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and her colleagues 12 have optimised a remote power system in the Peruvian
mountains using a number of micro-turbines and demonstrated considerable
potential savings in project cost.
Closely related to the estimation of electricity production is life cycle assess-
ment (LCA) which aims to quantify the energy and greenhouse gas cost of pro-
ducing the electricity. A simple outcome of LCA is the ratio of energy produced
over the turbine lifetime to that expended during manufacture, transport, and
installation. There are only a few such studies of small wind turbines. Allen et al.
[ 10 ] show that the ratio varies from 1.7 to 8.8 for a 600 W wind turbine depending
on the wind resource. This compares to about 3 for a small PV array, and about 16
for an offshore wind farm. The ratio of CO 2 saved over the turbine lifetime to that
expended in its production had a similar range of values.
12.4 Tower Raising and Lowering
It is impossible to give any general information on the transportation and related
loads under Load Case J of the IEC SLM, but the major loads during erection can
be estimated easily. Figure 12.6 is a simplified diagram of the tower raising or
lowering for a tilt-up tower, either guyed or not, using a gin pole. Figures 10.10
and 12.7 show actual raisings of small turbines. For simplicity, it is assumed that
the gin pole, of length L, is connected rigidly at the tower hinge point and the cable
runs through a pulley close to the end of the gin pole when the tower is vertical.
This may also be the anchor point for the separate guy-cable from the end of the
gin pole to the tower (which is not shown). h is the angle of the tower at any
height, so that 0 B h B 90. b is the angle between the tower and the gin pole,
which cannot be significantly less than 90. The following assumptions are made:
• The raising or lowering is done slowly, so inertial effects are ignored.
• The tower is raised or lowered only during calm weather so there are no wind
loads or turbine aerodynamic loads.
• The combined turbine, tower and gin pole has mass m, whose centre lies along
the tower at distance l from the hinge point. This distance is not shown in
Fig. 12.3 .
• The ground is flat.
• No mechanical advantage is used.
• If the tower is guyed, no guy wire is in tension during raising or lowering.
• The tower, gin pole, and cable remain in the same vertical plane so no torsional
loads occur, and
• The cable is massless (Fig. 12.6 ).
12 http://upcommons.upc.edu/e-prints/browse?rpp=20&order=ASC&value=Ferrer+Mart%C3%
AD%2C+Laia&type=author&locale=en .
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