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Life-cycle green-house gas emissions of onshore and offshore wind turbines
Journal of Cleaner Production 210 (2019) 804e810
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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
Life-cycle green-house gas emissions of onshore and offshore wind turbines Shifeng Wang a, c, Sicong Wang b, *, Jinxiang Liu a a
School of Urban Construction, Nanjing Tech University, No. 30 Puzhu Road, Nanjing, 211816, China Institute of Quantitative and Technical Economics, Chinese Academy of Social Sciences, Beijing, 100732, China c School of Engineering, Newcastle University, Newcastle, NE1 7RU, UK b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 June 2018 Received in revised form 22 September 2018 Accepted 3 November 2018
Onshore and offshore wind turbines may have different environmental sustainability due to their own characteristics, and this information is important for future growth of wind power. The paper uses life cycle assessment (LCA) to estimate the life-cycle greenhouse gas (GHG) emissions of onshore and offshore wind turbines with the nominal capacity of 2 MW, to advance our understanding of onshore and offshore wind energy and to inform policy, planning, and investment decisions for future growth of wind power. Results show that the life-cycle GHG emission intensity is 0.082 kg CO2-equivalent (eq)/Megajoule (MJ) for onshore wind turbine and is 0.130 kg CO2-eq/MJ for offshore wind turbine, respectively. Offshore wind turbine has larger life-cycle GHG emissions than onshore wind turbine, owing to the floating platform fixed in sea. Onshore and offshore wind turbines have much smaller life-cycle GHG emission intensity than coal power plants. If the installed wind turbines in 2014 displace coal, the saved GHG emissions can roughly reach 5.08 107 t CO2-eq, accounting for 0.09% of global GHG emissions in 2012. The sensitive analysis shows that the lifetime and energy production of wind turbine have large influences on the GHG emission intensity of both onshore and offshore wind turbines, implying that it is an effective way to prolong the lifetime of wind turbine and increase the energy production of wind turbine to reduce the GHG emission intensity of wind turbine. The sensitivity analysis further shows that the distance from wind turbine factory to wind farm site has more significant influence on the life-cycle GHG emission intensities of both onshore and offshore wind turbines than that from wind farm site to the recycling and landfill locations, suggesting that the wind farm site and the wind turbine factory should be as close as possible. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Wind energy Onshore wind turbine Offshore wind turbine GHG Life-cycle assessment
1. Introduction Wind power has rapid development worldwide, mainly owing to its perceived importance for combating climate change (Wang and Wang, 2015; Wang et al., 2015a) and meeting the energy service demand (e.g. electricity) in a sustainable way (Weinzettel et al., 2009). The global total installed wind power reaches 432.9 GW by the end of 2015 (GWEC, 2016). In contrast to gas and coal power plants, wind power plants have no significant greenhouse gas (GHG) emissions or resource consumption when converting kinetic energy of wind into electricity. Wind energy has already received the legislative support from government in several countries
* Corresponding author. (S. Wang). E-mail address: [email protected] (S. Wang). https://doi.org/10.1016/j.jclepro.2018.11.031 0959-6526/© 2018 Elsevier Ltd. All rights reserved.
including the UK (Wang et al., 2015a), and its huge boom in implementation and forecasts necessitates the further understanding of this power source (Pehnt et al., 2008). Although wind energy is a clean energy source, all stages of wind turbine life cycle (i.e. primary materials production, manufacturing of wind turbine parts, transportation, maintenance, dismantle and disposal) will consume energy and emit GHGs to the atmosphere. Some stages will give off less GHGs and consume less net energy; however other stages will give off more GHGs and consume more net energy. To reduce both GHG emissions and net energy inputs and to make wind energy more sustainable, it is therefore critically important to identify the dominant process (s) of the wind turbine which produces more GHGs emissions and consumes more net energy. The Life-Cycle Assessment (LCA) is an effective way to quantify the environmental impacts of energy technology. It has been already used to estimate the life-cycle GHG emissions of wind
S. Wang et al. / Journal of Cleaner Production 210 (2019) 804e810
turbine. Tremeac and Meunier (2009) estimate using LCA that the life-cycle GHG emissions for 250 W onshore wind turbine and 4.5 MW onshore wind turbine are 0.013 kg CO2-equivalent (eq)/MJ and 0.004 kg CO2-eq/MJ in France, respectively. The amount of lifecycle GHG emissions of wind turbine assessed by LCA will be impacted by system boundary and assumptions as well as the data quality. The life-cycle GHG emissions for both 850 kW onshore wind turbine and 3.0 MW onshore wind turbine are estimated as 0.003 kg CO2-eq/MJ in Australia by Crawford (2009), whereas the life-cycle GHG emissions for 1.25 MW onshore wind turbine are estimated as 0.002 kg CO2-eq/MJ in China by Chen et al. (2011). The life-cycle GHG emissions for six different 5 MW offshore wind turbines are estimated ranging 0.005e0.0095 kg CO2-eq/MJ (Raadal et al., 2014). The variation in the estimated life-cycle GHG emissions of wind turbines can be reduced using a meta-analytical process called “harmonization” which considers capacity factor, operating lifetime, system boundary, and global warming potentials (Dolan and Heath, 2012). The life-cycle GHG emissions of wind turbines after harmonization are 0.0033 kg CO2-eq/MJ for onshore wind turbines and 0.0031 kg CO2-eq/MJ for offshore wind turbines, respectively (Dolan and Heath, 2012). Although the life-cycle GHG emissions of both onshore and offshore wind turbines can be estimated through LCA (Tremeac and Meunier, 2009; Chen et al., 2011; Dolan and Heath, 2012; Arvesen and Hertwich, 2012; Raadal et al., 2014; Wang and Wang, 2015), the estimation of life-cycle GHG emissions for both onshore and offshore wind turbines are conducted separately. Typically, offshore is less covered compared to onshore. Recently, some but limited studies evaluate the life-cycle GHG emissions for both onshore and offshore wind turbines (Bonou et al., 2016; Kadiyala et al., 2017). Onshore and offshore wind turbines have their own characteristics. First, onshore wind turbines are deployed in land where it has low wind profile, and therefore have low energy outputs. In contrast, offshore wind turbines are deployed in shallow water or deep sea where it has relatively strong wind profile, and thereby have high energy output. Second, offshore wind turbines will need additional structure to fix the floating platform. This will require additional energy to produce the materials of structure and give off additional GHGs. All these different characteristics may result in different environmental sustainability of wind energy. Understanding the relevant processes for these can contribute to obtain more insights for the sustainable wind energy, and this information can be used by lenders, utility executives, and lawmakers to inform policy, planning, and investment decisions for future growth of wind power. This paper will examine the life-cycle GHG emissions for onshore and offshore wind turbines with the nominal capacity of 2 MW. In this paper, we first identify the dominant processes of both onshore and offshore wind turbines, and then use LCA to estimate the GHG emissions for each dominate process and for the whole life cycle processes for both onshore and offshore wind turbines. We then identify the dominant process (s) with more GHG emissions. In addition, we also perform a sensitive analysis to identify the interesting inputs which has the most significance on the life-cycle GHG emissions of wind turbine. 2. Materials and methods We use the process-based LCA, which follows the principle of ISO 14044, to calculate the life-cycle GHG emissions of both onshore and offshore wind turbines. The system boundary is shown in Fig. 1. The functional unit is set as 1 MJ electricity generated at the wind power plants with the selected turbines, and the GHG emissions are estimated for this functional unit. The GHG emissions are expressed as kg CO2-eq, and the GHG emission intensity, which is
805
Fig. 1. Life-cycle boundary and processes of onshore and offshore wind turbine.
