Multiphysics_Simulation Suplemento Iiee

7/31/2019 Multiphysics_Simulation Suplemento Iiee

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Special Advertising Section toIEEE Spectrum

Ju 2012

Multihic

iMulati

MetaMaterialsMake Physics

seeM like Magic

UPgrading thenUts and Bolts of

the electrical gridpage 3 page 22

page 8

Topology

opTimizaTion leads

To BeTTer Cooling

of eleCTroniCComponenTs

ponsored by


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S2 COMSOL MULTIPHYSICS JUN E 2012 + ONLINE: www.comsol.com/products/multiphysics/

TODAY SIMULATION ISubiquitous. It has been

embraced by virtually every industry that designsand innovates new products and services.

There has been remarkable progress in simula-

tion methods. In act, the perormance o improved

algorithms has matched that o improved hard-

ware over the last hal century. The combined eect

o these advances has been a huge increase in the

computing power available or simulation-based

design and optimization.

So we asked COMSOL, an innovator in multiphys-

ics simulation sotware and the creator o this special

supplement to lay out how the leap in computational

capability has changed what simulation sotware can

do today.

In the next ew pages you will fnd examples o

some truly remarkable work. These include devel-

oping metamaterials to achieve electromagnetic

cloaking, the shaping o ractallike pattern cold-

plates to cool power-electronics in hybrid cars, and

bringing superconducting ault current limiters tothe power grid.

I am sure you will fnd this supplement, spon-

sored by COMSOL, ascinating.

Please eel ree to contact me i you would like to

share your own experiences in pushing the limits

o simulation.

Email: [email protected]

SIMULATIONMORE THANMEETS THE EYE

ByJAMES A. VICK, SENIOR DIRECTOR, IEEE MEDIA;PUBLISHER, IEEE SPECTRUM

CONTENTS

3METAMATERIALS MAKEPHYSICS SEEM LIKE MAGICNASA Glenn Research Center,

Cleveland, OH USANaval Postgraduate School,

Monterey, CA USA

Duke University, Center forMetamaterials and IntegratedPlasmonics, Durham, NC USA

8NUMERICAL SIMULATION-BASED TOPOLOGYOPTIMIZATION LEADSTO BETTER COOLING OFELECTRONIC COMPONENTSIN TOYOTA HYBRID VEHICLES

Toyota Research Instituteof North America,

Ann Arbor, MI USA

ON THE COVER:Advanced heat sinks with optimized cooling channel topologyare being designed to cool power electronic components in Toyota hybrid vehicles.

M U L T I P H Y S I C S S I M U L A T I O N S p e c i a l A d v e r t i s i n g S e c t i o n

19MODELING SCAREFFECTS IN ELECTRICALSPINAL CORD STIMULATION

Lahey Clinic, Burlington, MA USA

12MODELING OPTIMIZES APIEZOELECTRIC ENERGYHARVESTER USED IN CAR TIRES

Siemens Corporate Technology,Mnich, Germany

16A SOLUTIONTO TREATINGNUCLEAR WASTECOMES VIAMODELING ANDSIMULATION

Idaho NationalLaboratory, IdahoFalls, ID USA

29LIGHTNING-PROOF

WIND TURBINESGlobal Lightning Protection

Services A /S, Lejre, Denmark

32MATHEMATICALMODELING: ANINDUSTRIAL PERSPECTIVE

DuPont Experimental Station,Wilmington, DE USA

22UPGRADING THE NUTSAND BOLTS OF THEELECTRICAL GRID FOR ANEW GENERATION

ABB AB Corporate ResearchPower Technologies,Vsters, Sweden

Florida State University, Centerfor Advanced Power Systems,Tallahassee, FL USA

26NUMERICAL MODELINGOF ELECTROSTATICPRECIPITATORS

Alstom Power Sweden AB,Vxj, Sweden


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S 3COMSOL MULTIPHYSICSJUN E 20 12

S p e c i a l A d v e r t i s i n g S e c t i o n

+ ONLINE: www.comsol.com/electrical

than those ound in natu-

rally occurring or chemi-

cally synthesized materials.

Manipulating the struc-

ture o the metamaterial

allows it to interact with

and control electromagnetic

waves. Just what an impact

this has comes into stark

relie when we take into

account the act that elec-

tromagnetic radiation can

have wavelengths that range

rom thousands o kilome-

ters to billionths o a meter.

Controlling electromag-netic waves lets us con-

trol whether objects can

be seen. For instance, the

wavelength o the electro-

magnetic waves that make

up visible light ranges

rom 400 to 750 nanome-

ters. But because the spac-

ing between atoms is much

smaller than thaton the

order o one-tenth o a

nanometer (an angstrom)we cannot resolve an image

o atoms rom visible light.

This leads to the exciting

prospect o using meta-

materials to make invisi-

ble objects visible and vis-

ible objects invisible.

All the ne details o the

medium are blurred on the

spatial scale o about one

wavelength, which allows

physicists to use an aver-aged description known as

eective medium theory.

The idea o metamateri-

als stems rom this simple

concept o eld averaging.

The many orders o mag-

nitude dierence between

the wavelength o visi-

ble, inrared, or microwave

radiation and the atomic

METAMATERIALS MAKEPHYSICS SEEM LIKE MAGICTo achieve this magical efect, one must havesimultaneous control over multiple physical phenomena

ByDEXTER JOHNSON, PROGRAM DIRECTOR, CIENTIFICA& BLOGGER, IEEE SPECTRUM ONLINE

Cylindrical metamaterial cloak for microwavefrequencies designed by the group of David R.Smith, Duke University.IMAGE: DAVID SCHURIG

David R. Smith(above), YaroslavUrzhumov, DukeUniversity.

M E T A M A T E R I A L S

THE FAMED SCIENCE ction author Arthur C. Clarke once remarked,

Any suciently advanced technology is indistinguishable rom magic.

I this idea indeed holds true, then the emerging eld o metama-

terials would have to be classied as a suciently advanced technol-

ogy. Metamaterials have been stunning both the layman and the sci-entist in recent years with their ability to render objects invisible (see

the cloak image above), leaving many to comment only hal in jest

that they must be magic.

Metamaterials are not magic, however. Instead, they are the result o a

science that requires an enormous amount o knowledge and control over

electromagnetic phenomena and other physical attributes o materials.

A metamaterial can be broadly dened as an articially structured

material abricated by assembling dierent objects so as to replace the

atoms and molecules that one would see in a conventional material.

The resulting material has very dierent electromagnetic properties


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S 4 COMSOL MULTIPHYSICS JUN E 2012

M E T A M A T E R I A L S

optical principle or the

manuacturing o lenses or

any other optical device that

bends or manipulates light.

All materials in nature

have a reractive index, or a

measurement o the speed

o light through that mate-rial. But some metamate-

rials are capable o achiev-

ing what is known as a

negative reractive index,

resulting in metamaterials

sometimes being reerred

to as let-handed or neg-

ative-index materials.

A material that has a

negative reractive index

is capable o bending light

in the opposite directiono what we would expect

based on typical reraction.

The method by which you

make a material that has

a negative reraction index

requires reversing the elec-

trical component (permit-

tivity) and the magnetic

component (permeability)

o a materials reractive

index. This is accomplished

by articially constructing

a material (Figure 5) that

possesses structures with

dimensions smaller than

the wavelengths o the light

it is intended to reract.

This causes the atoms and

the photons in the mate-

rial to resonate and reverse

the materials permittiv-ity and permeability.

These optical capabili-

ties o metamaterials are

important or understand-

ing the wide array o appli-

cations that exist or them.

APPLICATIONSFOR METAMATERIALSONE OF THE rst poten-

tial applications suggested

or metamaterials was asuperlens that would uti-

lize the negative reraction

o a metamaterial to pro-

vide much higher resolu-

tion than is possible with

lenses made rom nat-

ural materials, accord-

ing to Jerey D. Wilson,

a physicist at NASAs

Glenn Research Center.

Such a lens could enable

very high-resolution imag-

ing and lithography, with

scale creates a window o

opportunity or an eec-

tive medium consisting

o articial atoms that

are much larger than real

atoms but still signicantly

smaller than the wave-

length o the radiation.Such a medium is what sci-

entists call a metamaterial.

NEGATIVEREFRACTIVE INDEXAN IM PO RTAN T PROP ER TY

o metamaterials is the

phenomenon o negative

reraction. O course, were

all aware rom an early age

that reraction is the bend-

ing o light at the inter-section o two materials.

The most common exam-

ple o reraction at work is

the observation o underwa-

ter objects rom above the

water. In this case, rerac-

tion makes those objects

appear closer to the surace

than they actually are. So

reraction provides the basic


12 3

4 5

FIGURE 1: Full-wave simula-tion o a magnetic metama-terial disk levitating abovea current-carrying coil.

