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.
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Abbreviations
- 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
- c p exh :
-
Exhaust specific heat at constant pressure [W/(mK)]
- A avg :
-
Reference section area (m2)
- A fins :
-
Total area of the fins (m2)
- A no fins :
-
Total outer tube area excluding fin area (m2)
- A wall out :
-
Outer wall surface area (m)
- Biboil :
-
Boiling Biot number
- Biconv :
-
Convection Biot number
- C :
-
Constant used in Eq. 3
- D ext :
-
External tube diameter (m)
- Fo:
-
Fourier number
- h boil :
-
Boiling heat transfer coefficient [W/(m2K)]
- h conv :
-
Convection heat transfer coefficient [W/(m2K)]
- h conv 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
- k f :
-
Thermal conductivity of the fluid [W/(mK)]
- k metal :
-
Tube thermal conductivity [W/(mK)]
- L cond :
-
Active length of the condenser (m)
- L cond 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
- P avail :
-
Available (absorbable) exhaust power (W)
- P cond :
-
Thermal power absorbed by the condenser (W)
- P e :
-
Electric power produced by the thermoelectric modules (W)
- P evap :
-
Evaporator thermal power (boiling) (W)
- P exh :
-
Thermal power released by the exhaust air to the system (W)
- P ICE :
-
Instantaneous engine propulsion power (W)
- P Prop :
-
Instantaneous vehicle propulsion power (at the wheels) (W)
- Pr:
-
Prandtl number
- Prw :
-
Prandtl number evaluated at wall surface temperature
- P wall_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
- S n :
-
Transversal pitch between evaporator tubes (m)
- S p :
-
Longitudinal pitch between evaporator tubes (m)
- T exh avg :
-
Average Exhaust temperature (°C)
- T exh in :
-
Exhaust inlet temperature (°C)
- T exh out :
-
Exhaust outlet temperature (°C)
- T hp :
-
Temperature of the boiling water inside the tube (°C)
- T wall_in :
-
Inner evaporator wall surface temperature (°C)
- T wall_out :
-
Outer evaporator wall surface temperature (°C)
- u max :
-
Maximum fluid velocity (m/s)
- α :
-
Material diffusivity (m2/s)
- ΔT :
-
Temperature difference (°C)
- ΔT log :
-
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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Brito, F.P., Alves, A., Pires, J.M. et al. Analysis of a Temperature-Controlled Exhaust Thermoelectric Generator During a Driving Cycle. J. Electron. Mater. 45, 1846–1870 (2016). https://doi.org/10.1007/s11664-015-4258-7
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DOI: https://doi.org/10.1007/s11664-015-4258-7