Development of a thermoelectric generator for the exhaust of a vehicle

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UMinho | 2015

Escola de Engenharia

João Paulo Dourado Oliveira

Development of a Thermoelectric Generator

for the Exhaust of a Vehicle

Dezembro de 2015

João Paulo Dourado Oliveira

De

velopment of a Thermoelectric Generator for t

he Exhaus

t of a V

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Dissertação de Mestrado

Ciclo de Estudos Integrados Conducentes ao

Grau de Mestre em Engenharia Mecânica

Trabalho efetuado sob a orientação do

Professor Doutor Francisco Carrusca Pimenta Brito

e do

Professor Doutor José Carlos Fernandes Teixeira

João Paulo Dourado Oliveira

Development of a Thermoelectric Generator

for the Exhaust of a Vehicle

Dezembro de 2015

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DECLARAÇÃO

Nome: João Paulo Dourado Oliveira

Endereço eletrónico: joaopd_oliveira@hotmail.com Telefone: 911543102/253054800

Bilhete de Identidade/Cartão do Cidadão: 14234695

Título da dissertação: Development of a Thermoelectric Generator for the Exhaust of a Vehicle

Orientadores:

Professor Doutor Francisco Carrusca Pimenta Brito Professor Doutor José Carlos Fernandes Teixeira

Ano de conclusão: 2015

Mestrado Integrado em Engenharia Mecânica

É AUTORIZADA A REPRODUÇÃO INTEGRAL DESTA DISSERTAÇÃO APENAS PARA EFEITOS DE INVESTIGAÇÃO, MEDIANTE DECLARAÇÃO ESCRITA DO INTERESSADO, QUE A TAL SE COMPROMETE.

Universidade do Minho, _____/_____/_________

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Acknowledgments

I would like to express the deepest appreciation to Professor Francisco Brito for all the dedication, guidance and persistent help during the realization of this dissertation. I would like also to thank Professor José Carlos Teixeira for all the help and guidance provided.

A special thanks to Armando Alves, for all the help and support during the accomplishment of this work.

A thank you to Pedro Pinto, for his friendship and all the indispensable suggestions.

I would like to thank all my friends and colleagues for their friendship and support during this journey, in particular to Ricardo Leite, Nuno Vieira, Zé Nuno, João M. Oliveira, José P. Oliveira, João Soares, João F. Rodrigues, Almeno Antunes, Rui Vilas Boas and GRG crew.

Finally, I would like to express my eternal appreciation towards my parents, Irene and Paulo, for their unconditional support. I dedicate this work to them.

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Abstract

The automotive industry is facing increasingly tighter targets regarding the emissions of greenhouse gases and pollutants. Added to this, there is a need to achieve higher energetic efficiencies in automotive applications. One of the ways to achieve this is to implement technologies and devices that allow to recover waste energy. The thermal power released by the exhaust gases has a great potential of recovery due to the high exhaust temperatures. This thermal energy can be converted into electricity by the Seebeck effect using thermoelectric generator modules. The research group of the Internal Combustion Engines Lab of UMinho has been exploring a concept of such a generator which has the ability of controlling the temperature to which the modules are subjected. This is made through the use of a thermosiphon / heat pipe (HP) device placed as a thermal buffer between the heat source (exhaust gases) and the thermoelectric modules. It converts the heat source temperature down to the desired level for the modules (~250 ºC) by means of a phase change process. The temperature control is made by controlling the phase change temperature with the inner pressure of the HP buffer.

In the present work several sections of the thermoelectric generator concept have been modelled and assessed. This was made, on one hand, by improving and updating an existing MATLAB program which models the thermal and electric phenomena occurring in the generator through analytical and empirical analyses, and on the other hand, by modelling the exhaust heat exchanger through Computational Fluid Dynamics (CFD) techniques.

The main contributions of the present work were: the update of the existing model with an unsteady heat transfer model of the exhaust heat exchanger based on explicit and implicit finite difference methods (a computational time reduction of more than 90% for driving cycle simulations was achieved with the implicit method, and with lower accumulated errors); the modelling of the heat transfer at the water cooling system, in which a comparison in terms of heat transfer and pressure drop was made between series and parallel ducts configuration; the improvement of the thermosiphon /HP model in order to allow inner pressure and boiling temperature variation. This modelling allowed to assess impact of the expansion tank size on the generator’s operation, this component being responsible to stabilize the pressure and operating temperature; and the CFD modelling of an exhaust heat exchanger based on a staggered tube bundle configuration with the assessment of the influence of parameters such as wall vanes and tube fins. The final exhaust heat exchanger design allowed to achieve an average effectiveness around 84% for a highway driving cycle, with a negligible pressure drop for a car engine.

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Resumo

O setor automóvel é confrontado cada vez mais com metas mais exigentes relativamente às emissões de gases com efeitos de estufa e outros poluentes. Associado a isto, há necessidade de se atingir maiores eficiências energéticas em aplicações para o ramo automóvel. Isto pode ser conseguido recorrendo à recuperação de energia térmica desperdiçada. A potência térmica libertada pelos gases de escape de um automóvel tem um bom potencial de recuperação, devido à elevada temperatura dos gases libertados. Esta energia térmica pode ser convertida em eletricidade utilizando módulos termoelétricos que funcionam segundo o efeito Seebeck. O grupo de investigação do Laboratório de Motores Térmicos e Termodinâmica Aplicada tem desenvolvido um conceito de gerador com a capacidade de controlar a temperatura a que os módulos estão sujeitos, com recurso a um sistema de termossifão/heat pipes usado como um buffer (acumulador intermédio) de energia térmica entre a fonte de calor (gases de escape) e os módulos termoelétricos, através de mudança de fase. O controlo da temperatura de mudança de fase, que está no nível desejado para os módulos termoelétricos (~250 ºC), é feito através da regulação da pressão interna do buffer.

Neste trabalho são desenvolvidas e avaliadas várias partes constituintes do conceito do gerador termoelétrico. Isto foi feito, por um lado, pela melhoria e atualização do programa MATLAB existente, que faz a modelação térmica e elétrica dos fenómenos físicos que ocorrem no gerador através de análises analíticas e empíricas, e por outro lado, pela modelação do permutador de calor dos gases de escape através de análises de Computational Fluid Dynamics (CFD).

As maiores contribuições deste trabalho são: a melhoria do modelo existente através do desenvolvimento de um modelo de transferência de calor para regimes não-estacionários do permutador de calor dos gases de escape, baseado no método das diferenças finitas, explícito e implícito (redução do tempo de computação em mais de 90% para simulações em ciclos de condução pelo método implícito, e com erros acumulados menores); a modelação da transferência de calor no sistema de arrefecimento a água, usando diferentes configurações; o modelo termossifão/heat pipe foi atualizado de modo a permitir a variação da pressão interna e da temperatura de ebulição. E avalia o impacto que o tamanho do tanque de expansão tem no funcionamento do gerador; e a modelação CFD do permutador de calor dos gases de escape, usando uma configuração de tubos em quincôncio, com a avaliação da influência de parâmetros como, os defletores de parede e o uso de alhetas nas superfícies externas dos tubos. Este permutador de calor dos gases de escape obteve uma eficácia média de cerca de 84% para um ciclo de condução em autoestrada, com uma perda de pressão desprezável para o motor de um carro.

