The challenge of growth: Supply chain and wind turbine upscaling challenges

The global wind industry has grown rapidly in recent years, manufacturing capacity is increasing and the component supply chain is stretched to its limits. Eize de Vries reports on some of the challenges of this pace of growth, investigating some of the supply chain bottlenecks and revealing the importance of components suppliers teaming up with wind turbine suppliers as dedicated partners.

The global wind power market has entered into an exciting and historically unique accelerated growth path, a positive trend that is expected to continue in the years to come. As a consequence of this rapid market growth, especially in booming markets such as the US, Europe and China, wind turbine demand continues to tstrip the world’s cumulative supply capacity. Clearly this puts pressure on suppliers to construct new factories, and to and employ and train large numbers of additional staff. Transport, installation equipment (such as cranes) – the sector has growth challenges on all sides. For instance, a recent American Wind Energy Association (AWEA) supply chain workshop put a multiple focus on components supply, timely availability of construction equipment such as cranes, building materials such as sand and reinforcing steel, and transport logistics.

Each new wind market in turn creates its own specific demands for dedicated products and services, for which a wide variety of skills, along with well trained specialists, are required. Up until just four or five years ago there were only a handful of countries that could be considered significant wind markets. And there were only a couple of – mainly European – suppliers that dominated global wind turbine development and supply. That picture is now changing.

But within this framework, there are specific component shortage bottlenecks that keep on emerging as a negative side-effect of strong and sustained global wind turbine demand. Such bottlenecks are hard to pin down, being neither a transparent nor a static phenomenon. While some specific component shortage bottlenecks can indeed be resolved over time, other constraints have shown that they cannot. Sometimes it seems that the minute one weak link in the supply chain has been resolved, another immediately pops up somewhere else. It’s a fact that without the tight component supply that continues to put a brake on expanding global wind turbine output, today’s current unprecedented wind industry growth could have been even faster. Along with shortages of materials, such as steel, one of the major challenges has been scaling up manufacturing capacities to support a new generation of larger machines.

Focusing on the issues

In many European wind markets state-of-the-art 2 MW, 2.5 MW and 3 MW turbine models have taken over as the main volume product from the smaller 1.5 MW class, partly due to land restriction issues. The popular 2 MW class (here roughly defined at 2.0–2.4 MW) is now available with a range of rotor diameters from 71–95 metres. The 2 MW class itself is often referred to as the world’s first commodity-type turbine, an informal definition that covers a number of inter-related aspects. First, these are commercial products typically supplied in large volumes and available from a wide range of equipment suppliers. Secondly, the specific capacity class is likely to remain on the market for an extended period to come.

Another main wind turbine category that may soon qualify for commodity turbine status is the 2.5 MW class. A number of makes and types are now commercially available with rotor diameters up to 100 metres. Clipper, Fuhrländer, GE, and Nordex are some of the key manufacturers in this size segment.

Meanwhile, the 3 MW class still contains a rather limited number of suppliers, but the number of market entrants is growing steadily. The Vestas V90-3 MW with a rotor diameter of 90 metres is at the moment the most popular model in this class in terms of sales. A second 3 MW class model which has long been available is the WinWinD WWD-3, a pioneering Multibrid concept available with rotor diameters of either 90 metres or 100 metres. Recent newcomers in the 3 MW class include the 3 MW Alstom-Ecotécnia ECO 100, while prototypes announced in 2008 originate from Acciona (3.0 MW), REpower (3.3 MW) and ScanWind (3.5 MW). Enercon, meanwhile, is expected to erect 67 units of an E-82 model fitted with a newly-designed 3 MW generator in the next months in a Dutch project. A standard E-82 is 2 MW. Enercon is already operating several 3 MW prototypes and has done for an extended period of time, and the Eemshaven project is the first large commercial wind farm to be installed with this new direct drive turbine.

Each new power rating typically requires a largely original and dedicated supply chain that needs to be built largely from scratch before it arrives at the market. Furthermore, while these state-of-the-art wind turbine makes and types are all characterized by largely comparable main specifications, with similar power rating and rotor diameter ranges, components are rarely interchangeable.

