CHAPTER 8
CONCLUSIONS
A mesoscale combustor / thermoelectric (MSC-TE) power generation system was
investigated in this study. In the first part of the study, a mesoscale combustor was designed for
fuel-flexibility with liquid and gaseous fuels and incorporates heat-recirculation and porous inert
media for flame stabilization and improved combustion efficiency. The 12.8-cm³ mesoscale
combustor was operated on methane with heat release rate up to 330 W and it provided 99.7%
combustion efficiency and low NOx and CO emissions.
A computational analysis of the
mesoscale combustor showed heat loss of within 5% of the heat release rate. Detailed chemical
kinetics analysis identified major NOx formation mechanisms in this combustor. Experimental
and computational analysis demonstrated a low pressure drop through the mesoscale combustor.
In the second part of the study, an integrated 3D computational fluid dynamics / thermoelectric
(3D CFD-TE) power generation model was developed that permitted analysis of the interactions
among heat transfer, fluid flow and TE power generation and provided accurate analysis of TE
power generation systems. Analysis of current TE module design with the 3D CFD-TE model
provided insight into the importance of thermal strategies and their impact on TE system
efficiency. A novel TE module design was developed with the 3D CFD-TE model that yielded
improved system efficiency, ηs, by optimizing the design parameters to improve the heat input
ratio, QR, and TE module efficiency, ηm. The final part of the study was a computational
analysis of the integrated MSC-TE system.
The important conclusions of this study are
summarized below.
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
The mesoscale combustor achieved peak temperatures ranging from 2000 K to 2300 K for
m f = 16 g/hr to m f = 24 g/hr. The computational analysis, validated by experimental
data, demonstrated that heat loss was limited to 3 % - 6 % of the total heat release rate.

Heat recirculation along with PIM permitted high combustion efficiency with low
emissions. The computational analysis indicated high heat recirculation of 17 % - 18 % of
the HRR.

Experimental measurements demonstrated 85 Pa to 155 Pa of pressure drop through the
combustor with operation on methane. Through the entire system, including the fuel
injectors designed for liquid fuels, the pressure drop was 12 kPa to 31 kPa on methane
operation. The low mass flow rate of the reactants and low pressure drop indicate minor
pump work is required for combustor operation.

A CFD model incorporating TE power generation was developed that demonstrates the
importance of the interactions among heat transfer, fluid flow and TE power generation and
provides a tool for analysis of TE module design.

Computational analysis of current TE design, without and with fins, showed system
efficiencies of 0.1 % and 0.4 %, respectively, without heat recirculation. The low system
efficiency without and with fins is a result of low TE module efficiencies of 0.62 % and
0.86 % and low heat input ratios of 0.17 and 0.46. With heat recirculation, the system
efficiencies without and with fins increases to 0.13 % and 0.72 %. A water-cooled design
increased the system efficiency to 0.87 % without heat recirculation and 4.4 % with heat
recirculation showing the importance of thermal strategies. However, these efficiencies are
still low due to low heat input ratio and high thermal resistance between the TE module and
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working fluids and low temperature difference across the TE module due to the small
thickness (width) of current TE modules.

A novel TE design was developed to improve system efficiency by lowering the thermal
resistance between the TE module and working fluids and increasing the spacing between
the junctions of the TE module. With ideal conditions of an adiabatic divider between the
fluids and orthotropic conductor that limits axial conduction, the novel TE design achieved
ns,nr = 2.4 % without heat recirculation and ns,wr = 5.8 % with heat recirculation. The
computational parametric study showed the importance of TE design on system efficiency
as ns,nr ranged from 1.2 % to 2.4 % as design parameters varied. With heat recirculation,
ns,wr ranged from 1.4 % to 5.8 %.

As the temperature difference across the TE module increases, the temperature-dependent
TE material properties start having a greater effect on system efficiency. The Peltier effect
and Thomson effect are related due to the relationship between the Seebeck coefficient, α,
and Thomson coefficient, τ.
The magnitude of each effect would increase as the
temperature difference increased though the net Peltier effect resulted in the electric power
generation while the Thompson effect resulted in electric power loss.

A non-adiabatic divider between the fluids resulted in reduced system efficiency as it
diverted heat transfer from the TE module as well as increasing the total heat transfer
between the fluids which reduced the temperature difference between the fluids. While an
adiabatic divider is not achievable, it is important to limit the heat transfer to the TE
module as much as possible.

The isotropic conductor resulted in significant axial heat conduction, resulting in a nearly
uniform axial temperature profile. This negated the advantage of the counterflow heat
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exchanger as a significant portion of the heat transfer would occur near the inlets and
rapidly drops off downstream. The orthotropic conductor limits axial conduction and the
axial temperature profile of the conductor and junctions follow the trend of the bulkaveraged fluid temperatures. This resulted in better system efficiency and an effective
orthotropic conductor is possible with strips of conductor separated by gaps or insulator
such as the glass-mica composite.

Computational analysis of the integrated MSC-TE power generation system incorporated
both the mesoscale combustor design and novel thermoelectric module design. The MSCTE system achieved a system efficiency of 2%. Pumping losses, calculated from mass
flow rate and pressure drop, were only 2% of the electrical power generated.

The TE module temperatures were generally higher than the 1000 K - 1200 K temperature
range for optimum performance from the Si-Ge semiconductor materials and the TE
module design was not optimized for the temperature profiles in the combustor, so higher
system efficiency is likely possible.

While a longer MSC-TE system and TE module was expected to improve system
efficiency, the additional heat loss resulted in only minor improvement in system
efficiency. Thus, the smaller design with the 20-mm long TE module would be a better
choice to reduce the cost of TE material.
The research demonstrates that the MSC-TE power generation system is an option for
portable power generation and alternative to battery technology. Improvement in the individual
components can be utilized in other systems. The development of the integrated CFD-TE model
is an important tool in the analysis of TE systems. The novel TE design could be utilized in
systems other than portable power generation, such as waste heat recovery.
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Recommendations
The study has been a significant step in the development of a portable MSC-TE power
generation system.
Further study could improve the efficiency of the design as well as
understanding of the underlying physics. In particular, the following recommendations could be
investigated:

In-depth chemical kinetic analysis of the combustion process within the mesoscale
combustor. The computational model simplified the combustion process to heat release
rate in 1-mm thick disc representing surface combustion. Limited investigation is possible
of the interior combustion mode experimentally. A chemical kinetic model could aid
modeling interior combustion with CFD software. There are also transitional zones and
overlap, with respect to reactant flow rate and equivalence ratio, between interior and
surface combustion modes. Likewise, between surface combustion and blow-off, there is a
transition where a stable flamelet forms in the combustion tube.

Experimental analysis of the mesoscale combustor with liquid fuels to analyze the effect on
heat loss, pressure drop, and emissions. The high energy density (by volume) and high
specific energy (by mass) of liquid fuels is the reason the MSC-TE power systems could be
an alternative to battery technology, but further study is needed.

The development of the novel TE design has been limited to computational analysis.
Development of an experimental version of the novel TE design would validate and help
refine the computational model as well as identify any issues in the fabrication of the novel
TE design. Likewise, development and analysis of an effective orthotropic conductor with
alternating strips of conductor and insulator could help refine the model.
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
The integrated MSC-TE model provided promising results but the TE module temperatures
exceed the operational temperatures of current TE semi-conductor materials. While there
is research on high-temperature TE semi-conductor materials, it could be beneficial to
investigate methods of reducing the TE module temperature in the MSC-TE system while
minimizing the reduction in system efficiency.
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