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List of Figures

Figure 1.1- Comparison of CO2 targets for the EU and other countries [2]. ... 1

Figure 1.2- Scheme of an engine heat balance [7]. ... 2

Figure 1.3- Outline of the operation of a heat pipe heat exchanger (adapted from [13]). ... 3

Figure 1.4- Outline of the operation of the condenser under (a) low thermal load and (b) high thermal load (adapted from [12]).The orange/white regions of the HP interior are vapour/non-condensable gas. ... 4

Figure 1.5- Location of the buffer in the generator concept. ... 5

Figure 2.1- Seebeck effect principle shown in a circuit configured as a thermoelectric generator [20]. .. 9

Figure 2.2- Detailed thermoelectric module [24]. ... 10

Figure 2.3- Thermocouple configured for power generation [25]. ... 11

Figure 2.4- Figure of merit ZT for common TE materials, as a function of temperature [28]. ... 12

Figure 2.5- Gravitational heat pipe working principle [33]. ... 14

Figure 2.6- Limitations to heat transport in a HP [14]. ... 15

Figure 2.7- Hi-Z assembly of a 1 kW TEG [45]. ... 17

Figure 2.8- Hi-Z test of a 1kW TEG on a 14 L Cummins NTC 273 engine [45]. ... 18

Figure 2.9- Maximum output power in function of the hot side temperature with different thermoelectric materials [23] [51]. ... 19

Figure 2.10- TEG to be installed in a GMC Sierra Pickup truck [53] [54]. ... 19

Figure 2.11- TEG installed in a BMW 530i [57]. ... 20

Figure 2.12- Exhaust TEG system attached to the underside of a Volkswagen [58]. ... 20

Figure 2.13- Representation of the TEG installed in the exhaust system of a Chevrolet Suburban, composed by skutterudites and BiTe modules [59] [60]. ... 21

Figure 2.14- The GMZ's 1000 W TEG in testing [62]. ... 21

Figure 2.15- TEGs performance comparison under two urban and suburban driving cycles. (a) Heat transfer rate and (b) pressure drop [63]. ... 22

Figure 2.16- Advancement in the figure of merit ZT along the years [41]. ... 23

Figure 2.17- TEG installed in a vehicle with a bypass system [61]. ... 23

Figure 3.1- CFD modelling structure. ... 26

Figure 3.2- Representation of turbulent motion [74]. ... 27

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Figure 3.4- Cell types [78]. ... 31

Figure 3.5- Ideal and skewed cells [79]. ... 32

Figure 3.6- The velocity profiles for the law of the wall and for the logarithmic law with experimental data (adapted from [77]). ... 34

Figure 3.7- Near-wall model approach [77]... 35

Figure 3.8- Available solvers in ANSYS-FLUENT [78]. ... 36

Figure 4.1- Generator Concept used as reference geometry in the calculations (adapted from [17]).... 41

Figure 4.2- Improvements made to the exhaust heat exchanger model. ... 42

Figure 4.3- Representation of the geometric parameters and the minimum flow areas of a staggered arrangement... 43

Figure 4.4- Annular Fins (adapted from [10]). ... 45

Figure 4.5- The stencils for the implicit (red) and explicit (blue) methods. ... 48

Figure 5.1- Improvements made to the Heat Pipe model. ... 53

Figure 5.2- Simplified scheme of the heat pipe algorithm. ... 54

Figure 5.3- Outline of the operation of the condenser with the representation of temperatures and height of vapour (adapted from [12]). ... 57

Figure 5.4- Condenser region of the Generator Concept. ... 58

Figure 6.1- TEG module GM250-127-28-10 from European Thermodynamics. ... 61

Figure 6.2- TEG module dimensions [88]. ... 62

Figure 6.3- Detailed geometry and dimensions of a cooling plate. ... 63

Figure 6.4- Cooling plate prototype for testing... 63

Figure 6.5- Representation of the channels in series (a) and in parallel (b). ... 64

Figure 6.6- Determination of the shape factor using the software ANSYS-THERMAL. ... 65

Figure 6.7- Thermal resistances at the CPs. ... 65

Figure 7.1- Geometric parameters of the tubes interim arrangement, expressed in millimetres. ... 69

Figure 7.2- Representation of the smooth tube and its dimensions. ... 71

Figure 7.3- Representation of the spiral finned tube and its dimensions. ... 71

Figure 7.4- Representation of the final geometry and its dimensions, with flow vanes [17]. ... 72

Figure 7.5- Schematic representation of the final geometry, without vanes (adapted from [17]). ... 72

Figure 8.1- Geometry and named selections used in ANSYS-FLUENT for 4 rows (interim arrangement). ... 81

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Figure 8.3- Inflation at the tube boundaries of the 4 rows mesh. ... 82

Figure 8.4- Contours of velocity magnitude (above) and velocity vectors (below), for the simulation with 4 rows. ... 85

Figure 8.5- Flow characteristics over a tube. ... 85

Figure 8.6- Wake region represented by velocity vectors. ... 86

Figure 8.7- Contours of pressure, for the simulation with 4 rows. ... 86

Figure 8.8- Contours of temperature, for the 4 rows simulation... 87

Figure 8.9- Heat rate absorbed per row from the CFD results, for the influence of the number of tubes rows case. ... 88

Figure 8.10- Part of the generated meshes for the 4 rows geometry: (a) quadrilateral dominant (more refined); (b) quadrilateral dominant (original); (c) triangular. ... 89

Figure 8.11- Heat rate absorbed by the tubes and its increase rate as a function of the number of rows increasing, from theoretical and CFD results. ... 91

Figure 8.12- Average heat transfer coefficient and its increase rate as a function of the rows number increasing, for theoretical and CFD results. ... 92

Figure 8.13- Outlet and average exhaust temperatures as a function of the number of rows increasing, for theoretical and CFD results. ... 92

Figure 8.14- Pressure drop as a function of the number of rows increasing, for theoretical and CFD results. ... 93

Figure 8.15- Geometries and boundaries used in ANSYS-FLUENT for the: (a) smooth tube and (b) finned tube. ... 94

Figure 8.16- Generated meshes in a front view for the: (a) smooth tube and (b) finned tube. ... 95

Figure 8.17- Generated meshes in a top view for the: (a) smooth tube and (b) finned tube. ... 95

Figure 8.18- Inflation near the finned tube boundary. ... 96

Figure 8.19- Contours of the velocity magnitude for the smooth tube (above) and for the finned tube (below) geometries, perpendicular view to the tube axis. ... 98

Figure 8.20- Contours of temperature for the smooth tube (above) and for the finned tube (below) geometries, perpendicular view to the tube axis. ... 99

Figure 8.21- Contours of the velocity magnitude and temperature at the tube midplane, for the smooth tube (above) and for the finned tube (below) geometries, normall view to the flow direction. ... 100

Figure 8.22- Relative Increases and decreases for the parameters under study, for the fins effect case. ... 102

