Download display - Edge - Rochester Institute of Technology

Multi-Disciplinary Senior Design Conference
Kate Gleason College of Engineering
Rochester Institute of Technology
Rochester, New York 14623
Project Number: 11462
THERMOELECTRIC AND FAN SYSTEM FOR COOK
STOVE
Jared Rugg / Project Lead
Bradley Sawyer / Lead Engineer
Thomas Gorevski / Electrical Team
Fahad Masood / Electrical Team
Jeff Bird / Mechanical Team
ABSTRACT
A brief summary (often <200 words) should open the
paper. The purposes of an abstract are: (1) to give a
clear indication of the objective, scope, and results of
the paper so that readers may determine whether the
full text will be of particular interest to them; (2) to
provide key words and phrases for indexing,
abstracting, and retrieval purposes. The abstract should
not attempt to condense the whole subject matter into
a few words for quick reading. We can work on this
after the paper is finished.
NOMENCLATURE
TEG
Thermocouple
Kg
Min
In
CFM
L – Liter
C
INTRODUCTION
Billions of people around the world depend
on biomass for everyday cooking fuel. Cooking is
often performed on a semi-open stove or over a simple
three-stone fire. These methods of cooking are very
inefficient and lead to high fuel usage and airborne
pollution. Cooking is often performed indoors and
therefore the use of biomass cooking has a negative
impact on the health of people who must live in the
heavily polluted air conditions. Inefficient cooking
methods have especially had a major impact on Haiti,
the poorest country in the western hemisphere. Haiti
was once covered with forests, but is now less than 4%
forested. Much of the deforestation is a result of the
need for cooking fuel. The most popular fuel used in
Haiti is charcoal.
The introduction of oxygen to any
combustion process will, in general, allow for a more
efficient, complete and clean combustion. The primary
function of the thermoelectric and fan system is to
provide a forced airflow to the rocket-style cook stove
without the need of an external power source.
Furthermore, the unit utilizes a thermoelectric unit to
convert thermal energy taken from the combustion
process into electrical energy. Electrical energy
provided by the thermoelectric unit is used to power a
fan and also a UBS port which can be used to charge
the batteries of an ‘auxiliary’ device such as a cell
phone.
The thermoelectric, which is the heart of the
system, develops electrical power when a temperature
differential is created across its top and bottom
surfaces. The thermoelectric generator (TEG) takes
advantage of the Seebeck Effect to create electrical
power. The Seebeck Effect happens when a
temperature difference is created between two
different metals or semiconductors. A voltage
Copyright © 2008 Rochester Institute of Technology
Proceedings of the Multi-Disciplinary Senior Design Conference
differential is created between the metals, and if they
are connected in a loop (circuit) then a current will
flow. This configuration is also known as a
‘thermocouple.’ Figure 1 shows how the Seebeck
Effect works. Wires constructed of two different
metals (metal A, metal B) are joined at two points.
When a temperature differential between the two
junctions is present (T1, T2) a voltage is created.
FIGURE 1 – SEEBECK EFFECT
A thermoelectric generator consists of multiple “leg
pairs” of p-type and n-type semiconductors. Each leg
pair is a thermocouple which produces electrical
power when a temperature difference is created across
it. The leg pars are connected together in either a
series or parallel circuit through thin metal
interconnects. The leg pairs are sandwiched between
two thin wafers of ceramic which gives the
thermoelectric generator structural rigidity. The
ceramic substrate also provides a smooth contact
surface for thermal energy to be conducted to. Figure 2
shows a typical thermoelectric generator. This is the
style generator that is used in the thermoelectric fan
module.
Page 2
that the unit requires a minimal amount of actions
from the user in order to operate correctly. User
interactions can include adjustment of any knobs,
switches or any general physical interaction with the
unit. Simple operation of the unit also includes a
design that can easily be removed or connected to the
stove.
The design of the thermoelectric and fan
system has five main facets. The first facet is
transferring heat energy from the combustion chamber
of the rocket stove to the hot side surface of the TEG.
