Design of a Methanol to Hydrogen Microreformer for Fuel Cell Applications
(2006)
Available from
Jeremy Gernand's profile on Mendeley.
or
Abstract
A new design for a microchannel methanol-steam reformer has been developed to provide power in conjunction with a micro fuel cell for a portable, low- power device. The design is optimized for low pumping power and rapid operation as well as thermal efficiency, overall size, and complete generation of the available hydrogen. An iterative, implicit, finite element solution code, which locates the boundaries between liquid, two-phase, and gaseous flow, provides a complete solution of the fluid and heat transfer properties throughout the device. The solution employs experimentally verified microchannel fluid dynamics relations to develop accurate results, but this is the first application of those relations to a methanol- water mixture. Based on this analysis, the proposed microreformer design will have an efficiency of 42%, with a theoretical maximum of 70%.
Available from
Jeremy Gernand's profile on Mendeley.
Page 1
Design of a Methanol to Hydrogen Microreformer for Fuel Cell Applications
RICE UNIVERSITY
Design of a Methanol to Hydrogen Micro-Reformer
for Fuel Cell Applications
by
Jeremy Michael Gernand
A THESIS SUBMITTED
IN PARTIAL FULFILMENT OF THE
REQUIREMENTS OF THE DEGREE
Master of Science
APPROVED, THESIS COMMITTEE
________________________________
Yildiz Bayazitoglu, Chair
Harry S. Cameron Chair Professor
Mechanical Engineering and Materials Science
________________________________
Andrew Meade
Associate Professor
Mechanical Engineering and Materials Science
________________________________
Jun Lou
Assistant Professor
Mechanical Engineering and Materials Science
HOUSTON, TEXAS
JANUARY 2007
Design of a Methanol to Hydrogen Micro-Reformer
for Fuel Cell Applications
by
Jeremy Michael Gernand
A THESIS SUBMITTED
IN PARTIAL FULFILMENT OF THE
REQUIREMENTS OF THE DEGREE
Master of Science
APPROVED, THESIS COMMITTEE
________________________________
Yildiz Bayazitoglu, Chair
Harry S. Cameron Chair Professor
Mechanical Engineering and Materials Science
________________________________
Andrew Meade
Associate Professor
Mechanical Engineering and Materials Science
________________________________
Jun Lou
Assistant Professor
Mechanical Engineering and Materials Science
HOUSTON, TEXAS
JANUARY 2007
Page 2
ABSTRACT
Design of a Methanol to Hydrogen Micro-Reformer
for Fuel Cell Applications
by
Jeremy Michael Gernand
A new design for a microchannel methanol-steam reformer has been
developed to provide power in conjunction with a micro fuel cell for a portable, low-
power device. The design is optimized for low pumping power and rapid operation as
well as thermal efficiency, overall size, and complete generation of the available
hydrogen. An iterative, implicit, finite element solution code, which locates the
boundaries between liquid, two-phase, and gaseous flow, provides a complete
solution of the fluid and heat transfer properties throughout the device. The solution
employs experimentally verified microchannel fluid dynamics relations to develop
accurate results, but this is the first application of those relations to a methanol-
water mixture. Based on this analysis, the proposed microreformer design will have
an efficiency of 42%, with a theoretical maximum of 70%.
Design of a Methanol to Hydrogen Micro-Reformer
for Fuel Cell Applications
by
Jeremy Michael Gernand
A new design for a microchannel methanol-steam reformer has been
developed to provide power in conjunction with a micro fuel cell for a portable, low-
power device. The design is optimized for low pumping power and rapid operation as
well as thermal efficiency, overall size, and complete generation of the available
hydrogen. An iterative, implicit, finite element solution code, which locates the
boundaries between liquid, two-phase, and gaseous flow, provides a complete
solution of the fluid and heat transfer properties throughout the device. The solution
employs experimentally verified microchannel fluid dynamics relations to develop
accurate results, but this is the first application of those relations to a methanol-
water mixture. Based on this analysis, the proposed microreformer design will have
an efficiency of 42%, with a theoretical maximum of 70%.
Page 3
iii
ACKNOWLEDGEMENTS
My wife, Alison, and my manager, Cheryl Corbin, made possible this work.
Without their efforts, flexibility, and understanding, I could not have made it to the
end. I would also like to acknowledge my advisor, Dr. Yildiz Bayazitoglu, and thesis
committee, Drs. Andrew Meade and Jun Lou.
ACKNOWLEDGEMENTS
My wife, Alison, and my manager, Cheryl Corbin, made possible this work.
Without their efforts, flexibility, and understanding, I could not have made it to the
end. I would also like to acknowledge my advisor, Dr. Yildiz Bayazitoglu, and thesis
committee, Drs. Andrew Meade and Jun Lou.
Page 4
iv
TABLE OF CONTENTS
INTRODUCTION................................................................................................................... 9
Fuel Cells ........................................................................................................................9
Methanol Reforming ....................................................................................................11
Microchannel Device Manufacture.............................................................................11
LITERATURE REVIEW........................................................................................................14
DESIGN REQUIREMENTS.................................................................................................20
Power Requirement .....................................................................................................20
Heat Requirements and Efficiency .............................................................................22
ANALYSIS AND OPTIMIZATION ........................................................................................24
Preheat Region.............................................................................................................31
Phase Change Region..................................................................................................33
Superheat Region ........................................................................................................36
Catalytic Reactor Region .............................................................................................37
Geometric Effects ........................................................................................................39
RESULTS ......................................................................................................................58
MICROCHANNEL DEVICE MANUFACTURING ..................................................................77
CONCLUSIONS AND RECOMMENDATIONS ....................................................................80
Conclusions ..................................................................................................................80
Recommendations.......................................................................................................80
REFERENCES....................................................................................................................82
APPENDIX A: Silicon Wall Stress Analysis......................................................................86
APPENDIX B: Insulation and Thermal Efficiency Analysis ............................................91
TABLE OF CONTENTS
INTRODUCTION................................................................................................................... 9
Fuel Cells ........................................................................................................................9
Methanol Reforming ....................................................................................................11
Microchannel Device Manufacture.............................................................................11
LITERATURE REVIEW........................................................................................................14
DESIGN REQUIREMENTS.................................................................................................20
Power Requirement .....................................................................................................20
Heat Requirements and Efficiency .............................................................................22
ANALYSIS AND OPTIMIZATION ........................................................................................24
Preheat Region.............................................................................................................31
Phase Change Region..................................................................................................33
Superheat Region ........................................................................................................36
Catalytic Reactor Region .............................................................................................37
Geometric Effects ........................................................................................................39
RESULTS ......................................................................................................................58
MICROCHANNEL DEVICE MANUFACTURING ..................................................................77
CONCLUSIONS AND RECOMMENDATIONS ....................................................................80
Conclusions ..................................................................................................................80
Recommendations.......................................................................................................80
REFERENCES....................................................................................................................82
APPENDIX A: Silicon Wall Stress Analysis......................................................................86
APPENDIX B: Insulation and Thermal Efficiency Analysis ............................................91
Page 5
v
APPENDIX C: Microreformer Design Drawings..............................................................95
APPENDIX D: Fuel Cartridge Feasibility Analysis........................................................ 100
APPENDIX E: Effect of Pin Fins on Microreformer...................................................... 102
LIST OF TABLES
Table 1: Portable Electronic Devices and Their Approximate Average Power
Requirements..............................................................................................20
Table 2: Comparison of Channel Dimensional Orientation ....................................25
Table 3: Comparison of Channel Packing Orientations ..........................................26
Table 4: Optimization Criteria Summary ..................................................................57
Table B-1: Potential Insulating Materials and the Required Thicknesses to
Achieve Safe Surface Temperature ...........................................................91
LIST OF FIGURES
Figure 1: Proposed Microreformer Design................................................................13
Figure 2: Archimedes’ Spiral ......................................................................................26
Figure 3: Numerical Solution Cell Parameters..........................................................27
Figure 4: Design Concept for Microreformer (assembled view) ..............................28
Figure 5: Design Concept for Microreformer Assembly (exploded view) ................29
Figure 6: Channel Detail Diagram .............................................................................29
Figure 8: Solution Code Flowchart.............................................................................31
Figure 9: Linear and Exponential Chemical Reaction Models .................................39
Figure 10: Total Channel Length Required for Varying Channel Widths ...................41
Figure 11: Resultant Device Diameter for Varying Channel Widths ..........................42
APPENDIX C: Microreformer Design Drawings..............................................................95
APPENDIX D: Fuel Cartridge Feasibility Analysis........................................................ 100
APPENDIX E: Effect of Pin Fins on Microreformer...................................................... 102
LIST OF TABLES
Table 1: Portable Electronic Devices and Their Approximate Average Power
Requirements..............................................................................................20
Table 2: Comparison of Channel Dimensional Orientation ....................................25
Table 3: Comparison of Channel Packing Orientations ..........................................26
Table 4: Optimization Criteria Summary ..................................................................57
Table B-1: Potential Insulating Materials and the Required Thicknesses to
Achieve Safe Surface Temperature ...........................................................91
LIST OF FIGURES
Figure 1: Proposed Microreformer Design................................................................13
Figure 2: Archimedes’ Spiral ......................................................................................26
Figure 3: Numerical Solution Cell Parameters..........................................................27
Figure 4: Design Concept for Microreformer (assembled view) ..............................28
Figure 5: Design Concept for Microreformer Assembly (exploded view) ................29
Figure 6: Channel Detail Diagram .............................................................................29
Figure 8: Solution Code Flowchart.............................................................................31
Figure 9: Linear and Exponential Chemical Reaction Models .................................39
Figure 10: Total Channel Length Required for Varying Channel Widths ...................41
Figure 11: Resultant Device Diameter for Varying Channel Widths ..........................42
Page 6
vi
Figure 12: Total Heat Loss from Microreformer for Varying Channel Widths ...........44
Figure 13: Maximum Mach Number within Microreformer for Varying Channel
Widths..........................................................................................................46
Figure 14: Maximum Fluid Velocity within Microreformer for Varying Channel
Widths..........................................................................................................47
Figure 15: Maximum Reynolds Number within Microreformer for Varying