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Figure 8.23- Geometries and boundaries used in ANSYS-FLUENT without (above) and with (below) the use of wall vanes. ... 103 Figure 8.24- Generated mesh of the final geometry without wall vanes. ... 103 Figure 8.25- Generated mesh of the final geometry with wall vanes. ... 104 Figure 8.26- Mesh inflations at the tube and wall boundaries, for the final geometry with (left) and without (right) wall vanes. ... 104 Figure 8.27- Contours of the velocity magnitude for the final geometry case without (above) and with (below) wall vanes. ... 106 Figure 8.28- Contours of the velocity vectors for the final geometry case without (above) and with (below) wall vanes. ... 107 Figure 8.29- Contours of the pressure for the final geometry case without (above) and with (below) wall vanes. ... 108 Figure 8.30- Contours of the temperature for the final geometry case without (above) and with (below) wall vanes. ... 109 Figure 8.31- Relative Increases and decreases for the parameters under study, for the final geometry case. ... 110 Figure 8.32- Temperature of the cold side of TEGs as a function of the volume flow rate, of the CPs analysis case. ... 111 Figure 8.33- Average heat transfer coefficient as a function of the water flow rate, of the CPs analysis case. ... 112 Figure 8.34- Pressure drop as a function of the water flow rate, of the CPs analysis case. ... 113 Figure 8.35- Mass flow rate and temperature of the exhaust gases during the highway route (HR). ... 115 Figure 8.36- Evaporator thermal power absorbed during the HR driving cycle, using the explicit and the implicit methods. ... 116 Figure 8.37- Absolute errors of the evaporator thermal power during the HR driving cycle, using the explicit and the implicit methods (Reference: Implicit Method with 0.001 s). ... 117 Figure 8.38- Relative errors of the evaporator thermal power absorbed during the HR driving cycle, using the explicit and the implicit methods (Reference: Implicit Method with 0.001 s). ... 117 Figure 8.39- Air mass flow rate and temperature, and relative errors at a specific interval of time of the HR driving cycle, using the explicit and the implicit methods (Reference: Implicit Method with 0.001 s). ... 118 Figure 8.40- Total simulations times for the explicit and implicit methods, for the HR driving cycle. .. 119

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Figure 8.41- Thermal and electric powers during the HR driving cycle, of the MATLAB models comparison.

... 122

Figure 8.42- Temperature of water inside HPs during the HR driving cycle, of the MATLAB models comparison. ... 122

Figure 8.43- Active condenser length during the HR driving cycle, of the MATLAB models comparison. ... 123

Figure 8.44- HP pressure variation during the HR driving cycle for different expansion tank sizes. .... 125

Figure 8.45- Boiling temperature of water during the HR driving cycle for different expansion tank sizes. ... 126

Figure 8.46- Thermal and electric powers during the HR driving cycle, with a tank of 4 litres. ... 127

Figure 8.47- Active condenser and buffer lengths during the HR driving cycle, with a tank of 4 litres. 128 Figure B.1- Main algorithm scheme of the heat pipe model. ... 160

Figure B.2- Scheme of the volumes calculation function. ... 161

Figure B.3- Scheme of the height of vapour calculation function. ... 161

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List of Tables

Table 2.1- Characteristics of TEGs from various sources (Tmax in kelvin) [30]. ... 12

Table 3.1- Cell quality and skewness ranges [77]. ... 33

Table 4.1- Constants C and n from the Zukauskas correlation for the heat transfer in tube banks of 20 rows or more (adapted from [83]). ... 44

Table 4.2- Ratio of hconv for less than 20 longitudinal rows (adapted from [83]). ... 45

Table 6.1- Relevant performance parameters of the TEG modules from European Thermodynamics (adapted from [88]). ... 61

Table 6.2- Constants variables used for CPs calculations. ... 66

Table 7.1- Boundary conditions and number of rows for the study of transversal rows effects. ... 70

Table 7.2- Boundary conditions and number of rows for the study of spiral finned surfaces. ... 71

Table 7.3- Boundary conditions and number of rows of the final geometry study. ... 73

Table 7.4- Boundary conditions and parameters of the CPs analysis. ... 73

Table 7.5- Highway route details (adapted from [17]). ... 74

Table 7.6- Boundary conditions and simulation parameters for the explicit and implicit analysis. ... 75

Table 7.7- Boundary conditions and simulation parameters for the MATLAB model validation. ... 76

Table 7.8- Boundary conditions and simulation parameters for the tank size case analysis. ... 77

Table 8.1- Theoretical results of the influence of the number of tubes rows case. ... 80

Table 8.2- Values of skewness, nodes and elements of the generated meshes, for the influence of the number of tubes rows case. ... 83

Table 8.3- Boundary conditions and types applied in FLUENT to the influence of the number of tubes rows case. ... 83

Table 8.4- ANSYS-FLUENT results for the influence of the number of tubes rows case. ... 88

Table 8.5- Values of skewness, nodes and elements of the meshes used for validation. ... 90

Table 8.6- ANSYS-FLUENT results and its relative errors for different meshes. ... 90

Table 8.7- Values of skewness, nodes and elements of the generated meshes, for the fins effect case. ... 96

Table 8.8- Boundary conditions and types applied to the fins effect case. ... 96

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Table 8.10- ANSYS-FLUENT results for the fins effect case. ... 101 Table 8.11- Values of skewness, nodes and elements of the generated meshes for the final geometry case. ... 105 Table 8.12- Boundary conditions and types applied to the final geometry case. ... 105 Table 8.13- ANSYS-FLUENT results for the final geometry analysis. ... 109 Table 8.14- Theoretical results of the working point for both cases, of the CPs analysis case. ... 114 Table 8.15- Values of the maximum absolute and relative, and the accumulated errors of the evaporator thermal power. ... 118 Table 8.16- Total energies for the MATLAB model comparison. ... 120 Table 8.17- Total energies for the different expansion tank sizes. ... 124 Table 8.18- Required initial HP pressure for each expansion tank size. ... 125

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List of Abbreviations

1D One-Dimensional

2D Two-Dimensional

3D Three-Dimensional

CFD Computational Fluid Dynamics

CP Cooling Plate

DBCS Density-Based Coupled Solver

DNS Direct Numerical Simulation

FDM Finite Difference Method

FEM Finite Element Method

FSM Fractional Step Method

FVM Finite Volume Method

HE Heat Exchanger

HP Heat Pipe

HR Highway Route

LES Large Eddy Simulation

LMTD Logarithmic Mean Temperature Difference

NITA Non-Iterative Time Advancement

PBCS Pressure-Based Coupled Solver

RANS Reynolds Averaged Navier-Stokes

RNG Renormalization Group

SIMPLE Semi-Implicit Method for Pressure-Linked Equations

SIMPLEC Semi-Implicit Method for Pressure-Linked Equations-Consistent

TE Thermoelectric

TEG Thermoelectric Generator

TS Thermo-siphon

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List of Symbols

Variables

a Height of Cooling Plate’s Channel [m]

A Area [m2]

Asec cond Cross Sectional Area of Condenser [m2]

Bi Biot Number [-]

C Constant; [-]

C1ϵ, C2ϵ, C3ϵ Water and Air in different flow configurations [-]

cp Specific Heat [J/kg.K]

