The evolution of modern wind turbines is a story of engineering and scientific skill, coupled with a strong entrepreneurial spirit. In the last 20 years, turbines have increased in size by a factor of 100 (from 25 kW to 2500 kW and beyond), the cost of energy has reduced by a factor of more than five and the industry has moved from an idealistic fringe activity to an acknowledged component of the power generation industry.
At the same time, the engineering base and computational tools have developed to match machine size and volume. This is a remarkable story, but it is far from finished: many technical challenges remain and even more spectacular achievements will follow.
The concept of a wind driven rotor is ancient, and electric motors have been widely disseminated, both domestically and commercially, in the latter half of the 20th century. Making a wind turbine may seem simple, but it is a big challenge to produce a wind turbine that:
* Meets specifications (frequency, voltage, harmonic content) for standard electricity generation with each unit operating as an unattended power station;
* Copes with the variability of the wind (mean wind speeds on exploitable sites range from 5 m/s to 11 m/s with severe turbulence in the earth’s boundary layer and extreme gusts up to 70 m/s); and
* Competes economically with other energy sources.
The traditional ‘Dutch’ windmill had proliferated to the extent of about 100,000 machines throughout Europe at their peak usage in the late 19th century. These machines preceded electricity supply and were indeed wind-powered mills used for grinding grain. Use of the wind for water pumping also became common. The windmills were always attended, sometimes inhabited and largely manually controlled. They were also characterised by direct use of the mechanical energy generated on the spot. They were integrated within the community, designed for frequent replacement of certain components and efficiency was of little importance.
In contrast, the function of a modern power-generating wind turbine is to generate high quality, network frequency electricity. Each wind turbine must function as an automatically controlled independent ‘mini-power station’. It is unthinkable for a modern wind turbine to be permanently attended, and uneconomic for it to be frequently maintained. The development of the microprocessor has played a crucial role in enabling cost effective wind technology. A modern wind turbine is required to work unattended, with low maintenance, continuously for in excess of 20 years.
Although most of the largest wind turbines now employ active pitch control, in the recent history of wind turbine technology, the use of aerodynamic stall to limit power has been a unique feature of the technology. Most aerodynamic devices (aeroplanes and gas turbines, for example) avoid stall. Stall, from a functional standpoint, is the breakdown of the normally powerful lifting force when the angle of flow over an aerofoil (such as a wing section) becomes too steep. This is a potentially fatal event for a flying machine, whereas wind turbines can make purposeful use of stall as a means of limiting power and loads in high wind speeds.
The design requirements of stall regulation led to new aerofoil developments and also the use of devices, such as stall strips, vortex generators, fences, Gurney flaps, for fine tuning rotor blade performance. Even when not used to regulate power, stall still very much influences aerofoil selection for wind turbines. In an aircraft, a large margin in stall angle of attack, compared to the optimum cruising angle is very desirable. On a wind turbine, this may be undesirable and lead to higher extreme loads.
The power train components of a wind turbine are subject to highly irregular loading input from turbulent wind conditions, and the number of fatigue cycles experienced by the major structural components can be orders of magnitude greater than for other rotating machines. Consider that a modern wind turbine operates for about 13 years in a design life of 20 years and is almost always unattended. A motor vehicle, by comparison, is manned, frequently maintained and has a typical operational life of about 160,000 km, equivalent to 4 months of continuous operation.
Thus in the use rather than avoidance of stall and in the severity of the fatigue environment, wind technology has a unique technical identity and unique R&D demands.
The Development of Commercial Technology
An early attempt at large-scale commercial generation of power from the wind was the 53m diameter, 1.25 MW Smith Putnam wind turbine, erected at Grandpa’s Knob in Vermont, USA in 1939. This design brought together some of the finest scientists and engineers of the time (aerodynamic design by von Karman, dynamic analysis by den Hartog). The wind turbine operated successfully for longer than some megawatt machines of the 1980s.
