Page 54 - Energize January 2022

TECHNICAL

Such a system would require a matrix of purpose-designed wind Obstacles (the shelter effect)
recording stations, accurate terrain maps, and the computing power The presence of obstacles can affect wind to a height of three times
to give a reasonable interpretation of measured data at any site in the that of the obstacle, and to a distance of 30 to 40 times the obstacle
country. In addition, ongoing updating of data over the long-term scale height in the downwind direction.
is required. High future renewable energy penetration requires an
understanding of potential impact of climate change on future energy WASA: A numerical wind atlas for South Africa
generation. Substantial changes in the near-surface atmospheric flow A wind atlas is a generalized set of wind climate information at a
and storm climates in a GHG-warmed world can affect wind energy given location and is much more than a simple map containing mean
generation/spatial manifestations. Accurate forecasting is required, wind speed.
and the accuracy of the forecasting increases as the measurement The WASA project was undertaken by a consortium consisting
period increases. 3 of SANEDI, the South African Weather Services (SAWS), the Council
A number of systems that used satellite data and global wind for Scientific and Industrial Research (CSIR), the University of Cape
maps were available, but none related to actual ground-based Town (UCT) and DTU Wind Energy (the Danish Research Institute – a
measurements. To meet these requirements the WASA (Wind Atlas of recognised world leader in wind energy) with SANEDI as a coordinator.
South Africa) project was initiated. The project was conducted under The numerical wind atlas was constructed by establishing a
the auspices of SANEDI, with co-operation of DTU, CSIR, UTC, SU and generalised regional wind climatology and then extrapolating down to
other organisations. estimate the climatology of a specific location as shown in Figure 2.

Factors affecting wind speed
Wind speed close to ground level in the zone where wind turbines are
likely to be situated, is affected by the terrain in several ways. Knowledge
of the terrain can be used to estimate wind speed at a particular site.
In the past, such knowledge was not available to the level of detail
required, but the advent of geomatics and digital mapping has made
data of the required accuracy and granularity available.

Height above ground level
Wind speed increases with the height of the wind turbine above
ground level. A general rule is the 1/7 rule which states that vertical
th
extrapolation of wind follows the law:

U z = U ref (Z/Z ref )
1/7 1
Where U z is the wind speed at height Z, Z ref is the reference height and
U ref the reference wind speed.

Surface roughness
Surface roughness, which is determined by the size and distribution of
roughness elements, such as trees, vegetation, rocks and man-made
structures, leads to a retardation of wind speed close to the ground.
Accurate data on roughness is essential for estimating wind speed at a
particular site. Roughness can be dependant on seasons, particularly
where farmland is involved. It is characterised by a parameter known
as roughness length.

Terrain: hills, valleys, changes in elevation (Orography)
Wind speeds are affected by variations in the surface height and shape,
for instance hills, valleys, mountains and cliffs, and obstacles and features
which constrict the flow of wind. This fact can be taken advantage of
when siting wind turbines. Features do not increase the power in the
wind but serve to concentrate it in a smaller area. This can be explained
by understanding the same mass or volume of air has to pass through
a constricted space, and the only way for this to happen is that the air
speed increases. This leads to an increase of wind power per unit area,
but the same power is in fact available on both side of the anomaly. Figure 2: Wind Atlas methods 1,3

energize | January 2022 | 52