CR Capacity Ratio [W/K]

D Diameter [m]

E Energy [J]

Econd Energy of Condensation [J]

f' Friction Factor [-]

f1 Relaxation Factor [-]

Fo Fourier Number [-]

Gb Generation of Turbulence Kinetic Energy due to buoyancy [m2/s2]

Gk Generation of Turbulence Kinetic Energy due to the mean

velocity gradients

[m2/s2]

Gmax Mass Velocity at Minimum Flow Area [kg/m2.s]

h Heat Transfer Coefficient [W/m2.s]

I Bessel Function of the First Kind [-]

k Turbulent Kinetic Energy [J/kg]

K Bessel Function of the Second Kind [-]

l Length of Cooling Plate’s Channel [m]

L Vapour Height [m]

m Mass [kg]

ṁ Mass Flow Rate [kg/s]

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NL Number of Longitudinal Rows [-]

NT Number of Transversal Rows [-]

NTU Number of Transfer Units [-]

Nu Nusselt Number [-]

p Pressure [bar] or [MPa]

P Cross-sectional Perimeter [m]

Pe Electric Output Power [W]

Pr Prandtl Number [-]

̇ Heat Transfer Rate [W]

̇ Available Heat Rate in Exhaust Gases [W]

r Radium [m] R Thermal Resistance [K/W] Re Reynolds Number [-] S Source Term [-] Se Seebeck Coefficient [V/K] SF Shape Factor [m] Sn Transversal Pitch [m] Sp Longitudinal Pitch [m] t Time Unit [s] T Temperature [K] or [ᵒC]

tf Fin Tip Thickness [m]

u Fluid Velocity [m/s]

U Overall Heat Transfer Coefficient [W/m2.K]

u* Friction Fluid Velocity [m/s]

u+ Dimensionless Fluid Velocity [-]

V Volume [m3_]

w Width of Cooling Plate’s Channel [m]

̇ Pump Power [W]

x, y, z Rectangular Coordinates [m]

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YM Influence of the Fluctuating Dilatation in Compressible

Turbulence to the Overall Dissipation Rate

[m2_/s2_] Z Figure of Merit [K-1] ZT Figure of Merit [-] α Thermal Diffusivity [m2/s] γ Adiabatic Index [-] Γ Diffusion Coefficient [m2/s]

ΔhLV Latent Heat of Vaporization [J/kg]

Δp Pressure Drop [Pa]

Δt Time Step [s]

ΔTlog Logarithmic Mean Temperature Difference [K]

ΔV Voltage [V]

Δy First Layer Distance [m]

ε Turbulence Dissipation [J/kg.s]

ϵ Heat Exchanger Effectiveness [-]

η Efficiency [-] κ Electrical Conductivity [(Ω/m)-1_] Thermal Conductivity [W/m.K] Dynamic Viscosity [kg/m.s] Kinematic Viscosity [m2_/s] Π Peltier Coefficient [V]

ρ Volumetric Mass Density [kg/m3]

Turbulent Prandtl Number [-]

w Laminar Shear Stress Near-wall [Pa]

ϕ General Variable [-]

ω Specific Turbulence Dissipation [s-1]

Subscripts

0 Zero-order

1 First-order

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air Exhaust Gases

avg Average

b Average Free-stream

boil Boiling

c Cold Junction

cold Cold Fluid

cold side Cold Side of TEG modules

cond Condenser

condensed Condensed Water

conv Convection

CP Cooling Plate

cup _{Excess Power Condenser (“Cup”)}

d Diameter

dissipated Released by TEG modules

evap Evaporator

excess Excess Water Vapour

f Fluid

fins Finned Surface of the HPs

flange Connection Zone between Evaporator and Condenser

h Hot Junction

hot Hot Fluid

hot side Hot Side of TEG modules

HP Inside Heat Pipes

hyd Hydraulic

i Interior node

in Inlet

k Turbulent Kinetic Energy

L Longitudinal

m Material

n Node at the external wall of the HP

max Maximum

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min Minimum

n Node at the Internal Wall of Heat Pipe

no fins Unfinned Surface of the HPs

out Outlet

real Actual Flow

t Turbulent

T Transversal

TEG Thermoelectric Generator Module

to boiling Needed to Boling

total in Total Space Inside the HPs (evaporator, condenser, flange, buffer and expansion tank)

vap Water Vaporization

vapor Water Vapour

w At Wall Temperature

wall in Internal Walls of Heat Pipes

wall out External Walls of Heat Pipes

water Liquid Water

ε Turbulence Dissipation

Dynamic Viscosity

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1

1 Introduction

The automobile industry has a continuous demanding in the energy efficiency, not only due to the high fuel prices but also to the environmental impact concerns too. Vehicles are responsible for about 12% of the total European Union emissions of carbon dioxide (CO2) [1]. The

European Commission has established targets for the emission limits of new vehicles [1], which depend on the mass of the vehicle, in order to maintain the fleet average target at 130 grams of CO2 per kilometre (g/km) by 2015 and 95 grams of CO2 by 2021 (Figure 1.1). This also allows

manufacturers to be granted with emissions credits, with a maximum emissions saving of 7 g/km per year for vehicles with innovative technologies. The 2021 target represents reductions of 40% compared with 2007 new fleet average emissions (158.7 g/km of CO2).

Figure 1.1- Comparison of CO2 targets for the EU and other countries [2].

In order to achieve the imposed limits of CO2 emissions, the automobile companies are

required to use new materials and technologies that allow vehicles to be lighter and more efficient [3] [4]. That is the major cause for the increasing release of hybrid and electric vehicles to the market. There are some typical hybrid characteristics primarily used in energy savings such as [2] [5]: a smaller engine and an electric motor; a start-stop function; and the regenerative braking system where some of the kinetic energy during braking is converted into electricity. Other techniques are frequently used as the Atkinson cycle engines instead of Otto cycle to increase fuel economy [5].

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There are also promising technologies such as the recovery of the waste heat from the automobiles, which is the main subject of this dissertation [6].

1.1 Motivation

Despite the efforts that have been made to achieve a higher efficiency in internal combustion engines, the reality is that almost 70% of the fuel chemical energy released is unintentionally wasted as heat. This is about twice the mechanical power used for traction which is wasted through the exhaust and the engine cooling system (Figure 1.2) [7] [8]. In other words, roughly just 1/3 of the fuel energy content is used for mechanical power, while 2/3 are wasted through the exhaust and cooling systems in approximately equal parts. In practical terms, only the energy wasted through the exhaust is worthy of recovery due to its high temperature, often in excess of 650 ᵒC.

Figure 1.2- Scheme of an engine heat balance [7].

Some of the enthalpy of the exhaust gases can be recovered by using the over-expanded cycles [9], turbocharging and turbo compounding. Even using this kind of systems, the temperature of the exhaust gases is still much higher than that of the cooling system, allowing it to be recovered. Even if the recovered energy is just a small percentage, it could have a notable impact in the global energy efficiency of the vehicle [7]. This energy can be converted into electricity by using thermoelectric generators (TEGs), which use the principle of the Seebeck effect that consists on

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generating a voltage difference from two different material junctions possessing a temperature difference between them [10].

