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1 Introduction

The idea of using wind power to produce electricity dates back to the nineteenth century. The field is generally considered to have been pioneered by Charles F. Brush, an outstanding inventor, thinker, and entrepreneur of the time, known mainly for his dynamo and his arc lights. He registered over 50 patents and founded the Brush Electric Company, which after subsequent sales and mergers eventually became General Electric, now one of the world’s biggest power companies. Brush was the first man to install and operate a wind turbine to generate electricity (in 1888, in order to charge the batteries at his home in Cleveland, Ohio). It stood 60 ft (18.29 m) high, measured 56 ft (17.07 m) in diameter and was connected to a 12 kW dynamo. It charged the batteries at the house for 20 years. His turbines had 144 blades, whereas modern turbines have just 3 (Rivkin et al. 2012).

Renewable energy sources, and particularly wind power, have undergone considerable development in recent years. This new boom in wind power dates back to the early 1990s and is due mainly to the need to find viable alternatives to fossil fuels, reserves of which are finite and will in the long term be incapable of sustaining current levels and trends in consumption around the world.

The wind turbines now in use are models developed in recent years. There are numerous types of turbine capable of generating electricity, but most of those currently installed use horizontal-axis technology. Their rated power outputs range from 500 kW to 5 MW. They are used basically in direct connections to the electricity grid and are grouped into wind farms to take advantage of economies of scale and facilitate monitoring and maintenance tasks.

Figure 1 shows the world-wide trend over time in total generating capacity from wind power. There has been a clear, sustained year-by-year increase in capacity in recent years, to almost 200 GW in 2010. However, a breakdown of these data by regions (Fig. 2) clearly shows the influence of the worldwide recession, with less capacity being installed in 2010 than in 2009, especially in Europe and North America.

Fig. 1
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Global cumulative installed wind capacity and forecasting

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Fig. 2
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Annual installed capacity by region 2007–2010

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The basic idea underlying the generating of electricity via modern, horizontal-axis wind turbines of the kind that can be found in most wind farms around the world is to convert wind energy into mechanical energy, by using the force of the aerodynamic thrust of the blades to generate torque on the main shaft. That mechanical energy is then turned into electricity by means of a generator. The disadvantage of this system compared with other conventional generators is that it can only generate electricity when there is enough wind. Moreover, it is not currently possible, from the economical point of view, to store the electricity generated in order to use or to transfer it to the grid at a later time, because this type of energy is noncontrollable and is subject to fluctuations depending on changes in wind speed. From the viewpoint of electricity grids as a whole, the requirements for the connection of new wind farms are becoming increasingly stringent. Continual efforts are being made to improve the integration of this type of energy into the grid, with control systems being established to regulate the reactive energy produced by wind farms, to vary the voltage or frequency supplied by farms at the point of connection, or to prevent the complete disconnection of wind farms when there are small voltage gaps in the grid, etc. This partly offsets the negative effect of the noncontrollability of wind farms within the grid and at the same time increases the wind energy capacity installed in the system, acting as basic energy that enables the number of conventional power plants operating to be reduced, or at least reducing the amount of fossil fuel required for their operation.

2 Wind Turbines

As mentioned, most of the wind turbines in use today for large-scale energy production around the world are three-blade, horizontal-axis units. The basic components of such turbines are shown in Fig. 3:

Fig. 3
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Main components of a horizontal-axis wind turbine

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  • Foundations

  • Tower

  • Nacelle

  • Rotor

2.1 Foundation

For a wind turbine, to be stable, its foundations must be capable of bearing the loads to which the turbine is subjected. The type of foundations built depends on the type and consistency of the soil where the unit is to be erected, and on which type of unit.

Obviously, the foundation requirements are considerably more complex for off-shore wind farms than for land-based farms. Various types of off-shore foundations may be used, depending on the depth of the sea at the site where the turbines are erected:

  • Monopile (4–25 m): A hollow steel pile, plus grout injected between pile and transition piece.

  • Surface-level foundations: Made of concrete and steel with a large base which sits on the terrain

  • Jackets (30–35 m): Similar to lattice towers used in offshore oil and gas projects.

  • Tripods: Monopile adaptation, broadening the footprint by adding three piled connections close to the seabed.

  • Multipile (up to 40 m): Another monopole adaptation. Several piles, connecting above the water via a grouted transition piece, driven into the seabed forming a larger foundation footprint than a typical monopile.