The second facet is removing heat from the cold side
surface of the TEG, the third is providing a source of
forced airflow to the stove, the fourth is controlling
and managing the flow of electrical energy and the
fifth is packaging the components in an efficient and
simple manner. This thermoelectric and fan project
(P11462) is a second generation design as a previous
RIT Senior Design team attempted to fulfill the same
requirements. The design of the P11462 unit takes the
previous team’s design failures and successes into
account.
LITERATURE REVIEW
Project 11462 is a second generation project
for the thermoelectric and fan system. The previous
project (10462) left behind important information to
learn from. The main issues faced by P10462 were a
malfunctioning electrical system and insufficient
cooling of the TEG. The custom heat sink used by
team 10462 appears to have insufficient surface area
and overall architecture. This fact was echoed in their
technical write-up. Another issue that was mentioned
was a suspected problem with thermal resistances
between the ceramic substrate of the TEG and the
surfaces of the thermal bridge (heat conduction rod)
and heat sink. On the electrical front, team 10426
experienced multiple problems with managing the
power allowances between the electrical devices. An
important consideration that was discovered through
reading the research by previous thermoelectric teams
was that the thermoelectric must be operated in a
range where it can provide sufficient power.
FIGURE 2 – THERMOELECTRIC GENERATOR
PROCESS (OR METHODOLOGY)
The target market for this thermoelectric and
fan system is Haiti, therefore, one of the major design
objectives is to produce a low cost unit. In conjunction
with the low cost, the system should also be simple
and easy to use. The beauty of a thermoelectric unit is
that it is a solid-state device, meaning it has no moving
parts and therefore is intrinsically simple (from a
physical stand-point). Another design requirement is
The final proposed design was the result of
careful consideration of past team experiences,
pairwise comparisons of possible solutions, and first
principle feasibility calculations. The final design
consists of a square cross-section metal housing which
functions as an air duct. Inside the housing resides a
fin style aluminum heat sink along with a computer
Project P11462
Proceedings of the Multi-Disciplinary Senior Design Conference
case fan to provide air flow. Figure **** shows the
design of the thermoelectric and fan system. The air is
ducted axially from the top of the stove to the bottom
and is then turned 90 degrees to enter the bottom of
the stove. This axial design was implemented to fit the
system (mainly the large heat sink and fan) in close
proximity to the stove. This was desirable for both
package-ability purposes and also because keeping the
unit close to the stove has a minimal effect on the
stove’s center of gravity.
One of the most important parts of the design
is the heat sink which removes heat from the cold side
of the thermoelectric. The simple finned heat sink was
chosen because of its simplicity and also because
commercially available finned heat sink extrusions are
cheap and plentiful. The finned heat sink also creates a
minimal pressure drop when compared pin style heat
sinks. The heat sink is fastened to the metal housing
via four cap screws.
The manner in which thermal energy is removed
from the combustion process for use by the
thermoelectric module was an important point of
debate. In the end a “ruler style” thermal conduction
rod was selected which was to be made of steel.
Although the thermal conductivity of steel is
significantly less than that of aluminum, initial tests
suggested that the temperatures generated by the fire
would cause significant damage to aluminum. The
conduction rod enters the side of the stove through a
slot and protrudes into the combustion chamber much
like a large fin. The other end of the rod transfers
energy to the thermoelectric through a small pocket
machined in the face of the rod. Compression is
maintained on the thermoelectric via four, 6-32 cap
screws which thread into the heat sink. The thermal
bridge that these cap screws create between the
conduction rod and the heat sink is an issue. The
attempt at remediating this problem was to have no
contact with the threads of the cap screw and the hole
in the conduction rod in which they pass through.
Stainless steel washers sit underneath the cap of the
screws. A circular recess in the conduction rod accepts
the washers with a reasonably tight tolerance. These
washers serve a two-fold function. For one, stainless
steel has a relatively low (compared to steel) thermal
conductivity and therefore the washers help to
decrease the thermal bridging through the 6-32 cap
crews. The second function the washers provide is to
constrain the rod from any lateral motions which could
damage the TEG.