However there are some problems to deal with, particularly the high temperature of exhaust gases, up to 1000ᵒC. This can be problematic and creates additional challenges for systems design and there are no commercially available TEGs that can work at this temperature levels yet [11]. However, recent papers, namely from the UMinho research group, have shown various innovative prototypes developed using thermos-siphon (TS) devices, such as gravitational heat pipes (HPs), where it was possible to reduce, as minimum as possible, the thermal resistance of the system and control the operating temperature of TEGs, while ensuring there will be no overheating of the system [7] [12] [13].

The HPs heat exchangers (HEs) are simple heat transfer devices with a high heat transfer rate due to their basis principle of phase changing (Figure 1.3) The heat source (e.g. the exhaust gases of a vehicle, for instance) are at a higher temperature than the fluid inside HPs ( > _�), so they provoke the fluid to boil. The resulting vapour from boiling rises along the HP heigth, then releasing heat to a heat sink (e.g. TEG) due to the existing temperature difference ( _� > _ℎ ). This process will create a film condensation (through gravity) at the condenser zone, with the

condensed water returning to the

evaporator by gravity, restarting the process [12] [14].

One of the most important aspects of this operation is the control of the operating temperature, which is possible due to the use of variable conductance heat pipes (VCHPs) (Figure 1.4). The VCHPs take advantage of an expansion tank located at the top of the HP and the addition of a non-condensable gas such as air acting as a compressible gas. Like the name suggests, the

Figure 1.3- Outline of the operation of a heat pipe heat exchanger (adapted from [13]).

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expansion tank allows the vapour inside the HP to expand keeping the pressure inside the VCHP nearly constant, as well as the fluid boiling temperature. So, the desired operating temperature (fluid boiling temperature) can be defined by regulating the inner VCHP pressure. The active condenser length varies with the exhaust thermal load fluctuation. This means that under low thermal loads there will not be much vapour present and the non-condensable gas will occupy most of the condenser length. Under high loads, however, high amounts of vapour will be produced and they will reach the condenser region by pushing the non-condensable gas out of the condenser region and into the expansion tank [12].

Figure 1.4- Outline of the operation of the condenser under (a) low thermal load and (b) high thermal load (adapted from [12]).The orange/white regions of the HP interior are vapour/non-condensable gas.

Other aspect of this system includes the use of a buffer (a device located at the top of the condenser that can accommodate excess vapour production). This device offers the possibility of accumulating vapour in high thermal load events when the condenser exceeds its maximum capacity and then supplying this vapour excess under lower thermal load events to compensate for the lack of vapour production during those events. This way, the condenser will remain fully active for a longer time (Figure 1.5). If there is more vapour that the condenser and buffer can store, the excess vapour will be condensed in a secondary heat exchanger (“Excess Power Condenser”) and the condensates fall back to the evaporator.

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Figure 1.5- Location of the buffer in the generator concept.

Comparing this technology with the conventional purely convective/conductive systems, it has clear advantages. Namely, because the active thermoelectric modules will always be close to their optimal operating temperature and passively protected against over-heating. On the contrary, conventional systems are normally optimized for a given thermal load and need to deploy by-pass systems to reject excess thermal load.

One disadvantage of the constant temperature concept proposed is that it will only start absorbing heat once the boiling temperature/pressure is achieved by the phase change fluid inside the HP [15]. For practical terms, the inner HP pressure will be constant if the system volume is big enough for the range of thermal loads present. If the expansion tank volume is reduced, then the pressure will display some variation. This variation, if kept within certain limits, might inclusively be beneficial for the system operation. If the fluid masses and the system volumes are carefully selected, it might be possible to obtain a range of operating temperatures instead of a nearly fixed operating temperature so that the system will operate under a wider range of exhaust temperatures, namely low end ones. To design such system would require the actual calculation of the inner HP pressure under operation instead of assuming it constant. This calculation is something which was still missing in the existing models of the system.

There is also a need to carefully model and assess the performance of several components of the concept, such as the exhaust heat exchanger and the cooling system attached to the cold face of the thermoelectric modules. Also, the high variability of the exhaust thermal load during a real driving cycle would be better modelled with an unsteady heat transfer model instead of the

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quasi-steady state model that existed. So, the present work aims to address several of the aforementioned modelling challenges mentioned.

A progressive development of these systems, aligned with a higher performance of thermoelectric materials, might increase the overall efficiency of these type of devices, enabling their use in commercial vehicles and thereby improving the fuel economy in future vehicles.

1.2 Objectives of the Dissertation

The main purpose of this work is to develop a concept of a HE to absorb the heat released by the exhaust gases from an engine, with the maximum efficiency possible recurring to the HP working principle already referred. It includes the analysis of thermal and flow characteristics in the HE through an analytical and empirical analysis, and a computational fluid dynamics (CFD) approach, that has been applied to the HEs design [16], using the software ANSYS-FLUENT. It is intended that this HE could absorb from the exhaust gases a thermal power in excess of 12 kW.

In addition, this study will use the earlier version of a MATLAB theoretical model [17], that calculates the thermal behaviour and the electrical power output of a generator for the exhaust gases in steady-state regimes. The proposed approach is to improve this model with new capabilities, which are the following:

- To predict the thermal and flow characteristics of the concept HE to be developed, referred in the first paragraph;

- To incorporate the simulation of unsteady regimes during driving cycles;

- To develop a theoretical HP model that assumes pressure variation inside the HPs; - To assess and validate the approaches above.

- To study the water cooling process (geometrical arrangement, heat transfer, pressure loss, etc.) of the cold side of the TEGs, to maintain the desired cold face temperature of the TEGs using as less water flow rate and pumping power as possible.

These modifications will grant a more realistic prediction of the generator performance and more accurate data. Some of the analyses of the present work have already contributed for the

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design of a proof-of-concept thermoelectric generator and aim to help in the development of a new generation generator to be built in an upcoming National Project.

1.3 Structure

This dissertation is divided into 9 chapters, the first of all (“Introduction”) provides the general idea of the work scope. The next two chapters are, mainly, theoretical. “Literature Review” includes all the necessary framework in order to allow understanding the major contents of this dissertation, adding some important information about thermoelectricity, heat pipes and electric cooling devices.

In turn, “Computational Fluid Dynamics”, presents the basic theory behind every CFD tool, with a specific focus on the FLUENT software. The main simulation parameters used to run accurate and reliable CFD simulations are detailed.

Afterwards, there are three chapters intended to explain the theoretical models that describe the concept generator mode of operation. The “Exhaust Heat Exchanger Model” details the calculation procedure to predict the exhaust HE’s behaviour. The output variables are inserted in the chapter “Heat Pipe Model”, where the physical phenomena which occur inside HPs is detailed. The “Cooling Plates Model” is the last theoretical model covered and focuses on the description of the TEG cooling process design.

This dissertation presents seven case studies and their conclusions in chapters 7(“Cases of Study Definition”)and 8(“Results and Discussion”). Three of those cases studies regard to the exhaust HE’s geometry analysis, other three are related to the MATLAB model of the generator concept and another one to the cooling plates performance.

