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A comparative life-cycle analysis of tall onshore steel wind-turbine towers

A comparative life-cycle analysis of tall onshore steel wind-turbine towers Earth has lately been suffering from unforeseen catastrophic phenomena related to the consequences of the greenhouse effect. It is therefore essential not only that sustainability criteria be incorporated into the everyday lifestyle, but also that energy-saving procedures be enhanced. According to the number of wind farms installed annually, wind energy is among the most promising sustainable-energy sources. Taking into account the last statement for energy-saving methods, it is essential to value the contribution of wind energy not only in eliminating CO emissions when producing electricity from wind, but also in assessing the total environmental impact associated with the entire lifetime of all the processes related with this energy-production chain. In order to quantify such environmental impacts, life-cycle analysis (LCA) is performed. As a matter of fact, there are a very limited number of studies devoted to LCA of onshore wind-energy-converter supporting towers—a fact that constitutes a first-class opportunity to perform high-end research. In the present work, the life-cycle performance of two types of tall onshore wind-turbine towers has been investigated: a lattice tower and a tubular one. For comparison reasons, both tower configurations have been designed to sustain the same loads, although they have been manufactured by different production methods, different amounts of material were used and different mounting procedures have been applied; all the aforementioned items diversify in their overall life-cycle performance as well as their performance in all LCA phases examined separately. The life-cycle performance of the two different wind-turbine-tower systems is calculated with the use of efficient open LCA software and valuable conclusions have been drawn when combining structural and LCA results in terms of comparing alternative configurations of the supporting systems for wind-energy converters. Keywords: energy and environment; energy system and policy; wind energy in carbon dioxide release, primarily from fossil-fuels com- Introduction bustion, increasing concern on cost and security issues re- Some of the most catastrophic events recently have been lated to fossil-based energy has been observed [12 , ]. This associated with climate change due to global warming and has led to the exponential growth of renewable-energy consequences of the greenhouse effect. One of the pri- sources as an alternative to fossil fuels. Renewables being mary reasons for global warming is the excessive emission free of CO emissions are considered ideal for eliminating of CO combined with the parallel increase in energy de- greenhouse-effect consequences and limiting water and mand. Due to the fact that energy reports show an increase Received: 2 August 2019; Accepted: 5 October 2019 © The Author(s) 2019. Published by Oxford University Press on behalf of National Institute of Clean-and-Low-Carbon Energy This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/li- censes/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properl 1 y cited. For commercial re-use, please contact journals.permissions@oup.com Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz028/5639689 by DeepDyve user on 03 December 2019 2 | Clean Energy, 2019, Vol. XX, No. XX air contamination. Global energy demand is continuously different products, Ardente et  al. [10] investigated the air growing and renewable-energy production is becoming and water emissions of a wind farm and compared these more important than ever. This need is reflected in the results with the emissions of other energy-generation contemporary European Commission Directive, which systems. Several research teams have focused on the in- sets the goal of at least 27% total energy consumption vestigation of LCA for wind-generation systems. The LCA coming from renewables by 2030 [3]. Due to its nearly in- results for wind-turbine towers are commonly assessed by finite nature and great potential, wind is considered the calculating various environmental indicators (e.g. global- most promising renewable-energy source, holding second warming potential (GWP), acidification potential (AP), eu- place in the power-generation capacity installed in Europe trophication potential) and the energy-payback time. The in 2018. Today, wind energy accounts for 18.8% of the EU’s energy-payback time is conceived of as the time for which total installed power-generation capacity [4] and, in 2019, a wind-energy system must operate in order to generate it is predicted to overtake natural gas, which is in first pos- the amount of energy that was required for the entire life ition. While conventional power sources (fossil fuel, oil, of the structure, i.e. from production to dismantling. This coal) are expected to decommission more capacity than payback time is calculated as the ratio of the total primary they install, wind power has installed more capacity than energy requirements of the system throughout its life cycle any other form of power generation in the EU in 2018, ac- over the total annual power produced by it. In the majority counting for 48% of total power-capacity installations. Its of wind-turbine LCA cases, a lifetime of 20 years has been expansion in the last decade has been remarkable, trip- taken into consideration and the analyses are in compli- ling in power-generation capacity and, more specific- ance with ISO 14040 [11] and ISO 14044 [12]. In almost all ally, from ~66 GW in 2008 to 189 GW in 2018 according to the cases investigated in the literature, the energy-payback the European annual statistics [4]. When calculating the indexes for wind turbines are calculated to be lower than total environmental impact of power-generation installa-1 year [13, 14]. tions, one should not only take into account the operation LCA results are usually presented in percentage charts stage where renewables are advantageously emitting al- and are commonly grouped either per structural compo- most zero carbon dioxide, but also their manufacture, nent (e.g. foundation, tower, nacelle, rotor) or per life phase transportation, installation and dismantling stages. It is (e.g. manufacture, transport, erection, operation/mainten- recorded that, for renewable-energy-production installa- ance, dismantling). The highest environmental impacts in tions, the majority of their environmental impact results all cases investigated are detected in the manufacturing from the manufacture and installation processes [5]. Since stage of wind turbines followed by the transport phase all forms of energy generation are based on the conv-er [15]. The smallest impact is attributed to the operation sion of natural-resource inputs, there are subsequent en- stage of the turbines [16]. Thorough scientific work has vironmental impacts. When decisions for energy-system been conducted on the sustainability assessment of steel investment, planning and developing are made, it has to construction focused on offshore wind turbines [17–19]. be ensured that all aspects are taken into account during The assessment of energy and emissions related to the the assessment and comparison of alternative solutions production and manufacture of materials related to an off- [6]. Life-cycle analysis (LCA) is a holistic methodology that shore wind farm using an LCA model has been performed can be used as a tool in detecting these potential en-vir in the work of Schleisner [20]. Tremeac and Meunier [21] onmental impacts associated with energy systems and drew some valuable conclusions regarding the environ- in calculating their sustainability performance from their mental impact of wind energy by comparing payback early development stages [7]. In this methodology, final time and CO emissions of turbines with different power- products are examined and assessed in terms of their en- generation capacities, meaning a large 4.5-MW and a small vironmental impact all the way through their life cycle, 250-W wind turbine. The size of the wind turbines, though, from raw-material extraction until end of life [8, 9]. does not appear to be a decisive factor in optimizing their Wind farms as investments for energy production with life-cycle energy performance and the embodied energy high economical impact are usually assessed in terms component of wind turbines over their service life [22]. In of safety and robustness only, meaning that they are de- the case study presented herein, steel towers are under in- signed in order to withstand the wind loads in all phases vestigation. Although the amount of steel required for the from construction to operation and extreme wind circum- construction of wind turbines is great, the component with stances. Even when environmental-impact analyses were the highest environmental impact is the tower foundation, conducted for wind-power generators, the methodologies because the potential recycling or reusing of steel compo- deployed would take into account only a limited number nents can lead to reduced environmental impact and, with of life-cycle steps. LCA, being a holistic methodology, is contemporary technologies, almost 80% of a wind-power- capable of investigating and quantifying both direct and generation system can be recycled—practically everything indirect