, P. W., Simmons, A. J., Andrae, U., Bechtold, V. D. C., Fiorino, M., Gibson,  J.  K.,  Haseler,  J.,  Hernandez,  A., Kelly, G. A.,  Li,  X.,  Onogi,  K., Saarinen, S., Sokka, N., Allan, R. P., Andersson, E., Arpe, K.,  Balmaseda,  M.  A.,  Beljaars,  A.  C.  M., Berg, L. V. D., Bidlot, J., Bormann, N., Caires, S., Chevallier, F., Dethof, A., Dragosavac, M., Fisher,  M.,  Fuentes,  M.,  Hagemann,  S., Hólm,  E.,  Hoskins,  B.  J.,  Isaksen,  L., Janssen, P. A. E. M., Jenne, R., Mcnally, A. P., Mahfouf, J.-F., Morcrette, J.-J., Rayner, N. A.,  Saunders,  R.W.,  Simon,  P.,  Sterl,  A., Trenberth,  K.  E.,  Untch,  A.,  Vasiljevic,  D., Viterbo, P.,  Woollen, J.,  2005.  The ERA-40 reanalysis.  Quarterly  Journal  of  the  Royal Meteorological  Society  131  (612),  2961–3012.[2]. Mayr,  G.  J.,  Armi,  L.,  Gohm,  A.,  Zängl,  G., Durran,  D.  R.,  Flamant,  C.,Gabersek,  S., Mobbs, S., Ross, A., Weissmann, M., 2007. Gap  flows:  Results  from  the  Mesoscale  Alpine Programme.  Quarterly Journal  of  the Royal Meteorological Society 133 (625), 881–896.[3]. Mayr,  G.  J.,  Armi,  L.,  2008.  Föhn  as  a response  to  changing  upstream  and downstream air masses. Quarterly Journal of the Royal Meteorlogical  Society  134,  1357–1369.[4].Peterson, E. W., Hennessey, J. P., Mar. 1978. On the use of  power laws for  estimates  of  wind  power  potential. Journal  of  Applied Meteorology 17 (3), 390–394.[5].  Draxl,  C:  Wind  Energy  Potential  at  Wind Measurement  Sites in  the Tyrolean Alps.  A diploma thesis submitted to the Department of Meteorology and Geophysics, University of Innsbruck, Aug. 2004Figure 4: Vertical profile of horizontal wind speed at Sattelberg (SB in Fig. 5) from Doppler sodar measurements and automatic weather station (3 m AGL) 14 September - 27 October 2004. Overlaid is a logarithmic wind profile computed from power-law [4] with exponent of 1/7  starting from the median value of wind speed at 3 m AGL. Data availability (right) decreases with height.Appendix Station MSL Sensor Median Weibull In range Icing Median >=160 Extr. Pot. Output Data[m] [m] AGL [m/s] A [m/s] k [%] [%] [W/m2] [%] [kWh/ (am2)] [GWh/a] [a]SB 2108 3 7.1 8.8 1.62 79.7 3.5 *) 104 41.7 2927 5.23 4.59SJ 2165 18 7.4 8.9 1.77 82.4 7.7 *) 117 42.6 2835 5.23 1.87PA 2247 7 3.9 6.2 1.16 54.4 7.5 18 22.1 2673 3.01 9.85ES 2926 4 6.2 7.6 1.65 75.9 29.4 67 31.7 1794 4.08 5.55SS 3325 4 4.8 6.6 1.34 64.1 51.2 29 23.8 1612 3.32 7.18Table 1: Station name (as in Fig. 5), altitude, height of the wind sensor above ground, median of the wind speed, Weibull parameters, the percentage of wind speed data that are within the operating range of common wind turbines between 3.5 and 25 m/s, the percentage of icing (relative to the missing data) of the potential wind energy sites, where *) indicates that the icing relates to the prequality controlled data set with higher resolution data and refers to the whole time series of available data, Extractable power (median), percentage of extractable power above 160 W/m2, extractable wind energy potential and GWh/a with an exemplary wind turbine and the period of the available data. For the characteristics of the rest of the evaluated sites see [5].Figure 5: Topographical map (shading interval 500 m) of all the evaluated stations. The best potential wind energy sites are indicated with the labels SB = Sattelberg, SJ = Sandjoch, PA = Patscherkofel, ES = Elferspitze, SS = Schöntaufspitze. Names of valleys in light font. The black rectangle marks the region of the Wipp Valley (see chapter 3.2) with denser instrumentation for a period of 70 days. Inset shows the location of the map within central Europe.Figure 6: Wind statistics of the 4 stations of the Tyrolean routine networks with the highest wind energy  potential: Sattelberg, Patscherkofel, Elferspitze and Schöntaufspitze. (Left) Histogram and kernel-density fitted probability density function (solid) with fitted parametric Weibull distribution (dashed; A and k) and median value (m/s); (center) power curves indicating the extractable power duration curve (dashed, W/m2 and the machine productivity curve (solid, kW); (right) wind rose with measurement period(years) and data  availability (%). NB: different frequency scale (%; italics) among the stations.

Meteorological wind energy potential of the Alps using ERA40 and wind measurements of the Tyrolean Alps

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Meteorological wind energy potential of the Alps using ERA40 and wind measurements of the Tyrolean Alps

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