Finally, chapter 9, “Conclusion”, provides the final statements of the work developed in this dissertation and its contribution for the field of study. Some suggestions for future work are also proposed.

As a complement, the “References” used are presented. The “Appendices” present additional content which include a tutorial of ANSYS-FLUENT done by the candidate for the simulation of flows across tube banks; the detailed flowcharts describing the principle of the HP

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model; and the scientific article published (currently in-press), entitled “Analysis of a Temperature Controlled Exhaust Thermoelectric Generator during a Driving Cycle”. The article includes part of this work written by the author, it was presented in the 34th Annual International Conference on

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2 Literature Review

In this chapter the basic principles of the thermoelectricity and a brief bibliographic research of common thermoelectric materials applied to thermoelectric generator modules is introduced. A state of the art is presented, covering the early and existing automobile systems for waste heat recovery, in particular TEGs applications and other relevant aspects that are going to be studied and presented in this work such as HPs and electronic cooling devices.

2.1 Thermoelectricity

Some basic but extremely important theoretical concepts about thermoelectricity are described in this subchapter in order to understand the basic principle of electricity generation by these applications. As its name suggests, thermoelectricity is based on the conversion of a thermal energy into electric energy, or the reverse process. There are three major principles associated to this phenomena, the Seebeck Effect, the Peltier Effect, and the Thomson Effect.

Between 1822 and 1823, Thomas Johann Seebeck was the first person to observe that a circuit made of two dissimilar metals (A and B) with different temperatures (Tc and Th) at their

junctions would generate voltage differences [18] [19], as shown in Figure 2.1. The Seebeck coefficient (Se) characterizes this effect and obtained as follows (considering Th>Tc), expressed in

Volts per Kelvin:

= ∆

ℎ− (2.1)

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The Seebeck Effect is applied widely in sensors for temperature measurement in industrial and domestic applications, namely thermocouples. These devices measure a temperature difference so a reference temperature must be used, which is one of its disadvantages. The voltage versus temperature difference is dependent on the normalized thermocouple type.

The reciprocal process of the Seebeck Effect, the Peltier Effect, was discovered by Jean Peltier in 1834 [21]. According to this principle, the current originated by a voltage difference between two junctions of two dissimilar materials will provoke heating in one junction and cooling in the other junction. The thermal energy is proportional to the electric current and is defined by the Peltier Coefficient (Π). This coefficient is related with the Seebeck coefficient through:

Π= (2.2)

Where T is the absolute system temperature. This effect is mostly applied to cooling systems for diverse applications, although it can be used for heating too.

Discovered by William Thomson in 1854 [22], the Thomson Effect describes the heat generated when there is a passage of an electric current through a material and with a temperature difference along its length. In practical applications, this effect is not one of the most important in thermoelectric devices and is frequently ignored [23].

2.1.1 Thermoelectric Modules and Figure of Merit

When desiring the generation of electric energy using a temperature difference, the devices that can be used with this purpose are thermoelectric (TE) modules, represented in Figure 2.2.

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TE modules are composed by many pairs connected electrically in series and thermally in parallel (Figure 2.3) [25]. Each pair has two different elements, p-type with a positive � and hole charge carriers, and n-type with negative � and electron charge carriers. The temperature difference causes the carriers of both elements to diffuse in the cold side direction of the TEG module, and a thermoelectric voltage is generated.

TE modules have distinct advantages. They are

simple and reliable as they have no moving parts, not position-dependent and silent in operation [26].

The efficiency of a thermoelectric device for electricity generation is defined as [23]:

� = _ℎ _{ℎ ℎ} ℎ (2.3)

To evaluate the conversion efficiency from thermal to electric energy of TE materials, a dimensionless property called the figure of merit (ZT), is used:

=� (2.4)

Where � is the thermal diffusivity, the electrical conductivity, m the thermal conductivity

of the material and T is the absolute temperature of the material. The higher the figure of merit of a TE material the higher the electricity generation of a TE device. The maximum power efficiency (� ) of a TEG module is given by (2.5) [27].

� = − ℎ

− −

ℎ +

ℎ

(2.5)

Figure 2.4 displays the values of ZT for common and emerging TE materials. The most applied TE material is the bismuth telluride (Bi2Te3), although its ZT decreases significantly for

higher temperatures. Nevertheless, new combinations of different TE materials are emerging in order to have a higher ZT value at higher temperatures.

Figure 2.3- Thermocouple configured for power generation [25].

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Figure 2.4- Figure of merit ZT for common TE materials, as a function of temperature [28].

The common materials used in TEGs modules include the Bi2Te3 (bismuth telluride), the

PbTe (lead telluride alloys) and the Si80Ge20 (Silicon-Germanium alloys), which have a maximum

operating temperatures about 320ᵒC, 630ᵒC, and 1020ᵒC, respectively [29] [30]. Unfortunately, most high temperature TE materials have manufacture or reliability challenges. In Table 2.1, the evolution of the main characteristics of TEGs from some reliable sources are shown [30]. For example, the TEG made of bismuth telluride, the most popular TE material, has reached a maximum operating temperature of approximately 297ᵒC (570 K), which is still far from the more than 700ᵒC from the exhaust gases of a car engine [7]. Also, the use of bismuth telluride in large applications is conditioned due the cost and scarcity of tellurium [31].

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13 2.2 Gravitational Heat Pipes

Once that it is desirable to have a thermal control between the exhaust gases and the hot face of TEGs with the lowest thermal resistance possible, the thermo-siphon (TS) devices, such as gravitational HPs, are the most suitable choice for this purpose, and they were used too in previous prototypes by the group too [12]. Their heat transfer coefficients between the evaporator and the condenser zones are 103–105 W/m2 K [32], which is considerably high. The main idea of HP was

first suggested by Gaugler in 1942, although its properties and world development started to become popular only since its invention in 1960 by Grover [14].

The HP is a component with a significant high thermal conductivity and the systems where they are incorporated take advantage of this crucial characteristic to transfer heat in a vast range of applications. The working principle behind a gravitational HPs is represented in Figure 2.5, where a quantity of working fluid is placed in the evaporator, then it starts to vaporize due to the high temperature. Because the HP is sealed, the pressure rises and causes the water vapour moving to the condenser, where there is a heat sink. The temperature of the water vapour becomes lower and starts to condense. The condensated water return to the evaporator through the gravitational forces.

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Figure 2.5- Gravitational heat pipe working principle [33].

Other types of HPs, which do not have the evaporator at the lowest point, are not considered TS devices, once that they need a wick (placed at the internal walls of HPs) to return the condensed water to the evaporator, through capillary forces. In this work, there are used gravitational HPs placed vertically, without the wick.

The heat flux and operating temperature limitations are illustrated in Figure 2.6, which depend on the working fluid, the wick material and the geometry of the HP. As it is shown, the capillary limit is present only in HPs with wick and it is related with the capacity of HP to return the condensates to the evaporator due to capillary forces. The sonic limit can occur when the vapour velocity reaches sonic values during the start up, and the compressibility effects have a significant influence in the vapour pressure drop. The viscous limit is the most crucial at the start-up, at this limit, for a low temperature of the working fluid, there is a very low pressure at the evaporator, and the difference in vapour pressure is insufficient to overcome the viscous and gravitational forces. This occurs because the pressure in the condenser cannot be less than zero. The entrainment limit describes a decrease of performance when the vapour velocity is high enough to entrain the liquid returning to the evaporator. The boiling limit is reached when a vapour film starts to form at the evaporator walls of the HP [14].

