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www.DLR.de Chart 1 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013 Highly Integrated Heat Exchangers for Automotive Thermoelectric Generators (TEG) Methodical functional integration and numerical

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SLIDE 1

Highly Integrated Heat Exchangers for Automotive Thermoelectric Generators (TEG)

Methodical functional integration and numerical analysis

  • f 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
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SLIDE 2

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

www.DLR.de • Chart 2 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

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SLIDE 3

Evolution of TEG at DLR

www.DLR.de • Chart 3 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

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SLIDE 4

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 0,4 0,5 0,6 0,7 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

Figure of Merit zT Efficiency 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

T S U   

𝑨𝑈 = 𝑇2 ∗ 𝜏 𝜆 ∗ 𝑈 𝜃max = 𝜃Carnot ∗ 𝜃ex 𝜃Carnot = 𝑈h − 𝑈c 𝑈h 𝜃ex = 1 + 𝑨𝑈

𝑛 − 1

1 + 𝑨𝑈

𝑛 + 𝑈c/𝑈h

www.DLR.de • Chart 4 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

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SLIDE 5

0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 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

Figure of Merit zT Efficiency η [%] Hot Side Temperature T_h [°C]

η_Carnot η_ex Bi2Te3 η_max Bi2Te3 η_ex CoSb3 η_max CoSb3 zT Bi2Te3 zT CoSb3

Introduction Why use high temperature TE-Materials?

Cold Side Temperature T_c = 100°C

𝑨𝑈 = 𝑇2 ∗ 𝜏 𝜆 ∗ 𝑈 𝜃max = 𝜃Carnot ∗ 𝜃ex 𝜃Carnot = 𝑈h − 𝑈c 𝑈h 𝜃ex = 1 + 𝑨𝑈

𝑛 − 1

1 + 𝑨𝑈

𝑛 + 𝑈c/𝑈h

heiß kalt

n p +

  • +

Kühlmittel Abgas heiß kalt

n p +

  • +

Kühlmittel Abgas

T S U   

  • Higher efficiency at high temperatures mainly through

higher Carnot efficiency

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

www.DLR.de • Chart 5 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

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SLIDE 6

0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 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

Figure of Merit zT Efficiency η [%] Hot Side Temperature T_h [°C]

η_Carnot η_ex Bi2Te3 η_max Bi2Te3 η_ex CoSb3 η_max CoSb3 zT Bi2Te3 zT CoSb3

Introduction Why use high temperature TE-Materials?

  • Higher efficiency at high temperatures mainly through

higher Carnot efficiency

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

Cold Side Temperature T_c = 100°C

𝑨𝑈 = 𝑇2 ∗ 𝜏 𝜆 ∗ 𝑈 𝜃max = 𝜃Carnot ∗ 𝜃ex 𝜃Carnot = 𝑈h − 𝑈c 𝑈h 𝜃ex = 1 + 𝑨𝑈

𝑛 − 1

1 + 𝑨𝑈

𝑛 + 𝑈c/𝑈h

heiß kalt

n p +

  • +

Kühlmittel Abgas heiß kalt

n p +

  • +

Kühlmittel Abgas

T S U   

www.DLR.de • Chart 6 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

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SLIDE 7

Procedural method

VDI Guideline 2221

www.DLR.de • Chart 7 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

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SLIDE 8

DLR – test vehicle BMW 535i 3l, 6 cylinder, spark ignition 190kW @ 6600 1/min

A B C 210mm 400mm 440mm 290mm 170mm 270mm 190mm 150mm 170mm λ installation space length width height

List of requirements

e.g. Vehicle boundary conditions

www.DLR.de • Chart 8 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013 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.

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SLIDE 9

100 200 300 400 500 600 700 800 900 1000

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.

Exhaust gas temperatures [°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

www.DLR.de • Chart 9 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013 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)

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SLIDE 10

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)

ro

P 

pr

P 

co

P 

in

P 

www.DLR.de • Chart 10 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

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SLIDE 11

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 energy conduct heat convert heat conduct heat dissipate heat to coolant provide force distribute contact pressure smoothly

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. www.DLR.de • Chart 11 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

1)

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SLIDE 12

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. www.DLR.de • Chart 12 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

1)

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SLIDE 13

Wellenleistungsänderung [%] Streckenverbrauchsänderung [%].     

Change of shaft power [%] Change of fuel consumption [%]

Δ Δ Δ Δ Δ Δ

Overall system simulations

Results for design point 135 km/h

  • 1,92
  • 0,54
  • 0,95
  • 2,98
  • 0,79
  • 1,44
  • 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

A4 1,0 

   

Δ Δ Δ Δ Δ Δ

Legend: Change in fuel consumption additional system TEG as add on

  • ptimized total

vehicle system Effect on basic shaft power

  • el. TEG input

back pressure cooling load rolling resistance

  • pt

F % 

add

F % 

in

P % 

pr

P % 

co

P % 

ro

P % 

A B C Design 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)

www.DLR.de • Chart 13 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

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SLIDE 14

?

How can functional integration be done to reduce the TEG weight and thermomechanical stress?

?

www.DLR.de • Chart 14 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

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SLIDE 15

Module structure (acc. to VDI 2221) for functional integration within heat exchangers

www.DLR.de • Chart 15 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

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SLIDE 16

exhaust + heat coolant feed exhaust distribute heat smoothly heat transfer dissipate coolant dissipate electric energy dissipate exhaust exhaust + heat coolant + heat feed coolant electric energy conduct 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

www.DLR.de • Chart 16 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

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SLIDE 17

Module structure Hot gas heat exchanger

𝑛 hot gas Thermoelectric Generator (TEG) Module structure

(acc. VDI Guideline 2221)

Hot gas heat exchanger 2D - Simulation model Hot gas heat exchanger with TEM

Thermoelectric Module (TEM) Hot gas heat exchanger

www.DLR.de • Chart 17 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

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SLIDE 18

Analytic analysis of fin buckling

  • buckling formulation:

𝐺 = 𝜌²∗𝐹∗𝐽

𝑀²

  • > 𝑞 = 𝜌²∗𝐹∗𝐽

𝑀²∗𝐵

factor of safety s = 3 analytic pressure p = 5 MPa

  • Variation of
  • fin distance
  • fin thickness
  • heat exchanger height (fin leght)

www.DLR.de • Chart 18 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

p = 5MPa

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SLIDE 19

Analytic analysis of fin buckling

p = 5MPa

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 thickness [mm] 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

www.DLR.de • Chart 19 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

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SLIDE 20

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)

www.DLR.de • Chart 20 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

1)

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SLIDE 21

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]

www.DLR.de • Chart 21 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

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SLIDE 22

Results - Design with Reinforcements

  • Result:

Inhomogeneous contact pressure Contact pressure < 1 MPa High local stress at TEM

p = 5MPa

www.DLR.de • Chart 22 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

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SLIDE 23

ΔH = 1,7mm

Results - Oval Design

www.DLR.de • Chart 23 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

  • 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

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SLIDE 24

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

www.DLR.de • Chart 24 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

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SLIDE 25

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 thickness [mm] 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:

www.DLR.de • Chart 25 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

p = 5MPa

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SLIDE 26

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

www.DLR.de • Chart 26 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013

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SLIDE 27

Institut of Vehicle Concepts

Pfaffenwaldring 38-40 70569 Stuttgart Dipl.-Ing.(FH) Martin Kober Phone: 0049 - 711 6862 -457 martin.kober@dlr.de 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!

www.DLR.de • Chart 27 > Highly Integrated Heat Exchangers for Automotive TEG > M. Kober > 03.12.2013