defined as the amount of GHG emissions per unit of power generation, is expressed as kg CO2-eq/MJ, in order to compare to other global studies. Please note the MJ part of the unit of kg CO2-eq/MJ refers to the electrical power production. Fig. 1 shows the process flow chart of onshore and offshore wind turbines. The life-cycle processes of onshore and offshore wind turbines are organized into four stages: manufacturing, transport and erection, operation and maintenance, and dismantling and disposal. Each stage encompasses a number of processes. During the stage of manufacturing, the foundation, tower, nacelle, rotor and parts of the transmission grid are assumed being manufactured in wind turbine factory. Then these components will be transported from factory to election site and will be assembled and installed through crane work and other construction work at site at the stage of transport and installation. The processes at the stage of operation and maintenance include change of oil and lubrication, renovation of gear and generator, and transporting to and from the turbines for regular checking. Finally, at the stage of dismantling and disposal the main processes involve carnage for dismantling, transporting from erection place to the final disposal, and the further handling of the materials, either by recycling or by deposit. Fig. 2 shows the structure of onshore and offshore wind turbines. The wind turbine is mainly composed of: rotor, nacelle, tower, and foundation. The total GHG emissions are the sum of these four stages, described as
GHGTotal ¼ GHGM þ GHGTI þ GHGOM þ GHGDD
(1)
where the subscript M indicates Manufacturing, TI for transport and installation, OM for operation and maintenance, DD for dismantling and disposal. The GHG emissions for each stage can be calculated as
GHG ¼
X ðinputi Gi Þ
(2)
where Gi is the GHG-intensity coefficient of the ith input of wind turbine (including direct and indirect), and inputi is the amount of the ith input. In this analysis, the capacity size of both onshore and offshore
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S. Wang et al. / Journal of Cleaner Production 210 (2019) 804e810 Table 2 Inputs for 2 MW offshore wind turbine (Dones et al., 2004). Component
Material
Total mass (kg)
Tower
Steel, low alloyed Epoxy resin Concrete Reinforcing steel Glass fibre reinforced plastics Chromium steel Cast iron Steel, low alloyed Cast iron Chromium steel Cast iron Chromium steel Rubber Cast iron Chromium steel Aluminium Copper Chromium steel Chromium steel Glass fibre reinforced plastics Steel, low alloyed Chromium steel Chromium steel Chromium steel Lubricant Copper Lead Steel, low alloyed PVC
113,210 547 2,095,200 80,000 29,714
Foundation Rotor
Nacelle
blades extender hub Mechanic parts
shaft main bearing gearbox
generator Fig. 2. Basic structure of onshore wind turbine (left) and offshore wind turbine (right).
wind turbines is determined as 2 MW due to the data accessibility, though larger wind turbines are commercially used nowadays. The inputs for both onshore and offshore wind turbines are detailed in Table 1 and Table 2, respectively. In these tables, GFRP denotes glass fibre reinforced plastic. Table 3 shows the parameters of wind turbine. In this study, we only focus on wind turbine itself, so the transmission of electricity from wind turbine to other places such as transformer station is not considered. In addition, due to the unavailability to access the complete data about all the life-cycle processes of wind turbines, a number of important assumptions are made in this analysis which are described as follows. Assumption for manufacturing: All the components of wind turbine are assumed to be produced in the wind turbine factory. Assumption for transport and installation: In general, onshore wind turbine is installed in land and offshore wind turbine is in water. Such difference will result in different transport and installation strategies for both onshore and offshore wind turbines. In this analysis, the transport and installation strategies are treated
Table 1 Inputs for 2 MW onshore wind turbine (Elsam Engineering, 2004; Ghenai, 2012). Component Tower Nacelle
tower structure cathodic protection gears generator core generator conductors transformer core transformer conductors transformer conductors cover
Material
Total mass (kg)
Low carbon steel Zinc alloys Stainless steel Cast iron, gray Copper Cast iron, gray Copper Aluminium alloys GFRP, epoxy matrix (isotropic) Cast iron, ductile (nodular) Stainless steel
164,000 203 19,000 9000 1000 6000 2000 1700 4000
main shaft other forged components other cast components Cast iron, ductile (nodular) Rotor blades GFRP, epoxy matrix (isotropic) iron components Cast iron, ductile (nodular) spinner GFRP, epoxy matrix (isotropic) spinner Cast iron, ductile (nodular) Foundation pile & platform Concrete steel Low carbon steel Transmission conductors Copper conductors Aluminium alloys insulation Polyethylene (PE)
12,000 3000 4000 24,500 2000 3000 2200 805,000 27,000 254 72 1380
Casing
brake frame cover
Yaw system
ball bearing drive brake Hydraulic system Transmission
10,966 11,320 12,661 10,025 10,025 9139 9139 100 3382 8877 845 986 613 16,940 11,224 2389 1225 816 2042 150 3900 7575 8766 3500
Table 3 Parameters of wind turbine (Elsam Engineering, 2004).
Capacity size (MW) Life time (year) Electric power generation (MWh)
Onshore wind turbine
Offshore wind turbine
2 20 5634
2 20 8088
equivalent for both onshore and offshore wind turbines. This is reasonable because: (1) the offshore wind turbines considered is close to shore. For example, the Middelgrunden in Denmark is 3.5 km to the shore; and (2) the on-site construction activities for offshore wind turbine is rarely obtained. So we only need to describe the transport and installation strategy for onshore wind turbine. The same manner can also be applied to the stages of operation and maintenance, and dismantling and disposal. Transportation of wind turbine to wind farm site is assumed by road truck with fuel of diesel. The consumption intensity of the diesel is estimated as 0.05 L/(kg.km) with diesel density of 0.83 kg/L (Chen et al., 2011). The average transport distance from wind turbine factory to installation location is assumed as 1500 km. Assumption for operation and maintenance: Regular check-up of the turbines with diesel truck are assumed three times per year (Dolan and Heath, 2012; Haapala and Prempreeda, 2014). The fuel consumption of each regular check-up is 50 kg of diesel (Ardente et al., 2008). For the offshore case, 4 h of helicopter operation per year is added. The oil and lubricant changes are conducted for each check-up, while rotor blade, gearbox, and generator replacement are assumed once within the average lifetime of 20 years (Haapala and Prempreeda, 2014). Assumption for dismantling and disposal: For rotors and nacelle, 20% of blade materials will be recycled (Ardente et al., 2008),
S. Wang et al. / Journal of Cleaner Production 210 (2019) 804e810
and other materials will be sent to the dump nearby. For tower, an average material loss rate of 10% for materials undergoing a recycling process has been assumed (Elsam Engineering, 2004; Haapala and Prempreeda, 2014). It is assumed that the concrete of foundations is landfilled entirely and other materials of foundations will be recycled with an average material loss rate of 10% (Haapala and Prempreeda, 2014). It is assumed that the recycling and landfill locations are 100 km from the wind farm site, respectively. For the offshore case, additional 30 km is added (Bonou et al., 2016). The GHG-intensity coefficients for inputs are shown in Table 4. When calculating the life-cycle GHG emissions of wind turbine, the GHG emissions of land for foundation is ignored, as wind turbine foundations cover only a small area of land (Chen et al., 2011) and such data is unavailable as well. We fulfil a sensitivity analysis to identify the interesting inputs which has the most significance on life-cycle GHG emissions of wind turbine. Sensitivity analysis determines how the uncertainty in the model result can be apportioned to the value of a given input component. That is, does a small change in the input cause a significant change in the output? If this is the case, the model is termed sensitive to the input. We take account of the lifetime of wind turbine, energy production, degree of recycling and the distance from wind turbine factory to wind farm site and from wind farm site to the recycling and landfill locations as the interesting inputs. 3. Results Fig. 3 shows the life-cycle GHG emissions for 2 MW onshore and offshore wind turbines. The life-cycle GHG emissions for 2 MW onshore wind turbine are 33278.19 t CO2-eq for 20-year lifetime. So the annual life-cycle GHG emissions are 1663.91 t CO2-eq for onshore wind turbine. Given that the electric power generation of onshore wind turbine is 5634 MWh/year (Table 3), the life-cycle GHG emission intensity is 0.082 kg CO2-eq/MJ for onshore wind turbine. The life-cycle GHG emissions for 2 MW offshore wind turbine for 20-year lifetime are 75904.84 t CO2-eq. Therefore, the annual life-cycle GHG emissions of offshore wind turbine are 3795.24 t CO2-eq. The life-cycle GHG emission intensity of offshore wind turbine is 0.130 kg CO2-eq/MJ, provided that the electric Table 4 GHG intensity for materials. Material
GHG intensity (kgCO2-eq/kg)
Source
Aluminium Aluminium alloys Cast iron Cast iron, ductile (nodular) Cast iron, gray Chromium steel Concrete Copper Epoxy resin GFRP, epoxy matrix (isotropic) Glass fibre reinforced plastics Lead Low carbon steel Lubricant Polyethylene (PE) PVC Reinforcing steel Rubber Stainless steel Steel, low alloyed Zinc alloys Diesel
22 22 1.25 1.25 1.25 2.03 0.22 4.7 3.07 6.72 2.63 1.64 1.39 2.93 2.40 2.14 2.03 3.18 2.03 1.39 3.41 0.45
Gao et al. (2009) Gao et al. (2009) Chen et al. (2011) Chen et al. (2011) Chen et al. (2011) Chen et al. (2011) Chen et al. (2011) Chen et al. (2011) Chen et al. (2011) Li et al. (2013) Li et al. (2013) South End Plant (2011) Chen et al. (2011) South End Plant (2011) South End Plant (2011) Franklin Associate (2011) Chen et al. (2011) Li et al. (2013) Chen et al. (2011) Chen et al. (2011) South End Plant (2011) Chen et al. (2011)
807
Fig. 3. GHG emissions for onshore and offshore wind turbine.