FIGURE 2: Hydrodynamicalcloak: a porous metamate-rial shell that eliminates wake.Designed and modeled with

COMSOL Subsurace Flowand Optimization Modules.

FIGURE 3: A pair o coilstightly coupled througha negative-permeabil-ity metamaterial slab.

FIGURE 4: Unidirectionalacoustic cloak based onquasi-conormal transor-mation optics, modeledusing COMSOLs axisym-metric pressure acoustics.

FIGURE 5:A composite Split

Ring Resonator Thin WireArray metamaterial exhibit-ing negative index o rerac-tion in the microwave X-band;COMSOL simulation. Notethe additional blocks o reespace that acilitate multi-direc-tional S-parameter simulations.

IMAGES: YAROSLAV URZHUMOV(DUKE UNIVERSITY)


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one application being the

abrication o smaller and

aster computer chips.

Problems with abrica-

tion and attenuation issues

need to be solved, however,

beore this becomes practi-

cal. Attenuation issues that

severely limit the peror-

mance o negative-index

lenses are however less

severe in near-eld appli-

cations, where, instead o

negative index, one can use

either a negative permit-

tivity or negative permea-bility. The latter material

property - still rarely avail-

able in nature - is particu-

larly promising or applica-

tions requiring magnetic

eld enhancement and

ocusing, such as magnetic

levitation (Figures 1, 3).

The area in which elec-

tromagnetic metama-

terials have rst been

used or practical appli-cation is antenna tech-

nology, explains NASAs

Wilson. Metamaterials

have been used in antennas

to signicantly reduce size,

increase requency band-

width, and increase gain.

While antenna technol-

ogy has been the largest

application or metama-

terials, it is perhaps in the

area o cloaking that themost excitement and pub-

licity have been generated.

In cloaking, metama-

terials are used to divert

microwaves or optical

waves around an object

so that it appears invis-

ible. Most o the appli-

cations or this cloaking

eect involve the military.

Among the exciting

potential uture applica-

tions being discussed or

metamaterials is a seismic

metamaterial that could

be used to protect struc-

tures rom earthquakes,

according to Wilson.

Another area in which

metamaterials are gain-ing traction is in terahertz

(THz) technologies, in par-

ticular or imaging appli-

cations. THz imaging has

aroused interest in the con-

texts o security and medi-

cal imaging because o its

ability to penetrate non-

metallic materials and

abrics and do so without

damaging tissue or DNA.

THz waves have re-quencies that are higher

than those o microwaves

but lower than those o

optical radiation, explains

Wilson. However, the THz

requency band has been

essentially neglected and

is reerred to as the THz

gap o the electromagnetic

spectrum. The primary rea-

son or this is that currently

available compact THz

sources can produce only

at NPS have been develop-

ing metalms (thin lms

based on metamateri-als) that could enable less

expensive THz imaging

devices and total absorp-

tion o the THz waves.

The metalm we are

developing exhibits prop-

erties not ound in natural

materials, explains Alves.

It is obtained by placing

a periodic array o metal

cells close to a conduct-

ing plane with a dielectric

spacer in between to orman articial structure that

exhibits electromagnetic

properties such that its

impedance matches with

the surrounding media

(ree space in our case)

at a specic requency.

In this situation, ide-

ally there is no transmission

and no refection, result-

ing in total absorption. By

selecting appropriate mate-rials and geometry, it is pos-

sible to design lms with

near 100 percent absorption

in the desired requency.

small amounts o power

on the order o milliwatts.

Some companies have

developed airport scanners

that make use o THz imag-

ing but achieve their capa-

bilities by means o veryexpensive and complicated

imaging arrangements.

The problem has been

that the background ther-

mal energy in the THz

range o the electromag-

netic spectrum is small

compared with inra-

red, according to Fabio

Alves, a researcher rom

the Sensor Research Lab,

led by Proessor GamaniKarunasiri, at the Naval

Postgraduate School (NPS)

in Monterey, Cali. When

the THz waves have to

travel through open air, as

they do in airport imag-

ing technologies, most o

the radiation is absorbed

beore it reaches its target.

Alves and his colleagues

MICROWAVE DEVICE: Microwave Rotman lens whose size is substan-tially reduced with the aid o magnetic metamaterial lling the taperedtransmission lines in the center. IMAGE: JOHN HUNT (DUKE UNIVERSITY)

Simulationtools enable

us to be creativeand to quicklytest new ideasthat would bemuch more dif-cult, time-consum-ing, and expensiveto test in the lab.JEFFREY D. WILSON,NASA GLENNRESEARCH CENTER

S p e c i a l A d v e r t i s i n g S e c t i o n M E T A M A T E R I A L S


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rating its perormance (See

the compressed Rotman

lens on the previous page.).

Physical implementations ospace transormation ideas

almost invariably require

metamaterials with exotic

electromagnetic properties.

OBSTACLES INDESIGNING WITHMETAMATERIALSONE OF THE key design con-

straints researchers ace

when working with meta-

materials is the high ohmicloss o metamaterials due

to the ohmic losses in the

metal. This causes electro-

magnetic waves that pass

through the structure to

be strongly attenuated.

Not only are there design

constraints in working with

metamaterials, but they also

require a high level o con-

trol over their structure. In

act, a metamaterial derivesits propertiessuch as its

electromagnetic cloaking

rom its structure rather

than its chemical composi-

tion. So, as one might imag-

ine, being able to design and

then abricate these complex

structures is no easy eat.

Metamaterials usually

have a airly complex struc-

ture, with a large num-

ber o design parameters,

including architectural

parameters as well as elec-

tromagnetic properties o

the materials rom which

they are constructed,explains CMIPs Urzhumov.

Complex structure

leads to very complex elec-

tromagnetic response.

Frequency spectra o meta-

materials typically have

lots o interesting eatures,

most stemming rom elec-

tric and magnetic reso-

nances, says Urzhumov.

While analytical models

exist or a handul o simplegeometries and crude semi-

analytical estimates can

be made or certain other

types o structures by intro-

ducing approximations, it is

virtually impossible to pre-

dict the electromagnetic

response o complex struc-

tures without simulations,

according to Urzhumov.

The impact o model-

ing and simulation tools inthe eld o metamaterials

is not restricted to the sci-

ence. It can also extend to

business considerations, as

well as helping to push the

limits o our imagination.

Simulation tools enable

us to be creative and to

quickly test new ideas that

would be much more di-

cult, time-consuming, and

expensive to test in the lab,

explains NASAs Wilson.

The lms can be employed

in the abrication o

microbolometers and bima-terial ocal plane arrays,

where the absorption char-

acteristic can be engineered

to match the requency o

the source, signicantly

improving the eciency

o the imaging system.

One o the leading

research organizations in

metamaterialsand the

one perhaps most closely

associated with the cloak-

ing eects o metama-terialsis the Center

or Metamaterials and

Integrated Plasmonics

(CMIP) at Duke University,

led by David R. Smith.

CMIP is also working on

nding ways o compen-

sating near-eld decay in

ree space or open air.

In ongoing work at

CMIP, Yaroslav Urzhumov,

an assistant research pro-essor, and others are

working with Toyota

Corporation to abricate

magnetic metamaterials

or wireless power transer

or electrical vehicles (EVs).

When one imagines

how such a wireless trans-

er o power could be

achieved, one usually con-

jures up devices incor-

porating microwave or

laser technology. Both o

these technologies come

with the obvious inherent

risk o rying the device

being charged, however.

Just as Smith and his

CMIP colleagues devel-

oped metamaterials that

made it appear as thoughan object had disap-

peared using electromag-

netic cloaking, they have

now created a lens (Figure

3) made rom metamate-

rials that can ocus low-

requency elds in such a

way that it makes the dis-

tance between the power

source and the device

being charged disappear.

Making a source appearcloser than it really is with

the aid o metamaterial-

based lenses is just one o

the tricks that the novel

concept o transorma-

tion optics has predicted.

Transormation optics is an

engineering methodology

based on the idea o warp-

ing, bending, or squeezing

physical space, as electro-

magnetic waves or elds seeit. While cloaks and fat-

tened sh-eye lenses (see

the Maxwell sh-eye lens

above) are examples o space

warping, even more trivial

coordinate transormations

like space squeezing are o

tremendous practical use,

as they reduce the device

dimensions without deterio-

M E T A M A T E R I A L S S p e c i a l A d v e r t i s i n g S e c t i o n

A modied Maxwell sh-eye lens with two fattened suraces or two-dimensional microwave propagation, experimental sample. The fatten-ing o a normally circular Maxwell lens shape is accomplished with quasi-conormal transormation optics theory. IMAGE: JOHN HUNT (DUKE UNIVERSITY)

As or mepersonally, I

discovered entirely

in a COMSOLsimulation thatthese cloaks canperorm extremelywell in the short-wavelength limit.YAROSL AV URZHUMOV,DUKE UNIVERSITY


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When we nd an idea that

works, we can optimize the

desired eect and thus spec-

iy the design to be built.

