Road transportation constraints restrict common tubular steel towers to a maximum diameter to 4.2-4.3 metres, which, together with turbine size, puts a cap on practically achievable hub heights.
One option for retaining a road-transportable diameter, combined with hub heights typically above 100 metres is a "soft" tubular steel tower with advanced turbine control. But a much-quoted disadvantage is that steel-wall thickness increases rapidly, making the tower heavy and expensive.
An alternative is a steel tower with wide-base bottom sections vertically split into at least three coning segments. Such segments are much easier to transport on a flatbed truck and assembled into full circles on-site using bolt joints or grouting.
However, in specific markets - where ports have the necessary facilities and there are no road transport restrictions to the construction sites - substantial costs savings can be achieved through manufacturing the wide-base section(s) in full circles.
Vestas has been deploying its in-house large diameter steel tower (LDST) design since 2005. LDSTs come with base diameters of 6.2-6.5 metres, for a stiffer, stronger structure that enables higher hub heights for similar loads.
Steel usage is also reduced substantially thanks to steel sheet wall thicknesses in the 25-40mm range, as opposed to the 50-70mm for conventional tubular steel towers. The Vestas design comprises two bottom LDST-type sections, each of which are 28-33 metres long, supplemented by three standard tubular steel sections.
The LDST sections are initially manufactured in single full-circle elements, and then sliced in three 120-degree segments after welding three pairs of steel connecting flanges with pre-drilled holes inside. The sections are assembled on-site with bolt joints.
The Vestas V150-4.2MW will be offered with an LDST for a maximum hub height of 166 metres and tip height of 241 metres. A prototype is due to be installed late next year.
Lagerwey's 4.0-4.5MW L136 flagship turbine will become available with in-house developed wide-base modular steel towers (MST) for hub heights of 132 and 166 metres.
MSTs consist of 12-metre long coning pre-bent steel sheets with pre-manufactured holes, brought to site on flatbed trucks. These sheets are bolted together on-site into full-circle assemblies using maintenance-free bolts used in the bridge-building industry.
Individual assemblies are hoisted by crane, stacked and bolted to the previous level to the required hub height. A novel interlinked product is the self-climbing Lagerwey crane, now being tested. This is attached to add-on brackets incorporated on MST segments, moving up and down the tower enabled by hydraulic cylinders.
Senvion's new high tower concept was jointly developed with German tower specialist SIAG. It encompasses two vertically segmented bolted bottom sections, supplemented by a short steel adapter. Each full circle has ten coning panels, each about 20 metres long and 20-40mm thick.
The bottom-section panels are less than three metres wide, which gives a base diameter of roughly 9.2 metres for a 139-metre tower supporting a 3MW M122 turbine.
The upper section panels are narrower, and final assembly is conducted with maintenance-free bolted joints.
Each complete tower further comprises four or more conventional tubular steel sections depending on hub height. The new tower solution will become available for other Senvion turbines with hub heights up to at least 160 metres.
The new Nordex Delta4000 N149/4.0-4.5 features a 149-metre rotor diameter and will be initially available with four standard hub heights. Two tubular steel towers provide hub heights of 105 and 125 metres, and two concrete-hybrid towers go up to 145 and 164 metres. The tip height for a turbine on the tallest tower is 238.5 metres.
Enercon is the commercial frontfrunner in the 4MW-plus class and a concrete tower pioneer. The firm's new concrete-steel hybrid modular tower concept for the E-126 and E-141 EP4 models has a maximum hub height of 159 metres. Some 3MW E-115 EP3 turbines have also been installed on them.
The new structure differs fundamentally from Enercon's classic-shape towers - made in full concrete or as concrete-steel hybrids - which have been deployed for many years on multiple models, including the now discontinued 7.5MW E-126 EP8.
A main characteristic of the new design is a substantially reduced base diameter of around ten metres. Other key features are the combined use of standardised cylindrical and coning concrete and steel elements, of which the first 26 (cylindrical and coning) element layers are vertically split in 180 degrees to enable road transport.
Also noteworthy is a less costly concrete transition segment in place of the steel component used previously. The concrete sections "package" is post-tensioned by steel cables mounted inside the structure.
The steel tower sections supplementing the concrete part start with a longitudinal subdivided coning segment of three sections, with a 5.0-metre base diameter, that are assembled on-site.
The tower is completed with two cylindrical tubular-steel segments, each with identical flange mountings and a maximum 4.2-metre diameter. The ten-turbine Ahaus wind farm in Germany, close to the Dutch border, is one of the first projects featuring Enercon's E-141 flagship on 159-metre hybrid towers.
The EP4 E-126 for IEC I sites comes with 135- and 159-metre towers, but Enercon also offers a 99-metre steel tower mainly focused at export markets.
It comprises five sections in total, of which the two wide-base bottom sections can be longitudinally split into 120-degree segments, assembled on-site by using bolted joints or grouting. The modular designs allows flexible manufacturing of different tower variants at similar production lines across the world.
German market leader Max Bögl supplies concrete-steel hybrid towers up to a current maximum hub height of 164 metres.
The initial shape of the design closely resembled Enercon's classic concept, each comprising multiple 3.8-metre full-circle and vertically segmented concrete coning elements supplemented by tubular steel sections.
Likely unique to the Max Bögl product, thoug, are the precision-grinded concrete upper and lower joining surfaces, enabling weather-independent dry stapling without requiring mortar.
The company's latest concept for hub heights over 160 metres (called System 160+) consists of coning segments for the base sections, but cylindrical rings for the middle sections to gain extra height. An integrated systems supplier since 2012, Max Bögl also produces the tubular steel sections and the tower interior fittings.
For the installation, the firm uses a self-climbing revolving tower crane with a maximum hoisting height of 180 metres, built close to the tower using mounting fixings incorporated in the hybrid tower's foundation.
The first one-third tower-crane bottom section is assembled using a mobile crane, followed by a self-climbing process adding crane lattice-type segments during assembly of all the tower segments. Competitors use functionally comparable tower-crane solutions, now popular for space-constrained and forested sites.
Max Bögl has also developed an innovative 178-metre support structure for the four-turbine "Naturstromspeicher" project in Gaildorf, in the south German state of Baden-Württemberg, which incorporates an integrated pumped storage solution.
The design comprises a 40-metre-high storage basin with "active" varying water column height, a hybrid tower, and a 3.4MW GE-3.4-137 turbine with a record installation tip height of 246.5 metres.
Combined system features include active and passive energy storage, fast balancing power response for grid stabilisation, and a claimed boost of up to 25% in annual energy production due to the 40-metre extra hub height.
German supplier Ventur's prefab tower comes in full concrete for hub heights of up to 120 metres, and as a concrete-steel hybrid solution for up to 200 metres.
The concrete part has an octagonal coning shape, but a different base with more sides could be used for the tallest heights. At ground level, four flat ten-metre elements alternate with four five-metre elements, following by construction steps adding ten-metre elements until the required elevation is reached.
The "remaining" gaps are closed with five-metre elements, and placement of a transition piece for tubular steel mounting completes the concrete tower construction part. The Ventur design solution offers simple manufacture of flat elements, transportation on flatbed trucks, and installation requires only small cranes.
This (incomplete) overview shows a substantial range of different high-tower products with clear preferences for wide-base steel variants and concrete-steel hybrid solutions.
However, specific circumstances can boost alternative approaches. One example is Brazil, where high steel prices favours the use of full concrete towers, preferably with mobile factories for building on-site.
Tower development has shown itself capable of keeping pace with continuous wind-turbine upscaling, largely independent of supplier preferences in both materials and engineering solutions.