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Figure 2.6- Limitations to heat transport in a HP [14].

The temperature of HP will depend on the boiling temperature of the working fluid at the pressure inside the HP, that can change during the operation. The choice depends on the working fluid with a specific boiling temperature at a specific pressure.

By rising the pressure inside of HP, the boiling temperature of the working fluid rises too and vice-versa. However, there are some problems to control and to stabilize the temperature due to the increase of pressure during the phase change. The solution to this problem is to use a HP variant known as VCHPs (Figure 1.4), which by controlling the pressure allows to control the phase changing temperature [13]. This type of devices uses a tank at the top of the TEG system which allows the volume expansion during the water boiling.

2.3 Electronic Cooling Devices

Nowadays, the electronic equipment is applied in a wide range of devices, the performance of such components is crucial to the overall performance of the systems in which they are incorporated. Once that every type of these components has electric current passing inside, the resistance to the electricity caused by the wire produces a heat generation known as the Joule effect. The continuous evolution of this kind of components resulted in a significant increase in the amount of heat generated per unit volume, a higher temperature means a higher probability for the equipment failure [34]. The progressive increasing in power dissipation of electronic devices

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brings the necessity to explore new cooling techniques in order to maintain a desirable performance [35]. So, the temperature control is a crucial step in the electronic equipment design and operation. The most common techniques used are the conduction cooling, natural convection, radiation cooling, forced-air convection, liquid cooling, and immersion cooling [34].

One important aspect of the design of the cooling technique is the environment where the electronic devices will operate. An interface between the electronic equipment and the environment is used, it can be a fluid such as air, water, or a refrigerant [34]. Usually air is preferred due to its availability and harmless risk of leakage.

Most of the cooling methods use convection as the mechanism of heat transfer. Convection is defined as the transferred heat between a solid surface and an adjacent liquid or gas in motion, which are at different temperatures, and it involves the effects of conduction and fluid motion [34]. The higher motion of fluid, the higher heat transfer by convection. A forced convection (driven by external sources) has a much more efficient heat transfer than natural convection (caused by density differences) due to the existence of turbulence. Liquids have higher convection coefficients than gases due to their higher densities.

There is an emerging investigation to the nanofluids properties with a great potential of application in heat transfer including electronics cooling. Nanofluids are traditional heat transfer fluids, such as water and oil, containing suspending nanoparticles with average sizes below 100 nm with unique thermal properties [36]. The first experiments in electronic cooling using these fluids were apparently made by Chien [37] who showed that the use of nanofluids instead of deionized water as a working fluid in a disk-shaped heat pipe can reduce its thermal resistance by around 40 %. Authors as Tsai [38] and Kang [39], also demonstrated the advantages of nanofluids, the nanoparticles were constituted by gold and silver, respectively, instead of the deionized water to heat pipe application. There is still an ongoing research in order to manage the improvement of the thermal properties of nanofluids that allows to achieve greater energy efficiency, lower operating costs, and others [36].

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17 2.4 State of the Art

Thomas Johann Seebeck was the first person, in 1821-23, to realize that a difference of temperature could produce an electric current in a circuit made of two different metals. This phenomena became known as the Seebeck effect (See section “Thermoelectricity”).

During and after the world wars, the thermoelectricity was studied for applications in military technologies and civilian uses, although the generator efficiencies in the fifties were not higher than 5% [20]. Only in 1963 the first TEG was built and reported by Neil [40].

In the 1960s the discussion was that the figure of merit ZT (the measure of performance for thermoelectric materials) upper limit was below 1 and caused a slowdown in thermoelectric investigations by many organizations [20] [41]. Nevertheless, some small companies started to invest in this niche market of the thermoelectricity, one of them was Hi-Z Technology Inc. [41].

In 1988, the first attempt to implement a TEG in automobiles was published, it was made in the exhaust system of a Porsche 944, by Birkholz from the Universitat Karsruhe in Germany with Porsche collaboration [42].

Hi-Z Technology Inc. developed since 1990s an exhaust gas TEG for diesel truck engines with a goal for the output power of 1 kW. Their prototype is represented in Figure 2.7, part of a program from the U.S. Department of Energy and California Energy Commission Funding [43] [44].

Figure 2.7- Hi-Z assembly of a 1 kW TEG [45].

In the first test, the TEG produced approximately 400 W in a 14 L Cummins NTE 273 engine, shown in Figure 2.8, the low output power was due to the inexistence of a turbulent flow in the exhaust gas [45]. Several modifications were attempted to reach the 1 kW goal for the output

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power. This was achieved in 1995 by using a heat-diffuser with a conic shape that solved the heat transfer problem [45] [46] [47].

Figure 2.8- Hi-Z test of a 1kW TEG on a 14 L Cummins NTC 273 engine [45].

A heat exchanger was also tested with swirls fins type where a maximum power of 1068 W was produced with an engine power of 300 hp and operating at 1700 rpm [45].

In 1966, the Nissan Motor Group started its TEG research by developing a TEG prototype made of Si-Ge alloys with a ZT around 0.6. However, just a total of 35.6 W were obtained, with a 3000 cc gasoline engine under 60 km/h in a hill climbing [48]. They built and tested another TEG prototype in 1999 but using Bi2Te3, at this experiment a total of 193 W was produced, with a 2 to

3 L gasoline engine in a 5% hill at 60 km/h [49] [50].

During the 20th century, the bismuth telluride was the material with the best ZT (about 1)

around ambient temperatures which meant efficiencies from about 5 to 7% for a wide range of applications. However, for temperatures over approximately 230 ᵒC it decreases significantly its efficiency [41]. A new family of thermoelectric materials started to emerge, called skutterudites.

A joint program of universities and industries started in 1993 with the main goal of developing new thermoelectric materials to improve its properties and for implementing them in automobiles [51] [52]. One such program was coordinated from 1993 to 1999 by the Yamaguchi prefecture’s Supporting Program, and in 2000, 2002 and 2003 by the New Energy Development Organization Commission Program [23]. They developed a TEG prototype using skutterudites and bismuth telluride based materials, and evaluated their performance, as represented in Figure 2.9.

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Figure 2.9- Maximum output power in function of the hot side temperature with different thermoelectric materials [23] [51].

From this figure, it can be observed that for temperatures below 300 ᵒC the single module of Bi2Te3-based materials has the higher output power produced compared with the other materials.

For temperatures over 300 ᵒC, the segmented type module made of skutterudites and Bi2Te3-based

materials has the best performance, capable of generating about 25% more output power than the modules of skutterudites single type [51] [23]. 16 segmented type modules were incorporated in a TEG prototype to be used in a 2000 cc passenger vehicle. The operating conditions were a hot side temperature of 550 ᵒC and a cold side temperature of 23 ᵒC, which produced a maximum output electric power of 18 W with a module efficiency between 5 and 6% [52].

