Volume 2 wind energy 2 21 – trends prospects, and rd directions in wind turbine technology

Volume 2 wind energy 2 21 – trends, prospects, and rd directions in wind turbine technology Volume 2 wind energy 2 21 – trends, prospects, and rd directions in wind turbine technology Volume 2 wind energy 2 21 – trends, prospects, and rd directions in wind turbine technology Volume 2 wind energy 2 21 – trends, prospects, and rd directions in wind turbine technology Volume 2 wind energy 2 21 – trends, prospects, and rd directions in wind turbine technology

JK Kaldellis and DP Zafirakis, Technological Education Institute of Piraeus, Athens, Greece

© 2012 Elsevier Ltd All rights reserved

2.21.1 Brief Description of Wind Power Time Evolution

2.21.2 The Current Wind Turbine Concept

2.21.3 Size Evolution of Wind Turbines

2.21.4 Pitch versus Stall and Active-Stall Wind Turbines

2.21.5 Direct-Drive versus Gearbox

2.21.6 Blade Design and Construction

2.21.7 Innovative Concepts

2.21.8 Environmental Impact Reduction

2.21.9 Offshore Wind Parks

2.21.10 Vertical-Axis Wind Turbines

2.21.11 Small Wind Turbines

2.21.12 Building-Integrated Wind Turbines

2.21.13 Wind Energy Cost Time Evolution

2.21.14 Research in the Wind Energy Sector

2.21.15 Wind Energy Technological Problems and R&D Directions

2.21.16 Financial Support of Wind Energy Research Efforts

Capacity factor The capacity factor of a wind turbinerefers Union so as to support and encourage research in various

to the ratio of the actual energy production of the machine sectors,including also wind energy.

for a given time period to the respective potential energy Pitch control In pitch-controlled machines, the angle of production of the same machine if it had operated at its the blades is adjusted through signaling and the use of a

various life cycle stages of a system, for example, a wind Power (aerodynamic) coefficient A measure of the wind turbine, equally well restricted to a single stage such as turbine rotor’s ability to exploit the available kinetic

corresponds

the system in order to investigate whether they can be R&D The term is used to describe research and

Feed-in tariff A policy mechanism developed for the systematic basis in order to increase the stock of

knowledge and the use of

electricity produced by a renewable resource and fed into Stall control In stall-controlled machines, the angle of the

the grid Feed-in tariffs may vary on the basis of blades is fixed, while the blades are designed so that they technology, geographical location, and installation size can increasingly stall the angle of attack with the increase

Tip speed ratio The ratio of the linear speed at the tip

on the basis of Framework Programmes alone, comprise the blade to the wind speed upstream of the rotor

Uses: Power generation Type: Electric, gearless generators, permanent

magnets, no slip rings

Wind blades: Plastics (epoxy) or bioplastics, fail

safe pitch control

Towers: Extruded concrete (selfmounting) Erection: self erecting Uses: Power generation Twenty-first century Type: Electric, geared or gearless

generators, auxiliary excitation,

Wind blades: Plastics, pitch control Towers: Steel or concrete

Twentieth century

Uses:Power generation−grinding mills

Type: Electric/mechanical Wind blades: Wood/metal, stalled Towers: Wood (handycraft) Third generation

2.21.1 Brief Description of Wind Power Time Evolution

Wind energy development counts thousands of years, that is, from the starting point of the very first vertical-axis wind machines operating on the basis of drag forces, up until the current time, during which wind turbines under development have reached the scale of tens of MW (Figure 1)

Constant evolution of the wind power concept throughout this period may be reflected in the most straightforward way by the fact that we are now arguably entering the time of fourth-generation wind power machines (Figure 2) [1] From the early times of wind power exploitation, when the first vertical-axis windmills were used for grinding, to the times that electricity power generation lies on the rotation of huge epoxy-based blades reinforced with carbon fiber and the exploitation of offshore potential, humankind has encountered numerous types of wind machines and designs, which have always found an important place in the puzzle of technological development

It was in fact centuries ago when the technology of wind energy made its first actual steps – although simpler wind devices date back thousands of years – with the vertical-axis windmills found at the Persian–Afghan borders around 200 BC and the horizontal-axis windmills of the Netherlands and the Mediterranean following much later (AD 1300–1875) (Figure 3) [2–4] Further evolution and perfection of these systems was performed in the United States during the nineteenth century, when over six million small machines were used for water pumping between 1850 and 1970 (Figure 4)

Figure 1 Wind power evolution: From the very first vertical-axis machines to large-scale contemporary wind turbines

Figure 2 Wind power evolution: From the first to the fourth generation of wind power machines

Figure 3 The vertical-axis grain machines of the Persians and the horizontal-axis windmills of the Netherlands

Figure 4 From water pumping to the California outbreak

On the other hand, the first ‘large’ wind machine to generate electricity (a low-speed and high-solidity wind turbine of 12 kW) was installed in Cleveland, Ohio, in 1888, while during the late stages of World War I, the use of 25 kW machines throughout Denmark was widespread Much later, the first wind turbine feeding a local grid was installed in 1931 in the USSR in Balaklava, with the electricity generated being fed into a small grid that was supplied by a 20 MW steam power station Further development of wind generators in the United States was inspired by the design of airplane propellers and monoplane wings, while subsequent efforts in Denmark, France, Germany, and the United Kingdom during the period between 1935 and 1970 showed that large-scale wind turbines could work Note that during this period, emphasis was mainly given to the development of horizontal-axis wind machines (i.e., the shaft of rotation is parallel to the ground) operating on the top of adequately high towers and using a small number of blades (normally two or three)

Meanwhile, it was in 1931 that Georges Darrieus invented the vertical-axis wind turbine known as the ‘eggbeater’ windmill, introducing a new power generation concept for wind machines (Figure 5) European developments continued after World War II

In Denmark, the Gedser mill 200 kW three-bladed upwind rotor wind turbine operated successfully until the early 1960s [5], while

in Germany, a series of advanced horizontal-axis designs were developed, with both of the aforementioned concepts dictating the future horizontal-axis design approaches later emerging in the 1970s

One of the most important milestones of wind energy history coincides with the US government involvement in wind energy R&D after the oil crisis of 1973 [6–8] Following this, in the years between 1973 and 1986, the commercial wind turbine market evolved from domestic and agricultural (1–25 kW) to utility-interconnected wind farm applications (50–600 kW) In this context, the first large-scale wind energy penetration outbreak was encountered in California [9], where over 16 000 machines ranging from

