Table of Contents:
: Reducing the Carbon Footprint of Wind Energy: What Can Be Learned from Life-Cycle Studies?
A renewable energy source that has recently seen rapid growth is wind energy. U.S. wind capacity grew by 13% in 2017, resulting in 6.3% of U.S. power being produced by wind energy (American Wind Energy Association. 2017). Wind turbines can convert wind to electric power without generating greenhouse gas emissions; however, over their entire life cycle (raw material acquisition, manufacturing, operation, and end-of-life), turbines do contribute pollutants that cause climate change, including carbon dioxide and methane (Alsaleh, 2016). To compare the effects of wind energy generation with other energy options, carbon dioxide equivalent (CO,-e) emissions must be estimated over the complete life cycle.
A variety of life cycle assessment (LCA) and carbon footprint studies have been conducted regarding greenhouse gas (GHG) emissions and other environmental impacts of wind turbines (e.g., impacts on wildlife, noise, resource consumption). Wang and Wang (2015) reviewed 64 studies in terms of GHG emissions, as well as wind turbine impacts on noise pollution, bud and bat fatalities, and land surfaces. Saidur et al. (2011) reviewed 54 studies of wind turbine carbon dioxide emissions, as well as impacts on wildlife, water consumption, noise, and visual aesthetics. Davidsson et al. (2012) compared 12 life cycle assessments of wind energy systems in terms of methods used, energy use accounting, quantification of energy production, energy performance and primary energy, natural resources, and recycling. Leung and Yang (2012) reviewed 80 studies in terms of wind turbine impacts on noise and visual impacts, impacts on animals and buds, and local climate; however, impacts on global climate through greenhouse gas emissions were not included. Aivesen and Hertwich (2012) provided a comprehensive review of the literature up to 2012, categorizing 59 studies by on-shore vs. off-shore turbines, turbine size, turbine lifetime, geographic scope, and impact categories included. From the existing literature, they found a mean emission value of 19 (± 13) g CO,-e/kWh of electricity generated from wind turbines.
Previous studies can provide useful insights regarding the phase of the wind turbine life cycle that produces the most GHG emissions, and sensitivity of emissions to changes in materials, manufacturing processes, and other parameters. This chapter will, thus, summarize insights from the literature concerning effective approaches for reducing the life-cycle carbon footprint of wind energy.
Which Phase of a Wind Turbine’s Life Cycle Produces the Greatest CO,-e Emissions?
Figure 1 shows a flow chart of wind turbine life-cycle phases. Figure 2 shows the contribution of turbine life-cycle phases to overall CO,-e emissions for selected studies for large (> 1 MW) on-shore wind turbines. Every study shown in Figure 2 found the raw material/manufacturing phase (sometimes combined with the installation phase) to contribute the most to greenhouse gas emissions. Arvesen and Hertwich (2012), in their review of 59 studies that included smaller turbines < 1 MW, as well as offshore turbines, also found that the manufacturing phase dominates energy consumption and climate change indicators, sometimes up to 90%. Hence, the manufacturing phase should be targeted for reducing greenhouse gas contributions of wind energy. It will be discussed in more detail in the next section.
In Figure 2, the installation phase ranks 2nd or 3rd in importance, if it was included as a separate phase. Hence, after raw material acquisition/manufacturing, the installation phase should be targeted to reduce GHG emissions. The installation phase consists of the use of heavy-duty diesel equipment for digging/moving soil, pouring the foundation, and installing the turbine tower, rotor, and nacelle (Figure 3). Methods of reducing emissions from the installation phase would include the use of heavy- duty equipment which is more fuel-efficient or operates on alternative fuels.
In Figure 3, O&M ranks 2nd, 3rd, or 4th in importance, depending on the study. The operation and maintenance phase includes inspection trips, change of oil, lubrication of gears and the generator, replacement of turbine parts, and repair of the turbines when they break down. Interestingly, the parts targeted for replacement varied among the studies (brake system, generator, turbine blade, part of the nacelle), which may account for differences in rankings. Differences in emissions from the electricity mix used for operation and maintenance (global, USA, and Brazil) may also explain variations in the rankings.
Similar to O&M, transportation ranks 2nd, 3rd, or 4th, depending on the study. Tremeac and Meimier (2009), who found transportation to rank 2nd, included a concrete tower, which was heavier to transport compared to steel, used in most other studies. Interestingly, both Alsaleh and Sattler (2019) and Rajaei and Tinjum (2013) included overseas transport of turbine parts, but still found transportation to rank 4th. Rajaei and Tinjum (2013) also found that eliminating transportation of components from overseas, via local manufacture, could reduce transportation GHG emissions by 22%.