Channel Widths ...........................................................................................48
Figure 16: Maximum Knudsen Number within Microreformer for Varying
Channel Widths ...........................................................................................49
Figure 17: Maximum Slip Velocity within Microreformer for Varying Channel
Widths..........................................................................................................50
Figure 18: Maximum Mass Velocity for Varying Channel Widths...............................51
Figure 19: Fluid Transit Time for Varying Channel Widths .........................................52
Figure 20: Available Reactor Surface Area for Varying Channel Widths ...................53
Figure 21: Total Pressure Drop for Microreformer with Channel Widths between
20 and 210 Micrometers ...........................................................................55
Figure 22: Total Pressure Drop for Microreformer with Channel Widths between
210 and 400 Micrometers.........................................................................56
Figure 23: Microreformer Channel Width Optimization Plot Displaying Pressure
Drop, Heat Loss, Mach Number, and Transit Time...................................57
Figure 24: Pressure Drop per Unit of Distance for the Preheat Region ....................59
Figure 25: Pressure Drop per Unit of Distance for the Phase Change Region .........60
Figure 26: Pressure Drop per Unit of Distance for the Superheat Region................60
Figure 27: Pressure Drop per Unit of Distance for the Catalytic Reactor Region.....61
Figure 12: Total Heat Loss from Microreformer for Varying Channel Widths ...........44
Figure 13: Maximum Mach Number within Microreformer for Varying Channel
Widths..........................................................................................................46
Figure 14: Maximum Fluid Velocity within Microreformer for Varying Channel
Widths..........................................................................................................47
Figure 15: Maximum Reynolds Number within Microreformer for Varying
Channel Widths ...........................................................................................48
Figure 16: Maximum Knudsen Number within Microreformer for Varying
Channel Widths ...........................................................................................49
Figure 17: Maximum Slip Velocity within Microreformer for Varying Channel
Widths..........................................................................................................50
Figure 18: Maximum Mass Velocity for Varying Channel Widths...............................51
Figure 19: Fluid Transit Time for Varying Channel Widths .........................................52
Figure 20: Available Reactor Surface Area for Varying Channel Widths ...................53
Figure 21: Total Pressure Drop for Microreformer with Channel Widths between
20 and 210 Micrometers ...........................................................................55
Figure 22: Total Pressure Drop for Microreformer with Channel Widths between
210 and 400 Micrometers.........................................................................56
Figure 23: Microreformer Channel Width Optimization Plot Displaying Pressure
Drop, Heat Loss, Mach Number, and Transit Time...................................57
Figure 24: Pressure Drop per Unit of Distance for the Preheat Region ....................59
Figure 25: Pressure Drop per Unit of Distance for the Phase Change Region .........60
Figure 26: Pressure Drop per Unit of Distance for the Superheat Region................60
Figure 27: Pressure Drop per Unit of Distance for the Catalytic Reactor Region.....61
Page 7
vii
Figure 28: Ratio of Curvature to Channel Diameter .....................................................62
Figure 29: Vapor Quality vs. Channel Location in Phase Change Region .................63
Figure 30: Mean Fluid Velocity in the Preheat Region ...............................................64
Figure 31: Mean Fluid Velocity in the Phase Change Region ....................................65
Figure 32: Mean Fluid Velocity in the Superheat Region ...........................................66
Figure 33: Mean Fluid Velocity in the Catalytic Reactor Region ................................67
Figure 34: Temperature Map of the Microreformer (Side 1, Top; Side 2, Bottom) ..68
Figure 35: Temperature Curve in the Preheat Region................................................69
Figure 36: Temperature Curve in the Superheat Region ...........................................70
Figure 37: Fluid Slip Velocity by Channel Coordinate .................................................71
Figure 38: Fluid Mach Number by Channel Coordinate .............................................72
Figure 39: Fluid Knudsen Number by Channel Coordinate .......................................73
Figure 40: Convection Coefficient in the Preheat Region ..........................................74
Figure 41: Convection Coefficient in the Phase Change Region ...............................75
Figure 42: Convection Coefficient in the Superheat Region ......................................76
Figure 43: Convection Coefficient in the Catalytic Reactor Region ...........................77
Figure 44: Microreformer Manufacturing Process......................................................79
Figure A-1: Solid Model of Silicon Wall Test Configuration .........................................87
Figure A-2: Solid Mesh for Silicon Wall Stress Analysis...............................................88
Figure A-3: Close View of Solid Mesh for Silicon Wall Stress Analysis .......................88
Figure A-4: Displacement Results in Silicon Wall with Applied Pressure of 1.06
MPa..............................................................................................................89
Figure A-5: Results for Stresses in Silicon Wall with Applied Pressure of 1.06
MPa..............................................................................................................90
Figure 28: Ratio of Curvature to Channel Diameter .....................................................62
Figure 29: Vapor Quality vs. Channel Location in Phase Change Region .................63
Figure 30: Mean Fluid Velocity in the Preheat Region ...............................................64
Figure 31: Mean Fluid Velocity in the Phase Change Region ....................................65
Figure 32: Mean Fluid Velocity in the Superheat Region ...........................................66
Figure 33: Mean Fluid Velocity in the Catalytic Reactor Region ................................67
Figure 34: Temperature Map of the Microreformer (Side 1, Top; Side 2, Bottom) ..68
Figure 35: Temperature Curve in the Preheat Region................................................69
Figure 36: Temperature Curve in the Superheat Region ...........................................70
Figure 37: Fluid Slip Velocity by Channel Coordinate .................................................71
Figure 38: Fluid Mach Number by Channel Coordinate .............................................72
Figure 39: Fluid Knudsen Number by Channel Coordinate .......................................73
Figure 40: Convection Coefficient in the Preheat Region ..........................................74
Figure 41: Convection Coefficient in the Phase Change Region ...............................75
Figure 42: Convection Coefficient in the Superheat Region ......................................76
Figure 43: Convection Coefficient in the Catalytic Reactor Region ...........................77
Figure 44: Microreformer Manufacturing Process......................................................79
Figure A-1: Solid Model of Silicon Wall Test Configuration .........................................87
Figure A-2: Solid Mesh for Silicon Wall Stress Analysis...............................................88
Figure A-3: Close View of Solid Mesh for Silicon Wall Stress Analysis .......................88
Figure A-4: Displacement Results in Silicon Wall with Applied Pressure of 1.06
MPa..............................................................................................................89
Figure A-5: Results for Stresses in Silicon Wall with Applied Pressure of 1.06
MPa..............................................................................................................90
Page 8
viii
Figure A-6: Design Check in Silicon Wall with Applied Pressure of 1.06 MPa...........90
Figure B-1: Temperature Location Diagram for Microreformer Thermal Analysis .....92
Figure B-2: Microreformer Heat Loss as a Function of Channel Width ......................93
Figure B-3: COSMOS Analysis Plot of Insulation Optimization Results.......................94
Figure D-1: Schematic of Possible Portable Fuel Cartridge Design ......................... 100
Figure E-1: Top View of Pin Fins in the Microreformer Channel ............................... 103
Figure E-2: Detail Picture of Pin Fins in the Microreformer Channel........................ 103
Figure E-3: Plot of Pressure Drop at Various Pin Porosities...................................... 104
Figure E-4: Plot of Pressure Drop at Various Pin Diameters ..................................... 105
Figure E-5: Total Catalytic Surface Area Obtained by Use of Pin Fins ...................... 106
Figure E-6: Pressure Drop per Unit of Channel Length including Pin Fins............... 107
Figure A-6: Design Check in Silicon Wall with Applied Pressure of 1.06 MPa...........90
Figure B-1: Temperature Location Diagram for Microreformer Thermal Analysis .....92
Figure B-2: Microreformer Heat Loss as a Function of Channel Width ......................93
Figure B-3: COSMOS Analysis Plot of Insulation Optimization Results.......................94
Figure D-1: Schematic of Possible Portable Fuel Cartridge Design ......................... 100
Figure E-1: Top View of Pin Fins in the Microreformer Channel ............................... 103
Figure E-2: Detail Picture of Pin Fins in the Microreformer Channel........................ 103
Figure E-3: Plot of Pressure Drop at Various Pin Porosities...................................... 104
Figure E-4: Plot of Pressure Drop at Various Pin Diameters ..................................... 105
Figure E-5: Total Catalytic Surface Area Obtained by Use of Pin Fins ...................... 106
Figure E-6: Pressure Drop per Unit of Channel Length including Pin Fins............... 107
Page 9
9
INTRODUCTION
The storage of energy and the utilization of that energy at places or times far
away from the initial generation or capture of that energy is a critical discipline for
today’s society. Without this capability most economic activity would not be possible,
and many of the capabilities most individuals depend on for health, safety or general
well being would not exist. While there are many forms of energy storage and
utilization, there exists an obligation to find the most efficient methods possible so
that the positive benefits are most available and any detrimental effects are
minimized.
Mechanical, nuclear, thermal, electrical, and chemical methods are all utilized
to provide short term or long-term storage of energy. Of these, chemical methods,
which include combustible fuels and batteries, provide high power capabilities and
rapid availability along with long storage durations for a variety of critical portable
and stationary applications.
Fuel Cells
Batteries are the primary present day solution to these needs, but batteries
have significant limitations. Many battery chemistries in use today are mature
technologies reaching their theoretical limit in terms of power density. Many
batteries contain materials that are toxic and difficult to dispose of properly without
posing risks to health or the environment. Batteries usually require a period of 1 up
to 4 hours to fully recharge, which limits their utility and the distance that portable
devices can be taken away from the power grid.
INTRODUCTION
The storage of energy and the utilization of that energy at places or times far
away from the initial generation or capture of that energy is a critical discipline for
today’s society. Without this capability most economic activity would not be possible,
and many of the capabilities most individuals depend on for health, safety or general
well being would not exist. While there are many forms of energy storage and
utilization, there exists an obligation to find the most efficient methods possible so
that the positive benefits are most available and any detrimental effects are
minimized.
Mechanical, nuclear, thermal, electrical, and chemical methods are all utilized
to provide short term or long-term storage of energy. Of these, chemical methods,
which include combustible fuels and batteries, provide high power capabilities and
rapid availability along with long storage durations for a variety of critical portable
and stationary applications.
Fuel Cells
Batteries are the primary present day solution to these needs, but batteries
have significant limitations. Many battery chemistries in use today are mature
technologies reaching their theoretical limit in terms of power density. Many
batteries contain materials that are toxic and difficult to dispose of properly without
posing risks to health or the environment. Batteries usually require a period of 1 up
to 4 hours to fully recharge, which limits their utility and the distance that portable
devices can be taken away from the power grid.
Page 14
14
LITERATURE REVIEW
A successful design for a methanol microreformer requires the application of
several disciplines. Since traditional or classical fluid and heat transfer science may
not be applicable for all micro-scale applications, new studies are required to
accurately predict the behavior of various designs. Several different investigators
have studied various aspects of the phenomena involved in this problem, however
some areas remain open and in need of further study. The best available research is
outlined here and utilized for development analysis of this microreformer design.
Research into fuel cell technology is proceeding at a brisk pace, although
most new work examines automobile sized portable fuel cells. Results from these
studies do, however, provide information for our application in terms of operating
temperature, fuel supply requirements, and operating efficiency. A review of polymer
electrolyte fuel cells by Prater verifies the basic assumptions made about the
operational requirements of PEM fuel cells for this paper to ensure that the proposed
microreformer will successfully operate as part of a system [4].