Challenge of custom design

In 2006, independent rotor blade supplier LM Glasfiber of Denmark enjoyed a worldwide market share of over 27% on a MW basis, while it delivered rotor blades to nine out of the Top10 wind turbine suppliers from its geographically spread manufacturing facilities. Given its market dominance, LM determines the so-called blade root bolt circle and the number of bolts in each rotor blade for its capacity class, be it 1.5 MW, 2 MW, or 2.5 MW. These two main dimensions and the bolt diameter determine whether or not a rotor blade fits a given rotor hub.

A senior rotor blade design consultant explains that, as a result, independent rotor blade designers and/or smaller rotor blade manufacturers have no choice other than to comply with this industry semi-standard. Vertically integrated suppliers such as Enercon and Suzlon both manufacture all of their blade demand in-house and therefore have the freedom to determine their own rotor blade root design specifications. Enercon, for instance, applies a double ring of rotor blade bolts as standard, while the rest of the industry sticks to a single ring but with bigger diameter bolts. These two distinct rotor blade design approaches are clearly not interchangeable.

With regard to geared drive train systems, many suppliers in the 1.5 MW and 2 MW class apply a three-point gearbox suspension, mostly in a combination with a common doubly-fed induction generator. These drive systems comprise a main rotor bearing support in front and a main shaft that sticks directly into the gearbox input side. The gearbox features two so-called torque supports, one on each side. Gearboxes are usually not very easily interchangeable between wind turbine makes and types, especially because technical specifications like design loads, total gear ratio, and dedicated mechanical design features often differ.

In many of these state-of-the-art 1.5–2.5 MW class wind turbines a four-pole generator with a nominal speed of 1500 rpm is common. Generators of a given power rating are normally characterized by standardized dimensions for components such as the diameter of the generator shaft, generator mountings, and the vertical distance between the mounting and the centre shaft. Indeed, technically comparable products from different component suppliers can be made to fit into the same turbine type, as long as technical design choices like air-cooling or liquid cooling are also known. However, this is not the case when a given wind turbine type features specific design solutions and dimensions. For instance, the Vestas V90-3 MW features a special custom-developed gearbox with integrated main bearing that, as a unit, is flanged onto the cast main carrier.

Power ratings keep on heading upwards

As a general trend, average installed power ratings in most wind markets continue to creep upwards. A second somewhat surprising trend is the fact that the sub-megawatt class is enjoying a renaissance and is even attracting new market entrants, possibly in response to supply chain issues associated with larger machines.

Enercon is a good example of a Top 5 supplier that between 2004 and 2006 introduced three new sister turbines in the 800 kW–900 kW class, with three matching rotor sizes for specific wind climates. By the end of March, 2008, the cumulative operational total for these new models, the 900 kW E-44 plus 800 kW E-48 and E-53, stood at over 2100 units, with India and Germany two key markets, although for differing reasons. In India, wind speeds are generally modest, and imperfect transport logistics is a major issue. In Germany, the E-53 with the largest rotor, at 53 metres, is apparently well positioned for repowering projects and for other situations where physical conditions can make the installation of larger turbines problematic. Other high-potential newcomers in the sub-megawatt class include, among others, Conergy of Germany with its 900 kW PowerWind 56, and Unison of South Korea with its 750 kW U50-U57 series. An example of a third newcomer comes from Vergnet of France, which presented its innovative two-blade GEV HP 1 MW turbine at EWEC 2008 in Brussels recently. One of the novel features of this new concept is that the complete nacelle can be lowered to ground level as a damage precaution measure when the installation operates in hurricane prone areas.

Among the older designs of sub-megawatt class turbines, both Vestas and Gamesa still successfully market their 850 kW turbine models while Mitsubishi of Japan is a successful vendor of a ‘veteran’ 1 MW machine.

Components for sub-megawatt class turbines are usually less bulky and therefore lighter than their equivalents in larger machines. Sub-megawatt class equipment suppliers also claim that because their sourcing base substantially differs from that of larger machines, main components such as generators and gearboxes are generally easier to obtain.

