1 Introduction

Worldwide, the use of wind turbine generators (WTG) to produce electricity is an important topic of discussion when considering energy sources. Decreasing oil supplies and environmental and economic impacts of traditional electrical power sources have made wind energy a promising alternative.

In the last decade, many climate change studies have focused on temperature change and extreme precipitation events. Studies have shown that the move towards increased usage of renewable energy sources, especially wind power, can positively affect climate change in the long run (Sailor et al. 2008). An important contributor to the climate change discussion is the amount of greenhouse gas emissions (GHG) produced, especially in conventional power plants using coal and other fossil fuels (IPCC 2007). A feasibility assessment of wind energy in Taiwan concluded that 9.8 million metric tons of CO2 emissions can be eliminated by replacing coal-based fuel with wind energy for electricity generation (Yue et al. 2001). In Canada, research showed that use of solar and wind energy in place of fossil and gas fuels could reduce CO2 emissions up to 2.3 million metric tons annually (Granovskii et al. 2007). Another study showed that by using wind power to produce electricity in the mobile phone industry in Korea, CO2 emissions could be reduced up to 97 % (Weea and Choi 2010).

Although wind energy has numerous advantages such as power production without the consumption of water, a number of disadvantages also need to be addressed. Noise, visual impact, electromagnetic interference, and sunlight flicker for those in the tower’s shadow represent a few of the typical concerns. These issues can be addressed to some degree by technical design and appropriate site location selection (Yue et al. 2001). Finally, wind is a transient resource, and the overall electrical and transmission system must be able to accommodate and balance this variable source of energy.

2 Life Cycle Analysis of Wind Farms

Life cycle analysis can be a time- and resource-consuming process. Because climate change is one of the most important potential environmental impacts in many industrial activities such as power generation, analysis of life cycle GHG emissions is usually an important focus of LCA (Raadal et al. 2011). Various methodologies have been used to perform LCA, such as Process Analysis and Input/output Analysis (IOA) (Raadal et al. 2011).

GHGs can be expressed as equivalent emissions of CO2 (CO2(eq)). The CO2(eq) of a GHG is calculated by multiplying the weight of the GHG emission to its corresponding Global Warming Potential (GWP) factor. GWP is a relative index, showing the relative global warming effect of a unit weight of any GHG compared to a unit weight of CO2. Thus, the CO2(eq) of 1 kg of CO2 is equal to 1 kg, while the CO2(eq) of 1 kg CH4, which affects global warming 21 time more than CO2 (GWP = 21), is 21 kg (Department of Climate Change and Energy Efficiency 2010).

The effectiveness and efficiency of a wind farm in life cycle energy terms is determined using the energy payback ratio (EPR). The energy payback ratio of a power plant is defined as the total energy produced over the lifetime of the plant divided by the energy needed to construct, operate, and fuel the plant over the life time (Gagnon et al. 2002). A low ratio shows that the system consumes more energy than it produces, raising serious questions about the sustainability or utilization of such a system. The EPR is less affected by upstream choices of energy supply, thus minimizing the fluctuation in results when evaluating GHG emissions of various systems (Gagnon et al. 2002).

McChulloch et al. (2000) compared emissions from wind power, coal, and natural gas for electricity generation in Alberta, Canada. The studied wind farm was a 600 kW Vestas WTG with 20 % capacity factor and a 25 year life span. The environmental impacts covered in this study were GHG emission, acid rain potential, and ozone layer damage. Based on previous studies on GHG emissions in Canada, the authors reported that almost 47 % of GHG emissions from electricity generation in Alberta are related to the combustion of coal and natural gas. Results showed that natural gas power plants produced 775 kg CO2(eq) emissions more than a wind farm, while the difference increased to 1,080 kg CO2(eq) when comparing to a coal plant.

Van Wijk and Turkenburg (1992) investigated the potential costs savings of wind energy as the main source of energy in the Netherlands for the period of 1995–2010. The points of focus in this study were fuel consumption, capacity, and the emissions of CO2, SO2 and NOx. They used five different types of WTG with the data of eleven wind stations to simulate their model. Based on their prediction, the installed wind energy power would rise from 400 MW in 1995–2000 MW by 2010. Total fuel savings was predicted to increase from 7.27 PJ to 29.12 PJ during this same time period. An increasing trend in cost savings was also observed for all investigated emissions.

Schleisner (2000) presented a model of LCA for two wind farms in Denmark. The model consisted of the consumed energy for 1 kg of each type of material used in the WTG to be produced, transported, and manufactured. An off-shore farm with ten 500 kW WTGs and an onshore farm with 18,500 kW WTGs were analyzed. The resulting emissions over the 20-year life span of the projects were calculated as 16.5 g CO2/kWh for the off-shore site and 9.7 g CO2/kWh for the onshore one. The energy payback periods for the onshore and off-shore sites was calculated at 0.26 years and 0.39 years respectively.

