Concrete Wind Turbine Tower Construction

Utilizing concrete in wind turbine tower construction is becoming more prevalent as installations are drastically increasing in scale, from rotor size to hub height. Scale increases are due to an increasing market for less turbulent and stronger wind resources, leading to improved capacity factors (Lantz, 2017).  For 2018, the average rotor diameter of a turbine was 118m, with a corresponding average hub height of 135m (Keiler & Hauser, 2018).  In response, supporting structures such as the turbine tower must be designed to handle the increased loads and stresses, generally leading to larger towers as well. Steel monopole towers are facing limitations dictated by transportation requirements, such as road weight limits and height restrictions, creating the opportunity for concrete towers to become a viable option (Von Der Haar & Marx, 2015). This increasingly popular tower design is a result of the numerous benefits offered by this material selection including weight, stiffness, and sound transmission (Stewart, Lesson 6, 2018).

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This paper aims to explore benefits and detriments of concrete tower construction (both precast and cast-in-place) construction for onshore wind turbines as compared to traditional steel monopoles. An overview of construction and broad limitations will be provided before factors such as cost, longevity, and structural differences are examined. These comparisons will be used to determine where and when use of a concrete tower might be preferable to a steel monopole. Other impacts of a concrete tower, specifically social and environmental effects will be reviewed.  In addition to reviewing the efficacy of concrete towers, a newer concrete-steel hybrid construction that is still in the experimental stages will be briefly discussed (Pons, 2017). As this is not a widely used (there are some companies and countries testing this hybrid tower) or studied design, it is not fully compared to concrete or steel towers.

Construction and Limitations:

Primarily, wind turbine towers are constructed with steel, as either freestanding monopoles and lattices with concrete foundations, or guyed towers (for smaller turbines). For standard steel monopoles, tower heights typically range from 60-120m, though transportation difficulties create practical and economic limitations of approximately 80m or less (Pons, 2017). Concrete towers, whether assembled on-site or precast, have similar height ranges, from 60-115m for the former on-site assembly to 80-120 for the latter precast towers (Pons 2017).  Hybrid towers consist of a bottom portion constructed from concrete, and a top portion made of steel (Von Der Haar & Marx, 2015).   However, hybrid towers can have hub heights far greater than their steel or concrete counterparts, ranging from 80-146m (Pons, 2017). Steel monopole, precast, and hybrid tower designs have similar base diameters, from 3-5m while cast-in place concrete towers have a larger base diameter range between 5-8m.

Steel monopole towers are constructed offsite as rolled sheets of steel formed into conical sections, typically in 20-30m segments (Stewart, Lesson 1, 2018). These segments are then nested to minimize vehicles needed for transportation to the site, where the tower will be assembled using post-tensioning tendons (Von Der Haar & Marx, 2015).  Similarly, precast towers are formed in sections at an off-site plant, transported, and then assembled on site. However, unlike steel monopole sections which are produced in limited locations, plants for construction of precast sections can be established closer to a wind farm (Sritharan, 2015). Cast-in-place towers utilize an adjustable climbing framework that allows for continuous and swift construction (Von Der Haar & Marx, 2015).

Structural Comparison:

Primary structural concerns for all towers include designing against buckling in compression, strength under fatigue loading, and tower stiffness as related to natural frequency (Burton, 2011).  Buckling is a major concern for cylinders in axial compression, where weak points are introduced at tower joints. Beyond that, a wind turbine tower must be designed with the strength to withstand fatigue loads primarily caused by bending stresses (Burton, 2011). Finally, the natural frequency of a tower plays in the fatigue experienced by a tower as well. The closer a tower’s natural frequency to the excitation frequency of the rotor, or passing frequency of the blades, the higher the fatigue loads will be due to resonance.  Within this review, it is difficult to determine which load type is more critical than the other, as additional factors such as the turbine type (pitch or stall regulated) and project siting may create conditions where one load type will merit greater consideration (Burton, 2011). Hybrid towers are not reviewed within this section, as data regarding the performance of a combined system was not located.  

