Electrical Machines For Renewable Energy

Converters

Dr. Markus MuellerSchool of Engineering

University of Edinburgh

Presentation

Renewable Energy Systems Electrical Drivetrain Technology Challenges – engineering & economic Integrated Design Tools Electrical Machine Topologies Future Developments

Wind Energy

E uropean Wind E nergy Association

Wind Drivetrain

Direct Drive Wind

Enercon E112 4.5MW 220 tons Iron cored field wound

synchronous machine Large magnetic

attraction forces between stator and rotor.

Structural support >60% of total mass.

Wave Energy

Global Wave Energy Resource

EU Wave Energy Resource

Source : Future Energy Solutions

UK ResourceUK Resource

•Offshore: 50TWh/year Offshore: 50TWh/year (1/7(1/7th th UK Electricity UK Electricity Production) Production)

•Near-shore: 7.8TWh/yearNear-shore: 7.8TWh/year

•Shoreline: 0.2TWh/yearShoreline: 0.2TWh/year

Oscillating Water Column

Wave energy conversion device LIMPET on Islay since 2000 Turbines delivered to Mutriku, Spain 4MW scheme on Lewis consented

Oscillating Water Column (OWC)Shoreline and near shore

© Wavegen

Pelamis Full Scale – 750kW

Image © Aquatera.co.uk

Pelamis Wave Power

© Pelamis Wave Power

• Pelamis – sea snake• Designed for survivability• Uses off the shelf

components• A 750 kW device will be

150m long and 3.5m in diameter.

• Hydraulic PTO• Generators

• Squirrel Cage Induction Machine

Pelamis – power take off

Cylindrical sections connected by hinged joints

Motion of joints resisted by hydraulic rams, pumping high pressure oil through hydraulic motors via smoothing accumulators, ultimately driving induction machines

Power take off at each hinge

Power from all hinges fed into one sub-sea cable © Ocean Power Delivery

http://www.youtube.com/watch?v=F0mzrbfzUpM

Hydraulic Power Take Off

Aquamarine

Hydro Technology Pelton Turbine driving Synchronous

Machine© Aquamarine

Oyster – Testing at EMEC

© Aquamarine

Point Absorber: Archimedes Wave Swing

© AWS BV

Move with incident waves either in surge or heave mode and is veryMove with incident waves either in surge or heave mode and is very

small compared to the wavelength. small compared to the wavelength.

AWS Electrical Power Conversion

Linear PM synchronous machine Rating – 2MW Average – 400kW Stroke – 4 to 7m Velocity <= 2.2m/s Double sided Stator – 5.6m Translator – 8.4m Mass ?

© AWS BV

(Source: Dr. Henk Polinder, TU Delft)

Marine Current Turbines

Axial flow turbine SEAFLOW: 300 kW

unit; Lynmouth, North Devon Coast

SEAGEN: 1.2MW, Strangford Narrows, N. Ireland

Power take-off by geared induction generator

© Marine Current Turbines

Tidal Current Direct Drive:Open Hydro

Rim Generator Air-cored PM

generator Fully flooded 300kW device

tested at EMEC

ScotRenewables

Floating Turbine Hydraulic Power Take Off Induction Generator

Engineering Challenges

Low speed Wind: 1MW – 20rpm, 7MW – 10rpm Wave: recoprocating, 1-1.5 m/s peak Tidal Current: 1MW – 10rpm

Mechanical interface to step up speed Direct Drive

Physical size, weight, Permanent Magnet - Cost and Availability

June £77/kg, August £150/kg Environment – corrosion, vibration

Engineering Challenge

Variable prime mover Wave:

Challenge Example - Oyster

Diameter 6 m

Total Weight 118,6 t

Total Generator Cost £ 3,066k

Power Electronics Cost £ 116k

Total Cost £ 3,182k

Single Stage Gearbox

Direct Drive• Maximum Reliability• Larger generator mass & cost• Low utilization of magnetic material

(due to low speed)

Single Stage Gearbox• Reduced reliability

(but not as multi speed gearboxes)• Decreased generator mass & cost• Increased efficiency & electricity generation

