Project 2
This paper presents an in depth evaluation and comparison of three different drivetrain choices based on permanent‐magnet synchronous generator (PMSG) technology for 10‐MW offshore wind turbines. The life cycle approach is suggested to evaluate the performance of the different under consideration drivetrain topologies. Furthermore, the design of the drivetrain is studied through optimized designs for the generator and gearbox. The proposed drivetrain analytical optimization approach supported by numerical simulations shows that application of gearbox in 10‐MW offshore wind turbines can help to reduce weight, raw material cost, and size and simultaneously improve the efficiency. The possibility of resonance with the first torsional natural frequency of drivetrain for the different designed drivetrain systems, the influence of gear ratio, and the feasibility of the application for a spar floating platform are also discussed. This study gives evidence on how gearbox can mitigate the torque oscillation consequences on the other components and how the latter can influence the reliability of drivetrain.
1 INTRODUCTION
The capacity of offshore wind turbines and the distance from shores is rising rapidly, so that multigigawatt offshore wind farms based on multimegawatt floating turbines show potentials to be one of the dominant sources of power production in the future. The latter is due to availability of better wind resources, less turbulence, steadier winds, and less wind shear; easier transportation of larger turbines on the sea; technological developments in power electronic converters and direct current (DC) power transmission technologies; the establishments of required market infrastructures; and technological achievements in installation of turbines in deep waters, which lead to a considerable drop in the levelized cost of energy (LCOE) of offshore wind turbines. In spite of significant improvements, there are still no unanimous decision about the drivetrain system technology in offshore wind turbines.1, 2 Offshore wind reaches to 10‐MW turbines or even higher, but there are still different interests between manufacturers in the selection of drivetrain technology. One reason is that the drivetrain in wind originally comes from available experiences in other industries, which has been modified over time. The latter has then been upscaled for higher powers with some modifications to reduce the production costs and improve the dynamic response. Because the wind turbines are developed for a wide range of power in various onshore/offshore, fixed/floating, two‐/three‐bladed rotors, upwind/downwind, high/medium/low wind, fix/variable speed, stall/active yaw, and stall/active pitch applications, a customized design of the drivetrain for the cost reduction and performance improvement is inevitable. Therefore, there is a need for a special drivetrain design for each power class for different applications to have the lowest manufacturing, installation, and maintenance costs and simultaneously achieve the most reliable and efficient operation. The optimization of the drivetrain system also influences on the turbine design, because the size and weight of drivetrain is a main constraint in design of nacelle.
Even though the drivetrain system has an important share in manufacturing and maintenance of the turbine, and even if there are some researches conducted on the optimization of the components of the drivetrain, there is no analytical study about a system‐level optimization of the drivetrain for 10‐MW floating wind turbine applications as of the authors' knowledge. The proof for this claim is the different technology interests in offshore wind turbine industry. Because the drivetrain is a complex electromechanical system, the optimization of this system is a multidisciplinary task that calls for external system‐level and internal component‐level optimization problems. The other circumstance is that the optimizations need to be performed over the life cycle of the system, consisting of design, manufacturing and installation, and operation and maintenance (O&M). In this paper, different permanent magnet synchronous generator (PMSG)‐based drivetrain technologies, that is, direct‐drive permanent‐magnet synchronous generator (DDPMSG), medium‐speed permanent‐magnet synchronous generator (MSPMSG), and high‐speed permanent‐magnet synchronous generator (HSPMSG), are designed and compared. This work is a starting point for the optimization of drivetrain systems for large offshore wind turbines from design, manufacturing, and O&M perspectives. It will be shown that a thoughtful selection of technology can considerably reduce the drivetrain weight and cost, while improving the overall efficiency and dynamic response. The numerical results show that the drivetrain system based on MSPMSG could be the most promising choice for 10‐MW floating offshore wind turbines.
- There are some limitations such as availability of a technology, vendors, and raw materials that influence on companies' drivetrain selection. The latter will not be discussed in this research.
- The purpose of analytical design of the drivetrain components is to provide input data for the comparison study between different drivetrain technologies. Complementary component‐level detailed design steps such as finite element analysis for more detailed electromagnetic, structural, and thermal designs are not in the scope of this research.
- Reliability calculations for under consideration drivetrain systems are not in the scope of this paper, but how the failure modes are affected by the drivetrain technology is discussed.
- For each drivetrain technology, the conventional configuration is considered in this study. Some innovative wind turbine drivetrain systems, such as gearbox integrated main‐bearing (used eg in Areva Multibrid M5000 medium‐speed technology) and hub‐supported drivetrain (used eg in GE Haliade 150‐6MW direct‐drive technology) based on more compact and lightweight designs, are not the focus of this work.
- Drivetrain performance assessment using the life cycle approach is introduced.
- A new drivetrain optimization approach is proposed, which ensures an optimized overall cost and weight, improves the reliability and efficiency, and simultaneously assesses the feasibility of the design.
- A new analytical design approach is proposed for the optimized design of PMSG based on the optimization of the active material cost. The optimization problem is solved numerically, and the global optimizer is determined. The design is validated by ANSYS‐RMxprt software.
- An analytical design model of the gearbox is presented, and a new optimization approach is proposed to optimize the gearbox weight. KISSsoft software is used to validate the feasibility of the design.
- A comprehensive comparison between the design, raw material cost, weight, size, efficiency, and reliability of DDPMSG, MSPMSG, and HSPMSG drivetrain technologies is presented.
- The first torsional natural frequency of three under consideration drivetrain technologies is calculated, and the feasibility of application in floating offshore wind turbines is investigated.