Windtech: Tower technology reaches new heights

WORLDWIDE: New materials and tower designs, coupled with the latest long-bladed low-wind turbines, continue to push hub and tip heights to new levels.

Top tip… Nordex N131/3300 on Max Bögl tower at 229.5 metres

Earlier this year, Max Bögl installed an innovative hybrid prototype tower with an integrated pumped energy storage system at Gaildorf in south-west Germany.

GE supplied its latest 3.6-137 turbine for the prototype, with three more to come for the complete pilot project.

The 40-metre water-basin and tower provided a hub height of 178 metres and a total installation height of 246.5 metres, a new record although one that is unlikely to stay unbroken for long.

Wide-based tower designs built in steel appear to be on the increase. These concepts can be roughly subdivided into hybrid solutions, usually with two longitudinal segmented bottom sections supplemented by three or more "standard" road-transportable tubular steel sections, or tower designs completely built from segments.

The WindEnergy exhibition at Hamburg in September saw Vestas introduce a fully certified 166-metre large diameter steel tower (LDST), now available for its V136-3.45MW turbine. Maximum installation height is 234 metres and, according to Jan Hagen, vice-president for product strategy, LDSTs will become Vestas's sole high-tower product offering.

First seen in 2005 with the V90-3.0MW turbine, the LDST concept is characterised by longitudinal 120-degree segmentation of the bottom sections, pre-assembled onsite by maintenance-free bolt joints into full coning circles.

LDST towers typically comprise two bottom segmented sections with a maximum diameter base of 6.2-6.5 metres and lengths of 28-33 metres, supplemented by conventional tubular steel sections.

Steel designs

Enercon's new 4.24MW E-126 EP4 turbine is available with a newly developed 99-metre steel tower, similarly comprised of two 120-degree longitudinal segmented bottom sections with road-transportable tubular steel sections.

The standard joining method consists of dowel pins welded to the section inner wall edges with box-type steel reinforcement elements fitted over these pins at the joining surfaces. A steel sealing cover is mounted, followed by pouring grout to complete the assembly process.

For cold-climate applications, Enercon offers an alternative solution comprising flanges welded inside the segments' outer edges and maintenance-free bolted joints.

Senvion's new high tower concept is the outcome of an exclusive cooperation with German tower specialist SIAG. "The joint development project took about a year, including patent search," Stefan Philipp, Senvion's head of portfolio management, said at the Hamburg show, where the new tower concept was introduced.

"It encompasses two vertically segmented, bolted bottom sections, supplemented by a short steel adapter. Each tower further comprises four or more tubular steel sections - depending on hub height - with a maximum bottom diameter of 4.3 metres. Our main design objectives were ease of transport within a smart, visually pleasing overall concept," he said.

Each full circle has ten coning panels, with individual panels about 20 metres high and 20-40 millimetres thick. The bottom-section panels are less than three metres wide, which gives a base diameter of roughly 9.2 metres.

The upper section panels are obviously narrower, and final assembly is conducted with maintenance-free bolted joints. The tower base diameter might increase for hub heights beyond 139 metres, according to Philipp.

"We installed a 139-metre prototype tower with a 3.0M122 turbine in Landin (Germany) in September," said Philipp. "The new tower will also be available for the 3.6M140 with hub heights up to 160 metres. The main markets include Germany and Scandinavia,although Ontario in Canada is potentially attractive due to a combination of rather low wind speeds and high population density."

Dutch manufacturer Lagerwey Wind plans to install a prototype of its direct-drive 4MW L136 turbine early in 2017, which will come as standard with an in-house developed wide-base modular steel tower (MST), a bolted structure of pre-bent steel sheets.

The L136 will be offered with 120-, 132- and 166-metre hub heights, the last providing a tip height of 234 metres.

The innovative Lagerwey crane, another in-house development, is aimed at eliminating deployment of heavy crane to allow installation at space-constrained sites.