The need for a method in which to control the air
flow into the stove led to the implementation of the
“bypass” design. Initially the air flow into the stove
was going to be controlled by varying the voltage to
the fan, but this idea was scrapped because of the need
for a constant substantial airflow over the heat sink to
provide cooling to the TEG. The bypass is a simple
Page 3
hinged door located downstream of the fan and heat
sink. When opened, the bypass allows some air to be
ducted to atmosphere instead of into the stove which
allows for control of the strength of the combustion
process inside the stove.
The thermoelectric module used for this project is
a 4 watt module, meaning that 4 watts is the maximum
power output of this module. In order to achieve a 4
watt output the temperature differential across the unit
must be approximately 200 C, with a maximum
sustained hot side temperature of 300 C. With this data
in mind, the unit was designed to operate with a hot
side temperature of 300 C and a cold side temperature
of 100 C. Most thermoelectric modules, including this
one, are approximately 4% efficient. Therefore it was
assumed that for a desired output of 4 watts, 100 watts
of heat energy would need to be passed through the
TEG unit. A design value of 130 watts was selected to
account for thermal losses in the system and for losses
in the TEG unit. Once these design parameters were
selected, the conduction rod and heat sink could be
designed to pass 130 watts of energy through the TEG
while maintaining a 200C temperature differential
across the TEG.
The electrical systems were designed around
certain needs by the customer. From an electrical
standpoint the priority list is as follows: 1) Provide
power to the computer case fan which provides air
flow, 2) be able to charge a battery pack which will
allow the fan to run at startup when the TEG is not yet
producing power, 3) be able to charge a secondary
battery pack which will be able to power a USB port
to charge an auxiliary device.
FAN
Initially, the proposed maximum nominal
specification for air flow was 1.2 kg/min. Using STP,
this value converts to 36 CFM. The flow into the stove
will depend on both the resistance to flow of the stove
as well as the pressure drop provide by the fan. First,
the stove’s impedance needed to be determined; it was
decided that this would be determined empirically. A
pressure tap was placed in the inlet to the stove. Air
was then ducted into the stove with a smooth PVC
pipe with a flow meter inline. Using a pressure
transducer an outlet voltage was recorded and
converted to units of inches of water. Using the known
cross-section of the PVC the flow velocity from the
flow meter was converted into a flow rate. The flow
rate was varied and several data points were taken.
The stove’s impedance was then plotted as pressure
drop versus flow rate.
Copyright © 2008 Rochester Institute of Technology
Proceedings of the Multi-Disciplinary Senior Design Conference
Stove Impedence
Pressure Drop
(inches H2O)
0.60
0.50
0.40
0.30
0.20
0.10
0.00
0
10
20
30
40
50
Flowrate (CFM)
After examining this plot and a general overlay of an
estimate of a very efficient computer fan operating at 2
watts it quickly became apparent that the goal of 36
CFM of flow into the stove would not be plausible.
Subsequently, a desire to better understand the effect
of the air flow rate on the fire’s properties emerged. In
order to do this an additional test was run. Again using
the PVC with the flow meter inline to duct air into the
stove, a fire was started in the stove and the stove was
allowed to reach a “hot start” temperature. Next, 2 L
of water at room temperature (50 F) was placed in a
pot. After the charcoal was topped off to a set level the
pot was placed on the stove and the time to boil was
measured. This experiment was done at three different
flow rates: 20, 30, and 40 CFM. The results are shown
in Figure #.
Starting Water
Temp (deg F)
Flow Rate
(CFM)
Time to boil
(min)
51.1
20
3:54
48.6
30
3:15
50.9
40
3:13
As shown in the Figure decreasing the flow rate by
half had little effect on the time to boil. With this
information in mind, the air flow rate specification
was pushed down to a nominal max value of 0.7
kg/min.