However, in the case of a direct-drive turbine model, the sourcing base needs to be specialized for that specific wind technology. Since 1992, the direct-drive wind market segment has been dominated by Enercon, and the company among others manufactures its full generator demand, all power converters and power cabinets, and rotor blades in-house. New facilities have been added recently for processing cast iron components such as main carriers and rotor hubs, and the manufacture of aluminium nacelle covers. Additional manufacturing capacity has become operational for fabricating the huge welded steel inner section of the E-126 hybrid rotor blade. And, according to well informed industry sources, Enercon is planning its own foundry in the Aurich area.

In contrast, EWT of the Netherlands is an emerging direct-drive turbine manufacturer that chooses to outsource its 900 kW direct drive generators from channels outside the geared driven supply chain.

Bearing shortage

In recent years component shortages have occurred in a number of key supply chain areas including single main bearings, gearboxes, generators, main shafts, control cabinets, and complex castings such as hubs and mainframes. Larger bearings in particular, with outer ring diameters of about 200–250 mm and up are quoted as being in short supply at the moment. According to wind industry sources, the shortage situation is unlikely to be fully resolved within the next two or perhaps even three years.

Bearings can be found in gearboxes, generators, cooling systems, blade pitch and yaw systems, and rotor support systems among a number of major and heavy-duty wind turbine applications. However, bearings in a wide range of sizes and with specific design characteristics are applied in all kinds of rotating machinery and in multiple industries. The wind industry therefore has to compete with other booming sectors and applications such as agriculture, ship propulsion, rotating equipment for power stations, transportation, and mining machinery.

Gearbox failures and generator bearing problems continue to be primary areas for substantial down time in modern megawatt and multi-megawatt class gear driven wind turbines. One Dutch project developer recently estimated that about 50% of all geared wind turbines in the Netherlands suffer or have suffered from major gearbox problems. In addition, the developer says, a much higher percentage of all geared wind turbine generators have already been exchanged and/or have required one or more bearing retrofits. Wind technology drive train expert Jan van Egmond of Dutch engineering consultancy Quality in Wind confirms these dramatic figures and says they are in line with his own findings and experiences. One of the consequences of the current bearing shortages is that wind turbines in the field suffering from gearbox and/or generator failure can easily stand idle for months while awaiting components. Chief technical consultant for the Danish association of wind turbine owners Strange Skriver was recently quoted saying that 14–18 month delivery times for large bearings and gearboxes is the norm. There are no indications that these waiting periods will get shorter, but rather longer and Skriver therefore reportedly advises that manufacturers and large operators should stockpile spare gearboxes. Indeed, a Dutch Vestas turbine ‘users platform’ group established in 2007 is already stockpiling gearboxes for V66 and V80 turbine models as a preventative countermeasure. The group is also considering stockpiling full sets of gearbox bearings as a preventive countermeasure against long turbine outages during necessary gearbox repair and retrofit actions.

Major components

The largest bearing unit found in modern wind turbines is a single rotor bearing applied in the 5 MW Multibrid M5000 (first prototype in 2004) and the 5 MW BARD VM turbines (two prototypes in 2007). The Multibrid bearing features a 3200 mm outside diameter, a 2620 mm inner ring diameter, a width of 485 mm – or nearly half a metre – and a component mass of 7200 kg. In the latter trend-setting arrangement, the rotor hub is directly bolted to the main bearing, which is in turn attached to the integrated main carrier. This compact cast main housing itself also accommodates the single-stage gearbox and the generator. Another unique, and for wind turbine applications unusual, feature of the innovative Multibrid concept is that the shafts of the single-stage gearbox rotate in slide bearings. Slide bearings are widely used in the automotive industry, especially as crankshaft and camshaft bearing solutions. In common multi-stage wind turbine gearboxes, slide bearings are still rare and cylinder-roller, taper-roller and spherical-roller bearings dominate. The use of slide bearings in gearboxes according to Mutibrid among others offers the advantage of compactness, low mechanical noise, and relative insensitivity to vibrations and shock loads. But the application also places high demands on the structural stiffness of the gearbox housing and all rotating shafts.

During the M5000 design phase the large-diameter internal geared rings for the gearbox and bearing rings proved to be critical components. During bearing dimensioning key engineering challenges formed the maximum width regarding machining capacity and the maximum component diameter as determined by hardening limits.