Hybrid solutions

Initially, only Enercon and, from 2010, Max Bögl produced technologically and visually comparable hybrid concrete-steel towers. Both solutions comprise multiple layers of coning precast concrete ring-shape elements that are post tensioned with steel cables following assembly completion.

Turbine size and hub height to a large degree determine tower base width, and the bottom ring sections come in either 120-degree or 180-degree segments, while upper sections are full circles.

Both suppliers supplement the concrete bottom sections with an adapter and multiple tubular-steel tower elements. Max Bogl's concrete tower layers are all 3.8 metres high with 0.3-metre wall thickness.

Earlier this year, Nordex installed an N131/3300 turbine on a 164-metre Max Bogl concrete-steel hybrid tower in inland Germany with a 229.5 metres tip height.

Enercon has introduced a new hybrid tower concept for its 4.2MW EP4 platform, aimed at increased flexibility, reduced manufacturing cost, easier transport-logistics and lower installation costs.

Additional focus points were a high degree of component modularisation and meeting the latest hub-height demands in markets such as Germany.

The new tower has a very different appearance from that used by Enercon's 7.5MW E-126 flagship. That giant comes with the company's elegantly cone-shaped pre-fab concrete tower for a hub height of 135 metres and a tip height of 198.5 metres.

The EP4's tower combines a mix of cylindrical and coning concrete elements whereby diameters reduce with height. The final concrete section is a transition element for accommodating a longitudinally divided steel section, followed by several cylindrical tubular steel sections making up to the final hub height.

The E-126 and E-141 EP4 models are each available with two hybrid towers and a maximum 159-metre hub height. The latter configuration for the E-141 gives a maximum 229.5-metre tip height.

Concrete advances

Dutch hybrid and full-concrete tower-technology provider Advanced Tower Systems (ATS) this August announced a cooperation agreement with international building-materials leader LafargeHolcim and Brazilian tower supplier DTS. ATS has licensed its patented tower technology to several international partners.

Steel is very expensive in Brazil, which is the main reason for deploying ATS-type full concrete towers up to a popular 120-metre hub height. This tower type comprises a square-shaped coning bottom section, and an upper section made of circular concrete elements with 4-4.5 metres outer diameter in about 50:50 ratios.

Each prefabricated ATS-type tower has a square bottom section consisting of multiple slender concrete elements, all 0.5-4.0 metres wide and 16 metres high, dimensions allowing road transportation on standard flatbed trucks.

All square cross-section layers comprise four identical cylindrical-shaped 90-degree corner elements, plus four flat coning elements fitting in between. The flat elements decrease in width for each layer added, while all corner elements have standardised dimensions.

The company now develops a new, much lighter tower variant, by switching from conventional steel-reinforced concrete to ultra-high performance concrete (UHPC).

LafargeHolcim produces this advanced concrete material, incorporating about 10-millimetre long metal or polymer fibres, under the trade name Ductal. The multinational company is active in many countries and supplies complete technology packages, including mixers, equipment and training if necessary.

"The unique characteristics of Ductal are that it contains a large quantity of small fibres," said Frans Brughuis, wind business developer at ATS's parent company, Mecal.

"After mixing and element manufacture, these are randomly distributed in the concrete-fibre matrix, providing superior compression and tensile strength.

"This includes up to 300% of the compressive strength known from normal precast concrete, which allows substantial reduction in element wall thicknesses and approximately 50% lower mass of the precast concrete modules.

"However, it should be noted that we also do retain much-reduced steel reinforcements the in modules as well," he said.

Brughuis added that Ductal costs at least twice as much as normal concrete, but this is largely compensated by the fact that only half the quantity is needed for functionally similar structures.

"The roughly 50% reduced masses of the precast concrete elements will allow faster installation and the use of smaller, cheaper, and more easily available 400-500 tonne cranes instead of the currently required 600-tonne equipment.

"Alternatively, switching to UHPC does enable greater hub heights with increased energy generation through using the bigger cranes already available in Brazil." Brughuis envisages to introduce ATS-type towers based on UHPC in Brazil, with an initial prototype project during the first half of 2018.