highly integrated heat exchangers for automotive ... integrated... · outline • introduction •...

Highly Integrated Heat Exchangers for Automotive

Thermoelectric Generators (TEG)

Methodical functional integration and numerical analysis

of TEG heat exchangers

www.DLR.de • Chart 1 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

Institut of Vehicle Concepts

M. Kober H. Friedrich

Outline

• Introduction

• Methodical concept development acc. to VDI Guideline 2221

• Module structure used for functional integration

• Comparison between three heat exchanger approaches

• Numerical and analytic analysis with focus on

• Fin buckling

• Reduction of thermomechanical stress

• Homogenisation of contact pressure


Evolution of TEG at DLR


Introduction

Why use high temperature TE-Materials?

• Comparison between Bithmuth Telluride and Skutterudite

• Exemplarily Materials with zTmax = 1

0,0

0,1

0,2

0,3

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1,0

1,1

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

50%

55%

100 200 300 400 500

Figu

re o

f M

erit

zT

Effi

cien

cy η

[%

]

Hot Side Temperature T_h [°C]

η_Carnot

η_ex Bi2Te3

η_overall Bi2Te3

η_ex CoSb3

η_overall CoSb3

zT Bi2Te3

zT CoSb3

heiß

kalt

n p

+-

- +

Kühlmittel

Abgas

heiß

kalt

n p

++-

- ++

Kühlmittel

Abgas

TSU

𝑧𝑇 =𝑆2 ∗ 𝜎

𝜅∗ 𝑇

𝜂max = 𝜂Carnot ∗ 𝜂ex

𝜂Carnot =𝑇h − 𝑇c

𝑇h

𝜂ex =1 + 𝑧𝑇𝑚 − 1

1 + 𝑧𝑇𝑚 + 𝑇c/𝑇h


0,0

0,1

0,2

0,3

0,4

0,5

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0,9

1,0

1,1

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

50%

55%

100 200 300 400 500

Figu

re o

f M

erit

zT

Effi

cien

cy η

[%

]


η_Carnot

η_ex Bi2Te3

η_max Bi2Te3

η_ex CoSb3

η_max CoSb3

zT Bi2Te3

zT CoSb3

Introduction


Cold Side Temperature T_c = 100°C


𝜅∗ 𝑇



𝑇h

𝜂ex =1 + 𝑧𝑇𝑚 − 1


heiß

kalt

n p

+-

- +

Kühlmittel

Abgas

heiß

kalt

n p

++-

- ++

Kühlmittel

Abgas

TSU

• Higher efficiency at high temperatures mainly through

higher Carnot efficiency

• zTmax=1 leads to an exergy efficiency 𝜂ex ~ 17%


0,0

0,1

0,2

0,3

0,4

0,5

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0,8

0,9

1,0

1,1

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

50%

55%

100 200 300 400 500

Figu

re o

f M

erit

zT

Effi

cien

cy η

[%

]


η_Carnot

η_ex Bi2Te3

η_max Bi2Te3

η_ex CoSb3

η_max CoSb3

zT Bi2Te3

zT CoSb3

Introduction


• Higher efficiency at high temperatures mainly through

higher Carnot efficiency

• zTmax=1 leads to an exergy efficiency 𝜂ex ~ 17%

Cold Side Temperature T_c = 100°C


𝜅∗ 𝑇



𝑇h

𝜂ex =1 + 𝑧𝑇𝑚 − 1


heiß

kalt

n p

+-

- +

Kühlmittel

Abgas

heiß

kalt

n p

++-

- ++

Kühlmittel

Abgas

TSU


Procedural method VDI Guideline 2221


DLR – test vehicle

BMW 535i

3l, 6 cylinder, spark ignition

190kW @ 6600 1/min

A B C

210mm 400mm 440mm290mm 170mm 270mm190mm 150mm 170mm

speed / gear gas mass flow [g/s] λ

50km/h / 3 18,9 1,6 484 430 400

100km/h / 5 22,9 1,0 624 538 504

135km/h / 6 37,5 1,0 727 647 607

installation space

thermal boundary conditions

gas temperatures [°C]

length width height

List of requirements e.g. Vehicle boundary conditions


1) Kober, M. ; Häfele, C. ; Friedrich, H. E. (2012) Methodical Concept Development of Automotive Thermoelectric Generators (TEG). 3. International Conference 'Thermoelecrics goes Automotive', 2012, Berlin, Deutschland.

0

100

200

300

400

500

600

700

800

900

1000

Ab

gast

em

pera

ture

n [

°C]

Volllast 126 g/s

160 km/h, 6.Gang 55 g/s

145 km/h, 6.Gang 45 g/s

135 km/h, 6.Gang 39 g/s

120 km/h, 6.Gang 28 g/s

100 km/h, 6.Gang 23 g/s

70 km/h, X.Gang 17 g/s

50 km/h, 5.Gang 9 g/s

List of requirements e.g. Gas temperatures along exhaust system

Gas temperatures along exhaust system at different steady state driving

conditions with replaced NOx-catalyst.

