London, UK [Renewable Energy World Magazine] The erection of an innovative medium-speed 4.5 MW wind turbine prototype in Spain’s Saragossa region made 2008 an important year for Gamesa. In that year, the vertically integrated company ranked third among the world’s largest wind turbine suppliers, with manufacturing facilities in Spain, the US and Asia. Previously, the company had relied on its sub-megawatt class Gamesa G5X-850 kW and Gamesa G8X-2.0 MW series, both based upon Vestas technology designs but further developed and optimised in-house (see panel below). Gamesa’s global manufacturing capabilities include rotor blades, root joints, blade moulds, gearboxes, generators, converters and towers, as well as wind turbine assembly.
The main Gamesa G10X-4.5 MW design feature, is its segmented composite Gamesa-designed INNOBLADE rotor blades. In addition, the 128 metre rotor diameter represents a wind industry record, while the 250 tonne Top Head Mass (THM) can be highly regarded in its performance class. Gamesa’s G10X-4.5 MW project is a major multi-year product development undertaking, and the company’s first genuinely in-house developed wind turbine. The overall product development process involved more than 150 Gamesa engineers and other experts located in its Spanish and Danish R&D centres, working in close co-operation with external specialists and research organisations.
The company’s largest main volume series, the 2 MW Gamesa G8X-2.0 MW, has been commercially available since 2002 and features a conventional state-of-the-art fast-speed drive system. This arrangement includes a multi-stage gearbox and a doubly–fed generator. The series offers rotor diameters of 80–90 metres.
Compared to the Gamesa G8X-2.0 MW series, the new Gamesa G10X-4.5 MW represents a huge leap forwards in terms of rated capacity and rotor size. The rotor swept area has even grown by a factor two set against the currently largest G90-2.0 MW (rotor diameter 90 metres), which translates into a huge increase in terms of power generating potential per installation. Key development goals for the G10X-4.5 MW were maximum drive system reliability and being able to employ transportation and erection equipment logistics similar to the smaller Gamesa G8X-2.0 MW series.
Medium-speed Drive System Emerges From R&D
As the final outcome of an extensive multi-year drivetrain study and development process incorporating various alternative solutions, Gamesa’s reliability goal resulted in a medium-speed drive system. During the process many options were analysed, ranging from direct drive (no gearbox), to medium-speed and conventional high-speed geared solutions (see panel on page 66). Besides striving for maximised reliability, Gamesa formulated additional demands, including optimised systems flexibility and stability.
The project aimed to set new standards with regard to Life Cycle Costs (LCC) performance, explains Gamesa chief technology officer José Antonio Malumbres.* ‘Major direct drive system limitations, in our view, include complex and costly transport and erection logistics due to the annular generator size and weight. By choosing instead a medium-speed drive system we were able to overcome size and weight issues linked to direct drive. Simultaneously, a medium-speed drive system eliminates the trouble-prone high-speed gear stage, which is an integral part of three-stage gearboxes applied in conventional geared wind turbines. It was important that by choosing to eliminate the high-speed gear stage as a reliability enhancing measure it proved unnecessary to compromise on overall systems flexibility.’
The Gamesa G10X-4.5MW CompacTrain drivetrain comprises a main shaft with two main bearings and a two-stage planetary-type gearbox with 1:37 speed-up ratio, built as a single semi-integrated module. Like some of its main competitors (GE and Vestas), Gamesa made a switch away from a doubly-fed generator to a permanent magnet (PM)-type synchronous generator. Malumbres explains, ‘PM generators offer multiple advantages, include greater compactness, slip-ring elimination, superior partial-load efficiency and easier compliance with stringent future grid codes. Being a full converter technology it also offers greater project developer application flexibility at combined 50 Hz and 60 Hz wind markets.’
The full power converter system consists of modular units located in the rear of the nacelle and is subdivided into two levels. Major focus points were maximised reuse of components already applied in other Gamesa models, the preferred application of standard off-the-shelf components and solutions, and ease of serviceability, for example the mass of individually removable components is limited to 25 kg. A medium-voltage transformer is also fitted in the nacelle rear, but behind the converter, while a liquid-cooling system for converter, generator and gearbox is positioned above the converter.
Cyclic Blade Pitching Cuts THM
‘A main contributing factor to the favourable THM (nacelle + rotor) is the combination of an advanced MultiSmart turbine control system and the application of load-reducing cyclic rotor blade pitch technology. The latter continuously monitors blade root loads and adjusts the blade angle accordingly during each rotor revolution’, Malumbres continues. ‘As our highest priority was to commence soonest with controller testing and optimising, we decided to initially fit the prototype with a smaller provisional 107 metre rotor based upon conventional single piece blades.’ The segmented blades to be fitted on the prototype consist of carbon fibre and glass fibre reinforced epoxy composite inner and outer sections connected by a single bolted joint. ‘Eliminating any risk of a potential stiffness interruption occurring in the joining surface cross-section represented a major challenge for our rotor blade designers and required extensive research efforts’, he says.
Malumbres disagrees with some competitors that transporting single blades with lengths of 52 metres and more (for instance the latest generation 3 MW+ turbines) by road can still be accomplished without facing real constraints. He names Mediterranean markets as an example where transporting such blades would represent a real challenge.
