With increasing CO2
emissions, there is an urgent demand for environmental awareness and sustainable design and construction. A significant reduction of the carbon emissions can be achieved with the use of renewable energy sources, such as solar radiation, movement of water, geothermal and wind energy [1
]. Since the early 1980s, many wind parks have been constructed, making a remarkable contribution to a growth in the renewable energy generation across the world. In 2016, it was reported that 341,320 wind turbines were installed across the world and globally more than 637,000,000 tonnes of carbon emissions were averted [2
]. In Europe, there were annual installations of +10 GW of wind energy capacity since 2009, while in just the first half of 2019, 4.9 GW of new wind energy capacity was introduced in European Union (EU) [3
]. Further to onshore and offshore wind parks, small scale horizontal [4
] and vertical wind axis turbines [6
] are now also being installed in private properties. It is believed that combining repowering of wind farms with developing new ones will enable reaching EU targets. As stated in Wind Europe’s Central Scenario, by 2030, 323 GW of cumulative wind energy capacity would be installed in Europe that would correspond to 30% of the EU’s energy demands [3
In order to come up with the present energy demands, there is ongoing research on optimising the wind energy structural systems, the aeroelastic and mechanical performance, and thus the energy production. Research on wind energy systems is multidisciplinary and necessitates the collaboration and interaction of structural, electrical and mechanical engineering in order to fully understand the mechanics with the aim to maximise the energy generation and, at the same time, prevent any type of failures. Examples from recently researched topics include the structural robustness and the connections of towers [7
], the seismic assessment of onshore and offshore systems [9
], the minimisation of electrical failures of turbines via condition monitoring and maintenance [12
], and the optimisation of the environmental and economic performance [14
], to name a few.
At the same time, needs for taller wind turbines with bigger capacities, intended for places with high wind velocities or at higher altitudes, have led to new technologies in the wind energy industry. A recently reported new structural system for onshore wind turbine towers is the hybrid tower, which was investigated within the scope of a European research programme, named SHOWTIME (“Steel Hybrid Onshore Wind Towers Installed with Minimal Effort”). This tower combines efficiently steel lattice and tubular parts, thus allowing for taller hub heights and hence better exploitation of the wind energy at higher altitudes. The project examined structural configurations, successfully adhering to safety and durability design checks, while allowing for economically and environmentally sustainable solutions. Jovašević et al. [17
] performed a structural optimisation, examining a range of bracing systems, a number of connections and various dimensions of columns, thereby resulting in a series of optimised hybrid configurations. For the optimised geometries, an aeroelastic analysis was carried out [19
] and the structural performance of the wind turbine towers under normal and extreme operating conditions, ensuring adequate structural robustness, was investigated. Focusing on the critical transition piece, which aims to transfer the dynamic and wind loads from the tubular to the lattice part and subsequently to the foundation, a rigorous numerical study considering fatigue loading conditions was carried out [20
]. Given that these types of towers allow for hub heights over 180 m, an innovative erection procedure, minimising time and effort was also suggested [21
]. As these tall hybrid towers are a new structural system, with different erection process from the widely used tubular towers, comprehension of their environmental performance is missing and deemed essential. To examine the environmental performance of a system, life cycle assessment (LCA) is commonly adopted.
Life Cycle Assessment of Wind Turbine Towers
Life cycle assessment is a meticulous holistic technique for the evaluation and analysis of potential environmental impacts of a system throughout its life, starting from raw material production to the end-of-life. LCA comprises a conceptual framework that is also used by companies aiming for sustainable supply chain management and product development. It is a prolonged scientific procedure that necessitates deep understanding of the influencing parameters and the realisation of comprehensive computations. To facilitate its execution, a number of databases, software and tools are currently available. One such software will be used herein, as will be discussed in Section 2
. According to ISO 14040-44 [22
], life cycle assessment consists of the following four stages:
Definition of the analysis’ goal and scope, where the methodology, assumptions and limitations are established.