environmental impacts, taking into account all except the concrete foundation and the composite blades. the life-cycle steps of products and services. Using LCA’s Not only the power-generation capacity of a wind tur - advantages in comparing the environmental impact of bine can be considered responsible for producing different Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz028/5639689 by DeepDyve user on 03 December 2019 Stavridou et al. | 3 results, but also the LCA methods used, even for the same sense of erection without external cranes) that consists structure [23]. Martinez et al. [24 25 , ] investigated the de- of a new design of cross-sections that have been particu- pendency of results on the impact-assessment method- larly optimized to minimize the material use [32]. These ology implemented by conducting two different studies of self-rising lattice towers efficiently combine steel parts, al- the same turbine using the Eco-indicator 99 and the CML lowing them to fulfil the required safety, robustness and methodology. Browsing through the literature, it is difficult durability requirements, while keeping the solution poten- to compare LCA results of different wind turbines where tially economical and environmentally sustainable. different methodologies have been implemented, because Due to the fact that energy demands are constantly of the discrepancy of results among methodologies even increasing, contemporary installed wind turbines need to when investigating the same structure. have increased efficiency and power-generation capacity. The objective of the work presented in the current study In order to achieve this increased power-generation cap- is to perform a comparative LCA for two potential wind tur - acity, wind turbines are constructed using longer blades bines to be deployed in a wind park located in the UK. The and greater tower-hub height in order to take advantage of two potential wind-power generators carry the same wind the smoother wind flow combined with higher wind vel- turbine at their top: the Repower MM92 [26]. In Table 1, a ocities at greater heights. All alternative solutions, with number of LCAs for onshore wind-turbine towers around either the use of internal stiffening or the lattice con- the globe have been grouped, where the type of tower, the figuration, have been studied in terms of structural per - hub height, the wind-turbine size and the LCA software formance and have been proven to be robust enough to used are presented. One can easily note that the majority sustain the greater loads due to higher wind velocities and of studies have focused on tubular-steel or concrete towers the greater nacelle mass due to bigger rotors and longer with a hub height up to 124 m, leaving the environmental blades. The environmental impact of classic tubular-steel impacts of tall steel-lattice towers almost unexplored. wind-turbine towers increases exponentially, since both For onshore wind farms, the horizontal-axis wind tur - the amount of steel and the size of the tower foundation bines are the prevailing structural configuration, where increase. It is therefore very interesting to compare the the tower consists of cylindrical parts interconnected environmental impact of the tubular-tower solution with with bolted flanges by means of pre-stressed bolts [29]. the proposed lattice one since the innovative erection Although cylindrical shells have great advantages in terms approach leads also to energy saving and can result in a of load-bearing capacity to shell-thickness ratio, when solution that goes far beyond the decrease in the envir - getting more slender, local buckling phenomena can be onmental impact deriving from the minimization of ma- catastrophic; therefore, an increase in their thickness is in terial used. For onshore wind-turbine towers, the life-cycle most cases unavoidable. As an alternative solution to the stages usually taken into account are the following: manu- existing cylindrical-tower configuration, the implementa- facture, transportation, construction/erection, operation tion of internal stiffening of tubular wind-turbine towers and dismantling. When assessing the environmental im- has been the focus in the work of several research groups pacts of the various stages, the manufacturing stage is by [30, 31]. Although the solution of internal stiffening has far the one with the highest environmental impact, with been proved advantageous in terms of material use and the transportation stage following in second place. concurrent structural enhancement, the previous work of Since the tower and foundation appear to be the wind- the current research group has focused on the elaboration turbine components with the highest environmental im- of an alternative tower configuration that can permit wind pact, the present work focuses on the investigation of the turbines to reach even greater heights with less steel use environmental impact of two alternative tower configur - and smaller-scale foundations too. This new tower config- ations: one tubular and one lattice. The different tower uration is a self-rising lattice-tower configuration (in the configurations require different foundations, so valuable Table 1 LCA overview of onshore wind-turbine towers Publication year Tower type Hub height (m) Capacity (MW) Software 2008 [10] Tubular steel 55 0.66 – 2008 [13] Tubular steel 45, 46, 60 0.66, 0.60, 1.75 – 2009 [18] Tubular concrete 124 4.50, 0.25 Sima Pro 2009 [19] Tubular steel 60, 80 0.85, 3.0 – 2009 [21] Tubular steel 70 2.0 Sima Pro 2012 [16] Tubular steel 105, 65 1.8, 2.0 GEMIS 2013 [15] Tubular steel/concrete 80 1.4 – 2014 [27] Tubular steel/concrete 80, 100, 150 2.0, 3.6, 5.0 Gabi 2015 [28] Tubular steel 80 2.0 Gabi 2016 [14] Tubular steel 92.5 2.3, 3.2 Sima Pro Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz028/5639689 by DeepDyve user on 03 December 2019 4 | Clean Energy, 2019, Vol. XX, No. XX Table 2 Lattice-tower sections details Legs V-brace diagonals V-brace horizontals Height (m) Diameter (mm) Thickness (mm) Diameter (mm) Thickness (mm) Diameter (mm) Thickness (mm) Part-1 34.45 411 8 413 7 342 6 Part-2 55.53 371 8 385 7 282 5 Part-3 68.19 352 7 375 7 240 5 Part-4 75.64 340 7 363 7 216 4 Part-5 76.15 286 5 253 5 214 4 conclusions of the two most important tower parts are duration of a turbine’s lifetime is assumed to be 20 years. drawn. First, the scope of the study is presented, sup- The LCA results of the wind turbines are assessed in terms porting life-cycle inventory data are reported and the re- of various environmental factors like GWP, AP, the energy- sults of the life-cycle-impact assessment are discussed. In payback time, etc. The calculations are usually performed order for the results to be comparable, the structures share per lifetime stage and for all the structural components the same height and have the same loading being applied independently. The software and databases for performing at the hub height. Both tower configurations are designed LCA are various and, in the present study, GEMIS (Global in a manner to be capable of sustaining the same loads Emission Model for Integrated Systems) open-source soft- as proved in previous works. The analysis methodologies ware is selected due to its focus on construction, energy implemented are identical while the lattice tower has 35% and transport fields [35]. less steel, an almost 33% lighter foundation and many ad- vantages in terms of transportation and erection. 2 Tower models After having studied various cases of LCA on onshore wind- 1 Methodology for LCA turbine towers, in the present study, the life-cycle perform- LCA is a useful methodology for determining the total ance of two 76.16-m-tall wind-turbine towers is carried system impacts of a given technology and is realized by out. The investigation of tall wind turbines is here ex- associating all environmental impacts with the material-plored, whilst the investigation of super-tall wind turbines acquisition, processing, manufacturing, use and disposal or (e.g. greater than 150 m) is currently underway. Unlike the recycling at the end-of-life stage. This approach is valuable various investigations related to the LCAs of wind turbines, towards the sustainable design of systems and is there- the present study focuses on the tower configuration only, fore used by both policy makers and industrial partners for taking for granted that the nacelle and blades are identical product development and the management of sustainable in both turbine cases. To this end, the life-cycle perform- systems. In principle, when conducting LCA for systems or ance of two towers—one tubular tower and one lattice— products, the steps that have to be followed are: has been investigated. All life stages from production of the raw materials to the end of life have been taken into (i) the definitions of system boundaries, requirements account under the assumption of a 20-year lifetime. and assumptions; The steel parts of both towers are made of steel class (ii) the collection of resources for all system inputs and S355 and the foundation is assumed to be made from con- outputs; ventional concrete. The nacelle and blades that are sup- (iii) the definition of the parameters used to evaluate posed to be accommodated on both towers are identical and the environmental impacts related to the inputs and the structural analyses for both towers have been already outputs; verified. The lifetime stages taken into