1. Introduction
Despite the fact that progress is being made towards the seventh UN sustainable development goal [
1], with encouraging signs that energy is becoming more sustainable and widely available, the world still relies on traditional fossil energy. The latter is the dominant contributor to climate change, accounting for around 60% of the total global greenhouse gases emissions [
1].
According to the long-term climate change strategy recently agreed, universal access to affordable, sustainable energy services should be ensured by 2030, and society should be climate-neutral by 2050 [
2]. According to this, wind energy stands as a prominent renewable source that encourages the development of innovative wind energy systems with enhanced output generation capacity. Such new developments are floating wind turbines or very tall onshore turbines [
2].
Increasing environmental problems, energy costs, and diminishing nonrenewable energy sources oblige people to prefer renewable energy sources such as wind, solar, and hydropower [
3]. Renewable energy sources are provided from natural resources and are sustainable energies. Renewable energy sources are not only clean, safe, and environmentally friendly, but also cost-effective and inexhaustible [
4]. The negative impacts of renewable energy resources are limited and insignificant compared to the other traditional energy resources as sustainable energy has a limited negative impact on natural vegetation and human health, does not burn fossil sources, has no radioactive effect, and is experiencing rapid technological development [
5]. Wind energy is the foremost renewable energy, offering clean energy, economic advantages, and a clear way to reduce greenhouse gas emissions (GHG) [
5].
In 2020, wind energy production reached over 650 GW around the world, with 59.7 GW added in the previous year [
6]. In 2019, the growth rate of wind energy was 10.1% higher than the 2018 rate in terms of market size, but less than in 2017 and 2016. China and the US, having the biggest wind market size of the last five years, installed wind energy amounting to 27.5 and 9.1 GW, respectively, in 2019. Although European countries erected 15.4 GW of wind energy capacity in 2019, the growth rate of wind energy was 27% higher than the previous year but 10% less than the rate in 2017 [
6,
7,
8].
According to the 2019 report “
Wind Energy in Europe: Outlook to 2023” [
9], it is accepted that the wind energy capacity of Germany, Spain and the UK will sharply rise by 2023. Installation of 65 GW of wind energy is planned across nine European countries from 2019 to 2023. Germany will install 11.2 GW of onshore wind energy in these years and will be the leader in onshore wind energy in the European continent. Furthermore, Spain (8.7 GW), France (8.1 GW) and Sweden (7.5 GW) come after Germany in terms of onshore wind capacity. Concerning offshore wind energy, the UK will install 6.4 GW of wind energy over five years, followed by The Netherlands (4 GW), Germany (2.9 GW), Denmark (1.7 GW), and France (1.3 GW). The UK will become the leader in offshore wind energy in Europe [
9,
10].
Thanks to new technologies, wind turbines which have taller structures, longer blades, and set up different locations can be constructed and thus wind energy capacity is rising. Onshore wind turbines with new hybrid structures have taller hub heights and allow better operation of wind energy in higher locations [
11]. Floating wind turbines, being a recent high-innovation development in the wind energy sector, are located in deep waters where fixed-bottom offshore wind towers are not feasible because of the cost of the foundation and the lack of technology [
12]. Along with the design of these new wind turbine structures, the carbon emissions emitted during their manufacture, installation, operation, and disposal must also be considered. Given these new turbine structures, this study focuses on the environmental impacts of floating wind turbines using the life cycle assessment methodology.
2. Life Cycle Assessment—An Overview
Sustainability revolves around achieving an equal balance among the economic, environmental, and social factors throughout the life cycle of any given product [
13]. The LCA is a rigorous method that evaluates the environmental impact of energy, raw materials and waste, and emissions resulting from a product, process, or service based on the ISO 14040 and 14044 standards [
14,
15]. The LCA is carried out in the following four standard steps. In the first step, the purpose, scope, methodologies, and limits of the system are determined, and, in the second step, the life cycle inventory (LCI) is determined with inputs and outputs at the boundaries of the system. In the third step, a life cycle impact assessment (LCIA) is performed with environmental impact potentials determined using inventory data collected and compiled in the previous step, and the results are interpreted in the fourth step [
13].
