From lattice frame constructions to hybrid concrete-steel towers, the aim is to increase height to 150 metres and beyond. Manufacturers are investigating different types of materials such as UHPC and how to reduce costs and deal with fabrication and transport issues. Read about the solutions developed by major industry players such as GE, Alstom and Max Bogl in the article here: http://bit.ly/_Article_HigherWindTowers

1. Industry Article
Tower Concepts for Increased Hub Height
By Colin Pawsey
As the Wind Industry continues to grow, all trends are towards producing more power and lowering the
levelised cost of energy. The development of towers for onshore turbines is being driven by those same
issues, and one of the key areas of development is in tower height. It is much more of a factor onshore
than it is offshore, as the conditions on land tend to favour elevated hubs in terms of increased energy
production. In areas where wind speed is low, taller towers allow the turbine to access stronger winds at
height; and in forested areas, such as those in Northern Europe, the wind shear and turbulence created
by the terrain favours elevated hub heights.
Of course, it is not as simple as just fabricating ever
taller towers, as some of the conventional designs
are not scalable. Equally, any tower concept must
be economically feasible. There is a demand within
the industry for new, creative designs to come to
the fore that can take wind turbine towers beyond
150 metres and more.
Trends towards increased hub height
Data from Fraunhofer IWES Wind Monitor shows
the trend towards increased tower height. In 2013
33% of all new turbines installed in Europe had hub
heights in the range of 120-140 metres. While most
existing wind turbines have a hub height in the
range of 60-80 metres, only 10% of those construct-
ed during 2013 were within those heights. The
difference was also much more distinct between
coastal and non-coastal regions. Stronger winds
are found at lower height in coastal areas, and the
average tower height of turbines constructed in
these regions in 2013 was 60-100 metres. In low
mountain regions, where winds are stronger at
height, almost 80% of turbines constructed during
the year were in excess of 120 metres.
Current Concepts
There are a number of interesting concepts among
the key players in the industry, which are taking
tower design and construction to greater heights.
Here is a brief summary of some of the concepts
that have been introduced in recent years, with the
intention of increasing energy yield through better
tower design.
Enercon
In December 2013
Enercon installed a
prototype E-115
turbine in Northern
Germany, with a
precast concrete tower
at a hub height of
135 metres. The new
turbine became
available for series
production in mid-
2014, and can be
fabricated with a
hub height of up to
149 metres. In typical
inland conditions with a wind speed of 6.5 m/s and
a hub height of 149 metres, the E-115 is able to pro-
duce an annual output of approximately 9.3 million
kilowatt hours – an increase of 14% over the Ener-
con E-101.
Enercon’s precast concrete towers are fabricated as
separate elements with diameters up to 14.5 me-
tres. Segments with larger diameters are manufac-
tured in two or three half-shells so that they can be
easily transported to site. After assembly the seg-
ments are linked to each other via prestressing ten-
dons centered in the tower wall.The joints between
individual precast tower segments are sealed with
a special epoxy resin to transmit compressive loads
evenly from segment to segment. The prestressing
tendons are passed through sheathing pipes em-
bedded in the segments, then connected to the
foundations. Any gaps between the tendons and
www.windturbine-towers.com
Source: Enercon

2. Tower Concepts for Increased Hub Height
the sheathing pipes are then filled with grout to
provide long-term corrosion protection.
GE
GE’s Space Frame Tower with a hub height of 139
metres was officially introduced in 2014, after the
company had already installed a 97 metre proto-
type at its facility in Tehachapi, California. The five-
legged enclosed-lattice structure was designed by
GE to scale up towers with the caveat of keeping
costs down. The 10-metre diameter base will allow
a 120 metre tower to use 20-30% less steel than
a traditional 100 metre tubular steel tower, as the
broad base means less support is required from the
tower walls.
Lattice towers have been tried and were largely un-
successful in the past. In part due to bolts frequent-
ly rattling loose causing structural failures, and also
because birds took up perches in the structure
whichledtoincreasedavianfatalities.Geaddressed
both of these problems by using splined bolts to
eliminate the risk of structural failure, and enclos-
ing the structure in a translucent, non-weight-bear-
ing, PVC Polyester fabric coating.
Siemens
Siemens installed a Bolted Steel Shell Tower proto-
type as early as 2011, and offered the concept for
serial production in 2012, and it remains one of the
more interesting concepts. The tower consists of
multiple sections mounted on top of one another,
each made out of steel shells which are assembled
on site. The shells are produced from bended steel
plates and can be transported to site on standard
trucks, where they are joined together with HRC
(tension-controlled) bolts to form a tower section.
Siemens say that the design can produce towers in
excess of 140 metres, and that there is no limit on
height at all, provided a suitable crane is available
for installation.
In terms of cost reduction, Siemens say that the
concept has a lower steel requirement compared
with tubular steel towers due to larger tower di-
ameters, high-strength steel and reduced material
thickness. There is also the potential to use steel
coils which are cheaper than quarto steel plates,
while an automated shell production process en-
sures high quality and reduces cost.
Nordex
In 2013 turbine manufacturer Nordex installed two
prototype N90/2500 turbines at their site in Iven,
Germany. The tower used was a concrete/steel hy-
brid that can reach up to 120 metres in height. The
company says the hybrid design can reduce costs
by simplifying the transport and logistics processes
during construction.
The hybrid tower consists of a concrete tower of
60 metres, which is mounted directly on the base
and at the location and then prestressed. It bears
the three steel tower sections of the modular tower
which has a total height of a further 60 metres. The
advantage is that the concrete tube is produced on
site and doesn’t require expensive transportation,
while the steel tower sections can be carried on
standard vehicles.The concrete for the tower is pre-
pared at the site free of any joints and prestressed
over the entire height of the tower using external
tendons. This allows the tendons to be easily ex-
amined for damage, and replaced if necessary. The
lowest section of the steel tower is placed atop of
the concrete tower and fastened by means of an-
chor bolts.
www.windturbine-towers.com