But ultimately, science

considerations are para-

mount when working with

metamaterials. I one wants

metamaterial-based devices

to unction properly, precise

knowledge o the response

at each requency o inter-

est is needed, making accu-

rate requency-domain sim-

ulations a requirement.

Its become clear thatsimulation is absolutely

necessary in working with

structures that have arbi-

trary, inhomogeneous,

time-dependent, and non-

linear electromagnetic

properties, as seen in meta-

materials. But not all sim-

ulation tools have these

capabilitiesand i they

do, theyre quite limited.

According to NPSsAlves, modeling and sim-

ulation tools have been

exceptionally helpul in the

design and analysis o the

metalms he and his col-

leagues are developing.

One o the most signi-

cant design constraints in

our work is the lack o an

analytical model that com-

pletely explains the inter-

actions o all involvedparameters, explains

Alves. The numerical sim-

ulations ll this gap. The

fexibility o COMSOL

Multiphysics allows us to

deal with several degrees

o reedom simultane-

ously. Furthermore, mate-

rial properties can be

tuned by tting the mea-

sured and simulated

data, improving the accu-

racy o uture designs.

Flexibility and versatil-

ity are key requirements

or a modeling and sim-

ulation tool when work-

ing with metamaterials.

The use o COMSOL

Multiphysics allows us to

analyze the perormance o

the sensors in many ways,

(Figure 6) says Alves. In

the specic case o the

bimaterial sensor, RF sim-

ulations were conducted to

obtain the amount o radi-

ated power absorbed by

the metalms. This poweris converted into heat that

fows through the sensor

and is exchanged with the

environment. This phenom-

enon can be studied using

heat transer simulations.

Ultimately, structural

mechanics simulations eval-

uate the deormation in

the bimaterial structures,

which is the eect to be

probed by the external read-out, according to Alves. This

is all done in a single run.

This process would be

exceedingly dicult with-

out the help o multiphys-

ics simulations, says Alves.

We appreciate the ver-

satility o all boundary

conditions and excita-

tion types that can be used

in all types o studies in

COMSOL, says Urzhumov.One eature in particu-

larthe ability to speciy

a given background eld

and use it as an excitation

has been truly enabling

or many o our projects.

COMSOL Multiphysics

can do much more than just

modiy all boundary condi-

tions: It allows or changes

to the equations themselves.

I routinely insert addi-

tional polarization den-

DISCOVERIESAND APPLICATIONSENABLED BY MODELING

AND SIMULATIONURZHUMOV CREDITS

much o the success o his

research to being able to

use modeling and simu-lation tools to open up

new avenues o discovery.

According to

Urzhumov, one o unique

eatures o COMSOL

is its ability to per-

orm a sensitivity anal-

ysis semi-analytica lly,

which enables quick gra-

dient-based optimiza-

tion with a huge num-

ber o design parameters.With the help o the

numerical optimization in

COMSOL I could extend

my fuid cloak solu-

tion (Figure 2) into the

strongly nonlinear fow

regime, where analyti-

cal solutions are almost

impossible to obtain,

says Urzhumov.

FIGURE 6: COMSOL simulation (deormation analysis) o theBi-material sensor integrated with the THz sensitive metalm. Themetalm in the center absorbs THz and transer the heat to the mul-tiold legs that bend proportionally to the absorbed radiation. Theamount o bending can be accessed using optical readouts.IMAGE: FABIO ALVES (NAVAL POSTGRADUATE SCHOOL (NPS) )

sities that describe the

response o a dispersive

medium, such as a metal at

optical requencies, which

allows me to model neg-

ative-index metamateri-

als in the time domain,

says Urzhumov. Thisextra polarization den-

sity is merely an extra term

in the main electromag-

netics equation that cou-

ples it to an extra equation

describing the evolution o

that polarization density.

In act, the most noted

quality o metamateri-

als, their ability to cloak

objects electromagneti-

callythe so-called invisi-bility cloakwas predicted

entirely by simulation,

according to Urzhumov

As or me person-

ally, I discovered entirely

in a COMSOL simula-

tion that these cloaks can

perorm extremely well

in the short-wavelength

limit, says Urzhumov.

S p e c i a l A d v e r t i s i n g S e c t i o n M E T A M A T E R I A L S


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S 8 COMSOL MULTIPHYSICS JUN E 2012 + ONLINE: www.comsol.com/mechanical

cooling channel patterns

in an automated ash-

ion, using advanced sim-

ulation tools as opposed

to a traditional trial-and-

error design approach.

Dede carried out this

work as part o TRI-NAsmission to conduct accel-

erated advanced research

in the areas o energy and

environment, saety, and

mobility inrastructure.

TRI-NA is a division o the

Toyota Technical Center,

which in turn is part o

Toyota Motor Engineering

& Manuacturing North

America, in charge o R&D,

engineering design anddevelopment, and manuac-

turing activities or Toyotas

North American plants.

TRI-NAs Electronics

Research Department

ocuses on two main areas:

1) sensors and actuators and

2) power electronics. Among

its resources are powerul

modeling and simulation

capabilities and prototype

design tools, which enableits sta to develop eective

solutions in the compressed

time rames demanded

by the highly competi-

tive automotive markets.

HOT UNDER THE HOODTOYOTA HYBRID vehicles

have sophisticated electri-

cal systems in which many

ONE GLANCE UNDER the hood o a modern automobile is all it takes to realize that ree

space in the engine compartment is a thing o the past.

I carmakers could reduce the number, size, and weight o the components in there, bet-

ter uel economy would result. A case in point is the design and development o optimized

cooling structures, or advanced heat sinks, or thermally regulating the growing number o

power electronics components used in the electrical system o Toyota hybrid vehicles.

To save the time and expense associated with analytical design methods and trial-and-error physical prototyping, researchers at the Toyota Research Institute o North America

(TRI-NA) in Ann Arbor, Mich., instead used numerical simulation and multiphysics topol-

ogy optimization techniques to design, abricate, and test possible prototypes o a novel

heat sink or uture hybrid vehicle generations.

One prototype example combines single-phase jet impingement cooling in the plates

center region with integral hierarchical branching cooling channels to cool the periphery.

The channels radiate rom the devices center, where a single jet impinges, and carry liquid

coolant across the plate to dissipate heat evenly throughout, with minimal pressure loss.

Numerical simulations enabled Dr. Ercan (Eric) Dede, principal scientist in TRI-NAs

Electronics Research Department, and his colleagues to produce the optimized branching

NUMERICAL SIMULATION-BASEDTOPOLOGY OPTIMIZATIONLEADS TO BETTER COOLINGOF ELECTRONIC COMPONENTS

IN TOYOTA HYBRID VEHICLES

ByGARY DAGASTINE

The Toyota Research Instituteo North Americas topologyoptimization team includes

(rom let) Ercan Dede Ph.D.,principal scientist; JaewookLee Ph.D., researcher; andTsuyoshi Nomura Ph.D.,senior principal engineer.

E L E C T R O N I C S C O O L I N G S p e c i a l A d v e r t i s i n g S e c t i o n


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power diodes and power

semiconductors, such as

insulated gate bipolar tran-

sistors (IGBTs), are used or

power conversion and other

applications. These compo-

nents are standard planar

silicon devices measuring

a ew centimeters per side,

with high power dissipation.

In these hybrid vehicles,

they are mounted on alu-

minum heat sinks, or cold

plates, through which a

water-glycol coolant mix-

ture is pumped. In earliermodel years, the cold plate

design eatured a uid inlet

on one side o the plate and

an outlet on the other side;

in between were arrange-

ments o mostly straight

cooling channels through

which the coolant owed.

The long channels pro-

vided adequate heat trans-

er, but it came at the cost

o a signicant pressuredrop across the plate.

The technology road

map or these power com-

ponents, however, calls or

them to shrink to about hal

their current size while dis-

sipating the same amount

o power, meaning that heat

uxes will have to increase.

In addition, although they

have a 150 C maximum

operating temperature, sil-icon devices are normally

kept at lower tempera-

tures or greater component

reliability. Furthermore,

the role o such devices is

becoming more impor-

tant as the electrication o

vehicle systems increases.

All o these actors mean

that the thermal man-

agement o these devices

will become more difcult

than it has been to date.

It might seem reason-

able simply to redesign the

cold plates so that more

coolant can be pumped

through them. But that

would require more pump-

ing power, and with space

already at a premium in theengine compartment where

the pump is located, mov-

ing to a larger, more power-

ul pump or adding an addi-

tional pump is unacceptable.

Instead, Toyota decided

to look at re-engineering

the cold plate with an eye

toward achieving optimum

heat transer along with

negligible additional pres-

sure drop. I both could beachieved, thermal objectives

could be met with no need

to signicantly increase

system pumping capacity.

