Comparison of direct-drive and geared generator concepts for wind turbines

The objective of this paper is to compare five different generator systems for wind turbines, namely the electrically-excited and the permanent-magnet excited direct-drive generator system, the permanent-magnet and the doubly-fed induction generator system with single-stage gearbox and the doubly-fed induction generator system with three-stage gearbox. The comparison is based on cost and annual energy yield for a given wind climate. The doubly-fed induction generator with single-stage gearbox seems the most attractive in terms of energy yield divided by cost. The doubly-fed induction generator with three-stage gearbox is a cheap solution using standard components. The permanent-magnet direct-drive generator has the highest energy yield, but although it is cheaper than the electrically-excited direct-drive generator, it is more expensive than the generator systems with gearbox.