Site Assessment and Installation - Small Wind Turbines: Analysis, Design, and Application - page 261

Since l is usually greater than L, the ratio l/L may be termed the ''mechanical
disadvantage'' of the gin pole. The horizontal force on the foundation is

mgl cos h
L tan h þ b

F xb ¼ ð 12 : 8 Þ

ð ð Þ = 2 Þ

with a maximum value given by

mgl
L tan b = 2

F xb ; max ¼ ð 12 : 9 Þ

ð Þ

Example 12.1 The 18 m high, 530 kg tower in Fig. 10.2 has its centre of mass
7.46 m from the base. The turbine mass is 170 kg, the gin pole is 6.04 m long, and
the angle b as defined in Fig. 12.3 is 80. Estimate the maximum cable tension in
lowering or raising the turbine using Eqs. 12.4 - 12.9 . What are the maximum
vertical and horizontal loads on the foundation.

Answer First it is necessary to determine mgl = (530 9 7.46 ? 170 9 18) 9
9.81 = 68,805 Nm. From Eq. 12.6 f Tmax = 1/sin(40) = 1.56. Therefore T max =
1.56 9 68,805/6.04 = 17.72 kN which is less than 2 tonnes (= 2 9 1000 9
9.81 kN) in the units commonly used for cable tensions and lifting requirements.
From ( 12.7 ) F yb,max = (530 ? 170) 9 9.81 9 (1 ? 7.46/6.04) = 15.35 kN which
is over twice the vertical force on the foundation when the tower is upright.
Equation 12.8 gives F xb,max = 68,805/(6.04 9 tan(40)) = 13.58 kN which is
higher than the maximum horizontal wind force calculated in Chap. 9 .

The analysis of raising and lowering shows that minimising the foundation
loads, which should minimise foundation costs, is equivalent to minimising the
tower mass as was done in Chap. 10 .
An important aspect of tower installation and raising for small turbines is
related to the fact that most have their centre of mass displaced from the tower
axis. This implies that considerable care must be taken to ensure that the tower is
vertical once raised, otherwise there could be undesirable gravity effects in the
turbine yaw response and furling, if used. Figure 12.8 shows the leveling of a
centre-hinged tower for the Aerogenesis 5 kW turbine shown in Fig. 1.2 which
was installed using a crane. The foundation is a drilled concrete shaft comprising a
3.5 m long, 900 mm diameter steel reinforcement cage concreted in an auger-dug
hole with the square concrete pad visible on the surface. The bolts through the
tower baseplate were tied to the cage. In turn, these bolts have nuts below the
baseplate which were adjusted as shown to level the tower. Then the gap between
the baseplate and the concrete was later filled with grouting.
An alternative steel pile foundation is shown in Fig. 12.9 .