20 to 350 kW (a total of 1.7 GW) were installed between 1981 and 1990, as a result of the incentives (such as the federal investment and energy credits) given by the US government (Figure 4) In northern Europe, wind farm installations increased steadily through the 1980s and 1990s (Figure 6), with the higher cost of electricity and the excellent wind resources leading to the creation of a small but stable market

After 1990, most market activity shifted to Europe [10], with the last 20 years bringing wind energy to the front line of the global scene, with major players from all world regions Nevertheless, both the revival of interest in the United States and the recent dynamic introduction of the Chinese in the wind energy sector have much altered the up-to-now wind energy market situation

In summary, during these past 20 years, the wind energy sector has met tremendous growth, not only in terms of market share but also in terms of technological developments, with the latest achievements bringing about the era of offshore wind power

19 1997 1998 992000 2001 022003 2004200

5

2006 200720 20 2010

40.2 20.7

Year

Figure 5 Aspects of Darrieus vertical-axis wind machines

Figure 6 Danish stamp of 1989 and a present-day offshore wind farm

generation (Figure 6) [11] At this point, it should be noted that important advancements met in the field comprise the result of constant and unceasing research efforts, aiming at the development of innovative clean energy technologies

In fact, according to the latest figures, systematic efforts recorded throughout this period of growth correspond to a galloping global wind power capacity that recently managed to exceed 200 GW (Figure 7) and that is, according to market experts, anticipated to reach 450 GW by 2015 [12] As already implied (Figure 7), the cumulative installed wind power is

Figure 7 (a) Time evolution of installed wind power and (b) 2010 cumulative wind capacity distribution

nowadays mainly concentrated in the European Union, the United States, China, and India, while what should also be noted is that there is aremarkable activity recently recorded in offshore installations, with contemporary machines now reaching or even exceeding 5 MW

2.21.2 The Current Wind Turbine Concept

Being the result of strong competition among different design schools, techniques and manufacturers from all around the world, the vast majority of today’s wind turbines comprise the following main parts [13] (Figure 8):

A ‘rotor’ of diameter D, using three relatively thin blades placed upstream of the tower and rotating on the basis of a horizontal axis that is almost parallel to both the ground and the wind direction Rotational speed of the rotor nR is kept relatively low in order

to limit development of strong centrifugal stresses upon the blades [14], while it is the rotor that at the same time determines the power of wind to be exploited Pw (see also eqn [1]):

Upward gradient of the rotation axis to the horizontal level

Vertical line to the axis of rotation

Wind stream

Hub height

Orientation (yaw) system

Tower

Ground level

Rotation axis of the rotor

Blades

Conicity

Rotor diameter

Figure 8 Typical (simplified) contemporary wind turbine concept

of contemporary wind machines also come with more degrees of freedom in order to avoid excessive loading in the case of rather high wind speeds

A ‘nacelle’ that is directed toward the wind with the help of a ‘yaw mechanism’ [17] and includes the equipment components necessary to convert the mechanical power of the low-speed primary shaft (i.e., the rotational speed of the rotor) to electrical energy satisfying the requirements of consumption In this regard, there are two established concepts (see Section 2.21.5) to be considered

In the first case, a gearbox is used in order to achieve increased rotational speed of the secondary axis nG, which is directly connected to the electrical generator, in comparison with the low rotational speed of the primary axis nR, based on the gearbox ratio i:

At the same time, the electrical generator (either synchronous or induction machine) converts the mechanical energy of the secondary axis to electrical energy of a given voltage and frequency f (e.g., 50 or 60 Hz depending on the consumption require­ments), on the basis of the following equation:

p ⋅n

½3

60 where p is the number of pairs of poles of the electrical generator

The second design option uses no gearbox and is actually based on the exploitation of the primary shaft mechanical energy with the use of a variable-speed electrical generator, which, according to eqn [3], produces electrical energy of variable frequency Following this, the AC current is rectified (via an appropriate rectifier) and is then transformed again into AC of defined characteristics (frequency, voltage, etc.) with the use of an inverter

The nacelle also includes a series of electronic and electrical support subsystems, the mechanical brakes, and the hydraulic circuits, all together ensuring safe and smooth operation of the machine Finally, the overall installation is also supported by a number of monitoring instruments (e.g., for measuring wind speed and determining wind direction, for measuring air temperature, etc.) as well as the foundation structure of the machines, of which only a minor part is visible

2.21.3 Size Evolution of Wind Turbines

Wind turbine technology has since 1980 encountered a constant size evolution that transformed the sector of small-scale turbines of tens of Watts to the sector of MW machines During this time of evolution, the need for upscaling along with the urgency to exploit economies of scale managed to overcome every technological barrier appearing, resulting in the construction of rotors that nowadays even exceed 120 m The main drivers behind this unceasing trend of size increase concerned the need to exploit higher winds at higher altitudes, maximize area exploitation, and minimize system operational costs per unit power Size evolution is summarized in Figures 9–11, where the time evolution of the rotor diameter along with the evolution of hub height and nacelle mass in relation to the former provides an overview of swept area, height, and mass increase of wind turbines over time [18] More precisely, according to Figure 9, rotor diameter may be determined by an almost exponential trend that, however, results in

a typical S-shaped curve due to the stagnation of size development lately met in contemporary wind turbines at the level of 3 MW rated power Following the trend of rotor diameter increase, the increase of the nacelle mass is also analogous, which, on the other hand, seems to become lighter per unit swept area of the rotor (i.e., from 14.6 to 13.4 kg m−2) (Figure 10) The results are similar when considering hub height, with current numbers well exceeding 100 m (Figure 11), although it seems that there is now a trend toward shorter machines (hub height to rotor diameter below 1.0) that is partly justified by offshore developments (where taller towers do not make up for the need to build stronger foundations) In addition, according to the characteristics (thickness) of the

Time evolution of contemporary wind turbines’ diameter

Nacelle mass vs rotor diameter increase

Nacelle mass Nacelle mass per unit area

Rotor diameter (m)

Figure 10 Relation between nacelle mass and rotor diameter

Hub height vs rotor diameter increase

Hub height Hub height-to-rotor diameter ratio

Rotor diameter (m)

Figure 11 Relation between hub height and rotor diameter

atmospheric boundary layer, wind speed increase at heights greater than 120 m is not as appreciable and, thus, there is no actual energy gain with further increase of the tower height

What is also interesting to note in terms of structure is that size increase has in general brought about a downgrade of manufacturing energy requirements in the area of 2–3 MWh per kW of rated power (Figure 12), which is actually a key factor for the overall life-cycle energy performance of contemporary wind turbines [19–21]