Exh

au

st ga

s te

mp

era

ture

s [°C

]

Full load 126 g/s

160 km/h, 6. Gear, 55 g/s

145 km/h, 6. Gear, 45 g/s

135 km/h, 6. Gear, 39 g/s

120 km/h, 6. Gear, 28 g/s

100 km/h, 6. Gear, 23 g/s

70 km/h, X. Gear, 17 g/s

50 km/h, 5. Gear, 9 g/s



1)

Interactions of TEG and vehicle system

electrical TEG input power

( )

back pressure / cooling of exhaust

( )

cooling load ( )

(el. power for cooling water

pump and cooling fan)

rolling resistance ( )

(weight increase) roP

prP

coPinP


TEG concept development – Function structure

exhaust +

heat

coolant

feed

exhaust

distribute heat

smoothly

heat

transfer

dissipate

coolant

dissipate

electric energy

dissipate

exhaust

exhaust +

heat

coolant +

heat

feed

coolant

electric

energyconduct

heat

convert

heat

conduct

heat

dissipate heat to

coolant

provide

force

distribute contact

pressure smoothly



1)

1 2 3 4 5 6 7

feed/dissipate exhaust

heat transfer

conduct heat

distribute heat smoothly

dissipate electric energy

conduct heat

feed/dissipate coolant

provide force

distribute contact pressure

smoothly

sub-functions

sub-solutions

A2 E1 A4 B2 B1 C1 A1 D1 A3

TEG concept development – Sub-solutions

A4 1) Kober, M. ; Häfele, C. ; Friedrich, H. E. (2012) Methodical Concept Development of Automotive Thermoelectric Generators (TEG). 3. International Conference 'Thermoelecrics goes Automotive', 2012, Berlin, Deutschland.


1)

-1,9

2

-0,5

4

-0,9

5

-2,9

8

-0,7

9

-1,4

4

-4,0

-3,5

-3,0

-2,5

-2,0

-1,5

-1,0

-0,5

0,0

0,5

1,0

A B C

Wel

len

leis

tun

gsä

nd

eru

ng

[%

]

Stre

cken

verb

rau

chsä

nd

eru

ng

[%

]. A4

-0,3

8 -0,0

9

-0,5

5 -0,1

2

nic

ht

in B

au

rau

m B

inte

gri

erb

ar

-4,0

-3,5

-3,0

-2,5

-2,0

-1,5

-1,0

-0,5

0,0

0,5

1,0

A B C

Wel

len

leis

tun

gsä

nd

eru

ng

[%

]

Stre

cken

verb

rau

chsä

nd

eru

ng

[%

].XXXXXXXX XXXXXXXX

XXXXXXXX XXXXXXXX

XXXXXXXX XXXXXXXX

B1

*EineP%

*KüeP%

addSB%

DreP%

RoeP%

optSB%

Change o

fsh

aft

pow

er

[%]

Change o

ffu

elc

onsu

mption

[%]

Δ%Fadd Δ%Fopt

Δ%Pin

Δ%Pco

Δ%Ppr

Δ%Pro

no

t in

teg

rable

in in

stalla

tion

po

sition

B

Overall system simulations Results for design point 135 km/h

-1,9

2

-0,5

4

-0,9

5

-2,9

8

-0,7

9

-1,4

4

-4,0

-3,5

-3,0

-2,5

-2,0

-1,5

-1,0

-0,5

0,0

0,5

1,0

A B C

Wel

len

leis

tun

gsä

nd

eru

ng

[%

]

Stre

cken

verb

rau

chsä

nd

eru

ng

[%

]. A4

-0,3

8 -0,0

9

-0,5

5 -0,1

2

nic

ht

in B

au

rau

m B

inte

gri

erb

ar

-4,0

-3,5

-3,0

-2,5

-2,0

-1,5

-1,0

-0,5

0,0

0,5

1,0

A B C

Wel

len

leis

tun

gsä

nd

eru

ng

[%

]

Stre

cken

verb

rau

chsä

nd

eru

ng

[%

].

XXXXXXXX XXXXXXXX

XXXXXXXX XXXXXXXX

XXXXXXXX XXXXXXXX

B1

*EineP%

*KüeP%

addSB%

DreP%

RoeP%

optSB%

Chan

ge o

fsh

aft

pow

er

[%]

Chan

ge o

ffu

elc

onsu

mption

[%]

Δ%Fadd Δ%Fopt

Δ%Pin

Δ%Pco

Δ%Ppr

Δ%Pro

no

t in

teg

rable

in in

stalla

tion

po

sition

B

Legend:

Change in fuel consumption

additional system

TEG as add on

optimized total

vehicle system

Effect on basic shaft power

el. TEG input

back pressure

cooling load

rolling resistance

optF%

addF%

inP%

prP%

coP%

roP%

A B C

Design A4


1)


?

