Integrated Design Of Direct-Drive Linear Generators For

Wave Energy Converters

A. S. McDonald, R. Crozier, S. Caraher, M. A. Mueller and J. P. Chick.Supergen Marine

Institute for Energy SystemsJoint Research Institute for Energy

Edinburgh Research Partnership in Engineering and MathematicsSchool of Engineering & Electronics

The University of Edinburgh

Agenda

� Introduction to Direct-Drive and Linear Generators

� Design approach

� Structural and magnetic design

� Bearing design

� Future Work

Direct Drive Linear Generators� Reduction of Failure Points� Potential for Maintenance

Free Operation� Potential for Submerged

Operation� Flexibility and Potential for

Improvement� Generators can be large and

heavy� Permanent magnets are

expensive� Large forces in the machine –

lots of structural material required

� No in-built energy storage

Fig 1: Mueller, M. A., Baker, N. J. (2005). "Direct drive electrical power take-off for offshore marine energy converters." Proceedings of the I MECH E Part A Journal of Power and Energy 219: 223-234.

Direct Drive Machines For Wave Energy

Linear PM Synchronous Machine (Double-Sided)

Linear Air-Cored PM Synchronous Machine

� Iron Core� High Power� Large Forces� Low Power Factor

� Air Core� Lower Power� Lower Forces� High Power Factor

Typical Design Approach

Wave FrequencyDistribution

WECmodel

Electricalmodel

Structuralmodel

Criterioncalculation

5 MW3 MW2 MW

Thermalmodel

Electrical DesignOptimisation

DesignCheck

FinalDesign

Improved Design Approach

Wave FrequencyDistribution

WECmodel

Generatormodel

Electricalmodel

Structuralmodel

Criterioncalculation

5 MW3 MW2 MW

Thermalmodel

DesignOptimisation

FinalDesign

Direct Drive Machines For Wave Energy

Linear PM Synchronous Machine (Double-Sided)

� Iron Core� High Power� Large Forces� Low Power Factor

Pictures from Polinder et al, “Linear PM Generator System for Wave EnergyConversion in the AWS”, IEEE transactions on energy Conversion, september 2004

Iron

Structural Steel magnet

Undesirable displacement

Pow

er P

rodu

cing

Mot

ion

Change In Forces With Displacement

Structural Design - Beams

� The forces must be withstood by some support structure

� The proposed structure is a series of beams running transverse to the direction of motion

� The I-Beam is chosen for its efficiency in its use of material – i.e. lower cost

� The beams are laid with no concern for the division by electrical pole

Iron-CoredI-Beam

Air-Cored

Impact on Capital Cost� The beam cross-section necessary to withstand the forces can

be determined by iteratively determining the forces on the beam as it deflects

� The choice of I-Beam sections can then be made from the following standards: BS4, Continental IPA, IPEA and IPN

276309

276243

276177

224299

224233

224167

172289

172223

172157

15

237284

237218

237152

185274

185208

185142

132264

132198

132132

10

198258

198192

198127

146248

146182

146117

94238

94172

94106

5

30, 930, 630, 320, 920, 620, 310, 910, 610, 3Copper

Cost €/kg

Magnet, Iron Cost €/kg

Top figure in each table cell is the cost of the ai r-cored machine, the second lower figure is the cost of the iron-cored machine.

237152

Bearings

� The structural analysis assumes that the material between the translator and stator is infinitely stiff

� In reality a bearing system must be designed to approximate this over a given lifetime

� This presents many challenges due to the harsh environment and high loadings in the machine

Internal Loads External Loads

� Wave loads were approximated using WAMIT giving a force of 177kN for a submerged WEC in depth of 30m and high frequency seas of 1.5Hz.

Net Force seen by the bearings

Airgap closing force in iron-cored machine

WEC length 21m,Ø9m.

Forces on machines:227kN on Iron Cored.177kN on Air Cored.

Forces to expect are higher the closer the WEC is to the surface.

Source: Gregory Payne

Bearing Options� Rolling contact , Thrust and Angular Ball Bearings.

� Need oil as lubricant – degrades over time� Lifespan calc’s can give years of operation� Reaction to shock loading is limited� Lubrication intervals -max yearly

� Linear Slides and Profile rails. � Maintenance and constant operation problems

� LBCT ,greasing every 800hrs, long life under low load.� LLHRC, cleaning stroke daily, life of ~600km.

� Sealing for linear rails difficult� Misalignment from rail over a long stroke.

� Water lubricated systems - Promising� Fluid film options� Contact sliding bearings from marine suitable polymers

� No seals, no pollution, fluid film reduces surface contact

(SKF.com)

SXL (Thordon Inc)

T814 (Tenmant Ltd.)

