turbine towers, Navarra, Spain
© 1999 Soren Krohn
The tower of the wind turbine carries the nacelle and the
Towers for large wind turbines may be either
tubular steel towers, lattice towers, or concrete towers. Guyed
tubular towers are only used for small wind turbines (battery
large wind turbines are delivered with tubular steel towers,
which are manufactured in sections of 20-30 metres with flanges
at either end, and bolted together on the site. The towers are
conical (i.e. with their diameter increasing towards the base)
in order to increase their strength and to save materials at
the same time.
Photograph © NEG-Micon A/S 1998
Lattice towers are
manufactured using welded steel profiles. The basic advantage
of lattice towers is cost, since a lattice tower requires only
half as much material as a freely standing tubular tower with
a similar stiffness. The basic disadvantage of lattice towers
is their visual appearance, (although
that issue is clearly debatable). Be that as it may, for aesthetic
reasons lattice towers have almost disappeared from use for large,
modern wind turbines.
Photograph © Nordex A/S 1998
small wind turbines are built with narrow pole towers supported
by guy wires. The advantage is weight savings, and thus cost.
The disadvantages are difficult access around the towers which
make them less suitable in farm areas. Finally, this type of
tower is more prone to vandalism, thus compromising overall safety.
Photograph © Soren Krohn 1999
The price of a tower for a wind turbine is generally around 20
per cent of the total price of the turbine. For a tower around
50 metres' height, the additional cost of another 10 metres of
tower is about 15,000 USD. It is therefore quite important for
the final cost of energy to build towers as optimally as possible.
Lattice towers are the cheapest to manufacture,
since they typically require about half the amount of steel used
for a tubular steel tower.
Generally, it is an advantage to have a tall tower in areas with
high terrain roughness, since the wind speeds increases farther
away from the ground, as we learned on the page about wind
Lattice towers and guyed pole towers have
the advantage of giving less wind shade than a massive tower.
The rotor blades on turbines with relatively short
towers will be subject to very different wind speeds (and thus
different bending) when a rotor blade is in its top and in its
bottom position, which will increase the fatigue
loads on the turbine.
Between Low and Tall Towers
Obviously, you get more energy from a larger wind turbine than
a small one, but if you take a look at the three wind turbines
below, which are 225 kW, 600 kW, and 1,500 kW respectively, and
with rotor diameters of 27, 43, and 60 metres, you will notice
that the tower heights are different as well.
Clearly, we cannot sensibly fit a 60 metre rotor to a tower
of less than 30 metres. But if we consider the cost of a large
rotor and a large generator and gearbox, it would surely be a
waste to put it on a small tower, because we get much higher
wind speeds and thus more energy with a tall tower. (See the
section on wind resources). Each
metre of tower height costs money, of course, so the optimum
height of the tower is a function of
- tower costs per metre (10 metre extra tower will presently
cost you about 15,000 USD)
- how much the wind locally varies with the height above ground
level, i.e. the average local terrain
roughness (large roughness makes it more useful with a taller
- the price the turbine owner gets for an additional kilowatt
hour of electricity.
Manufacturers often deliver machines where the tower height
is equal to the rotor diameter. aesthetically, many people find
that turbines are more pleasant to look at, if the tower height
is roughly equal to the rotor diameter.
The choice of tower type has consequences for occupational safety:
This is discussed in detail on the page on Wind
Turbines and Occupational Safety.