The design relies on the fan running at the
max flow rate at all times. This is due to the fact that
the proper operation of the heat sink relies on good air
flow through it. The amount of air entering the fire is
controlled by a bypass. Some of the air flow can be
ducted out to atmosphere while the rest is ducted into
the stove. Currently, this is done through the use of a
pivoting door. Because of this design and the
engineering specification of being able to produce a
range of flow rates into the stove, the fan must be able
to produce the maximum flow rate at all times.
A computer case fan was chosen to provide
the air flow to the stove. The computer case fan was
chosen for a myriad of reasons. First and foremost,
Page 4
computer case fans are cheap and plentiful which fits
into the constraint that the system must be cheap and
easily producible. An equally important reason is that
most computer case fans are relatively efficient and
thus require low power to operate. The fan must
operate at a low power because power is at a premium
in the system. The computer case fan is also desirable
because it is well packaged and self-contained which
makes it easy to integrate into the system. The life
span of a computer case fan is also of importance. A
long fan life span is desirable and a computer case fan
provides this. The only real issue present with using a
computer case fan is that they do not provide large
pressure drops.
Once the parts had arrived the flow rate and
pressure drop capabilities of the fan needed to be
verified. This was done using a test rig designed to
specifically test computer case fans. The test rig is a
PVC tube with several pressure taps downstream of
the fan. The flow of the fan can be varied using a cone
at the outlet of the PVC tube. The closer the cone is to
the outlet of the tube the greater the resistance to flow
and thus higher the pressure drop. The flow rate was
measured using a hot wire anemometer that was
inserted into the middle of the pipe. Because the fan
speed is also a function of input voltage the fan speed
was held constant at 12 volts during the test (12 volts
is the recommended operating voltage of the fan). The
pressure drop was varied and the resulting flow rate
recorded. This data was plotted the stove impedance
curve. Despite being one of the most efficient fans
available the operating point of the system was still
too low. In order to combat this problem the inlet
holes to the combustion chamber were enlarged. This
modification should decrease the pressure drop into
the stove and shift the optimal point to a higher flow
rate. This will allow a higher flow rate of air into the
combustion chamber of the stove.
ROD
The design of the heat conduction rod involved many
factors. To start our process we developed a
temperature desired for the end of the rod based on the
temperature difference that would provide us with the
most power possible. Also an analysis of the
thermoelectric module provided us with an
approximate heat flux through the module. This piece
of information allowed constraining the design to sizes
that would allow for that level of heat conduction. A
test was also run to determine the U value of the fire
(need test name) and a general fire temperature was
assumed. The assumptions were used to generate a
desired temperature at the inner wall of the stove
assuming the section from that point back to the
module to be perfectly insulated. So the heat loss from
the inner chamber of the stove back was due only to
Project P11462
Proceedings of the Multi-Disciplinary Senior Design Conference
the k value of the material, in this case 1018 steel. The
general set of heat transfer equations used for the
portion of the rod in the fire was of an adiabatic tip fin.
Using these equations a Matlab program was
developed to test different lengths and widths for a
given thickness to match the amount of heat absorbed
by the fire to the desired temperature at the wall of the
inner chamber, with that temperature first being
calculated with each cross-sectional area and the
desired temperature at the thermoelectric. From the
different lengths and widths one was picked that was
considered the best fit for our design.
RESULTS AND DISCUSSION
This section should describe your final product or
process, whether it met specs (results of testing), and
how you evaluated its success. Most conference
papers include enough information for your work to be
reproducible.
CONCLUSIONS AND RECOMMENDATIONS
This section should include a critical evaluation of
project successes and failures, and what you would do
differently if you could repeat the project. It’s also
important to provide recommendations for future
work.
REFERENCES
Within the text, references should be cited in
numerical order by order of appearance. The
numbered reference should be enclosed in brackets.
For example: “It was shown by Prusa [1] that the
width of the plume decreases under these conditions.”
In the case of two citations, the numbers should be
separated by a comma [1,2]. In the case of more than
two references, the numbers should be separated by a
dash [5-7].