JET IMPINGEMENT ANINCOMPLETE SOLUTION

MANY RESEARCHERS work-

ing on diverse applications

have identied jet impinge-

One solution to this

problem is to combine jet

impingement with a periph-

eral channel structure to

increase the area-aver-

age heat transer. Its in

your interest to make thosechannels short to keep pres-

sure drop to a minimum,

but short, straight channels

arent efcient enough or

our needs, Dede explains.

Our goal was to come up

with a combination jet

impingement/channel ow

based cold plate with opti-

mally designed branch-

FIGURE 1: Optimal cooling channel topology, with uid streamlines colored blue (let), normalized temper-

ature contours (center), and normalized pressure contours (right).

FIGURE 2: Isometric views o the derived hierarchical micro-channel cold plate, without a jet plate (let) and with a jetplate, which is shown transparent or clarity (right).

E L E C T R O N I C S C O O L I N G

ment as an attractive way

to cool suraces, says Dede.

But while jet impinge-

ment perorms well with

respect to heat dissipa-

tion close to the jet, its less

than optimal as you moveaway rom the orice.

The reason is that the

greatest heat transer occurs

close to the jet entrance,

where the uid is the cool-

est and velocity is the high-

est. As a result, much heat-

transer capability is lost by

the time the coolant reaches

the exit o the cold plate.


10/32

S 10 COMSOL MULTIPHYSICS JUN E 2012 MATLAB is a registered trademark of The MathWorks, Inc.

ing study. Dedes group had

separately perormed such

studies, so his assump-

tions were well inormed.

Ultimately, these

numeric simulations pro-

duced an optimal coolingchannel topology with uid

streamlines in branching

channels (see Figure 1).

Because these chan-

nels efciently distrib-

ute coolant throughout

the plate and create rela-

tively uniorm tempera-

ture and pressure distribu-

tions that are a unction o

branching complexity, this

ractal-like topology wasin turn used to guide the

design o a cold plate pro-

totype (see Figure 2). The

size o the plate was set to

approximately 60 by 45

millimeters, with a mid-

dle cooling zone cover-

ing a 25- by 15-mm area

to match a specic heat

source. The plates base

ing channels to uniormly

remove the most heat with

the least pressure drop.

The CFD and Heat

Transer Modules o

COMSOL Multiphysics

sotware were essential to

the numerical simulations

at the heart o this work.

COMSOLs LiveLink or

MATLAB also enabled

Dede to work with the mul-

tiphysics simulations in a

high-level scripting lan-

guage as he went about

the task o optimizing thecold plates topology.

He examined how topol-

ogy inuenced such vari-

ables as steady-state con-

vection-diusion heat

transer and uid ow. He

did this using well-estab-

lished material interpo-

lation techniques and a

method o moving asymp-

totes (MMA) optimizer,

moving back and orthbetween COMSOL and

MATLAB in an iterative

ashion to investigate cool-

ing channel layouts. (MMA

is a convex-approximation

strategy to aid in optimiz-

ing physical structures.)

Although the aspect

ratio o the channels (i.e.,

the ratio o height to width)

is quite important, to sim-

pliy the numerical sim-ulations Dede assumed

a thin 3-D structure and

then urther attened it.

Once an initial channel

topology was derived, the

height o the ns that sep-

arate the cooling chan-

nels could be investigated

and incorporated with a

separate parametric siz-

FIGURE 3: Prototype aluminum cold plates with (let) and with-out (right) the hierarchical microchannel topology.

FIGURE 4: Comparison o cold plate unit thermal resistance (top) andpressure drop (bottom).

FIGURE 5: Multichip applica-tion (let) and multipass confg-uration or single-chip package.

4X device

4X substrate

4X cooling cell (single pass)

Device Substrate

Cooling cell (multi-pass)

E L E C T R O N I C S C O O L I N G S p e c i a l A d v e r t i s i n g S e c t i o n


11/32


12/32


(TPMS) driven by motion.

TPMSs are tradition-

ally powered by batteries,

they tend to be mounted

on the wheel rim. With

no reliance on a bat-

tery, such a system couldbe placed inside the tire

(see Figure 1) and would

be in a position to mea-

sure much more than

pressure. It could moni-

tor temperature, riction,

wear, and torque; assist

with optimal tracking and

engine control; and con-

vey all this critical inor-

THE DESIRE TO eliminate batteries and power lines is moti-

vating a wide range o research. In the quest or systemsthat are energy autonomous, the concept o energy har-

vesting is attracting a great deal o attention. Combine this

idea with operation at the micro level, and the what i

scenarios become even more enticing.

For researchers at Siemens Corporate Technology in

Munich, exploring the potential o an energy-harvesting

microelectromechanical system (MEMS) generator holds

strong appeal. As Ingo Kuehne, a senior engineer explains,

Our mandate is broad. We are looking to develop platorm

technologies or tomorrow rather than specic products;

MODELINGOPTIMIZES APIEZOELECTRICENERGYHARVESTERUSED INCAR TIRESSiemens is using uid-struc-ture interaction simulationto ensure the cost efective

optimization o a cantileverin a MEMS generatordesigned to power a tirepressure monitoring system.

ByJENNIFER HAND

FIGURE 1: Two TPMS mounting options: on the rim or on the innerlining o the tire.

FIGURE 2: Schematic o the piezoelectric MEMS generator energyharvester. The cantilever is made o two materials, and electricalenergy is transerred through the circuit rom the cantilever.

however, it makes sense to

demonstrate the value o

our research. Together with

our partner Continental

AG, we decided to ocus on

an application with clear

commercial potential. Ourultimate goal is to design

the MEMS generator to be

as small, light, and strong

as possible, with enough

energy to power a sys-

tem under a range o con-

ditions. The researchers

chose to design a microgen-

erator or an innovative tire

pressure monitoring system

TPMS MOUNTING

RIM

TIRETREAD

SHUFFLE

Attherim

valvehole

On inner linerof the tire

CARRIER

PIEZO

CANTILEVER

M E M S E N E R G Y H A R V E S T E R S S p e c i a l A d v e r t i s i n g S e c t i o n


13/32


14/32


M E M S E N E R G Y H A R V E S T E R S

excitation, says Alexander

Frey, a senior engineer.

We had to adopt an uncon-

ventional approach and

avoid mass and its concen-

tration. This in turn gave

us a more serious problem,

because damping becomesmuch more critical.

The big question or the

Siemens team was how

to optimize the design

o the cantilever in order

to minimize damping. It

appeared that air damp-

ing was the dominant

eect, and the aerody-

namic prole was a criti-

cal parameter. A lthough

the cantilever area waslimited to 100 square mil-

limeters, the layer thick-

nesses were design param-

eters that could be reely

changed. We needed

to nd suitable values

or these parameters so

that we could ensure that

the mechanical oscilla-

tion would continue or

FIGURE 4: 2-D simulations o FSI on a cantilevers defec-tion at a gas pressure o 1 bar or various carrier thicknesses.

FIGURE 6: A 3-D FSI simulation showing the defection o the triangular cantilever.

FIGURE 5: 3-D simulations o FSI on a cantilevers defection asa unction o gas pressure, with a carrier thickness o 250 m.



15/32



M E M S E N E R G Y H A R V E S T E R S

as long as possible and

transer as much o the

mechanical energy as

possible to the electrical

domain, says Frey. We

really needed a numeri-

cal tool to determine the

optimal structure and

ensure that enough energy

was being produced.

FLUID AND STRUCTURE:AN OPEN RELATIONSHIPHAVING IDENTIFIED THE

transer o mechanical

energy to the surround-ing air as a critical process,

the team rst conducted a

fuid-structure interaction

(FSI) analysis o the can-

tilever. Kuehne explains:

We started with static sim-

ulations, and these gave us

some initial values. Then

a time-dependent anal-

ysis allowed us to see a

range o physical eects

and understand the impacto the surrounding air on

the damping o the cantile-

ver. (See Figures 3 and 4.)

Members o the team

went on to conduct a 3-D

FSI simulation and to con-

sider the cantilever defec-

tion as a unction o exter-

nal pressure and carrier

thickness (see Figures 5

and 6). They examined the

maximum stress required

or initial defection at

each thickness. With this

analysis, Frey says, we

conrmed quantitatively

that increasing the thick-

ness o the cantilever led

to an improvement in

the damping behavior o

the MEMS harvester.

OPTIMIZING THE CANTI-LEVERS SIZE AND SHAPE

WITH COMSOL Multiphysics

simulation sotware, we

learned how to numeri-cally describe the behav-

ior o our structure, which

allowed us to conduct

research in the labora-

tory, says Kuehne. In

order to compare the simu-

lated behavior with experi-

ments, the cantilever was

periodically excited, and

the piezoelectric voltage

generated was recorded.