power generation of offshore wind turbine is 8088 MWh/year (Table 3). In order to identify the dominant process (s) of the wind turbine, a breakdown to the life-cycle GHG emissions of wind turbine is conducted for both onshore and offshore wind turbines, respectively. The breakdown shows that for onshore wind turbine the transport and installation accounts for large proportion of life-cycle GHG emissions (91.86%), followed by the dismantling and disposal (5.06%) and the manufacturing (2.41%), while the operation and maintenance represents only 0.67%. The large proportion of lifecycle GHG emissions for the transport and installation is due to that the total mass of onshore wind turbine (which is a large amount) has to be transported from wind turbine factory to wind farm site. For offshore wind turbine, the breakdown shows that the transport and installation makes up the largest fraction of the lifecycle GHG emissions, up to 90.98%, whilst the operation and maintenance accounts for only 0.27%. The argument for onshore wind turbine can be applied here to explain the large fraction of life-cycle GHG emissions in the transport and installation. Fig. 3 demonstrates that for the same capacity of 2 MW the lifecycle GHG emissions of offshore wind turbine are more than two times of that of onshore wind turbine, and the life-cycle GHG emission intensity of offshore wind turbine is larger than that of onshore wind turbine. This is because offshore wind turbine needs a floating platform fixed in sea. The floating platform needs more materials, and therefore results in more GHG emissions in the manufacturing, transport and installation, and dismantling and disposal (Fig. 3). In addition to the operation and maintenance, all other stages have larger GHG emissions for offshore wind turbine than for onshore wind turbine. The relative rate of difference in GHG emissions between offshore wind turbine and onshore wind turbine is the largest for the dismantling and disposal (140%), followed by the transport and installation (125.90%) and manufacturing (43.73%). This is mainly due to the floating platform of offshore wind turbine which has to be fixed in sea and thereby needs more concrete than onshore wind turbine. The amount of concrete of offshore wind turbine is over two-folds of that of onshore wind turbine (Tables 1 and 2). Fig. 4 shows the sensitivity analysis results for the life-cycle GHG emissions of wind turbines. The lifetime and energy production of wind turbine and the distance to wind farm site are demonstrated having larger influences on GHG emission intensity of both onshore and offshore wind turbines. Therefore, in order to reduce GHG emissions of wind turbine, these factors should draw more attentions. 4. Discussion Rapidly growing population and climate change are two main
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S. Wang et al. / Journal of Cleaner Production 210 (2019) 804e810
Fig. 4. Sensitivity analysis for wind turbine. The lines for lifetime of turbine and energy production are identical and are overlay for both onshore and offshore wind turbines.
drivers of renewable energy adoption. Wind energy is an important source of renewable energy, and wind energy technology has been already identified as one of the most important energy technologies in the UK DECC's Renewable Energy Roadmap that have either the greatest potential to help the UK meet the 2020 target in a cost effective and sustainable way, or offer great potential for the decades that follow (DECC, 2011). Since the early 1980's the United States of America (USA) has already used wind energy commercially to produce energy services such as electricity to satisfy a growing demand for electricity (Wang and Wang, 2015; Wang et al., 2015a; Kunz et al., 2007; Pasqualetti et al., 2004). Wind energy technology is expected to give off less GHG emissions while generating the demanded energy services, when compared to conventional energy technologies such as coal and fossil fuel technologies. The life-cycle GHG emission intensities of both onshore and offshore wind turbines are much smaller than that of coal power plants which have a typical life-cycle GHG emission intensity of 0.22 kgCO2-eq/MJ (Chen et al., 2011). The life-cycle GHG emission intensity of coal power plant is about 3 times of that of onshore wind turbine and 2 times of that of offshore wind turbine. Globally there are totally 369.6 GW installed wind turbines in 2014 among which offshore wind turbines account for 8.8 GW (GWEC, 2015). If these wind turbines displace coal, the saved GHG emissions can roughly reach 5.08 107 t CO2-eq, accounting for 0.09% of global GHG emissions of 5.35 1010 t CO2-eq in 2012 (JRC/PBL, 2014). Therefore, generating electricity from wind energy can save, to a great extent, the GHG emissions to the atmosphere when displacing coal, and mitigates the adverse impacts of coal on climate change. Offshore wind turbine has larger life-cycle GHG emissions and larger GHG emission intensity than onshore wind turbine, although they have the same capacity of 2 MW. Even if the wind turbine is rated at 2 MW, it will produce this power only with the right wind conditions. In general, the wind speed over ocean is much larger than that over land, due to less friction over the water. The stronger wind speed over ocean can make offshore wind turbine run more full-load hours per year than onshore wind turbine, resulting in more energy service outputs and thereby potentially smaller GHG emission intensity. In addition, offshore wind turbine will also
reduce land disturbance relative to onshore wind turbine. However, due to the fact that offshore wind turbine has floating platform, these advantages are offset in terms of GHG emissions. Therefore, it is important to design better floating platform for offshore wind turbine to achieve the equivalent GHG emissions to onshore wind turbine. Transportation (in the transport and installation and in the dismantling and disposal) has much larger GHG emission contribution to the life-cycle GHG emissions of wind turbines (onshore and offshore). In order to reduce the life-cycle GHG emissions of wind turbine, it is thus important to revisit transportation strategy. Transportation GHG emissions are impacted heavily by the distance and type of transport. Truck, train and ship have different GHG emissions and different economic costs. Tremeac and Meunier (2009) carried out a sensitivity analysis for the impacts of distance and type of transport on the life-cycle GHG emissions of wind turbine for two kinds of wind turbines: 250 W and 4.5 MW, and concluded that train was more environmentally friendly than truck if the train station was not too far from the wind farm site. Furthermore, train had more impact on life-cycle GHG emissions for larger capacity wind turbine. The sensitivity analysis conducted in this study further shows that the distance from wind turbine factory to wind farm site has more significant influence on lifecycle GHG emissions for both onshore and offshore wind turbines than that from wind farm site to the recycling and landfill locations. Therefore, the wind turbine factory and the wind farm site should be as close as possible. The sensitivity analysis shows that the lifetime and energy production of wind turbine are demonstrated having larger influences on GHG emission intensity for both onshore and offshore wind turbines. Therefore, it is an effective way to prolong the lifetime of wind turbine and increase the energy production of wind turbine to reduce the GHG emission intensity. This has informative implication for wind turbine manufacturing factory. There have four types of wind turbine: fixed-speed wind turbine (type 1), variable-slip wind turbine (type 2), doubly-fed induction generator wind turbine (type 3), and full-converter wind turbine (type 4). Type 2 can extract more energy from a given wind regime than type 1. However, it has more power lost as heat in the rotor resistance than type 1. Types 3 and 4 overcome this problem and will have higher energy production than type 2. Therefore, type 3 and type 4 will be the best option. The life-cycle GHG emissions of wind turbine will be impacted by the boundary of LCA, characteristics of wind turbine (e.g. wind turbine technology, size of wind turbine, lifetime of wind turbine), locations, and others. Table 5 shows the published life-cycle GHG emissions of wind turbine. It can be seen from Table 5 that the lifecycle GHG emission intensity is different for various capacity sizes and types of wind turbines. Furthermore, the life-cycle GHG emission intensity of wind turbine is not linearly proportional to the capacity size of wind turbine.
