Page 141 - Fluid Power Engineering

118 Chapter Seven

Description of WAsP Model
The main objective of the WAsP model or any wind assessment model
is to use measured wind data (or reanalysis data) and create wind
resourceestimatesfordesiredareasandfordesiredheights.Inessence,
the models extrapolate (or interpolate) wind data from sites where it
is available to sites with no wind data. In addition to the extrapola-
tion along the horizontal plane, wind speed data is extrapolated along
the vertical direction at multiple heights. This is the process of spatial
extrapolation, along all three spatial axes. In sequel, sites with wind
data are referred to as site A and sites where wind data is desired
are referred to as site B. Site A may be a collection of measurement
locations or a single measurement location with wind measurement
instruments. Site B may be an area that is being prospected or a collec-
tion of proposed wind turbine locations. If site B is an area, then a grid
has to be defined on which wind resources are estimated. For exam-
ple, site B may be a 20 km × 20 km area with a 200 m × 200 m grid and
wind resources are computed at all grid points. When multiple sites of
type A are involved, then a weighted average of these sites is normally
used to estimate wind speed at site B. One option is to use weights
that are based on inverse distance between site B and multiple site As.
In Fig. 7-2, all the computational activities on the left-hand side of
the diagram occur at site A, while all the activities on the right-hand
side happen at site B. WAsP’s approach is to sequentially remove
the affects of obstacles, roughness, and elevation in order to derive
a regional wind climate (RWC). The up arrow in Fig. 7-2 indicates
this. The RWC is then translated to site B. The RWC is made location
specific by applying elevation, roughness, and obstacles in sequence,
as indicated by the down arrow in Fig. 7-2.
WAsP model takes as input:

Measured wind data at site A. Typical sources of datasets are on-
site measurement data from met-towers with instruments at
multiple heights, airport data at 10 m, or any other source of
onsite wind data.
Position and dimensions of obstacles near site A and near the as-
sessment site, site B. Obstacles are manmade structures like
buildings, towers, row of trees planted as windbreakers, or
naturally occurring structures like isolated trees and tall rock
formations. The obstacles of relevance are structures taller
1
than / 4 the hub height. In most cases, an area of radius 1 km
is sufficient for modeling obstacles. Obstacles outside this area
will not have a significant impact on the wind conditions.
Most wind resource applications provide methods to specify
dimension and orientation of obstacles.
Terrain roughness at sites A and B. Roughness is a method to pa-
rameterize different types of terrains. An example of rough-
ness classifications is found in Table 2-5. Most applications