Dr John Coultate/ R&D and Consultancy Dept. Leader/ Romax Technology Ltd.
Dr Ashley Crowther/VP Engineering/Romax Technology Inc., www.romax.com
The wind industry is in an exciting stage of development. Turbines are getting bigger and the whole supply chain is working together to reduce the cost of energy. As the next generation of turbines launch, 6 to 8 MW machines will become more common and many of the reliability issues that plagued earlier multi-megawatt machines will be alleviated. The drivetrain described here is critical to the success of these next generation machines.
The Romax Butterfly drivetrain platform is an innovative medium-speed design scalable from 3 to 10 MW that provides an unprecedented level of supply-chain flexibility and options for integration into a nacelle.
A drivetrain is at the core of a wind turbine and encompasses everything from the rotor shaft to generator. Historically, drivetrain components have been critical to wind turbine reliability. Detailed cost analysis shows that they are crucial to achieving low Cost of Energy (CoE). The recent platform has been designed with CoE as a primary design driver, taking a holistic approach to the turbine rather than a traditional isolated view of subcomponent design. The following considerations add flexibility to the design.
A significant challenge faced by turbine manufacturers when selecting drivetrain components is the supply chain and availability of alternative suppliers. Various solutions exist for off-the-shelf gearbox and drivetrain designs, but in practically all other instances, these would restrict the turbine manufacturer to a single supplier and come with potentially restrictive IP conditions.
More recent drivetrain thinking is to offer complete flexibility in the supply chain. This means gearbox components can be manufactured under license anywhere in the world, thereby removing the headache of one-location manufacturing in a global business. Similarly, the generator can be sourced independently from any number of manufacturers or preferred suppliers. These may include permanent magnet generators, doubly-fed induction generators, or even axial-flux machines. For the Butterfly platform, Romax controls the overall specification of the generator including details of the generator interface, and by collaborating with generator manufacturing partners provides a global supply chain already in place. This significantly reduces the time-to-market for a prototype or mass production drivetrain.
Not all nacelle concepts are equal, so flexibility in a drivetrain design is essential. The Butterfly platform, for example, offers options for configurations ranging from two rotor-shaft bearings to a single ‘moment’ bearing. Consequently, the drivetrain easily integrates into any nacelle with a variety of mainframe designs.
At the forefront of any turbine-design project is the Cost of Energy or CoE, and rightly so. CoE was therefore the primary driver behind selecting a medium speed, geared drivetrain concept for the platform. The Cost of Energy table shows a relative comparison of CoE for four alternative drivetrain technologies, including direct drive, medium-speed drivetrains with single-stage and two-stage gearboxes, and a conventional high-speed drivetrain. This analysis was based on accurate capital expenditure (CAPEX) models for the drivetrain components combined with a detailed operation expenditure (OPEX) cost model including operation and maintenance (O&M) costs. Results indicate that the lowest CoE comes from a medium-speed geared drivetrain incorporating a two-stage gearbox. Studies by other manufacturers have come to similar conclusions, most notably demonstrated by Vestas’ selection of a medium-speed drivetrain for their V164 turbine.
To achieve a low CoE, it is not just the CAPEX that is important, the operational expenditure is also a significant part of the lifetime turbine cost. The pie chart shows the CoE breakdown between CAPEX and OPEX for a generic 500 MW wind farm, based on cost data for a 6 MW turbine. OPEX constitutes just over one third of the CoE in this example, but the drivetrain CAPEX is only 4% of the total CoE. There are many cases where increasing the CAPEX cost of a drivetrain (or other equipment in the turbine) can yield a much greater reduction in OPEX over the life of the turbine, hence the focus on reducing CoE.
Diving deeper into the cost equations identifies a few key sensitivities. Component providers are often under pressure to reduce prices when developing a new turbine, but this can be counter-productive when it impacts the reliability of the product. In fact, reducing the cost of the drivetrain by 1% would only yield a 0.04% reduction in CoE. However, if more time and effort is invested up front to develop a reliable drivetrain that delivers low CoE – the overriding principle behind the Butterfly platform – then its possible to have a big impact on operating expendatures. A reliable design will deliver higher availability, and comprehensive studies indicate that availability is the most sensitive parameter in the CoE equation. For instance, the cost analysis mentioned shows that a:
- 1% increase in availability, reduces CoE by 1.04%
- 1% increase in annual energy production (AEP) reduces CoE by 0.96%
- 1% reduction in turbine CAPEX reduces CoE by 0.30%
- 1% increase in operating life reduces CoE by 0.25%
- 1% reduction in O&M cost reduces CoE by 0.15%, and
- 1% reduction in drivetrain CAPEX reduces CoE by 0.04%
The wind industry is going through a challenging period that will judge it on whether it can deliver a cost of energy comparable with other power sources. The recent Butterfly drivetrain platform has been designed with cost of energy as its overriding principle and, through an innovative approach to supply-chain management, is positioned to meet the requirements of the next generation of large wind turbines. WPE
Filed Under: Featured, Gearboxes, Generators, News, Turbines