4.5 Prediction of the wind regime
4.5.1 Wind Atlas Analysis and Application Programme
The WAsP (Wind Atlas Analysis and Application Programme) was developed for the energy yield prediction of single wind turbines and wind farms [5].
With the years WAsP became a standard tool for the Micro-siting of wind farms, but its application has limits which are well known by now.
The software allows to analyze the wind regime at potential sites and to support the site selection. It is possible to transform the wind speed from one point A to an arbitrary point B, Fig. 4-43 [5]. For this purpose, in a first step all influences from orography, roughness and obstacles on the surface wind at point A are removed.
The resulting wind represents the geostrophic wind which is then assumed to be uniform in a large-scale area. In the second step, the local influences at the point B are applied which gives the local wind at this site. The energy yield is then deter- mined using the sectorised wind distributions and the power curve of the chosen wind turbine (cf. Fig.s 4-22 and 4-25).
The strongly simplified flow model applied in WAsP is based on the Navier- Stokes equations. The basic assumptions are:
- The atmospheric stratification is approximately neutrally stable.
- Thermal driven winds are neglected.
- The surface inclination is small, so there is no flow separation (inclination be low approx. 30%, i.e. 17°. But the critical inclination depends in reality e.g.
on the surface roughness and the atmospheric stability).
These basic assumptions allow the linearization and the solution of the Navier- Stokes equations. This model is relatively simple and requires quite small compu- tational resources.
The more complex the topography and climatology of a site, the higher are the uncertainties of this simplified calculation. Furthermore, the radius around point A in which this method is valid for application depends also on the quality of the input data. Own wind measurements close to the site are the favoured input data, so the radius for calculation results is a few hundred meters.
4.5 Prediction of the wind regime 162
Fig. 4-43 Principle of conversion of the wind statistics between different locations according to WAsP [5]
One of the biggest problems of WAsP is that the large-scale dynamic effects in mountainous terrain are not considered and implemented up to now. These effects are among others the dependency of the vertical wind speed profile on the free convection. The heating and cooling of the ground leads to different lift forces which influence the dynamics of turbulence, cf. section 4.1.3. If on one hand the ground is cooling down at night (stable stratification) turbulence decreases, and therefore the wind gradient increases. On the other, the heating of the ground dur- ing the day leads to an increased turbulence and mixing of the air masses (unstable stratification), so the wind speed gradient decreases. In order to take account for this effect the model in WAsP is based on a simplified input of the climatologic mean and the standard deviation. If the annual cycle at the site differs from the basic assumptions of WAsP there may be a false estimation of the wind profile.
One way to quantify the complexity of a site is the ruggedness index (RIX) which describes the percentage of the area around a site which exceeds a critical inclina- tion of e.g. 17°. Thus, the RIX serves for a rough estimation of the degree of flow separation and therefore to which extent the assumptions of the linearised model are violated.
50 40 30 20 10 0 -10 -20 -30 -40
Wind speederrorin %
-30 -20 -10 0 10 20 30 'RIX in %
50 40 30 20 10 0 -10 -20 -30 -40
Wind speederrorin %
-30 -20 -10 0 10 20 30 'RIX in %
Fig. 4-44 Prediction errors of WAsP versus difference between 'RIX of the investigated site and the reference site [6]
Comparing the RIX at the planning site (chosen wind turbine) with the RIX of the reference site (measuring mast), the relative difference ǻRIX may serve as an indicator for the prediction error of WAsP [6]. Fig. 4-44 illustrates the strong influence of flow separation on the prediction error: If the complexity of the terrain at the planning site and the reference site is quite similar (ǻRIX = 0), the prediction error is small. If the planning site is located in a more complex terrain than the reference site (ǻRIX > 0), WAsP tends to over-estimate the wind speed.
On the contrary, if the planning site is less complex than the reference site (ǻRIX < 0), WAsP tends to under-estimate the wind speed.
The advantages and disadvantages of WAsP are briefly summarized as follows:
Advantages Disadvantages x Simple operation x Valid only for a defined atmospheric
stratification close to neutrally stable atmosphere
x Cost effective and fast x Problems with turbulent flow separa- tion in complex terrain
x Validated method, ap- plication limits are known
x Problems with the wind speed calcu- lation outside the Prandtl layer (sur- face boundary layer)
4.5 Prediction of the wind regime 164