account are iden- (iv) the assessment of the results; in order to perform an tical for both towers: manufacturing, transportation and LCA, thorough research has to be performed in order erection, operation and dismantling. For the dismantling to identify the factors that contribute to the environ- stage, two different scenarios are investigated for each ma- mental impact of the system; for its calculation, a se- terial: the recycle/reuse scenario and the landfill scenario. ries of LCA software, tools and databases can be used In the manufacturing stage, both the production of the in accordance with the global or European standards raw materials and the energy consumed for their fabrica- that govern the sustainability-assessment procedures tion are taken into account. For the transportation stage, [33, 34]. it was assumed that the lattice-tower subparts have been As far as the wind-turbine towers are concerned, the stages produced in the factory 100 km away from the construc- that are usually taken into account when performing LCA tion site and transported there. On the other hand, for the are the manufacturing stage, the transportation, the erec- tubular subparts, the assumption was made that they are tion/construction, operation and dismantling, while the produced in north Germany in 30-m-long parts and they Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz028/5639689 by DeepDyve user on 03 December 2019 Stavridou et al. | 5 are transported on site by ship and truck. For the erection stage, large-scale cranes are used for the mounting of the tubular tower, whilst, for the lattice one, only small-scale cranes are used, since the tower can be erected by the pre- viously mentioned innovative self-rising approach without using tall cranes. For the dismantling stage, the materials are recycled when possible; otherwise, the landfill scenario was implemented. Further details are presented later on. 2.1 Tubular tower The tubular tower is 76.15 m tall and consists of three subparts of 21.8, 26.6 and 27.8 m from bottom to top. The 15 tower under investigation is an actually constructed tower [36]. The subparts are fabricated in the factory by hot rolling steel plates of varying thicknesses and forming rings about 3 m wide. The rolled plates are welded longitudinally to form 3-m rings connected to each other by means of cir - cumferential welds. The final tower subparts are trans- ported on site and are connected to each other with the aid of flanges by means of pre-stressed bolts. For tubular towers, the use of large-scale cranes and heavy machinery is mandatory, since the subparts are quite long and heavy. 20 The tower is not purely cylindrical, as the lower diameter of the cross-section of the tower is 4.3 m and the top one is 3 m. The thickness of the shell wall is also not constant, starting from 12 mm at the top to 30 mm at the bottom. The tower is embedded into a reinforced-concrete foundation that is anchored to the ground and, therefore, the tower can be considered and modelled as fixed at the foundation. The shell-thickness distribution along the height of the tower is presented in Fig. 1. The total tower weight is 127 t. 2.2 Lattice tower Fig. 1 Tubular-tower shell-thickness distribution The lattice tower is of square base shape consisting of five subparts along its height. The heights of the various using the freely available software GEMIS [30]. As men- subparts appear in Table 2. The tower is composed of three tioned above, an LCA study is completed in four stages discrete structural sub-systems: the legs, the face-bracing and, more specifically, the following in our case. trusses and horizontal braces and secondary bracings ar - ranged inside the plane of the face-bracing trusses. The 2.3.1 Analysis goal and scope definition connections between the structural members are bolted The goal of the present study is to compare the life-cycle connections with conventional steel bolts. The total tower environmental impacts of two wind-turbine towers. This weight for the lattice tower is 77.47 t and circular hollow investigation contributes to determining and quantifying sections of varying shell thicknesses and diameters are the impacts of a potential wind park located in UK. Both used, as presented in Fig. 2. The tower subparts are manu- onshore wind turbines have a power-generation cap- factured in conventional factories near the construction acity of 2.0 MW. They have similar function and technical site; they are transported on site and erected to their final specifications. However, the tower configurations differ positions with the aid of small-scale cranes. The final in the manner explained in Sections 2.1 and 2.2 of the tower shape and distribution of the steel sections are pre- present paper. sented in in Fig. 2 below. The scope definition of an LCA includes a description of the product under investigation in terms of the system boundaries. In the case study presented here, all life stages 2.3 Research methodology from production of materials to the end of life of the struc- In order to assess the environmental impacts of the two ture are under consideration. The wind-turbine stages in- towers under investigation, the LCA method is applied vestigated are presented in Fig. 3 and are the component 76,15 Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz028/5639689 by DeepDyve user on 03 December 2019 6 | Clean Energy, 2019, Vol. XX, No. XX manufacturing, transportation and erection, operation and Height (m) then dismantling or recycling, depending on the scenario Part-1 34.45 taken into account. Part-2 55.53 The connection to the grid is out of the scope of the Part-3 68.19 present study and has been neglected. The lifetime of the Part-4 75.64 turbines is set to be 20 years. The functional unit must be defined so that alternative solutions can be compared in Part-5 76.15 a meaningful way. The energy-payback comparison takes into account the amount of energy generated over the as- sumed 20-year lifetime. 2.3.2 Life-cycle inventory (LCI) analysis 3D view Wind turbines are complex structures that consist of many structural, mechanical and electrical assemblies, which comprise many sub-components. The most crucial stage in an LCA is data gathering and the maximum de- tail possible should be included so that the accuracy of the obtained results is not sacrificed. The LCI is the gathering of information related to primary components of a wind- turbine tower. For the manufacturing stage of the wind turbine, the two tower systems were analysed as detailed in the previous paragraphs. As far as the turbine itself is concerned, it is a 2.0-MW, three-bladed upwind horizontal- axis wind turbine with a hub height of 76.15 m. The hub and nose cone of the turbine are generally made of cast iron and fibreglass-reinforced polyester, respectively; the blades are made of a composite material consisting of 60% glass fibre and 40% epoxy; the generator is basically made of steel and copper, while the gearbox is made of cast iron and stainless steel. The energy-consumption calculation for all the manufacturing of the wind turbine has been based on data on material component weights available. Fig. 2 Lattice-tower configuration Raw material extraction Manufacturing Transportation & Erection Recycling Landfill Operation Fig. 3 LCA boundaries Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz028/5639689 by DeepDyve user on 03 December 2019 Stavridou et al. | 7 For the transportation and erection stage, the turbine is More specifically, it is a relative scale of how much a green- assumed to be produced in Germany and transported to house gas contributes to global warming and compares it the UK by boat and truck. The lattice-tower components to the same mass of carbon dioxide. Its measure is kgCO / are produced in a factory near the site, whilst the tubular- kWh (CO equivalent) and is used for the assessment of the tower subparts are again produced in Germany and trans- two wind-tower systems. The basic term for assessing the ported to the UK by boat and truck. The distance for the energy-generation part of an LCA for the wind-energy sys- components to reach the port in Germany is estimated to tems studied in this work is the total cumulative energy be 110 km, the distance at sea is estimated to be 1020 km, requirement that contains the overall energy needed to con- while the transportation of the components from the port struct a wind turbine. Finally, the energy-payback-time ratio to the site is assumed to be 130 km. The freight-transport is implemented in the present study in order to measure services are imported in GEMIS software in tkm (ton- the duration of operation needed in order for the wind tur - kilometre(s)) so, if the track and boat transport the tubular-bine to generate the amount of energy required for its entire tower components of 127 t over the distance of 110 km, life. It is calculated in the form of a ratio of total primary en- this equals a transport service of 13 970 tkm. ergy requirements of the wind turbine throughout its entire As far as the operation and maintenance stages are con- life over the total annual energy generated by the turbine. cerned, there is a certain level of energy input required for The total cumulative energy requirements for its entire life starting the turbine, for the break-system operation, for yaw- comprise the energy needed for production, transportation, and rotor-pitch control, etc. This energy input is