CHAPTER 1
INTRODUCTION
Mesoscale power generation fueled by liquid fuels has the potential for delivering higher
energy density (per unit volume) or specific energy (per unit mass) than battery technology. In
this study, a complete mesoscale power generation system is investigated that incorporates a
mesoscale combustor with high efficiency and low emissions and a novel thermoelectric (TE)
module to directly generate electrical power from the heat released by combustion. First, the
mesoscale combustor and TE power generation are analyzed independently and then as a
complete power generation system.
The important aspects of the present mesoscale combustor are high combustion
efficiency, low emissions, low heat loss and low pressure drop. An annular, fuel-flexible, heatrecirculating mesoscale combustor utilizing porous inert media was redesigned to reduce heat
loss and improve structural strength. Chapter 2 covers the experimental and computational
analysis of this mesoscale combustor.
Thermoelectric devices can offer reliable power for applications such as waste heat
recovery and portable systems.
Current devices provide low TE element/module/system
efficiency, primarily due to the poor heat transfer between the working fluids and TE junctions.
Chapter 3 covers the development of an integrated 3D Computational Fluid Dynamics –
Thermoelectric (3D CFD-TE) model to analyze complex fluid-solid interactions in a TE system.
The model offers insight into the combined effects of fluid flow, heat transfer, and material
properties on TE power generation.
The 3D CFD-TE model was utilized to analyze current TE module designs to understand
interactions among fluid flow, heat transfer, TE material properties and TE power generation. A
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simple TE system is compared with a finned TE system and water-cooled TE system in Chapter
4. Results show significant increases in system efficiency with the finned TE system because of
reduction of thermal resistance by convection, which improved both the heat input ratio, QR, and
TE module efficiency, ηm. The heat input ratio is the ratio of heat input rate into the TE module,
ΣQh, to the heat input rate to the TE system, Qin, as shown in Equation 1-1. TE module
efficiency, ηm, is the fraction of ΣQh converted into electric power, ΣWe, as shown in Equation 12.
QR 
m 
 Qh
(1-1)
Qin
We
 Qh
(1-2)
The system efficiency, represents the fraction of heat input into the system converted into
electric power, is the product of these two terms, i.e:
 s   m  QR 
We
Qin
(1-3)
The water-cooled TE system showed significantly higher system efficiency because of
the reduction of thermal resistance by convection and higher heat capacity of water. The watercooled system demonstrated that good thermal strategies that increase ΣQh and the temperature
difference across the TE module can improve system efficiency even when TE material
performance represented by the TE figure-of-merit, ZT, is low. The TE figure-of-merit, ZT, is a
dimensionless combination of three material properties: Seebeck coefficient (α), electrical
resistivity (ρe), and thermal conductivity (k), as well as the absolute temperature (T), as shown in
Equation 1-4.
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ZT 
2
T
e  k
(1-4)
where T = (Th+Tc)/2 is the algebraic mean of the hot (Th) and cold (Tc) junction temperatures.
Computational analyses of the current TE module designs led to the development of a
novel TE module design to increase the TE system efficiency. This design seeks to increase the
temperature differential between the TE junctions while increasing the heat transfer rate by
increasing heat transfer surface area between the TE module and working fluids. The parametric
study of the novel TE module design, in Chapters 5 and 6, provide fundamental understanding of
the interactions among fluid flow, heat transfer, TE material properties, and TE power
generation. Several improvements to the initial design were made based upon this study.
The final aspect of the study combined the computational models of the mesoscale
combustor and novel TE module design in Chapter 7. The integrated mesoscale combustor thermoelectric (MSC-TE) system allows interactions among fluid flow, heat transfer,
combustion, TE material properties and TE power generation to evaluate the overall system
performance. The parametric study presents detailed flow and temperature fields as reactant
mass flow rate, equivalence ratio, and system length are varied. Effects of operating system
parameters on heat input ratio, TE module efficiency, and TE system efficiency are discussed
and major heat loss pathways are identified.
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