How can functional integration be done

to reduce the TEG weight

and thermomechanical stress?

?


Module structure (acc. to VDI 2221)

for functional integration within heat exchangers


exhaust +

heat

coolant

feed

exhaust

distribute heat

smoothly

heat

transfer

dissipate

coolant

dissipate

electric energy

dissipate

exhaust

exhaust +

heat

coolant +

heat

feed

coolant

electric

energyconduct

heat

convert

heat

conduct

heat

dissipate heat to

coolant

provide

force

distribute contact

pressure smoothly

Module structure (acc. to VDI 2221)

for functional integration within heat exchangers

functional integration of thermal and mechanical functions

within the hot gas heat exchanger


Module structure

Hot gas heat exchanger

𝑚 hot gas

Thermoelectric Generator

(TEG)

Module structure (acc. VDI Guideline 2221)


2D - Simulation model


with TEM

Thermoelectric Module (TEM)



Analytic analysis of fin buckling

• buckling formulation:

𝐹 = 𝜋²∗𝐸∗𝐼

𝐿² -> 𝑝 =

𝜋²∗𝐸∗𝐼

𝐿²∗𝐴

factor of safety s = 3

analytic pressure p = 5 MPa

• Variation of

• fin distance

• fin thickness

• heat exchanger height (fin leght)


p = 5MPa

Analytic analysis of fin buckling p = 5MPa

0,05

0,10

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0,25

0,30

0,35

0,40

0,45

0,50

0,55

4 6 8 10 12 14 16 18 20

fin

th

ickn

ess

[m

m]

heat exchanger height [mm]

fin distance 1mm

fin distance 1,5mm

fin distance 2mm

fin distance 2,5mm

fin distance 3mm

Compromises for functional integration between

thermal and mechanical functions are too high

• Thermal functions:

thin fins required

• Mechanical functions:

thick fins required


Approaches

• Homogenisation of contact

pressure

• Reduction of thermomechanical

stress at thermoelectric modules

(TEM)

• Integration of thermal and

mechanical functions within the

heat exchanger fins

1.) Design with Reinforcements

2.) Oval Design

3.) Functional Integration

(Two or more levels of fins – Approach of this work)

1) Patent: DE102010042603 A1 2) Bürkle, A. ; et al. : Numerical optimisation of contact pressure with respect to the heat exchange properties of a thermo-electric generator. 2. International Conference 'Thermoelecrics goes Automotive', 2010, Berlin, Deutschland.

2)


1)

Numerical Analysis

Procedure

• Quarter of a heat exchanger

module structure is simulated

• Symmetry in x- and

y-direction

• Constant pressure or

deformation load

Goals

• Avoidance of fin buckling

• Homogeneous contact pressure

• Min. contact pressure of 1MPa

• Low mechanical stress at the TEM

X Y

p [MPa] or ΔH [mm]


Results - Design with Reinforcements

• Result: Inhomogeneous contact pressure

Contact pressure < 1 MPa

High local stress at TEM

p = 5MPa


ΔH = 1,7mm

Results - Oval Design


• Fins do not homogenise the contact pressure

significantly by reason of buckling

Analysis simplification: modeling without fins

• Result: Inhomogeneous contact pressure

Contact pressure < 1 MPa

High local stress at TEM

p = 5MPa

Results - Functional Integration

• Reduction of fin length through two fin layers

Thermal and machanical functions integrated

within the fins without functional compromises

• Result: Homogeneous contact pressure

Contact pressure > 1 MPa

Low stress at TEM


Analytic analysis of fin buckling

in multilayer fin structures

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0,45

0,50

0,55

4 6 8 10 12 14 16 18 20

fin

th

ickn

ess

[m

m]

heat exchanger height [mm]

fin distance 1mm

fin distance 1,5mm

fin distance 2mm

fin distance 2,5mm

fin distance 3mm

fin distance 1mm

fin distance 1,5mm

fin distance 2mm

fin distance 2,5mm

fin distance 3mm

• Required fin thickness in a two layer fin structure:


p = 5MPa

Summary

• Functional integration by using the module structure of VDI Guidline 2221

• Comparison of three approaches to homogenise contact pressure

• Multilayer fins is the only approach that achieve the requirements:

• Homogeneous contact pressure

• Low mechanical stress at TEM

• Successful integration of thermal/mechanical functions within heat exchanger


Institut of Vehicle Concepts Pfaffenwaldring 38-40 70569 Stuttgart Dipl.-Ing.(FH) Martin Kober Phone: 0049 - 711 6862 -457 [email protected] www.DLR.de/fk

Acknowledgement

This work is supported by the Ministry of Finance and Economics of

Baden-Württemberg by funds of the Baden-Württemberg Stiftung.

Thank you for your attention!


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