Back to the programme printer.gif Print




Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Innovative concepts for drive train components' taking place on Thursday, 13 March 2014 at 11:15-12:45. The meet-the-authors will take place in the poster area.

Ainhoa Pujana Tecnalia Research & Innovation, Spain
Co-authors:
Ainhoa Pujana (1) F P José María Merino (1) Gustavo Sarmiento (1) Santiago Sanz (1) Iker Marino (1) José Luis Villate (1)
(1) Tecnalia Research & Innovation, Derio, Spain

Printer friendly version: printer.gif Print

Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Ainhoa Pujana received the M.Sc. degree in Industrial Engineering from Escuela Técnica Superior
de Ingenieria de Bilbao in 2003. She has been working in the area of electric machines since 2004,
five years in the company ANDIA GROUP ELECTRIC APPLICANCES, SAL, as technical director of small
rotating electric machines projects for special applications. In 2010 she joined to TECNALIA,
where she currently is a researcher in the Marine Energy Area, involved in activities related to
the design of electrical machines for offshore wind turbines and wave generators.
She has contributed to several conference proceedings in this field

Abstract

DESIGN, OPTIMIZATION AND INTEGRATION OF A DIRECT DRIVE SUPERCONDUCTING GENERATOR FOR LARGE WIND TURBINE

Introduction

Offshore wind market demands higher power rate and reliable turbines in order to optimize capital and operational
cost. In this way, wind turbine industry is taking advantage of Direct Drive (DD) systems in order to avoid the
gearbox, one of the main sources of unavailability, reliability and losses. However, the use of this solution with
conventional electric generators yields to huge machines in size and weight. Therefore, the application of
superconductivity is a promising option that permits more powerful, more compact and lighter machines.

Approach

Offshore wind energy is having a rapid development motivated by the stronger and more regular winds at sea,
as well as the availability of large areas with fewer restrictions than sites on land. According to the International
Energy Agency, global offshore wind cumulative capacity could reach 100 GW in 2020 and 375 GW in 2030 [1].
However, the more expensive marine foundations and installation procedures, as well as the restricted access
during construction and O & M due to weather conditions, is demanding more powerful and reliable wind turbines.
Nowadays, direct-drive synchronous generators (electrically excited or with permanent magnet) are difficult to
scale up to 10 MW and beyond. Their huge size and weight drives up the cost of both fixed and floating
foundations as well as installation and operation and maintenance costs. New solutions to provide better power
scalability, topside weight reduction and reliability are needed. Superconductivity combines such features and
allows scaling to 10 MW and beyond by radical reduction of the head mass.
The use of superconducting materials has recently appeared as a real alternative to conventional resistive materials
such as copper. These materials allow constructing higher power generators with lower volumes and weights
driven by the following advantages. They can achieve stronger magnetic fields by means of DC field
superconducting windings with very low losses. Less iron in the magnetic circuit is required, providing more
space for AC stator winding. And they permit the achievement of higher air-gap shear stress, which allows
smaller and lighter direct-drive machine design.
In conjunction with this, an optimal integration of the generator in top of the wind turbine will improve performance
of the whole wind turbine.