Design ConsiderationsAir- Cored

Machine LoadsIron Cored

Machine Loads

Bearing loads

Sliding Contact

Rolling Contact

E.H.LH.L B.L

Life span Prediction

f

Wave LoadingOn Machine

Hydrostatic/Hydrodynamic

wear δT

Hydrodynamic lubrication

- Full film between parts in contact

Elasto –Hydrodynamic-lubrication- v. thin film between parts in contact

Boundary lubrication- Asperite contact

5000hrs travel8000km/ yr

Bearings for flooded generators

Industrial Machinery� IGUS,

� GGB, Glacier Garlock Bearings

� Dry or Water Lubricated� IGLIDUR UW, f 0.12 - 0.5 or 0.04� GGB DUB, f 0.02 - 0.12

Marine applications� Thordon SXL, propeller shafts� Tenmat T814, hydroelectric dams

� Benefits in Linear applications� Low abrasive wear, corrosion resistant� Low modulus of elasticity, ability to handle shock loads� High Load ratings – up to 40MPa

SXL

(Thordon Inc)

T814(Tenmant Ltd.)

Bearing Summary � For fully non-contact operation:

� Hydrostatics or Magnetic Levitation� Active systems with higher power demand.� Additional control needed, fluid flow or field control � Length of translator stroke makes running clearance hard to maintain

� Contact back-up system:� Water lubricated polymer materials.� Testing due to start on these materials to verify low wear claims.

� Options for Iron cored Machine are limited � Hybrid of non-contact bearings and a back up contact system

� Air Cored Machine more achievable� Water lubricated contact bearings are a promising option

Future Work

� Design and integrate thermal models

� Develop optimisation method� Have investigated Genetic Algorithms as a promising

option

� Completion of a test rig and physical testing of bearing materials

� Systemise bearing design for inclusion in cost and feasibility models of linear generators

The Following Slides are for Reference if Necessary

Hydrostatic Bearings

� Benefits-� Low friction

� f 10-3 – 10-6

� Low maintenance� Cooling effects on coils � Less stringent sealing if

flooded with water

� Difficulties-� High attractive forces in machines� Materials/methods for construction� Use of seawater

� Corrosion� Low Viscosity

Important Relationships

• Pad load

• Pumping Power

3zwh

Qγα

3

2

h

QH p

γ∝

“two moving surfaces separated by thin film of pres surized liquid “

Suitability for D-D Machines� Water provides thin fluid films

� Again – low viscosity of water lessens the achievable effect.

� Machining surfaces to a fine finish over lengths required is costly

� Power demand increases hugely with increased film.� Lower load carrying capability in comparison to oil.

� Reliability with water in an iron cored machine is low� Surface Contact may be expected due to small films.

� Operation in air cored machines is more promising� Power demand is lower.

Pressure Drop across pad

Schematic of Hydrostatic system

PM synchronous Linear GeneratorOperating considerations:

• Wave amplitude, 4 m

• Wave period, 10s

•6 strokes/min

• Constant sinusoidal operation

• Distance to traverse per year > 7,033km

(inc. redundancy of 40%)

External Forces

Challenges in Hydrostatic operation

Problems with sea water;� Corrosion & clogging if not filtered

� Clearances are extremely low� Operating range 12 - 50 µm in

comparison to oil ~150 - 300µm� Precision Machining methods for

surfaces add cost

� Implementing bearings into design means� Creating compliant translator� Single pad & opposed pad fluid

control using minimal pumps

Conclusion on Pad geometry

• Circular pads are favoured geometry

•Larger operating clearances

•Less power loss at optimum

•Less edge effects on flow

5..

If oil is used;

•Extend the clearance

•Operating points for B4 , ro - 0.15m•hopt ~ 0.5mm•Hp 7.5 W/pad

•Reliance on tight seals•Chance of leakage to sea

•Cooling benefits not as great

Bearing placement

Pad Geometry Investigation -Rectangular Pads

� Pad stiffness highest at low clearance

� Pads 2,3,4 with x - 0.049m most potential� Lower leakage at larger pad sizes� Lower operating clearances

Operation with seawater as lubricant

Pad 3 chosen for further analysis35 pads/sidePr = 0.304 MPaPressure to provide “lift”

Operation with oil as lubricant

Pad Geometry Investigation -Circular pads

� As before stiffness highest at low clearance

� Most promise - Pads ØD 0.15 and 0.12 with ri/ro 0.5� Lower leakage at smaller pad sizes� Lower operating clearances

Operation with oil as lubricantOperation with seawater as lubricant