The fluid passing through this microreformer design will experience many flow
conditions, and a much wider variety will be experienced when considering all of the
geometric variations studied to arrive at an optimized design. Relations for single-
and two-phase flow for liquids and gasses in microchannels and minichannels will all
be required to conduct this study.
Morini and others have studied the laminar flow of liquid through
microchannels [5]. Among the significant findings is that classical Navier-Stokes
equations remain valid for microchannels with hydraulic diameters as low as 30 µm.
While, this will not hold true for gasses, it will result in less complexity for the analysis
LITERATURE REVIEW
A successful design for a methanol microreformer requires the application of
several disciplines. Since traditional or classical fluid and heat transfer science may
not be applicable for all micro-scale applications, new studies are required to
accurately predict the behavior of various designs. Several different investigators
have studied various aspects of the phenomena involved in this problem, however
some areas remain open and in need of further study. The best available research is
outlined here and utilized for development analysis of this microreformer design.
Research into fuel cell technology is proceeding at a brisk pace, although
most new work examines automobile sized portable fuel cells. Results from these
studies do, however, provide information for our application in terms of operating
temperature, fuel supply requirements, and operating efficiency. A review of polymer
electrolyte fuel cells by Prater verifies the basic assumptions made about the
operational requirements of PEM fuel cells for this paper to ensure that the proposed
microreformer will successfully operate as part of a system [4].
The fluid passing through this microreformer design will experience many flow
conditions, and a much wider variety will be experienced when considering all of the
geometric variations studied to arrive at an optimized design. Relations for single-
and two-phase flow for liquids and gasses in microchannels and minichannels will all
be required to conduct this study.
Morini and others have studied the laminar flow of liquid through
microchannels [5]. Among the significant findings is that classical Navier-Stokes
equations remain valid for microchannels with hydraulic diameters as low as 30 µm.
While, this will not hold true for gasses, it will result in less complexity for the analysis
Page 15
15
of the liquid preheating regime, which will form only a small part of the overall design.
Other researchers including Bontemps [38] have also investigated liquid flow through
microchannels and the classical correspondence.
For flows of thin gasses, the classical equations do not continue to have such
a strong hold on the phenomena of fluid flow in microchannels. Many investigators
have studied the flow of gasses in microchannels and a review by Yener, et al [6]
provides a good description of the current state of research in this area. Relations
identified by this review as well as those developed by Turner, et al [7] will be used to
analyze the flow conditions in the superheat and catalytic reactor regions of the
microreformer. Additionally, Bayazitoglu and Kakac describe the flow regimes and
characteristics of gases in microchannels including the most common variations from
expected classical flow conditions [33]. Most research on laminar gas flow in
microchannels focuses on thin, single-constituent gasses, or refrigerants. For
example, see Morini’s study [32]. In some cases steam is investigated, but little in
the way of anything approximating a water-methanol mixture, so the study produced
here will be employing these experimentally derived relations on new ground,
although not outside of their prescribed flow conditions.
There is a need for verification of the flow effects of curved channels at the
micro scale. For normal scales, the effects on flow separation, pressure drop, and
other effects are known for pipe or channel bends and corners. The experimental
correlations have been repeatedly demonstrated for a variety of scales, but most
microchannel studies involve only straight channels. This is a current weakness in
the microchannel data, so the analysis presented here will utilize the macro relations
for this effect [8].
of the liquid preheating regime, which will form only a small part of the overall design.
Other researchers including Bontemps [38] have also investigated liquid flow through
microchannels and the classical correspondence.
For flows of thin gasses, the classical equations do not continue to have such
a strong hold on the phenomena of fluid flow in microchannels. Many investigators
have studied the flow of gasses in microchannels and a review by Yener, et al [6]
provides a good description of the current state of research in this area. Relations
identified by this review as well as those developed by Turner, et al [7] will be used to
analyze the flow conditions in the superheat and catalytic reactor regions of the
microreformer. Additionally, Bayazitoglu and Kakac describe the flow regimes and
characteristics of gases in microchannels including the most common variations from
expected classical flow conditions [33]. Most research on laminar gas flow in
microchannels focuses on thin, single-constituent gasses, or refrigerants. For
example, see Morini’s study [32]. In some cases steam is investigated, but little in
the way of anything approximating a water-methanol mixture, so the study produced
here will be employing these experimentally derived relations on new ground,
although not outside of their prescribed flow conditions.
There is a need for verification of the flow effects of curved channels at the
micro scale. For normal scales, the effects on flow separation, pressure drop, and
other effects are known for pipe or channel bends and corners. The experimental
correlations have been repeatedly demonstrated for a variety of scales, but most
microchannel studies involve only straight channels. This is a current weakness in
the microchannel data, so the analysis presented here will utilize the macro relations
for this effect [8].
Page 19
19
occur during microchannel flow [21], and so strengthening mechanisms or bundles
should be employed if this avenue is investigated further.
Another option for increasing surface area may be the micro-pin fins
investigated by Peles et al [22]. While a new manufacturing technique may be
required to affix catalyst to the sides of these pin fins, they provide a means to create
added surface area in the channel with a single process no different from the actual
formation of the channel. A single piece of silicon could then be employed for the
microchannel device and the surface area enhancement, which would increase
reliability and decrease the manufacturing cost.
occur during microchannel flow [21], and so strengthening mechanisms or bundles
should be employed if this avenue is investigated further.
Another option for increasing surface area may be the micro-pin fins
investigated by Peles et al [22]. While a new manufacturing technique may be
required to affix catalyst to the sides of these pin fins, they provide a means to create
added surface area in the channel with a single process no different from the actual
formation of the channel. A single piece of silicon could then be employed for the
microchannel device and the surface area enhancement, which would increase
reliability and decrease the manufacturing cost.
Page 23
23
TmCq Perheat ∆= &sup (8)
W 030.0sup =erheatq
The total power utilized to heat the fluid and sustain the chemical reaction is
0.446 W. This is approximately one-third of the total energy available in the hydrogen
fuel stream generated by this microreformer and reserves ample power for a device
such as a cell phone. The overall efficiency of the reformer should be maintained
near 50%, which means that the heat loss should be maintained below 0.304 W for
this flow rate. Both the size of the microreformer and the thickness of the insulation
material affect the heat loss. Increases in the flow rate will result in more power
output in the form of Hydrogen gas, but will also increase the power necessary for the
heating and conversion of the fuel.
The microreformer heater should have an output of 0.446 W plus the heat
lost to the environment to balance the inputs and outputs. A number of devices
already in existence are capable of providing the necessary heat within a small
volume. If commercial small-scale heating elements were not desired, a simple trace
applied to the reverse side of the microreformer could provide the necessary source
through resistive heating. The concept modeled in this paper includes a disk-type
heating element.
Since the thermal mass of the microreformer including the insulation is fairly
small, the transient response of the system to startup of the heater should be rapid.
However, as the channel changes geometry the fluid velocity changes and the transit
time for the fluid varies significantly. The transient heat up time and the fluid transit
time should each be no longer than 2 seconds.
TmCq Perheat ∆= &sup (8)
W 030.0sup =erheatq
The total power utilized to heat the fluid and sustain the chemical reaction is
0.446 W. This is approximately one-third of the total energy available in the hydrogen
fuel stream generated by this microreformer and reserves ample power for a device
such as a cell phone. The overall efficiency of the reformer should be maintained
near 50%, which means that the heat loss should be maintained below 0.304 W for
this flow rate. Both the size of the microreformer and the thickness of the insulation
material affect the heat loss. Increases in the flow rate will result in more power
output in the form of Hydrogen gas, but will also increase the power necessary for the
heating and conversion of the fuel.
The microreformer heater should have an output of 0.446 W plus the heat
lost to the environment to balance the inputs and outputs. A number of devices
already in existence are capable of providing the necessary heat within a small
volume. If commercial small-scale heating elements were not desired, a simple trace
applied to the reverse side of the microreformer could provide the necessary source
through resistive heating. The concept modeled in this paper includes a disk-type
heating element.
Since the thermal mass of the microreformer including the insulation is fairly
small, the transient response of the system to startup of the heater should be rapid.
However, as the channel changes geometry the fluid velocity changes and the transit
time for the fluid varies significantly. The transient heat up time and the fluid transit
time should each be no longer than 2 seconds.
Page 26
26
Figure 2: Archimedes’ Spiral
Table 3: Comparison of Channel Packing Orientations
Spiral
Square
Outer Dimension
(mm) 31.6 28.5
Total Area
(mm2) 785.6 812.3
Area Efficiency
(ε) 44.6 % 43.1 %
Equivalent Length
due to Bends
(Le/D)
1308 2280
Additional
Pressure Drop
(mm H2O)
117.5 204.9
While many spiral configurations exist, the concentric spiral or Archimedes’
spiral has been chosen to provide the layout for this microreformer design. The
Archimedes’ spiral is described by the following equation:
θar = . (10)
Figure 2: Archimedes’ Spiral
Table 3: Comparison of Channel Packing Orientations
Spiral
Square
Outer Dimension
(mm) 31.6 28.5
Total Area
(mm2) 785.6 812.3
Area Efficiency
(ε) 44.6 % 43.1 %
Equivalent Length
due to Bends
(Le/D)
1308 2280
Additional
Pressure Drop
(mm H2O)
117.5 204.9
While many spiral configurations exist, the concentric spiral or Archimedes’
spiral has been chosen to provide the layout for this microreformer design. The
Archimedes’ spiral is described by the following equation:
θar = . (10)
Page 27
27
An Archimedes’ spiral increases its radius by a constant amount with each full
revolution. This allows the microreformer to contain a constant channel width and a
constant wall thickness. If any variation were desired, such as a constantly
expanding channel, another Archimedean spiral could be employed.
( )[ ]22 1ln1
2
1 θθθθ ++++= as (11)
Ch_w
Rn
Rn+1
Ch_w
Ch_h
=β · Ch_w
twall
SIDE VIEW TOP VIEW
twall
θn+1 θ
sn
Ln
Figure 3: Numerical Solution Cell Parameters
An additional factor to consider in the optimal layout of the channel in the
microreformer is the wall thickness required. This wall thickness must resist the
internal pressure applied to the fluid to force it through the reformer. A thickness
An Archimedes’ spiral increases its radius by a constant amount with each full
revolution. This allows the microreformer to contain a constant channel width and a
constant wall thickness. If any variation were desired, such as a constantly
expanding channel, another Archimedean spiral could be employed.