normally maintenance, operation and decommissioning. estimated as 1% of the total electricity generated by the - tur bine [37]. For both turbines, maintenance of the mechanical 2.3.4 Result interpretation parts is assumed to be carried out three times a year and The LCA results can be obtained from GEMIS software in the distance for the service team is assumed to be 100 km tabular and graphic format. In the next section, the results per trip. The life of the turbines is assumed to be 20 years. As of the LCA are presented. First, the material requirements far as the dismantling and recycling stages are concerned, per structural component are presented in a tabular format. again, there is an energy input taken into account that is The CO equivalent per lifetime stage for the two towers is assumed to be 2% of the total electricity generated [37]. In presented in a graphic form. Then, in a comparative graph, Table 3, the possible recycling scenarios for the main mater - the cumulative energy requirements of the two towers are ials included in the present study are presented. presented with specific data per stage. Finally, the compo- nent contribution to energy demand of the two towers is 2.3.3 Life-cycle impact assessment depicted. The life-cycle inventories for the two 2.0-MW wind-turbine models were used to support the life-cycle assessment in terms of the basic impact-assessment categories. GEMIS 3 Results software is a freely available database used widely in In the present study, the LCA of two onshore steel wind- Europe. It enables a detailed description of all the process turbine-tower configurations is performed. Both towers are steps of an energy system and has been successfully used of 76.15 m height and their structural analysis results have in previous work for the calculation of the primary energy proved that they can both accommodate the same rotor needed in the process, the emissions and the mass and en- with similar efficiency. A study is performed where all life ergy flows of materials. The database includes >1000 prod- stages from the production of the raw materials to the end ucts, 10  000 processes and >130 scenarios covering data of life have been considered. The connection of the tur - from >50 countries, whilst there is freedom for the user to bines to the grid is not examined in the present study, while import additional data. Upon LCA performance, GWP, green- the lifetime of the turbines is set to be 20 years. In Table 4, house gases, water effluents, solid waste and many more the material requirements for the main components of the can be obtained in tabulated or graphic format. The GWP is two towers under investigation are presented. an indicator of the impact of any process on climate change. Table 4 Material requirements for the tower components Table 3 Possible recycling scenarios [38] Tubular tower Lattice tower Material End-of-life treatment Weight Weight Concrete Landfill 100% Component Mass (t) fraction Mass (t) fraction Cast Iron Recycling with 10% loss Copper Recycling with 5% loss Rotor 34 0.04 34 0.07 Epoxy Incinerated 100% Nacelle 55 0.06 55 0.11 Fibreglass Incinerated 100% Tower 127 0.13 77.47 0.15 Plastic Incinerated 100% Foundation 750 0.78 350 0.68 Stainless steel Recycling with 10% loss Total 966 516.47 Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz028/5639689 by DeepDyve user on 03 December 2019 8 | Clean Energy, 2019, Vol. XX, No. XX In Fig. 4, the distribution of the CO emissions per life- Life Cycle Cumulative Energy Requirements 3.50E+05 time stage is presented, where the construction stage with the steel components and the concrete foundation has 3.00E+05 been proved to have the highest environmental impact. It 2.50E+05 is worth mentioning that the manufacturing phase con- 2.00E+05 stitutes 82% of the total equivalent CO emissions of the 2 1.50E+05 tubular tower. 1.00E+05 In Fig. 5, the distribution of the CO emissions per life- 5.00E+04 time stage for the lattice tower is presented. Again, in this 0.00E+00 tower configuration, the highest environmental impact ap- LATTICE TUBULAR pears to derive from the manufacturing stage of the tower. Transportation Manufacturing Dismantling Operation In Fig. 6, the cumulative energy requirements of the two Fig. 6 Life-cycle cumulative energy requirements towers are presented, where it is obvious that the lattice tower has a much lower energy requirement compared to the tubular one. For the tubular tower that can be better compared, the cumulative energy is compatible with reduced foundation required for a lattice structure com- similar 2.0-MW turbines in the work of Guezuraga et  al. pared to a tubular one and the transportation and erec- [16]. This can be attributed to three factors: the smaller tion advantages that the proposed self-rising system is amount of steel used for the tower construction, the offering. This can be better observed in Fig. 7, where the contribution of each wind-turbine component is presented TUBULAR TOWER for the two towers. Transportation Manufacturing Dismantling Operation The largest cumulative energy requirements contribu- tion comes from the manufacturing stage in both the lat- 2% 9% 7% tice and the tubular towers, reaching values between 75% and 82% of the total life cycle of the turbines. In absolute values, though, the manufacturing/construction stage of the tubular tower is much larger compared to that of the lattice one. The average share from each tower compo- nent is shown in Fig. 7, where it is proved that the tower manufacturing and foundation construction is a larger part of the total energy requirement for the tubular tower. In both tower cases, the smallest contribution is derived from the operation phase, which accounts for only 2% of 82% the total energy requirements. An indicative ratio presented in the above text that can better picture the environmental impact of the two tower Fig. 4 Distribution of CO emissions per life stage for the tubular tower configurations is the energy-payback time. The energy generated by both turbines, since they are hypothetically TUBULAR TOWER positioned at the same spot, sharing the same hub height Transportation Manufacturing Dismantling Operation and same rotor, would be for a 2.0-MW turbine ~6.12 GWh assuming a 35% capacity factor as assumed in Haapala and Prempreeda [38]. The results of the payback time for both towers are presented in Table 5. 3% 8% The energy-payback time for the tubular tower is 14% 5–6  months whilst, for the lattice, it has been calculated to 4  months. These results for the tubular tower are pro- portional to similar 2.0-MW turbines [16] and very close to the results of the same capacity and same hub-height turbines [38]. The lattice structure has proved to be more advantageous in terms of both the material used and the transportation and erection methods applied. The steel tower fabrication along with the foundation construction 75% are much less energy-consuming procedures compared to the tubular structure and the energy-payback period is also 15% less. Fig. 5 Distribution of CO emissions per life stage for the lattice tower GWh Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz028/5639689 by DeepDyve user on 03 December 2019 Stavridou et al. | 9 Component Contribution to the total energy demand the energy-payback time. The most impactful stage in the 70% lifetime of a wind turbine is the manufacturing phase. The 60% analysis conducted shows that the lattice structure is 32% less impactful on the environment in terms of equivalent 50% CO emissions. The energy-payback time for the tubular- 40% steel tower is 5–6 months, whilst the lattice self-rising steel 30% tower has an energy payback time of 4 months. The present 20% study shows that only by saving material from the foun- 10% dation and tower, which are the most energy-consuming components of the wind structure, could the equivalent 0% Rotor Nacelle TowerFoundation CO emissions be reduced. This research study aims at LATTICE TUBULAR obtaining an initial approach of the environmental impact of the proposed manufacturing and erection procedure Fig. 7 Distribution of CO emissions per structural component and this is the reason why a tower of 76.15-m hub height was selected, since comparable data were available. After having proved that the new tower system is robust enough Table 5 Energy-payback-time calculation and less environmentally impactful, a further more com- Tubular Lattice prehensive study of taller structures is to follow. Units tower tower Cum. energy requirements GWh 2.96 2.00 Annual energy generated GWh 6.12 6.12 Acknowledgements Energy-payback time – 0.48 0.33 This project has received funding from the European Union’s Energy-payback time Months 5–6 4 Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie Grant Agreement No. 747921 that the au- thors acknowledge with thanks. 4 Conclusions Constantly growing global energy needs require the im- Conflict of Interest plementation of additional power-generation systems worldwide. Fossil-fuel shortage and consequences of the None declared. greenhouse effect have led to greener energy-production methods and wind turbines are among the most advanta- References geous. 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A comparative life-cycle analysis of tall onshore steel wind-turbine towers

Clean Energy , Volume Advance Article – Apr 4, 2020

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© The Author(s) 2019. Published by Oxford University Press on behalf of National Institute of Clean-and-Low-Carbon Energy