Comparison o the sim-ulation with physical test-

ing revealed that the over-

all damping behavior

was actually higher, says

Kuehne. The obvious

explanation was that we

were losing energy because

o intrinsic losses in the

material. We assumed an

accepted value or this

internal damping, and

out. Testing takes a urther

two months. In particu-

lar, the extra expense o a

clean room inrastructure

results in development costso more than 100,000 or

a single prototype run over

six months. In contrast, you

can measure a 2-D simu-

lation in hours and a 3-D

simulation in days. In that

amount o time it is easy

to simulate the peror-

mance o up to 2,000 di-

erent prototypes within

COMSOL Multiphysics.

Frey concludes:Without COMSOL and the

option o numerical mod-

eling, we would have to

make numerous physical

structures, which would

have been time-consum-

ing and expensive. Instead,

we were able to get on with

the process o optimiz-

ing the MEMS design.FIGURE 7: Prototype o a piezoelectric MEMS energy-harvesting module and the surrounding system.

ater taking these correc-

tion actors into account,

we arrived at the same

results. This reassured us

that our simulation pro-cess with COMSOL was

reliable and that we could

continue to investigate the

perormance o the cantile-

ver using dierent param-

eter values. The team was

then able to move on to

optimizing system com-

ponents and system inte-

gration (see Figure 7).

The use o COMSOL

was critical to the develop-ment o the physical proto-

types. According to Kuehne,

it takes three people our

months to do one techno-

logical run, which typi-

cally consists o one batch

o up to 25 waers. One

run usually results in a

couple o complete proto-

types, depending on lay-

Kuehne holding one o the waers used in the production o theMEMS energy harvester prototypes.

Energy-managementASIC

PiezoelectricMEMSharvester


16/32


and operation o a pro-

cess to retrieve and treat

the INL calcine. The proj-ect is known as the Calcine

Disposition Project (CDP).

HOT ISOSTATICPRESSINGTHE CDP PROCESS will

remove the calcine rom

the storage bins, place it

in stainless-steel contain-

ers, and process it using a

THE IDAHO NATIONAL Laboratory (INL), owned by the U.S.

Department o Energy (DOE), has been engaged since

1992 in the Idaho Cleanup Project (ICP) to clean up and

dispose o radioactive material stored at the site. The

ICP is the result o the INL having received spent nuclear

uels rom reactors all over the world starting in 1952.

From 1953 to 1992, the INL operating contractors recov-

ered unused, highly enriched uranium rom the spent

nuclear uel via a procedure known as uel reprocessing.Fuel reprocessing at this acility consisted o dissolving

the spent uel to generate an aqueous solution consisting o

the dissolved uel cladding, unused uranium, and the s-

sion/activation products. The uranium was then separated

rom the aqueous solution via a solvent extraction technique.

Ater the uranium was separated, the ranate (waste solu-

tion) was temporarily stored in underground tanks; then

it was solidied using a high-temperature drying process

known as calcination. The calcination process produced a

small, granular product (0.30.7 millimeters) known as cal-

cine. Calcination o the uel reprocessing ranate occurred

rom 1962 through 2000 and generated 4,400 cubic meterso calcine that are stored in several large storage bins.

The INL calcine contains the bulk o the ssion and acti-

vation products originally in the spent uel. As a result, it is

highly radioactive and is classied as high-level waste. In

addition, the calcine contains some Resource Conservation

and Recovery Act (RCRA) metals. The combination o

radionuclides and RCRA metals makes calcine a mixed

waste. Some o the radionuclides and RCRA metals in the

calcine are in a leachable orm, and thereore this calcine

is not in an acceptable state or waste disposal. Prior to

A SOLUTION TO TREATINGNUCLEAR WASTE COMES VIAMODELING AND SIMULATIONSimulations enable nuclear waste solutionto come faster and cheaper than expected

ByDEXTER JOHNSON, PROGRAM DIRECTOR, CIENTIFICA & BLOGGER, IEEE SPECTRUM ONLINE

FIGURE 1: Representation o HIP Can lledwith Calcine or COMSOL Analysis.

N U C L E A R W A S T E T R E A T M E N T S p e c i a l A d v e r t i s i n g S e c t i o n

disposal, the calcine must

be retrieved rom its cur-

rent storage location andtreated so as to immo-

bilize the radionuclides

and RCRA constituents.

Currently, DOE has con-

tracted with CH2M-WG

Idaho, LLC (commonly

known as CWI) to per-

orm conceptual design

and test work to support

the uture construction

Cooling jacket

Pressure vessel

Pressurized gas

Heater

HIP can

HIP can

protective cage

Thermocouple


17/32



hot isostatic press (HIP).

The HIP process creates

a glass-ceramic (mineral-

like) waste orm that is sta-

ble, immobilizes the radio-

nuclides and RCRA metals,

and can be placed in a dis-

posal repository with min-

imal risk to the environ-

ment. The HIP process is

airly common in manu-

acturing. It simultane-

ously applies heat and pres-

sure to powdered materials

like metals and ceramics

to create various shapesthat are dicult or impos-

sible to orge or cast.

The CDP is compli-

cated by signicant time

and budget constraints.

The CDP aces dead-

lines imposed by a settle-

ment agreement between

the state o Idaho and

DOE to have the calcine

processed and ready to

leave the state by 2035.At this stage o the CDP,

a stainless-steel container

must be designed that

can hold the radioactive

calcine and then be sub-

jected to the HIP process.

The resulting container

is reerred to as the HIP

can among the engineers

working on the project.

TESTING WITH ARADIOACTIVE MATERIAL

ITS QUITE DIFFICULT to

actually work with the

radioactive calcine. There

are obvious risks and costs

involved in handling it,

making it very dicult

to veriy whether the HIP

process will be efective

by working directly with

the radioactive material.

This is why CDP has

adopted a virtual test-

ing program, validated by

physical tests with surro-

gate calcine, which departs

rom traditional meth-

ods o testing and design,explains Vondell J. Balls,

project engineer with the

CDP. Beore the advent

o high-perormance com-

puters and sophisticated

analysis programs, engi-

neers would have an idea

and then go to a shop to

build it, test it, and break

it and then go back into

the design shop and make

changes and then go back

out again to build and test

it until they iterated onto

their nal design. What

were doing or the HIP can

development and design is

to use COMSOL and other

analysis sotware as our

virtual test platorm. We

model, simulate, and test

virtually, and then we per-

orm physical tests with

surrogate calcine to veriy

and validate the model.

Because the radioactivematerial is so cost-prohib-

itive to work with, the rst

ull-scale HIP can con-

taining the INLs radioac-

tive calcine will probably

be created when the HIP

plant rst comes on line

10 to 15 years rom now.

Currently, the benchmark-

ing that is done involves

nonradioactive simulated

calcine, which permitsaccurate predictions about

the treatment o radioac-

tive calcine beore actual

processing begins. Small-

scale tests with radioac-

tive calcine will then be

perormed to conrm

the results obtained with

the surrogate calcine.

While COMSOLs mod-

eling and simulation sot-

ware allows or testing andverication that would be

dicult in a real-world

environment, it has also

demonstrated that the

design o the HIP contain-

ers can be ar simpler than

was originally anticipated.

When we started this

project a couple o years

ago, experts were telling us

FIGURE 2: Densication and temperature using derivedvolume change coefcient

What weredoing for the

HIP can develop-ment and design isto use COMSOLand other analysissoftware as ourvirtual test platform.VONDELL J. BALLS ,PROJECT ENGINEER,CDP ENGINEER

N U C L E A R W A S T E T R E A T M E N T


18/32


19/32



+ ONLINE: www.comsol.com/electrical

SINCE THE 1960S, spinal

cord stimulation (SCS)

has been used to alleviate

chronic back and leg pain.

The process involves surgi-cally implanting a series o

electrodes, which are used

to apply electrical poten-

tials directly to the spine

(see Figure 1). Although

approximately 30,000 such

procedures are perormed

each year, there is still not

a precise understanding o

SCSs mode o action. SCS

somehow intereres with

the human pain signal-ing circuitry. In the past

15 years, researchers have

begun to develop a more

detailed understanding

o the eects o this stim-

ulation. What makes the

method attractive is that it

is known to have benecial

results without many o the

side eects o long-term

pharmacological treatment.

One o the phenomenaassociated with this treat-

ment is that it remains

eective or many years,

although over the course

o time the stimulation

generally has to be repro-

grammed to modiy the

original parameters. As

early as our to six weeks

ater the electrode is

MODELING SCAR EFFECTS INELECTRICAL SPINAL CORD STIMULATION

B I O M E D I C A L E L E C T R O M A G N E T I C S

Discussing the spinal cord stimulation modeling results. From the let Mr. Kris Carlson, Dr. Jerey Arle,and Dr. Jay Shils. All with the Neuromodulation Group at Lahey Clinic in Burlington, MA.

FIGURE 1: X-ray image o astimulator electrode array onthe spinal cord or treatment ochronic back pain.