( )[ ]22 1ln1
2
1 θθθθ ++++= as (11)
Ch_w
Rn
Rn+1
Ch_w
Ch_h
=β · Ch_w
twall
SIDE VIEW TOP VIEW
twall
θn+1 θ
sn
Ln
Figure 3: Numerical Solution Cell Parameters
An additional factor to consider in the optimal layout of the channel in the
microreformer is the wall thickness required. This wall thickness must resist the
internal pressure applied to the fluid to force it through the reformer. A thickness
Page 28
28
greater than is necessary will increase the size of the device making the system less
efficient. In order to determine the optimal thickness, a stress analysis was
conducted as described in Appendix A. The minimum feasible wall thickness for
current MEMS manufacturing techniques in silicon is 35 µm [3]. The analysis found
that the internal walls should have a thickness of 40 µm, while the exterior walls
require a thickness of 70 µm. These values maintain a factor of safety of at least 5.
Figure 4: Design Concept for Microreformer (assembled view)
greater than is necessary will increase the size of the device making the system less
efficient. In order to determine the optimal thickness, a stress analysis was
conducted as described in Appendix A. The minimum feasible wall thickness for
current MEMS manufacturing techniques in silicon is 35 µm [3]. The analysis found
that the internal walls should have a thickness of 40 µm, while the exterior walls
require a thickness of 70 µm. These values maintain a factor of safety of at least 5.
Figure 4: Design Concept for Microreformer (assembled view)
Page 34
34
( ) ( )
=
h
f
fotp d
kBo
Nu
Nuh 714.0857.0
4
3 Re30 (20)
Additionally, the following expression (21) developed by Warrier et al [41]
predicts the heat transfer coefficient of two phase flow. This expression will be
compared against that of Lazarek and Black above to evaluate the microreformer
design. In the expression below, Vq is the vapor quality of the two phase mixture, and
is equal to 1, when all the liquid has been vaporized.
( )( )( )
−−++=
h
liq
qtp D
kVBoBoNuh 65.016/13 85513.560.1 (21)
The following two-phase heat transfer correlation (22) was developed by
Kandlikar [13]. This relation is compared against those of (21) and (20) above in the
analysis that follows. Co in this relation is the convection number, a measure of the
effective specific volume of the flow at a particular mixture of liquid and vapor. This
relation takes into account two boiling regimes or mechanisms that can occur at
different points in the flow path as the fluid conditions change. The maximum
function in expression (22) evaluated on expressions E (24) and S (26) provides this
capability.
),max(
4
3 SEh
Nu
Nuh sptp = (22)
( ) ( )
h
liq
liqliqsp D
kh 4.08.0 PrRe023.0= (23)
7.02.0 10586683.0 BoCoE += − (24)
−
=
liq
gas
q
q
V
VCo
ρ
ρ9.01 (25)
( ) ( )
=
h
f
fotp d
kBo
Nu
Nuh 714.0857.0
4
3 Re30 (20)
Additionally, the following expression (21) developed by Warrier et al [41]
predicts the heat transfer coefficient of two phase flow. This expression will be
compared against that of Lazarek and Black above to evaluate the microreformer
design. In the expression below, Vq is the vapor quality of the two phase mixture, and
is equal to 1, when all the liquid has been vaporized.
( )( )( )
−−++=
h
liq
qtp D
kVBoBoNuh 65.016/13 85513.560.1 (21)
The following two-phase heat transfer correlation (22) was developed by
Kandlikar [13]. This relation is compared against those of (21) and (20) above in the
analysis that follows. Co in this relation is the convection number, a measure of the
effective specific volume of the flow at a particular mixture of liquid and vapor. This
relation takes into account two boiling regimes or mechanisms that can occur at
different points in the flow path as the fluid conditions change. The maximum
function in expression (22) evaluated on expressions E (24) and S (26) provides this
capability.
),max(
4
3 SEh
Nu
Nuh sptp = (22)
( ) ( )
h
liq
liqliqsp D
kh 4.08.0 PrRe023.0= (23)
7.02.0 10586683.0 BoCoE += − (24)
−
=
liq
gas
q
q
V
VCo
ρ
ρ9.01 (25)
Page 36
36
Superheat Region
This zone consists of heating the saturated vapor mixture at the boiling point
to a superheated vapor mixture at 200°C.
This value for the Reynolds number indicates that the flow is laminar in this
region even considering the lower laminar to turbulent transition value for
microchannel flow. Microchannel laminar-turbulent transition begins at Re = 1600
[13].
The Nusselt number for fully developed laminar flow in a microchannel [6] is
given by:
62.0Re=Nu . (34)
Therefore, the convection coefficient for this zone is given by [24]:
k
DhNu h= (35)
And, applying this to the overall convection equations to find the required
length of the channel in this zone:
( )
∆
∆
∆−∆
=∆−∆
i
o
io
SoiP
T
T
TTAhTTCm
ln
& (36)
h
B
PD
TkKn 22piσ
= (37)
The slip velocity which is determined by the Knudsen number magnitude is
calculated by the following equation, evaluated at y = 1:
Superheat Region
This zone consists of heating the saturated vapor mixture at the boiling point
to a superheated vapor mixture at 200°C.
This value for the Reynolds number indicates that the flow is laminar in this
region even considering the lower laminar to turbulent transition value for
microchannel flow. Microchannel laminar-turbulent transition begins at Re = 1600
[13].
The Nusselt number for fully developed laminar flow in a microchannel [6] is
given by:
62.0Re=Nu . (34)
Therefore, the convection coefficient for this zone is given by [24]:
k
DhNu h= (35)
And, applying this to the overall convection equations to find the required
length of the channel in this zone:
( )
∆
∆
∆−∆
=∆−∆
i
o
io
SoiP
T
T
TTAhTTCm
ln
& (36)
h
B
PD
TkKn 22piσ
= (37)
The slip velocity which is determined by the Knudsen number magnitude is
calculated by the following equation, evaluated at y = 1:
Page 37
37
Kn
Kn
d
y
uu m 81
41
2
3
2
+
+
−
= (38)
The thermal temperature jump at the wall can be determined by the following
expression. However, this effect is not likely to be significant, since the Knudsen
number is extremely small in the superheat region and little heat transfer is taking
place in the catalytic reactor region, and all of that heat transfer happens at a
constant, equal temperature between the wall and the fluid.
y
T
F
FTT
T
T
WS ∂
∂
+
−
=−
Pr1
22 γ
γ
γ (39)
The effects of slip flow also impact the pressure drop. This effect can be
evaluated via the following expression. Again, if this has a significant effect, it will
likely occur only in the catalytic reactor region
2
2
1
61
1
Re
96
m
h
V
DKndx
dp ρ
+
=− . (40)
Catalytic Reactor Region
This zone consists of chemically converting the superheated vapor mixture of
methanol and water into a mixture of hydrogen and carbon dioxide. The key
parameter in this zone is the dwell time of the vapor mixture in the presence of the
Cu/ZnO catalyst. Previous research has demonstrated that a period of 750
milliseconds is required to fully convert the mixture [23]. The length of the channel in
this section is governed entirely by having the fluid experience the necessary dwell
time of 750 milliseconds. However, the fact that the mixture is changing chemical
constituents results in other chemical properties such as density, speed of sound,
Kn
Kn
d
y
uu m 81
41
2
3
2
+
+
−
= (38)
The thermal temperature jump at the wall can be determined by the following
expression. However, this effect is not likely to be significant, since the Knudsen
number is extremely small in the superheat region and little heat transfer is taking
place in the catalytic reactor region, and all of that heat transfer happens at a
constant, equal temperature between the wall and the fluid.
y
T
F
FTT
T
T
WS ∂
∂
+
−
=−
Pr1
22 γ
γ
γ (39)
The effects of slip flow also impact the pressure drop. This effect can be
evaluated via the following expression. Again, if this has a significant effect, it will
likely occur only in the catalytic reactor region
2
2
1
61
1
Re
96
m
h
V
DKndx
dp ρ
+
=− . (40)
Catalytic Reactor Region
This zone consists of chemically converting the superheated vapor mixture of
methanol and water into a mixture of hydrogen and carbon dioxide. The key
parameter in this zone is the dwell time of the vapor mixture in the presence of the
Cu/ZnO catalyst. Previous research has demonstrated that a period of 750
milliseconds is required to fully convert the mixture [23]. The length of the channel in
this section is governed entirely by having the fluid experience the necessary dwell
time of 750 milliseconds. However, the fact that the mixture is changing chemical
constituents results in other chemical properties such as density, speed of sound,
Page 40
40
of the microreformer considering different initial geometric conditions. Several
conclusions can be drawn from the following information to arrive at the optimal
channel geometry. All quantities considered in this section are evaluated from a
minimum width of 20 µm to a maximum width of 400 µm.
The first property to consider is the total channel length required. In the
preheat, phase change and superheat regions the channel length is determined by
the heat transfer coefficient, while the length of the catalytic reactor is determined by
the dwell time. Figure 10 below displays the channel length required for a variety of
channel widths. The necessary channel length varies from a maximum of 2244
meters for a 20 µm channel to a minimum of 1.6 meters for a 400 µm wide channel.
The overwhelming majority of the channel length is devoted to the catalytic reactor
region. On average, less than 1% of the total channel length is devoted to the
preheat, phase change, and superheat regions combined.
of the microreformer considering different initial geometric conditions. Several
conclusions can be drawn from the following information to arrive at the optimal
channel geometry. All quantities considered in this section are evaluated from a
minimum width of 20 µm to a maximum width of 400 µm.
The first property to consider is the total channel length required. In the
preheat, phase change and superheat regions the channel length is determined by
the heat transfer coefficient, while the length of the catalytic reactor is determined by
the dwell time. Figure 10 below displays the channel length required for a variety of
channel widths. The necessary channel length varies from a maximum of 2244
meters for a 20 µm channel to a minimum of 1.6 meters for a 400 µm wide channel.
The overwhelming majority of the channel length is devoted to the catalytic reactor
region. On average, less than 1% of the total channel length is devoted to the
preheat, phase change, and superheat regions combined.
Page 41
41
Figure 10: Total Channel Length Required for Varying Channel Widths
The total channel length directly affects the total size of the device, but not in
a simple linear fashion. Since, the wider channel tends to increase the device
diameter and also decrease the length required, which decreases the device
diameter, these factors must be balanced. In the end, the channel length effect is
much more significant, and wider channels can result in small devices. This plot
assumes that the device is 2-sided with an equal length of spiral channel cut into
both sides of the device. One can observe that the packing efficiency of the spiral
configuration is quite high as over 2200 meters of channel can be held on a circular
surface only 10 cm in diameter.