Table 5 Life-cycle GHG emissions of wind farm. Location
Turbine type
GHG emission intensity (kg CO2-eq/MJ)
Reference
China Australia Australia France France Germany Italy Worldwide worldwide
1.25 MW-onshore 850 kW-onshore 3.0 MW-onshore 250 W-onshore 4.5 MW-onshore 5.0 MW-offshore 660 kW-onshore
0.002 0.003 0.003 0.013 0.004 0.006 0.002e0.004 0.008e0.123 0.002e0.123
Chen et al. (2011) Crawford (2009) Crawford (2009) Tremeac and Meunier (2009) Tremeac and Meunier (2009) Pehnt et al. (2008) Ardente et al. (2008) Lenzen and Munksgaard (2002) Kubiszewski et al. (2010)
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There have certain uncertainties in the calculation of life-cycle GHG emissions of wind turbines. The uncertainties mainly source from the uncertainty in data related to the GHG intensity of materials and the assumptions concerning the life-cycle processes of wind turbine. Different processes of material production will result in different GHG emissions. The GHG intensity of material will vary with locations due to the level of technology and management. The GHG intensity of material tends to be higher in developing countries than in developed countries. Therefore, it would be very important to work together worldwide, to reduce the GHG emissions (and even broad ecosystem services) of material production (Wang et al., 2015b). Due to that certain detailed information is not available or accessed, a number of assumptions have to be made. Different assumptions will result in different life-cycle GHG emissions and thus the life-cycle GHG emission intensity (Dolan and Heath, 2012 and Table 5). If the assumptions are related to the processes to which the results are sensitive, the results would have large uncertainty. The calculated life-cycle GHG emissions and life-cycle GHG emission intensities of onshore and offshore wind turbines are sensitive to the transport strategy. Chen et al. (2011) assumed that the transport distance from manufacturer to wind site was 2225 km for wind rotors and was 2143 km for transformers, and did not consider the transport distance from wind site to recycling and landfill locations. Their calculated life-cycle GHG emission intensity of 0.002 kg CO2-eq/MJ is smaller than that of this study. The paper mainly investigates the life-cycle GHG for both onshore and offshore wind turbines. In addition to GHG, there are some other environmental impacts of wind turbines which have been investigated by other studies. These mainly includes energy payback time, noise and impacts from infrastructure materials and so on (Wang and Wang, 2015). Among them, the energy payback time is extensively examined (Weinzettel et al., 2009; Bonou et al., 2016). The energy payback time refers to the amount of time that the system needs to run in order to produce the amount of energy equivalent to the energy consumed throughout the life time of system. Weinzettel et al. (2009) estimated the energy payback time for Sway 5 MW floating wind power plant as 5.2 months. Bonou et al. (2016) estimated that the energy payback time for both onshore and offshore wind turbines was less than 1 year, and offshore wind turbine had longer energy payback time than onshore wind turbine. 5. Conclusion The paper examines the life-cycle GHG emissions for 2 MW onshore and offshore wind turbines using process-based LCA, in order to advance our understanding of GHG emissions of wind turbine and provide information used in policy discussions concerned with future energy sources growth. Results show that the life-cycle GHG emission intensity is 0.082 kg CO2-eq/MJ for onshore wind turbine, and is 0.130 kg CO2-eq/MJ for offshore wind turbine, respectively. Offshore wind turbine has larger life-cycle GHG emissions than onshore wind turbine. This is because offshore wind turbine needs a floating platform fixed in sea which needs more materials and thus results in more GHG emissions. Transportation contributes to a large proportion of life-cycle GHG emissions of both onshore and offshore wind turbines, suggesting that it is important to revisit transportation strategy to reduce the life-cycle GHG emissions of wind turbines. When revisiting the transportation strategy, it is suggested to pay more attention to the distance from wind turbine factory to wind farm site than that from wind farm site to the recycling and landfill locations. Results also indicate that the wind turbines, including onshore and offshore, have much smaller life-cycle GHG emission intensity
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than coal power plants. The life-cycle GHG emission intensity of coal power plant is about 3 times of that of onshore wind turbine and 2 times of that of offshore wind turbine. If the installed wind turbines in 2014 displace coal, the saved GHG emissions can roughly reach 5.08 107 t CO2-eq, accounting for 0.09% of global GHG emissions in 2012. Therefore, wind energy is expected as a promising energy source to mitigate climate change while meeting the energy service demand and diversifying the energy portfolio. Results further show that the lifetime and energy production of wind turbine are demonstrated having large influences on the GHG emission intensity of both onshore and offshore wind turbines. This has informative implication for wind turbine manufacturing factory, and suggests that it is an effective way to prolong the lifetime of wind turbine and increase the energy production of wind turbine to reduce the GHG emission intensity of wind turbine. Acknowledgments The study is supported by UK EPSRC award (Grant no.: EP/ K012398/1). SC Wang is supported by young investigator program of Institute of Quantitative and Technical Economics, Chinese Academy of Social Sciences (Grant no.: IQTE2018QN-04). References Ardente, F., Beccali, M., Cellura, M., Brano, V.L., 2008. Energy performances and life cycle assessment of an Italian wind farm. Renew. Sustain. Energy Rev. 12, 200e217. Arvesen, A., Hertwich, E.G., 2012. Assessing the life cycle environmental impacts of wind power: a review of present knowledge and research needs. Renew. Sustain. Energy Rev. 16, 5994e6006. Bonou, A., Laurent, A., Olsen, S.I., 2016. Life cycle assessment of onshore and offshore wind energy-from theory to application. Appl. Energy 180, 327e337. Chen, G., Yang, Q., Zhao, Y., 2011. Renewability of wind power in China: a case study of non-renewable energy cost and greenhouse gas emission by a plant in Guangxi. Renew. Sustain. Energy Rev. 15, 2322e2329. Crawford, R.H., 2009. Life cycle energy and greenhouse emissions analysis of wind turbines and the effect of size on energy yield. Renew. Sustain. Energy Rev. 13, 2653e2660. Dolan, S.L., Heath, G.A., 2012. Life-cycle greenhouse gas emissions of utility-scale wind power: systematic review and harmonization. J. Ind. Ecol. 16, S136eS154. Department of Energy and Climate Change (DECC), 2011. UK Renewable Energy Roadmap. https://www.gov.uk/government/uploads/system/uploads/ attachment_data/file/48128/2167-uk-renewable-energy-roadmap.pdf. (Accessed 10 June 2015). Dones, R., Faist, M., Frischknecht, R., Heck, T., Jungbluth, N., 2004. Life Cycle Inventories of Energy Systems: Results for Current Systems in Switzerland and Other UCTE Countries. Final Report Ecoinvent 2000 No. 5. Paul Scherrer Institut, Villigen, Duebendorf, Switzerland. http://ecolo.org/documents/documents_in_ english/Life-cycle-analysis-PSI-05.pdf. (Accessed 29 June 2015). Elsam Engineering A/S, 2004. Life Cycle Assessment of Offshore and Onshore Sited Wind Farms. Report by Vestas Wind Systems A/S of the Danish Elsam Engineering. http://www.apere.org/manager/docnum/doc/doc1252_LCA_V80_ 2004_uk%5B1%5D.fiche%2042.pdf. (Accessed 10 July 2015). Franklin Associate, 2011. Cradle to Gate Life Cycle Inventory of Nine Plastics Resins and Four Polyurthane Precursors. Prepared for the plastics division of the American chemistry council. http://plastics.americanchemistry.com/LifeCycleInventory-of-9-Plastics-Resins-and-4-Polyurethane-Precursors-Rpt-Only. (Accessed 12 August 2015). Gao, F., Nie, Z., Wang, Z., Li, H., Gong, X., Zuo, T., 2009. Greenhouse gas emissions and reduction potential of primary aluminum production in China. Sci. China E 52, 2161e2166. Ghenai, C., 2012. Life cycle analysis of wind turbine. In: Ghenai, C. (Ed.), Sustainable Development-energy, Engineering and Technologies-manufacturing and Environment. InTech. ISBN: 978-953-51-0165-9. Global Wind Energy Council (GWEC), 2016. Global Wind Statistics 2015. http:// www.gwec.net/wp-content/uploads/vip/GWEC-PRstats-2015_LR.pdf. (Accessed 28 September 2016). Global Wind Energy Council (GWEC), 2015. Global Wind Statistics 2014. http:// www.gwec.net/wp-content/uploads/2015/02/GWEC_GlobalWindStats2014_ FINAL_10.2.2015.pdf. (Accessed 19 August 2015). Haapala, K.R., Prempreeda, P., 2014. Comparative life cycle assessment of 2.0 MW wind turbines. Int. J. Sustain. Manuf. 3, 170e185. Joint Research Centre (JRC)/PBL, 2014. Emission Database for Global Atmospheric Research (EDGAR), Release Version 4.2. http://edgar.jrc.ec.europa.eu/overview. php?v¼GHGts1990-2012. (Accessed 20 August 2015). Kadiyala, A., Kommalapati, R., Huque, Z., 2017. Characterization of the life cycle
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Raadal, H.L., Vold, B.I., Myhr, A., Nygaard, T.A., 2014. GHG emissions and energy performance of onshore wind power. Renew. Energy 314e324. South End Plant, 2011. Winnipeg SEWPCC Treatment Programme. In: http://www. winnipeg.ca/finance/findata/matmgt/documents//2012/682-2012//682-2012_ Appendix_H-WSTP_South_End_Plant_Process_Selection_Report/Appendix% 207.pdf. (Accessed 12 August 2015). Tremeac, B., Meunier, F., 2009. Life cycle analysis of 4.5 MW and 250 W wind turbines. Renew. Sustain. Energy Rev. 13, 2104e2110. Wang, S., Wang, S., Smith, P., 2015a. Ecological impacts of wind farms on birds: questions, hypotheses, and research needs. Renew. Sustain. Energy Rev. 44, 599e607. Wang, S., Wang, S., 2015. Impacts of wind energy on environment: a review. Renew. Sustain. Energy Rev. 49, 437e443. Wang, S., Wang, S., Smith, P., 2015b. Quantifying impacts of onshore wind farms on ecosystem services at local and global scales. Renew. Sustain. Energy Rev. 52, 1424e1428. Weinzettel, J., Reenaas, M., Solli, C., Hertwich, E.G., 2009. Life-cycle assessment of a floating offshore wind turbine. Renew. Energy 34, 742e747.