Furthermore, although it cannot be attributed to upscaling of the machines alone, the fact that the mean annual worldwide capacity factor presents an increasing trend (Figure 13), among others (e.g., better wind resource assessment and better siting), also reflects the ability of larger machines to exploit the available wind potential more efficiently (Figure 14) [22]

Meanwhile, although stagnation has been recently noted in the wind energy industry around the scale of 2–3 MW machines, manufacturers are still increasing the size of commercial models Note that a similar situation was encountered during the end of the 1990s as well, see Figure 9, with significant technological improvements of that period eventually pushing commercial machines to exceed the limit of 1 MW As a result, according to the latest market data, the largest commercial wind turbine is at the moment the Enercon E-126, with a rated power of 7.5 MW [23], while one may also encounter the Repower 6 M model of 6 plus

MW [24] In this context, in Figure 15, the 10 giants of wind energy industry are gathered in order to obtain a first idea of current commercial sizes (2010–11), although it should be noted that models under development at the moment correspond to wind machines that may start from 7 MW (e.g., Vestas V164) and even reach up to 10 MW (e.g., Sway project, Clipper Windpower and American Superconductor)

There have been strong arguments during recent years that the current horizontal-axis concept is at its peak, with certain studies predicting a stagnation point around 5 MW [25] The response to this question of whether the horizontal-axis concept has indeed little to give in the following years – as a result of both structural limitations and increased costs associated with the shift to 10 MW

Time evolution of the mean annual capacity factor on the European Union and the global level

Figure 12 Manufacturing energy requirements of wind turbines in relation to their rated power

Figure 13 Time evolution of mean annual capacity factor

Specific annual energy production of wind turbines in relation to the respective rotor diameter

Figure 15 The 10 commercial wind turbine giants of the present day (2011)

plus machines – is expected by the results of the UPWIND project [26], aiming at the development of 20 MW wind turbines, addressing all issues involved in such an outstanding effort

In conclusion, it is the present day that the current concept of wind turbines faces the greatest challenge in more than 30 years, during which constant size increase led to the establishment of a tremendously growing industry that allowed wind energy to dominate On the other hand, what must be noted is that even if proceeding with the status quo of commercial wind turbines, achieving power outputs in the order of 10 MW, future progress of wind power is still thought to be rather encouraging

2.21.4 Pitch versus Stall and Active-Stall Wind Turbines

Unless the power production of a wind machine is regulated at high wind speeds, the occurring overloading may even cause failure of certain components or of the entire structure Power regulation is applied with the use of two main concept mechanisms, that is, stall control and pitch control [27]

In the first case, power regulation is achieved through the exploitation of the stall phenomenon (Figure 16(a)) and the aerodynamic design of the rotor blades itself More precisely, once a certain threshold of wind speed is exceeded, the airfoil angle

Figure 16 Wind flow over an airfoil when stalling and illustration of the pitch-control mechanism

Time evolution of pitch-and-stall regulated

Figure 17 Gradual dominance of pitch-control over stall-control regulation

of attack is increased and the lift force previously occurring upon the blades is now reduced due to a progressive and not abrupt passive stalling In the second case, pitch-control regulation is based on the rotation of blades along their longitudinal axis in order

to ensure power regulation within the desired limits (Figure 16(b)) More specifically, through electronic control, power output of the machine is controlled several times per second and blade angle changed to either obtain maximum power for a given wind speed or reduce power output after the point that rated power has been obtained

A third option that has emerged during recent years, and is mostly applied to machines in the scale of MW, is the active stall-control concept [28] In this case, pitchable blades are used by the machine, which are, however, operated in a somewhat different way than in the classic pitch-control design More specifically, although at low wind speeds blade control is similar to the case of the classic pitch-control operation, the situation becomes different when rated power is exceeded At that point, electronic controls decide to increase the angle of attack, which in turn leads to stalling, which is opposite to the case of pitch control, where reduction of the angle of attack is performed so as to reduce power output at the levels required An immediate advantage of this solution is that by ensuring active stall in wind speeds above rated power, deep stall – as in the classic stall-control option – is avoided, thus allowing for greater wind energy exploitation

Considering also the interaction between power regulation and rotational speed of the machine, there are actually three common options currently used in power regulation, that is, constant speed of the rotor and fixed blades using stall regulation (e.g., NM 1000/60), constant speed of the rotor and pitchable blades using pitch/active-stall regulation (e.g., WindMaster 750/48 and Vestas V82), and, finally, variable speed of the rotor and pitchable blades using pitch regulation (e.g., Enercon E126), with the latter concept being the one that has dominated in large MW-scale machines (Figure 17) The opposite trend was noted during the 1980s, when stall control found itself to be the dominant solution, with the respective three-bladed fixed-speed machines (Danish concept) actually comprising the only option during that time

Between the two, due to the fact that the pitch fixed speed solution was never successful, many of the manufacturers decided to introduce only some level of variability to begin with, with systems such as OptiSlip and Flexi-slip allowing for a maximum slip in the area of 10–17% [29] In the following years, attributes of variable speed such as considerable flexibility and high quality of power offered to the grid were much appreciated, although high costs compared with relatively small energy gains always comprised

a barrier for total acceptance of the concept

2.21.5 Direct-Drive versus Gearbox

Up until 1990, contemporary wind turbine models were based on the classical drivetrain concept, according to which the rotor (or the primary shaft) speed has to be increased with the use of a multistage gearbox in order to allow operation of the electrical generator at high rotational speeds Since 1991, however, direct-drive design was also introduced (with the generator being directly coupled to the rotor), providing an alternative option for wind turbine manufacturers Despite the breakthrough characteristics of the direct-drive design, however, the dominance of geared machines in terms of commercial use is still evident, since the only company that has, up to now, supported the specific solution is Enercon In fact, according to the latest official data concerning the time period between 2001 and 2008, the market share of gearless machines reached only 15% [30], with the rest of the delivered machines corresponding to the classical geared concept (Figures 18 and 19)

During recent years, great interest has been demonstrated in the R&D of direct-drive concepts, much stimulated by the shift of wind power technology to offshore applications, with less complex direct-drive configurations implying significantly lower

Geared vs gearless wind turbines’ time evolution (2001−08)

Figure 18 Market share of geared and gearless machines (number of wind turbines)

Figure 19 Market share of geared and gearless machines (delivered capacity)