BY EDWARD BROWN

implanted, scarring occurs

at the interace o the elec-

trode and the surrounding

tissue. Paradoxically, while

this helps keep the paddle

that holds the electrodes

securely in place, it alters

the electrical characteris-

tics o the system, so thatthe stimulation has to be

reprogrammed. The repro-

gramming is generally done

through trial and error.

Research into this phe-

nomenon was perormed

by Jerey Arle, a neuro-

surgeon with a degree in

computational neurosci-

ence; Jay Shils, who has

a background in electri-

cal engineering and com-

putational neurophysiol-

ogy; and Kris Carlson, who

has expertise in program-

ming and along with Shils

has become an expert in

the use o COMSOL sot-

ware. They are all with the

Neuromodulation Group at

Lahey Clinic in Burlington,

Mass., and have con-

cluded a study based on the

hypothesis that the orma-

tion o relatively higher-resistance scar tissue alters

the impedance seen by the

implanted electrodes, which

in turn alters the pattern o

the electric eld distribu-

tion. It was their thesis that

a 3-D mathematical model

could be used to accurately

predict these changes and

dene the necessary cor-


20/32


B I O M E D I C A L E L E C T R O M A G N E T I C S

are either activated or not, based on

the strength o electrical stimulation,

which is governed by the gradient

o the potential eld. This is signi-

cant because its the axons that carry

the pain control signals to the brain.

The team wanted to learn just how

the implanted electrodes treated the

pain. So part o the process was mod-

eling the circuitry in the spinal cord

and the eects o the electricity on that

circuitry. In order to get to that stage,

they had to understand exactly what

in the spinal cord actually gets stimu-

lated. Thats where the COMSOL sot-

ware came into model the electricelds rom the electrodes them-

selves and all o the tissue character-

istics they pass through, says Arle.

The spinal cord is essentially foat-

ing in cerebrospinal fuid (CSF), which

is in turn surrounded by a tube-like

membrane called the dura. The stim-

ulating electrodes sit outside the dura,

which is tough and electrically resistive.

The dierent materials have very di-

erent conductivities, that o the dura

being low and that o the CSF being acouple o orders o magnitude higher.

To study the electrical environment,

the team created a nite element

model o the gray and white matter in

the cord, dura, cerebrospinal fuid, epi-

dural tissue, scar tissue, and stimu-

lator electrodes. The gradients o the

system are aected by the relative con-

ductivities o these dierent materials.

One reason an accurate model is

required is that the potential eld

can vary along the length o the spi-nal cord. Its possible that at one point

there isnt a high enough potential

gradient to generate an activation

potential at the neuron, while 0.5 or

1 millimeter away you may have that

critical gradient. These variations can

occur or a number o dierent rea-

sons. The electrode geometry may be

dierent; the material may not be

uniorm, or instance, the dura itsel

rective modication o the stimulus

pattern. Reprogramming the stimu-

lation accordingly could then reverse

the deterioration in the perormance

o the treatment that is oten observed.

MODELINGDRAWING ON EXTENSIVE earlier work

done by their group, the three had agreat deal o data that included pre-

cise measurements o the spinal cord

segments and estimations o the num-

bers o neurons in each and the num-

bers o each type o neuron. They were

also able to draw on detailed published

work on the so-called white matter,

(or axonsthe wiring that carries

signals rom neurons in the spine up

to the brain). Their plan was to store

this data as a digital database so that

it could be accessed and manipulated.This database was then used to build a

3-D model o the spinal cord, one that

was much more accurate than any-

thing that had been done in the past.

The basic structure o the model

was built using the SolidWorks CAD

platorm. The SolidWorks model

could then be imported into COMSOL

Multiphysics so as to solve some o the

critical problems encountered with

SCS. The great advantage o COMSOL

is that you can not only import CAD

rom SolidWorks, but you can subse-

quently make changes in these geome-

tries, press a button, and these changes

appear in the COMSOL model with-

out losing any o the settings or mate-rial properties, says Carlson.

COMPUTINGTHE GOAL OF this particular proj-

ect was to examine to what extent

scar ormation aects the electrical

eld distribution between the elec-

trodes and axons. This is important

because the axons running through

the spinal cord (the white matter)


FIGURE 2: The geometry created inSolidWorks (let), imported into COMSOLMultiphysics, yields the most com-plex model o its kind created to date.Most work by medical device compa-

nies is done in 2-D. The 3-D model cre-ated by Lahey Clinic has 432 possible con-fgurations o scar and electrode.

FIGURE 3: View o the stimulator array onthe spinal cord, with scars, electric feld

isopotential, and contour lines. Scar con-ductivity is initially set to that o tis-sue, and anodes (red) properly shieldcathode (blue) feld rom unwantedregions and toward center target.


21/32


might not be uniorm across the cord;

or there may be material such as scar-

ring that could give one area o the

cord a higher resistivity than another.

There are also variations o the poten-

tial eld in the cerebrospinal fuid

along dierent parts o the cord.

Using SolidWorks and COMSOL

together made it very easy to change

the geometry with SolidWorks and

study the resulting changes in con-

ductivities and permittivities with

COMSOL, says Shils. This meant

that the output o the simulation

could be spatially added to a model

that we have in-house o a neural net-work. This gives us a more accurate

understanding o where the action

potentials are occurring in the spi-

nal cord, and given some o the com-

plexities, it was nice to be able to

show that a little change in one place

could really shit the energy gradient.

Carlson explains how the team uses

simulation in its work: We decided

to do very sophisticated geometry,

much more so than anything that had

been done in the past. Not only 3-Das opposed to 2-D, but a much more

accurate prole o the spinal cord.

In the model, we set all the material

parameters. We can play with those

mainly the conductivities, and then the

physicswe change the voltages, pulse

widths, and requencies o the vari-

ous electrodes. So or the scar study

that the two doctors designed a year or

so ago, we have an incredibly sophis-

ticated geometry. There are 64 di-

erent pieces o scar and 64 electrodepositions scattered on the surace o

the spine, and each o those is very

easy to manipulate in the sotware.

Another great eature is that ater we

run the simulation, we perorm a huge

amount o post-processing. With the

graphic eatures, we can run all di-

erent kinds o ltering criteria and

also export the data and perorm ur-

ther post-processing in tools dedi-

cated or the purpose (See Figure 3).

The ormation o scar tissue

changes the playing eld, says Arle.

Usually, the programmer is let not

knowing what the scar looks like

exactly and trying to move the stim-ulation around to get the best treat-

ment or the patient. Now, by add-

ing only a little bit to the model, we

can begin to see the distortion o the

electrical elds caused by the scar

ormation. The procedure is prov-

ing to be extremely eective in imme-

diately relieving pain once the pro-

gramming is on target (See Figure 4).

Its very important that you

understand what youre doing with

COMSOL Multiphysics, says Shils.You have to understand the phys-

ics o what youre usingwhy youre

using a certain model as opposed

to another. The way you choose the

meshing, which COMSOL allows you

to do with great fexibility and pre-

cision, is a critical part o the analy-

sis. You choose the proper elements

and then gure out what the edges

are supposed to be. The next step

is to choose the appropriate equa-

tions, starting points, and meshing.

Mesh quality is o particular impor-

tance, especially around the curves o

the axons, which is where most o the

activation is located. I mesh resolu-tion is inadequate, we could miss the

high points o the eld and gradients.

WHATS NEXT?BY IMPORTING COMSOL data show-

ing which nerve bers red into the

groups own neural circuitry simula-

tion sotware, they intend to unravel

how SCS produces relie rom pain.

Arle sums up his eelings about the

project this way: In biological sys-

tems in human anatomy and physiol-ogy systems, theres not a huge amount

o work done on this kind o thing, as

opposed to more engineering-based

projects. You really need to understand

the anatomy, the physiology, and the

neuroscience, and then ramp this up to

understand the mathematics and the

physics. People are beginning to realize

that you need to take this approach to

really understand what were doing.

FIGURE 4: Slice plot and contour values o electric feld projecting into the spinal cord.Top: Cross section o electrode array on the spinal cord. Let: Control with no scar.Right: Scar under let electrode reduces proper feld symmetry. Contrary to expectation, scartissue can result in higher feld values projecting into the spinal cord and too much stimulation.

SolidWorks is a registered trademark of Dassault Systmes SolidWorks Corp.

S p e c i a l A d v e r t i s i n g S e c t i o n B I O M E D I C A L E L E C T R O M A G N E T I C S


22/32

S 22 COMSOL MULTIPHYSICS JUN E 2012 + ONLINE: www.comsol.com/electrical

THE HUGE ENGINEERING project o migrating the electrical

grid to a smart grid mostly gets discussed in terms o IT

issues or embedded systems, but the orgotten part o thestory is updating the nuts and bolts o the grid.

The issue o modernizing items like transormers, cable

joints, terminations, bushings, and ault current limiters

(FCLs) are critical elements in what may turn out to be one o

the largest engineering projects o the next decade.