2.2 Tower

Towers must be built not just to bear the weight of the nacelle and rotor but also to absorb the loads caused by variations in wind speed.

Various types of tower can be used:

  • Tubular steel towers: Most large wind turbines have tubular steel towers built in 20–30 m sections with flanges at each end, which are bolted together on site. They are tapered, i.e., their diameter decreases toward the top, to increase their strength and save on materials.

  • Concrete towers built on site: These towers are subject to height limitations. They have the advantage that there is no need to transport the tower sections to the point of installation of the turbine.

  • Prefabricated concrete towers: These are made of segments that are placed one on top of another.

  • Lattice towers: These are made of steel sections. Their main advantage is their cost, as they require only half as much material as a tubular tower with no additional supports and provide the same degree of stiffness. Their main disadvantage is their appearance. For esthetic reasons, lattice towers are hardly ever used in large, modern wind turbines. They are common, however, in India and can also be found elsewhere, e.g., in the USA and in Germany.

  • Hybrid towers: Towers can also be built using combinations of the above methods. The most common variant is to build the bottom part of cement and the top of tubular steel. Indeed, this seems likely to be the tendency for the large, multimegawatt turbines of the future.

The height of the towers used for modern wind turbines varies as shown in Table 1.

Table 1 Tower height depending on rated power and rotor diameter
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2.3 Rotor

The rotor converts wind energy into mechanical rotational movement. It comprises the turbine blades and the hub (which joins the blades to the main shaft). The hub is the center of the rotor. It is made of cast steel or iron. Depending on the type of wind turbine, hubs may be connected to the low-speed shaft of the gearbox or directly to the generator if the turbine has no gearbox. A high percentage of the turbines installed today are still fitted with gearbox to raise the rotor revolutions to the rated speed of the generator. However, gearboxes can be problematic, so there is a tendency for modern multimegawatt turbines to be built without them in order to reduce maintenance requirements.

Currently, most rotors have three blades and a horizontal shaft and are between 40 and 90 m in diameter. Conventional windmill rotors for pumping water use 16 or more blades and are made of metal, but three-blade rotors are more efficient for generating power in large turbines; they also make for better weight distribution, which allows more stable rotation.

Rotor blades are made mainly of fiberglass or carbon fiber reinforced with plastic. In profile, they are similar to aircraft wings, and they work on the same principles of thrust. The wind passing over the bottom of the blade generates high pressure, while low pressure is generated at the top. This force, plus the force of resistance, causes the rotor to rotate. Each manufacturer makes blades according to its own concepts and characteristics.

2.4 Nacelle

The nacelle encases and supports all the machinery of the wind turbine and must be able to turn in order to face the wind direction. It is therefore attached to the tower by bearings. Nacelle designs vary from one manufacturer to another in line with the designs of the turbines and the location of the powertrain components (main shaft, gearbox, generator, coupling, and brake).

The gearbox converts the 18–50 rpm rotation of the rotor to the rated rotation speed of the generator, i.e., approximately 1,750 rpm. The generator rotation speed depends on the frequency of the electrical current and on how many pairs of poles the machine has. Apart from enabling low rotor rotation speeds to be coupled to high generator rotation speeds, the gearbox also enables the unit to withstand a wide range of variations in wind speed.

The generator is what turns mechanical energy into electricity. Double-fed asynchronous generators are the most widespread type in use on today’s high-power wind turbines.

Along with the main components mentioned previously, it is also necessary to install a coupling between the generator and the gearbox. Flexible couplings are usually utilized for this purpose. Mechanical brakes are also fitted to the powertrain.

The standards usually applied in wind turbine design require turbines to be equipped with two independent braking systems, so both aerodynamic and mechanical brakes are usually fitted: the aerodynamic system is located at the tip of the blades or over the whole rotor blades to change the pitch angle. On most turbines, the second system is usually a mechanical disk brake, which is intended mainly to be used should the aerodynamic system fail, and when the turbine is stopped for repair or maintenance work.

