sin αst

ROTOR
Table 6.3 provides a material cost comparison based on approximate prices per tonne,
given in [58]. The prices are derived relatively to the price of steel in 2005, set to
$100/tonne. The actual values are out of date and they are unlikely to represent the prices
in development countries. However the prices are considered valid for a relative compari-
son of the price differences.
46
Material Steel Aluminium CFRP GFRP Wood
Relative material cost
($/tonne)
Relative volume cost
($/m 3 )
Table 6.3: Material cost comparison
100 400 20000 1000 200
785 1080 28000 1700 76
In addition to material cost, it is necessary to take into account manufacturing costs. As a
general rule it can be stated that manufacturing processes which require little or no auto-
matic machining are most advantageous. For the present project the price of manufactur-
ing is not scrutinised further, as the majority of it will be carried out by volunteers.
When taking into consideration the available data, the material selection is neither
straightforward nor unambiguous and the right choice of blade material will, to a great
extent, depend on the conditions at the specific destination where the wind turbine is to be
built. For the current wind turbine design wood is chosen as blade material, primarily due
to its generally high availability in developing countries, its low price and its low require-
ments for production facilities. The choice in material is further sustained by the fact that
wood in its nature is designed for resisting wind loads in bending, which is confirmed by
the overall excellent material properties in terms of strength and stiffness. Other material
options may prove superior in certain cases, e.g. where high quality wood is expensive or
difficult to find. The blade attachment method described in section 6.2.4 enables blades of
other materials to be attached in future designs without impacting the overall design con-
cept.
An in-depth description of the engineering properties of the chosen blade material may be
found in appendix E.
6.2.2 Airfoil and geometry
The cross-section of the wooden blades has the shape of an airfoil, which optimises the
aerodynamic performance. A number of terms are used to characterise airfoils, as shown
on figure 6.5. The geometric parameters have great influence on the aerodynamic per-
formance of the airfoil. Not shown on figure 6.5 is the span of the airfoil, which is perpen-
dicular to the shown cross-section.