Effect of the Sequence of the Thermoelectric Generator and the Three-Way Catalytic Converter on Exhaust Gas Conversion Efficiency. NANDO pdf

Journal of ELECTRONIC MATERIALS, Vol. 42, No. 7, 2013
DOI: 10.1007/s11664-012-2454-2
Ó 2013 TMS
Effect of the Sequence of the Thermoelectric Generator
and the Three-Way Catalytic Converter on Exhaust Gas
Conversion Efficiency
CHUQI SU,1 NAIQIANG TONG,1,2 YUMAN XU,1 SHAN CHEN,1
and XUN LIU1
1.—Hubei Key Laboratory of Advanced Technology for Automotive Components, Automobile
Engineering Institute, Wuhan University of Technology, Hongshan District, 205 Luoshi Road,
Wuhan 430070, China. 2.—e-mail: [email protected]
The potential for thermoelectric exhaust heat recovery in vehicles has
increased with recent improvements in the efficiency of thermoelectric generators (TEGs). The problem with using thermoelectric generators for vehicle
applications is whether the device is compatible with the original vehicle
exhaust system, which determines the quality of the exhaust gas treatment
and the realization of energy conservation and emission reduction. Based on
ANSYS CFX simulation analysis of the impact of two positional relationships
between the TEG and three-way catalytic converter in the exhaust system on
the working efficiency of both elements, it is concluded that the layout with
the front three-way catalytic converter has an advantage over the other layout
mode under current conditions. New ideas for an improvement program are
proposed to provide the basis for further research.
Key words: Thermoelectric generator (TEG), three-way catalytic converter,
impact on efficiency, ANSYS CFX simulation analysis
INTRODUCTION
Currently, about 30% of the total energy from
automotive engines is used to drive the vehicle,
whereas another 30% is consumed by the cooling
system, and the remaining 40% is emitted as heat
by the exhaust system. A normal sedan wastes
20 kW to 30 kW of heat at uniform speed when the
engine is under common working conditions.1 Part
of this waste heat can be recovered and reused to
reduce the fuel consumption of the vehicle as well as
environmental pollution. Automobile exhaust waste
heat thermoelectric power generation systems can
be used on vehicles to recover part of this energy.
Although not reaching zero emissions, such systems
are appropriate for the current state of energy
recuperation technology.2,3
An automobile exhaust waste heat thermoelectric
power generation system consists of a hot-side heat
(Received July 7, 2012; accepted December 28, 2012;
published online February 9, 2013)
exchanger, a cold-side water tank, and a clamping
device.4 Thermoelectric modules made of semiconductor material are placed between the hot-side
heat exchanger that is connected to the engine
exhaust pipe and the cold-side water tank, enabling
the conversion of the temperature difference
between the hot and cold side into power.5 Based on
the Seebeck effect, thermoelectric modules can
make use of a difference in temperature between
their two ends to generate power. In this work, we
use low-temperature modules made of Ti2Be3,
which are capable of withstanding hot temperatures
below 300°C.6
However, such addition of a thermoelectric generator (TEG) to the automotive exhaust system will
affect the overall system, especially the three-way
catalytic converter.7 Since the TEG and three-way
catalytic converter perform properly when the
temperature reaches a certain level, simulation
analysis of the relative positional relationship of
these two elements was carried out to determine a
reasonable layout. This research also provides the
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Su, Tong, Xu, Chen, and Liu
foundation for future
improvement programs.
study
directions
and
ANALYSIS ON THE RELATIVE POSITIONAL
RELATIONSHIP BETWEEN THE TEG
AND THREE-WAY CATALYTIC CONVERTER
There are two positional relationship types: Fig. 1
shows one type in which the TEG is placed in front
of the three-way catalytic converter, in which the
exhaust gas goes through the TEG first and then
the three-way catalytic converter; Fig. 2 shows the
other type in which the TEG is placed behind the
three-way catalytic converter, in which exhaust gas
goes through the three-way catalytic converter first
and then the TEG. When the TEG is in front, it
absorbs some heat from the exhaust gas. Consequently, the remaining heat of this gas is insufficient to raise the temperature of the three-way
catalytic converter to the required standard; when
the three-way catalytic converter is in front, the
temperature of the exhaust gas rises after the
reaction in the three-way catalytic converter. As a
result, the temperature of the hot side of the TEG
exceeds the maximum upper temperature of the
thermoelectric modules, possibly damaging them.