3 Wind Turbine LCA

3.1 State of the Art

The relevant scientific literature contains various life-cycle analyses of wind power and wind turbines (Ardente et al. 2008; Ben et al. 2008; Góralczyk 2003; Tryfonidou and Wagner 2004; Wiese and Kaltschmitt 1996; Gürzenich et al. 1999; Uchiyama 1995; Nadal 1998; Haack 1981; Krohn 1997; Uchiyama 1996; Schleisner 2000; Lenzen and Munksgaard 2002; Lenzen and Wachsmann 2004; Crawford 2009; Weinzettel et al. 2009; Martínez et al. 2009). Most of them focus on energy analysis and CO2 emissions, and their results vary considerably. This should come as no surprise in view of the significant developments in the field of wind energy in recent years: in a very short time, wind turbines have been developed with widely differing electricity generating capacities, ranging from rated power outputs of just a few kW to the latest large multimegawatt turbines (7.5 MW). Moreover, issues such as estimated useful lifetime and equivalent hours of annual production significantly affect the final results of any study conducted. In spite of the variability observed, a clear tendency is found in which CO2 emissions per kWh produced decrease as the rated power of the turbine increases. The results also show levels of emissions and environmental impact that are far lower than those of other conventional sources of electricity generation.

With this in mind, the present study focuses on current, multimegawatt wind turbines, seeking to obtain LCA data for wind power at the present time.

3.2 LCA of a 2 MW Wind Turbine

This wind power LCA focuses on the type of wind turbine most widely used all over the world at this time: the multimegawatt double-fed induction generator (DFIG).

3.2.1 System Limits

Cutoff criteria for the system must be defined if the results of the analysis are to be understood and interpreted correctly. The criteria applied in this case include the following:

  • The construction phase of the main components of the wind turbine.

  • Transportation of the wind turbine to its specific site in the wind farm.

  • Direction, installation, and commissioning of the turbine at its final operating site.

  • Decommissioning of the turbine and subsequent treatment of the waste produced.

For a diagram showing these system cutoff criteria, see Fig 4.

Fig. 4
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LCA system limits (Martínez et al. 2009)

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All the electrical switchgear for the electricity generated by the turbine, which is used to distribute that energy to the grid, is excluded from the system. This includes the following:

  • Medium voltage wiring

  • The substation transformer

  • The electrical distribution and transmission network

3.2.2 Functional Unit

The functional unit defined here for the LCA of the electricity generated by a wind turbine is the production of 1 kWh of electricity. Choosing a functional unit that is appropriate for the task being performed is of course an important factor in ensuring that the results obtained are meaningful. What is being studied here is the environmental impact of wind energy in the context of other types of renewable and nonrenewable energy sources, so selecting 1 kWh as the functional unit gives a clearer outlook on results, and facilitates comparisons, if desired, with the environmental impacts resulting from the generation of 1 kWh with such other sources.

3.2.3 Data Used

When an LCA is conducted on a wind power system, data need to be collected on each of the main components of the wind turbine and on the various subcomponents that make up those main components. These components are highly varied in their nature and characteristics and may include various types of mechanical, electrical, and electronic parts. This makes it difficult to obtain all the information needed from the various suppliers in order to perform an LCA on each and every part used in the turbine. It is therefore necessary to draw up a suitable life-cycle inventory including the most important and significant components of the unit, such as the foundations, the tower, the nacelle, and the rotor. Most of the inventory data have been obtained directly from the companies that produce the wind turbines (generator, gearbox, blades, etc.). The exception corresponds to the dry-type transfomer, where the information was obtained from a third company that manufactures dry transformers similar in volume, power, and tension. In individual cases, where it was not possible to obtain reliable and verified data, commercial databaseEcoinvent (Boustead and Hancock 2003; Frischknecht and Rebitzer 2005; Frischknecht et al. 2005) was utilized.

Data on energy expenditure and materials used in manufacturing the main components of a wind turbine were supplied by Gamesa, a major worldwide manufacturer. Table 2 gives a general summary of the materials used in the main components of turbines and the energy used in the manufacturing processes associated with those main components.

Table 2 Summary of the materials and manufacturing energy used in the main components (Martínez et al. 2009)
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Along with materials and energy consumption, the transportation of components from their places of manufacture to the final location of the turbine on the wind farm must also be taken into account. Once the wind turbine has been erected on site and commissioned, it will require preventive and corrective maintenance to ensure that it remains in working order for most of its useful lifetime. Logically, all these operations must also be taken into account in the LCA conducted, and information must be compiled on how much oil and grease is used, on how many filters are replaced, on the transportation of materials and workers, etc.

