Session: 01-11-03: Wind Energy

Paper Number: 110872

110872 - A Critical Evaluation of Large Scale Direct-Drive Wind Turbine Electrical Machines Manufacturing Techniques Considering Sustainability Aspects and Recyclability Issues 

Direct-drive generators are recognised for their low maintenance, comparative to conventional geared drivetrains - failing within the region of 40% less, due to their fewer working parts. However, consequent to their low-speed operation, considerably higher torques are required to generate the same power output and as such, these machines are substantially larger and heavier than their counterparts.

Significant electromagnetic forces desire to close the air-gap separating the rotor and stator, which is typically 1/1000^th of its diameter. A partial closing of this air-gap could result in catastrophic failure of the generator and thus rigorous stiffness requirements are imperative, further necessitating large direct-drive generator structural masses. The National Renewable Energy Laboratory’s 15MW Reference Turbine was released specifically to facilitate enabling-research on turbines of a magnitude which does not yet exist. This report details the structural optimisation of the “NREL” reference turbine rotor, through parameter and topology optimisation, with consideration of its mechanical, modal and its thermal performances. Optimised under the inherent electrical machine mechanical loads, two 15MW potential rotor structures were generated, the first through parameter optimisation and the second through both parameter and topology optimisation. Both rotors met structural and modal performance requirements, but with a large difference in added inactive mass.

An initial thermal analysis showed that the operating temperature of the rotor has a large proportional contribution to deformation through thermal expansion, breaching stress and deformation limits for the optimised device. Necessitating more in-depth thermal analysis, a Computational Fluid Dynamics study was then carried out, applying radiant heat from generator losses to the rotor and airflow from a cooling fan. The Computational Fluid Dynamics study was then linked as input to the final Finite Element structural analysis of the rotor. This study found that as mass increases, thermal stress becomes the dominant contributor to deformation, and in some cases, prohibitive under presumed operating conditions. This further emphasises the need for lightweight generator design and the requirement to design with heat in mind. It was also found that heatsinks can reduce internal operating temperatures and consequently deformation resulting from thermal expansion, whilst also contributing to the rotors structural stiffness. A life cycle analysis and a detailed cost estimation of the resulting structure have also been included.

 

Presenting Author: Pablo Jaen Sola Edinburgh Napier University

Presenting Author Biography: Dr Pablo Jaen Sola is an Assistant Professor in Mechanical Engineering in the School of Computing, Engineering and the Built Environment at Edinburgh Napier University (UK). He holds a PhD from the UK EPSRC Wind Energy Systems Doctoral Training Centre and his research is focused on the design and optimisation of powertrain mechanical components for renewable energy devices.

Authors:

Pablo Jaen Sola Edinburgh Napier University
Magnus Bichan Edinburgh Napier University
Andrew Jack Edinburgh Napier University