Raadal et al. (2011) published an overview of recent studies and literature with regards to GHG emissions during the life cycle of electricity generation in wind farms and hydro power plants. The reviewed studies showed that wind farms and hydro power plants can exhibit a wide range of GHG emission levels (from 0.2 to 152 g CO2(eq) per kWh) depending on the type of power plant and specifications. Analyzing data from several types of wind turbines (100 kW to 5 MW), they found that the 30 kW turbines had the most GHG emissions, while the 3 MW WTG had the minimum level of emissions, corresponding to values of 55.4 and 4.6 g CO2(eq) per kWh, respectively.

3 The Glacier Hills Wind Energy Park

The State of Wisconsin is the site to a number of operational wind farms. The largest wind farm is the Glacier Hills Wind Park, located in the towns of Randolph and Scott in Columbia County. This wind farm covers about 7,020 hectares, including 7,720 hectares of agricultural land and 1,300 hectares of woodland, wetlands, buildings and roads (WE Energies 2009). Only one population center, the Village of Friesland, is located inside the project boundaries. An ethanol plant is also located within the project area (RENEW Wisconsin 2011). The expected life span of the wind farm used in the project economic analysis is 26 years, which was used for this LCA (WE Energies 2009).

Annual capacity factors for wind generating facilities in the Midwest range from approximately 25–40 % (WE Energies 2009), due to inconsistancy and intermitancy in wind velocity. The Glacier Hills Wind Park environmental impact report (WE Energies 2009) used a 25 % capacity factor to calculate approximate power output of the plant. The Wisconsin wind resource map at 80 m elevation shows that the Glacier Hills Wind Park falls within the region with average wind velocity of 6.5–7.0 m/s (US DOE and NREL 2012). Areas with annual average wind speeds around 6.5 m/s and greater at an 80-m height are generally considered suitable for wind development.

The name plate generating capacity of the Glacier Hills Wind Park is in the range of 162 MW, which is the product of 90 WTGs and the WTG rating of 1.8 MW. If the WTGs generate at rated capacity for an entire month, they would produce approximately 117 GWh. However, at a capacity factor of 25 %, as used in the Glacier Hills Wind Park environmental impact report (WE Energies 2009), the facility would produce approximately 29 GWh during the month.

4 Analysis Methodology and Assumptions

One of the main objectives of the present study is to calculate CO2 emissions and energy consumption in wind farm construction with respect to the Glacier Hills Wind Park. An interactive spreadsheet was developed to enable a user to choose a turbine type from the spreadsheet database, from which the spreadsheet would calculate the emissions and energy consumption as a function of material used, transportation of material, and volume of earth work and construction activities. Input parameters and environmental, emission, and energy content and consumption values for various construction activities, transportation, manufacturing, and production of materials from “cradle-to-gate” were gathered from the Ecoinvent v.2 database (Frischknecht and Jungbluth 2007), as provided by the SimaPro version 7.3 LCA software, as well other sources of literature and environmental impact reports. The gathered data and the developed interactive spreadsheet were used to conduct a “cradle-to-grave” life cycle assessment of the Glacier Hills Wind Park, spanning from the time the raw material and resources were extracted and produced, to the end of the operational design life of the wind park.

The main components of a typical wind turbine are shown in Table 1. For each category, the material type and associated weight for a Vestas V90 turbine, as used in the Glacier Hills Wind Park, is shown. These values were derived from multiple sources for a relatively complete inventory of materials used in this product (WE Energies 2008; Vestas 2009, 2010).

Table 1 Vestas V90 turbine main components, materials and weights
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The primary source for values of CO2(eq) emissions and production energy through the life cycle of each of the major listed material types used in the construction of the wind farm was the SimaPro “Ecoinvent v.2 database.” This database includes various material types and processes from different national and international sources. For each material type, a network of connections and processes from the “cradle-to-gate” are analyzed, thus enabling the definition of the materials emissions and energy consumption to the point of use. These values are listed in Table 2 and were used to calculate the CO2(eq) emissions and production energy per unit weight of material, as defined previously in Table 1.