It was a landmark in technological development and provided valuable information about quality input to design, machine dynamics, fatigue, siting and sensitivity. However, preceding the oil crisis of the 1970s, there was no economic incentive to pursue the technology further in the immediate post war years.
The next milestone in wind turbine development was the Gedser wind turbine. With assistance from Marshall Plan post war funding, a 200 kW, 24m diameter wind turbine was installed during 1956 and 1957 in the town of Gedser in the southeast of Denmark. This machine operated from 1958 to 1967 with about a 20 per cent capacity factor.
In the early 1960s, Professor Ulrich Hütter developed high tip speed designs, which had a significant influence on wind turbine research in Germany and the US.
1970 TO 1990
In the early 1980s, many issues of rotor blade technology were investigated. Steel rotors were tried but rejected as too heavy, aluminium as too uncertain in the context of fatigue endurance, and the wood-epoxy system developed by Gougeon Brothers in the US was employed in a number of both small and large wind turbines. The blade manufacturing industry has, however, been dominated by fibreglass polyester construction, which evolved from LM Glasfiber, a boat building company, and became thoroughly consolidated in Denmark in the 1980s.
By 1980 in the US, a combination of state and federal, energy and investment tax credits had stimulated a rapidly expanding market for wind in California. Over the 1980-1995 period, about 1700 MW of wind capacity was installed, more than half after 1985 when the tax credits had reduced to about 15 per cent.
Tax credits attracted an indiscriminate overpopulation of various areas of California (San Gorgonio, Tehachapi and Altamont Pass) with wind turbines, many of which were ill-designed and functioned poorly. However, the tax credits created a major export market for European (especially Danish) wind turbine manufacturers, who had relatively cost effective, tried and tested hardware available. The technically successful operation of the later, better designed wind turbines in California did much to establish the foundation on which the modern wind industry has been built. The former, poor quality turbines conversely created a poor image for the industry, which has taken a long time to shake off.
1990 TO PRESENT
The growth of wind energy in California was not sustained, but there was striking development in European markets, with an installation rate in Germany of around 200 MW per annum in the early 1990s. From a technological standpoint, the significant outcome was the development of new German manufacturers and of some new concepts. The introduction of innovative direct drive generator technology by German manufacturer Enercon is noteworthy. Subsequently, a huge expansion of the Spanish market occurred, including wind farm development, new designs and new manufacturers.
Over this period there have been gradual, yet significant, new technological developments in direct drive power trains, in variable speed electrical and control systems, in alternative blade materials and in other areas. However, the most striking trend in recent years has been the development of ever larger wind turbines, leading to the current commercial generation of multi-megawatt onshore and offshore machines.
Significant consolidation of design has taken place since the 1980s, although new types of electrical generators have also introduced further diversification.
Vertical axis wind turbine (VAWT) designs were considered, with expected advantages of omni-directionality and having gears and generating equipment at the tower base. However, they are inherently less efficient (because of the variation in aerodynamic torque with the wide range in angle of attack over a rotation of the rotor). In addition, it was not found to be feasible to have the gearbox of large vertical axis turbines at ground level because of the weight and cost of the transmission shaft.
The vertical axis design also involves a lot of structure per unit of capacity, including cross arms in the H-type design. The Darreius design is more efficient structurally. The blade shape is a so-called ‘troposkein curve’ and is loaded only in tension, not in bending, by the forces caused as the rotor spins. However, it is evident that much of the blade surface is close to the axis. Blade sections close to the axis rotate more slowly and this results in reduced aerodynamic efficiency. The classic ‘egg-beater’ shaped Darrieus rotors also suffered from a number of serious technical problems, such as metal fatigue related failures of the curved rotor blades. These disadvantages have caused the vertical axis design route to disappear from the mainstream commercial market. FlowWind, previously the main commercial supplier of vertical axis turbines, stopped supplying such machines over a decade ago.
Although there is not yet any substantial market penetration, there has recently been a remarkable resurgence of innovative VAWT designs in the category of small systems for diverse applications, especially on roof tops of buildings, and also some innovative designs have been made for large-scale offshore applications.