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2515-4230
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2515-396X
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10.1093/ce/zkz028
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Abstract

Earth has lately been suffering from unforeseen catastrophic phenomena related to the consequences of the greenhouse effect. It is therefore essential not only that sustainability criteria be incorporated into the everyday lifestyle, but also that energy-saving procedures be enhanced. According to the number of wind farms installed annually, wind energy is among the most promising sustainable-energy sources. Taking into account the last statement for energy-saving methods, it is essential to value the contribution of wind energy not only in eliminating CO emissions when producing electricity from wind, but also in assessing the total environmental impact associated with the entire lifetime of all the processes related with this energy-production chain. In order to quantify such environmental impacts, life-cycle analysis (LCA) is performed. As a matter of fact, there are a very limited number of studies devoted to LCA of onshore wind-energy-converter supporting towers—a fact that constitutes a first-class opportunity to perform high-end research. In the present work, the life-cycle performance of two types of tall onshore wind-turbine towers has been investigated: a lattice tower and a tubular one. For comparison reasons, both tower configurations have been designed to sustain the same loads, although they have been manufactured by different production methods, different amounts of material were used and different mounting procedures have been applied; all the aforementioned items diversify in their overall life-cycle performance as well as their performance in all LCA phases examined separately. The life-cycle performance of the two different wind-turbine-tower systems is calculated with the use of efficient open LCA software and valuable conclusions have been drawn when combining structural and LCA results in terms of comparing alternative configurations of the supporting systems for wind-energy converters. Keywords: energy and environment; energy system and policy; wind energy in carbon dioxide release, primarily from fossil-fuels com- Introduction bustion, increasing concern on cost and security issues re- Some of the most catastrophic events recently have been lated to fossil-based energy has been observed [12 , ]. This associated with climate change due to global warming and has led to the exponential growth of renewable-energy consequences of the greenhouse effect. One of the pri- sources as an alternative to fossil fuels. Renewables being mary reasons for global warming is the excessive emission free of CO emissions are considered ideal for eliminating of CO combined with the parallel increase in energy de- greenhouse-effect consequences and limiting water and mand. Due to the fact that energy reports show an increase Received: 2 August 2019; Accepted: 5 October 2019 © The Author(s) 2019. Published by Oxford University Press on behalf of National Institute of Clean-and-Low-Carbon Energy This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/li- censes/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properl 1 y cited. For commercial re-use, please contact journals.permissions@oup.com Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz028/5639689 by DeepDyve user on 03 December 2019 2 | Clean Energy, 2019, Vol. XX, No. XX air contamination. Global energy demand is continuously different products, Ardente et  al. [10] investigated the air growing and renewable-energy production is becoming and water emissions of a wind farm and compared these more important than ever. This need is reflected in the results with the emissions of other energy-generation contemporary European Commission Directive, which systems. Several research teams have focused on the in- sets the goal of at least 27% total energy consumption vestigation of LCA for wind-generation systems. The LCA coming from renewables by 2030 [3]. Due to its nearly in- results for wind-turbine towers are commonly assessed by finite nature and great potential, wind is considered the calculating various environmental indicators (e.g. global- most promising renewable-energy source, holding second warming potential (GWP), acidification potential (AP), eu- place in the power-generation capacity installed in Europe trophication potential) and the energy-payback time. The in 2018. Today, wind energy accounts for 18.8% of the EU’s energy-payback time is conceived of as the time for which total installed power-generation capacity [4] and, in 2019, a wind-energy system must operate in order to generate it is predicted to overtake natural gas, which is in first pos- the amount of energy that was required for the entire life ition. While conventional power sources (fossil fuel, oil, of the structure, i.e. from production to dismantling. This coal) are expected to decommission more capacity than payback time is calculated as the ratio of the total primary they install, wind power has installed more capacity than energy requirements of the system throughout its life cycle any other form of power generation in the EU in 2018, ac- over the total annual power produced by it. In the majority counting for 48% of total power-capacity installations. Its of wind-turbine LCA cases, a lifetime of 20 years has been expansion in the last decade has been remarkable, trip- taken into consideration and the analyses are in compli- ling in power-generation capacity and, more specific- ance with ISO 14040 [11] and ISO 14044 [12]. In almost all ally, from ~66 GW in 2008 to 189 GW in 2018 according to the cases investigated in the literature, the energy-payback the European annual statistics [4]. When calculating the indexes for wind turbines are calculated to be lower than total environmental impact of power-generation installa-1 year [13, 14]. tions, one should not only take into account the operation LCA results are usually presented in percentage charts stage where renewables are advantageously emitting al- and are commonly grouped either per structural compo- most zero carbon dioxide, but also their manufacture, nent (e.g. foundation, tower, nacelle, rotor) or per life phase transportation, installation and dismantling stages. It is (e.g. manufacture, transport, erection, operation/mainten- recorded that, for renewable-energy-production installa- ance, dismantling). The highest environmental impacts in tions, the majority of their environmental impact results all cases investigated are detected in the manufacturing from the manufacture and installation processes [5]. Since stage of wind turbines followed by the transport phase all forms of energy generation are based on the conv-er [15]. The smallest impact is attributed to the operation sion of natural-resource inputs, there are subsequent en- stage of the turbines [16]. Thorough scientific work has vironmental impacts. When decisions for energy-system been conducted on the sustainability assessment of steel investment, planning and developing are made, it has to construction focused on offshore wind turbines [17–19]. be ensured that all aspects are taken into account during The assessment of energy and emissions related to the the assessment and comparison of alternative solutions production and manufacture of materials related to an off- [6]. Life-cycle analysis (LCA) is a holistic methodology that shore wind farm using an LCA model has been performed can be used as a tool in detecting these potential en-vir in the work of Schleisner [20]. Tremeac and Meunier [21] onmental impacts associated with energy systems and drew some valuable conclusions regarding the environ- in calculating their sustainability performance from their mental impact of wind energy by comparing payback early development stages [7]. In this methodology, final time and CO emissions of turbines with different power- products are examined and assessed in terms of their en- generation capacities, meaning a large 4.5-MW and a small vironmental impact all the way through their life cycle, 250-W wind turbine. The size of the wind turbines, though, from raw-material extraction until end of life [8, 9]. does not appear to be a decisive factor in optimizing their Wind farms as investments for energy production with life-cycle energy performance and the embodied energy high economical impact are usually assessed in terms component of wind turbines over their service life [22]. In of safety and robustness only, meaning that they are de- the case study presented herein, steel towers are under in- signed in order to withstand the wind loads in all phases vestigation. Although the amount of steel required for the from construction to operation and extreme wind circum- construction of wind turbines is great, the component with stances. Even when environmental-impact analyses were the highest environmental impact is the tower foundation, conducted for wind-power generators, the methodologies because the potential recycling or reusing of steel compo- deployed would take into account only a limited number nents can lead to reduced environmental impact and, with of life-cycle steps. LCA, being a holistic methodology, is contemporary technologies, almost 80% of a