Inventory analysis, where the system’s inputs and outputs are assembled.
Impact assessment, where various indicators such as global warming, energy requirements etc. are determined.
Interpretation, where the system’s environmental impact is estimated and discussed.
LCA can be carried out in order to assess the eco-friendly performance of renewable energy systems [24
] and thus has been applied to study the life cycle performance of wind turbine towers around the world. Collated research of LCA studies on onshore wind turbine towers are shown in Table 1
, where the structural material of the tower (i.e., steel, concrete, composite), the hub height in m, the wind turbine size in MW, the assumed installation location, the adopted software and the main drawn conclusions are presented. Herein, focus is primarily placed on onshore wind turbines, while research on offshore wind turbine towers [25
] or small-scale wind turbines [26
] are out of the scope of this study and are not included in the table.
For most of the examined research studies, a lifetime of 20 years has been considered. The results of the LCA of the wind turbine towers are usually assessed by determining (a) the global warming potential (GWP), in which the emissions of greenhouse gases are evaluated; (b) the abiotic depletion (AD), which is one of the most prevalent impact categories of LCA and includes the depletion of non-renewable resources; and (c) the energy payback time (EPT), which shows the duration the wind energy system has to operate in order to produce the amount of energy that was necessitated throughout its entire life. The results are commonly provided in percentage charts and grouped either per life stage or per structural component, therefore allowing to draw conclusions on the most critical part or process.
Garrett and Rønde [27
] studied the LCA of an onshore wind plant leading to the conclusion that the energy it produces for the society is 22 to 30 times more than the energy it consumes. The calculated energy payback indexes found for onshore wind turbine towers were generally lower than 1 year, while in some cases only 1.3 months [28
]. The biggest environmental impacts have been related to the manufacturing stage [29
], whilst the lowest contribution belongs to the operation phase [32
]. Martínez et al. [33
] discussed the contributions of the copper and fiberglass of the Rotor-Nacelle-Assembly (RNA), whereas Razdan and Garrett [34
] suggested the use of iron, steel, aluminium and concrete as primary contributors to environmental impacts. The component which affects most importantly the environment was reported to be the foundation [35
], whereas other studies estimated the steel tower and the nacelle as the components with highest LCA footprints [36
]. The effect of the size of the wind turbine was investigated by Crawford [37
] who reported no significant variation in the energy yield between small and large wind turbines, while on the other hand Smoucha et al. [38
] stated that the installation of higher-rated over lower-rated turbines allows for greater environmental benefits. In order to enhance the environmental performance, Xu et al. [39
] suggested optimisation of the structural design and raw materials application, whilst Schreiber et al. [40
] recommended replacement of material components in order to control the environmental impacts. Tremeac and Meunier [41
] emphasised maximising recycling during decommissioning and Bonou et al. [42
] proposed comprehensive examination of end-of-life treatment technologies and recycling technologies for composite materials. Demir and Taskin [43
] stated that wind turbines with high hub heights (i.e., installed in optimum wind speed regions) can lead to lower environmental impacts. In addition, even though steel towers have been reported to comprise large contributions of the total carbon emissions [44
], the fact that steel can be reused and recycled [45
] suggests that it can be the preferred solution for towers with increased hub heights and larger rotor blades [46
As can be observed in Table 1
, past studies have mainly dealt with the environmental effects of towers with hub height up to 100 m, while there are few reported studies on hub heights up to 150 m, leaving environmental impacts of taller structures still unexplored. On this direction, the present study will examine even taller (185 m) steel towers. Moreover, focus has been previously placed upon common forms of tubular towers, while any research results on hybrid towers has not been reported. Aiming to address this knowledge gap, this paper presents a comprehensive LCA targeting in a better understanding of the environmental performance of the recently introduced tall onshore hybrid steel wind turbine towers. The methodology implemented for this study is presented in Section 2
. The results are discussed in Section 3
and the main conclusions are summarised in Section 4