NUMBER OF BLADES
Small-scale, multi-bladed turbines are still in use for water pumping. They are of relatively low aerodynamic efficiency but, with the large blade area, can provide a high starting torque (turning force). This enables the rotor to turn in very light winds and suits a water pumping duty.
Most modern wind turbines have three blades, although in the 1980s and early 1990s some attempt was made to market one and two-bladed wind turbine designs.
The single-bladed design is the most structurally efficient for the rotor blade, as it has the greatest blade section dimensions with all the installed blade surface area in a single beam. It is normal to shut down (park) wind turbines in very high winds, in order to protect them from damage. This is because they would generally experience much higher blade and tower loads if they continued to operate. The one-bladed design allows unique parking strategies – with the single blade acting as wind vane upwind or downwind behind the tower – which may minimise storm loading impact. However, there are a number of disadvantages. With a counterweight to balance the rotor statically, there is reduced aerodynamic efficiency and complex dynamics requiring a blade hinge to relieve loads. The designs of Riva Calzoni, MAN, Messerschmidt and others were of too high a tip speed to be acceptable in the modern European market from an acoustic point of view.
The two-bladed rotor design is technically on a par with the established three-bladed design. In order to obtain a potentially simpler and more efficient rotor structure with more options for rotor and nacelle erection, it is necessary either to accept higher cyclic loading or to introduce a teeter hinge, which is often complex. The teeter hinge allows the two blades of the rotor to move as a single beam through typically ±7° in an out-of-plane rotation. Allowing this small motion can much relieve loads in the wind turbine system, although some critical loads return when the teeter motion reaches its end limits. The two-bladed rotor is a little less efficient aerodynamically than a three-bladed rotor.
In general, there are small benefits of rotors having increasing number of blades. This relates to minimising losses that take place at the blade tips. These losses are, in aggregate, less for a large number of narrow blade tips than for a few wide ones.
In rotor design, an operating speed or operating speed range is normally selected first, taking into account issues such as acoustic noise emission. With the speed chosen, it then follows that there is an optimum total blade area for maximum rotor efficiency. The number of blades is, in principle, open but more blades imply more slender blades for the fixed (optimum) total blade area. This summarises the broad principles affecting blade numbers.
Note also that it is a complete misconception to think that doubling the number of blades would double the power of a rotor. Rather, it would reduce power if the rotor was well designed in the first instance.
It is hard to compare the two- and three-bladed designs on the basis of cost-benefit analysis. It is generally incorrect to suppose that, in two-bladed rotor design, the cost of one of three blades has been saved, as two blades of a two-bladed rotor do not equate with two blades of a three-bladed rotor. Two-bladed rotors generally run at much higher tip speed than three-bladed rotors, so most historical designs would have noise problems. There is, however, no fundamental reason for the higher tip speed and this should be discounted in an objective technical comparison of the design merits of two versus three blades.
The one-bladed rotor is perhaps more problematic technically, whilst the two-bladed rotor is basically acceptable technically. The decisive factor in eliminating the one-blade rotor design from the commercial market, and in almost eliminating two-bladed design, has been visual impact. The apparently unsteady passage of the blade or blades through a cycle of rotation has often been found to be objectionable.
PITCH VERSUS STALL
This section discusses the two principal means of limiting rotor power in high operational wind speeds – stall regulation and pitch regulation.
Stall regulated machines require speed regulation and a suitable torque speed characteristic intrinsic in the aerodynamic design of the rotor. As wind speed increases and the rotor speed is held constant, flow angles over the blade sections steepen. The blades become increasingly stalled and this limits power to acceptable levels, without any additional active control. In stall control, an essentially constant speed is achieved through the connection of the electric generator to the grid. In this respect, the grid behaves like a large flywheel, holding the speed of the turbine nearly constant irrespective of changes in wind speed.
Stall control is a subtle process, both aerodynamically and electrically. In summary a stall-regulated wind turbine will run at approximately constant speed in high wind without producing excessive power and yet achieve this without any change to the rotor geometry.