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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
Life-cycle green-house gas emissions of onshore and offshore wind turbines Shifeng Wang a, c, Sicong Wang b, *, Jinxiang Liu a a
School of Urban Construction, Nanjing Tech University, No. 30 Puzhu Road, Nanjing, 211816, China Institute of Quantitative and Technical Economics, Chinese Academy of Social Sciences, Beijing, 100732, China c School of Engineering, Newcastle University, Newcastle, NE1 7RU, UK b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 June 2018 Received in revised form 22 September 2018 Accepted 3 November 2018
Onshore and offshore wind turbines may have different environmental sustainability due to their own characteristics, and this information is important for future growth of wind power. The paper uses life cycle assessment (LCA) to estimate the life-cycle greenhouse gas (GHG) emissions of onshore and offshore wind turbines with the nominal capacity of 2 MW, to advance our understanding of onshore and offshore wind energy and to inform policy, planning, and investment decisions for future growth of wind power. Results show that the life-cycle GHG emission intensity is 0.082 kg CO2-equivalent (eq)/Megajoule (MJ) for onshore wind turbine and is 0.130 kg CO2-eq/MJ for offshore wind turbine, respectively. Offshore wind turbine has larger life-cycle GHG emissions than onshore wind turbine, owing to the floating platform fixed in sea. Onshore and offshore wind turbines have much smaller life-cycle GHG emission intensity than coal power plants. If the installed wind turbines in 2014 displace coal, the saved GHG emissions can roughly reach 5.08 107 t CO2-eq, accounting for 0.09% of global GHG emissions in 2012. The sensitive analysis shows that the lifetime and energy production of wind turbine have large influences on the GHG emission intensity of both onshore and offshore wind turbines, implying that it is an effective way to prolong the lifetime of wind turbine and increase the energy production of wind turbine to reduce the GHG emission intensity of wind turbine. The sensitivity analysis further shows that the distance from wind turbine factory to wind farm site has more significant influence on the life-cycle GHG emission intensities of both onshore and offshore wind turbines than that from wind farm site to the recycling and landfill locations, suggesting that the wind farm site and the wind turbine factory should be as close as possible. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Wind energy Onshore wind turbine Offshore wind turbine GHG Life-cycle assessment
1. Introduction Wind power has rapid development worldwide, mainly owing to its perceived importance for combating climate change (Wang and Wang, 2015; Wang et al., 2015a) and meeting the energy service demand (e.g. electricity) in a sustainable way (Weinzettel et al., 2009). The global total installed wind power reaches 432.9 GW by the end of 2015 (GWEC, 2016). In contrast to gas and coal power plants, wind power plants have no significant greenhouse gas (GHG) emissions or resource consumption when converting kinetic energy of wind into electricity. Wind energy has already received the legislative support from government in several countries
* Corresponding author. (S. Wang). E-mail address: [email protected] (S. Wang). https://doi.org/10.1016/j.jclepro.2018.11.031 0959-6526/© 2018 Elsevier Ltd. All rights reserved.
including the UK (Wang et al., 2015a), and its huge boom in implementation and forecasts necessitates the further understanding of this power source (Pehnt et al., 2008). Although wind energy is a clean energy source, all stages of wind turbine life cycle (i.e. primary materials production, manufacturing of wind turbine parts, transportation, maintenance, dismantle and disposal) will consume energy and emit GHGs to the atmosphere. Some stages will give off less GHGs and consume less net energy; however other stages will give off more GHGs and consume more net energy. To reduce both GHG emissions and net energy inputs and to make wind energy more sustainable, it is therefore critically important to identify the dominant process (s) of the wind turbine which produces more GHGs emissions and consumes more net energy. The Life-Cycle Assessment (LCA) is an effective way to quantify the environmental impacts of energy technology. It has been already used to estimate the life-cycle GHG emissions of wind
S. Wang et al. / Journal of Cleaner Production 210 (2019) 804e810
turbine. Tremeac and Meunier (2009) estimate using LCA that the life-cycle GHG emissions for 250 W onshore wind turbine and 4.5 MW onshore wind turbine are 0.013 kg CO2-equivalent (eq)/MJ and 0.004 kg CO2-eq/MJ in France, respectively. The amount of lifecycle GHG emissions of wind turbine assessed by LCA will be impacted by system boundary and assumptions as well as the data quality. The life-cycle GHG emissions for both 850 kW onshore wind turbine and 3.0 MW onshore wind turbine are estimated as 0.003 kg CO2-eq/MJ in Australia by Crawford (2009), whereas the life-cycle GHG emissions for 1.25 MW onshore wind turbine are estimated as 0.002 kg CO2-eq/MJ in China by Chen et al. (2011). The life-cycle GHG emissions for six different 5 MW offshore wind turbines are estimated ranging 0.005e0.0095 kg CO2-eq/MJ (Raadal et al., 2014). The variation in the estimated life-cycle GHG emissions of wind turbines can be reduced using a meta-analytical process called “harmonization” which considers capacity factor, operating lifetime, system boundary, and global warming potentials (Dolan and Heath, 2012). The life-cycle GHG emissions of wind turbines after harmonization are 0.0033 kg CO2-eq/MJ for onshore wind turbines and 0.0031 kg CO2-eq/MJ for offshore wind turbines, respectively (Dolan and Heath, 2012). Although the life-cycle GHG emissions of both onshore and offshore wind turbines can be estimated through LCA (Tremeac and Meunier, 2009; Chen et al., 2011; Dolan and Heath, 2012; Arvesen and Hertwich, 2012; Raadal et al., 2014; Wang and Wang, 2015), the estimation of life-cycle GHG emissions for both onshore and offshore wind turbines are conducted separately. Typically, offshore is less covered compared to onshore. Recently, some but limited studies evaluate the life-cycle GHG emissions for both onshore and offshore wind turbines (Bonou et al., 2016; Kadiyala et al., 2017). Onshore and offshore wind turbines have their own characteristics. First, onshore wind turbines are deployed in land where it has low wind profile, and therefore have low energy outputs. In contrast, offshore wind turbines are deployed in shallow water or deep sea where it has relatively strong wind profile, and thereby have high energy output. Second, offshore wind turbines will need additional structure to fix the floating platform. This will require additional energy to produce the materials of structure and give off additional GHGs. All these different characteristics may result in different environmental sustainability of wind energy. Understanding the relevant processes for these can contribute to obtain more insights for the sustainable wind energy, and this information can be used by lenders, utility executives, and lawmakers to inform policy, planning, and investment decisions for future growth of wind power. This paper will examine the life-cycle GHG emissions for onshore and offshore wind turbines with the nominal capacity of 2 MW. In this paper, we first identify the dominant processes of both onshore and offshore wind turbines, and then use LCA to estimate the GHG emissions for each dominate process and for the whole life cycle processes for both onshore and offshore wind turbines. We then identify the dominant process (s) with more GHG emissions. In addition, we also perform a sensitive analysis to identify the interesting inputs which has the most significance on the life-cycle GHG emissions of wind turbine. 2. Materials and methods We use the process-based LCA, which follows the principle of ISO 14044, to calculate the life-cycle GHG emissions of both onshore and offshore wind turbines. The system boundary is shown in Fig. 1. The functional unit is set as 1 MJ electricity generated at the wind power plants with the selected turbines, and the GHG emissions are estimated for this functional unit. The GHG emissions are expressed as kg CO2-eq, and the GHG emission intensity, which is
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Fig. 1. Life-cycle boundary and processes of onshore and offshore wind turbine.