Table 2 Ecoinvent v.2 database values CO2(eq) emissions and production energy per unit weight
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4.1 Fuel and Fuel Consumption

An important component of the analysis of a wind farm system is the amount of energy required during the construction of the wind farm. Fuel consumption during transportation of equipment and components, as well as construction activities (e.g., excavation and compaction) can account for a notable share of energy consumption. The other major source of energy consumption is energy consumed during the production of material used in the construction of the wind farm. Fuel consumption rates for the main transportation modes were derived from the Department of Energy’s “Transportation Energy Handbook” (US DOE 2008). The conversion of BTU to liters of diesel fuel was accomplished by using BTU/liter values provided by Hofstrand (2007), thus the liters of diesel per kg km values were calculated for each of these transportation modes, as shown in Table 3.

Table 3 Fuel consumption per transportation mode type
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For the construction and earthwork equipment (e.g., excavators, bulldozers, compactors, etc.), values of productivity per hour were derived from the PaLATE software documentation, based on manufacturer-provided data (Horvath 2004). The productivity values were converted to kg of CO2 emissions and MJ of consumed energy per ton of material processed.

4.2 Transportation

A wind farm construction project, like any other major construction activity, entails a large number of transportation trips for supplies, material, and equipment mobilization. An additional factor in wind farm construction is the need to transport prefabricated turbine sections, consisting of loads with unstandard dimensions and weights to the construction site, often over considerable distance. For the construction of the Glacier Hills Wind Park, an estimated 8200 truck trips for construction and support purpose, and 630–900 trips for delivery of large parts were undertaken (WE Energies 2009).

Eleven trains from Colorado delivered the 45-m-long fiberglass blades along with nacelles. More than 800 trucks delivered the steel towers from Manitowoc, WI, each of which are made of four flanged sections bolted together for the 80-m hub height (Dvorak 2011). Vestas has factories in Colorado providing the required components including a blade factory in Windsor, and nacelle factory in Brighton (Jokela and Innis 2010). Thus, the approximate delivery distance for the blades by train would be 1,600 km. Truck delivery distances for the nacelle and tower components would be 1,600 and 165 km, respectively.

The generators used in this project were transported from Vestas factories in Tianjin, China. This factory also provided the nacelle components assembled in the Vestas manufacturing plant in Brighton, CO. As information is not available on the transportation of these components from China, only approximate calculations are possible. The approximate waterborne freight distance from Tianjin to the Port of Los Angeles is 11,000 km (Farnel Capital Inc. 2012), followed by 1,600 km of ground shipping to Brighton, CO.

The truck configurations for wind farm component transportation are designed and assembled specifically for the dimensions and weights of the tower or blade parts to be hauled, and for the specific haul routes that would be used. Trucks for the nacelle, blades, or tower sections could range from 25- to 53-m long. Most of the trucks hauling the turbine components and large cranes are over standard weight and/or over standard size. The Glacier Hills WTG towers were manufactured and shipped in four sections (WE Energies 2009).

4.3 Foundation

In the Glacier Hills project, 16-m-diameter, 2.5-m-embedded, shallow octagonal foundations were used for each WTG structure. The design concrete volume for each foundation was 240 m3 (Lang 2012).

A minimum 0.5 m of additional space is required between the edge of the foundation and the edge of the excavation footprint for most shallow octagonal, WTG foundation designs. The slope of the foundation excavation boundary is commonly limited to a 2–1 ratio (depending on soil type) to prevent caving or collapsing of the foundation excavation. Thus, based on these constraints, the approximate excavation area for each foundation can be calculated (Lang 2012). The overall volume of excavated soil for each foundation site would be approximately 685 m3. Based on the 240 m3 of foundation concrete that will be placed in the excavation site, the remaining 445 m3 will be backfilled with soil and leveled to the existing land grade.

4.4 Power Transmission

Energy produced by each WTG in a wind farm is first transferred to local transformers, through which the energy is transformed to the appropriate voltage levels and then transferred to the central substation, which in turn transfers this energy to the area’s existing power transmission system. Underground collector systems at wind energy sites use jacketed, concentric neutral conductors. Main branch circuits from the substation utilize 1,000 kc-mil cable. As the cables are spliced for branches to turbine sub-circuits, the conductor sizes would be reduced to 750 kc-mil, 500 kc-mil, 4/0 and 1/0 cables, depending on the ampacity requirements of the circuit branch (WE Energies 2009). These products along with quantity values were used in the analysis.

5 Discussion and Results

5.1 Greenhouse Gas Emissions

The GHG emissions are evaluated based on equivalent grams of CO2(eq) produced during various stages of the production and operation. The values were calculated for each main category, results of which are shown in Figs. 1–4. Figure 1 shows that although material production was responsible for most GHG emissions, the transportation and construction activates also resulted in significant emissions through the design life. Figure 2 shows CO2(eq) of material production, broken down by component. The highest contributors to the material production CO2(eq) category were the foundation and tower components, due to the high volume of steel and concrete used in these components. Figure 3 shows that the crane pads had the highest relative CO2(eq) emissions in the construction of this wind park, due to the volume of crushed stone material and excavation depth needed to prepare the crane pads at the required bearing capacity of 215 kPa (WE Energies 2009).