The main alternative to such a stall regulated operation is pitch regulation. This involves turning the wind turbine blades about their long axis (pitching the blades) to regulate the power extracted by the rotor. In contrast to stall regulation, pitch regulation requires changes of rotor geometry by pitching the blades. This involves an active control system, which senses blade position, measures output power and instructs appropriate changes of blade pitch.
The objective of pitch regulation is similar to stall regulation, namely to regulate output power in high operational wind speeds. A further option, active stall regulation, uses full span pitching blades. However, they are pitched into stall in the opposite direction to the usual fine pitching where the aerofoil sections are rotated leading edge into wind direction. This concept, like the conventional fine pitch solution, uses the pitch system as a primary safety system, but also exploits stall regulation characteristics to have much reduced pitch activity for power limiting.
VARIABLE SPEED VERSUS FIXED SPEED
Initially, most wind turbines operated at fixed speed when producing power. In a start-up sequence the rotor may be parked (held stopped), and on release of the brakes would be accelerated by the wind until the required fixed speed was reached. At this point, a connection to the electricity grid would be made and then the grid (through the generator) would hold the speed constant. When the wind speed increased beyond the level at which rated power was generated, power would be regulated in either of the ways previously described, by stall or by pitching the blades.
Subsequently, variable speed operation was introduced. This allowed the rotor and wind speed to be matched, and the rotor could thereby maintain the best flow geometry for maximum efficiency. The rotor could be connected to the grid at low speeds in very light winds and would speed up in proportion to wind speed. As rated power was approached, and certainly after rated power was being produced, the rotor would revert to nearly constant speed operation, with the blades being pitched as necessary to regulate power. The important differences between variable speed operation, as employed in modern large wind turbines and the older conventional fixed speed operation are:
* Variable speed in operation below rated power can enable increased energy capture; and
* Variable speed capability above rated power (even over quite a small speed range) can substantially relieve loads, ease pitch system duty and much reduce output power variability
The design issues of pitch versus stall and degree of rotor speed variation are evidently connected.
In the 1980s, the classic Danish, three-bladed, fixed speed, stall-regulated design was predominant. Aerodynamicists outside the wind industry (such as for helicopters and gas turbine) were shocked by the idea of using stall. Yet, because of the progressive way in which stall occurs over the wind turbine rotor, it proved to be a thoroughly viable way of operating a wind turbine. It is one of the unique aspects of wind technology.
Active pitch control is the term used to describe the control system in which the blades pitch along their axis like a propeller blade. Superficially, this approach seemed to offer better control than stall regulation, but it emerged through experience that pitch control of a fixed speed wind turbine at operational wind speeds that are a lot higher than the rated wind speed (minimum steady wind speed at which the turbine can produce its rated output power) could be quite problematic. The reasons are complex, but in turbulent (constantly changing) wind conditions it is demanding to keep adjusting pitch to the most appropriate angle and under high loads, and excessive power variations can result whenever the control system is ‘caught out’ with the blades in the wrong position.
In view of such difficulties, which were most acute in high operational wind speeds (of say 15-25 m/s), pitch control in conjunction with a rigidly fixed speed became regarded as a ‘challenging’ combination. Vestas initially solved this challenge by introducing OptiSlip (which allows a certain degree of variable speed using pitch control in power limiting operation, which allows about 10 speed variation using a high slip induction generator). Suzlon presently use a similar technology, Flexslip, with a maximum slip of 17 per cent. Speed variation helps to regulate power and reduces demand for rapid pitch action.