wind-power- capable of investigating and quantifying both direct and generation system can be recycled—practically everything indirect environmental impacts, taking into account all except the concrete foundation and the composite blades. the life-cycle steps of products and services. Using LCA’s Not only the power-generation capacity of a wind tur - advantages in comparing the environmental impact of bine can be considered responsible for producing different Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz028/5639689 by DeepDyve user on 03 December 2019 Stavridou et al. | 3 results, but also the LCA methods used, even for the same sense of erection without external cranes) that consists structure [23]. Martinez et al. [24 25 , ] investigated the de- of a new design of cross-sections that have been particu- pendency of results on the impact-assessment method- larly optimized to minimize the material use [32]. These ology implemented by conducting two different studies of self-rising lattice towers efficiently combine steel parts, al- the same turbine using the Eco-indicator 99 and the CML lowing them to fulfil the required safety, robustness and methodology. Browsing through the literature, it is difficult durability requirements, while keeping the solution poten- to compare LCA results of different wind turbines where tially economical and environmentally sustainable. different methodologies have been implemented, because Due to the fact that energy demands are constantly of the discrepancy of results among methodologies even increasing, contemporary installed wind turbines need to when investigating the same structure. have increased efficiency and power-generation capacity. The objective of the work presented in the current study In order to achieve this increased power-generation cap- is to perform a comparative LCA for two potential wind tur - acity, wind turbines are constructed using longer blades bines to be deployed in a wind park located in the UK. The and greater tower-hub height in order to take advantage of two potential wind-power generators carry the same wind the smoother wind flow combined with higher wind vel- turbine at their top: the Repower MM92 [26]. In Table 1, a ocities at greater heights. All alternative solutions, with number of LCAs for onshore wind-turbine towers around either the use of internal stiffening or the lattice con- the globe have been grouped, where the type of tower, the figuration, have been studied in terms of structural per - hub height, the wind-turbine size and the LCA software formance and have been proven to be robust enough to used are presented. One can easily note that the majority sustain the greater loads due to higher wind velocities and of studies have focused on tubular-steel or concrete towers the greater nacelle mass due to bigger rotors and longer with a hub height up to 124 m, leaving the environmental blades. The environmental impact of classic tubular-steel impacts of tall steel-lattice towers almost unexplored. wind-turbine towers increases exponentially, since both For onshore wind farms, the horizontal-axis wind tur - the amount of steel and the size of the tower foundation bines are the prevailing structural configuration, where increase. It is therefore very interesting to compare the the tower consists of cylindrical parts interconnected environmental impact of the tubular-tower solution with with bolted flanges by means of pre-stressed bolts [29]. the proposed lattice one since the innovative erection Although cylindrical shells have great advantages in terms approach leads also to energy saving and can result in a of load-bearing capacity to shell-thickness ratio, when solution that goes far beyond the decrease in the envir - getting more slender, local buckling phenomena can be onmental impact deriving from the minimization of ma- catastrophic; therefore, an increase in their thickness is in terial used. For onshore wind-turbine towers, the life-cycle most cases unavoidable. As an alternative solution to the stages usually taken into account are the following: manu- existing cylindrical-tower configuration, the implementa- facture, transportation, construction/erection, operation tion of internal stiffening of tubular wind-turbine towers and dismantling. When assessing the environmental im- has been the focus in the work of several research groups pacts of the various stages, the manufacturing stage is by [30, 31]. Although the solution of internal stiffening has far the one with the highest environmental impact, with been proved advantageous in terms of material use and the transportation stage following in second place. concurrent structural enhancement, the previous work of Since the tower and foundation appear to be the wind- the current research group has focused on the elaboration turbine components with the highest environmental im- of an alternative tower configuration that can permit wind pact, the present work focuses on the investigation of the turbines to reach even greater heights with less steel use environmental impact of two alternative tower configur - and smaller-scale foundations too. This new tower config- ations: one tubular and one lattice. The different tower uration is a self-rising lattice-tower configuration (in the configurations require different foundations, so valuable Table 1 LCA overview of onshore wind-turbine towers Publication year Tower type Hub height (m) Capacity (MW) Software 2008 [10] Tubular steel 55 0.66 – 2008 [13] Tubular steel 45, 46, 60 0.66, 0.60, 1.75 – 2009 [18] Tubular concrete 124 4.50, 0.25 Sima Pro 2009 [19] Tubular steel 60, 80 0.85, 3.0 – 2009 [21] Tubular steel 70 2.0 Sima Pro 2012 [16] Tubular steel 105, 65 1.8, 2.0 GEMIS 2013 [15] Tubular steel/concrete 80 1.4 – 2014 [27] Tubular steel/concrete 80, 100, 150 2.0, 3.6, 5.0 Gabi 2015 [28] Tubular steel 80 2.0 Gabi 2016 [14] Tubular steel 92.5 2.3, 3.2 Sima Pro Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz028/5639689 by DeepDyve user on 03 December 2019 4 | Clean Energy, 2019, Vol. XX, No. XX Table 2 Lattice-tower sections details Legs V-brace diagonals V-brace horizontals Height (m) Diameter (mm) Thickness (mm) Diameter (mm) Thickness (mm) Diameter (mm) Thickness (mm) Part-1 34.45 411 8 413 7 342 6 Part-2 55.53 371 8 385 7 282 5 Part-3 68.19 352 7 375 7 240 5 Part-4 75.64 340 7 363 7 216 4 Part-5 76.15 286 5 253 5 214 4 conclusions of the two most important tower parts are duration of a turbine’s lifetime is assumed to be 20 years. drawn. First, the scope of the study is presented, sup- The LCA results of the wind turbines are assessed in terms porting life-cycle inventory data are reported and the re- of various environmental factors like GWP, AP, the energy- sults of the life-cycle-impact assessment are discussed. In payback time, etc. The calculations are usually performed order for the results to be comparable, the structures share per lifetime stage and for all the structural components the same height and have the same loading being applied independently. The software and databases for performing at the hub height. Both tower configurations are designed LCA are various and, in the present study, GEMIS (Global in a manner to be capable of sustaining the same loads Emission Model for Integrated Systems) open-source soft- as proved in previous works. The analysis methodologies ware is selected due to its focus on construction, energy implemented are identical while the lattice tower has 35% and transport fields [35]. less steel, an almost 33% lighter foundation and many ad- vantages in terms of transportation and erection. 2 Tower models After having studied various cases of LCA on onshore wind- 1 Methodology for LCA turbine towers, in the present study, the life-cycle perform- LCA is a useful methodology for determining the total ance of two 76.16-m-tall wind-turbine towers is carried system impacts of a given technology and is realized by out. The investigation of tall wind turbines is here ex- associating all environmental impacts with the material-plored, whilst the investigation of super-tall wind turbines acquisition, processing, manufacturing, use and disposal or (e.g. greater than 150 m) is currently underway. Unlike the recycling at the end-of-life stage. This approach is valuable various investigations related to the LCAs of wind turbines, towards the sustainable design of systems and is there- the present study focuses on the tower configuration only, fore used by both policy makers and industrial partners for taking for granted that the nacelle and blades are identical product development and the management of sustainable in both turbine cases. To this end, the life-cycle perform- systems. In principle, when conducting LCA for systems or ance of two towers—one tubular tower and one lattice— products, the steps that have to be followed are: has been investigated. All life stages from production of the raw materials to the end of life have been taken into (i) the definitions of system boundaries, requirements account under the assumption of a 20-year lifetime. and assumptions; The steel parts of both towers are made of steel class (ii) the collection of resources for all system inputs and S355 and the foundation is assumed to be made from con- outputs; ventional concrete. The nacelle and blades that are sup- (iii) the definition of the parameters used to evaluate posed to be accommodated on both towers are identical and the environmental