Optimized Generator Designs for the DTU 10-MW Offshore Wind Turbine using GeneratorSE

Abstract

Compared to land-based applications, offshore wind imposes challenges for the development of next generation wind turbine generator technology. Direct-drive generators are believed to offer high availability, efficiency, and reduced operation and maintenance requirements; however, previous research suggests difficulties in scaling to several megawatts or more in size. The resulting designs are excessively large and/or massive, which are major impediments to transportation logistics, especially for offshore applications. At the same time, geared wind turbines continue to sustain offshore market growth through relatively cheaper and lightweight generators. However, reliability issues associated with mechanical components in a geared system create significant operation and maintenance costs, and these costs make up a large portion of overall system costs offshore. Thus, directdrive turbines are likely to outnumber their gear-driven counterparts for this market, and there is a need to review the costs or opportunities of building machines with different types of generators and examining their competitiveness at the sizes necessary for the next generation of offshore wind turbines. In this paper, we use GeneratorSE, the National Renewable Energy Laboratory's newly developed systems engineering generator sizing tool to estimate mass, efficiency, and the costs of different generator technologies satisfying the electromagnetic, structural, and basic thermal design requirements for application in a very large-scale offshore wind turbine such as the Technical University of Denmark's (DTU) 10-MW reference wind turbine. For the DTU reference wind turbine, we use the previously mentioned criteria to optimize a direct-drive, radial flux, permanent-magnet synchronous generator; a direct-drive electrically excited synchronous generator; a medium-speed permanent-magnet generator; and a high-speed, doubly-fed induction generator. Preliminary analysis of leveled costs of energy indicate that for large turbines, the cost of permanent magnets and reliability issues associated with brushes in electrically excited machines are the biggest deterrents for building direct-drive systems. The advantage of medium-speed permanent-magnet machines over doubly-fed induction generators is evident, yet, variability in magnet prices and solutions to address reliability issues associated with gearing and brushes can change this outlook. This suggests the need to potentially pursue fundamentally new innovations in generator designs that help avoid high capital costs but still have significant reliability related to performance.

Method for holistic wind turbine drivetrain comparison exemplarily applied to geared and direct drive systems

“…susp. [12] Mid speed PMSG [23] DFIG [24] Susp. [25] DFIG [26] Low/mid speed PMSG and complexity by increasing the rotational generator input speed while having a small gearbox might have the potential to result in a lighter system compared to geared and direct drive concepts.…”

Evaluation of PMSG‐based drivetrain technologies for 10‐MW floating offshore wind turbines: Pros and cons in a life cycle perspective

“…Superconducting direct‐drive synchronous generators are still far from being commercialized regarding standardization and some open paths to explore regarding cost and reliability . An overview on the commercialized turbines (see Table ) and the reference turbines developed by research institutions and universities shows a unanimous interest in PMSG for high‐power offshore wind turbine drivetrain systems. Higher efficiency due to external excitation circuit removal, higher reliability and availability, and less maintenance costs due to the compactness and lightweight design and brushes elimination are the motivations .…”

“…An overview on the commercialized turbines (see Table ) and the reference turbines developed by research institutions and universities shows a unanimous interest in PMSG for high‐power offshore wind turbine drivetrain systems. Higher efficiency due to external excitation circuit removal, higher reliability and availability, and less maintenance costs due to the compactness and lightweight design and brushes elimination are the motivations . Different designs for permanent‐magnet generators are proposed in the literature.…”

Co-Design Optimization of Direct Drive PMSGs for Offshore Wind Turbines Based on Wind Speed Profile

“…Generally, the working cycle is not taken into account in the design process. In most cases, the generator is only designed for the rated power [3][4][5][6]. Such a method can lead to oversize the generator, particularly when it works in a variable thermal regime.…”