Figure 20 Current wind turbine drivetrain concepts

maintenance and operation requirements, which are of primary importance when it comes to offshore wind parks One should also take into account the so-called hybrid solutions, which manage to combine the two above-mentioned edges through the incorporation of a low gear ratio gearbox and a permanent magnet generator

In summary, current drivetrain options [31] (Figure 20) include the following three solutions

• ‘The classical geared concept’, with high gear ratios allowing for generator speeds in the order of 1200–1800 rpm Its main benefits are the reduced size of electrical generators used and the up-to-now lower purchase costs, and its main drawbacks are the inherent energy losses and the need for considerable maintenance needs [32]

• ‘The hybrid solution’ (e.g., Multibrid [33]), in which the common synchronous electrical generator met in the direct-drive design

is replaced by a permanent magnet generator that eliminates excitation losses A single- or two-stage gearbox is also incorporated

in order to downsize the need for the huge generators required in large-scale direct-drive machines [34]

• ‘The direct-drive concept’ (e.g., Enercon machines), with the introduction of low-speed generators that are directly connected to the machine rotor [31] Its main advantages are the higher efficiency and the minimization of maintenance requirements, and its main disadvantages are the fact that a bulkier/heavier system should be considered, which also comes along with higher purchase costs Current R&D efforts are mainly concentrated on the combination of direct-drive systems and permanent magnet generators, destined

to support the development of large-scale machines (in the order of 5 MW), especially for offshore applications On the other hand, next-generation advanced solutions concerning drivetrain systems [35] also include options such as superconductive drivetrains, continuously variable transmissions, and other innovative concepts, with their characteristics being synopsized in Table 1

2.21.6 Blade Design and Construction

Possibly the most important component of a wind machine, that is, its rotor blades, is the result of elaborate design on the basis of multiple tests carried out (Figure 21) [36], aiming at maximum exploitation of the available wind energy potential Because the

Table 1 Main characteristics of next-generation drivetrains

Capital costs, reliability, energy capture, supply chain security Energy capture

Capital costs, reliability, energy capture, safety and serviceability

Technical, perception, scaling Technical

Technical, commercial, perception, certification

Subcomponent reliability testing, demonstration project deployment, engage independent third parties

Laboratory testing

Materials science research, trade-off analysis, cost/benefit analysis, IEC standards development

System design and topology Power electronics

Capital costs, reliability, energy capture, safety and serviceability Capital costs, reliability, performance

Technical, commercial, certification Technical, perception, commercial

Laboratory research, cost/benefit analysis, IEC standards development

Laboratory research, workforce education and training, cost/benefit analysis Diagnostics and

maintenance

Reliability Reliability, operator

training, certification

Reliability research, workforce education and training, IEC standards development Variable trains

Innovative

Frictional contact drives Fluid drives

Uptower DC

Capital costs, reliability, energy capture Capital costs, reliability, energy capture, energy storage

Reliability, grid benefits

Technical, scalability, materials Technical, scalability, materials, perception Technical

Cost/benefit analysis, design and modeling tools, demonstration projects, component development and testing

Cost/benefit analysis, design and modeling tools, component development and testing, demonstration projects

Feasibility studies, equipment survey concepts generator

Ground-level Reliability Technical Feasibility studies generators

Rim-drive turbines Tandem generators Complete uptower gearbox reparability Other innovative drivetrains

Reliability Availability, scalability Reliability

Capital costs, reliability

Technical Commercial Technical

Technical, perception

Feasibility studies Feasibility studies Feasibility studies

Design studies

Blade weight trends in relation to manufacturing

material and rotor diameter

Figure 21 Aspects of full-scale tests for evaluation of blade performance

power output of a wind turbine is found to be analogous to the swept area of the rotor (or the square of the rotor diameter), blade manufacturers try to meet the challenge of creating larger and larger wind turbine blades that may ensure higher power output per unit machine In this case, it was only recently [37] that LM Wind Power (former LM Glasfiber) announced the construction of a 73.5 m blade (equivalent to a 24-storey building) for the Alstom 6 MW offshore wind turbines, demonstrating advancements made

in the specific sector during recent years

Higher energy output requiring larger blades also comes with the drawback of increased weight and loads that affect the entire machine structure As a result, efforts concerning the evolution of blade design are more focused on weight reduction, with the use

of lighter materials being the research subject of many blade evolution projects during the past (e.g., WindPACT, the NREL-sponsored Turbine Rotor Design Study, the Sandia-sponsored Blade System Design Studies, etc.), with emphasis given on the development of jointed blade designs and the introduction of carbon fiber composites [38–41]

Meanwhile, according to Hau [42], there are five basic blade designs that must be considered, at least in terms of material used:

• Fiberglass/polyester: These comprise old-design heavy blades (in the order of 2–3 kg m−2 of swept area; see Figure 22), mainly used in older Danish machines, with built-in aerodynamic spoilers, being suitable for comparatively low tip speed ratios

• Laminated fiberglass/polyester: Mostly used in the past, destined to serve small- to medium-scale wind turbines and medium to high tip speed ratios, with a specific weight ranging from 1.5 to 2.5 kg m−2

• Fiberglass/epoxy: With the use of epoxy resin, construction of lighter blades that are mostly used in large-scale machines has become possible, also supporting reduction of the specific weight, even under 1 kg m−2 for the smallest of machines

• Wood epoxy: Such blades when introduced provided a temporary solution that was much lighter than the older fiberglass/ polyester Nevertheless, with the use of more advanced composites accordingly, the specific solution is not as common nowadays

• Fiberglass/carbon: With the introduction of carbon fibers, extremely lightweight blades gave an answer to the challenge of developing very large-scale machines, employing rotors of more than 80 m In this regard, many of today’s companies use carbon

Figure 22 Specific weight of blades in relation to materials used and rotor diameter

Blade mass and specific weight in relation to blade length for different class machines (39 designs)

Blade mass

Blade length (m)

Figure 23 Blade mass and specific weight of blades in relation to blade length

fiber, normally at a given proportion of 750–1000 kg MW−1, while the weight reduction achieved is supposed to counterbalance increased costs, especially when considering extremely long blades