impacts related to the inputs and the structural analyses for both towers have been already outputs; verified. The lifetime stages taken into account are iden- (iv) the assessment of the results; in order to perform an tical for both towers: manufacturing, transportation and LCA, thorough research has to be performed in order erection, operation and dismantling. For the dismantling to identify the factors that contribute to the environ- stage, two different scenarios are investigated for each ma- mental impact of the system; for its calculation, a se- terial: the recycle/reuse scenario and the landfill scenario. ries of LCA software, tools and databases can be used In the manufacturing stage, both the production of the in accordance with the global or European standards raw materials and the energy consumed for their fabrica- that govern the sustainability-assessment procedures tion are taken into account. For the transportation stage, [33, 34]. it was assumed that the lattice-tower subparts have been As far as the wind-turbine towers are concerned, the stages produced in the factory 100 km away from the construc- that are usually taken into account when performing LCA tion site and transported there. On the other hand, for the are the manufacturing stage, the transportation, the erec- tubular subparts, the assumption was made that they are tion/construction, operation and dismantling, while the produced in north Germany in 30-m-long parts and they Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz028/5639689 by DeepDyve user on 03 December 2019 Stavridou et al. | 5 are transported on site by ship and truck. For the erection stage, large-scale cranes are used for the mounting of the tubular tower, whilst, for the lattice one, only small-scale cranes are used, since the tower can be erected by the pre- viously mentioned innovative self-rising approach without using tall cranes. For the dismantling stage, the materials are recycled when possible; otherwise, the landfill scenario was implemented. Further details are presented later on. 2.1 Tubular tower The tubular tower is 76.15 m tall and consists of three subparts of 21.8, 26.6 and 27.8 m from bottom to top. The 15 tower under investigation is an actually constructed tower [36]. The subparts are fabricated in the factory by hot rolling steel plates of varying thicknesses and forming rings about 3 m wide. The rolled plates are welded longitudinally to form 3-m rings connected to each other by means of cir - cumferential welds. The final tower subparts are trans- ported on site and are connected to each other with the aid of flanges by means of pre-stressed bolts. For tubular towers, the use of large-scale cranes and heavy machinery is mandatory, since the subparts are quite long and heavy. 20 The tower is not purely cylindrical, as the lower diameter of the cross-section of the tower is 4.3 m and the top one is 3 m. The thickness of the shell wall is also not constant, starting from 12 mm at the top to 30 mm at the bottom. The tower is embedded into a reinforced-concrete foundation that is anchored to the ground and, therefore, the tower can be considered and modelled as fixed at the foundation. The shell-thickness distribution along the height of the tower is presented in Fig. 1. The total tower weight is 127 t. 2.2 Lattice tower Fig. 1 Tubular-tower shell-thickness distribution The lattice tower is of square base shape consisting of five subparts along its height. The heights of the various using the freely available software GEMIS [30]. As men- subparts appear in Table 2. The tower is composed of three tioned above, an LCA study is completed in four stages discrete structural sub-systems: the legs, the face-bracing and, more specifically, the following in our case. trusses and horizontal braces and secondary bracings ar - ranged inside the plane of the face-bracing trusses. The 2.3.1 Analysis goal and scope definition connections between the structural members are bolted The goal of the present study is to compare the life-cycle connections with conventional steel bolts. The total tower environmental impacts of two wind-turbine towers. This weight for the lattice tower is 77.47 t and circular hollow investigation contributes to determining and quantifying sections of varying shell thicknesses and diameters are the impacts of a potential wind park located in UK. Both used, as presented in Fig. 2. The tower subparts are manu- onshore wind turbines have a power-generation cap- factured in conventional factories near the construction acity of 2.0 MW. They have similar function and technical site; they are transported on site and erected to their final specifications. However, the tower configurations differ positions with the aid of small-scale cranes. The final in the manner explained in Sections 2.1 and 2.2 of the tower shape and distribution of the steel sections are pre- present paper. sented in in Fig. 2 below. The scope definition of an LCA includes a description of the product under investigation in terms of the system boundaries. In the case study presented here, all life stages 2.3 Research methodology from production of materials to the end of life of the struc- In order to assess the environmental impacts of the two ture are under consideration. The wind-turbine stages in- towers under investigation, the LCA method is applied vestigated are presented in Fig. 3 and are the component 76,15 Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz028/5639689 by DeepDyve user on 03 December 2019 6 | Clean Energy, 2019, Vol. XX, No. XX manufacturing, transportation and erection, operation and Height (m) then dismantling or recycling, depending on the scenario Part-1 34.45 taken into account. Part-2 55.53 The connection to the grid is out of the scope of the Part-3 68.19 present study and has been neglected. The lifetime of the Part-4 75.64 turbines is set to be 20 years. The functional unit must be defined so that alternative solutions can be compared in Part-5 76.15 a meaningful way. The energy-payback comparison takes into account the amount of energy generated over the as- sumed 20-year lifetime. 2.3.2 Life-cycle inventory (LCI) analysis 3D view Wind turbines are complex structures that consist of many structural, mechanical and electrical assemblies, which comprise many sub-components. The most crucial stage in an LCA is data gathering and the maximum de- tail possible should be included so that the accuracy of the obtained results is not sacrificed. The LCI is the gathering of information related to primary components of a wind- turbine tower. For the manufacturing stage of the wind turbine, the two tower systems were analysed as detailed in the previous paragraphs. As far as the turbine itself is concerned, it is a 2.0-MW, three-bladed upwind horizontal- axis wind turbine with a hub height of 76.15 m. The hub and nose cone of the turbine are generally made of cast iron and fibreglass-reinforced polyester, respectively; the blades are made of a composite material consisting of 60% glass fibre and 40% epoxy; the generator is basically made of steel and copper, while the gearbox is made of cast iron and stainless steel. The energy-consumption calculation for all the manufacturing of the wind turbine has been based on data on material component weights available. Fig. 2 Lattice-tower configuration Raw material extraction Manufacturing Transportation & Erection Recycling Landfill Operation Fig. 3 LCA boundaries Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz028/5639689 by DeepDyve user on 03 December 2019 Stavridou et al. | 7 For the transportation and erection stage, the turbine is More specifically, it is a relative scale of how much a green- assumed to be produced in Germany and transported to house gas contributes to global warming and compares it the UK by boat and truck. The lattice-tower components to the same mass of carbon dioxide. Its measure is kgCO / are produced in a factory near the site, whilst the tubular- kWh (CO equivalent) and is used for the assessment of the tower subparts are again produced in Germany and trans- two wind-tower systems. The basic term for assessing the ported to the UK by boat and truck. The distance for the energy-generation part of an LCA for the wind-energy sys- components to reach the port in Germany is estimated to tems studied in this work is the total cumulative energy be 110 km, the distance at sea is estimated to be 1020 km, requirement that contains the overall energy needed to con- while the transportation of the components from the port struct a wind turbine. Finally, the energy-payback-time ratio to the site is assumed to be 130 km. The freight-transport is implemented in the present study in order to measure services are imported in GEMIS software in tkm (ton- the duration of operation needed in order for the wind tur - kilometre(s)) so, if the track and boat transport the tubular-bine to generate the amount of energy required for its entire tower components of 127 t over the distance of 110 km, life. It is calculated in the form of a ratio of total primary en- this equals a transport service of 13 970 tkm. ergy requirements of the wind turbine throughout its entire As far as the operation and maintenance stages are con- life over the total annual energy generated by the turbine. cerned, there is a certain level of energy input required for The total cumulative energy requirements for its entire life starting the turbine, for the