Yet another major Multibrid challenge was the production of its 25–40 tonne heavy castings for the main housing and rotor hub. Besides their huge size and complexity, these castings are also characterized by a substantial variation in wall thicknesses. In order to prevent material shrinkage cracks during cooling, this process has to take place under highly controlled conditions over a period of up to six weeks. The stringent requirements of the cooling process means that in a single mould a foundry can only complete seven or eight unit per year. At the same time investment costs in these large complex moulds are by definition also very high. According to the aerodyn engineers responsible for the Mutibrid turbine design, the shear size of a 25–40-tonne casting is not the main challenge. The major issue is meeting the high quality demands regarding texture and material defects in combination with component size. Pioneering German ‘super’ class turbine designers and suppliers such as aerodyn, Enercon, and REpower all indicate that initially at least, only a few European foundries were capable of manufacturing their 4.5–5 MW casting jobs.

The 70-tonne main frame of the 5 MW Bard VM offshore turbine is the heaviest casting currently applied in the wind industry. The German press reports that in its latest vertically integrated supply chain capacity expansion, BARD Engineering plans a new foundry in the town of Leer. Under the trade name EMS Guss GmbH the new foundry will commence business activities in the spring of 2009 and employ 320 workers. In future the castings will be applied in BARD type turbines in which the power rating will be scaled up to 7 + x MW.

The component casting challenges are not a lone example of the engineering challenges associated with larger machines. In general, during a major product size scale up – and depending on complexity – the initial number of potential suppliers for main components is limited. But the number of qualified component suppliers tends to expand once a series product matures, volumes increase and all parties in the supply chain gain experience.

The distinct steps in scaling up from 500 kW–600 kW to the 1.5 MW class seen in 1995 and subsequently from 1.5 MW to 4.5 MW–5 MW from 2002–2004 are illustrative for all such projects. In 1992–1993 four European pioneers – Enercon, Vestas, and Nordtank and Tacke, which no longer exist – were each engaged in the development of a 1.5 MW class turbine and each faced many challenges. The power rating of their new flagship models for instance increased 2.5–3 fold compared to their previous largest 500 kW–600 kW volume model. Simultaneously, their latest proven wind technology know-how base was from 1992-1993 and validated only for the much smaller 450kW-600 kW class turbines. Some engineering implications of several major scaling up steps are summarized in Table 1.



The table indicates that rotor revolutions during each of the two scaling up steps roughly drop by half, while installed capacity during each of the steps increases threefold. Shifting from 0.5 MW initially to 4.5 MW therefore results in a torque-increase by a factor 36 in the example given. Increased rotor torque by definition means bigger loads and this translates in engineering terms into larger size bearings, wider gears, thicker shafts, larger gearbox housings with increased wall thicknesses and so on.

Partner co-operation

In a traditional wind turbine manufacturer and component supplier model, the former places a product order, to be delivered in time and by meeting all previously agreed terms and conditions. Both parties are individually responsible for carrying out their own part of the agreement, but there is inherently little incentive to share learning and other experiences with each other. In a common scenario, in this business model the turbine supplier chooses the component supplier on the basis of a lowest price criterion. The component supplier, in a worst-case scenario, tries to maximise profits by meeting only the minimum quality standards and other agreed terms and conditions.

However, when a wind turbine manufacturer selects a number of dedicated component suppliers and all agree to cooperate as partners, a superior business model with substantial added benefits to all participants potentially emerges. It can, for instance, be mutually agreed that component improvements or optimisations achieved by one supplier are passed on to all other partners. The turbine manufacturer in turn can agree to provide certain preferred-supplier guarantees to specific component suppliers. In a long-term partnering relationship the partners can even decide to bear new product development costs and other risks together.

Especially dedicated partnerships that aim at long-term cooperation have often proven to be an essential precondition and critical success factors during complex high-risk and costly wind turbine development projects. In the critical first development stage of such projects partners can assist each other by setting objectives and meeting major challenges during the design and prototype stages. In the prototype testing stage they can again team up during product and process optimization. When the partners finally engage in series production, they continue reaping benefits from learning by doing and sharing know-how and experience. Having gone through a complete product development cycle as dedicated partners the joint experience can thus serve as an excellent preparation for the next challenges to be tackled together.

Eize de Vries is Wind Technology Correspondent of Renewable Energy World Magazine



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