Variable speed has some attractions, but also brings cost and reliability concerns. It was seen as a way of the future, with expected cost reduction and performance improvements in variable speed drive technology. To some extent this has been realised. However, there was never a clear case for variable speed on economic grounds, with small energy gains being offset by extra costs and also additional losses in the variable speed drive. The current drive towards variable speed in new large wind turbines relates to greater operational flexibility and concerns about power quality of traditional stall regulated wind turbines. Two-speed systems emerged during the 1980s and 1990s as a compromise, improving the energy capture and noise emission characteristics of stall regulated wind turbines. The stall-regulated design remains viable, but variable speed technology offers better output power quality to the grid and this is now driving the design route of the largest machines. Some experiments are underway with the combination of variable speed and stall regulation, although variable speed combines naturally with pitch regulation. For reasons related to the methods of power control, an electrical variable speed system allows pitch control to be effective and not overactive.
Another significant impetus to the application of pitch control, and specifically pitch control with independent pitching of each blade, is the acceptance by certification authorities that this allows the rotor to be considered as having two independent braking systems acting on the low speed shaft. Hence, only a parking brake is required for the overall safety of the machine.
Pitch control entered wind turbine technology primarily as a means of power regulation, which avoided stall when stall, from the experience of industries outside wind technology, was seen as problematic if not disastrous. However, in combination with variable speed and advanced control strategies, stall offers unique capabilities to limit loads and fatigue in the wind turbine system and is almost universally employed in new large wind turbine designs. The load-limiting capability of the pitch system improves the power to weight ratio of the wind turbine system and compensates effectively for the additional cost and reliability issues involved with pitch systems.
Design Drivers for Modern Technology
The main design drivers for current wind technology are:
* Low wind and high wind sites;
* Grid compatibility;
* Acoustic performance;
* Aerodynamic performance;
* Visual impact; and
Although only some 1.5 per cent of the world’s total installed capacity is currently offshore, the latest developments in wind technology have been much influenced by the offshore market. This means that, in the new millennium, the technology development focus has been mainly on the most effective ways to make very large turbines. Specific considerations are:
* Low mass nacelle arrangements;
* Large rotor technology and advanced composite engineering; and
* Design for offshore foundations, erection and maintenance.
A recent trend, however, is the return of development interest to new production lines for the size ranges most relevant to the land-based market, from 800 kW up to about 3 MW. Of the other main drivers, larger rotor diameters (in relation to rated output power) have been iintroduced in order to enhance exploitation of low wind speed sites. Reinforced structures, relatively short towers and smaller rotor diameters in relation to rated power are employed on extremely high wind speed sites.
Grid compatibility issues are inhibiting further development of large wind turbines employing stall regulation. Acoustic performance regulates tip speed for land-based applications and causes careful attention to mechanical and aerodynamic engineering details. Only small improvements in aerodynamic performance are now possible (relative to theoretical limits), but maximising performance without aggravating loads continues to drive aerodynamic design developments. Visual impact constrains design options that may fundamentally be technically viable, for example, two-bladed rotors.
Architecture of a Modern Wind Turbine
Many developments and improvements have taken place since the commercialisation of wind technology in the early 1980s, but the basic architecture of the mainstream design has changed very little. Most wind turbines have upwind rotors and are actively yawed to preserve alignment with the wind direction.
The three-bladed rotor proliferates and typically has a separate front bearing, with low speed shaft connected to a gearbox that provides an output speed suitable for the most popular four-pole (or two -pole) generators. Commonly, with the largest wind turbines, the blade pitch will be varied continuously under active control to regulate power in higher operational wind speeds.
Support structures are most commonly tubular steel towers tapering in some way, both in metal wall thickness and in diameter from tower base to tower top. Concrete towers, concrete bases with steel upper sections and lattice towers, are also used but are much less prevalent. Tower height is rather site specific and turbines are commonly available with three or more tower height options.
The drive train shows the rotor attached to a main shaft driving the generator through the gearbox. Within this essentially conventional architecture of multi-stage gearbox and high speed generator, there are many significant variations in structural support, in rotor bearing systems and in general layout. For example, a distinctive layout has been developed by Ecotècnia (Alstom), which separates the functions of rotor support and torque transmission to the gearbox and generator. This offers a comfortable environment for the gearbox, resulting in predictable loading and damping of transients, due to its intrinsic flexibility. Among the more innovative of a large variety of bearing arrangements is the large single front bearing arrangement adopted by Vestas in the V90 3 MW design. This contributes to a very compact and lightweight nacelle system.