break-system operation, for yaw- comprise the energy needed for production, transportation, and rotor-pitch control, etc. This energy input is normally maintenance, operation and decommissioning. estimated as 1% of the total electricity generated by the - tur bine [37]. For both turbines, maintenance of the mechanical 2.3.4 Result interpretation parts is assumed to be carried out three times a year and The LCA results can be obtained from GEMIS software in the distance for the service team is assumed to be 100 km tabular and graphic format. In the next section, the results per trip. The life of the turbines is assumed to be 20 years. As of the LCA are presented. First, the material requirements far as the dismantling and recycling stages are concerned, per structural component are presented in a tabular format. again, there is an energy input taken into account that is The CO equivalent per lifetime stage for the two towers is assumed to be 2% of the total electricity generated [37]. In presented in a graphic form. Then, in a comparative graph, Table 3, the possible recycling scenarios for the main mater - the cumulative energy requirements of the two towers are ials included in the present study are presented. presented with specific data per stage. Finally, the compo- nent contribution to energy demand of the two towers is 2.3.3 Life-cycle impact assessment depicted. The life-cycle inventories for the two 2.0-MW wind-turbine models were used to support the life-cycle assessment in terms of the basic impact-assessment categories. GEMIS 3 Results software is a freely available database used widely in In the present study, the LCA of two onshore steel wind- Europe. It enables a detailed description of all the process turbine-tower configurations is performed. Both towers are steps of an energy system and has been successfully used of 76.15 m height and their structural analysis results have in previous work for the calculation of the primary energy proved that they can both accommodate the same rotor needed in the process, the emissions and the mass and en- with similar efficiency. A study is performed where all life ergy flows of materials. The database includes >1000 prod- stages from the production of the raw materials to the end ucts, 10  000 processes and >130 scenarios covering data of life have been considered. The connection of the tur - from >50 countries, whilst there is freedom for the user to bines to the grid is not examined in the present study, while import additional data. Upon LCA performance, GWP, green- the lifetime of the turbines is set to be 20 years. In Table 4, house gases, water effluents, solid waste and many more the material requirements for the main components of the can be obtained in tabulated or graphic format. The GWP is two towers under investigation are presented. an indicator of the impact of any process on climate change. Table 4 Material requirements for the tower components Table 3 Possible recycling scenarios [38] Tubular tower Lattice tower Material End-of-life treatment Weight Weight Concrete Landfill 100% Component Mass (t) fraction Mass (t) fraction Cast Iron Recycling with 10% loss Copper Recycling with 5% loss Rotor 34 0.04 34 0.07 Epoxy Incinerated 100% Nacelle 55 0.06 55 0.11 Fibreglass Incinerated 100% Tower 127 0.13 77.47 0.15 Plastic Incinerated 100% Foundation 750 0.78 350 0.68 Stainless steel Recycling with 10% loss Total 966 516.47 Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz028/5639689 by DeepDyve user on 03 December 2019 8 | Clean Energy, 2019, Vol. XX, No. XX In Fig. 4, the distribution of the CO emissions per life- Life Cycle Cumulative Energy Requirements 3.50E+05 time stage is presented, where the construction stage with the steel components and the concrete foundation has 3.00E+05 been proved to have the highest environmental impact. It 2.50E+05 is worth mentioning that the manufacturing phase con- 2.00E+05 stitutes 82% of the total equivalent CO emissions of the 2 1.50E+05 tubular tower. 1.00E+05 In Fig. 5, the distribution of the CO emissions per life- 5.00E+04 time stage for the lattice tower is presented. Again, in this 0.00E+00 tower configuration, the highest environmental impact ap- LATTICE TUBULAR pears to derive from the manufacturing stage of the tower. Transportation Manufacturing Dismantling Operation In Fig. 6, the cumulative energy requirements of the two Fig. 6 Life-cycle cumulative energy requirements towers are presented, where it is obvious that the lattice tower has a much lower energy requirement compared to the tubular one. For the tubular tower that can be better compared, the cumulative energy is compatible with reduced foundation required for a lattice structure com- similar 2.0-MW turbines in the work of Guezuraga et  al. pared to a tubular one and the transportation and erec- [16]. This can be attributed to three factors: the smaller tion advantages that the proposed self-rising system is amount of steel used for the tower construction, the offering. This can be better observed in Fig. 7, where the contribution of each wind-turbine component is presented TUBULAR TOWER for the two towers. Transportation Manufacturing Dismantling Operation The largest cumulative energy requirements contribu- tion comes from the manufacturing stage in both the lat- 2% 9% 7% tice and the tubular towers, reaching values between 75% and 82% of the total life cycle of the turbines. In absolute values, though, the manufacturing/construction stage of the tubular tower is much larger compared to that of the lattice one. The average share from each tower compo- nent is shown in Fig. 7, where it is proved that the tower manufacturing and foundation construction is a larger part of the total energy requirement for the tubular tower. In both tower cases, the smallest contribution is derived from the operation phase, which accounts for only 2% of 82% the total energy requirements. An indicative ratio presented in the above text that can better picture the environmental impact of the two tower Fig. 4 Distribution of CO emissions per life stage for the tubular tower configurations is the energy-payback time. The energy generated by both turbines, since they are hypothetically TUBULAR TOWER positioned at the same spot, sharing the same hub height Transportation Manufacturing Dismantling Operation and same rotor, would be for a 2.0-MW turbine ~6.12 GWh assuming a 35% capacity factor as assumed in Haapala and Prempreeda [38]. The results of the payback time for both towers are presented in Table 5. 3% 8% The energy-payback time for the tubular tower is 14% 5–6  months whilst, for the lattice, it has been calculated to 4  months. These results for the tubular tower are pro- portional to similar 2.0-MW turbines [16] and very close to the results of the same capacity and same hub-height turbines [38]. The lattice structure has proved to be more advantageous in terms of both the material used and the transportation and erection methods applied. The steel tower fabrication along with the foundation construction 75% are much less energy-consuming procedures compared to the tubular structure and the energy-payback period is also 15% less. Fig. 5 Distribution of CO emissions per life stage for the lattice tower GWh Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz028/5639689 by DeepDyve user on 03 December 2019 Stavridou et al. | 9 Component Contribution to the total energy demand the energy-payback time. The most impactful stage in the 70% lifetime of a wind turbine is the manufacturing phase. The 60% analysis conducted shows that the lattice structure is 32% less impactful on the environment in terms of equivalent 50% CO emissions. The energy-payback time for the tubular- 40% steel tower is 5–6 months, whilst the lattice self-rising steel 30% tower has an energy payback time of 4 months. The present 20% study shows that only by saving material from the foun- 10% dation and tower, which are the most energy-consuming components of the wind structure, could the equivalent 0% Rotor Nacelle TowerFoundation CO emissions be reduced. This research study aims at LATTICE TUBULAR obtaining an initial approach of the environmental impact of the proposed manufacturing and erection procedure Fig. 7 Distribution of CO emissions per structural component and this is the reason why a tower of 76.15-m hub height was selected, since comparable data were available. After having proved that the new tower system is robust enough Table 5 Energy-payback-time calculation and less environmentally impactful, a further more com- Tubular Lattice prehensive study of taller structures is to follow. Units tower tower Cum. energy requirements GWh 2.96 2.00 Annual energy generated GWh 6.12 6.12 Acknowledgements Energy-payback time – 0.48 0.33 This project has received funding from the European Union’s Energy-payback time Months 5–6 4 Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie Grant Agreement No. 747921 that the au- thors acknowledge with thanks. 4 Conclusions Constantly growing global energy needs require the im- Conflict of Interest plementation of additional power-generation systems worldwide. Fossil-fuel shortage and consequences of the None declared. greenhouse effect have led to greener energy-production methods and wind turbines are among the most advanta- References geous. 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Journal

Clean EnergyOxford University Press

Published: Apr 4, 2020

References