Wind towers are currently limited in height by the limits placed on steel towers. Concrete and hybrid towers however, offer the possibility of on-site construction and innovative methods to reach hub heights of 120 m or more.
Conventional steel towers used with wind turbines are limited by various factors to a hub height of approximately 80 m. Wind speed increases with height, and larger hub height also allows larger blade size, leading to a greater swept area and lower cut-in speeds. Increased height also leads to reduction in variability, as cut out speeds are lower for the same energy production. The main advantage of taller wind turbines is that they are exposed to a higher average wind speed and a more constant wind profile over the height of the rotor.
To take advantage of better performance at higher hub heights, tower manufacturers have had to move to different tower construction materials. Concrete towers have been in use for many years for other applications, and the construction technology is well established. Concrete microwave and radio transmitter masts are a common sight in this country, and have proven to be durable. Using concrete towers for wind turbines poses different challenges from a load and stress point of view, but the challenge has been surmounted. Cost is a major factor for renewable energy, but the industry has been able to produce designs whose overall cost is not much more than steel towers, and in many cases may even be lower .One consequence of using taller wind towers is the need to increase the structural strength and stiffness required to carry both increased turbine weight and bending forces under wind action on the rotors and the tower, and to avoid damaging resonance from excitation by forcing frequencies associated with the rotor and blades passing the tower. This, in turn, may require the tower to have a larger cross sectional diameter.
Advantages of concrete towers over steel
Transport and on-site manufacture assembly
The biggest advantage of concrete towers over steel towers is in manufacture and transport. Concrete segments and sections can be manufactured on site or at a location near to the site (in the nearest town for instance). Normal reinforced concrete techniques, using shutters or moulds are used to manufacture the components, and machinery and moulds can be moved from site to site. Transportation cost of tower segments can account for a large percentage of the total cost of the tower, particularly when wind sites are remote or served by poor transportation networks. Companies regularly use mobile precast concrete factories for the production of tower segments. This has two advantages: firstly, transportation cost is reduced significantly, and secondly local labour can be used. Construction involves transporting the component materials to the manufacturing site, rather than assembled parts.
The sizes of the larger steel sections, which are transported by heavy-load vehicles, started to exceed the road transport limitations, especially the clearance height of bridges, so that concrete towers became a more attractive alternative for the latest wind turbine support structures. The width of the base sections increases as height increases, and limitations on transport restricts the diameter to approximately 5 m, which equates to an 80 m high mast .
The deflection on a steel tower of 100 m is nearly 1000 mm. The deflection of a comparable concrete precast tower is about 100 mm. Over time, deflection increases stress and fatigue on the structure, ultimately requiring replacement or repair .
Over time, painted steel towers will deteriorate and begin to rust. At that point they will need to be repainted, which is expensive. Precast towers are made with integral colour and a maintenance free exterior. The addition of photocatalytic cement can make the concrete self-cleaning by causing the tower surface to interact with sunlight to break down organic matter .Vibration damping
Because of the thickness and rigidity of precast concrete, concrete towers greatly reduce vibrations when compared to steel towers. Higher vibration levels are hard on the equipment, and the nuisance noise levels created are a major issue, often proving to limit where wind farms can be placed. Precast towers will enable wind turbines to be located closer to cities and towns and increase their overall viability. The combination of reduced deflection and lower vibration allow the internal mechanical design to be simplified .
Design and construction flexibility
Concrete’s versatility enables design solutions with no restriction on height or size to meet challenges influenced by site conditions and accessibility. Designs can be adapted to both in-situ and precast construction methods and offer a wide range of construction flexibility to suit site conditions, availability of specialised plant and labour and other local or market circumstances.
The increased stiffness and reduced vibration of concrete towers greatly reduces the stiffness requirements of the foundation. This helps to reduce uncertainties associated with the deformation of the ground and allows significant savings in the foundation, particularly in places with soft ground. The connection with the foundation is made without interfaces; it’s simpler, cheaper, and more reliable. The great weight of the tower makes it more stable and allows for a significant reduction of the foundation, making it more economical .
Concrete tower configurations
Concrete towers are constructed of segments which may be conical or circular in shape. The segments may be cast in one piece or may be assembled from precast panels. Tower segments may also be cast in situ. Typical wall thicknesses are between 250 and 350 mm.