Designs with Different Gear Ratios

Integrated Design

Wind speeddistribution

Wind turbinemodel

Generatormodel

Axial-flux

Electricalmodel

Structuralmodel

Criterioncalculation

Radial-flux

5 MW3 MW2 MW

Thermalmodel

Hydrodynamicmodel

Generatormodel

Electricalmodel

Structuralmodel

Criterioncalculation

Thermalmodel

DesignOptimisation

FinalDesign

Wave Energy Converter

Wave FrequencyDistribution

Structural Modelling of Direct Drive

2θ

R

R1Ro

hyr

What does this modelling tell us? Structural material is dominant

Optimal aspect ratios are larger

Optimal airgap lengths are larger

Stator

Rotor

Stator

Rotor

Structural Optimisation

Integrated Electromagnetic-Structural Optimization

A FEA optimisation tool was created to further decrease the weight of the direct drive generator

The FEA optimisation tool removes peaces of the predefined structure based on the major forces that apply on it

The rotating part was optimised separately from the stationery one

The new structures are up to 15% lighter compared to the original ones

Original Structure “New” StructurePartial FEA Optimisation

Induction Generator Modelling for OWCs -

Wavegen

Airflow and generator power recorded during OWC operation

Recorded casing and winding temperatures and 1 minute average generator power during operation

Solutions to Challenges

Power Density or Mass Transverse Flux Machine Air-cored Machines Novel Structures Superconducting Machines

Low speed Magnetic Gearing - SNAPPER

PM Magnets Switched Reluctance Machines

Armature

Translator

Springs

Copper Winding / Coil

Stack Length, ls

Magnetic Gearing: SNAPPER

Fdrive

Fspring

Fdrive

Fspring

Phase 1

Spring force is less than magnetic attraction force:

Translator and armature move in same direction.

Phase 2

Spring force matches magnetic attraction force:

Armature movement ceases

Phase 3

Armature becomes decoupled from translator and begins to move at high velocity relative to the translator.

Dry Testing

Dry Testing Video

Dry Testing video

Economic – PM availability

Switched Reluctance No permanent magnet material Coils on stator only. Rotor consists of iron laminations only. Versatile in terms of control. Requires grid connection for excitation Small airgaps required for high

performance.

Experimental Prototype20 kW at 100 rpm

Switched Reluctance with Segmental Rotor

Prof Barrie Mecrow, University of Newcastle

TOPOLOGIES FOR WOUND-FIELD THREE-PHASE SEGMENTED-ROTOR FLUX-SWITCHING MACHINES A. Zulu, B.C. Mecrow, M. Armstrong, IET PEMD, Brighton, 2010

Switched Reluctance with Segmental Rotor

65% improvement in Torque Density (Nm/kg) compared conventional.

“Optimised Segmental Rotor Switched Reluctance Machines with a Greater Number of Rotor Segments Than Stator

Slots” J.D. Widmer and B.C. Mecrow, IEEE IEMDC, Niagara, Canada, 2011.

Transverse Flux Machines

High Shear Stress at the airgap 200kN/m2 reported by Weh 4-5 times conventional PM synchronous

machine Construction is challenging Power Factor is an issue

Surface mounted TFM – pf ~0.2 Flux concentrating TFM - pf ~0.5

EEC

What type of TFPM machine ?A number of TFPM machine types have been proposed.

It is necessary to find the most suitable type. How?

PM

Core

Winding

Secondary part

Primary part

Core

EEC

PM

Core

Winding

Secondary part

Primary part

Core

CorePM

Winding

Secondary part

Primary part

Core

PM

Core

Winding

Primary part

Core

Secondary part

PM

Core

Winding

Primary part

Core

Secondary part

a) RFPM machine b) TFPM machine-1 c) TFPM machine-2

d) TFPM machine-3 e) TFPM machine-4

Comparative design of PM machines

EEC

Generator parameter

Generator power, P 5.56 MW

Rotational speed, rpm 12

Number of phase, m 3

Nominal current, is 675 A

No-load voltage, ep 2746 V

Air gap length, lg 6.14 mm

Air gap diameter, Dg 6.14 m

Material parameter

Remanent flux density of the magnets (T)

1.2

Recoil permeability of the magnets 1.06

Resistivity of copper at operating temperature (μΩm)