The LCA methodology adopted for this study is depicted in
Figure 1. The system boundary (
Section 3.2) includes the following six stages that have been considered in all analyses: manufacture, transportation, erection, operation and maintenance, disposal and recycling. With reference to the six stages of the wind turbines’ LCA (
Figure 1), the following should be noted:
During the production and manufacture stage, the materials and parts are selected and made for the wind turbine, i.e., the tower, blades, nacelle, foundation, etc. are produced.
During transportation, the manufactured parts are transferred to the area where the system will be installed. Here, the distance between the factory and the installation area is the critical impact factor.
During the erection, the installation of the system is completed. The wind turbine parts must be modular and of movable size.
Concerning the operation and maintenance of the system, a periodic maintenance of the system carried out systematically that has to be taken into account.
For the disposal, a turbine that has completed its life cycle is dismantled.
During the recycling, any recyclable materials are submitted to the manufacturer and the remaining materials are sent to landfill [
14,
15].
Until now, LCA studies have been focused on different wind turbine designs and sizes of onshore and offshore wind turbines. Demir and Taskin [
16] studied the environmental impact of onshore wind turbines having different heights and sizes. They recommend large-size wind turbines with alternative environmentally friendly materials to decrease the environmental emissions. Guezuragaet et al. [
17] conducted the LCA of 2 and 1.8 MW steel wind towers. They highlighted higher environmental emissions during the manufacturing phase. Gervásio et al. [
18] focused on the LCA of concrete, steel, and composite wind turbine towers having different heights and sizes. It has been pointed out that the environmental impact of using steel towers is less than other towers. Gkantou et al. [
11] focused on the environmental impact of two tall hybrid towers. The towers consisted of two parts, a top tubular part and the bottom lattice part with either four or six legs. The four-legged hybrid tower exhibited a lower environmental impact than the six-legged one. Stavridou et al. [
19] analysed comparatively a 2 MW tall tubular tower and a lattice wind tower and concluded that the lattice tower has lower environmental impact and energy-payback time. Alsaleh and Sattler [
20] studied the environmental impact of large onshore wind farms in the US. The manufacturing stage was more than 60% of the total CO
2 contribution. Moreover, extension of the lifetime of the wind farm, such as to 25 and 35 years, was analysed. These extended lifetimes have lower impacts per kWh of electricity generated. Lanzen and Wachsmann [
21] compared the LCA of wind turbines in different geographical locations (e.g., Brazil and Germany) considering the manufacturing locations of the components. They considered the distance from the manufacturing area of wind turbine components to the place of operation of the wind turbine in five different scenarios, such as production in Germany and operation in Brazil. The scenario of production and operation in Brazil has a lower kg CO
2/kWh than other scenarios. In addition, Kaldellis and Apostolou [
22] studied the life cycle energy and CO
2 emission comparison of offshore and onshore wind energy systems. Offshore wind turbines have a large carbon footprint; however, they are the best choice considering their high energy efficiency. Huang et al. [
23] evaluated the LCA of offshore aeolian farms considering two different substations (onshore and offshore). They highlighted that the high environmental impact corresponds to the offshore substation; they also concluded that the impact could be moderated using recycled materials. Considering twenty past studies, Bhandari et al. [
24] investigated the GHG emissions and the annual energy yields (AEY) of single onshore and offshore wind turbines. The results of Bhandari et al. highlighted a correlation between the GHG to the AEY for onshore wind farms and single wind turbines. Kasner et al. [
25] investigated the energy efficiency and environmental effects of wind turbines with a lifespan of 25 years and 50 years, using the sustainable modernization method. To increase the lifetime of the wind turbine to 50 years, it was extended by replacing components, such as rotor, blades, structure parts etc., and maintaining them at the required time. In this study, they analysed the environmental impacts of a wind turbine with a life of 50 years and a new wind turbine that has completed its 25-year lifetime and which will operate again for 25 years. They highlighted that the greenhouse gas emission of the wind turbine with a lifetime 50 years, is lower by 40–50% than that of two wind turbines during their 25-year life periods [
25].
LCA of onshore and offshore wind turbines has been investigated in the past as shown in
Table 1. These past studies have focused on a wind turbine in a specific area, comparing other wind turbines or renewable energy sources (solar), different heights, wind tower materials (steel, concrete, etc.), and wind turbine size. The results of these LCA studies, environmental impacts and energy payback time of a wind turbine have been calculated and these results have been interpreted.