Fig. 1
figure 1

Glacier Hills Wind Park CO2(eq) emissions categorized by activity

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Fig. 2
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Glacier Hills Wind Park CO2(eq) emissions from material production by component

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Fig. 3
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Glacier Hills Wind Park CO2(eq) emissions categorized by earthwork and construction component

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Fuel consumption during international shipping of WTG components was a major contributor to the GHG emissions in the transportation category, as apparent by the relatively high transportation emissions for the nacelle and generator components shown in Fig. 4. Although economic considerations may make overseas production of WTG components desirable, domestic production can lead to reductions in environmental life cycle impacts of a wind farm project. For the Glacier Hills Wind Park domestic production and shipping of all components would have resulted in up to 22 % reduction in total transportation CO2(eq) emissions.

Fig. 4
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Glacier Hills Wind Park CO2(eq) emissions from transportation by component

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The average CO2(eq) emissions per kWh of expected power generation over the life time of the wind farm is usually calculated as a means of assessing and comparing global warming potential and greenhouse gas generation of power plants. For the case of the Glacier Hills Wind Park, the calculated grams of CO2(eq) per kWh produced over the life time was 16.9 g CO2(eq)/kWh. A typical coal power plant may generate between 800 and 1,000 g CO2(eq)/kWh depending on coal type, or down to 115 g CO2(eq)/kWh if CO2 capture is used in plant (Spath et al. 1999; Ruether et al. 2004).

5.2 Production Energy and Payback

The majority of the energy consumed during the transportation and construction stages in the life cycle assessment of the Glacier Hills Wind Park was due to fuel consumption. Increasing the fuel efficiency of transportation and heavy construction machinery, as well as use of recycled crushed material for crane pads and access roads, are possible methods to influence the total life cycle energy use of the wind farm by reducing the fuel and energy consumption during construction and material production.

The EPR for the Glacier Hills Wind Park was calculated as part of the present study by dividing the total expected life time energy production of the wind farm using a 25 % capacity factor, as was used in the Glacier Hills Wind Park environmental impact report for estimation of the plant power output (WE Energies 2009). The analysis resulted in an EPR of 25.5. This EPR demonstrates the high energy efficiency and net gain for use of wind energy as a power source. Case studies of various wind farms have resulted in EPR values typically ranging between 15 and 35. In comparison, coal power plants have been reported to have EPRs of 2.5–7.0, while natural gas plants may have EPRs of 2.5–5.0, depending on coal and natural gas type and the transportation distance of fuel to the plant (Gagnon 2005).

The energy payback time (EPT) represents the duration of wind farm operation and power generation required to compensate for production energy of the wind farm. This value is highly dependent on the rate of power generation, which in a wind farm is directly related to the capacity factor. For the case of the Glacier Hills Wind Park, using a capacity factor of 25 %, the energy payback time would be 12.2 months, after which any subsequent wind farm energy generation in the remaining 25 years of intended operation life would result in a net gain in life cycle energy of the wind farm.

6 Conclusions

Life cycle assessment methodology was used to perform a quantitative analysis and rating of the construction and operation of a wind energy plant through the case study of the Glacier Hills Wind Park. The energy payback ratio (25.5), energy payback time (12.2 months), and the total grams of CO2(eq) per kWh of energy (16.9) produced over the life time of the Glacier Hill Wind Park were calculated. The resulting values show that the Glacier Hill wind farm as a power generation source will lead to considerable saving in terms of GHG emissions and energy consumption compared to other conventional power sources.

Analysis of the result also shows that up to 22 % of the energy consumed and CO2(eq) produced during the life cycle transportation stage is due to the overseas shipping of major wind turbine components to the United States, either for direct installation at site or to domestic manufacturing plants. These results indicate the potential need to evaluate the source of wind turbine technology not only in economic terms, but in terms of environmental impact through minimization of transportation distances for major components.

In the material production category, the considerable emissions from the production of the large volume of Portland cement used in the concrete foundation may be significantly reduced by replacing a portion of Portland cement with cementitious fly ash, a byproduct of coal combustion.

Furthermore it was observed that after the material production and manufacturing category, the earthwork and construction category accounted for the highest GHG emissions in the project. These values are heavily influenced by fuel combustion during mechanical crushing and transportation of haul road and crane pad base layer material. Increasing the fuel efficiency of heavy machinery, as well as increasing the usage of recycled crushed material in haul roads and crane pads can potentially lead to a significant decrease in the GHG emission and environmental impact of this stage.