0.025

Cost modeling

Laminations cost (€/kg) 3

Copper cost (€/kg) 15

Magnet cost (€/kg) 25

Design parameters

EEC

PM

Core

Winding

Secondary part

Primary part

Core

CorePM

Winding

Secondary part

Primary part

Core PM

Core

Winding

Primary part

Core

Secondary part

PM

Core

Winding

Primary part

Core

Secondary part

Comparison

EEC

PM

Core

Winding

Secondary part

Primary part

Core

CorePM

Winding

Secondary part

Primary part

Core

PM

Core

Winding

Primary part

Core

Secondary part

PM

Core

Winding

Primary part

Core

Secondary part

a) RFPM machine b) TFPM machine-1 c) TFPM machine-2

d) TFPM machine-3 e) TFPM machine-4

Comparative design of PM machines

PM Air-cored Machines

Stator winding contains no iron. Elimination of magnetic attraction

forces between stator and PM rotor Benefits in terms of

Machine structural mass Assembly and manufacture

PM machines

Copper

Steel

PM

Stator

Rotor

Rotor

Iron-cored machines:

High flux density and shear stress

Large attractive forces between rotor and stator

Air-cored machines:

Lower flux density and shear stress

No attractive forces between rotor and stator

Air cored PM: SLIM & Goliath

Goliath – 250kW

Spoked Structure Airgap Winding, steel surrounding

winding

Open Hydro

Air-cored Machines:C-GEN

Mild steel C-core

Magnets

C-GEN modular assemblyC-GEN modular assembly

Rotor

Stator

C-GEN final assemblyC-GEN final assembly

• PM GeneratorsPM Generators– Assembly is difficult and Assembly is difficult and

dangerousdangerous– Large forces of attraction Large forces of attraction

between rotor and statorbetween rotor and stator

• C-GEN stator can be C-GEN stator can be simply and easily simply and easily lowered into placelowered into place– No forcesNo forces– Assembles with an engine Assembles with an engine

hoisthoist– Production savings for Production savings for

large generatorslarge generators

C-GEN Mk I: 20 kW C-GEN Mk I: 20 kW Prototype ResultsPrototype Results

80%

85%

90%

95%

100%

0 5 10 15 20 25

Power (kW)

Eff

icie

ncy

100rpm90rpm80rpm70rpm60rpm50rpm

PowerPower 21.5 kW21.5 kW Outer radiusOuter radius 502 mm502 mm

EfficiencyEfficiency 93 %93 % Machine Machine lengthlength

500 mm500 mm

SpeedSpeed 100 rpm100 rpm Total massTotal mass 949 kg949 kg

C-GEN MkII: 15kW C-GEN MkII: 15kW resultsresults

85

90

95

100

0 200 400 600 800 1000 1200

Torque (Nm)

50

96

107

115

120

rpm

Linear C-GEN for Wave

50kW (pk) Vpk = 2m/s Machine Length =

3m Stroke = 2m

High Temperature Superconducting Machines

American Superconductor Coorp

36.5 MW, 120 rpm (U.S. Navy, AMSC)

HTS Context

Larger Offshore Wind Turbines (>5MW)

Gearboxes unfeasible Direct Drive

Low Speed – High Torque More Reliable High Generator Mass

• High Installation Cost

M. Lesser, J. Müller, “Superconductor Technology – Generating the Future of Offshore Wind Power,”

Types of HTS Machines

Rotating DC Superconducting Field Most Common Type Transient Torques on HTS wire Cryocooler Coupler + Brushes

Low Reliability Cooling Times

Magnetized Bulk HTS Very Difficult to Handle Demagnetization

All Superconducting Machines AC Losses on HTS wire

HTS Machines – Claw Pole

Stationary HTS coil to provide field excitation

Air-cored winding Claw Pole Rotor

steel construction modulates the

field

Claw Pole HTS Generator

Future

Low speed Direct Drive Single Stage Gear Box

Direct Drive HTS, Air-cored machines, Novel Support

Structures Permanent Magnet Issue

Switched reluctance segmented rotor machine Integrated Design Tools

Electromagnetic, structural, thermo-fluid Operational Environment Design for Reliability

Acknowledegements

Scottish Enterprise The Carbon Trust npower juice EPSRC Supergen Marine EU FP6 UPWIND EU FP7 SNAPPER Project NGenTec Fountain Design Ltd, TUV NEL, & Hopewell Wind Ltd PhDs & RAs

Ozan Keysan, Richard Crozier, Alasdair McDonald, Aris Zavvos

Professor Ed Spooner (Goliath & Open Hydro) Dr. Henk Polinder (TU Delft)