The first floating wind turbine was built in Scotland in 2017 by Equinar and Masdar Companies [
38]. This newly developed wind turbine attracted the interest of researchers who perform LCA on wind turbines. Currently, four studies related to LCA of floating wind turbines have been published [
Table 2]. Weinzettel et al. [
39] focused on comparative LCA of a 5 MW sway floating wind power plant, a 2 MW offshore turbine, and a natural gas electricity system. The energy payback time and the CO
2 emissions of the floating wind power plant were calculated at 5.2 months and 3 ×10
−4 kg, respectively. This comparative study shows that the sway floating wind power plant has less environmental impact than the 2 MW offshore wind power plant and a natural gas electricity system. Randal et al. [
40] compared the GHG emission and energy performance of six different wind turbines. These wind turbines have different foundations and mooring designs: spar, two tension-leg-buoy (MIT and UMaine TLB), sway, semisubmersible floating, and jacket bottom-fixed designs. According to the results of Randal et al., the lowest GHG emission was of the MIT TLB at 18.0 g CO
2 eq./kWh, while the semisubmersible design had a higher value (31.4 gCO
2 eq./kWh). Furthermore, they stated that wind turbines with a higher energy payback ratio and lower energy payback time have the best energy performance. Taking account of that shows that MIT TLB and jacket offshore wind turbines have the best performance, and steel platforms and anchor cables have high contributions to the total CO
2 emissions. Elginoz and Bas [
41] focused on the life cycle assessment of a floating multiuse offshore platform farm that combines a wind and a wave energy system, considering the overall environmental impact. In addition, this study includes a comparative analysis of a spar platform with a single-use semisubmersible one over a lifespan of 25 years. As a result of their research, for the semisubmersible floating wind turbine they concluded that the amount of terrestrial ecotoxicity, freshwater aquatic ecotoxicity and eutrophication is high. Kausche et al. [
42] investigated the economic and environmental impact of a tension leg platform floating wind turbine. The first objective of their study was to investigate a possible reduction of the economic impact and investment cost, and the second one was the curtailment of CO
2 emissions during the manufacturing process of the system. Three types of floating wind turbines, steel-concrete, steel-reinforced concrete, and steel structure, were designed and analysed considering the economic and environmental impact. Concerning the CO
2 emissions, the steel-concrete turbine has a lower value of about 395 t/MW, while the best economic result corresponds to the steel-concrete wind turbine [
42]. As can be observed in
Table 2, the published literature on floating wind turbine towers has been collected and presented in chronological order. The four studies focused on the LCA of five different types of floating platforms, sway, spar, TLB, semisubmersible, and TLP.
As a result of a comprehensive literature review, the LCAs of wind turbines were analysed considering parameters such as turbine size, height, design, location, type of turbine. In the light of the literature review, the present study aims to analyse the environmental impacts of the barge-type floating wind turbine and compare it to the environmental impacts of the spar floating, the onshore, and the jacket offshore wind turbines.
5. LCA Results for the Barge-Type Floating Wind Turbine
The LCA of the barge-type floating wind turbine included all life cycle stages from raw materials to the end of its life. The lifespan of the floating wind turbine is assumed to be 20 years [
11].
Table 5 contains a summary of the LCA of the barge-type floating wind turbine. The majority of the past studies focused on the following four environmental impacts: global warming potential (GWP), acidification potential (AP), abiotic depletion potential for fossil fuels (ADPF), and energy payback time, where
Abiotic depletion potential for fossil fuels focused on the non-renewable resource is measured in MJ;
Global warming potential is related to CO2 emissions measured in CO2-equivalent;
AP values show aggregated acid air emissions measured in SO2-equivalent;
The energy payback time, a ratio of primary energy to annual energy produced by a wind turbine, is calculated in months and years [
11].