In Figure 2, end-of-life consistently ranks last, regardless of whether the parts not recycled were combusted (Martinez et al., 2009) or landfilled (Tremeac and Meimier, 2009; Alsaleh and Sattler, 2019). In a number of studies, CO,-e emissions from the end-of-life phase were considered negative because benefits of recycling the turbine parts were included.
Figure 1. Wind turbine life cycle phases (adapted from D’Souza et al., 2011).
Figure 2. Contributions of wind turbine life cycle phases to CO,-e emissions.
Figure 3. Major turbine parts docera.com/wiki.
In summary, according to Figure 2, to reduce life cycle carbon emissions, the raw material acquisition/' manufacturing phase should be targeted, followed by installation. Impacts of raw material acquisition/ manufacturing for specific materials will be discussed in the following section.
Which Turbine Material Generates the Most CO,-e Emissions during Manufacturing?
Table 1 shows an example of materials used for a turbine for a Gamesa G83 2-MW turbine. The mass percentages are of the entire turbine mass, including the nacelle, rotor, tower, wiring, and foundation. The
Table 1. CO,-e emissions from turbine material manufacturing for Gamesa GS3 2 MW turbines (Gamesa, 2013).
substation is excluded. CO,-e emissions estimates were generated using SimaPro Software version 8.3.2, with all databases selected, including Ecoinveut 3, ELCD, EU & DK Input Output Database, Industry data 2.0, Methods, Swiss Input Output Database, US-EI 2.2, and USLCI.
According to Table 1, the material with the highest mass is concrete (76%), but manufacturing it produces only 11% of the CO,-e emissions of the materials. This is because the average emissions intensity (kg of CO,-e emitted per kg of material manufactured) for concrete is low, at 0.15 (the value for cement is 0.89; concrete contains water and aggregate mixed with cement). Carbon emission intensity values for other major turbine components are higher than that for concrete: 6.1 for stainless steel, 0.88 to 3.29 for other steel, 1.51 for cast non. 2.6 for fiberglass, and 8.14 for aluminium (Wirmepeg, Canada. 2012). Manufacture of low alloy steel, for example, which makes up 15.5% of the materials by mass, generates 39% of the CO,-e emissions. According to Table 1, manufacture of low alloy steel, corrugated steel, and high alloy steel together produces 58.1% of CO,-e emissions.
Most (83%) of the low alloy steel shown in Table 1 is used in the tower of the turbine. Accordingly, Alsaleh and Sattler (2019) found that the manufacture of the tower contributed > 40% of greenhouse gas emissions. Similarly, for a 1.5 MW turbine in France, Oebels and Расса (2013) found the steel tower to contribute > 50% of the CO, emissions from the manufacturing phase. Guezuraga et al. (2011) and Ardeute et al. (2008) also found the steel tower to contribute over 50% and around 50%, respectively, of energy consumption over the turbine's lifetime.
Hence, in order to reduce climate change contributions of turbines, steel used in the tower should be targeted for emission reductions. Methods of accomplishing this will be discussed in detail in the next section.
A few wind turbine studies (Martinez et al., 2009; Rajaei and Tinjum, 2013) har e found the foundation to produce the greatest CO, emissions, primarily due to the cement. Bemdt (2015) examined the influence of concrete mix design on turbine life cycle GHG emissions. Replacing 40 MPa class concrete with 32 MPa class concrete reduced CO, emissions by 11%. Replacing 65% of the cement with blast furnace slag reduced CO, emissions by around 44%. Use of recycled concrete aggregate reduced emissions moderately.
How can CO, Emissions from Steel Manufacturing for Turbine Parts be Reduced?
Using recycled steel would substantially reduce CO, emissions, as would the use of renewable energy in steel manufacturing. In addition, a number of changes and innovations in the steel-making process can lower steel’s carbon footprint. Basic oxygen furnace (BOF) steel-making emits 4 times more CO, than electric furnace steel-making: hence, replacing BOF processes with electric furnace processes would reduce GFIG emissions by 75% from one of the most-energy intensive parts of the steel-making process (Turner, 2011). Using charcoal to replace part of the coal and coke used in steelmaking can reduce CO, emissions by about 50% without requiring substantial modification of the steelworks (CSIRO, 2015).
Oebels and Расса (2013) found that replacing the steel tower with a cement tower, with reinforcing steel, decreased CO, emissions by 6.4% overall. Although manufacturing emissions decreased by more than 6.4%. transportation emissions increased, due to the greater weight of concrete.
Ardente et al. (2008) found that replacing 40% of the steel in the turbine transformer with copper reduced CO, emissions. Substituting carbon fibers for glass fibers in reinforced plastics increased energy consumption by 12%, but substituting flax fibers decreased energy consumption by 1%.