Wind in complex terrain. A comparison of WAsP and two ... - WindSim

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Figure 10: Outer domain (red) and inner domain(blue) of the meso-scale simulation.The present analysis shows that, despite thecomplex terrain, WAsP compares better than theCFD-models to the observations when results for thesectors south and north are considered. Thisconclusion is valid both for the vertical wind profilesand the annual average wind speed levels. The meanabsolute error for sector south is 11%, 10% and 28%for WAsP, WindSim and 3DWind respectively. Forsector north the corresponding numbers are 14%,24% and 24%. The RMS-differences in the windspeeds between the two CFD-models have beencalculated for an area where wind energy exploitationis realistic. For both sector south and north anaverage RMS-difference of about 1.5 m/s is found.Meso-scale simulations with a completemeteorological model indicate large wind variationswithin the island due to the mountains to the south.For one particular case with southerly flow, windspeed variations of 2-3 m/s at 100 m height areencountered moving from the western to the easternside of the island. A dependence of the vertical windprofile on vertical stability is also found in the mesoscalemodel simulations. One disadvantage of WAsPand the two CFD-models is that meso-scale windvariations are not taken into account in the modeling.In particular, the effects of the vertical stability onboth the meso-scale and the micro-scale flow may beanticipated to be important (see for example [8]), butit is not accounted for in the micro-scale models. Thisbecomes a limitation of the three models whenapplied to a complex terrain site such as Gurskøy.An advantage of the CFD-models compared toWAsP, is the explicit calculation of the turbulencefield. The comparison of the measured and modeledturbulence intensities shows that 3DWindoverestimates the turbulence while WindSim tends togive too low values. The directional classificationuses 12 sectors of 30°. The model runs show thatlarge variation in the turbulence level may occurwithin a 30° sector when the terrain is complex. Thussmaller sectors should be employed, although thisincreases the computational efforts of the CFDmodels.Through the analysis of the modeledturbulence fields a better understanding of themeasurements at Gurskøy is obtained. For example,the 3DWind runs clearly indicate an influence of thefirst mountain to the south on the turbulence level(and consequently also the average wind speed level)at the measuring sites. This mountain is of the sameheight as sites 2 and 3 and located about 3 km to thesouth.The results of this study emphasize theimportance of high quality measurements in complexterrain for a reliable wind resource mapping. Still,both micro- and meso-scale models are important forthe interpretation of the measurements and for aninvestigation of the turbulence and speed-up effects incomplex terrain. The data material of this report islimited, and therefore further validations of themicro- and meso-scale models in complex terrain arerecommended. This is anticipated to give increasedinsight into the potential and limitations of themodels for wind farm development.
Figure 11: Wind speed at 50 m height at 03 UTC 01.01.2005. WRF-run with 1 km horizontal resolution. Heightcontours (black lines) are drawn every 30 m. The vertical profiles are from the positions 1 (SW) and 2 (SE).Figure 12:Vertical profiles of wind speed and potential temperature at the SW corner (red lines) and SE corner(blue lines of the island Gurskøy. Height represent the heights above sea level.References[1]. WAsP Manual: Wind Analysis and ApplicationProgram (WAsP). Vol 2: Users Guide. RisøNational Laboratory, Roskilde, Denmark, ISBN87-550-178, 1993.[2]. Bowen, A. J. and Mortensen, Niels G. Exploringthe limits of WAsP the wind atlas analysis andapplication program. In Proceedings of EWEC-1996, Gøteborg, Sweden, 1996.[3]. Berge, E., Nyhammer, F.K., Tallhaug, L. andJakobsen, Ø. An evaluation of the WAsP modellat a coastal mountainous site in Norway, 2006.
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Wind in complex terrain. A comparison of WAsP and two ... - WindSim

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