Towers reach for the sky

WORLDWIDE: The development of tall, cost-effective towers is widely recognised as the next revolution in the wind industry because of their potential in boosting yields and cutting energy costs. Eize de Vries reviews the latest trends, challenges and opportunities in tall tower design.

Hidden… GE’s new lattice tower has a plastic fabric coating for protection of the internal parts
Hidden… GE’s new lattice tower has a plastic fabric coating for protection of the internal parts

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In 2006 German turbine maker Fuhrländer (now FWT Trade) installed a 2.5MW turbine with a 90-metre rotor diameter atop a 160-metre SeeBa tower. The resulting maximum tip height of 205 metres made it the world's tallest wind turbine until earlier this year, when Vestas installed a V-164-8.0MW prototype on a 140-metre tower, producing a tip height of 222 metres. This is some way above the 200 metres that several turbine suppliers still retain as an informal maximum height limit.

Onshore wind turbine towers can be divided into six categories: tubular steel; "conventional" lattice steel; concrete-steel hybrid; full concrete; new-generation lattice steel; and bolted steel-shell.

Tubular steel towers are considered an economic overall solution and typically come in two to four bolted sections. Maximum tower foot diameter has until recently been typically limited to 4.1-4.3 metres to allow for road transport logistics, accessing railway crossings and tunnels. This limitation in practice curbs hub height to about 100 metres. Where suppliers raise the hub to around 120 metres, the steel tower walls must be thickened, which have a negative impact on the mass of the structure and the cost.

A new industry trend is for tubular steel towers with a large foot diameter. The 8MW Vestas prototype's tower, for example, measures 6.5 metres at the base and 4.5 metres at the top. These measurements are only feasible when traditional road transport bottlenecks are not involved, such as at coastal locations where access is easy by vessels and unobstructed roads. At least one supplier other than Vestas has indicated considering high, wide-base tubular steel towers for coastal forested projects in Scandinavia.

Clothed spaceframe

Traditional four-legged lattice steel towers enjoyed some popularity in the past in Europe for combining high hubs with uncomplicated design, easy transport logistics and favourable mass and costs. But the assembly process is time-consuming and regular bolt torque checks add to the lifecycle cost. The appearance further hampered the spread of lattice towers in Europe, but they proved popular in the US.

A new steel lattice tower design is seen in GE's five-legged spaceframe tower, which offers a 139-metre hub height for the company's latest 2.75-120 low-wind model. Its maintenance-free bolt joints used in bridge-building do not require regular re-tensioning or checks. Unlike the lattice towers of the past, GE's spaceframe is clothed in plastic fabric to protect the elevator, power electronics, and either the transformer or battery storage system or both from the weather.

Enercon has pioneered the use of concrete towers. After initially applying modular cylindrical towers and in-situ manufactured coning concrete towers, a final switch was made to in-house manufacture of full concrete towers. These designs comprise multiple stapled prefab coning concrete rings, a steel adapter on top, and total assembly post-tensioning by steel cables.

Earlier this year Alstom introduced a 199-metre full concrete tower for its ECO 122 low-wind turbine, comprising 11 concrete sections and a base diameter of 7.2 metres. The novel installation method, again using bridge-building techniques, starts with the top sections, which it raises to insert lower section below.

Acciona's current main volume model is the AW116/3000 for IEC class IIA, available with a 120-metre concrete tower and representing over 50% of total sales. The use of concrete is successful in local content markets such as Brazil, where steel is expensive.

Enercon applies full concrete towers with steel adapter for its 7.5MW-E-126 turbine, but a concrete-steel hybrid is generally preferred for its smaller machines. Enercon offers its 3MW E-101 and E-115 series with hybrid towers for a hub height of 149 metres.

Max Bögl hybrid towers, which offer hub heights of up to 150 metres, dominate the independent high tower supply market, currently used by Alstom, GE, Nordex, Senvion and Vestas. The modular concrete-steel tower system bears a strong technological similarity to Enercon's prefab design, including the use of coning concrete rings in full or segmented circles to manage road transport constraints. Max Bögl sections comprise multiple coning rings 3.8 metres high and 0.3-metre wall thickness, which, unusually, are stapled with a precision dry-joint method.

Concrete-steel hybrid towers typically comprise a bottom concrete section that extends to an elevation, where it mergers with a "standard" tubular-steel top section of 4.1-4.3 metre bottom diameter.

Easing transportation

Founded in 2005, Advanced Tower Systems (ATS) offers prefab concrete towers for hub heights up to 150 metres, and in full concrete up to 120 metres. They are widely used by German developer Juwi, while product licenses have been awarded to partners in Spain and Mexico.

ATS towers consist of multiple slender concrete elements, no more than four metres wide and 16 metres long for easy transportation on standard flatbed trucks. Each square cross-section comprises four identical standardised cylindrical-shaped 90-degree corner elements, with four flat coning elements in between, which decrease in width for each layer added.

Germany's Drossler Unwelttechnik has developed Ventur, a prefab tower in full concrete for hub heights up to 120 metres, and a concrete-steel hybrid for up to 200 metres. Nordex is one of the first Ventur customers.

The concrete part is an octagonal shape, but a different base shape with more sides could be used for hub heights around 200 metres, said a company spokesman.

At ground level, four ten-metre long elements alternate with four five-metre elements, followed by construction steps adding ten-metre elements until reaching the required elevation. The four "remaining" gaps are closed with five-metre elements, and placement of a transition piece for tubular steel mounting is added. At base level, the width is typically tapered in the three-metre range, but that reduces with height. The company claims the Ventur design offers simple manufacture of flat elements, transportation on flat-back trucks and installation with only small cranes.

Working with a Danish partner, Siemens developed a patent-protected bolted steel-shell tower, which it introduced in 2012. It aims to offer an alternative to concrete and hybrid towers for hub heights over 100 metres. The main benefits are ease of transportation and handling, elimination of height limits, much reduced crane limitations and a fast, foolproof construction process, says Siemens. The wide-base tower consists of 12 inward-facing U-shaped fabricated sections, all pre-assembled with a maintenance-free bolt system in full circles at ground level, whereby each added tower level has a smaller "footprint". A compressible rubber line pre-fitted along the vertical contact surfaces prevents the seeping-in of moisture.

Dutch supplier Lagerwey Wind earlier this year installed a 2.5MW turbine atop a patented 136-metre bolted steel-shell prototype tower. These towers are circular in shape and comprise multiple pre-bent steel sheets, each 2.8 metres wide and 12 metres long, and with precision prefabricated bolt holes. The prototype tower has a diameter of 9.0 metres at the foot and 2.3 metres on top. The bolt joints are again maintenance free. Optimal materials usage for this specific turbine-tower combination resulted into a tower mass of only around 400 tonnes, according to Lagerwey.

For greater hub heights one or more bottom sections are added. For a higher-rated turbine in development with hub heights up to 160 metres, the tower foot diameter will be increased too.

Design challenge

This overview of different solutions shows that an ultimate high-tower solution is still to be found. Market demand dictates both opportunities and challenges whereby turbine manufacturers at least prefer to have a choice between various tower solutions and suppliers because a lack of competition often results in high prices. Each individual tower solution must be compared and evaluated for specific benefits, but especially on lifecycle-based cost of energy whenever possible, considering the still limited track record of many solutions.

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