Buckling in an axially compressed thin-walled cylinder occurs if stored membrane strain energy is transferred into bending energy (Sun, 1995).  Given an imperfect structure, such as a jointed wind turbine tower, initial deformations may occur slowly, followed by a sudden “snap-through” to a post-buckle state after the maximum load value of the material is reached (Sun, 1995).  Within a traditional steel monopole tower, the weakest load-bearing portion are the tower joints.  The critical buckling stress for a typical steel monopole tower is dependent on the strength of the welds. In turn, the strength of the welds is dependent on the fabrication quality (Burton, 2011). There are similar concerns with tower joints in precast concrete towers, though use of post-tensioning cables and a joint mortar with a higher tensile strength minimizes risk of failure occurring in the joints (Von Der Haar & Marx, 2015)(Paredes, 2011). Construction of a cast-in-place tower also eliminates joint buckling concerns, as this method eliminates joints in the structure and improves compressive strength through post-tensioning (Von Der Haar & Marx, 2015).

Due to the repeated and inconsistent loading experienced by a wind turbine tower, another major concern in design is the tower fatigue load. Fatigue loads are aggravated by a tower’s deflection. Steel’s residual fatigue strength is interpreted as unlimited beyond an initial cutoff limit reached in the first months of a turbine life (Harte, 2007).  Observation of the behavior of concrete wind turbine towers exposed to high-cycle loads appears to be lacking, and it is theoretically assumed that there is no cutoff limit for concrete (Harte, 2007).  However, concerns remain with fatigue and damage predictions in concrete towers, as steel can assume a linear damage accumulation relationship, while concrete cannot (Paredes, 2011). If tower damage occurs in the form of cracks, concrete tower durability is compromised and may affect the expected life of the turbine (Harte, 2007).  To combat this, lightweight engineered cementitious composite suited for application to concrete wind tower exteriors is being developed (Jin, 2019). This material has the benefit of high tensile strength and ductility, as well as controlled and predicable cracking patterns that minimize corrosion of interior structural material (Jin, 2019). 

Finally, as noted earlier in this section, natural frequency of a tower is a serious design factor that becomes a larger issue as tower heights increase (Find Source!). The wider base of a concrete tower allows for greater tower stiffness when compared with either steel or hybrid towers.  Due to the increased stiffness and mass density of a concrete tower as compared to a steel monopole, concrete towers have higher natural frequency, further away from expected frequencies caused by the rotor or passing blades.

Longevity Comparison

While a number of other components may experience failure before the wind tower itself, a steel wind tower is expected to have a useful life of 20-25 years (Sritharan, 2015).  Due to benefits of minimal deflection, briefly touched upon in the fatigue portion of the structural comparison, modeling of concrete towers has indicated that the useful life may be significantly higher, whether precast or cast in place (Sritharan, 2015) (Rycroft, 2017). These assumptions regarding longevity of concrete towers is promising, though longevity may also be severely limited by tower cracking as described below:

“While concrete make tall wind turbine towers feasible, its tendency of cracking raises durability concerns [6,7]. Cracks on the concrete cover provide pathways for aggressive chemicals such as chlorides to reach the steel reinforcement and accelerate concrete deterioration [8–10]. As a result, both the durability and long-term performance of a tall concrete wind turbine tower can be compromised, and expensive maintenance and undesirable downtime are required. These concerns are exacerbated by flexural and fatigue loading, and especially if the tower is located in an aggressive environment such as a coastal region for capturing high-speed wind.” (Jin, Page 87, 2019).

Economic Comparison:

Unlike certain components that benefit from the economies of scale associated with larger turbine systems, wind turbine tower costs have been shown to increase significantly with size and are predicted to become a larger share of wind turbine costs as sizes increase (Lantz, 2017), (Ashuri, 2016).

Table 1: Cost Share for the Optimized 5, 10, and 20 MW turbines (assembled from data in figure 14 of Multidisciplinary design optimization of large wind turbines— Technical, economic, and design challenges, page 66, Ashuri, 2016).

 Despite this positively correlated relationship between cost and tower size, improvements in tower design such as the concrete or hybrid tower design present an opportunity to reduce these cost increases. Increased stiffness and weight of a concrete tower reduce foundation stiffness requirements, as well as eliminating foundation-to-tower interfaces, lowering the design complexity and cost of the tower foundation (Rycroft, 2017).  Additionally, steel prices have generally risen sharply since the early 2000’s, causing a 38% increase in raw material costs for a standard steel monopole tower, on top of the additional material that is required to construct a larger tower (Lantz, 2017). The figure below highlights a cost comparison for the raw materials of a wind turbine tower, if designed as a steel monopole or steel and concrete hybrid.