Whilst rotor technology is set amongst the leading commercial designs and the upwind three-bladed rotor prevails generally, more unconventional trends in nacelle architecture are appearing. The direct drive systems of Enercon are long established, and many direct drive designs based on permanent magnet generator (PMG) technology have appeared in recent years. A number of hybrid systems, such as Multibrid, which employ one or two gearing stages, and multi-pole generators have also appeared. These developments are discussed in Technology Trends. It is far from clear which of the configurations is the optimum. The effort to minimise capital costs and maximise reliability continues – the ultimate goal is to minimise the cost of electricity generated from the wind.
Growth of Wind Turbine Size
Modern wind technology is available for a range of sites: low and high wind speeds and desert and arctic climates can be accomodated. European wind farms operate with high availability (97 per cent) and are generally well integrated with the environment and accepted by the public.
At the start of the millennium, an ever increasing (in fact mathematically exponential) growth in turbine size with time had been documented by manufacturers, such as Siemens Wind Power (earlier Bonus AS), and was a general industry trend. In the past three or four years, although interest remains in yet larger turbines for the offshore market, there has been a levelling of turbine size at the centre of the main, land-based market and a focus on increased volume supply in the 1.5 to 3 MW range.
The past exponential growth of turbine size was driven by a number of factors. The early small sizes, around 20-60 kW, were very clearly not optimum for system economics. Small wind turbines remain much more expensive per kW installed than large ones, especially if the prime function is to produce grid quality electricity. This is partly because towers need to be higher in proportion to diameter to clear obstacles to wind flow and escape the worst conditions of turbulence and wind shear near the surface of the earth. But it is primarily because controls, electrical connection to grid and maintenance are a much higher proportion of the capital value of the system.
Also, utilities have been used to power in much larger unit capacities than the small wind turbines, or even wind farm systems of the 1980s, could provide. When wind turbines of a few hundreds of kW became available, these were more cost-effective than the earlier smaller units, being at a size where the worst economic problems of very small turbines were avoided. However, all the systems that larger wind turbines would require were also needed and the larger size was the most cost effective. It also became apparent that better land utilisation could often be realised with larger wind turbine units, and larger unit sizes were also generally favourable for maintenance cost per kW installed. All these factors, the psychology of ‘bigger is better’ as a competitive element in manufacturers’ marketing, and a focus of public research funding programmes on developing larger turbines contributed to the growth of unit size through the 1990s.
Land-based supply is now dominated by turbines in the 1.5 and 2 MW range. However, a recent resurgence in the market for turbines around 800 kW size is interesting and it remains unclear, for land-based projects, what objectively is the most cost-effective size of wind turbine. The key factor in maintaining design development into the multi-megawatt range has been the development of an offshore market. For offshore applications, optimum overall economics, even at higher cost per kW in the units themselves, requires larger turbine units to limit the proportionally higher costs of infrastructure (foundations, electricity collection and sub-sea transmission) and lower the number of units to access and maintain per kW of installed capacity.
The future challenges in extending the conventional three-bladed concept to size ranges above 5 MW are considerable, and are probably as much economic as engineering issues. Repower, exploiting reserve capacity in design margins, has up-rated its 5 MW wind turbine to 6 MW and BARD Engineering has announced a similar up-rating of its 5 MW design to 6 MW and later 7 MW. Clipper Windpower has announced a 7.5 MW prototype to be purchased by the UK Crown Estates (an unprecedented type of investment for them), with no specific timeline for development, but suggestions of production by around 2012. The interest in yet larger wind turbines, especially for offshore markets, is reflected in the UPWIND project. This major project of the EU 6 Framework Programme addresses a wide range of wind energy issues, including up-scaling, by evaluating the technical and economic issues in developing unit wind turbines of 10 and 20 MW capacity. The Magenn airborne wind energy concept is one of a number of speculative new concepts for large capacity wind energy systems that is reviewed later in the chapter (Future Innovations).