These parts o the grid will ultimately prove just as key

to enabling the next-generation smart grid as any other

aspect o it. And though these parts may seem humble

on their own, it in act requires a lot o engineering to get

them right.

UPGRADING THE NUTS ANDBOLTS OF THE ELECTRICALGRID FOR A NEW GENERATIONFrom traditional to emerging technologies, the parts thatmake up the electrical grid are on a continuous path ofimprovement, supported by simulation and modeling tools

ByDEXTER JOHNSON, PROGRA M DIRECTOR, CIENTIFICA

& BLOGGER, IEEE SPECTRUM ONLINE

S M A R T P O W E R G R I D S p e c i a l A d v e r t i s i n g S e c t i o n

Large power trans-ormers, like thisrom ABB, are anexample o thecritical equipmentneeded to distrib-ute electricity inan ecient way.One type o eedbushings can beseen on top o thetransormer tank.IMAGE: COURTESYOF ABB

HIGH-VOLTAGE CABLE JOINTS,TERMINATIONS, AND BUSHINGSITEMS INVOLVED WITH high-voltage

cables, such as cable joints, cable termina-

tions, and bushings, are oten overlooked.

Cable joints are used to connect two

power transmission cables (AC or DC).

Cable terminations are used as end

plugs or a cable that may later be con-

nected to another cable or some added

external equipment.

Finally, bushings are devices that let

conductors pass through a grounded

wall. Bushings prevent ashover or

breakdown when a high-voltage con-

ductor is penetrating a metal wall. Inother words, each part o the grid is

capable o bringing at least part o it

down i its not properly engineered.

The area o bushings and connectors

is a eld that Gran Eriksson, a scien-

tist with ABB AB Corporate Research

Power Technologies in Sweden, has

been addressing in his research.

In particular, Eriksson has been

looking at the problem caused by the

use o increased voltages in mod-

ern transmission systems. The aimo increasing the voltage is to reduce

line current and the resulting resis-

tive loss in the cables.

Unortunately, the straightorward

engineering solution o using larger

equipment to avoid ashover or dielec-

tric breakdown in insulators brings

higher business costs. While there are

always increasing demands or higher

voltages and power ratings, at the

same time there is a strong pressure to

reduce the size and cost o equipment.

ACCOMMODATING BUSINESS ANDTECHNOLOGICAL CONSIDERATIONSTHROUGH DESIGNONE METHOD ENGINEERS have

employed or keeping the size o

transmission systems to a minimum

is the use o so-called eld grading

materials (FGM), which have an elec-

tric conductivity dependent on the


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local electric eld strength.

While employing FGMs more

evenly distributes eld than when no

FGM is used, it is still necessary to

ollow a careul and detailed optimi-

zation procedure to keep the cost and

size o the insulation to a minimum.

When designing joints, terminations,

and bushings correctly, problems arisethat are both electrical and thermal in

nature, according to Eriksson. (Figures

1-5 illustrate the diferent coupled phe-

nomena involved in the simulation o

an oil cooled DC bushing.)

In all cases, there is a large poten-

tial diference between the inner high-

voltage conductor and the end o the

grounded cable shield or the grounded

metal wall, explains Eriksson. Very

high electric elds are created that

could result in a ashover or breakdowni no measures are taken (Figure 5).

With the high eld and current lev-

els, there will also be substantial resis-

tive heating in these devices (Figure 2).

In many cases, it is cable joints, termi-

nations, and bushings that are the most

stressed components in a transmission

system, and their reliability is thereore

crucial or overall perormance.

The complexity o the problem

necessitates the use o simulation and

modeling tools, according to Eriksson.

There is a strong connection among

electromagnetic, thermal, and uid

phenomena in the behavior o these

systems, so the physics o the systems

become quite involved.

The physics are very complex

and truly multiphysical, explainsEriksson. Many o the material

parameters are dependent on the

local electric eld strength and the

local temperature.

The electrical and thermal prob-

lems are thereore strongly coupled.

In addition, the thermal problem is

requently coupled to the equations

describing the ow o a cooling liquid

or gas, which transports and removes

the heat generated inside the device

(Figures 3-4). For very large, high-voltage bushings there may also be

mechanical considerations involved.

With so many material and geo-

metrical parameters involved, nding

an optimized solution by experimen-

tal prototyping and testing becomes

practically impossible, besides

becoming ar too costly and time-con-

suming, says Eriksson. By employ-

ing simulations instead, its possible

S M A R T P O W E R G R I D

FIGURE 1: Shows the axisymmetricgeometry. The main current is owingalong the inner high voltage conduc-tor (red). The interrupted groundedshield o the connected cable ismarked with black while blue denotesthe metallic oil container and the wall(both grounded). The FGM layer isshown as purple and grey denotesvarious non-ideal insulating materi-als. Finally, the upper boundary o the(yellow) oil volume connects this vol-

ume to a much larger oil container.An open boundary condition or theuid ow is thereore applied there.

to make ull-parameter optimizations

and to evaluate proposed design con-

cepts in a short time.

The results obtained by using

COMSOLs Multiphysics tool to improve

the bushings have been dramatic.

The component size can be signi-

icantly reduced compared to when

noor only simpliedsimulationsare carried out, says Eriksson. Also,

the occurrence o any unwanted elec-

trical and thermal hot spots, which

tend to reduce reliability, can be bet-

ter predicted and kept under control.

In cases, measurement o phys-

ical prototypes is not a realistic

FIGURE 2:Plots theresistive lossdistributioninside thehigh volt-age conduc-tor. This dis-tribution isthen used asa heat source

input in theheat balanceequation.

FIGURE 3:Displays thetemperaturedistributionin the device(color plot),the conductiveheat ow pathsin the solids(blue) and theconvective heat

ow in the oiluid (arrowplot).

By employingsimulations,

its possible to makefull-parameter optimizationsand to evaluate proposeddesign concepts in ashort time.GRAN ERIKSSON, ABB ABCORPORATE RESEARCHPOWER TECHNOLOGIES

SolidsGroundedshield


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S M A R T P O W E R G R I D

option, according to Eriksson. This

is because o the large associated

costs in terms o time, money, and lab

resources. In act, some important

parameters may not even be accessi-

ble using only measurements.

To quantiy just how much impact

the use o simulations can have on anengineering issue, in a similar case

Eriksson encountered it was possible

to reduce the size o the eed-through

device by almost 30 percent compared

with the original design proposal.

SUPERCONDUCTINGFAULT CURRENT LIMITERSWHILE ENGINEERS ARE improving the

traditional parts o the grid, like the

joints, terminations, and bushings

o electrical cables, work is also pro-gressing on bringing emerging tech-

nologies into the grid.

One area where the power utilities

would like to nd an improved solu-

tion is in ault currentlimiting (FCL)

devices, which respond to the condi-

tion o the system and insert increased

impedance in the event o a ault.

FCLs protect electrical equipment

and the grid inrastructure rom ault

currents caused by short circuits, which

typically result rom lightning strikes.

The simplest condition-based FCL

device is a use, explains Dr. Michael

Mischa Steurer, a scientist at the

Center or Advanced Power Systems at

Florida State University (FSU-CAPS).

The major disadvantage o a use, ocourse, is that it has to be replaced

when blown in order to restore power

ow on the afected circuit.

The solution o uses and use-

based devices also runs into problems

because they are not readily avail-

able or voltages much above 36 kilo-

volts. Its because o this that there is a

strong interest by the utility industry

in the development o condition-based

FCLs, which reset by themselves, pre-

erably under load current ow.A possible solution or develop-

ing FCL devices has been the appli-

cation o superconducting materials.

According to Steurer, most supercon-

ducting ault current limiters (SFCLs)

exploit the substantial resistance

increase o the superconductor when

the transport current, the external

magnetic eld, and/or the temperature

exceed their respective thresholds.


FIGURE 4:The velocityamplitude isillustrated bycolors togetherwith the owstreamlines. Thestrong upward

uid ow isgenerated by theheated solid parts.

FIGURE 5: Shows theelectric feld ampli-tude (color plot) andthe equipotentialcurves (white) dueto the potential di-erence between thehigh voltage conduc-tor and the groundedparts. From such a

plot one can easilyidentiy areas withtoo high stress levels.

OBSTACLES TO WIDERADOPTION OF SFCL TECHNOLOGYTHE DISCOVERY OF high-tempera-

ture superconductors (HTSs) ush-

ered in a period o intense excitement

and optimism in the development o

superconductor-based applications.

Nevertheless, even with HTSs the

challenge o developing a cost-efec-

tive SFCL solution has proved to be

daunting, and progress toward com-

mercialized devices has been slow.