Further investigation of weight trends may be obtained from the Energy Research Centre of the Netherlands (ECN) [43], where several machines are compared, with the results designating a power trend with the blade mass being analogous to the radius square (a power factor around 2 is to be expected; see Figure 23), whereas specific weight is found to vary between 1.2 and 1.6 kg m−2 Gradual employment of lighter materials may be reflected by the results of Figure 24, where different period curves illustrate advancements made in the field, with weight difference being more and more pronounced as the blade length increases The issue of number of blades employed seems to have been solved long ago, with three-bladed machines being – as already seen – the almost exclusive option in contemporary wind turbines The comparatively higher power coefficient of three-bladed machines, although not ensuring a priori sufficient energy yield to outweigh the cost of an extra blade, is also accompanied by other positive attributes More precisely, operation of one- and two-bladed machines at higher tip speeds also implies higher levels of aerodynamic noise to consider Furthermore, asymmetry introduced in terms of aerodynamics from the one- and two-bladed machines entails higher loading, which requires more complex components to be introduced in the machine (e.g., teeter hinge), while, finally, visual asymmetry is also a matter of concern, with three-bladed machines being more easily adapted to inhabited environments Nevertheless, if the costs of facing additional loads – as a result of higher rotational speeds – become comparable to

Figure 24 Blade mass and specific weight of blades for different time periods

Blade mass and specific weight in relation to bladelength for machines of different time period (58 wind turbines)

Specific weight cost vs overall cost contribution in

relation to blade length

Figure 25 Specific weight cost and blade cost contribution in relation to blade length

the cost of an extra blade, two-bladed machine applications may actually find their place in the wind energy market (e.g., in offshore applications where noise may not comprise such a problem and the potentially easier assembly of a two-blade rotor may prove more cost-effective [44])

At the same time, economies of scale seem to apply in the blade industry as well (Figure 25), with larger blades ensuring some level of cost reduction [45], although this can be quite sensitive and much depends on the mix of materials to be used On the other hand, gradual reduction of this cost was not found to be analogous to the overall cost reduction of wind turbines, with the contribution share of rotor blades found to increase with the size increase of the machines

Nevertheless, this trend is expected to change in the years to come, since the diffusion to offshore wind machines will signal a shift of major costs to other components such as foundations In fact, according to experts, it is this trend that will also allow for carbon fiber to be largely adopted in new wind turbine giants

At the same time, R&D in the field is ongoing, with the latest efforts being carried out by the Wind Energy Department of Sandia Laboratories, where exploration of a 100 m blade, potentially for a 13.2 MW offshore machine, comprises the latest challenge

2.21.7 Innovative Concepts

Although the three-bladed horizontal axis is by far the most common wind turbine concept, new design concepts constantly emerging demonstrate the ongoing research interest in the wind power field More precisely, innovative designs presently being examined include the following:

• Magenn Air Rotor System (MARS): MARS [46] is a tethered wind turbine that is lighter than air and rotates around a horizontal axis

in order to produce electrical energy (Figure 26) MARS is elevated with the use of helium, which allows the machine to ascend to very high altitudes (up to 300 m), where exploitation of high wind speeds is possible Furthermore, MARS may operate within the broad range of speeds from 2 up to 28 m s−1, while one of its main advantages is its ability to remain mobile and be

Figure 26 Aspects of the MARS design (prototype, operation principle and future MARS wind farms)

Figure 27 Aspects of the Maglev design (1 GW machine conceptualized and Maglev machine with guide vanes)

non-site-specific (i.e., at such high altitudes high-quality wind potential is a priori given, thus eliminating the need to seek for

‘good’ sites) According to its manufacturers, initial targets of MARS include mini-grid applications (e.g., small island regions), rapid deployment in areas where electricity is urgently required (taking advantage of its mobility), and other off-grid applications, with a 100 kW MARS being the first commercial size expected

• Maglev: The specific vertical-axis concept [47] uses full-permanent magnets to almost eliminate friction through levitation of the blades above their base (Figure 27) According to its designers, the concept is able to provide enormous turbines in the order of

1 GW, requiring an area of almost 100 acres, while at the same time the machine is able to start operating at wind speeds of only 1.5 m s−1 At the moment, some first efforts to provide the first industrial product have been recorded in China, where developer Zhongke Hengyuan Energy Technology has invested a total of US$55 million so as to build a massive Maglev turbine of 1 GW

• Kite Gen: The Kite Gen design [48–50] is based on the configuration of a carousel of tethered kites (Figure 28) centrally operated

by a control system with two distinct operational phases More precisely, when each of the kites pulls (i.e., when energy can be generated by the system), the control unit, which is attached on a vertical-axis rotor that is connected to an electrical drive, makes the drive act as a generator However, when the kites need to be dragged (when no energy generation is available), the electric drive acts as a motor, with the proportion of energy required for dragging supposed to be minor In use, according to rough estimations [50], the employment of 100 kites (total area of 500 m2), on a 1500 m radius carousel at an average wind speed of

12 m s−1, may even achieve a power output in the order of 1 GW

In addition to the medium- and large-scale innovative designs targeted at the production of novel machines with power output from hundreds of kW to hundreds of MW, small-scale wind turbine concepts are also developed, with the rebirth of the specific market encouraging the development of several patents Two examples of such concepts that have managed to go commercially are presented here, that is, aeroturbines and Windpods

• Aeroturbines: The turbines [51] comprise small-scale vertical-axis machines destined to satisfy urban applications Among their unique features are the modular/stackable cage that allows almost any kind of installation (horizontally, vertically, or diagonally) and the helical rotor (Figure 29), with available models reaching up to 2.5 kW of rated power

• Windpods: These are also vertical-axis micro-turbines [52], basing their operation on three different sections, each with blade positions offset 60° from the next one (Figure 29) Commercial modules reach 500 W, with the design’s main advantages being its modularity and building applicability

Figure 28 Carousel and small-scale prototype configurations for the Kite Gen concept

Figure 29 Aspect of the aeroturbine and the Windpod designs

Figure 30 Diffuser-augmented wind turbines (DAWTs): From the early Vortec design to the recent FloDesign and WindTamer machines

Finally, one of the most popular concepts that drew much attention during the past was the so-called diffuser-augmented wind turbine [53, 54] Such wind turbines mainly evolved during the mid-1990s under Vortec, although the concept was much older; nevertheless, the specific type never managed to establish itself in the wind energy market Such machines had a diffuser placed upstream of the rotor (Figure 30) in order to augment power output through achieving wind speed increase without increasing hub height, and claimed augmentation that was, however, never validated The additional weight put upon the machine along with increased costs in order to provide sufficient support condemned the specific solution quite early on, although there are still some investing efforts, aiming at the rebirth of such wind turbines [55, 56]