The ‘top ten’ wind turbine manufacturers
Vestas has long been the world’s leading supplier of wind turbines. Key volume products are the V80 and V90 series. Vestas technology is generally particularly lightweight. Blades made using high strength composites in the form of prepregs, and innovation in nacelle systems design, has contributed to this characteristic. According to industry sources, Vestas is developing a new offshore wind turbine model.
GE is now focusing increasingly on their 2.5 MW 2.5XL series, which entered series production in the summer of 2008. This is seen as its next generation of turbines to succeed to the proven 1.5 MW series and be produced at high volumes. It is interesting to note the change in design to a permanent magnet generator with full converter retains a high speed generator with a multi-stage planetary gear system. Doubly-fed induction generator (DFIG) technology has been challenged recently by more stringent network requirements, the ‘fault ride-through’ requirement in particular, and adaptations have been made to respond to these issues. The DFIG solution is undoubtedly cheaper in capital cost than systems with full converters. However, the implications for part-load efficiency can make it inferior in cost of energy to efficient systems with PMG and full converter, and the value of capitalised losses should not be underestimated. This may in part be the motivation for GE’s change in design route and adoption of a PMG generator in their latest turbine series. Note also that a synchronous PMG can be applied without design hardware modifications in both 50Hz and 60Hz network regions. This greatly increases flexibility for international developers operating in multiple wind markets.
Their latest design, the Gamesa G10X, 4.5 MW, 128 m diameter prototype, is presently being developed. Key features of this design include:
* a two-bearing arrangement, integrated with a two-stage planetary type gearbox;
* a low mass sectional blade (inserts bonded into carbon pultruded profiles are bolted on site);
* a hybrid tower with concrete base section and tubular steel upper section; and
* an attached FlexFitTM crane system that reduces the need for large externalcranes.
Gamesa is also using state-of-the art control and converter technology in this design. The main shaft is integrated into a compact gearbox, limited to two stages and providing a ratio of 37:1. It would appear that their system will have a synchronous generator with fully rated power converter. The nacelle of the G10X is shown in Figure 3.11.
Enercon has dominated supply of direct drive turbines. They have favoured wound-rotor generator technology in their designs, although permanent magnet technology is now the choice of most manufacturers developing new direct drive designs. A direct drive generator, with a wound field rotor is more complex, requiring excitation power to be passed to the rotor, but it benefits from additional controllability.
Enercon has perhaps the aerodynamic design that gives most consideration to flow around the hub area, with their blade profile smoothly integrated with the hub cover surface in the fine pitch position. Their latest designs achieve a very high rotor aerodynamic efficiency which may be due both to the management of flow in the hub region and tip winglets (blade tip ends curved out of the rotor plane), which can inhibit tip loss effects. Enercon have quite diverse renewable energy interests, which include commercially available wind desalination and wind-diesel systems. In addition, they have involvement in hydro energy systems and Flettner rotors for ship propulsion.
SUZLON ENERGY AND REPOWER
Suzlon produce wind turbines in a range from 350 kW to 2.1 MW. It has developed its technology through acquisitions in the wind energy market and is targeting a major share in the US market.
Recent additions to the range include the S52 – 600 kW for low wind speed Indian sites and S82 , a 1.50 MW wind turbine. The S52 employs a hydraulic torque converter that can allow up to 16 per cent slip, thus providing some of the benefits of variable-speed operation.
In 2007, after a five-month takeover battle with the French state-owned nuclear company Areva, Suzlon took a controlling stake in REpower, with 87 per cent of the German wind company’s voting rights.
REpower, which had previously acquired the blade supplier Abeking & Rasmussen, continue to expand their manufacturing facilities in Germany and also rotor blade production in Portugal. The company cooperates with Abeking & Rasmussen ROTEC, manufacturing rotor blades at Bremerhaven in a joint venture, PowerBlades GmbH.
