Abstract
Heat pipe (HP)-based heat exchangers can be used for very low resistance heat transfer between a hot and a cold source. Their operating temperature depends solely on the boiling point of their working fluid, so it is possible to control the heat transfer temperature if the pressure of the HP can be adjusted. This is the case of the variable conductance HPs (VCHP). This solution makes VCHPs ideal for the passive control of thermoelectric generator (TEG) temperature levels. The present work assesses, both theoretically and experimentally, the merit of the aforementioned approach. A thermal and electrical model of a TEG with VCHP assist is proposed. Experimental results obtained with a proof of concept prototype attached to a small single-cylinder engine are presented and used to validate the model. It was found that the HP heat exchanger indeed enables the TEG to operate at a constant, optimal temperature in a passive and safe way, and with a minimal overall thermal resistance, under part load, it effectively reduces the active module area without deprecating the temperature level of the active modules.
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Abbreviations
- 1D:
-
One-dimensional
- 3D:
-
Three-dimensional
- EREV:
-
Range extended electric vehicle
- HEV:
-
Hybrid electric vehicle
- HP:
-
Heat pipe
- IC:
-
Internal combustion
- ORC:
-
Organic rankine cycle
- TE:
-
Thermoelectric
- TEG:
-
Thermoelectric generator
- VCHP:
-
Variable conductance heat pipe
- c p :
-
Specific heat at constant pressure (J kg−1 K−1)
- g :
-
Acceleration of gravity (m s−2)
- h c :
-
Contact heat transfer coefficient (W m−2 K−1)
- H L :
-
Enthalpy of vaporisation (J kg−1)
- I :
-
Electric current (A)
- k :
-
Thermal conductivity (W m−1 K−1)
- k l :
-
Thermal conductivity of liquid (W m−1 K−1)
- L c :
-
Active length of the condenser (m)
- \({\dot{m}}\) :
-
Mass flow rate (kg s−1)
- N pairs :
-
Number of (P–N) pairs in a thermoelectric module
- n teg :
-
Number of thermoelectric modules
- P max :
-
Electrical output power at matched load (W)
- \({\dot{Q}}\) :
-
Thermal power (W)
- \({\dot{q}}\) :
-
Thermal power generated per unit volume (W m−3)
- R :
-
Thermal resistance (K W−1)
- \({Ri_{{{\rm{total}}_{{{\rm{system}}_{\rm{e}} }} }}}\) :
-
Total electrical resistance of the module (Ω)
- S :
-
Shape factor (m−1)
- T :
-
Temperature (K)
- T s :
-
Saturation temperature of the working fluid
- T cond_w :
-
Temperature of the condenser wall
- V o :
-
Open circuit voltage (V)
- α :
-
Seebeck coefficient (V K−1)
- η :
-
Effectiveness of heat exchanger
- μ l :
-
Dynamic viscosity of the liquid (Pa s)
- ρ :
-
Electrical resistivity (Ω m)
- ρ c :
-
Contact resistivity (Ω m2)
- ρ l :
-
Density of the liquid (kg m−3)
- BiTe:
-
Thermoelectric material/legs
- cold:
-
Cold side of the thermoelectric generator
- coolant:
-
Liquid for cooling the cold face of TEG
- downstream:
-
Downstream of the module (in terms of heat flux direction)
- e:
-
Electric
- evap:
-
Evaporator (sector1)
- exh:
-
Exhaust gases
- hot:
-
Hot side of the thermoelectric generator
- hot junction:
-
Corresponding to the power leaving the hot junction
- HP:
-
Heat pipe
- in:
-
At the inlet
- Joule total:
-
Total Joule power generated within all the legs
- l :
-
Liquid
- out:
-
At the outlet
- Peltier:
-
By Peltier effect
- Sector1:
-
Evaporator region
- Sector2:
-
Condenser region (including coolant system)
- TEG:
-
Thermoelectric generator module
- total:
-
Corresponding to all legs, not just one leg
- upstream:
-
Upstream of the module (in terms of heat flux direction)
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Acknowledgements
Project ThinHarvest (FCOMP-01-0124-FEDER-041343/EXPL/EMS-ENE/1023/2013) and post doctoral grant SFRH/BPD/89553/2012, financed by FEDER funds through Programa Operacional Fatores de Competitividade—COMPETE and National funds through PIDDAC and FCT—Fundação para a Ciência e a Tecnologia; Luso-American Foundation/National Science Foundation (FLAD/NSF) 2013 PORTUGAL—U.S. Research Networks Program, Project “Waste Exhaust Energy Recovery of Internal Combustion Engines”.
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Brito, F.P., Martins, J., Hançer, E. et al. Thermoelectric Exhaust Heat Recovery with Heat Pipe-Based Thermal Control. J. Electron. Mater. 44, 1984–1997 (2015). https://doi.org/10.1007/s11664-015-3638-3
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DOI: https://doi.org/10.1007/s11664-015-3638-3