Main body of abstract

The present work falls in the SUPRAPOWER EU FP7 cofounded research project that started in December 2012
and which is expected to finish at the end of 2016, covering the design of a 10 MW 8.1 rpm salient pole
synchronous generator with superconducting field coils according to the patented concept by Tecnalia [2]
resumed in .
The generator operation is based on the use of superconducting filed coils of MgB2 wire, in the form of sandwich
tape with outer Cu stabilization layer, specifically designed, manufactured and characterized for this generator.
MgB2 selected wire is an industrial solution with a very competitive cost, several times lower than other HTS wires.
The cooling system for the MgB2 coils make use of a rotating cryogen free solution consisting basically of two parts,
one modular cryostat able to accommodate one coil and a thermal collector which links all the modules. The heat
is extracted by conduction through two stage G M cryocoolers which rotate jointly with the rotor, through two high
conductivity thermal circuits enclosed by the thermal collector. One will be connected to the cryocoolers first stage
and to the thermal shield (T~80K), and the other will be thermally linked to the second stage of the cryocoolers
and to the superconducting coils, maintaining them at operation temperature (T~20K) [3].
Described superconducting system implies some conditions in the design of the electric machine that must be
taken in account: the cryostat will be modular and will contain only the coils, keeping the poles at room temperature
and requiring so a large inter-pole room; the stator teeth will be removed to avoid high inductions in them,
resulting in a stator air-gap winding that involves the design of an innovative mechanical support for it; inter-polar
leakage must be carefully considered as it is critical in the definition of the superconducting wire operation
conditions.
The design has started with analytical electromagnetic calculations, resulting in a machine with a polar pitch of
about 600 mm and a magnetic length of about 500 mm. The stator winding concept is not different from a
conventional one, with the exception that the stator teeth will be eliminated to prevent saturation in them.
Due to it, winding fixation is a challenge to solve, having devised solutions for it. Along the study, one
requirement is the modularity of the system, allowing if necessary the transport of modules and a final assembly
on site if necessary.
This first design has been optimized with 2D FEM simulations. One of the most critical parts to be checked is
the work conditions of the field superconducting coils, whose operation parameters have to be below their
critical ones (Bc, Tc, Ic). One result of these simulations is shown in .
Once having an optimized electromagnetic circuit, a calculation of weight and costs has been made, comparing
the result with values of a previously studied Permanent Magnet (PM) DD generator of similar characteristics
and obtaining an improvement in the Levelized Cost of Energy (LCOE) value as well as a substantial weight
reduction, which would simplify commissioning, decommissioning and O&M works for both land and offshore
wind farms.
In addition to the electromagnetic parts, the generator needs structural elements to transmit torque and forces
both from the blades to the generator and from generator to the nacelle and tower. An optimal integration, that
must be different to that adopted in generators driven by gearbox, will eliminate the duplication of mechanical
elements lightening the wind turbine top mass, being the main aspects that dealt in the contribution now
described. Mechanical air gap must absorb misalignments at large diameters, implying regularity around its
periphery. Transmission of torque will be made along an optimal defined path. Minimization of the distance
between wind turbine axis and nacelle bearing plane will avoid strengths over it. A structure of arms will support
the generator rotor to the wind turbine one and transmit mechanical torque.


Conclusion

An optimized design of a 10 MW electric generator for wind turbines applying superconducting technology has
been made. The rotor has a warm iron poles configuration, which means that each modular cryostat only encloses
the superconducting coils, so the mass to be cooled is reduced and the high torque (11.8 MN•m) transmission
will be shared between the cold part (superconducting coils) and the warm one (iron poles). In addition, the
modular cryostat solution makes the construction, assembly and maintenance operations affordable in offshore
conditions. Air-gap winding design and support system allow transporting the generator in modules, making if
necessary the final assembly on site.
The design is fully oriented to the offshore environment, having reduced maintenance requirements and with very
competitive life cycle costs, critical issues in sites dependent of weather windows.
Weight and cost values have been calculated for such a generator, making too a comparison with same values of a
PM DD generator of similar characteristics. It has been concluded that the concept of DD synchronous salient pole
electric generator with cryogen-free cooling system applied to MgB2 field coils is a feasible technology that will allow
a reduction of weight over 30% with respect to a PM DD synchronous one, obtaining a substantial reduction of
LCOE value, too.
The integration study of the generator into the nacelle results in an improved performance of the whole wind
turbine, removing duplicated structural elements, distributing efficiently the efforts and simplifying the design
of fixing or floating platforms, depending of the water depth where the wind farm will be installed.



Learning objectives
Basics of cryogen-free superconducting technology applied to a MW-class electric generator.
Detailed study of weight and costs of a superconducting electric generator.
Details of integration of a direct drive generator in nacelle.



References
[1] International Energy Agency 2010 World Energy Outlook 2010 ISBN 978 92 64 08624 1
[2] European Patent Application EP 2 5251 252 A1: “Direct-action superconducting synchronous generator for
a wind turbine”; 7.12.2012; PCT/ES2009/070639
[3] S Sanz, et al. 2013 Superconducting light generator for large offshore wind turbines. EUCAS 2013