The global warming potential, AP, and ADPF of each component and life cycle stage are presented in
Figure 5 and
Figure 6. As can be observed in
Figure 5, it can be seen that the foundation component has the largest percentage of all the components in measures of global warming potential, acidification potential, and abiotic depletion potential for fossil fuels. The foundation component constitutes 81% of the total equivalent GWP of the floating wind turbine. This could be related to the use of steel, concrete, nylon fibre and polyurethane, and the long usage of the crane and tugs. Likewise, the ADPF and acidification potential percentage of the foundation component is higher than the other 78% and 79%, components respectively. The second highest GWP and ADPF is the tower component, at 10% and 11%, respectively. The second highest AP is reported at 9% for both the tower and nacelle components. The main reason for this high value for the nacelle is using large amounts of iron and steel in its production/manufacture stage. On the other hand, the lowest value of GWP, AP and ADPF is obtained at nearly 4% for the rotor component.
As far as the share among the life cycle stages is concerned,
Figure 6 demonstrates that production/manufacture is the stage with the biggest contribution of global warming potential (CO
2), acidification potential (SO
2), and abiotic depletion potential for fossil fuels (MJ). GWP of the production/manufacture stage is 94%. As a matter of fact, in line with past studies [
11,
16,
17,
18,
19,
20,
21,
22,
23,
24,
25],
Table 1 and
Table 2 show the largest GWP contribution comes from the production/manufacture stage in wind turbines. It can be decreased using alternative materials with the lowest CO
2 emissions or reducing the amounts of concrete and steel for the production/manufacture stage. The lowest contribution to GWP, AP and ADPF are derived from the transportation stage. It can be clearly seen that the erection stage is much higher compared to that of other stages, considering the contribution to GWP, AP, and ADPF. Given that the erection stage entails consumption of fuel, long-time usage of the crane and tugs, the amounts of GWP, AP, and ADPF are expected to be higher than other stages in the erection stage. Regarding energy performance, the energy payback time, as defined in this section, has been calculated as 1.13 years. In the aforementioned studies, EPT values were reported between 1.6 and 2.7 years. As a matter of fact, EPT and energy performance of the wind energy have opposite correlations. Hence, it is expected that the smaller the EPT, the better the energy performance.
5.1. Comparison of the Barge-Type Floating Wind Turbine LCA Results with Those of Other Types of Wind Turbines
In this section, the results of the LCA of the barge-type floating wind turbine are compared with those from onshore (2MW-398 tonne), offshore (2MW-979 tonne) and floating (5MW-4225 tonne) wind turbines. The data for the LCAs for these wind turbines have been already published in [
37,
39]. The LCA of these turbines correspond to g/kWh of electricity produced. Abiotic depletion potential for fossil fuels (measured in MJ), GWP (gCO
2eq./kWh), AP (gSO
2eq./kWh) and the energy payback time (months) of all turbines have been included. Regarding the results of GWP shown in
Figure 7, the largest GWP contribution is 18.6 gCO
2 eq./kWh for the barge-type floating wind turbine, while the lowest contribution is from the 2MW onshore wind turbine (representing 7.09 gCO
2eq./kWh). One of the most important factors is that the barge-type floating wind turbine platform is made using a high amount of concrete and steel. Therefore, the respective value was expected to be high. Furthermore, the installation of floating and offshore wind towers takes a longer time, and uses heavy cranes, hydraulic hammers, heavy-duty forklifts, pile drivers, vessels, and tugboats. The operation of this equipment consumes fossil fuel resources, having, as consequence of this consumption, high CO
2 emissions to the atmosphere.
As can be observed in
Figure 8, there are four comparable different types of wind turbines with regard to the AP contribution. Although the AP of the barge-type floating wind turbine is nearly 15 times larger than the onshore and jacket offshore wind turbine, the AP value of the floating wind turbine (5 MW) has a higher value than the barge-type floating wind turbine (representing 0.11 and 0.05 gCO
2 eq./kWh, respectively). The reason for the high AP value is due to the usage of iron and steel in all components (nacelle, tower and foundation) and construction. These differences between the on-/offshore and floating wind turbines are due to the high usage of iron in the platform, the mooring system and the nacelle. Since the sway floating turbine (5 MW) is large-scale, it consists of a high amount of iron material in the nacelle part, so the amount of iron used causes the AP value to be increased.
Energy payback time of the barge-type floating wind turbine is slightly higher, as shown in
Figure 9. This value was expected because the total mass of the barge-type floating wind turbine, and the duration of the erection stage and the transportation stage, are larger than the others. It should be noted that although the barge-type floating wind turbine has a large mass and long erection time, the energy payback time and CO
2 emissions of the floating wind turbine are not higher than the other ones (
Figure 7 and
Figure 9). The EPT of the onshore turbine is lower than the others. That is due to the turbine having less mass, a short erection time, and less fuel consumption.
