Wind tower advances lift hub height restrictions

Increasing hub heights and larger rotor diameters on today's wind turbines bring greater loads on the rotor and more tower bending per unit of tower length.

A wide tower base diameter contributes to optimised load transfer from tower to foundation
A wide tower base diameter contributes to optimised load transfer from tower to foundation

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Conical tubular steel towers require larger base diameters to cope with this load, which restricts hub heights since towers are limited in size according to what can be transported. Across Europe, as well as in the US, the diameter should not exceed 4.3-4.4 metres to allow for road bottlenecks such as railway crossings and, for example, the Elbe tunnel in Germany.

One way to reduce the diameter required for a tower base is to use a smaller diameter tower with thicker steel walls, called a soft tower, which is heavier and more expensive than the stiff tower design. Danish manufacturer Vestas offers a steel tower with a 119-metre hub height for its new flagship 3MW V112-3.0 turbine, with a maximum 4.2-metre base diameter.

Patented design

In the early 1990s, German wind turbine manufacturer Enercon pioneered a modular prefabricated concrete tower design for its 500kW E-40 turbines. This design consisted of several cylindrical reinforced concrete modules, manufactured in rotating moulds.

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Enercon later switched to in situ concrete towers built onsite with the more traditional sliding-shuttering method taken from factory chimney construction. Concrete is poured into pre-positioned casings, which move up as the build progresses. This was used for the towers of many 1.5-2MW E-66 series turbines and has been used by other suppliers, such as Areva Wind (formerly Multibrid), to build concrete-steel hybrid towers. However, it is labour intensive and time consuming.

For the last decade, Enercon has been using its own design of prefabricated concrete towers for several of its models, including the latest and largest E-126. This is different from a steel-hybrid structure, as it consists only of a short steel connector on top that functions as a transition between the concrete tower and the nacelle yaw bearing. The E-126 tower, with a 135-metre hub height, is composed of 35 tapering concrete rings. The base is 14.5 metres in diameter, narrowing gradually to a slender 4.1 metres at the top.

The largest lower tower rings are manufactured in sections of 120 degrees for size and logistical reasons. Some upper rings are made in halves, with the smallest at the top, made as one full-ring piece. After completing this construction phase, all tower elements are post-tensioned with steel cables that pass through them to the concrete foundation, and the space between cables and concrete wall is sealed. With this method, Enercon has developed its current highest tower with 138-metre hub height.

Enercon manufactures the elements for its concrete towers under controlled industrial conditions in several major wind markets around the world. The Emden plant in Germany is one of the newest with an advanced level of production automation and minimal labour input requirements. Among several distinct production technology features are unmanned overhead rail carts that continuously top up the liquid concrete reservoir of a distribution device that fills the different-sized conical concrete element moulds.

Structural foundation expert Axel Jacobs of leading Dutch engineering consultancy ABT says that a wide tower base diameter contributes favourably to an optimised load transfer from the tower into the foundation. That, in turn, offers the potential for a smaller, cheaper foundation design.

ATS concrete-steel hybrid tower

In May last year, Dutch firm Advanced Tower Systems (ATS) built a prototype concrete and tubular-steel hybrid tower at a test field in Germany. A 2.3MW Siemens nacelle with 93-metre rotor diameter and 133-metre hub height completed the wind turbine. The patented structure has a square-sided tapering concrete lower section with rounded corners and a tubular steel section on top. One cost-saving design feature is that the matching tubular steel towers are off-the-shelf products.

Plans for the design began in 2002, says managing director Frans Brughuis: "We first commenced design calculations with a decagonal (10-sided) cross-sectional coning shape, but this was rejected for mainly visual reasons. A search for alternatives resulted in a coning circular shape, but construction required many different moulds, making it only viable for large series."

The company tried a triangular shape and finally arrived at the current version. The ATS tower consists of multiple slender, prefabricated elements 0.5 to 4 metres wide and 16 metres high, which can be transported by standard trucks. The sections comprise four cylindrical 90-degree corner elements and four flat coning elements that fit in between. The coning elements decrease in size and the corner elements remain the same. The prototype tower base sides measure 8.3 metres square.

For tubular steel tower mounting, each ATS concrete structure incorporates a square-sided adaptor bolted onto the top section. Once installation of the concrete section is ready, the structure is post-tensioned with steel cables inside the tower walls.

Brughuis says: "A reduced number of elements is a main advantage, resulting in a minimum number of production moulds." The patent has had some success, with Inneo of Spain licensing the segmented circular-shape tower and a second supplier, Postensa of Mexico, manufacturing triangular-shaped towers under license to ATS.

The ATS tower solution is said to be economically viable for hub heights up to 150 metres and from 1.5MW. For larger power rating, the tower typically features a shorter tubular steel section than equivalent designs for smaller power ratings or rotor diameters, again because of the higher bending loads of large turbines and transportation limits of wide tubular-tower bases. Brughuis suggests that the company may switch to a five-sided structure with growing turbine size in order to stay within its elements’ transportation limits.

Other innovations

At a wind power exhibition this year, Germany’s Repower displayed a scale model of a slender concrete-steel hybrid tower jointly developed with a tower specialist, which is in final design stages. The first turbines using this tower concept are expected by the end of this year. 

Gamesa of Spain has developed a trunk-conical-tubular tower with 120-metre hub height for the 4.5MW G128 onshore turbine. According to product specification, this concrete tower hybrid comprises a post-tensioned prefabricated concrete lower part and a structural carbon steel upper part, both designed in easy-to-transport sections.

Nordex of Germany is also offering a new 2.5MW Gamma series with hub heights up to 140 metres. Another hybrid, this one comprises an 80-metre concrete lower section and a steel top. The concrete structural part is a batch-type solution composed of multiple three-metre-long sections and the company says it can be built in any weather condition. After build is complete, an eight-week period follows for foundation construction and post-stressing of the concrete structure with internal steel cables. Nordex has built two 120-metre towers using this design.  

Boosting yield. Bigger blades and taller towers

In order to extract more energy from the wind at low- and medium-speed wind sites per megawatt of installed power, there is a choice between two partially complementary options.

Bigger rotors

The first is increasing the rotor size for a given power rating — already a wind industry trend. The difference in rotor-swept area between a 3MW Vestas V90 (90-metre rotor diameter) and the later 3MW V112 (112 metre) is 55%. Vestas says that for a typical Dutch wind location with an average annual wind speed of 6.7 metres per second (m/s) at hub height, the annual energy yield difference between a V90 and V112 is 38%. However, at higher wind speeds, this difference gradually narrows.

Taller towers

A second option is to put wind turbines on taller towers. Enercon has conducted a comparative yield calculation, using data from two existing wind farm locations, A and B (6km internal spacing) plus data calculated from a third fictional location, F. All were ten-turbine wind farms, sited in similar locations. These wind farms operate under comparable wind speed and site conditions, but with different hub heights. The outcomes, shown in the table below, demonstrate the benefits of greater hub heights compared with a relatively modest rotor-swept growth area.

The energy yields achieved grew with the hub height: 100%, 140% and 190% (A > B > F). Rotor-swept area also increased from 100% > 103% > 133%. It should be mentioned that the E-70 and E-82 models feature Enercon’s high-yielding rotor blades. 

For maximum yield, the rotor diameter as well as the hub height should be optimised according to the specific site conditions.

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