One key challenge to SFCL adoption

that remains is the associated cost o

cooling. Usually, liquid nitrogen (LN2)

acts as a coolant. Heat inux rom theambient and losses in the SFCL (e.g., in

copper leads, AC losses in the super-

conductor or substrate, and core losses

i the core is in contact with LN2 and is

penetrated by the magnetic eld) cause

some LN2 to boil of. This requires

LN2 rells or reliqueaction.

In order to appreciate the other

technological hurdles that SFCLs ace,

one has to discuss the main SFCL

technologies.

SFCLs may be classied intoquench and nonquench types, accord-

ing to Steurer. A quench-type FCL

ofers efectively zero impedance due

to a superconducting state under nor-

mal power system conditions. But

when there is increased current ow

in the power system due to a ault,

impedance increases because the

superconducting FCL quenches

transitions rom a superconducting to

a resistive state.

Steurer adds that there is a subset oquenching FCLs called resistive FCLs.

These come in various packages in

which the superconductor carries the

network current, and there must be

power leads into and out o the cryo-

genic tank where the superconductor

is housed.

As one might suspect, it is a chal-

lenge to keep heat rom conduct-

ing into the cold environment, says

Equipotential curves


25/32


Tim Chiocchio, a research assistant at

FSU-CAPS. Another challenge comes

rom the act that the resistive SFCL

initiates its current limitation through

the quenching o its superconductor.

Another type o SFCL, the satu-

rated iron core SFCL, acts like a vari-

able inductor. The superconductor

does not quench but is employed as

a DC magnet that saturates the iron

core during normal operation. With

the iron core in saturation, the induc-

tance is small, but it becomes signi-

icantly larger as high ault current

drives the core into the linear region

o the iron cores characteristic mag-netizing curve.

One issue with this technology is

preventing transient currents rom

being induced in the DC magnet.

Another challenge is minimizing the

weight and size o the iron core while

maintaining the required reactance

under system ault conditions.

The shielded iron core SFCL also

acts like a variable inductor. During

normal operation the superconduc-

tor acts as a magnetic shield, prevent-ing the iron core rom exposure to

the magnetic eld o the AC windings

connected to the grid. In the event

o a ault, the magnetic eld exceeds

the critical eld o the superconduc-

tor, and this leads the superconduc-

tor to quench. The superconductor

then ceases to behave as a shield, and

inductance rises sharply as the mag-

netic eld reaches the iron core.

As with the use o higher voltages

in transmission systems, the issuesare not always technological. They

can be business-oriented as well.

Perhaps the biggest universal chal-

lenge is to compete with more tra-

ditional approaches such as cur-

rent-limiting reactors, or CLRs, says

Chiocchio. It is important to keep

costs low and to provide a signicant

perormance advantage with respect

to CLR-based solutions.

MODELING AND SIMULATION IS ACRITICAL TOOL IN SFCL DEVELOPMENT

A TE AM OF researchers at FSU-CAPS

unded by Bruker Energy & Supercon

Technologies (BEST) is trying to over-

come the major design challenges ac-

ing SFCLs in order to bring them to

the high-voltage grid.

The collaboration agreement

between BEST and FSU-CAPS is

ocused specically on urther devel-oping BESTs shielded iron core induc-

tive ault current limiter (iSFCL).

Computer modeling and simula-

tion o the devices behavior have been

indispensable tools in this work. The

multidisciplinary aspects o the system,

including the iSFCL and the electrical

grid with all the disparate components

that make it up, demand a multiphys-

ics environment in which to carry out

the simulations and modeling.

Devices such as the iSFCL are

embedded in a power system con-

sisting o power lines, transormers,

rotating machinery, capacitor banks,

circuit breakers, and surge arrestors,

says Dr. Lukas Graber, a postdoctoral

research associate at FSU-CAPS. It is

important to model the iSFCL in the

appropriate environment, i.e., cou-

pling a model o the power system

with the nite element analysis, or

FEA, model. COMSOL Multiphysics

lets us couple electric circuitsresis-

tors, capacitors, inductors, and

sourceswith electromagnetic FEA.Graber was impressed with how

easy it was to couple an electromag-

netic FEA with an electric circuit. A

tutorial rom the COMSOL model

library helped him understand and

implement this type o coupling.

Also very impressive was the act

that the simulation model awlessly

converged to a correct solution even

though it included a domain with

almost zero electrical resistivity1015

ohmmeters in the superconductor,says Graber. I expected numerical

problems with a model that includes

such extremely low resistivity.

The FSU-CAPS team published its

model at the COMSOL Conerence

2011, which included a model o a

benchtop FCL integrated with an

equivalent circuit o the driving

power electronic inverter and the out-

put transormer.

Graber says the team will use the

setup in uture tests to do in-the-looppower hardware experiments. The

researchers would also like to use

modeling to explore more complex

congurations o SFCLs and to opti-

mize geometries and dimensions. This

will let them simulate the conditions

a real SFCL would see in the power sys-

tem. Again, COMSOL should allow us

to implement an even more complex

equivalent circuit, says Graber.

FIGURE 6:(Top) Simulation model showingthe magnetic ux density o the bench-topault current limiter under normal operation.

(Bottom) Same but under ault condition.

S p e c i a l A d v e r t i s i n g S e c t i o n S M A R T P O W E R G R I D


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S 26 COMSOL MULTIPHYSICS JUN E 2012 + ONLINE: www.comsol.com/electrical

WITH COALS ABUNDANCE and relatively low cost,

it has become the primary source o electric-

ity generation around the world. Global coal

demand has almost doubled since 1980, driven

mainly by increases in Asia, where demand

rose by more than 400 percent rom 1980 to

2010. In the United States, coal is used to gener-

ate about hal o the electricity and remains the

largest domestically produced source o energy.

A natural result o burning coal is the

emission o y ash, consisting o ne parti-

cles derived rom mineral matter in the uel.

Increasingly strict emission and environmen-

tal standards dictate that virtually all o the

dust resulting rom coal combustion must be

removed. Particle emission limits in the rangeo 1030 mg/m3 in the exiting ue gas are com-

mon today. Nearly all power plants and many

industrial processes employ either electrostatic

precipitators (ESPs) or abric lters to sepa-

rate these particles rom the ue gas. ESPs

are popular, due to their low operating and

maintenance costs, as well as their robust-

ness towards process variations (see Figure 1).

Particle removal eciencies o 99.9 percent are

common, and the world ESP industry has an

annual turnover o several billion U.S. dollars.

THE PRINCIPLES AT WORKAN ES P US ES electrical orces to remove particles

rom the ue gas. High-voltage discharge elec-

trodes, typically operating at 70100 kV, pro-

duce a corona discharge, which is an ioniza-

tion o the gas in the vicinity o the discharge

electrode. The ions then ollow the electric eld

lines and attach themselves to airborne parti-

cles in the ue gas that ows through the ESP,

essentially charging them. The charged par-

ticles then migrate in the electric eld and are

collected on grounded metal plates, called col-lecting electrodes, where they build up to orm

a dust cake, which is periodically cleaned of.

An ESP typically consists o rames with dis-

charge electrodes placed between large metal

curtains, acting as collecting electrodes (see

Figure 2). The exterior dimensions o an ESP

can be as large as 50 by 50 by 25 meters, divided

into many independently, energized sections.

Increasing the ESP collection eciency,

reducing power consumption, and optimiz-

F L U E G A S C L E A N I N G S p e c i a l A d v e r t i s i n g S e c t i o n

NUMERICALMODELING OFELECTROSTATICPRECIPITATORSDespite all the press given to alternativesources o power, we still rely heavily oncoal or energy production. Alstom designselectrostatic precipitators that are used toclean fue gas, and researchers are usingboth experiments and simulations to studyand optimize these units.

ByANDRE AS BCK, ALSTOM POWER SWEDEN AB,AND JOEL CRAMSK Y, ALVELID ENGINE ERING AB

FIGURE 1: A recently commissioned Alstom electrostatic precipitator placed ateran 850 MW lignite-red boiler in Poland.


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ing the design rom a cost

perspective are part o

the work at Alstom Power

Sweden AB. The Alstom

technical center in Vxj

serves as the global R&D

execution center or the

companys studies o envi-

ronmental control tech-

nologies, including par-

ticle separation, ue gas

desulurization, catalytic

NOX conversion, and CO2

abatement. ESP devel-

opment has traditionally

been an experimental andempirical science, although

some numerical studies on

selected precipitator phe-

nomena have also attracted

interest. With COMSOL

Multiphysics, it was easy

and straightorward to cre-

ate mathematical mod-

els that provided a deeper

and more detailed under-

standing o the behavior

occurring inside the ESP.

MODELING THE ESPTO ACHIEVE THE mechanical

stability required or a tall

collecting plate, it must be

proled or shaped. Because

the electric eld strength

at the plate surace deter-

mines when a spark-over

(short-circuiting) occurs,

it is very important to

have smooth curvaturesthat do not create points

o exceptionally high eld

strength. We studied this

us

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