20
wind systems have been installed and communi-
ties are generally unfamiliar with them, you may
face some obstacles in gaining permission to in-
stall a unit. We appreciate the pioneering spirit
and resolve demonstrated by our customers and
we stand ready to help out in any way that we
can.
B. Towers
The smooth flow of the wind over the land is inter-
rupted by obstructions and topographical varia-
tions. These interruptions bring about two impor-
tant phenomena:
wind shear
and
turbulence
.
Wind shear describes the fact that close to the
ground the wind is slowed down by friction and
the influence of obstacles. Thus, wind speed is
low close to the ground and increases with in-
creasing height above the ground. Wind shear is
more pronounced over rough terrain and less pro-
nounced over smooth terrain. Turbulence is es-
sentially rough air caused by the wind passing
over obstructions such as trees, buildings, or ter-
rain features. Turbulent air reduces energy output
and puts greater strain on the wind turbine.
The effects of both wind shear and turbulence
diminish with height and can be largely overcome
simply by putting the machine sufficiently high
above the ground. Taller towers usually will pro-
vide better economics because the power in the
wind increases as the cube of the wind velocity (P
= V
3
; e.g., a 26% increase in wind speed doubles
the energy output). A small increase in average
wind speed will result in a large increase in long-
term energy output.
Table 2 shows the influence that tower height can
have on annual energy output for the BWC XL.1
wind turbine under typical DOE Class 2 inland site
conditions with a shear exponent of 0.20. Wind
speed may increase more radically with tower
height in hilly or wooded areas. In flat open ar-
eas, power production will increase less signifi-
cantly with tower height.
The BWC XL.1 wind turbine must be placed on a
tower that is tall enough to give the rotor proper
exposure to the wind. Putting a wind turbine on a
tower that is too short is like installing a solar sys-
tem in the shade. As a “rule-of-thumb” the BWC
XL.1 should be 9 m (30 ft) above obstacles within
50 m (160 ft), particularly in the prevailing wind
direction. So, the minimum recommended tower
height is 9 m (30 ft.).
Tower
Height
(meters)
Average
Wind
Speed
(m/s)
Relative
Energy
Production
9
m 4.8 100%
13 m
5.2
121%
19 m
5.6
147%
25 m
5.9
165%
32 m
6.2
186%
Table 2: Variation in wind speed and ex-
pected relative energy output with tower
height.
We do not recommend mounting the BWC XL.1 to
a home and we suggest caution if installing one
on a larger, more substantial, building. Our con-
cerns are 1) the forces on the turbine and mount-
ing system are substantial and homes are not de-
signed structurally for them, 2) the air flow around
and over a home or building is complex and can
cause considerable turbulence, and 3) the wind
turbine will cause vibrations that could be trans-
mitted through the home’s structure.
BWC offers a guyed-tubular tilt-up tower, the
Tilt.Tower, for the XL.1 in heights form 9 m (30 ft)
to 32 m (104 ft). The Tilt.Tower is cost-effective
and is designed to be installable by non-experts.
The installation of these towers is covered in the
BWC XL.1 Tilt.Tower Installation Manual. BWC is
working to expand the range of tower options, in-
cluding self-supporting towers that do not require
guy wires.
Customers can also supply their own towers.
These towers have to meet certain criteria for
strength and blade clearance (see Appendix), and
a mounting adapter for the XL.1 wind turbine will
need to be designed and fabricated. Customer
supplied towers are not covered by the BWC war-
ranty and any damage to the XL.1 wind turbine
resulting from a customer supplied tower is ex-
cluded from the turbine warranty coverage.
C. Location
The size and layout of the installation site may
limit the tower location, height, or type. More of-
ten than not, however, there are several potential
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