2.21.8 Environmental Impact Reduction

The considerable increase of both installed wind power capacity and contemporary wind turbine size, and the need to operate wind farms in areas that are close to available electricity grids and are determined by sufficient infrastructure, have raised issues of social acceptance, as a result of impacts caused to the environment and local societies by the operation of wind farms In this regard, although

it is a common true that any type of impact caused by the operation of a wind farm is by far less important and more restricted (normally within a relatively limited area of some square kilometers in the vicinity of the wind farm location) than that caused by the majority of power stations (nuclear, thermal power, large hydropower, etc.), much attention has been given during the recent years to limit (if not to eliminate) the environmental impacts of wind energy [57] What must be noted, however, is that with the current levels

of social acceptability [58, 59] and appreciation of social benefits deriving from the operation of wind farms, installation of wind farms may well keep up with the up-to-now progress, allowing for integration of wind energy in all areas of the globe

Among the most important environmental impacts caused by the operation of a single or more wind turbines is noise production (especially aerodynamic noise since mechanical noise has already been much limited on the basis of past efforts) [60] As a result, emphasis is currently given on both the advanced design of blades in order to reduce noise produced and the optimum siting of the machines in an area of given characteristics (see Section 2.21.16) Special attention has also been given during recent years to the operational mode of the machines (e.g., reduction of the rotor rotational speed and variation of the angle of attack) in order to avoid annoyance Finally, a significant part of the current research efforts has been concentrated on the noise propagation through and upon water [61], as a result of the growth met in offshore applications

Moreover, one of the most important issues determining levels of social acceptability of wind turbines is also their aesthetic adaptation Taking into account the extreme size of contemporary wind turbines (with the blade tip height above the ground even approaching 200 m), it becomes apparent that adaptation concerns have become critical For this purpose, there are various calculation and photorealistic models currently developed [62], aiming at the minimization of the visual impact

In addition to the above, one of the issues that has long since been considered as a negative attribute of wind energy is the impact

of wind turbines on birds For this purpose, there are serious attempts carried out nowadays so as to interpret behavior of birds in a

Global distribution of cumulative offshore wind power capacity at the end of 2010 in MW

given area and also record the paths of migratory populations [63], using suitable systems that are able to monitor and record the courses followed by birds and thus eliminate the already reduced fatalities [64]

As far as the rest of the wind energy environmental impacts are concerned, it must be noted that land occupation has considerably improved over the course of time, with the use of higher and larger-scale machines both offering greater power output per square area and allowing for land activities to occur even next to the wind turbine foundations Besides, the gradual shift toward offshore applications has also much contributed to the amelioration of the land use impact Local issues such as shadow flicker and interference with telecommunication signals can be thought of as resolved, especially if practices and regulations already available are properly applied

Finally, special emphasis has been given during the past 20 years to the issue of communication with local societies as well as

to the development of integrated strategies that may assess impacts and better comprehend the factors that configure the behavior

of local inhabitants [65, 66] One should also consider the involvement of various types of scientists, originating from different subject areas such as communication, sociology, psychology, biology, and strategic planning Interaction of all these different experts then focuses on comprehending the opinion of the local societies, as well as determines the benefits accruing from the operation of wind energy in relation to the avoided costs deriving from the operation of alternative, usually conventional power sources In addition, considerable effort should also be spent in the development of strategies that will allow the approach of local inhabitants and help them to both overcome their worries and become active participants in the development of new wind energy applications [67]

What should always be taken into account is that during every stage of developing a new wind park, all parties involved should acknowledge and appreciate the fact that wind energy comprises a sustainable energy solution that has the ability to sufficiently support the energy needs of contemporary societies

2.21.9 Offshore Wind Parks

The future of wind energy is gradually heading seawards, with the following decade expected to be determined by the establishment

of offshore wind farms as the next generation of wind power Although the first offshore wind park was actually built in 1991, there were only seven additional farms installed up until 2001, resulting in a total of almost 100 MW capacity, which did not live up to the expectations of an impressive start Nevertheless, following this infancy period, the offshore concept managed to develop rapidly during the next decade, that is, from 2001 to 2011, leading to a total of almost 2.4 GW installed capacity worldwide [68] Distribution of cumulative installed capacity on the basis of the most recent official data may be obtained from Figure 31, where the leading role of the United Kingdom and Denmark may be noted, together holding approximately 72% of the overall capacity Furthermore, the number of in-operation offshore wind farms corresponding to the above-mentioned overall capacity is 42

however, in the offshore field during recent years, one should not disregard the fact that, at the moment, offshore wind farms represent only a very small percentage of the global wind power capacity, in the order of 1–1.5%

Although not in its infancy period, offshore wind power may be still thought of as an immature technology [70], and exploration

of prospects and technological trends [71] is of primary importance for determining its ability to compete with conventional onshore wind farms The evaluation of available offshore wind energy potential and mapping of the most suitable regions is the first step toward the promotion of the offshore concept Relative to this, based on the results of the European Environment Agency report

on the evaluation of the European offshore wind energy potential [72], the available as well as the restricted (i.e., when considering restricted areas such as Natura 2000) and the economically feasible wind energy potential are depicted in Figure 33, for a time horizon of up to 2030 As one may see, the available offshore potential is quite high, reaching almost 30 000 TWh yr−1, although

Figure 31 Distribution of global offshore wind power capacity in market-leading countries (end of 2010)

Onshore 2020 Onshore 2030 Offshore 2020 Offshore 2030 Onshore 2020 Onshore 2030 Offshore 2020 Offshore 2030 Onshore 2020 Onshore 2030 Offshore 2020 Offshore 2030

Figure 32 Main characteristics of in-operation offshore wind farms (end of 2010)

Figure 33 Evaluation of the European offshore wind energy potential

what is estimated as economically feasible is eventually much less, that is, 3400 TWhe, covering, in rough numbers, around 80% of that time’s EU electricity demand (i.e., much less modest than the European Wind Energy Association (EWEA) target, aiming at approximately 600 TWhe of production by 2030) [73]

Similar results have also been published for the United States, where, although no offshore wind parks are still in operation, remarkable offshore wind power penetration could be stimulated in the years to come More precisely, according to the figures provided by certain studies [68], there is an offshore wind power potential – in an area of less than 50 nautical miles from shore, which is also determined by mean annual wind speeds that are greater than 7 m s−1 – that well exceeds 4 TW or is almost 4 times the installed electrical capacity of the country (Figure 34)

Considering the above, new installations that are either in the stage of permission, or already approved, and under construction may be obtained from Figure 35, where the concentration of offshore wind farms in the European territory in the years to come

Estimated offshore wind energy potential of the United States (< 50 nautical miles from shore and > 7 m s–1 areas)