Configurations and size of segments are determined by the limitations of lifting gear on site. Ideally the optimum size of crane for erection should be determined by the nacelle weight, which is typically in the range of 70 to 150 t depending on the turbine rating. These limitations may be resolved by using in-situ concrete techniques or precast concrete solutions that lend themselves to being split into a greater number of segments, or combinations of the two. While an increased number of segments might result in an increased number of transport movements and some increase in construction time, this approach may offer a cost-effective solution to site constraint challenges .
The tower is assembled from tapered cone shaped segments which are either cast as whole segments or assembled on site from panels. Conical segments are assembled from precast panels to form a unit. Three types of precast construction are used:
One advantage of the smaller sections is that they can be transported on normal trucks and lifted into place by relatively small cranes. The large number of segments significantly increases the number of joints which in turn increases the cost. The choice between large and small precast segments will be strongly influenced by the specific site.Fig. 1 shows a long conical section tower being assembled. Fig. 2 illustrates the construction sequence for a short conical section tower.
Precast step segmented ring
Segments are cast as rings of various sizes and the tower assembled as steps of rings of the same size. The height of the rings is limited by transport requirements. A typical ring-type tower would consist of more than 30 rings. Ring segments are also sometimes cast in a single piece on site where conditions allow this.
Cast in place towers
This method uses ring type segments which are cast in place on the tower, the shutter moving upwards as the tower grows.
Double walled segments
A team at TU Wien (a technical university in Vienna) has developed a new tower-construction method which combines the key benefits of the existing methods. Double-wall elements are initially joined together on the ground to form large double-walled concrete rings. These rings are then lifted one on top of the other and finally filled with concrete. The construction of tall wind turbine towers is said to be faster and less expensive using this technique.
Towers consisting of a concrete lower section topped with a steel upper section are in use and have proven to be useful in many circumstances. The use of smaller steel sections overcomes the transport limitations of fully steel constructions. The lower concrete section consists of a standard concrete design with a height of up to 100 m and topped by two or more steel sections with heights of up to 60 m. The design combines the best properties of concrete and steel constructions .
Pre- and post-tensioning
The required strength and stiffness is assured by pre- and post-tensioning using cables which may be external to the concrete, or run within the concrete structure. The use of tensioning ensures that the compressive strength of concrete is used to the optimum extent.
In general the construction cost of a concrete tower will be higher than that of a steel tower. This is offset by the higher wind production, and longer life of the concrete structure. Overall costs for concrete towers exceeding 80 m in height are claimed to be lower than steel towers of the same height, if all factors are taken into account.Case study: Gouda wind farm
Forty-six precast concrete towers were deployed for the construction of one of South Africa’s largest wind farms, situated on farmland in the Gouda district of the Western Cape. Each tower is 100 m tall and supports a 3 MW turbine. The towers were manufactured according to a specification set by Acciona, a renewable energy company, by Cape Town-based precast concrete producer Concrete Units, in a joint venture with Windtechnic, a specialist engineering company, and Concrete Growth, an engineering company which specialises in concrete materials.
The towers are constructed from five 20 m long cylindrical pre-assembled units. Final assembly includes placement onto an in-situ foundation and post-tensioning. There are 17 individual panels per tower. Five re-useable moulds were used to cast 782 panels. Each segment weighs 60 t. Segments consist of between two and four panels depending on the tower’s diameter. The towers were constructed as follows:
Precasting of the panels took place over ten months and used 16 500 m3 of concrete.
 A Bromage and A Tricklebank: “Concrete Towers for Onshore and Offshore Wind Farms”, www.concretecentre.com/Publications-Software/Publications/Concrete-Towers-for-Onshore-and-Offshore-Wind-Farm.aspx
 J Jimeno: “Concrete Towers for Multi-Megawatt Turbines”, www.windsystemsmag.com/article/detail/334/concrete-towers-for-multi-megawatt-turbines
 S Gouws: “Concrete Towers – a business case for sustained local investment”, Concrete growth, www.slideshare.net/SantieGouws/concrete-towers-a-business-case-for-sustained-investmentrev-5
 M Froese: “Nordex installs the world’s tallest wind turbine”, Windpower engineering, 28 June 2016, www.windpowerengineering.com/featured/business-news-projects/nordex-installs-worlds-tallest-wind-turbine/
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