Figure 10: Total Channel Length Required for Varying Channel Widths
The total channel length directly affects the total size of the device, but not in
a simple linear fashion. Since, the wider channel tends to increase the device
diameter and also decrease the length required, which decreases the device
diameter, these factors must be balanced. In the end, the channel length effect is
much more significant, and wider channels can result in small devices. This plot
assumes that the device is 2-sided with an equal length of spiral channel cut into
both sides of the device. One can observe that the packing efficiency of the spiral
configuration is quite high as over 2200 meters of channel can be held on a circular
surface only 10 cm in diameter.
Page 42
42
Since, a portable-sized device is required and this value for device diameter
does not include insulation, which will be considered later, the channel widths below
50 µm can be ruled out at this point. A stronger substrate material could make the
increase in diameter less steep at the lower end, or alternatively, a multi-layered
device could be considered if a strong need for the smaller channels were apparent.
Figure 11: Resultant Device Diameter for Varying Channel Widths
Another reason that the device diameter is important is that it affects the
amount of heat lost to the environment, and thus can decrease the efficiency of the
reformer in providing more power than it uses. Figure 12 below displays the amount
of heat loss based on the device diameter and also the channel width. The heat loss
is calculated by considering the device outer surface at an insulated temperature
Since, a portable-sized device is required and this value for device diameter
does not include insulation, which will be considered later, the channel widths below
50 µm can be ruled out at this point. A stronger substrate material could make the
increase in diameter less steep at the lower end, or alternatively, a multi-layered
device could be considered if a strong need for the smaller channels were apparent.
Figure 11: Resultant Device Diameter for Varying Channel Widths
Another reason that the device diameter is important is that it affects the
amount of heat lost to the environment, and thus can decrease the efficiency of the
reformer in providing more power than it uses. Figure 12 below displays the amount
of heat loss based on the device diameter and also the channel width. The heat loss
is calculated by considering the device outer surface at an insulated temperature
Page 43
43
around 45°C, the maximum permitted for safe continuous skin contact [25]. Using a
typical free convection model for air, the following values for heat loss were derived.
In reality, some of this heat could be used to maintain the temperature of the fuel
cell, or some heat could be recaptured from waste heat given off by the supplied
devices electronics. But, the chart should be considered a good estimate of potential
losses, although probably at the high end.
As proposed, this microreformer is designed to produce hydrogen equivalent
to 1.5 W of power. The heat required by the fluid for conversion is more than 0.4 W.
Therefore, on the basis of this data, channels smaller than 100 µm can be
eliminated.
around 45°C, the maximum permitted for safe continuous skin contact [25]. Using a
typical free convection model for air, the following values for heat loss were derived.
In reality, some of this heat could be used to maintain the temperature of the fuel
cell, or some heat could be recaptured from waste heat given off by the supplied
devices electronics. But, the chart should be considered a good estimate of potential
losses, although probably at the high end.
As proposed, this microreformer is designed to produce hydrogen equivalent
to 1.5 W of power. The heat required by the fluid for conversion is more than 0.4 W.
Therefore, on the basis of this data, channels smaller than 100 µm can be
eliminated.
Page 45
45
The complication to the Mach number calculation in the catalytic reactor
region is the changing chemical constituents of the fluid. As methanol vapor, steam,
hydrogen, and carbon dioxide have different sonic values, the sonic value of the fluid
account must take into account what stage of the conversion is in effect for the
particular cell being analyzed.
The analysis of Mach number is important since it determines whether or not
assumptions of incompressibility are valid for the observed flow. Figure 13 below
displays the maximum Mach number versus channel width for the microreformer. A
good rule of thumb to avoid compressibility effects is to maintain the maximum Mach
number below 0.15 [39]. In order to comply with this criteria, the microreformer
channel width must be 85 µm or greater.
The complication to the Mach number calculation in the catalytic reactor
region is the changing chemical constituents of the fluid. As methanol vapor, steam,
hydrogen, and carbon dioxide have different sonic values, the sonic value of the fluid
account must take into account what stage of the conversion is in effect for the
particular cell being analyzed.
The analysis of Mach number is important since it determines whether or not
assumptions of incompressibility are valid for the observed flow. Figure 13 below
displays the maximum Mach number versus channel width for the microreformer. A
good rule of thumb to avoid compressibility effects is to maintain the maximum Mach
number below 0.15 [39]. In order to comply with this criteria, the microreformer
channel width must be 85 µm or greater.
Page 46
46
Figure 13: Maximum Mach Number within Microreformer for Varying Channel
Widths
A related quantity is the maximum mean fluid velocity within the
microreformer. That value is plotted against the channel width on Figure 14 below.
At the low end, the velocity effects of the choked flow are obvious. While no direct
decisions can be made on the choice of optimum channel width as a result of this
information, the maximum mean fluid velocity directly affects several other quantities
including the Mach number and overall channel length seen on previous figures.
Figure 13: Maximum Mach Number within Microreformer for Varying Channel
Widths
A related quantity is the maximum mean fluid velocity within the
microreformer. That value is plotted against the channel width on Figure 14 below.
At the low end, the velocity effects of the choked flow are obvious. While no direct
decisions can be made on the choice of optimum channel width as a result of this
information, the maximum mean fluid velocity directly affects several other quantities
including the Mach number and overall channel length seen on previous figures.
Page 48
48
Figure 15: Maximum Reynolds Number within Microreformer for Varying Channel
Widths
In considering flows of thin gasses in very small channels, one must also be
aware of the effects of rarefied flow conditions. While the gasses in this
microreformer are not particularly thin, the molecule size of methanol is rather large,
and so the same effects may be encountered in small channels as for thin gasses.
These rarefied flow conditions challenge the continuum assumption made in most
calculations.
The Knudsen number provides a measure of how applicable the continuum
assumption is for the flow being studied with the assumption becoming less valid as
Figure 15: Maximum Reynolds Number within Microreformer for Varying Channel
Widths
In considering flows of thin gasses in very small channels, one must also be
aware of the effects of rarefied flow conditions. While the gasses in this
microreformer are not particularly thin, the molecule size of methanol is rather large,
and so the same effects may be encountered in small channels as for thin gasses.
These rarefied flow conditions challenge the continuum assumption made in most
calculations.
The Knudsen number provides a measure of how applicable the continuum
assumption is for the flow being studied with the assumption becoming less valid as
Page 50
50
One of the continuum assumptions made in fluid equations is the no-slip
condition. As the Knudsen number increases that condition becomes invalid and slip
observed at the channel wall can be come a significant factor. As explained for
expression (38), the maximum slip velocity is plotted versus channel width below in
Figure 17. In comparing Figures 12 and 9, one can see that the slip velocity is very
small and remains insignificant for the flow conditions observed in this
microreformer. If one were still considering the smallest channel widths that
determination may need to be reconsidered, but those have already been ruled out
for other reasons.
Figure 17: Maximum Slip Velocity within Microreformer for Varying Channel
Widths
One of the continuum assumptions made in fluid equations is the no-slip
condition. As the Knudsen number increases that condition becomes invalid and slip
observed at the channel wall can be come a significant factor. As explained for
expression (38), the maximum slip velocity is plotted versus channel width below in
Figure 17. In comparing Figures 12 and 9, one can see that the slip velocity is very
small and remains insignificant for the flow conditions observed in this
microreformer. If one were still considering the smallest channel widths that
determination may need to be reconsidered, but those have already been ruled out
for other reasons.
Figure 17: Maximum Slip Velocity within Microreformer for Varying Channel
Widths
Page 52
52
The total time fluid takes to traverse the entire device is of importance to the
operation of the microreformer. If an end user has to wait for 10 seconds after
turning a system on before it reaches full power and becomes operational that may
be too long. The following chart, Figure 19, displays the total transit time for the fluid
in different channel widths. The transit time increases as the channel width
increases, but this quantity should be minimized for ideal operation. A maximum
transit value of 2 seconds is established which limits the channel width to less than
290 µm.
Figure 19: Fluid Transit Time for Varying Channel Widths
The total time fluid takes to traverse the entire device is of importance to the
operation of the microreformer. If an end user has to wait for 10 seconds after
turning a system on before it reaches full power and becomes operational that may
be too long. The following chart, Figure 19, displays the total transit time for the fluid
in different channel widths. The transit time increases as the channel width
increases, but this quantity should be minimized for ideal operation. A maximum
transit value of 2 seconds is established which limits the channel width to less than
290 µm.
Figure 19: Fluid Transit Time for Varying Channel Widths
Page 53
53
Another quantity to consider is the available reactor surface area. By the
present proposed method of depositing a layer of catalyst on the bottom, the surface
area in the lower surface of the channel can be easily compared across the possible
geometries. Since a greater amount of catalyst reduces the possibility of fouling and
increases the efficiency of the conversion, this surface area quantity should be
maximized. That leads one to favor the smallest possible channel width allowed by
the other constraints.
Figure 20: Available Reactor Surface Area for Varying Channel Widths
Another quantity to consider is the available reactor surface area. By the
present proposed method of depositing a layer of catalyst on the bottom, the surface
area in the lower surface of the channel can be easily compared across the possible
geometries. Since a greater amount of catalyst reduces the possibility of fouling and
increases the efficiency of the conversion, this surface area quantity should be
maximized. That leads one to favor the smallest possible channel width allowed by
the other constraints.
Figure 20: Available Reactor Surface Area for Varying Channel Widths
Page 55
55
Commercially produced air guns can be powered with compressed carbon
dioxide cartridges, which are small steel pressure vessels. These are produced
inexpensively and in large quantities. These small pressure vessels are typically
charged to 5.88 MPa, but can accept pressures as high as 8.27 MPa. Although
pressure can be produced by other means (e.g. pumps) or at higher or lower values,
this point should provide a good criterion with which to optimize our channel widths
for pressure drop.
Choosing only those channel widths that produce a pressure drop of less than
8.27 MPa leads one to select a channel width greater than 185 µm.
Figure 21: Total Pressure Drop for Microreformer with Channel Widths between
20 and 210 Micrometers
Commercially produced air guns can be powered with compressed carbon
dioxide cartridges, which are small steel pressure vessels. These are produced
inexpensively and in large quantities. These small pressure vessels are typically
charged to 5.88 MPa, but can accept pressures as high as 8.27 MPa. Although
pressure can be produced by other means (e.g. pumps) or at higher or lower values,
this point should provide a good criterion with which to optimize our channel widths
for pressure drop.
Choosing only those channel widths that produce a pressure drop of less than
8.27 MPa leads one to select a channel width greater than 185 µm.
Figure 21: Total Pressure Drop for Microreformer with Channel Widths between
20 and 210 Micrometers
Page 57
57
Table 4: Optimization Criteria Summary
Criterion Description Limit on Channel Width
Limit total pressure drop to less than
8.27 MPa > 185 µm
Limit heat loss to less than 0.5 Watts > 235 µm
Limit fluid transit time to less than 2
seconds < 295 µm
Maintain Mach number below 0.15 > 85 µm
Figure 23: Microreformer Channel Width Optimization Plot Displaying Pressure
Drop, Heat Loss, Mach Number, and Transit Time
Upon the choice of a channel width of 255 µm, a further detailed analysis of
the microreformer will be reviewed in the next section.