SIEMENS WIND POWER
Siemens (formerly Bonus) is among a few companies who is increasingly successful in the offshore wind energy market. Its 3.6 MW SWT turbines of 107 m diameter are now figuring prominently in offshore projects. Senior management in Siemens have indicated the end of a trend of exponential growth in turbine size (considering, year by year, the turbine design at the centre of their commercial supply). The stabilisation of turbine size has been a significant trend in the past three or four years and, although there is much discussion of larger machines and developments on the drawing board and a few prototypes, there is some evidence that, at least for land-based projects, turbine size is approaching a ceiling.
In a four year period, Acciona has become the seventh ranked world manufacturer in terms of MW supplied. The company presently has four factories, two located in Spain and one in both the US and China. In total, this amounts to a production capacity of 2,625 MW a year.
Acciona’s latest design is a new 3 MW wind turbine to be commercially available in 2009 and delivered into projects in 2010. The new turbine is designed for different wind classes (IEC Ia, IEC IIa and IEC IIIa). It will be supplied with a concrete tower of 100 or 120 m hub height and will have three rotor diameter options of 100, 109 and 116m, depending on the specific site characteristics. The rotor swept area for 116 m diameter of 10,568 m2, the largest in the market of any 3 MW wind turbine, will suit lower wind speed sites.
Electricity is generated at medium voltage (12 kV), aiming to reduce production losses and transformer costs. The main shaft is installed on a double frame to reduce loads on the gearbox and extend its working life. The AW-3000 operates in variable speed with independent blade pitch systems.
In North America, the AW-3000 will be manufactured at the company’s US-based plant, located in West Branch, Iowa. Both the AW-1500 and AW-3000 machines will be built concurrently. A present production capacity of 675 MW/year is planned to be increased to 850 MW/year.
Goldwind is a Chinese company in the wind industry providing technology manufactured under licences from European suppliers. Goldwind first licensed REpower’s 48kW to 750 kW turbine technology in 2002, and then acquired a licence in 2003 from Vensys Energiesysteme GmbH (Saarbrücken, Germany) for the Vensys 62 1.2 MW turbine. When Vensys developed a low wind speed version, with a larger 64 m diameter rotor that increased output to 1.5 MW, Goldwind also acquired the licence for this turbine and is currently working with Vensys to produce 2.0 MW and 2.5W turbines. Goldwind won a contract to supply 33 wind turbines (1.5 MW Vensys 77 systems) for the 2008 Olympic Games in Beijing. The company operates plants in Xinjiang, Guangdong, Zhejiangand HebeiProvincesand is building plants in Beijing and Inner Mongolia. In 2008, Goldwind signed a six-year contract with LM Glasfiber (Lunderskov, Denmark) to supply blades for Vensys 70 and 77 turbines and develop blades for Goldwind’s next generation of 2 MW and larger turbines at LM’s factory in Tianjin.
Nordex is developing new control techniques and has a condition monitoring system, which monitors component wear, also incorporating ice sensors and an automatic fire extinguishing system.
In March 2007, the AMSC (American Superconductor Corporation) signed a multi-million dollar contract with Sinovel Wind, under which 3 and 5 MW wind turbines would be developed. Sinovel is continuing to manufacture and deploy the 1.5 MW wind turbines it began producing in 2005. The 1.5 MW wind turbines also utilise core electrical components produced by AMSC.
Earlier in 2007, AMSC had acquired the Austrian company Windtec to open opportunities for them in the wind business. Windtec has an interesting history, originating as Floda, a company that developed ground-breaking variable speed wind turbines in the latter part of the 1980s. Based in Klagenfurt, Austria, Windtec now designs a variety of wind turbine systems and licenses the designs to third parties.
In June 2008, AMSC received a further $450 million order from Sinovel for core electrical components for 1.5 MW wind turbines. The contract calls for shipments to begin in January 2009 and increase in amount year by year until the contract’s completion in December 2011. According to AMSC, the core electrical components covered under this contract will be used to support more than 10 GW of wind power capacity, nearly double China’s total wind power installed base at the end of 2007.