defined as the amount of GHG emissions per unit of power generation, is expressed as kg CO2-eq/MJ, in order to compare to other global studies. Please note the MJ part of the unit of kg CO2-eq/MJ refers to the electrical power production. Fig. 1 shows the process flow chart of onshore and offshore wind turbines. The life-cycle processes of onshore and offshore wind turbines are organized into four stages: manufacturing, transport and erection, operation and maintenance, and dismantling and disposal. Each stage encompasses a number of processes. During the stage of manufacturing, the foundation, tower, nacelle, rotor and parts of the transmission grid are assumed being manufactured in wind turbine factory. Then these components will be transported from factory to election site and will be assembled and installed through crane work and other construction work at site at the stage of transport and installation. The processes at the stage of operation and maintenance include change of oil and lubrication, renovation of gear and generator, and transporting to and from the turbines for regular checking. Finally, at the stage of dismantling and disposal the main processes involve carnage for dismantling, transporting from erection place to the final disposal, and the further handling of the materials, either by recycling or by deposit. Fig. 2 shows the structure of onshore and offshore wind turbines. The wind turbine is mainly composed of: rotor, nacelle, tower, and foundation. The total GHG emissions are the sum of these four stages, described as
GHGTotal ¼ GHGM þ GHGTI þ GHGOM þ GHGDD
(1)
where the subscript M indicates Manufacturing, TI for transport and installation, OM for operation and maintenance, DD for dismantling and disposal. The GHG emissions for each stage can be calculated as
GHG ¼
X ðinputi Gi Þ
(2)
where Gi is the GHG-intensity coefficient of the ith input of wind turbine (including direct and indirect), and inputi is the amount of the ith input. In this analysis, the capacity size of both onshore and offshore
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S. Wang et al. / Journal of Cleaner Production 210 (2019) 804e810 Table 2 Inputs for 2 MW offshore wind turbine (Dones et al., 2004). Component
Material
Total mass (kg)
Tower
Steel, low alloyed Epoxy resin Concrete Reinforcing steel Glass fibre reinforced plastics Chromium steel Cast iron Steel, low alloyed Cast iron Chromium steel Cast iron Chromium steel Rubber Cast iron Chromium steel Aluminium Copper Chromium steel Chromium steel Glass fibre reinforced plastics Steel, low alloyed Chromium steel Chromium steel Chromium steel Lubricant Copper Lead Steel, low alloyed PVC
113,210 547 2,095,200 80,000 29,714
Foundation Rotor
Nacelle
blades extender hub Mechanic parts
shaft main bearing gearbox
generator Fig. 2. Basic structure of onshore wind turbine (left) and offshore wind turbine (right).
wind turbines is determined as 2 MW due to the data accessibility, though larger wind turbines are commercially used nowadays. The inputs for both onshore and offshore wind turbines are detailed in Table 1 and Table 2, respectively. In these tables, GFRP denotes glass fibre reinforced plastic. Table 3 shows the parameters of wind turbine. In this study, we only focus on wind turbine itself, so the transmission of electricity from wind turbine to other places such as transformer station is not considered. In addition, due to the unavailability to access the complete data about all the life-cycle processes of wind turbines, a number of important assumptions are made in this analysis which are described as follows. Assumption for manufacturing: All the components of wind turbine are assumed to be produced in the wind turbine factory. Assumption for transport and installation: In general, onshore wind turbine is installed in land and offshore wind turbine is in water. Such difference will result in different transport and installation strategies for both onshore and offshore wind turbines. In this analysis, the transport and installation strategies are treated
Table 1 Inputs for 2 MW onshore wind turbine (Elsam Engineering, 2004; Ghenai, 2012). Component Tower Nacelle
tower structure cathodic protection gears generator core generator conductors transformer core transformer conductors transformer conductors cover
Material
Total mass (kg)
Low carbon steel Zinc alloys Stainless steel Cast iron, gray Copper Cast iron, gray Copper Aluminium alloys GFRP, epoxy matrix (isotropic) Cast iron, ductile (nodular) Stainless steel
164,000 203 19,000 9000 1000 6000 2000 1700 4000
main shaft other forged components other cast components Cast iron, ductile (nodular) Rotor blades GFRP, epoxy matrix (isotropic) iron components Cast iron, ductile (nodular) spinner GFRP, epoxy matrix (isotropic) spinner Cast iron, ductile (nodular) Foundation pile & platform Concrete steel Low carbon steel Transmission conductors Copper conductors Aluminium alloys insulation Polyethylene (PE)
12,000 3000 4000 24,500 2000 3000 2200 805,000 27,000 254 72 1380
Casing
brake frame cover
Yaw system
ball bearing drive brake Hydraulic system Transmission
10,966 11,320 12,661 10,025 10,025 9139 9139 100 3382 8877 845 986 613 16,940 11,224 2389 1225 816 2042 150 3900 7575 8766 3500
Table 3 Parameters of wind turbine (Elsam Engineering, 2004).
Capacity size (MW) Life time (year) Electric power generation (MWh)
Onshore wind turbine
Offshore wind turbine
2 20 5634
2 20 8088
equivalent for both onshore and offshore wind turbines. This is reasonable because: (1) the offshore wind turbines considered is close to shore. For example, the Middelgrunden in Denmark is 3.5 km to the shore; and (2) the on-site construction activities for offshore wind turbine is rarely obtained. So we only need to describe the transport and installation strategy for onshore wind turbine. The same manner can also be applied to the stages of operation and maintenance, and dismantling and disposal. Transportation of wind turbine to wind farm site is assumed by road truck with fuel of diesel. The consumption intensity of the diesel is estimated as 0.05 L/(kg.km) with diesel density of 0.83 kg/L (Chen et al., 2011). The average transport distance from wind turbine factory to installation location is assumed as 1500 km. Assumption for operation and maintenance: Regular check-up of the turbines with diesel truck are assumed three times per year (Dolan and Heath, 2012; Haapala and Prempreeda, 2014). The fuel consumption of each regular check-up is 50 kg of diesel (Ardente et al., 2008). For the offshore case, 4 h of helicopter operation per year is added. The oil and lubricant changes are conducted for each check-up, while rotor blade, gearbox, and generator replacement are assumed once within the average lifetime of 20 years (Haapala and Prempreeda, 2014). Assumption for dismantling and disposal: For rotors and nacelle, 20% of blade materials will be recycled (Ardente et al., 2008),
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and other materials will be sent to the dump nearby. For tower, an average material loss rate of 10% for materials undergoing a recycling process has been assumed (Elsam Engineering, 2004; Haapala and Prempreeda, 2014). It is assumed that the concrete of foundations is landfilled entirely and other materials of foundations will be recycled with an average material loss rate of 10% (Haapala and Prempreeda, 2014). It is assumed that the recycling and landfill locations are 100 km from the wind farm site, respectively. For the offshore case, additional 30 km is added (Bonou et al., 2016). The GHG-intensity coefficients for inputs are shown in Table 4. When calculating the life-cycle GHG emissions of wind turbine, the GHG emissions of land for foundation is ignored, as wind turbine foundations cover only a small area of land (Chen et al., 2011) and such data is unavailable as well. We fulfil a sensitivity analysis to identify the interesting inputs which has the most significance on life-cycle GHG emissions of wind turbine. Sensitivity analysis determines how the uncertainty in the model result can be apportioned to the value of a given input component. That is, does a small change in the input cause a significant change in the output? If this is the case, the model is termed sensitive to the input. We take account of the lifetime of wind turbine, energy production, degree of recycling and the distance from wind turbine factory to wind farm site and from wind farm site to the recycling and landfill locations as the interesting inputs. 3. Results Fig. 3 shows the life-cycle GHG emissions for 2 MW onshore and offshore wind turbines. The life-cycle GHG emissions for 2 MW onshore wind turbine are 33278.19 t CO2-eq for 20-year lifetime. So the annual life-cycle GHG emissions are 1663.91 t CO2-eq for onshore wind turbine. Given that the electric power generation of onshore wind turbine is 5634 MWh/year (Table 3), the life-cycle GHG emission intensity is 0.082 kg CO2-eq/MJ for onshore wind turbine. The life-cycle GHG emissions for 2 MW offshore wind turbine for 20-year lifetime are 75904.84 t CO2-eq. Therefore, the annual life-cycle GHG emissions of offshore wind turbine are 3795.24 t CO2-eq. The life-cycle GHG emission intensity of offshore wind turbine is 0.130 kg CO2-eq/MJ, provided that the electric Table 4 GHG intensity for materials. Material
GHG intensity (kgCO2-eq/kg)
Source
Aluminium Aluminium alloys Cast iron Cast iron, ductile (nodular) Cast iron, gray Chromium steel Concrete Copper Epoxy resin GFRP, epoxy matrix (isotropic) Glass fibre reinforced plastics Lead Low carbon steel Lubricant Polyethylene (PE) PVC Reinforcing steel Rubber Stainless steel Steel, low alloyed Zinc alloys Diesel
22 22 1.25 1.25 1.25 2.03 0.22 4.7 3.07 6.72 2.63 1.64 1.39 2.93 2.40 2.14 2.03 3.18 2.03 1.39 3.41 0.45
Gao et al. (2009) Gao et al. (2009) Chen et al. (2011) Chen et al. (2011) Chen et al. (2011) Chen et al. (2011) Chen et al. (2011) Chen et al. (2011) Chen et al. (2011) Li et al. (2013) Li et al. (2013) South End Plant (2011) Chen et al. (2011) South End Plant (2011) South End Plant (2011) Franklin Associate (2011) Chen et al. (2011) Li et al. (2013) Chen et al. (2011) Chen et al. (2011) South End Plant (2011) Chen et al. (2011)
807
Fig. 3. GHG emissions for onshore and offshore wind turbine.