Table 4: Optimization Criteria Summary
Criterion Description Limit on Channel Width
Limit total pressure drop to less than
8.27 MPa > 185 µm
Limit heat loss to less than 0.5 Watts > 235 µm
Limit fluid transit time to less than 2
seconds < 295 µm
Maintain Mach number below 0.15 > 85 µm
Figure 23: Microreformer Channel Width Optimization Plot Displaying Pressure
Drop, Heat Loss, Mach Number, and Transit Time
Upon the choice of a channel width of 255 µm, a further detailed analysis of
the microreformer will be reviewed in the next section.
Page 58
58
RESULTS
Based on the analysis of different microchannel geometries, the best option is
based on a rectangular channel having a width of 255 µm. The detailed results of
the analysis for a microreformer utilizing that particular channel size are presented in
this section.
The next four charts, Figure 24, Figure 25, Figure 26, and Figure 27 display
the pressure drop per unit length for each cell analyzed by the numerical solution
code. By position, this could be considered in instantaneous rate of pressure drop.
These must be considered per unit length since the cells are not all the same length,
but are smaller near the entrance of the fluid to the microreformer and slowly grow
larger as the spiral spins outward to the catalytic reactor region.
A few of the qualities of these charts are worthy of further discussion. The
pressure drop per meter in the Preheat Region displays a decreasing trend due to the
decrease in fluid viscosity in this region that occurs as the fluid temperature
increases. The Phase Change Region exhibits a U-shaped curve as the relative
makeup of the liquid and vapor in this region intermingle their respective effects and
the equivalent length due to the curvature radius plays an increasing role. The
pressure drop rate in the Superheat Region climbs as the gas mixture expands. Also,
the effect of the curvature on the pressure drop decreases throughout this region
and the rate of pressure drop nears a constant at the end of the region. In the
Catalytic Reactor Region the alteration of the chemical makeup of the fluid results in
the decreasing trend as a result of changes in viscosity. In terms of scale, the
Catalytic Reactor Region dominates all others, as the length of this region is so much
RESULTS
Based on the analysis of different microchannel geometries, the best option is
based on a rectangular channel having a width of 255 µm. The detailed results of
the analysis for a microreformer utilizing that particular channel size are presented in
this section.
The next four charts, Figure 24, Figure 25, Figure 26, and Figure 27 display
the pressure drop per unit length for each cell analyzed by the numerical solution
code. By position, this could be considered in instantaneous rate of pressure drop.
These must be considered per unit length since the cells are not all the same length,
but are smaller near the entrance of the fluid to the microreformer and slowly grow
larger as the spiral spins outward to the catalytic reactor region.
A few of the qualities of these charts are worthy of further discussion. The
pressure drop per meter in the Preheat Region displays a decreasing trend due to the
decrease in fluid viscosity in this region that occurs as the fluid temperature
increases. The Phase Change Region exhibits a U-shaped curve as the relative
makeup of the liquid and vapor in this region intermingle their respective effects and
the equivalent length due to the curvature radius plays an increasing role. The
pressure drop rate in the Superheat Region climbs as the gas mixture expands. Also,
the effect of the curvature on the pressure drop decreases throughout this region
and the rate of pressure drop nears a constant at the end of the region. In the
Catalytic Reactor Region the alteration of the chemical makeup of the fluid results in
the decreasing trend as a result of changes in viscosity. In terms of scale, the
Catalytic Reactor Region dominates all others, as the length of this region is so much
Page 61
61
Figure 27: Pressure Drop per Unit of Distance for the Catalytic Reactor Region
The “bump” in the center of Figure 27 above is explained by the change of the
flow in the microreformer from side 1 to side 2. Since, the radius of curvature of the
spiral effects the pressure losses due to the bend, there is a change that occurs at
this point from one of flow spiraling outward on side 1 to one of flow spiraling inward
on side 2. Figure 28 below displays the change in the radius of curvature ratio to the
hydraulic diameter of the channel.
Figure 27: Pressure Drop per Unit of Distance for the Catalytic Reactor Region
The “bump” in the center of Figure 27 above is explained by the change of the
flow in the microreformer from side 1 to side 2. Since, the radius of curvature of the
spiral effects the pressure losses due to the bend, there is a change that occurs at
this point from one of flow spiraling outward on side 1 to one of flow spiraling inward
on side 2. Figure 28 below displays the change in the radius of curvature ratio to the
hydraulic diameter of the channel.
Page 64
64
well below the sonic velocities and out of any concern over compressible flow
regimes, as will also be explained in the discussion of Figure 38.
Figure 30: Mean Fluid Velocity in the Preheat Region
well below the sonic velocities and out of any concern over compressible flow
regimes, as will also be explained in the discussion of Figure 38.
Figure 30: Mean Fluid Velocity in the Preheat Region
Page 67
67
Figure 33: Mean Fluid Velocity in the Catalytic Reactor Region
The following chart, Figure 34, is a map displaying the position and
temperature of one half of the cells in the numerical model of the microreformer.
Only one-half of the cells are displayed in the map to save on computer resources in
generating the plot. Each point in the plot indicates the position of the node and the
fluid temperature at that node. The map makes obvious the fact that all of the
temperature increases of the fluid occur very close to the entrance point. Side 2 of
the microreformer would produces a plot almost identical to this one, although no
temperature variation takes place.
Figure 33: Mean Fluid Velocity in the Catalytic Reactor Region
The following chart, Figure 34, is a map displaying the position and
temperature of one half of the cells in the numerical model of the microreformer.
Only one-half of the cells are displayed in the map to save on computer resources in
generating the plot. Each point in the plot indicates the position of the node and the
fluid temperature at that node. The map makes obvious the fact that all of the
temperature increases of the fluid occur very close to the entrance point. Side 2 of
the microreformer would produces a plot almost identical to this one, although no
temperature variation takes place.
Page 70
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Figure 36: Temperature Curve in the Superheat Region
Figure 36: Temperature Curve in the Superheat Region
Page 78
78
feature size being limited to at least tens of micrometers [3]. While many materials
could be employed for the manufacture of this microreformer, the design utilizes
silicon due the greater supply of manufacturing facilities and expertise to encourage
future experimentation and prototype evaluation.
This microreformer employs a minimum feature size of 35 µm for the wall
thickness between adjacent loops of the spiral channel to ensure adequate margin
for the pressurized fluid that the walls must resist. This is well within the standard
technology capabilities for visible light photolithography and silicon manufacturing.
The other components of the microreformer are assembled by bonding. The
fourth side of the microchannel is specified as Pyrex glass; however, silicon or
another material could also be employed. The Pyrex is useful because of its thermal
stability, low cost, and visibility advantages for experimentation purposes. The
thermal insulation and outer packaging of the device are selected only on the basis
of thermal properties and cost.
The manufacturing process of this microreformer consists of multiple steps,
which include etching the microchannels into a silicon wafer, then depositing the
Cu/ZnO catalyst within the microchannels, and then finally assembling the device.
The multiple-step manufacturing process for the microreformer is detailed in Figure
44 below.
feature size being limited to at least tens of micrometers [3]. While many materials
could be employed for the manufacture of this microreformer, the design utilizes
silicon due the greater supply of manufacturing facilities and expertise to encourage
future experimentation and prototype evaluation.
This microreformer employs a minimum feature size of 35 µm for the wall
thickness between adjacent loops of the spiral channel to ensure adequate margin
for the pressurized fluid that the walls must resist. This is well within the standard
technology capabilities for visible light photolithography and silicon manufacturing.
The other components of the microreformer are assembled by bonding. The
fourth side of the microchannel is specified as Pyrex glass; however, silicon or
another material could also be employed. The Pyrex is useful because of its thermal
stability, low cost, and visibility advantages for experimentation purposes. The
thermal insulation and outer packaging of the device are selected only on the basis
of thermal properties and cost.
The manufacturing process of this microreformer consists of multiple steps,
which include etching the microchannels into a silicon wafer, then depositing the
Cu/ZnO catalyst within the microchannels, and then finally assembling the device.
The multiple-step manufacturing process for the microreformer is detailed in Figure
44 below.
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79
Step 1: Apply photo-resistive mask to silicon wafer
Silicon
Mask
Step 2: Expose mask to light according to microchannel pattern
Pattern
Step 3: Remove exposed mask
Step 4: Etch silicon wafer to desired depth
Step 5: Remove mask
Step 6: Separate silicon microchannel chips, Bond Pyrex wafer to close channel,
Bond heater and 2nd microchannel chip to reverse, Machine through-holes
Pyrex
Heater
Step 7: Bond mini-tubes to input and output channels, Assemble insulation and
packaging (this step not shown)
Figure 44: Microreformer Manufacturing Process
Step 1: Apply photo-resistive mask to silicon wafer
Silicon
Mask
Step 2: Expose mask to light according to microchannel pattern
Pattern
Step 3: Remove exposed mask
Step 4: Etch silicon wafer to desired depth
Step 5: Remove mask
Step 6: Separate silicon microchannel chips, Bond Pyrex wafer to close channel,
Bond heater and 2nd microchannel chip to reverse, Machine through-holes
Pyrex
Heater
Step 7: Bond mini-tubes to input and output channels, Assemble insulation and
packaging (this step not shown)
Figure 44: Microreformer Manufacturing Process
Page 80
80
CONCLUSIONS AND RECOMMENDATIONS
Conclusions
The microreformer design presented here has proven feasible based on the
analyses performed to provide a safe, clean hydrogen source for a micro fuel cell.
The scale and power requirements of this design are consistent with the needs of
small-scale personal electronic devices. The cost of the materials involved is low, as
well as the required assembly tasks. The flow conditions at each point of the
microreformer have been determined by a finite element code developed especially
to optimize the geometry and flow conditions of this design. This analysis tool has
demonstrated its value in that any changes to geometry, flow conditions, or thermal
characteristics can be quickly evaluated for their effect on the fluid at each point
along its travel and transformation process.
While this prototype-class design could likely be implemented into a portable
power system, the current size and efficiency may only be sufficient for highly
specialized portable electronic equipment for use by surveyors, military, adventurers,
and pipeline or power line inspectors. Further optimization should increase the
efficiency and usefulness for a greater variety of consumer applications.
This design can play a significant role in the implementation of small scale
fuel cells as viable portable electronic power sources.
Recommendations
Experimentation should be conducted on this design and variations to test the
validity of the analyses techniques presented here. Good agreement between
experimental results and the analytical results found herein would allow rapid virtual
CONCLUSIONS AND RECOMMENDATIONS
Conclusions
The microreformer design presented here has proven feasible based on the
analyses performed to provide a safe, clean hydrogen source for a micro fuel cell.
The scale and power requirements of this design are consistent with the needs of
small-scale personal electronic devices. The cost of the materials involved is low, as
well as the required assembly tasks. The flow conditions at each point of the
microreformer have been determined by a finite element code developed especially
to optimize the geometry and flow conditions of this design. This analysis tool has
demonstrated its value in that any changes to geometry, flow conditions, or thermal
characteristics can be quickly evaluated for their effect on the fluid at each point
along its travel and transformation process.
While this prototype-class design could likely be implemented into a portable
power system, the current size and efficiency may only be sufficient for highly
specialized portable electronic equipment for use by surveyors, military, adventurers,
and pipeline or power line inspectors. Further optimization should increase the
efficiency and usefulness for a greater variety of consumer applications.
This design can play a significant role in the implementation of small scale
fuel cells as viable portable electronic power sources.
Recommendations
Experimentation should be conducted on this design and variations to test the
validity of the analyses techniques presented here. Good agreement between
experimental results and the analytical results found herein would allow rapid virtual
Page 81
81
experimentation by use of the model developed through this research. Experimental
data would also permit evaluation of the various microchannel relations for the
conditions occurring in this design. Additionally, further tuning of the modeling code
could be accomplished with sufficient data over a variety of flow conditions.
For future improvements to this application the most important would be an
effective catalyst that operates at lower temperatures. Some ongoing, but
unpublished research may prove that the reaction temperature can be reduced to
120°C with the use of new noble metal catalysts. The lower temperature would
greatly reduce the size and increase the thermal efficiency of this microreformer.
experimentation by use of the model developed through this research. Experimental
data would also permit evaluation of the various microchannel relations for the
conditions occurring in this design. Additionally, further tuning of the modeling code
could be accomplished with sufficient data over a variety of flow conditions.
For future improvements to this application the most important would be an
effective catalyst that operates at lower temperatures. Some ongoing, but
unpublished research may prove that the reaction temperature can be reduced to
120°C with the use of new noble metal catalysts. The lower temperature would
greatly reduce the size and increase the thermal efficiency of this microreformer.
Page 82
82
REFERENCES
[1] Dyer, C.K. “Fuel Cells for Portable Applications.” J. of Power Sources. Vol.106.
No.1. April 2002. pp.31-4.
[2] DeGaspari, J. “Beyond Silicon.” Mechanical Engineering. July 2005. pp. 30-3.
[3] Upadhye, H. R., and Kandlikar, S. G. “Optimization of Microchannel Geometry
for Direct Chip Cooling Using Single Phase Heat Transfer.” Proc. of
ICMM2004-2398. June 2004.
[4] Prater, K.B. “Polymer Electrolyte Fuel Cells: A Review of Recent
Developments.” J. of Power Sources. Vol.51. No.1. 1994. pp.129-144.
[5] Morini, G., “Laminar Liquid Flow Through Silicon Microchannels.” J. of Fluids
Engr. Vol. 126. May 2004. pp485-9.
[6] Yener, Y., Kakac, S., Avelino, M., and Okutucu, T., “Single Phase Forced
Convection in Microchannels: A State of the Art Review,” Microscale Heat
Transfer. Eds.: Kakac, S., Vasiliev, L., Bayazitoglu, Y., and Yener, Y. pp. 1-24.
Springer. The Netherlands. 2005.
[7] Turner, S., Lam, L., Faghri, M., and Gregory, O. “Experimental investigation of
Gas Flow in Microchannels.” J. of Heat Transfer. Vol 126. pp. 753-63. Oct.
2004.
[8] Flow of Fluids Through Valves, Fittings, and Pipe. Technical Paper No. 410.
Crane. The Woodlands, TX. 1999.
[9] Qu, W., and Mudawar, I. “Flow Boiling Heat Transfer in Two-Phase
Microchannel Heat Sinks—I. Experimental Investigation and Assessment of
Correlation Methods.” Intl. J. of Heat and Mass Transfer. 46 (2003) 2755-
2771.
[10] Qu, W., and Mudawar, I. “Flow Boiling Heat Transfer in Two-Phase
Microchannel Heat Sinks” Intl. J. of Heat and Mass Transfer. 46 (2003) 2773-
2784.
[11] Qu, W. and Mudawar, I., “Measurement and prediction of pressure drop in
two-phase microchannel heat sinks.” Intl. J. of Heat and Mass Transfer. 46
(2003) 2737-2753.
[12] Lazarek, G.M., and Black, S.H. “Evaporative Heat Transfer, Pressure Drop and
Critical Heat Flux in a Small Vertical Tube with R-113.” Intl. J. of Heat Mass
Transfer. Vol.25. 1982. pp.945-60.
REFERENCES
[1] Dyer, C.K. “Fuel Cells for Portable Applications.” J. of Power Sources. Vol.106.
No.1. April 2002. pp.31-4.
[2] DeGaspari, J. “Beyond Silicon.” Mechanical Engineering. July 2005. pp. 30-3.
[3] Upadhye, H. R., and Kandlikar, S. G. “Optimization of Microchannel Geometry
for Direct Chip Cooling Using Single Phase Heat Transfer.” Proc. of
ICMM2004-2398. June 2004.
[4] Prater, K.B. “Polymer Electrolyte Fuel Cells: A Review of Recent
Developments.” J. of Power Sources. Vol.51. No.1. 1994. pp.129-144.
[5] Morini, G., “Laminar Liquid Flow Through Silicon Microchannels.” J. of Fluids
Engr. Vol. 126. May 2004. pp485-9.
[6] Yener, Y., Kakac, S., Avelino, M., and Okutucu, T., “Single Phase Forced
Convection in Microchannels: A State of the Art Review,” Microscale Heat
Transfer. Eds.: Kakac, S., Vasiliev, L., Bayazitoglu, Y., and Yener, Y. pp. 1-24.
Springer. The Netherlands. 2005.
[7] Turner, S., Lam, L., Faghri, M., and Gregory, O. “Experimental investigation of
Gas Flow in Microchannels.” J. of Heat Transfer. Vol 126. pp. 753-63. Oct.
2004.
[8] Flow of Fluids Through Valves, Fittings, and Pipe. Technical Paper No. 410.
Crane. The Woodlands, TX. 1999.
[9] Qu, W., and Mudawar, I. “Flow Boiling Heat Transfer in Two-Phase
Microchannel Heat Sinks—I. Experimental Investigation and Assessment of
Correlation Methods.” Intl. J. of Heat and Mass Transfer. 46 (2003) 2755-
2771.
[10] Qu, W., and Mudawar, I. “Flow Boiling Heat Transfer in Two-Phase
Microchannel Heat Sinks” Intl. J. of Heat and Mass Transfer. 46 (2003) 2773-
2784.
[11] Qu, W. and Mudawar, I., “Measurement and prediction of pressure drop in
two-phase microchannel heat sinks.” Intl. J. of Heat and Mass Transfer. 46
(2003) 2737-2753.
[12] Lazarek, G.M., and Black, S.H. “Evaporative Heat Transfer, Pressure Drop and
Critical Heat Flux in a Small Vertical Tube with R-113.” Intl. J. of Heat Mass
Transfer. Vol.25. 1982. pp.945-60.
Page 85
85
[39] Anderson, J.D. Modern Compressible Flow with Historical Perspective. 3rd Ed.
McGraw Hill. Boston. 2004.
[40] Lockhart, R.W. and Martinelli, R.C. “Proposed Correlation of Data for
Isothermal Two-Phase, Two-Component Flow in Pipes.” Chem. Engr. Prog.
Vol.45. 1949. pp.39-48.
[41] Warrier G.R., Dhir, V.K., and Momoda, L.A. “Heat Transfer and Pressure Drop
in Narrow Rectangular Channel.” Exp. Therm. Fluid Sci. Vol.26. 2002. pp.53-
64.
[42] Madou, M. J. Fundamentals of Microfabrication: The Science of
Miniaturization, 2nd ed.; CRC Press: Boca Raton, 2002.
[43] Rodgers, J.A., Paul, K.E., Jackman, R.J., and Whitesides, G.M. “Using an
elastomeric phase mask for sub-100 nm photolithography in the optical near
field.” Applied Physics Letters. Vol.70. 1997. pp.2658-2660.
[44] Grzybowski, B. A.; Haag, R.; Bowden, N.; Whitesides, G. M. Anal. Chem. 1998,
Vol. 70, pp.4645-4652.
[39] Anderson, J.D. Modern Compressible Flow with Historical Perspective. 3rd Ed.
McGraw Hill. Boston. 2004.
[40] Lockhart, R.W. and Martinelli, R.C. “Proposed Correlation of Data for
Isothermal Two-Phase, Two-Component Flow in Pipes.” Chem. Engr. Prog.
Vol.45. 1949. pp.39-48.
[41] Warrier G.R., Dhir, V.K., and Momoda, L.A. “Heat Transfer and Pressure Drop
in Narrow Rectangular Channel.” Exp. Therm. Fluid Sci. Vol.26. 2002. pp.53-
64.
[42] Madou, M. J. Fundamentals of Microfabrication: The Science of
Miniaturization, 2nd ed.; CRC Press: Boca Raton, 2002.
[43] Rodgers, J.A., Paul, K.E., Jackman, R.J., and Whitesides, G.M. “Using an
elastomeric phase mask for sub-100 nm photolithography in the optical near
field.” Applied Physics Letters. Vol.70. 1997. pp.2658-2660.
[44] Grzybowski, B. A.; Haag, R.; Bowden, N.; Whitesides, G. M. Anal. Chem. 1998,
Vol. 70, pp.4645-4652.
Page 86
86
APPENDIX A: Silicon Wall Stress Analysis
In order to optimize the design of the microreformer for overall component
volume, the minimum amount of material should be utilized. To minimize the
material and exterior dimensions, the operating stresses must be balanced against
the operating pressure within the device. The internal channels must only withstand
the pressure differential caused by fluid flow around one rotation of the spiral.
Therefore, these internal channel walls will have a thickness of 40 µm, which is near
the minimum state of the art thickness for silicon micro-electromechanical systems
[3].
Below, Figure A-1 contains the three-dimensional solid model constructed in
SolidworksTM [27] utilized to analyze the stress and displacement of the silicon
structure under operating conditions. The radius of the wall matches the outer
radius of the overall device design. The thickness of this wall assuming a constant
inner dimension will be the optimized variable.
This solid model was analyzed by the use of COSMOSTM [27] a commercial
finite element analysis software package.
APPENDIX A: Silicon Wall Stress Analysis
In order to optimize the design of the microreformer for overall component
volume, the minimum amount of material should be utilized. To minimize the
material and exterior dimensions, the operating stresses must be balanced against
the operating pressure within the device. The internal channels must only withstand
the pressure differential caused by fluid flow around one rotation of the spiral.
Therefore, these internal channel walls will have a thickness of 40 µm, which is near
the minimum state of the art thickness for silicon micro-electromechanical systems
[3].
Below, Figure A-1 contains the three-dimensional solid model constructed in
SolidworksTM [27] utilized to analyze the stress and displacement of the silicon
structure under operating conditions. The radius of the wall matches the outer
radius of the overall device design. The thickness of this wall assuming a constant
inner dimension will be the optimized variable.
This solid model was analyzed by the use of COSMOSTM [27] a commercial
finite element analysis software package.
Page 87
87
Figure A-1: Solid Model of Silicon Wall Test Configuration
Figures A-2 and A-3, below, displays the solid mesh created to analyze the
stresses and displacement of the silicon wall under load. The mesh is optimized to
provide the maximum number of cells along the wall and not the base material.
The restraints applied to the model include fixing the lower surface of the
base material and fixing the top surface of the wall, which in the final assembly will
be bonded to a Pyrex wafer sealing the top of the microchannel surface.
A pressure of 1.06 MPa is applied perpendicularly to the inner surface of the
wall. This pressure is equivalent to the total pressure drop experienced by the fluid
within the assembly. This is a conservative value as much of this pressure drop will
be internally resolved within the device by the motion of the fluid. The pressure drop
of the final spiral rotation will be lower than that of the overall device.
Figure A-1: Solid Model of Silicon Wall Test Configuration
Figures A-2 and A-3, below, displays the solid mesh created to analyze the
stresses and displacement of the silicon wall under load. The mesh is optimized to
provide the maximum number of cells along the wall and not the base material.
The restraints applied to the model include fixing the lower surface of the
base material and fixing the top surface of the wall, which in the final assembly will
be bonded to a Pyrex wafer sealing the top of the microchannel surface.
A pressure of 1.06 MPa is applied perpendicularly to the inner surface of the
wall. This pressure is equivalent to the total pressure drop experienced by the fluid
within the assembly. This is a conservative value as much of this pressure drop will
be internally resolved within the device by the motion of the fluid. The pressure drop
of the final spiral rotation will be lower than that of the overall device.
Page 89
89
The results of this analysis are presented in Figures A-3, A-4 and A-5 below.
As found by analyzing wall thicknesses in 5 µm increments, the optimum thickness
was discovered to be 70 µm. At this thickness the minimum factor of safety is 5.2,
and the maximum displacement is 530 nm or 0.53 µm.
These results have been utilized to optimize the minimum wall thickness of
the microreformer. Due to the geometry of fitting the spiral channel within the
circular enclosure, the actual outer wall thickness will be greater than 70 µm in all
locations except one, where the minimum thickness point is reached at the point in
the assembly where the fluid passes to the second side of the device.
Figure A-4: Displacement Results in Silicon Wall with Applied Pressure of 1.06
MPa
The results of this analysis are presented in Figures A-3, A-4 and A-5 below.
As found by analyzing wall thicknesses in 5 µm increments, the optimum thickness
was discovered to be 70 µm. At this thickness the minimum factor of safety is 5.2,
and the maximum displacement is 530 nm or 0.53 µm.
These results have been utilized to optimize the minimum wall thickness of
the microreformer. Due to the geometry of fitting the spiral channel within the
circular enclosure, the actual outer wall thickness will be greater than 70 µm in all
locations except one, where the minimum thickness point is reached at the point in
the assembly where the fluid passes to the second side of the device.
Figure A-4: Displacement Results in Silicon Wall with Applied Pressure of 1.06
MPa
Page 92
92
T∞
T2
T1
insulation
microreformer
Figure B-1: Temperature Location Diagram for Microreformer Thermal Analysis
Other options for reducing the outer surface temperature include using fuel
pre-heating or enclosing the reformer in some type of evacuated capsule with
radiation barriers (this second option could be prohibitively expensive). While the
table above includes calculations assuming a single insulating material, in reality a
two material system is incorporated into the design: an inner insulating material, and
an outer exterior material for protection and durability.
At a surface temperature of 45°C, the power lost from the reformer surface
through natural convection to the air is roughly 0.31 W (based on an approximately
quarter sized microreformer, R=10mm). If the same size microreformer had a
surface temperature of 200°C, the heat loss would be approximately 2.75 W. This
means that the reformer must be sized to process enough fuel to make up for this
loss of thermal energy as well as powering the device to which it is connected.
As shown in Figure B-2 below, as the channel size increases and the overall
device size decreases, the heat lost as waste decreases.
T∞
T2
T1
insulation
microreformer
Figure B-1: Temperature Location Diagram for Microreformer Thermal Analysis
Other options for reducing the outer surface temperature include using fuel
pre-heating or enclosing the reformer in some type of evacuated capsule with
radiation barriers (this second option could be prohibitively expensive). While the
table above includes calculations assuming a single insulating material, in reality a
two material system is incorporated into the design: an inner insulating material, and
an outer exterior material for protection and durability.
At a surface temperature of 45°C, the power lost from the reformer surface
through natural convection to the air is roughly 0.31 W (based on an approximately
quarter sized microreformer, R=10mm). If the same size microreformer had a
surface temperature of 200°C, the heat loss would be approximately 2.75 W. This
means that the reformer must be sized to process enough fuel to make up for this
loss of thermal energy as well as powering the device to which it is connected.
As shown in Figure B-2 below, as the channel size increases and the overall
device size decreases, the heat lost as waste decreases.
Page 93
93
Figure B-2: Microreformer Heat Loss as a Function of Channel Width
Since, the microreformer only generates sufficient hydrogen for a total of 1.5
W of electrical power, it is obvious that channel widths below 150 µm become
unfeasible. At the proposed dimension of 255 µm, the microreformer will lose
approximately 0.45 W of heat. In a mature design including the actual fuel cell,
which must operate at an elevated temperature of 80°C or higher, not all of that
heat would be wasted, but could be utilized to increase the efficiency of the fuel cell.
Figure B-2: Microreformer Heat Loss as a Function of Channel Width
Since, the microreformer only generates sufficient hydrogen for a total of 1.5
W of electrical power, it is obvious that channel widths below 150 µm become
unfeasible. At the proposed dimension of 255 µm, the microreformer will lose
approximately 0.45 W of heat. In a mature design including the actual fuel cell,
which must operate at an elevated temperature of 80°C or higher, not all of that
heat would be wasted, but could be utilized to increase the efficiency of the fuel cell.
Page 94
94
Figure B-3: COSMOS Analysis Plot of Insulation Optimization Results
This analysis indicates that an insulating system of 6 mm of foam with a
thermal conductivity of 0.02 W/m-K and 1 mm of plastic (e.g. polyamide) packaging
can reduce the outer surface temperature from 473 K to 321 K or 48°C. Due to the
plastic rather than metal materials in the outer casing and the fact that an operator
will likely not have the opportunity to directly touch the microreformer packaging in
normal operations, this temperature should be low enough for safety purposes.
Including the energy required to heat the fluid and sustain the chemical
reaction within the microreformer, the total device has an efficiency of 42%. This
could be increased by the incorporation of some additional insulation, or utilizing
waste heat appropriately. The theoretical maximum thermal efficiency of this device
is 70%.
Figure B-3: COSMOS Analysis Plot of Insulation Optimization Results
This analysis indicates that an insulating system of 6 mm of foam with a
thermal conductivity of 0.02 W/m-K and 1 mm of plastic (e.g. polyamide) packaging
can reduce the outer surface temperature from 473 K to 321 K or 48°C. Due to the
plastic rather than metal materials in the outer casing and the fact that an operator
will likely not have the opportunity to directly touch the microreformer packaging in
normal operations, this temperature should be low enough for safety purposes.
Including the energy required to heat the fluid and sustain the chemical
reaction within the microreformer, the total device has an efficiency of 42%. This
could be increased by the incorporation of some additional insulation, or utilizing
waste heat appropriately. The theoretical maximum thermal efficiency of this device
is 70%.
Page 95
95
APPENDIX C: Microreformer Design Drawings
APPENDIX C: Microreformer Design Drawings
Page 100
100
APPENDIX D: Fuel Cartridge Feasibility Analysis
The microreformer design presented here requires a fuel supply at a constant
pressure of 1.06 MPa and a constant mass flow rate of 1.46×10-7 kg/s. Since, the
primary application for a microreformer of this design would be small electronic
devices; it is desirable to understand whether the fuel supply as well as the actual
reformer is capable of achieving certain size and weight restrictions.
Rechargeable batteries for portable electronic devices provide full power for
3-6 hours typically. The volume of methanol-water mixture fuel must be 6.07 mL to
ensure full power or 1.5 V for at least 10 hours. That is 98% of the volume for a
single AA sized battery.
To overcome the flow resistance in the microreformer, a fuel supply pressure
of 1.06 MPa is required. To provide this pressure within a small recyclable fuel
container a mechanical pressurization system is desired. Either a propellant or
spring energized piston could be employed to provide the desired pressure without
changing the quality of the fuel. See the diagram in Figure D-1 below for a graphical
representation.
Propellant
Methanol/Water
Mixture
Piston Fuel Cartridge Casing MEMS Valve
Valve
Control
0.5 mm Fuel Line
Figure D-1: Schematic of Possible Portable Fuel Cartridge Design
APPENDIX D: Fuel Cartridge Feasibility Analysis
The microreformer design presented here requires a fuel supply at a constant
pressure of 1.06 MPa and a constant mass flow rate of 1.46×10-7 kg/s. Since, the
primary application for a microreformer of this design would be small electronic
devices; it is desirable to understand whether the fuel supply as well as the actual
reformer is capable of achieving certain size and weight restrictions.
Rechargeable batteries for portable electronic devices provide full power for
3-6 hours typically. The volume of methanol-water mixture fuel must be 6.07 mL to
ensure full power or 1.5 V for at least 10 hours. That is 98% of the volume for a
single AA sized battery.
To overcome the flow resistance in the microreformer, a fuel supply pressure
of 1.06 MPa is required. To provide this pressure within a small recyclable fuel
container a mechanical pressurization system is desired. Either a propellant or
spring energized piston could be employed to provide the desired pressure without
changing the quality of the fuel. See the diagram in Figure D-1 below for a graphical
representation.
Propellant
Methanol/Water
Mixture
Piston Fuel Cartridge Casing MEMS Valve
Valve
Control
0.5 mm Fuel Line
Figure D-1: Schematic of Possible Portable Fuel Cartridge Design
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