What Other Factors can Reduce Turbine CO,-e Emissions?

Turbine design

Changes in design of wind turbines can affect GHG emissions over their lifetimes. Several studies have described such changes, as described in this section.

Guezuraga et al. (2011) compared GHG emissions from a 1.8 MW gearless turbine with a 2.0 MW turbine with gearbox. The gearless turbine generated 8.8 g CO,-e/kWh compared with 9.7 for the geared turbine. Ozoemena et al. (2018) found that a turbine design with advanced rotors and reduced tower mass increased CO,-e emissions by 6%; however, a design with drivetram improvements using permanent magnet generators decreased CO,-e emissions by 13%.

Vertical axis wind turbines have lower power ratings compared to horizontal axis turbines (Uddin and Kumar. 2014). However. Rashedi et al. (2013) found vertical axis wind turbines to har e lower CO,-e emissions per kWh compared to horizontal axis turbines.

Simons and Cheung (2016) developed a method to quantify wind farm carbon dioxide emissions and energy production in order to enable the design of more efficient wind farms by varying turbine material, hub height, and blade length.

Turbine size

Generally, turbines in the MW size range emit less GHG emissions per kWh of electricity generated than turbines in the kW size range. Crawford (2007) found an 11% increase in energy yield for a 3 MW turbine compared to an 850 kW turbine. Tremeac and Meuuier (2009) found that a smaller wind turbine (250 W) produced greater CO,-e emissions per kWh electricity produced than a 4.5 MW turbine. Juugblutz et al. (2005) found the environmental impact per kWh to decrease for larger wind turbines, an effect of scaling. Raadal et al. (2011) surveyed 63 life cycle studies conducted between 1990 and 2019, and found that GHG emissions from wind power varied from 4.6 to 55.4 g CO,-e/kWh. The low value was for a 3 MW turbine; GHG emissions were found to decrease with increasing turbine size, reflecting economies of scale.

Turbine lifetime

Currently, horizontal axis wind turbines are generally assumed to har e a lifespan of 20 years (Simons and Cheong, 2016). However, in practice, lifetimes of 30 years and over have been achieved (Garrett and Ronde, 2013). When the turbine lifespan is extended via proactive maintenance, GHG emissions from maintenance increase; however, this can be outweighed by the additional energy production, so that the overall GHG emissions per kWh of electricity generated decreases, because the pollutants from manufacturing are distributed over more years.

Alsaleh and Sattler (2019) examined changes in CO,-e.kWh power produced and net energy production when the turbine life span was extended via increased maintenance. They found that C'O.-e kWh power produced decreased from 0.053 to 0.042 to 0.035 when the turbine life span was extended from 20 to 25 and 30 years, respectively. They also found that for the 20, 25, and 30-year life spans, the turbines produced 3.2,4.2, and 5.1 times more energy than they consumed over then life cycle, respectively.

Turbine lifetimes could also be increased via the use of newly developed materials designed to be stronger and/or require less maintenance. For example, nickel-catalyzed growth of iron-aluminium microparticles and nanoparticles at defect sites in a low-density, aluminium-rich steel were found to strengthen the steel without compromising its pliability (Kim et al., 2015).

Wind farm location and layout

The greater the power production from a given wind turbine per time, the lower the CO,-e emissions per kWh (Garrett and Ronde. 2013). Wind farm location and layout can, thus, be chosen to maximize energy production and decrease GHG emissions per kWh power produced.

Kusiak and Song (2010) developed algorithms to predict wind farm power production based on weather forecasting data. The use of these algorithms would enable selection of the wmdfann location to maximize power production based on higher wind speeds. Larsen and Rethore (2013) developed the TOPFARM system to optimize wind farm layout based on power production, as well as cost and fatigue loads. Barthelmie and Jensen’s (2010) research findings proposed that placing horizontal wind turbines close together reduces their efficiency; specifically, for each distance equivalent to a rotor diameter that the turbines are placed closer together, there is a 1.3% loss in efficiency.

Arveseu and Hertwich (2012), from then review of 59 studies, report that off-shore turbines show comparable or slightly higher emissions than on-shore systems comprised of large turbines, despite higher wind capacity factors due to higher resource requirements.

Manufacturing location

Manufacturing location can also be chosen in order to take advantage of renewable energy for manufacturing, and to minimize transport distances and, thus, minimize GHG emissions due to transport. Guezuraga et al. (2011) found that manufacture of turbine components in China produced more CO,-e/kWli than manufacture of components in Germany or Denmark, due to longer required transport distances and greater use of black coal for component production in China. Oebels and Расса (2013) found 7.1 g/kWli for a 1.5 MW wind turbine, which is relatively low, based on the clean energy mix of Brazil (87% renewables).

End-of-life recycling

Compared to recycled materials, acquisition and processing of virgin materials requires greater energy and, thus, produces more GHG emissions when the energy is supplied by fossil fuels. Turbine materials, including metals (steel, iron, aluminium), plastics, and concrete, are excellent candidates for recycling. The greater the amount of material recycled, the lower the end-of-life CO,-e emissions (Martinez et al., 2010).

Table 2 summarizes disposal information included in several large (> 1 MW) wind turbine studies. All 3 studies assume a 90% or greater recycling percentage for metals, two assume 90% recycling of plastics, and one assumes 90% recycling of concrete. Martinez et al. (2009) and Tremeac and Meunier

Table 2. Disposal information included in various large wind turbine studies.

Note: R: Recycling, L: Landfilling, C: Combustion.

(2009) found CO,-e emissions from the end-of-life phase to be negative because the benefits of recycling the turbine pails were allocated to that phase, rather than to the product, which is subsequently made from the recycled materials, as was done by Alsaleh and Sattler (2019).

Conclusions and Recommendations

Among the phases of a turbine's life cycle, studies consistently report that manufacturing produces the greatest CO,-e emissions. Among turbine materials, steel causes the most GHG emissions during the manufacturing process, according to most studies. Using recycled steel, as well as changes in the steel production process—use of renewable energy, use of an electric furnace rather than a basic oxygen furnace, and substitution of charcoal for a portion of coal and coke—can reduce steel’s carbon footprint. Further research to reduce CO, emissions from steel production is recommended.

Additional strategies for reducing GHG emissions from wind energy include changes in turbine design, use of turbines in the MW range rather than the kW range, extending turbine lifetimes via proactive maintenance or use of new materials, selection of wind farm location and layout to maximize energy production, selection of manufacturing location to take advantage of renewable energy resources and minimize transport distances, and eud-of-life recycling of turbine materials.

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