power generation of offshore wind turbine is 8088 MWh/year (Table 3). In order to identify the dominant process (s) of the wind turbine, a breakdown to the life-cycle GHG emissions of wind turbine is conducted for both onshore and offshore wind turbines, respectively. The breakdown shows that for onshore wind turbine the transport and installation accounts for large proportion of life-cycle GHG emissions (91.86%), followed by the dismantling and disposal (5.06%) and the manufacturing (2.41%), while the operation and maintenance represents only 0.67%. The large proportion of lifecycle GHG emissions for the transport and installation is due to that the total mass of onshore wind turbine (which is a large amount) has to be transported from wind turbine factory to wind farm site. For offshore wind turbine, the breakdown shows that the transport and installation makes up the largest fraction of the lifecycle GHG emissions, up to 90.98%, whilst the operation and maintenance accounts for only 0.27%. The argument for onshore wind turbine can be applied here to explain the large fraction of life-cycle GHG emissions in the transport and installation. Fig. 3 demonstrates that for the same capacity of 2 MW the lifecycle GHG emissions of offshore wind turbine are more than two times of that of onshore wind turbine, and the life-cycle GHG emission intensity of offshore wind turbine is larger than that of onshore wind turbine. This is because offshore wind turbine needs a floating platform fixed in sea. The floating platform needs more materials, and therefore results in more GHG emissions in the manufacturing, transport and installation, and dismantling and disposal (Fig. 3). In addition to the operation and maintenance, all other stages have larger GHG emissions for offshore wind turbine than for onshore wind turbine. The relative rate of difference in GHG emissions between offshore wind turbine and onshore wind turbine is the largest for the dismantling and disposal (140%), followed by the transport and installation (125.90%) and manufacturing (43.73%). This is mainly due to the floating platform of offshore wind turbine which has to be fixed in sea and thereby needs more concrete than onshore wind turbine. The amount of concrete of offshore wind turbine is over two-folds of that of onshore wind turbine (Tables 1 and 2). Fig. 4 shows the sensitivity analysis results for the life-cycle GHG emissions of wind turbines. The lifetime and energy production of wind turbine and the distance to wind farm site are demonstrated having larger influences on GHG emission intensity of both onshore and offshore wind turbines. Therefore, in order to reduce GHG emissions of wind turbine, these factors should draw more attentions. 4. Discussion Rapidly growing population and climate change are two main
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Fig. 4. Sensitivity analysis for wind turbine. The lines for lifetime of turbine and energy production are identical and are overlay for both onshore and offshore wind turbines.
drivers of renewable energy adoption. Wind energy is an important source of renewable energy, and wind energy technology has been already identified as one of the most important energy technologies in the UK DECC's Renewable Energy Roadmap that have either the greatest potential to help the UK meet the 2020 target in a cost effective and sustainable way, or offer great potential for the decades that follow (DECC, 2011). Since the early 1980's the United States of America (USA) has already used wind energy commercially to produce energy services such as electricity to satisfy a growing demand for electricity (Wang and Wang, 2015; Wang et al., 2015a; Kunz et al., 2007; Pasqualetti et al., 2004). Wind energy technology is expected to give off less GHG emissions while generating the demanded energy services, when compared to conventional energy technologies such as coal and fossil fuel technologies. The life-cycle GHG emission intensities of both onshore and offshore wind turbines are much smaller than that of coal power plants which have a typical life-cycle GHG emission intensity of 0.22 kgCO2-eq/MJ (Chen et al., 2011). The life-cycle GHG emission intensity of coal power plant is about 3 times of that of onshore wind turbine and 2 times of that of offshore wind turbine. Globally there are totally 369.6 GW installed wind turbines in 2014 among which offshore wind turbines account for 8.8 GW (GWEC, 2015). If these wind turbines displace coal, the saved GHG emissions can roughly reach 5.08 107 t CO2-eq, accounting for 0.09% of global GHG emissions of 5.35 1010 t CO2-eq in 2012 (JRC/PBL, 2014). Therefore, generating electricity from wind energy can save, to a great extent, the GHG emissions to the atmosphere when displacing coal, and mitigates the adverse impacts of coal on climate change. Offshore wind turbine has larger life-cycle GHG emissions and larger GHG emission intensity than onshore wind turbine, although they have the same capacity of 2 MW. Even if the wind turbine is rated at 2 MW, it will produce this power only with the right wind conditions. In general, the wind speed over ocean is much larger than that over land, due to less friction over the water. The stronger wind speed over ocean can make offshore wind turbine run more full-load hours per year than onshore wind turbine, resulting in more energy service outputs and thereby potentially smaller GHG emission intensity. In addition, offshore wind turbine will also
reduce land disturbance relative to onshore wind turbine. However, due to the fact that offshore wind turbine has floating platform, these advantages are offset in terms of GHG emissions. Therefore, it is important to design better floating platform for offshore wind turbine to achieve the equivalent GHG emissions to onshore wind turbine. Transportation (in the transport and installation and in the dismantling and disposal) has much larger GHG emission contribution to the life-cycle GHG emissions of wind turbines (onshore and offshore). In order to reduce the life-cycle GHG emissions of wind turbine, it is thus important to revisit transportation strategy. Transportation GHG emissions are impacted heavily by the distance and type of transport. Truck, train and ship have different GHG emissions and different economic costs. Tremeac and Meunier (2009) carried out a sensitivity analysis for the impacts of distance and type of transport on the life-cycle GHG emissions of wind turbine for two kinds of wind turbines: 250 W and 4.5 MW, and concluded that train was more environmentally friendly than truck if the train station was not too far from the wind farm site. Furthermore, train had more impact on life-cycle GHG emissions for larger capacity wind turbine. The sensitivity analysis conducted in this study further shows that the distance from wind turbine factory to wind farm site has more significant influence on lifecycle GHG emissions for both onshore and offshore wind turbines than that from wind farm site to the recycling and landfill locations. Therefore, the wind turbine factory and the wind farm site should be as close as possible. The sensitivity analysis shows that the lifetime and energy production of wind turbine are demonstrated having larger influences on GHG emission intensity for both onshore and offshore wind turbines. Therefore, it is an effective way to prolong the lifetime of wind turbine and increase the energy production of wind turbine to reduce the GHG emission intensity. This has informative implication for wind turbine manufacturing factory. There have four types of wind turbine: fixed-speed wind turbine (type 1), variable-slip wind turbine (type 2), doubly-fed induction generator wind turbine (type 3), and full-converter wind turbine (type 4). Type 2 can extract more energy from a given wind regime than type 1. However, it has more power lost as heat in the rotor resistance than type 1. Types 3 and 4 overcome this problem and will have higher energy production than type 2. Therefore, type 3 and type 4 will be the best option. The life-cycle GHG emissions of wind turbine will be impacted by the boundary of LCA, characteristics of wind turbine (e.g. wind turbine technology, size of wind turbine, lifetime of wind turbine), locations, and others. Table 5 shows the published life-cycle GHG emissions of wind turbine. It can be seen from Table 5 that the lifecycle GHG emission intensity is different for various capacity sizes and types of wind turbines. Furthermore, the life-cycle GHG emission intensity of wind turbine is not linearly proportional to the capacity size of wind turbine.