3.2.4 Assumptions

When conducting an LCA on a system as complex as a wind energy generation system, limitations must be set on the level of detail applied in the compilation of data. Only thus can the LCA be completed within reasonable limits in terms of effort, resource requirements, and time with no loss of quality in the final results obtained.

As mentioned above, the first step is to establish cutoff criteria for the system to be studied. The study conducted here includes all the elements of the foundations on which the wind turbine is supported, plus the other main components (tower, nacelle, and rotor). It excludes all the elements that comprise the system of connection with the electricity distribution and transmission network (medium voltage lines and transformer substation).

The main cutoff criterion used is the weight of each component in relation to the total weight of the system under study. Despite the use of this cutoff criterion, which would be not valid in general cases, when applying it we have taken into account the necessity of not to exclude the components or materials which, in spite of its relatively low weight in the system, may present a significant environmental impact. This considerably reduces the number of small components that need to be analyzed individually and for which specific data must be compiled, but does not appreciably affect the final results. It also makes the LCA more flexible, enabling major changes to be incorporated into the system more quickly and easily.

The data used to define and characterize each of the main components are taken from a database supplied by the component manufacturer. The main data collected are the following: raw materials required for manufacture, energy consumed in the associated manufacturing processes, and details of the transportation of materials and components.

The basic assumptions made in the wind power system LCA are the following:

  • The weight of the main components is used as the cutoff criterion to determine whether or not they are included in the life-cycle inventory. The sum total of the components considered in the analysis accounts for 95 % of the weight of the foundations, 95 % of that of the tower, 85 % of that of the nacelle, and 85 % of that of the rotor.

  • All data on the emissions from and characteristics of the energy used in the manufacturing processes are taken from the Ecoinvent database and refer to Spain, because that is where the turbine manufacturer’s plants are located.

  • Modern, multimegawatt wind turbines are assumed to have a useful lifetime of 20 years.

  • The estimates made in regard to the decommissioning of the turbine at the end of its useful lifetime and the subsequent processing of waste products are based on decommissioning projects prepared by the company that holds the operating rights to the turbine. Basically, it is considered that 90–95 % of the metal (iron, steel, and copper) is recycled, PVC plastics, fiberglass and concrete are land-filled, and oils and other types of plastic are incinerated.

  • The annual output from the wind turbine is worked out on the basis of an appropriate figure in equivalent hours of production to ensure the economic viability of a wind farm, i.e., 2,000 equivalent hours of production per year. For a turbine with a rated power of 2 MW, this is equivalent to an annual output of 4 GWh.

  • In the context of corrective maintenance work on the turbine, in the course of its useful lifetime, it is estimated that one replacement generator will need to be installed due to malfunction (the complete new generator is considered, including manufacturing and assembly).

Finally, allocation as per the recommendations of standard ISO 14044 (which sets out the characteristics to be met by life-cycle analyses) is not used (Guinée et al. 2001; ISO ISO 2006a, b). The only function considered in the system analyzed and in all its components is that of generating electricity, so all the environmental impacts associated with the system are allocated to electricity generation. It is considered that the material recycled at the end of the useful life replaces virgin material used in the manufacturing stage of the turbine.

3.2.5 Methodology and Impact Categories Analyzed

When conducting an LCA, an environmental impact assessment method must be selected, and it must be decided which impact categories are to be analyzed. In this case, the assessment method chosen is CML Leiden 2000 (Guinée et al. 2001), and the categories are the following:

  • Abiotic depletion: this category is linked to the extraction of minerals and fossil fuels associated with the inputs of the system under analysis and their effects on human health and the ecosystem.

  • Acidification: this category is linked to the effect of various acidifying substances on the soil, groundwater, surface water, organisms, ecosystems, etc.

  • Eutrophication: this category is linked to excess micronutrients in the environment as a result of emissions of nutrients into the air, soil, and water.

  • Freshwater aquatic ecotoxicity: this category is linked to the effects of emissions into the air, soil, and water on freshwater ecosystems.

  • Global warming (GWP100): this category is linked to the effects of greenhouse gas emissions on human health and the environment.

  • Human toxicity: this category is linked to exposure to toxic substances and their effects on human health.

  • Marine aquatic ecotoxicity: this category is linked to the effects of emissions on marine ecosystems.