Figure 1: Evolution of Cost of Material for Towers – Traditional Steel Monopole versus Hybrid Tower (Reproduced from “Trends, Opportunities, and Challenges for Tall Wind Turbine and Tower Technologies”, Lantz, 2017)

Beyond the increasing raw material cost of steel for this type of tower, a large portion of the total cost of the tower may be comprised of the transportation costs, particularly for remote or undeveloped locations (Rycroft, 2017).  Over the 80m tower height, transportation and assembly costs become prohibitive for steel towers, while precast towers must also contend with road weight restrictions (Pons, 2017). Though precast concrete also has associated transportation and assembly costs, the ability to create a plant close to the site may prove to be more economical than transporting steel sections (Sritharan, 2015).  Cast-in-place concrete towers have low transportation costs, as only components are brought to a site, versus unwieldy assemblies (Rycroft, 2017).   However, a comparison must be made as to the cost of setting up a concrete plant on-site versus transportation costs of preassembled tower pieces (Stewart, Lesson 6, 2018)

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However, as noted by figure 1, other factors must be considered in tower costs, such as construction and maintenance. Both steel monopoles and precast towers may be assembled fairly quickly, though cast-in-place towers require a longer construction period that is dependent on-site weather conditions. The proposed engineered cementitious concrete detailed in the structural comparison has several economic advantages in addition to structural benefits for cast-in-place towers. This material may be precast and utilized as the mold in which a cast-in-place tower is formed, and the added material strength reduces the amount of steel reinforcement needed for crack control, as well as maintenance costs (Jin, 2019).

Non-Economic Impact Comparisons:

Beyond basic design compliance requirements and economic considerations, secondary impacts, such as those on the society and environment must be considered. Issues with visual impacts (from tower shadowing and flickering, to scenic view disruption) noise, and habitat interruption are considered. Broadly speaking, wind turbine has a generally low environmental impact of all wind energy when compared to other forms of generation, from non-renewable and other renewable sources.  Direct comparison of the three main tower types reviewed (steel monopole, cast-in-place concrete, and precast concrete) proved to be difficult, as little literature beyond steel monopole and lattice towers seems to exist on the topic.

Wind turbines have been shown to interfere with radar and telecommunication facilities, and generators also generate electrical and magnetic fields (Saidur, 2011). While not located in current research, investigation into the impact of non-metallic towers on generated fields might be worthwhile.

From a social perspective, both steel and concrete monopile towers have a larger tower shadow effect than a lattice structure. However, concrete towers have greater noise dampening properties than steel monopoles, leading for the potential for wind farm locations sited closer to populated areas (Burton, 2011), (Rycroft, 2017).   Additionally, the natural color of concrete, a neutral gray, is aesthetically pleasing, and eliminates need for painting as is done in steel monopiles.

Environmentally wind turbines have some of the lowest impacts of all forms of power generation, though much of the total greenhouse gas emissions occur during the transportation and installation stages (Other Source) (Wang, 2019). Utilizing cast-in-place towers may have the potential to cut down on these emissions.  In addition to emissions generated by a wind farm, construction disrupts habitat (Saidur, 2011). Creating an on-site concrete plant will increase the footprint of an installation, providing further habitat disruption that may need to be considered if there are sensitive plant or animal populations in the area. Water use requirements for the production of towers is another important impact that may vary depending on selected tower technology.

Conclusions:

After exploring a brief history of wind turbine trends, this paper conducted a comparison of traditional steel monopole towers, concrete towers, and, to a limited extent, hybrid towers. Structural, economic, and other impacts of tower material construction were explored, though no definitive conclusions can be reached regarding optimal tower technologies. Like wind turbine selection, tower selection is highly dependent upon the location-specific needs. Through investigation of this topic, it is evident that additional research is clearly needed in numerous areas, from long-term concrete or hybrid tower structural performance, to economic and environmental evaluations.

References:

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