References to original sources should be listed
together at the end of the paper, and should include
papers, technical reports, books, prior team projects,
personal discussions, websites (not Wikipedia), and
software. References should be arranged in numerical
order according to the sequence of citations within the
text. Each reference should include the last name of
each author followed by his initials.
(1) References to journal articles and papers in serial
publications should include: last name of each author
followed by their initials, year, full title of the article
in quotes, full name of the publication (abbreviated),
volume number (if any) in bold (do not include the
abbreviation, "Vol."), issue number (if any) in
Page 5
parentheses (do not include the abbreviation, "No."),
inclusive page numbers using “pp.".
(2) Reference to textbooks and monographs should
include: last name of each author followed by their
initials, year of publication, full title of the publication
in italics, publisher, city of publication, inclusive page
numbers using "pp.", chapter number (if any) at the
end of the citation following the abbreviation, "Chap."
(3) Reference to individual conference papers, papers
in compiled conference proceedings, or any other
collection of works by numerous authors should
include: last name of each author followed by their
initials, year of publication, full title of the cited paper
in quotes, individual paper number (if any), full title of
the publication in italics initials followed by last name
of editors (if any) followed by the abbreviation, "eds.",
city of publication, volume number (if any) in
boldface – include, "Vol." if part of larger identifier
(e.g., "PVP-Vol. 254") – inclusive page numbers of
using "pp.".
(4) Reference to theses and technical reports should
include: last name of each author followed by their
initials, year of publication, full title in quotes, report
number (if any), publisher or institution name, city.
Example References:
[1] Ning, X., and Lovell, M. R., 2002, "On the Sliding
Friction Characteristics of Unidirectional Continuous
FRP Composites," ASME J. Tribol., 124(1), pp. 5-13.
[2] Barnes, M., 2001, "Stresses in Solenoids," J. Appl.
Phys., 48(5), pp. 2000–2008.
[3] Jones, J., 2000, Contact Mechanics, Cambridge
University Press, Cambridge, UK, Chap. 6.
[4] Lee, Y., Korpela, S. A., and Horne, R. N., 1982,
"Structure of Multi-Cellular Natural Convection in a
Tall Vertical Annulus," Proc. 7th International Heat
Transfer Conference, U. Grigul et al., eds.,
Hemisphere, Washington, DC, 2, pp. 221–226.
[5] Hashish, M., 2000, "600 MPa Waterjet Technology
Development," High Pressure Technology, PVP-Vol.
406, pp. 135-140.
[5] Watson, D. W., 1997, "Thermodynamic Analysis,"
ASME Paper No. 97-GT-288.
[6] Tung, C. Y., 1982, "Evaporative Heat Transfer in
the Contact Line of a Mixture," Ph.D. thesis,
Rensselaer Polytechnic Institute, Troy, NY.
[7] Kwon, O. K., and Pletcher, R. H., 1981,
"Prediction of the Incompressible Flow Over A
Rearward-Facing Step," Technical Report No. HTL26, CFD-4, Iowa State Univ., Ames, IA.
ACKNOWLEDGMENTS
Be sure to acknowledge your sponsor and customer as
well as other individuals who have significantly
Copyright © 2008 Rochester Institute of Technology
Proceedings of the Multi-Disciplinary Senior Design Conference
helped your team throughout the project.
Acknowledgments may be made to individuals or
institutions.
Who to thank: Edward Hanzlik, Robert Stevens, John
Wellin,
Hoople,
Rob
Kraynik
Project P11462
Page 6
Requirements and Advice
Construct an outline
A proper outline is the framework upon which a good paper is readily written. In the process of making the outline,
ideas are classified and thoughts are ordered into a logical sequence that is readily transformed into complete
sentences. In outline form, the sequence of the various items and the progression of thought can easily be adjusted
and readjusted until the desired order is obtained.
Length
Your paper should not exceed 8 printed pages, all inclusive!
Style. The chief purpose of the paper is to convey information to others, many of whom may be less familiar with
the general subject than the author. Care should be taken, therefore, to use simple terms and expressions and to make
statements as concise as possible. If highly technical terms or phraseology are necessary, they should be adequately
explained and defined. The use of the first person and reference to individuals should be made in a manner that
avoids personal bias. Company names should be mentioned only in the acknowledgments.