With Gamesa’s segmented blades the maximum individual blade section length is in contrast less than 35 metres. Gamesa G10X nacelle erection is similar to the G8X-2.0 MW erection process and again, depending on hub height, (only) requires a standard 800–1000 tonne crane. Malumbres says, ‘Due to the modular Gamesa G10X concept with easily removable drivetrain and other main components, a relatively inexpensive crane can be employed to hoist an empty nacelle on top of a matching tower. After job completion this crane can then be moved to the next turbine erection location, while our new Gamesa FlexiFit nacelle-mounted crane system takes over and installs all main components.’
The FlexiFit crane is a ground-assembled self-mounting system able to either hoist-in or exchange even a complete drive system. Malumbres believes it will help to substantially lower G10X-4.5MW erection costs and increase operational availability. A second prototype G10X-4.5MW is envisaged in 2010 near to the first unit.
The next two prototypes are planned for the second quarter of 2011, perhaps 60 Hz versions, followed by series delivery to the US and Canada, Spain, Italy, Germany, France and China, all seen as key markets. Gamesa is also considering entering the offshore market, where it sees a relatively low THM as a key product specification. However, the envisaged offshore turbine will not represent the G10X-4.5 MW in its present form and its design will be evaluated further, says Malumbres.
- G10X-4.5MW chief engineer Rafael Hernà¡ndez and marketing manager Juan Diego Dàaz also participated in this interview and supplied some of the attributed quotes.
Company Profile: Gamesa
Gamesa’s history as a wind turbine manufacturer began in 1994 when Grupo Auxiliar Metalurgico SA (Gamesa’s parent company) established a joint venture with Vestas of Denmark. The joint venture, named Gamesa Eólica, consisted of Gamesa’s project development arm Gamesa Energàa with a 51% share, Vestas and the regional government of Navarra’s own industrial holding company Sodena. The joint venture with Vestas gave Gamesa ‘exclusive rights to manufacture, assemble and sell Vestas technology in Spain.’ The technology transfer arrangement proved to be highly successful and by 1997 Gamesa Eólica controlled some 70% of the Spanish wind turbine market.
In November 2001 Gamesa Eólica reached a deal with Vestas to buy out its 40% share of the company. Simultaneously it maintained the intellectual property rights to continue to utilize and build upon Vestas’ technology up until the 2 MW V80-2.0 MW in both the Spanish and world market. The deal between the two companies did not include the lightweight Vestas V90-3.0 MW.
Medium Speed a New Trend?
There are three distinct approaches to wind turbine drive systems. Nearly all operate with pitch-controlled variable speed. A majority of wind turbines on the market are fitted with a conventional fast speed geared drive system. This typically comprises a multi-stage step-up gearbox and either a fast rotating four-pole or six-pole generator. The nominal speed of a four-pole generator (most common) lies in the 1500 rpm range against a typical 1000 rpm for a six-pole equivalent.
Both figures relate to origins in fixed-speed installations in which the drive system is directly connected to the grid at 50 Hz or 60 Hz. With variable speed systems, by contrast, the generator is grid-connected with the aid of a frequency converter and generator frequency varies with generator speed, allowing greater drive system configuration freedom. When rotor sizes increase, rotor speed needs to go down in order to curb blade tip speed to limit aerodynamic noise. As a consequence, the total gearbox step-up ratio increases with rotor size, potentially raising overall gearbox complexity and increasing internal mechanical losses.
One option to limit the total gear ratio required for multi-megawatt class turbines is to apply a six-pole or even eight-pole generator. However, switching to a generator with more poles can mean an increase in the mass of the component. Investment costs per MW associated with more generator poles may work out higher than for a four pole type.
A second drive system alternative is direct drive, or ‘gearless’. In this arrangement a gearbox is absent and the turbine rotor directly drives an annular generator. Rotor speed and generator speed are identical. Since 1992, Enercon of Germany has been a direct drive pioneer and market leader, with over 15,500 systems between 30 kW–7.5 MW in operation worldwide. More recently several new ambitious entrants as well as major established suppliers have also made moves into the direct drive turbine segment, which is expected to gain importance offshore and on.
During the 1990s German engineering consultancy aerodyn Energiesysteme patented a 5 MW hybrid drive solution that fits between high-speed geared and direct drive systems. This concept, aimed at offshore applications, was named ‘Multibrid’ (Multi-megawatt hybrid). The commercial 5 MW Multibrid M5000 model features a fully integrated drive system comprising a single main bearing, a single-stage planetary type gearbox with 1:9.92 step-up ratio and a 28-pole medium-speed PMG. All three main components are integrated into a compact single cast load-transmitting structure. Areva is the main shareholder of the renamed Areva Multibrid, with WinWinD of Finland a second Multibrid technology licensee.
Multibrid represents a specific type of fully-integrated medium speed turbine technology, but Gamesa follows a different design principle (see main article). Multiple wind industry sources suggest several other wind turbine suppliers/developers are actively developing medium-speed geared wind turbine models, a design philosophy some regard as a major new wind technology trend.