According to the characteristics of the two layout
modes discussed above, the following assumptions
are made:
1. When the TEG is in front, one analyzes whether
the temperature of the exhaust gas could reach
the standard value demanded by the three-way
catalytic converter, regardless of heat release in
the catalytic reaction;
2. When the three-way catalytic converter is in
front, one analyzes whether the TEG can work
normally using the hotter gas after its passage
through the three-way catalytic converter.
SIMULATION
Simulation of the TEG in Front Layout
Boundary Condition Setting
The popular computational fluid dynamics (CFD)
software ANSYS CFX was adopted for the simulation. The boundary conditions must be specified,
and in this case are set as follows:
Fig. 1. Model with the TEG first.
Inlet boundary condition: average velocity of
exhaust, 15 m/s; exhaust temperature, 800 K. Outlet boundary condition: static pressure, 0 MPa. To
simplify the simulation model, it assumed in this
case that the outlet is connected to the surrounding
environment and the air pressure is equal to
atmospheric pressure correspondingly.
Convective heat transfer coefficient of the surface
of the heat exchanger of the TEG: 20 W/(m2 K). The
three-way catalyst parameters under the assumption that the reaction therein is uniform are: the
medium is set as common ceramic materials; energy
source value, 1 9 106 W/m3; convective heat transfer coefficient, 10 W/(m2 K).
Gas in this system is considered incompressible,
the flow is fully turbulent, and the molecular viscosity can be assumed negligible in the simulation
model. All of these are in line with the application of
k–e turbulence model equations.
Simulation Results
Figures 3–5 show the results obtained from the
CFX simulations. Figure 3 presents the temperature distribution of the layout with the TEG in
front. The highest temperature on the surface of the
heat exchanger is 540.7 K (282.6°C), and the lowest
one is 458.6 K (237°C). Due to the exhaust speed,
which is set as 15 m/s, and the exhaust direction
from the heat exchanger to the three-way catalyst,
the effect caused by the downstream three-way
catalyst on the convective heat transfer of the
upstream heat exchanger is slight.
The surface heat flux density distribution of the
whole model is shown in Fig. 4. Negative values
indicate that the surface of the model is exothermic,
and the value represents the heat flux density. The
distribution of the heat flux density is similar to
that of the temperature distribution. The higher the
temperature, the larger the heat flux density value.
The average heat flux density of the available top
and bottom surface of the heat exchanger based on
the calculation results is 3910 W/m2.
Figure 5 shows the temperature of the medium of
the three-way catalyst. Because of the in-front
position of the heat exchanger, the exhaust temperature of the three-way catalyst inlet is low, as is
Fig. 2. Model with the three-way catalyst first.
Effect of the Sequence of the Thermoelectric Generator and the Three-Way Catalytic Converter
on Exhaust Gas Conversion Efficiency
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Fig. 3. Temperature distribution with the TEG first.
Fig. 4. Heat flux distribution with the TEG first.
Fig. 5. Three-way catalyst carrier temperature distribution with the TEG first.
the whole temperature of the three-way catalyst.
The highest temperature is 260°C, which occurs at
the center of the medium inlet; correspondingly the
lowest temperature is 164°C, locating at the end of
the medium and near the three-way catalyst shell.
The average temperature of the whole medium is
230°C, which is lower than the ignition temperature
(250°C) of the harmful gas in the vehicle exhaust.
This means that, when the vehicle exhaust flows
through the medium of the three-way catalyst,
the catalytic reaction only happens in some of the
medium rather than uniformly throughout the
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Su, Tong, Xu, Chen, and Liu
Fig. 6. Temperature distribution with the three-way catalyst first.
Fig. 7. Heat exchanger local temperature distribution with the three-way catalyst first.
whole medium. It is easy to conclude that the layout
with the TEG in front is unfavorable for normal
operation of the three-way catalyst.
Simulation of the Three-Way Catalyst
in Front Layout
Simulation Results
Figure 6 presents the temperature distribution of
the system with the three-way catalyst in front
as obtained from the CFX simulations. Due to the
in-front location of the three-way catalyst and the
large amount of heat released by the catalytic
reaction therein, high-temperature areas are centralized at the shell of the three-way catalyst.
