Wind resource and site assessment - Wind Power Generation and Wind Turbine Design - page 71

Figure 13: Reduction of wind speed in percent due to shelter by a two-dimensional
obstacle [ 17 ] .
and using separate physics, the wake interaction between the wind turbines. The
use of such tools allows the energy production of different layouts, turbine types
and hub heights to be rapidly established once the model has been set up. Site fl ow
calculations are commonly undertaken using the WAsP model, which has been
widely used within the industry over the past decades.
However in the last few years use of CFD codes is increasing, although CFD
tools are typically used in addition to and not instead of more simple tools, to
investigate specifi c fl ow phenomena at more complex sites. CFD tools must be
used with care, as the results are quite sensitive to modelling assumptions and to
the skill of the user of the code. Typical use of CFD tools is, fi rstly, to give another
estimate of the local acceleration effects at the sites, and secondly, to identify hot
spots, in other words areas where the wind conditions are particularly diffi cult for
wind turbines. In particular, such tools are starting to be used to assist in the micro-
siting of wind turbines on more complex sites [1]. Often it is the only means by
which turbulence and shear across the site can be estimated.
4.3.2 WAsP
The challenge is to take a topographical map and the long-term corrected wind
climate at a known point and use this information to calculate the long-term wind
speed at all points on the map. WAsP is accomplishing this task using a double
vertical and horizontal extrapolation. The idea behind this is quite simple. The
local measurements on site are cleaned from local effects like obstacles, roughness
and orography to calculate the geostrophic wind climate. Having determined the