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Journal of Electronic Materials

, Volume 45, Issue 3, pp 1846–1870 | Cite as

Analysis of a Temperature-Controlled Exhaust Thermoelectric Generator During a Driving Cycle

  • F. P. BritoEmail author
  • A. Alves
  • J. M. Pires
  • L. B. Martins
  • J. Martins
  • J. Oliveira
  • J. Teixeira
  • L. M. Goncalves
  • M.J. Hall
Article

Abstract

Thermoelectric generators can be used in automotive exhaust energy recovery. As car engines operate under wide variable loads, it is a challenge to design a system for operating efficiently under these variable conditions. This means being able to avoid excessive thermal dilution under low engine loads and being able to operate under high load, high temperature events without the need to deflect the exhaust gases with bypass systems. The authors have previously proposed a thermoelectric generator (TEG) concept with temperature control based on the operating principle of the variable conductance heat pipe/thermosiphon. This strategy allows the TEG modules’ hot face to work under constant, optimized temperature. The variable engine load will only affect the number of modules exposed to the heat source, not the heat transfer temperature. This prevents module overheating under high engine loads and avoids thermal dilution under low engine loads. The present work assesses the merit of the aforementioned approach by analysing the generator output during driving cycles simulated with an energy model of a light vehicle. For the baseline evaporator and condenser configuration, the driving cycle averaged electrical power outputs were approximately 320 W and 550 W for the type-approval Worldwide harmonized light vehicles test procedure Class 3 driving cycle and for a real-world highway driving cycle, respectively.

Keywords

Thermoelectric generator exhaust heat recovery heat exchanger model variable conductance heat pipes thermosiphon driving cycles automotive heat recovery 

Nomenclature

1D

One-dimensional

3D

Three-dimensional

ATDC

After top dead centre

BMEP

Brake mean effective pressure

BSFC

Brake specific fuel consumption

CFD

Computational fluid dynamics

EGR

Exhaust gas recirculation

EVO

Exhaust valve opening

FMEP

Friction mean effective pressure

GPS

Global positioning system

HP

Heat pipe

HW

Highway driving cycle

IC

Internal combustion

ICE

Internal combustion engine

IMEP

Indicated mean effective pressure

LaMoTA

Laboratory of thermal engines and applied thermodynamics

MBT

Maximum brake torque

MEP

Mean effective pressure

NEDC

New European driving cycle

NTU

Number of transfer units

OEM

Original equipment manufacturer

PMEP

Pumping mean effective pressure

TE

Thermoelectric

TEG

Thermoelectric Generator

VHCP

Variable conductance heat pipe

VSP

Vehicle specific power

WLTP

Worldwide harmonized light vehicles test procedure

WOT

Wide open throttle

Variables

cp exh

Exhaust specific heat at constant pressure [W/(mK)]

Aavg

Reference section area (m2)

Afins

Total area of the fins (m2)

Ano fins

Total outer tube area excluding fin area (m2)

Awall out

Outer wall surface area (m)

Biboil

Boiling Biot number

Biconv

Convection Biot number

C

Constant used in Eq. 3

Dext

External tube diameter (m)

Fo

Fourier number

hboil

Boiling heat transfer coefficient [W/(m2K)]

hconv

Convection heat transfer coefficient [W/(m2K)]

hconv corr

Corrected convective heat transfer coefficient [W/(m2K)]

i

Index of spatial node located at a depth x from the outer surface of the tubes

kf

Thermal conductivity of the fluid [W/(mK)]

kmetal

Tube thermal conductivity [W/(mK)]

Lcond

Active length of the condenser (m)

Lcond max

Full length of the condenser (m2)

Loadcond

Condenser Load (%)

m

Index of instant t

\(\dot{m}_{\rm{exh}} \)

Exhaust mass flow rate (kg/s)

n

Constant used in Eq. 3

N

Number or tube rows (longitudinally)

p

Pressure (MPa) as used in Eq. 4

Pavail

Available (absorbable) exhaust power (W)

Pcond

Thermal power absorbed by the condenser (W)

Pe

Electric power produced by the thermoelectric modules (W)

Pevap

Evaporator thermal power (boiling) (W)

Pexh

Thermal power released by the exhaust air to the system (W)

PICE

Instantaneous engine propulsion power (W)

PProp

Instantaneous vehicle propulsion power (at the wheels) (W)

Pr

Prandtl number

Prw

Prandtl number evaluated at wall surface temperature

Pwall_out

Thermal power absorbed from exhaust gases at outer evaporator wall surface (W)

Red,max

Reynolds number based on the maximum velocity achieved at the smallest cross section area of the tube banks

Sn

Transversal pitch between evaporator tubes (m)

Sp

Longitudinal pitch between evaporator tubes (m)

Texh avg

Average Exhaust temperature (°C)

Texh in

Exhaust inlet temperature (°C)

Texh out

Exhaust outlet temperature (°C)

Thp

Temperature of the boiling water inside the tube (°C)

Twall_in

Inner evaporator wall surface temperature (°C)

Twall_out

Outer evaporator wall surface temperature (°C)

umax

Maximum fluid velocity (m/s)

α

Material diffusivity (m2/s)

ΔT

Temperature difference (°C)

ΔTlog

Mean logarithmic temperature difference (°C)

Δt

Time step used in evaporator model (s)

Δx

Space step used in evaporator model (ms)

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Copyright information

© The Minerals, Metals & Materials Society 2015

Authors and Affiliations

  1. 1.Mechanical Engineering DepartmentUniversidade do MinhoGuimaraesPortugal
  2. 2.Industrial Electronics DepartmentUniversidade do MinhoGuimaraesPortugal
  3. 3.Department of Mechanical EngineeringUniversity of Texas at AustinAustinUSA

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