Italy Japan Maldives The Netherl

25411

Country

Figure 34 Evaluation of the US offshore wind energy potential

Figure 35 Under development/pending permission vs in-operation offshore wind farms

becomes evident In fact, there is a remarkable integration of offshore wind farms to be expected in Germany (> 25 GW) and the United Kingdom (> 6 GW), followed by the Netherlands (∼4 GW), Sweden (> 3.4 GW), and Italy (> 2.5 GW), while, despite its outstanding offshore wind energy potential, the US investors have only announced a total of 2 GW As a result, it seems that the target set by the EWEA, pushing for a total of 40 GW by 2020 and 150 GW by 2030 for the European region (which along with onshore wind park developments aspire to lead to the remarkable installed capacity of 400 GW by 2030), could actually be rather realistic [74] (Figure 36)

Apart from the excellent opportunities in terms of available wind energy potential and the ambitious targets set at least at the European level, offshore wind energy is faced with many technological challenges such as the configuration of the most appropriate support structures for the safer and most cost-effective installation and operation of new offshore wind machines At this point, it should be noted that according to current trends, contemporary offshore wind machines suggest very large-scale wind turbines (in the order of 5 MW; see Table 2) that advance installation and operation requirements to a whole new level Up to now, support structures have been based on the concepts of monopole and gravity base, as well as on jackets and tripods In order for deeper sea migration to be accomplished, however, investing in floating concepts is thought to be imperative (Figure 37)

Deep waters, large energy potential

Subject to wave loading and fatigue, expensive

Weight and cost, stability, little experience

Cost-effective, combination of proven methods,

less noise, industrialization is possible

Distance from shore (m)

Figure 36 European targets concerning onshore and offshore wind energy installations

Turbine

manufacturer

Turbine model

Rated (MW)

GE 3.6 SWT2.3 V90 SWT3.6

5 M M5000 SL3000 BARD

GE 4.0/110

2.0 2.0 3.6 2.3 3.0 3.6 5.0 5.0 3.0 5.0 4.1

45 m water depth demonstration; Commercial at Alpha Ventus, Thornton Bank

Onshore 2005; Commercial at Alpha Ventus, 2009 First Chinese offshore project; 102 MW installed Installations at BARD Offshore 1 project; began in March 2010 Commercial sales announced; no prototype experience

Figure 37 Characteristics of existing offshore support structure designs

Variation of specific installation cost in relation to

the water depth

Figure 38 Operational depths and distances from shore for existing offshore applications

More specifically, up to now, both inadequate support structures and the need to operate as close as possible to the shore (so as

to minimize grid connection costs) have restricted installation area to the 20  20 rectangular (i.e., 20 km distance from the shore and 20 m depth; see Figure 38), which in order to be overcome will need both more advanced support structures and other cost components’ cost reduction (Figures 39 and 40) For this to happen, however, sufficient time needs to be given to the offshore technology to establish itself and thus proceed to a gradual smoothing of specific costs (Figure 41 [73]), which at the moment tend

to increase, exactly due to the fact that new farms need to go both further and deeper

Nevertheless, apart from the support structure advancements, the concept of offshore wind energy encompasses several other R&D directions that also need to be mentioned, with the main components being wind turbines, grid infrastructure, installation and assembly, and operation and maintenance

First of all, the operation of wind turbines at sea poses a completely different design than that of onshore machines, with additional requirements to consider As a result, there are considerable efforts carried out in issues such as developing larger machines (even

10 MW) and higher tip speeds along with improved electrical equipment that will ameliorate grid connection problems

Furthermore, electricity infrastructure may need to both alter and advance, due to the requirements that will be posed by the large-scale deployment of offshore parks High-voltage direct current (HVDC) transmission, which will probably progress alongside the offshore concept, and modification/strengthening as well as extension and interconnection [74] of existing electricity grids so as to absorb the available offshore wind energy yield should certainly be expected In addition, facilitating large-scale penetration of offshore wind power requires all the hidden needs deriving from the actual birth of a new specialized industry to support offshore wind power (e.g., advanced logistics) [75], while adjustment to the unique operational features of these wind farms assumes advancements in the monitoring field as well [76], in both procedures and equipment (e.g., helicopters, platforms, optimization of maintenance plans, etc.)

Figure 39 Specific cost of offshore wind parks for different water depths

Variation of specific installation cost in relation to

the distance from the shore

3000

Others Grid connection Installation Foundation Turbine

Figure 40 Specific cost of offshore wind parks for different distances from the shore

Figure 41 Future targets concerning EU offshore wind park installations and energy production in relation to the expected specific investment cost

In conclusion, offshore wind energy comprises a rather promising technology that is expected to succeed the gradually and, after all, inevitably stagnating onshore concept, in order for both immediate and future ambitious targets of the wind energy industry to

be accomplished For this to happen, however, efforts in the R&D field need to be constant and equally intense in all directions involved in the promotion of offshore wind power

2.21.10 Vertical-Axis Wind Turbines

Although the vertical-axis design was actually the first concept of wind power exploitation, establishment of the various types emerging over time never managed to support a solid market structure that would allow large-scale deployment of similar machines However, recent progress in the field of small wind turbines [77] provides excellent grounds for the revival of the almost abandoned vertical-axis machines, while of great interest are recent efforts seeking to put the vertical-axis concept on the map of multi-MW machines

There are two main types of vertical-axis machines that should be considered, that is, the Savonius type [78] and the Darrieus type [79] The Savonius machine (Figure 42), comprising a wind turbine of two half drums driven by drag forces, was invented by S.J Savonius in 1929 Such machines normally operate at a maximum power coefficient of 20% and are thus not suitable for

Figure 42 Examples of the original and the spiral-shaped Savonius type

high-power applications, although they can prove rather effective in water pumping and other low-power applications More modern Savonius designs suggest the introduction of spiral-shaped blades, which produce a torque performance that is more favorable during the whole rotation cycle [80], and are also determined by less vibrations to consider

The Darrieus machine, comprising a wind turbine that is driven by lift forces and that may achieve a maximum power coefficient

in the order of 40%, is based on the rotation of two or more aerofoil-shaped blades attached to a vertical axis It was invented in

1931 by George Jeans Mary Darrieus and comes with two main design concepts, that is, the eggbeater or curved-bladed machine

blades (introduction of variable pitch allows for the management of the low starting torque inherent characteristic) Actually, it was because of the fact that the original Darrieus concept suffered from severe drawbacks such as strong vibrations and relatively high noise levels that led to the development of straightened blades later on, with considerable efforts carried out in the United Kingdom during the period between 1970 and 1980 [81] In parallel, considerable research efforts were also undertaken in the United States and Canada that eventually resulted in the development of the 4.2 MW Eole C machine installed in Canada, while one should also underline the research work of Sandia National Laboratories, further promoting the vertical-axis concept at this time [82]