Papers should be concise. Long quotations should be avoided by referring to sources. Illustrations and tables are
desirable but they should be kept to a reasonable minimum. Detailed drawings, lengthy test data and calculations,
and photographs that may be interesting, but which are not integral to the understanding of the subject, should be
omitted. Equations should be kept to a reasonable minimum.
Originality
Only original contributions are accepted for publication. Under certain circumstances, reviews, collations, or
analyses of information previously published may be acceptable.
Accuracy
All technical, scientific, and mathematical information contained in the paper should be carefully checked. SI units
of measurement should be used. When U.S. customary units are given preference, the SI equivalent should be
provided in parentheses or in a supplementary table. Similarly, when preference is given to SI units, the U.S.
customary units should be provided in parentheses or in a supplementary table.
Headings
Headings and subheadings should appear throughout the paper to divide the subject matter into logical parts.
Headings assist the reader in following your thought process and in forming a mental picture of the points of chief
importance.
Tabulations/Enumerations
It is often advantageous to put related items in tabular or enumerative form, one after the other, rather than simply
running them into the text. This arrangement, in addition to emphasizing the items, creates a graphic impression that
aids the reader in accessing the information. It is customary to identify the individual items as (1), (2), (3), etc., or
as (a), (b), (c), etc. Although inclusion of such elements makes the text livelier, care should be taken not to use this
scheme too frequently, as it can make the reading choppy.
Mathematics
Equations should be numbered consecutively beginning with (1) and including any appendices. The number should
be enclosed in parentheses and placed on the right-hand-side of the equation. It is this number that should be
referenced within the text. An example is shown below in Eq. (1).
 2T 1 T

x 2  t
(1)
Formulas and equations should be created to clearly distinguish capital letters from lowercase letters. Care should be
taken to avoid confusion between the lowercase "l"(el) and the numeral one, or between zero and the lowercase "o."
All subscripts, superscripts, Greek letters, and other symbols should be clearly indicated.
In all mathematical expressions and analyses, any symbols (and the units in which they are measured) not previously
defined in nomenclature should be explained. If the paper is highly mathematical in nature, it may be advisable to
develop equations and formulas in appendices rather than in the body of the paper.
Figures
All figures (graphs, line drawings, photographs, etc.) should be numbered consecutively and have a caption
consisting of the figure number and a brief title or description. This number should be used when referring to the
figure in text. Figures should be referenced within the text as "Fig. 1." When a reference begins a sentence the
abbreviation "Fig." should be spelled out (e.g., “Figure 1”). You may use single column figures or, if needed, the
figure may span both columns.
Figures should be embedded electronically into your paper. Be sure to include the actual figure and not a link to
another file. We will produce your paper from the electronic format that you provide. All photographs should be
submitted as good quality jpeg or tiff files embedded in the Word file.
Tables
All tables should be numbered consecutively and have a caption consisting of the table number and a brief title. This
number should be used when referring to the table in text.
Tables may be inserted as part of the text, or included on a separate page immediately following or as close as
possible to its first reference, with the exception tables included in an appendix.
Format
Papers must be submitted in Microsoft Word format. Your paper will be printed and prepared directly from your
electronic media. Do not assume that any additional layout work will be performed, although we do reserve the right
to make layout changes and editorial changes as needed to meet the publication turn-around time.
Check all page headings to be sure that dates and paper numbers are current. Also:
 Math expressions must be created using the Equation Editor supplied with Microsoft Word. Otherwise, the
integrity of these special characters will be lost.
 All graphics should be embedded in the text file. Make sure the image is INCLUDED in the paper – not
hyperlinked to a separate file.
 All Tables must be created using the Table utility provided with Microsoft Word. Tables created by use of the
tab keys will not convert properly.
 No styling is necessary. However, inclusion of italics and roman script is necessary to indicate math and other
special characters.
 Label the electronic file clearly and submit it to the program office.