Table 5 Life-cycle GHG emissions of wind farm. Location
Turbine type
GHG emission intensity (kg CO2-eq/MJ)
Reference
China Australia Australia France France Germany Italy Worldwide worldwide
1.25 MW-onshore 850 kW-onshore 3.0 MW-onshore 250 W-onshore 4.5 MW-onshore 5.0 MW-offshore 660 kW-onshore
0.002 0.003 0.003 0.013 0.004 0.006 0.002e0.004 0.008e0.123 0.002e0.123
Chen et al. (2011) Crawford (2009) Crawford (2009) Tremeac and Meunier (2009) Tremeac and Meunier (2009) Pehnt et al. (2008) Ardente et al. (2008) Lenzen and Munksgaard (2002) Kubiszewski et al. (2010)
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There have certain uncertainties in the calculation of life-cycle GHG emissions of wind turbines. The uncertainties mainly source from the uncertainty in data related to the GHG intensity of materials and the assumptions concerning the life-cycle processes of wind turbine. Different processes of material production will result in different GHG emissions. The GHG intensity of material will vary with locations due to the level of technology and management. The GHG intensity of material tends to be higher in developing countries than in developed countries. Therefore, it would be very important to work together worldwide, to reduce the GHG emissions (and even broad ecosystem services) of material production (Wang et al., 2015b). Due to that certain detailed information is not available or accessed, a number of assumptions have to be made. Different assumptions will result in different life-cycle GHG emissions and thus the life-cycle GHG emission intensity (Dolan and Heath, 2012 and Table 5). If the assumptions are related to the processes to which the results are sensitive, the results would have large uncertainty. The calculated life-cycle GHG emissions and life-cycle GHG emission intensities of onshore and offshore wind turbines are sensitive to the transport strategy. Chen et al. (2011) assumed that the transport distance from manufacturer to wind site was 2225 km for wind rotors and was 2143 km for transformers, and did not consider the transport distance from wind site to recycling and landfill locations. Their calculated life-cycle GHG emission intensity of 0.002 kg CO2-eq/MJ is smaller than that of this study. The paper mainly investigates the life-cycle GHG for both onshore and offshore wind turbines. In addition to GHG, there are some other environmental impacts of wind turbines which have been investigated by other studies. These mainly includes energy payback time, noise and impacts from infrastructure materials and so on (Wang and Wang, 2015). Among them, the energy payback time is extensively examined (Weinzettel et al., 2009; Bonou et al., 2016). The energy payback time refers to the amount of time that the system needs to run in order to produce the amount of energy equivalent to the energy consumed throughout the life time of system. Weinzettel et al. (2009) estimated the energy payback time for Sway 5 MW floating wind power plant as 5.2 months. Bonou et al. (2016) estimated that the energy payback time for both onshore and offshore wind turbines was less than 1 year, and offshore wind turbine had longer energy payback time than onshore wind turbine. 5. Conclusion The paper examines the life-cycle GHG emissions for 2 MW onshore and offshore wind turbines using process-based LCA, in order to advance our understanding of GHG emissions of wind turbine and provide information used in policy discussions concerned with future energy sources growth. Results show that the life-cycle GHG emission intensity is 0.082 kg CO2-eq/MJ for onshore wind turbine, and is 0.130 kg CO2-eq/MJ for offshore wind turbine, respectively. Offshore wind turbine has larger life-cycle GHG emissions than onshore wind turbine. This is because offshore wind turbine needs a floating platform fixed in sea which needs more materials and thus results in more GHG emissions. Transportation contributes to a large proportion of life-cycle GHG emissions of both onshore and offshore wind turbines, suggesting that it is important to revisit transportation strategy to reduce the life-cycle GHG emissions of wind turbines. When revisiting the transportation strategy, it is suggested to pay more attention to the distance from wind turbine factory to wind farm site than that from wind farm site to the recycling and landfill locations. Results also indicate that the wind turbines, including onshore and offshore, have much smaller life-cycle GHG emission intensity
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than coal power plants. The life-cycle GHG emission intensity of coal power plant is about 3 times of that of onshore wind turbine and 2 times of that of offshore wind turbine. If the installed wind turbines in 2014 displace coal, the saved GHG emissions can roughly reach 5.08 107 t CO2-eq, accounting for 0.09% of global GHG emissions in 2012. Therefore, wind energy is expected as a promising energy source to mitigate climate change while meeting the energy service demand and diversifying the energy portfolio. Results further show that the lifetime and energy production of wind turbine are demonstrated having large influences on the GHG emission intensity of both onshore and offshore wind turbines. This has informative implication for wind turbine manufacturing factory, and suggests that it is an effective way to prolong the lifetime of wind turbine and increase the energy production of wind turbine to reduce the GHG emission intensity of wind turbine. Acknowledgments The study is supported by UK EPSRC award (Grant no.: EP/ K012398/1). SC Wang is supported by young investigator program of Institute of Quantitative and Technical Economics, Chinese Academy of Social Sciences (Grant no.: IQTE2018QN-04). References Ardente, F., Beccali, M., Cellura, M., Brano, V.L., 2008. Energy performances and life cycle assessment of an Italian wind farm. Renew. Sustain. Energy Rev. 12, 200e217. Arvesen, A., Hertwich, E.G., 2012. Assessing the life cycle environmental impacts of wind power: a review of present knowledge and research needs. Renew. Sustain. Energy Rev. 16, 5994e6006. Bonou, A., Laurent, A., Olsen, S.I., 2016. Life cycle assessment of onshore and offshore wind energy-from theory to application. Appl. Energy 180, 327e337. Chen, G., Yang, Q., Zhao, Y., 2011. Renewability of wind power in China: a case study of non-renewable energy cost and greenhouse gas emission by a plant in Guangxi. Renew. Sustain. Energy Rev. 15, 2322e2329. Crawford, R.H., 2009. Life cycle energy and greenhouse emissions analysis of wind turbines and the effect of size on energy yield. Renew. Sustain. Energy Rev. 13, 2653e2660. Dolan, S.L., Heath, G.A., 2012. Life-cycle greenhouse gas emissions of utility-scale wind power: systematic review and harmonization. J. Ind. Ecol. 16, S136eS154. Department of Energy and Climate Change (DECC), 2011. UK Renewable Energy Roadmap. https://www.gov.uk/government/uploads/system/uploads/ attachment_data/file/48128/2167-uk-renewable-energy-roadmap.pdf. (Accessed 10 June 2015). Dones, R., Faist, M., Frischknecht, R., Heck, T., Jungbluth, N., 2004. Life Cycle Inventories of Energy Systems: Results for Current Systems in Switzerland and Other UCTE Countries. Final Report Ecoinvent 2000 No. 5. Paul Scherrer Institut, Villigen, Duebendorf, Switzerland. http://ecolo.org/documents/documents_in_ english/Life-cycle-analysis-PSI-05.pdf. (Accessed 29 June 2015). Elsam Engineering A/S, 2004. Life Cycle Assessment of Offshore and Onshore Sited Wind Farms. Report by Vestas Wind Systems A/S of the Danish Elsam Engineering. http://www.apere.org/manager/docnum/doc/doc1252_LCA_V80_ 2004_uk%5B1%5D.fiche%2042.pdf. (Accessed 10 July 2015). Franklin Associate, 2011. Cradle to Gate Life Cycle Inventory of Nine Plastics Resins and Four Polyurthane Precursors. Prepared for the plastics division of the American chemistry council. http://plastics.americanchemistry.com/LifeCycleInventory-of-9-Plastics-Resins-and-4-Polyurethane-Precursors-Rpt-Only. (Accessed 12 August 2015). Gao, F., Nie, Z., Wang, Z., Li, H., Gong, X., Zuo, T., 2009. Greenhouse gas emissions and reduction potential of primary aluminum production in China. Sci. China E 52, 2161e2166. Ghenai, C., 2012. Life cycle analysis of wind turbine. In: Ghenai, C. (Ed.), Sustainable Development-energy, Engineering and Technologies-manufacturing and Environment. InTech. ISBN: 978-953-51-0165-9. Global Wind Energy Council (GWEC), 2016. Global Wind Statistics 2015. http:// www.gwec.net/wp-content/uploads/vip/GWEC-PRstats-2015_LR.pdf. (Accessed 28 September 2016). Global Wind Energy Council (GWEC), 2015. Global Wind Statistics 2014. http:// www.gwec.net/wp-content/uploads/2015/02/GWEC_GlobalWindStats2014_ FINAL_10.2.2015.pdf. (Accessed 19 August 2015). Haapala, K.R., Prempreeda, P., 2014. Comparative life cycle assessment of 2.0 MW wind turbines. Int. J. Sustain. Manuf. 3, 170e185. Joint Research Centre (JRC)/PBL, 2014. Emission Database for Global Atmospheric Research (EDGAR), Release Version 4.2. http://edgar.jrc.ec.europa.eu/overview. php?v¼GHGts1990-2012. (Accessed 20 August 2015). Kadiyala, A., Kommalapati, R., Huque, Z., 2017. Characterization of the life cycle
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