5.2. LCA of the Barge-Type Wind Turbine Transportations Scenarios
In this section, the transportation stage of the barge-type floating wind turbine are analysed considering different scenarios, such as, truck, train and vessel. The three manufacture locations were Spain, The Netherlands and Belgium. Truck, vessel, train, and hybrid models have been considered as transportation stage scenarios. These scenarios were modelled to investigate their effect on environmental impacts. The hybrid model was analysed for the LCA of the barge-type floating wind turbine (see
Section 5.1). In the truck, train, and vessel scenarios, all components of the barge-type floating wind turbine were transported by road, railway, and seaway, respectively. Moreover,
Table 6 shows total distance of components transportation such as road, railway and sea distances. Concerning the results, the total CO
2 emissions (tonnes) and CO
2e (g/kWh) emissions for the whole the LCA of the barge-type floating wind turbine are shown in
Table 7. As a result of these scenarios, the total distance of the train scenario was higher than other scenarios, while, regarding total CO
2 and g CO
2eq./kWh, the train scenario had lower values, 20.6 tonnes, and 0.172 g/kWh, respectively. Although the total transportation distance is the shortest by sea, it is the transportation mode with the highest carbon emissions and g CO
2eq./kWh. This was analysed for the hybrid scenario in the LCA of the barge-type floating wind turbine. The main reason for this is that the parts such as blades, nacelle, and towers are large. Moreover, the seaway is preferred in order to transport these parts without any damage and to avoid jeopardizing other traffic.
6. Conclusions
Previous studies have focused on the LCA of sway, spar, TLB, semisubmersible, and TLP wind turbines excluding the barge-type floating one. In this paper, the life cycle assessment of the barge-type floating wind turbine was performed, and the LCA of the barge-type floating wind turbine was compared to the LCA of 2 MW, onshore, offshore and 5 MW sway-type floating wind turbines by considering global warming potential, acidification potential, and energy payback time. The LCA of the barge-type floating wind turbine illustrates that the manufacture stage has high GWP and AP. The principal reason for this is the use of a high quantity of steel, nylon fibre and concrete. The other LCA stages do not exceed 6% of the total value of GWP contribution.
CO2 emissions of all turbines varies from 7.09 to 22.3 g CO2 eq./kWh. The energy payback time varies from 7.9 to 20.3 months. The barge-type floating wind turbine has a higher CO2 and energy payback time when compared with other wind turbines, whilst the results of the onshore wind turbine show less CO2 and SO2 and low energy payback time. Regarding acidification potential, the highest contribution to SO2 emissions was calculated for the 5 MW floating wind turbine. This highest value is related to the use of cast iron for the mooring system of the floating wind turbine. Given these results, the 2 MW onshore wind turbine is the most environmentally friendly and has the best energy performance.
GWP and energy payback time can be decreased by using alternative materials, components and recycling materials. Specifically, it is recommended during the development of the manufacturing stage to decrease environmental impacts. In this study, GWP and energy payback time were found to be the highest in the barge-type floating wind turbine. Thus, further research is recommended on whether GWP and energy payback time can be decreased and annual energy yield can be increased. In light of the results of the present study, the following recommendations can be made:
The energy capacity of the barge-type floating wind turbine should be increased. With this rise, the GWP emissions will reduce, and energy payback time will decrease while increasing the annual energy yield.
The platform and the tower, which are the most energy-consuming parts of the structure, should be designed optimally to save materials. The platform of the barge-type floating wind turbine is made of concrete and steel. Hence, a type of concrete having less CO2 emissions or an advanced high-strength concrete (saving material) could be preferred.
As floating wind turbines are located in deep water, a combination of a platform of the barge-type wind turbine that includes a wave energy system should be developed. This combination could decrease the equivalent CO
2 emissions by increasing the annual energy yield as described in [
24].
The lifetime of the wind turbine could be extended, e.g., to 25 or 30 years. Thus, the environmental impact (CO2 and SO2) per kWh of electricity generated would diminish.