In 1999, a collaboration between the Clarkson University, Hi-Z Technology Inc., GM and the Delphi Corporation studied the development of a TEG, with a goal of producing an electric power output between 300 and 330 W, and capable of recharging batteries (of 12 and 42V) to power the vehicle lights and other electric devices [53] [54]. The vehicle used was a GMC Sierra Pickup truck with a gasoline V8 220 hp engine. The exhaust heat recovery system is shown in Figure 2.10. The results presented a power output of 255 W (of the 300 W expected) with an inlet coolant temperature of 25ᵒC.

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At the DEER1 Conference in 2005, the BSST, BMW and other organizations published tests

results using a TEG in a BMW 530i with a gasoline 3 L 190 kW engine (Figure 2.11). One of the targets of the project was to obtain 10% improvement in vehicle fuel economy [55] [56] [57]. The thermoelectric materials applied to the TEG were segment elements comprised of skutterudites, TAGS (Te/Sb/Ge/Ag nanostructures), PbTe and BiTe, and the average TEG is intended to produce an electric output power about 750 W [56]. There are no reports to conclude if the goal of 10% improvement in vehicle fuel economy was achieved.

Figure 2.11- TEG installed in a BMW 530i [57].

Volkswagen showed the first demonstration of a TEG installed in one of their models (Figure 2.12) in 2008, at the Thermoelektrik-Eine Chance Fur Die Atomobillindustrie in Berlin [58]. A production of 600 W was claimed and the additional electric energy was claimed to power 30% of the car’s electric needs. According to [58], due to the lower mechanical load of the alternator, there was a reduction of more than 5% in fuel consumption.

Figure 2.12- Exhaust TEG system attached to the underside of a Volkswagen [58].

In 2011, new thermoelectric generator prototypes were assembled in commercial cars and in a SUV2. The TEGs installed in the vehicles of BMW and Ford were made by the BSST in California,

and for the Chevrolet SUV by GM in Michigan [59] [60] [61], as shown in Figure 2.13. Alternative

1_{Directions in Energy-Efficiency and Emissions Research} 2Sport Utility Vehicle

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materials were used by both groups instead of the common materials used for TEGs, bismuth telluride (more expensive and only works up to a maximum temperature of about 250 ᵒC). BSST used another group of thermoelectric materials, blends of hafnium and zirconium, which can work with higher temperatures and consequently higher efficiency of the generator. GM used in their prototypes skutterudites, they are cheaper than bismuth telluride and have better performance for higher temperatures. They are manufactured through complex and time-consuming processes, and their incorporation into devices is complex. The computational model previsions of GM for the Chevrolet Suburban showed a production of 350 W. Both BSST and GM researchers had claimed that they needed to find cheaper ways of doing a production of larger volumes of the new class of materials.

Figure 2.13- Representation of the TEG installed in the exhaust system of a Chevrolet Suburban, composed by skutterudites and BiTe modules [59] [60].

Another company in the thermoelectric generation field, the GMZ Energy, did a successful demonstration in June of 2014 of its 1000 W TEG (Figure 2.14), connected with an exhaust of a 15 L V8 diesel engine and using its nanotechnology approach in their thermoelectric modules, made of half-heusler alloys [62]. GMZ modules are able to operate up to 600 ᵒC and the goal is to apply the GMZ TEGs into military vehicles of the U.S. military in the next phase of the project, which is administrated by the U.S. Department of Energy.

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In 2014, Bai [63] analysed different structures of exhaust HEs in automobile TEGs, by numerical and experimental methods. Six different configurations were tested, the heat transfer and pressure drop were evaluated for typical driving cycles for a 1.2 L gasoline engine. The objective was to use different structures to evaluate the heat transfer and pressure drop of each one, and find a commitment between these two parameters. The results of the comparison are shown in Figure 2.15. For example, the pipe structure was the second one with greatest pressure drop, and the fourth with greatest heat transfer rate. The CFD data were validated for the pipe structure by the experimental results.

Figure 2.15- TEGs performance comparison under two urban and suburban driving cycles. (a) Heat transfer rate and (b) pressure drop [63].

Besides the appearance of new TE materials applied to TEGs, the ZT with a value of about 1 is still a barrier to better TEG performances. However, nowadays the appearance and study of the nanotechnology in TE applications is promising to break this barrier [41]. In recent years, there have been organizations as the MIT’s Lincoln Laboratory, the Research Triangle Institute and the Hi-Z Technology Inc. that reported TE materials with a ZT higher than 2 by using nanotechnology

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approaches [64] [65]. The progressing in the figure of merit ZT of the TE materials is represented in Figure 2.16. The figure shows that there are already reports of TE materials with a ZT clearly higher than 1.

Figure 2.16- Advancement in the figure of merit ZT along the years [41].

There have been theoretical predictions of the performance increase using nanostructured materials, that was also experimentally verified [29]. The high performance thermoelectric properties depend on the nanostructure, synthesis approach and device assembly [66]. There is an ongoing research on thermoelectric materials using nanostructures, and it has been shown improvement of performance of TE materials using cost-effective way of nanostructure synthesis methods that allow to be scaled for large productions [67].

In short, new TEG prototypes have been developed and advancements have been made. But there are still some issues to be solved, such as, the ZT values, the thermal stability of thermoelectric materials, viable manufacturing costs and the design of a heat exchanger with the capacity of absorbing a large amount of heat from the exhaust gases at a controlled operating temperature [23]. The

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last one is one of the major problems, most of the TEGs developed use bypass systems to avoid the risk of TE modules overheating (Figure 2.17).

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3 Computational Fluid Dynamics

In this chapter, the necessary theory about the CFD simulations and its procedure will be presented, by using the software ANSYS-FLUENT. This software allows several options for the user to predict various thermodynamic processes. The user can define if the flow is laminar or turbulent, in a steady or transient state, if the fluids are compressible or incompressible, and if phase changes and heat transfer are present.

A tutorial is presented in Appendix A with the detailed simulation steps for cases involving flow across tube banks.

The CFD is a useful tool to the HEs design due to the capacity of analysing different HEs with the same boundary conditions [63], and have been applied to various types of HEs design, using FLUENT [16] [68] [69]. The modern CFD started in the early 1950s, and it is based on basic tools, the finite difference method (FDM), the finite element method (FEM) and finite volume method (FVM), which are the earlier methods to determine the solutions of differential equations applied in CFD [70]. The first FDM solution presented was in 1910 for the stress analysis of a masonry dam, and the first FEM analysis was in 1956 for an aircraft stress analysis [70].

3.1 Modelling Structure

Before starting modelling in the software, the user must perform the problem and domain identifications, defining the physical models to use in the analysis, the assumptions that are possible to make in order to simplify the problem, the degree of accuracy, etc. FLUENT is structured in three general groups (Figure 3.1): Pre-Processing, where geometry, mesh, physical properties and solver settings are defined; Solver, where the solution is computed and the discretised conservation equations are solved iteratively until convergence; Post-Processing, where it is possible to visualize and examine results, and extract useful data. After scrutinizing the results, the user may update the model in some of the described steps and re-run it in order to improve the reliability and accuracy of the results [71] [72].