  • Ozone layer depletion (ODP): this category is linked to the proportion of UV-B radiation that reaches the surface of the Earth.

  • Photochemical oxidation: this category is linked to reagents, mainly ozone, whose appearance in the environment may have effects on human health and ecosystems and may damage crops.

  • Terrestrial ecotoxicity: this category is linked to the effects of emissions on land-based ecosystems.

4 Analysis of Results and Future Trends

The results for the production of 1 kWh of electricity via wind power are shown in Table 3, broken down into the various impact categories studied in the LCA and at the characterization stage. The intention is to prevent the potential subjectivity associated with other phases of the LCA and to facilitate the comparison of results with other LCA in the fields of renewable and conventional energy sources.

Table 3 Results of the LCA by impact categories (Martínez, et al., 2009)
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For example, a comparison with the environmental impact associated with electricity generation according to the Spanish energy mix for the year 2000 shows that the figures obtained here are between 89 and 99 % lower, according to the impact category considered. In the specific case of CO2 emissions associated directly with climate change (see the Global Warming category), the figure for 1 kWh of electricity generated via wind power is 98 % lower than the figure for the Spanish energy mix.

Another interesting result can be provided by an examination of how long wind turbines need to operate to offset their environmental impact from the time of their manufacture through their operational lifetime and maintenance to their decommissioning, and the processing of the resulting waste. Taking as the starting point for this analysis the fact that the energy generated by a wind turbine avoids the need for an equivalent amount of energy to be produced via conventional sources, another comparison with the Spanish energy mix can be made. The Spanish energy mix is used for the purposes of comparison because the turbine studied is located in Spain and the manufacturer is Spanish, but the results can be extrapolated to any other country or location. Under this premise, the environmental impact caused by the turbine is offset in between 53 and 784 days, depending on the impact category considered.

A more detailed examination of the results obtained reveals that the main components with the greatest impact on the environment are the rotor, the tower, and the nacelle. The impact of the rotor is derived mainly from the amount of fiberglass used in the blades and in the nose cone that covers the hub. The impact associated with this material is increased by the fact that it is not recycled at the end of the turbine’s useful lifetime. In this area, there is a clear potential for reducing environmental impact if the possibility of recycling fiberglass is considered in the future, even if it is only as a replacement for other types of plastic in applications other than wind turbine blades.

In the case of the nacelle, one of the elements that have the most environmental impact is the copper used in the wiring, and others include the electronic components and the fiberglass used in the casings that cover and protect the components that make up the powertrain and the associated systems on the turbine.

The main component of the tower is the steel from which it is manufactured. Much of the impact associated with it is offset by the fact that this steel is recycled at the end of the turbine’s useful lifetime.

All these results refer to state-of-the-art wind turbines currently available on the market, but the trend in wind power is toward even larger turbines with greater rotor diameters and higher power ratings. It is therefore reasonable to assume that the increased scale of future wind turbines will make for even greater reductions in environmental impact in electricity generated in this way.

Moreover, wind power technology is gradually maturing, and more and more efforts are being devoted into improving the operating and maintenance conditions of wind turbines. For example, more and more commercial wind turbines without gearboxes are being installed because gearboxes are among the most failure prone components in turbines. This is done in an attempt not only to reduce maintenance and breakdown costs but also to increase the effective production time of turbines over the course of their useful lifetimes.

5 Conclusions

This study looks at wind power from the viewpoint of life-cycle assessment. Such analyses have, of course, been conducted at various times throughout the development of wind power, and their results have varied as the designs and main characteristics of wind turbines have evolved. For that reason, the latest DFIG multimegawatt turbines are considered here, as theirs is the most numerous type currently in use on wind farms. On that basis, a comprehensive LCA is conducted on a wind turbine, covering all phases from its manufacture to its decommissioning and the processing of waste at the end of its useful lifetime.

The results clearly show how low the environmental impact of wind power is in the various impact categories studied, especially when compared to the figures for other, conventional sources of electricity generation. This confirms the positive nature of wind power in all environmental and climate change-related aspects, but does not take into account other essential elements such as the financial and technical viability of installing such systems within a specific electrical grid. LCA can thus be confirmed as a potentially important tool in the field of energy provided that it is used as just one of the means of support for decision making by the relevant authorities and other players in the field of energy system development.