Meanwhile, the heat exchanger is located downstream in the exhaust system and does not have a
heat source. The temperature of the heat exchanger
is low and changes little; the temperature distribution is shown in Fig. 7.
The highest temperature on the surface of the heat
exchanger is 510 K (265.6°C), and the lowest one is
439 K (226.1°C). The highest temperature is not
higher than the maximum temperature that the
thermoelectric modules can withstand, and is even
Fig. 8. Model of the TEG with two heat exchangers.
lower than for the layout with the TEG in front. The
average heat flux density of the available top and
bottom surface of the heat exchanger based on the
calculation results is 3510 W/m2. Thus, the heat
emission of the chemical reaction in the three-way
catalyst is less than the heat dispersed by the exhaust.
The largest useful surface area of one surface of
the heat exchanger is about 0.15 m2. According to
the simulation analysis, when the TEG is in front,
the total energy exchange of the heat exchanger
surfaces is 1173 W; the temperature of the threeway catalyst is only 230°C under this condition,
which cannot meet the normal working requirements for the three-way catalyst.
Effect of the Sequence of the Thermoelectric Generator and the Three-Way Catalytic Converter
on Exhaust Gas Conversion Efficiency
1881
Fig. 9. Heat flux distribution of the TEG with two heat exchangers.
When the three-way catalyst is in front, it can
perform properly, but the total energy of the TEG
heat exchanger’s surfaces is 1053 W, which is lower
than the previous condition by 120 W.
Optimization of the Three-Way Catalyst
in Front Layout
In the three-way catalyst in front system, the heat
emitted by the chemical reaction in the three-way
catalyst is less than the heat dispersed by the
exhaust. Thus, less heat can be recovered by the
TEG located in the downstream exhaust system.
Expanding the area for heat transfer is an effective
way to strengthen the heat convection, so it is practicable to increase the number of heat exchangers in
the TEG in order to recover more exhaust heat.
In this study, a three-dimensional model of a TEG
with two heat exchangers connected by a Y-type
fitting was designed as shown in Fig. 8. Considering
the restricted computational capacity available,
only a double-layer heat exchanger was simulated.
The temperature and velocity of the outlet gas of
the three-way catalyst are regarded as the inlet
boundary conditions of the double-layer heat
exchanger, with the other boundary conditions kept
unchanged. The simulation result is shown in
Fig. 9.
The simulation results indicate that the heat
flux density of each available surface of the heat
exchanger is basically consistent, and the average
surface heat flux is 2200 W/m2. Although the average heat flux density of a single surface of the
double-layer heat exchanger is less than that of the
single-layer heat exchanger, the total energy
recovery is increased by 25%. In comparison with
the heat recovered by the system with the singlelayer heat exchanger in front, the heat converted by
the system with the three-way catalyst with doublelayer heat exchanger in front is increased by 12.5%.
CONCLUSIONS
The challenge with onboard TEG use is to ensure
that the three-way catalyst and TEG can operate
normally at their corresponding working temperatures. According to the presented simulation analysis,
the results for the two considered layouts are as follows:
1. With the TEG in front, the three-way catalyst
cannot work normally, as its temperature is
lower than its working temperature, while the
TEG can recover more energy because of the high
temperature.
2. If the three-way catalyst is in front, it will work
normally. Although the chemical reactions give
off a large amount of heat, the temperature at the
TEG is lower than the condition with the TEG in
front. The heat recoverable through the thermoelectric power generation system is lower than in
the first condition.
Therefore, each of the two design has its own advantages. In the existing conditions, the layout with the
TEG after the three-way catalyst seems more practical. For both conditions, changing the materials of the
catalytic carrier and the heat exchanger or increasing
the number of heat exchangers will improve the thermal energy recovery efficiency. These measures will be
studied in future research.
ACKNOWLEDGEMENTS
This work is funded by Grant No. 2007CB607501
from International Science &Technology Cooperation Program of China, No. 2011DFB60150 developed by General Motors Corporation and Wuhan
University of Technology.
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3. K.T. Chau and Y.S. Wong, Energy Convers. Manage. 42,
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