It was also during this time that the simpler H-rotor type (Figure 43) established itself against more complex straight-blade designs that required feathering mechanisms (variable-geometry machines) to control overspeeding of the rotor, while in the following years, passive stall regulation was designated as the most appropriate solution for power control of such machines [78] Meanwhile, experimental efforts to evaluate the performance of the H-rotor type led to very interesting results [83–87], designating the ability

of such machines to compete not only with the original Darrieus concepts but also with horizontal-axis machines (Table 3)

Figure 43 Examples of the original Darrieus (eggbeater) and the H-rotor machines

Table 3 Experimental results of different studies on H-rotor vertical-axis wind turbines

Power coefficient Cpmax Tip speed ratio λ (at Cpmax)

Rated power (kW) Country Year 0.40

0.38 0.39 0.38 0.43

3.8 4.3 5.5 4.4

3

100 -

2–4

14

1

Great Britain USA USA Japan USA

As certain groups argue during the time being, it is likely that horizontal-axis machines will peak in the next few years, largely due

to blade limitations and their effects on the overall machine structure As a result, there may be actual room for vertical-axis machines to proceed to utility-scale machines of 10 MW and beyond, with the first prototypes of H-rotor machines already being underway [90] Besides, past experience has demonstrated the ability of the concept to reach the multi-MW level long ago (e.g., the Canadian Eole C of 4.2 MW in the 1980s already mentioned; see Figure 44)

Finally, of great interest are the recent research efforts in the field of offshore developments (Figure 45), where developers of UK concepts such as Aerogenerator X (10 MW) and Nova (9 MW) claim that the specific models are capable of producing the same power output as horizontal-axis machines of double to triple their size [91]

In summary, although not as evident, there are signs at the moment that the vertical-axis concept may find its place in the broad wind energy market by actually covering the two extreme edges, that is, small-scale machines destined to serve built environment applications and large-scale multi-MW machines exceeding the expected capacity limits of horizontal-axis machines For this to occur,

Figure 44 From the small-scale building-integrated turbines to the large-scale multi-MW machines (Eole-C 4.2 MW at Quebec)

Figure 45 Multi-MW vertical-axis offshore concepts (Aerogenerator X and Nova)

Wind home system Off-grid applications

On-grid applications

Wind hybrid system

Wind diesel system Building integrated

Single wind turbine

Mini wind farm

however, considerable research efforts are required in order to integrate and adapt some of the experience gained during this 30-year period in the field of horizontal-axis machines to the field of vertical-axis wind turbines and also develop existing concepts even further

2.21.11 Small Wind Turbines

Small wind turbines were for a long time period considered as the best example of wind potential exploitation all around the planet, used in several areas for water pumping and electricity generation and in many other applications However, ever since emphasis has been given to medium- and large-scale applications in order to take advantage of the so-called economies of scale, this has much restricted the evolution of small-size machines

Due to the rebirth of distributed generation patterns as well as to the fact that during recent years integration of renewable energy sources (RES) in urban environments has actually become a reality, a growing interest has been recently recorded in the field of small-scale wind turbines Both on-grid and off-grid applications (Figure 46), which include building integration, mini wind farms, and single turbine installations for the first and wind-battery plus wind-based hybrid systems for the second, have established a niche market that aspires to go even further Common applications include installation on sailboats and remote houses, farms, water pumping, desalination systems, distributed generation, street lighting, building-integrated machines, and so on

In contrast to large-scale wind turbines, small wind turbines are much simpler machines and are usually divided into three main categories in terms of power scaling, that is, micro wind, small wind, and small–medium wind, corresponding to the power scales of

0–1 or 1.5 kW, 1 or 1.5–10 kW, and 10–100 kW, respectively, although different classification may also encompass the class of pico wind (< 1 kW), micro wind (1–7 kW), mini wind (7–50 kW), and small wind (50–100 kW), with the respective application range given

[92], among which the majority are three-bladed machines featuring a generator – included in the hub – and a tail (Figure 47), although in the category of micro wind one may also encounter several vertical-axis models, mostly integrated in building structures

According to the latest official data, the number of active manufacturers in the field nowadays is found to exceed 250, with the majority (almost 40%) of them found in the United States (Figure 48), where one may also meet the biggest national market More specifically, the sector of small wind turbines in the United States (Figure 49) has during a decade’s time managed to put in operation a total of more than 55 000 machines yielding a total of more than 70 MW capacity (2009 data) [93] In addition, the progress seen in the UK market is analogous (Figure 49), where, according to the latest estimates [94], the number of operating

Figure 46 Range of applications for small scale wind turbines

Figure 47 Examples of applications for small horizontal- and vertical-axis wind machines

USA Japan

Cana

ChinGermanyThe Netherlands

Spai

n SwedenSouth Africa

Figure 48 Distribution of small wind turbine manufacturers around the globe

Recent increase of small wind turbines’ installed capacity

in the United Kingdom and the United States (2005–09)

Figure 49 Evolution of small wind turbines’ installed capacity in the United Kingdom and the United States

machines is expected to exceed 20 000 in 2011, with the total installed capacity reaching 43 MW at the end of 2011 and anticipated

to skyrocket to 1.3 GW by 2020

Contribution shares of different power scale machines in these two countries may be obtained from Figure 50, where large-scale deployment of machines up to 10 kW summing up to MWs of installed capacity reflects the considerable market integration of micro and small wind turbines Furthermore, based on the results of Figure 51, one may also obtain – as an example – the ratio between off-grid and on-grid applications as well as the ratio between horizontal- and vertical-axis wind turbines employed in the United Kingdom Note that the gradual increase of interest toward on-grid applications encountered in the United Kingdom

number of off-grid machines in 2009 was triple that of on-grid machines Finally, what is interesting to note is that although they are still found under the shadow of horizontal-axis machines, vertical-axis wind turbines have gradually taken their place in the market

Economies of scale, however, in the case of small wind turbines, are still thought to comprise a major obstacle to the diffusion of small wind turbines, with the current specific costs being at least double (or even 6 times) that of large-scale machines (Figure 52) Besides, the results of past studies based on European experience [95] are also analogous, where the determination of the specific cost Pr (€ kW−1) of small wind turbines may be provided with the use of the following semi-empirical equation, with No being the rated power of the machine: