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Multi-Disciplinary Senior Design Conference
Kate Gleason College of Engineering
Rochester Institute of Technology
Rochester, New York 14623
Project Number: P10462
THERMOELECTRIC POWER SYSTEM FOR IMPROVED
HAITIAN COOKSTOVE
Shawn Hoskins/ME
Dan Scannell/ME
Luke Poandl/EE
Young Jo Fontaine/ME
ABSTRACT
The primary goal of the Thermoelectric Power System
project was to design and build a power system for a
cookstove intended for use by rural Haitians. The
power system will power a fan on the stove, recharge
batteries, and provide auxiliary power for uses such as
charging a cell phone. Power will be provided by a
thermoelectric generator, a solid state device that
converts thermal energy (via a temperature difference)
into electrical power. Dr. Robert Stevens, of the
Mechanical Engineering Department at Rochester
Institute of Technology, was the main customer and
guided the team through the project. The project team
consisted of three mechanical engineers and two
electrical engineers. The three mechanical engineers
were divided into three areas of responsibility: airflow,
heat transfer (hot side), and heat transfer (cold side).
One electrical engineer was responsible for battery
charging and auxiliary loads. The other electrical
engineer was responsible for power delivery. The final
product consists of a mechanism for transferring heat
to the thermoelectric, a heat sink for cooling the
thermoelectric, a fan for providing airflow to the stove
and for cooling, and the electrical system. This paper
will describe in detail the design, fabrication, and
testing of the final product.
Dan Higgins/EE
CS – Cold Side
HS – Hot Side
ISC – Short Circuit Current
Li-Ion – Lithium Ion
MOSFET – Metal Oxide Semiconductor Field Effect
Transistor
NiCd – Nickel Cadmium
NiMH – Nickel Metal Hydride
R – Resistance
TEG – Thermoelectric Generator
VOC – Open Circuit Voltage
INTRODUCTION (OR BACKGROUND)
Haiti faces two major problems: deforestation and the
risk of respiratory infection due to smoke inhalation.
Currently, rural Haitians cook indoors over inefficient
open fires that utilize wood charcoal as fuel. The
objective of an improved cookstove is to promote a
cleaner and more complete combustion so that there is
relatively little smoke released into the atmosphere
and less fuel is used. A better combustion will
accomplished by introducing airflow into the
combustion chamber through use of a fan. The fan will
be powered by a TEG module, a solid state device that
converts thermal energy to electricity.
PROCESS (OR METHODOLOGY)
NOMENCLATURE
BAT – Battery / Batteries
BB – Buck Boost (Converter)
Overall Design
The final design selected was chosen through an
iterative process. Multiple concepts were generated
Copyright © 2010 Rochester Institute of Technology
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Proceedings of the Multi-Disciplinary Senior Design Conference
and then rated and ranked based on feasibility and
estimated performance. A major factor in the design
process was simplicity. The system had to be
simplistic so that it could be easily used and
constructed by those in Haiti. Also, the design had to
be affordable. Most Haitians live off only a $2 daily
income; therefore, the stove had to be inexpensive so
that the purchase would be justified. Any design that
was deemed too complicated or expensive was
immediately ruled out. The final design concept
selected utilizes an aluminum rod and flat plate
(thermal bridge) to transfer heat from the cookstove’s
fire to the TEG. The cold side of the TEG is
maintained by an aluminum heat sink through which
airflow from the fan is directed. The fan and heat sink
are surrounded by a square aluminum duct that would
direct the airflow into the combustion chamber.
Figure 2: Final System Design
Figure 2 does not show the L brackets that mount the
stove to the heat sink or the L brackets that mount the
fan to the heat sink. The top plate is secured to the
heat sink with four screws that use insulating washers
to prevent heat transfer directly to the heat sink. The
rod in Figure 2 is set into the combustion chamber and
is heated from the flame. This rod’s placement was
determined through testing the fire inside the stove as
multiple forms of biomass where burned to determine
the most steady source of heat generation.
Figure 1: Original Design Concept
This design, however, evolved over the course of
construction and testing of the system. The duct was
eliminated and it was instead decided to use the heat
sink for both cooling and directing airflow. The fan is
mounted inside of the heat sink, which serves as a
duct.
The system for the heat transfer and air flow was
designed to have the fewest number of parts to make it
easier to acquire for the Haitian people. The heat sink
was also used as a air duct for the fan. Fans require air
ducts to create pressure in the working fluid so that it
can provide the system with airflow. The current
system model is shown below in Figure 2.
Figure 3: Flat Plate
The device pictured in Figure 3 transfers heat from the
rod to the hot side of the TEG. The heat block was
designed this way after testing a non-machined flat
plate that screwed into the heat sink. This plate
theoretically would still achieve the same temperature
as the smaller block used for initial testing but would
require a longer ramp up time. In testing the larger flat
plate was not providing enough heat to the hot side of
the TEG so the plate was modified to reduce all the
area that was not in contact with the TEG. This
modification allows the block to develop heat faster
and has less convection heat losses with the outside
environment.
Airflow Verification
To provide optimal air to the combustion chamber
allowing complete gasification of the biomass being
burned the fan must provide adequate airflow to the
combustion process. The cook stove provided had the
fan mounted underneath the combustion chamber to
Project P10462
Proceedings of the Multi-Disciplinary Senior Design Conference
allow unrestricted airflow to the combustion chamber.
To redesign the system to allow for the fan to not only
provide sufficient air to the fire but also cool the heat
sink the fan had to be mounted on the side of the
stove. Before the stove was redesigned the airflow
with the fan being input from the side and through the
heat sink had to be analytically compared to the stove
without modification. The drop in pressure from the
system was used as the gauge to show that proper
airflow was being obtained.
To characterize the airflow the flow rate of the air
from the fan was tested by testing the time it took the
fan to fill a predetermined volume. This experiment
was tested five times to provide accurate data and
enough iterations to show that the fan could
consistently perform at the same level. With the flow
rate of the fan determined the pressure drop could be
calculated from the system using major and minor loss
equations for pump work.
With these calculations done the flow through the heat
sink and outer duct where calculated for comparison.
The greatest loss in the system was due to the orifice
loss at the inlet to the combustion chamber. The
addition of the major head losses through the heat sink
where minimal in comparison to the already existent
orifice losses. Figure 3 shows the comparison in
pressure drop through the system for the stock stove
and the modified model.
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10
Current
Design
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Figure 4: Comparison of Airflow
With this acceptable level of pressure drop the stove
was modified to accept airflow in through the heat
sink and into the side of the combustion chamber.
Through testing the stove operates at the same level as
before the modification.
Thermal Bridge
The decision for the placement of the thermal bridge
was obtained by conducting a heat test. In this test we
placed thermocouples along the side of the combustion
Page 3
chamber, at different heights, and on the bottom of the
combustion chamber. This test allowed us to see
where the largest amount of heat is located and the
most consistent numbers as well. Based on the results
of this test we concluded that placing a thermal bridge
in the side of the chamber at a height of about 2.75”
would be the optimal placement. By deriving base
equations and using the values found from thermal
testing, the design for the thermal bridge was selected.
Several calculations were conducted: one for a model
of what the convection heat transfer between the fire
and the rod was created. Next based on the convection
from the fire a base temperature was derived. Another
calculation using the base temperature of 315°C was
conducted to discover what the resistance of the
thermal bridge needed to be in order to have the hot
side of the TEG be 230°C. Finally another situation
involving an estimated maximum temperature of 500
°C at the inside wall, and what the resultant hot side
TEG temperature would be. The hot side temperature
of the TEG was selected because this would allow a
ΔT of 160 °C and allow our cold side to be as high as
70 °C, which was constrained by our heat sink
selection. The required qh was determined through
calculations using TEG specifications and our desired
temperature difference between the hot and cold side
of the module.
Initial design calculations showed that the rod should
only be inserted into the combustion chamber 1.5”
with a length of 1.33” protruding from the outside wall
of the stove. The rod was also designed with a
diameter of ½”. After testing, it was decided that the
thermal bridge would perform better if the rod were
inserted entirely into the combustion chamber with the
flat plate flush against the outside wall of the stove.
Heat Sink
The heat sink was originally designed using heat
transfer calculations and comparing the results to
commercially available heat sink extrusions. A heat
sink that was deemed suitable was ordered, but it was
discovered that the purchased heat sink would be
impractical to use for our purposes. Therefore, it was
decided to fabricate a custom heat sink. Through an
iterative process, a heat sink was fabricated and tested.
Results from testing showed that the heat sink would
have to be modified to improve its efficiency. In order
to improve the efficiency of the heat sink and increase
in surface area was needed. Due to the time required to
construct a new heat sink, the heat sink previously
machined was improved. After brainstorming through
different ways to optimize the surface area it was
determine that the easiest and most effective way
would be to drill semi-circles into the fins (as seen
below). Further testing was conducted and the heat
sink still proved incapable of producing the cold side
Copyright © 2010 Rochester Institute of Technology
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Proceedings of the Multi-Disciplinary Senior Design Conference
temperature needed to obtain the desired ∆T. A second
model of the heat sink was created to better understand
what the geometry of the heat sink should look like to
produce the desired cold temperature. This model took
into account the inlet velocity of the air to determine
the convection coefficient, and used the Qc derived
from the calculations on the TEG. This model was
designed to allow the geometry of the heat sink to
change. This allows for a better understanding of how
each variable affects the efficiency of the heat sink.
Electrical Source and Load
The primary focus behind the electrical design of this
project is how to deliver power from source to load.
The fan is the most important electrical load because
efficient operation of the stove requires it to be
running at all times. The novel feature of this project
is its ability to convert “waste heat” into electricity
which is accomplished by the Thermoelectric
Generator (TEG). The selected TEG is capable of
outputting up to 5.9W steady state assuming a
sufficient temperature differential is present. In the
case the thermal output is insufficient; a battery system
is implemented to run the fan when the TEG is
incapable of doing so.
The output of the TEG can be characterized as
dynamic; that is it will vary depending upon the
thermal operating conditions on its hot and cold sides
(thus the differential - ∆T) as well as the electrical
load placed on it. Ideally a matched load will be
provided to the output of the TEG for a given ∆T
resulting in maximum power transfer. Because the
total electrical load only differs when discrete loads
are interchanged, reaching this maximum power
transfer is an ideal scenario that cannot be met within
the scope of this project.
In addition to the fan motor, other electrical loads are
present in this system. The battery pack cannot simply
be wired into the circuit; it needs to be recharged and
discharged at proper times depending on variable
operating conditions. In addition, a circuit needs to be
implemented to allow charging at the proper rate and
level. The solution to this need is an off-the-shelf
battery charging chip. This device monitors and
controls current in and out of the battery pack; it
discharges when battery charge is high enough and at
other times trickle charges the battery pack from an
outside source. Thus, when the chip is supplying
current to charge the battery, it becomes a load with
respect to the TEG.
A secondary need in this project is the ability to
supply a small DC source to charge a cell phone
battery or similar device. Assuming the fan and
battery system are sufficiently powered any additional
power can be output at a 5V level. This requires two
additional considerations in the electrical design: a
constant output voltage level and load switching based
on TEG power.
Voltage Level Conversion
The TEG output voltage and therefore output power
are directly proportional to the ∆T across it as seen in
(1), minus the internal resistance factor. Consideration
is also given to other thermal and electric properties
however the temperature gradient is of most
importance.
(1)
Because this voltage is dynamic, a method is needed to
ensure a constant current is provided to the load. This
is implemented by using DC-DC Buck Boost
converters; one will provide a constant 5V output and
the other a constant 3.3V output. Figure 1 provides a
block diagram of the electrical system components to
explain the needs for voltage conversion. From this it
is clear the 2 voltage platforms are needed: a 3.3V
level to run the fan and switching reference (explained
later) and a 5V level to run the battery charging circuit
and provide USB-level auxiliary power.
Battery Array
Discharge
TEG
5V Buck
Boost
Converter
Switching
Charge
Aux
Power
3.3V Buck
Boost
Converter
FAN
Figure 5: Electrical System Block Diagram Note:
sampling & switching interconnects not shown
The 5V Buck Boost converter is capable of delivering
a constant 5 Volts to a load when an input range of
1.8-5.5V is available. It is important to consider
instantaneous energy (i.e. power) conservation since
output voltage and current will vary from input levels
yet the total power must be equivalent or less at the
load side (2). While this may seem obvious from a
physical perspective, it must be kept in active
consideration because of the dynamics of switching
the loads.
Project P10462
Proceedings of the Multi-Disciplinary Senior Design Conference
Load Switching
The proposed autonomous switching system will
sample the open circuit voltage to determine operating
point of the TEG; this corresponds to the maximum
instantaneous electrical power the unit can deliver. An
accumulating clock will effectively open the loaded
circuit of the TEG to allow the terminal voltage to
reach its maximum open circuit value. This will be
done every 60 seconds for a duration of approximately
30 ms; this time should be sufficient to allow any
transient switching behavior to stabilize.
The circuit is opened by use of a P-Channel MOSFET
(PMOS) or a latching relay; these options should be
explored based on cost on performance in testing. The
TEG’s VOC is sampled by comparators at its terminal
node(s); the reference value is derived through the
batteries & 3.3V BB through a voltage divider
network of resistors. The comparator outputs are fed
into D-type Flip Flops which latch the value stored on
the input until the next clock pulse. Therefore, if a the
comparator allocated to the fan sends a high-level out
upon sampling, that value will be stored on the output
of the DFF. This value is sent into an NMOS to sink
current to the actual load.
Comparison is done for four different load levels: an
“off” state, fan, battery recharging, and auxiliary
power. These loads are cumulative, meaning if the
auxiliary power is on; the battery recharging and fan
are also enabled. This system is powered through the
3.3V BB converter from the batteries meaning it is
always supplied with a steady current and voltage.
U14
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V2
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5Vdc
0
Figure 6: Load-Source switching diagram based on
sampling levels at various circuit nodes
Battery
One of the requirements that was determined very
early on was a battery would be required to power the
system while the fire was coming up to full
temperature. This need was twofold; the first was to
Page 5
increase combustion, reducing emissions during
startup. The other was to ensure that the heat sync was
able to ensure the temperature difference needed for
TEG operation. There are several common
rechargeable battery types available in today’s market.
Nickel Metal Hydride, Nickel Cadmium and Lithium
Ion are by far the most common place. Of these the
NiCd batteries were immediately ruled out due to their
limited performance and high environmental costs.
The remaining two were close competitors due to their
strong attributes. NiMh are commonly available for
use in mobile devices, are relatively cheap and do not
require complicated charging circuitry. Li-Ion batteries
are widely available for their use in cellular devices,
have high capacities and long shelf lives. However, in
the end it was determined that Li-Ion were not a
suitable choice due to their higher cost and the
requirement for more complicated charging. A series
of three Nickel Metal Hydride batteries would be used
for their higher capacity and suitable voltage.
Furthermore, the use of NiMh would allow the
possibility of several different charging schemes.
Auxiliary Power
The ability to use the additional power for other uses
was a design requirement from the beginning but the
exact requirements had never been formulated. In
early stages several possible uses were thought of;
lighting, computers, radios, and cell phones. Of all of
those the most useful was deemed the ability to charge
a cell phone. In this area there is no infrastructure for
AC electricity distribution, so charging is done from
twelve voltage car battery. This introduced the first
option of a adding a twelve volt output. However,
analyzing the charger it was deemed an impractical
use of power. Li-Ion batteries require a five volt
source to charge, so this adapter steps the twelve volt
input down. As the system runs at a much lower
voltage, not having to step up to a much higher voltage
would avoid unnecessary power loss. Several, off the
shelf products specifically designed to provide a
constant five volt source for powering a USB source
were evaluated. However, as with the car charger the
majority of products were designed for use with a
twelve to twenty-four volt source. Several with the
correct input range were found but the required output
current would not be met. However, a suitable
alternative already existed. A five volt buck-boost
converter was already being used to supply the battery
charger with the proper voltage; the auxiliary input
could just be utilized with the existing system with a
USB output cable.
RESULTS AND DISCUSSION
Copyright © 2010 Rochester Institute of Technology
Page 6
Proceedings of the Multi-Disciplinary Senior Design Conference
The two systems (mechanical and electrical) were
initially tested separately. After testing was conducted
and theories were verified on each system, the systems
were integrated for full system testing.
Mechanical System Testing
The mechanical system was initially tested with a
dummy TEG module to provide thermal resistance.
The testing was conducted over a live fire in the
cookstove. Also, each component of the mechanical
system was tested individually. Instead of using a heat
sink, a cold plate was used to simulate a consistent
cold temperature similar to what the intended final
heat sink would provide. Initial tests showed that the
aluminum rod that was selected was insufficient for
providing the required thermal conduction needed to
provide a temperature of 230°C at the hot side of the
TEG. Therefore, it was decided to increase the surface
area of the rod by increasing its diameter from ½” to
5/8”. This increase allowed the thermal system to
reach its required hot side temperature.
remove enough heat from the system. To compensate
for this inadequacy the heat sink was modified to have
the scallop shell designed fins that would increase the
surface area of the fins without the addition of more
fins. This heat sink was made in house and a better
alternative could be used from an extruded aluminum
heat sink however that was not an available option in
the time allotted.
Electrical System Testing
After the hot side of the system was verified, the full
mechanical system was tested with the addition of the
heat sink. The originally fabricated heat sink was
found to not be sufficient, therefore it was modified to
increase its surface area and thus provide better
cooling for the TEG.
Figure 8: Long Run Stability Test
When the heat is maintained for a long period of time,
the system is able to adequately maintain the voltage
output required for the fan. This data was recorded
over the course of six hours, during which one sample
was taken every second.
Figure 7: Final Heat Sink Design
The heat sink in Figure 7 was created to cool the cold
side of the TEG while mounting the fan for the
gasification process inside. Initially the team tried to
order a heat sink, unfortunately the lead time for a
custom heat sink and the cost of buying a bulk
extrusion would not have met the team’s goal of
having a working prototype for the EPA P3
conference. This heat sink was designed initially with
flat fins but from thermal testing that design could not
Project P10462
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Proceedings of the Multi-Disciplinary Senior Design Conference
∆T (°C)
50
60
80
100
110
120
130
140
150
160
170
Voc (V)
2.035
2.428
3.369
4.18
4.59
5.08
5.51
5.89
6.29
6.65
7.02
Isc (A)
0.928
1.083
1.453
1.731
1.883
2.066
2.211
2.345
2.464
2.581
2.712
Table 1: Results of TEG Characterization
The results show linear correlations between the
current and voltage output of the modules. When the
information is plotted as seen in Figure 9, a further
understanding of operation can be ascertained.
To accurately characterize the module, an existing
facility with thermostatically controlled hot and cold
plates was utilized. For predetermined temperature
difference, the open circuit voltage and short circuit
currents were measured and recorded. Furthermore,
the output was recorded when a fixed load was
applied.
∆T
50
60
80
100
110
120
130
140
150
160
170
Vmax (V)
1.018
1.214
1.685
2.090
2.295
2.540
2.755
2.945
3.145
3.325
3.510
Imax (A)
0.464
0.542
0.727
0.866
0.942
1.033
1.106
1.173
1.232
1.291
1.356
Pmax(W)
0.472
0.657
1.224
1.809
2.161
2.624
3.046
3.453
3.875
4.291
4.760
Table 2: Determination of Maximum Power
From the IV curve, the peak operating point of the
module can be found. If the system can be operated at
this point the module is able to provide the most
power.
Figure 10: System Efficiency as Function of ΔT
Figure 9: Voltage Current Characteristic of TEG
Full System Testing
After initial theories were verified through individual
subsystem testing and the necessary modifications
were made, the entire system was tested as a whole. At
this point, a data acquisition system (DAQ) was used
for all necessary measurements. Measurements
monitored included hot and cold side temperatures,
TEG voltage, and load voltage. With the ability to
monitor operating conditions in real-time, a better
Copyright © 2010 Rochester Institute of Technology
Proceedings of the Multi-Disciplinary Senior Design Conference
understanding of the system was obtained. The full
system tests yielded different results than what would
have been expected based on the findings from the
subsystem laboratory tests. For instance, the
temperatures along the thermal bridge and heat sink
varied drastically when an actual load from the TEG
was realized. Also, the TEG did not yield the expected
corresponding power output at the temperature
differences measured. Alterations were made to the
flat plate of the thermal bridge in order to achieve a
higher hot side temperature and to allow it to heat up
quicker. The way in which the flat plate was mounted
was also changed in order to improve the contact with
the thermoelectric module.
Figure 11: Complete Prototype Thermal System
Page 8
With these changes the temperature difference across
the TEG was able to reach 140°C corresponding to an
output voltage of 3.7 volts. Although this voltage
should have been high enough to at least power the
fan, problems with other components in the electrical
system prohibited proper operation. Ongoing testing is
in progress to determine the exact source of the error.
Despite this current setback it has been proven that the
system can operate within the given operating range.
CONCLUSIONS AND RECOMMENDATIONS
The project had many successes as well as failures.
The new stove design was able to provide proper
airflow and intermittently provided self-sustaining
power from the TEG. Unfortunately, the electrical
system currently does not provide enough power. This
may be due to a failed component, such as a buckboost converter. Also, the temperature difference
required is not at the ideal specification.
Future iterations of the project should include
investigation of the contact resistance between the
TEG and the two thermal surfaces. There could be a
loss of potential power because the module is not
making sufficient contact with the other surfaces. The
heat sink design should also be investigated and
optimized for the required temperature difference.
This may be able to solve the current problem with
insufficient temperatures. Also, an autonomous control
system should be integrated into the electrical system
so that the stove switches operating conditions
automatically. Currently, the stove’s operating
conditions are switched manually. In order to keep the
system feasible for use and construction in Haiti,
attempts should be made to construct the system out of
readily available materials.
ACKNOWLEDGMENTS
The team would like to thank the people who have
helped us over the course of this project. Thank you to
our faculty advisors, Dr. Rob Stevens and Dr. Rick
Lux, for their guidance and support throughout the
project process. Also, special thanks go to all who
assisted with the design and construction of our
project: Dr. Christopher Hoople, Dr. Robert Bowman,
Mr. Rob Kraynik, Mr. Steve Kosciol, Dr. James
Myers,
and
the
H.O.P.E.
organization.
Figure 12: System Response to Test Fire
Project P10462
Proceedings of the Multi-Disciplinary Senior Design Conference
Copyright © 2010 Rochester Institute of Technology
Page 9