1. Pittsburg State University / National Science Foundation -
Research Experience for Undergraduates and Teachers
Summer 2002
Initial test on Pittsburg State University's
Fossil Fuel burning Thermophotovoltaic system
Prepared by:
Curtis E. Williams
Dr. Bob Backes, Advisor
Dr. Chuck Blatchley, Advisor
Department of Physics
Submitted to:
Dr. Christopher Ibeh
PSU/NSF-REU/RET Director
Plastics Engineering Technology
Pittsburg State University
Pittsburg, KS 66762
August 1, 2002
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2. Table of Contents
I. Abstract……………………………………………………………..……………..3
II. Introduction………………………………………………………………………..3
A. Definitions…………………………………………………………………3
B. Problem…………………………………………………………………....3
C. Objectives………………………………………………………………....4
III. Literature Review…………………………………………..…………………..…4
A. Literature Reviews………………………………………………..…....….4
B. Problem and Significance……………………………...………...…….….5
C. Projected Contributions……………………………………………….…..5
IV. Thermophotovoltaics……………………..…………………….…………………5
A. The Emitter………………………………………….…………………….5
B. Fuels for Heating the Emitter………………………………………..……6
C. The gallium antimonide Photocell……………..…………………………6
V. Methodology………………………………………………………………………6
A. Equipment…………………………………………………………………6
B. Materials…………………………………………………………………..7
C. Procedure………………………………………………………………….7
VI. Evidence…………………………………………………………………………..9
A. Temperature vs. Length…………………………………………………...9
B. Power vs. Temperature………………………………………..….……...10
C. Temperature vs. Efficiency………………………………………….…..10
VII. Results…………………………….……………………………………………...11
A. Temperature vs. Length………………………………………………….11
B. Power vs. Temperature…………………………………………………..11
C. Efficiency vs. Temperature………………………………………………11
VIII. Conclusions………………………………………………………………………11
IX. Recommendations………………………………………………………………..11
X. References………………………………………………………………………..13
XI. Appendix…………………………………………………………………………14
A. Temperature vs. Length………………………………………………….14
B. Power vs. Temperature…………………………………………………..17
C. Pictures…………………………………………………………………..20
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3. Abstract
This study focused on the process of how to burn a Fossil Fuel inside a cylindrical
silicon carbide emitter. The emitter will be tested for temperature distribution and
efficiency using a Fossil Fuel. Set-up and use of a Fossil Fuel heating source is crucial in
the development of Pittsburg State University (PSU) Thermophotovoltaic (TPV) system.
The emitter for the TPV system must also be able to withstand temperatures of at least
1200 degrees Celsius (C) and emit uniformly on the surface of the rod. From this
research, an emitter can then be "fixed" to selectively emit in the IR range, which would
drastically increase the efficiency
Introduction
Definitions
Thermophotovoltaics is the conversion of thermal energy to electrical energy.
This is achieved by an emitting source giving off heat or infrared radiation as photons,
and a photocell converting that heat into DC electricity. The emitter must give off these
photons with a minimum energy equivalent to that of the band gap of the photocell. Any
photon energies less than the band gap energy will just produce excess heat that will
lower the efficiency of the photocell. The PSU TPV system uses a gallium antimonide
(GaSb) photocell with band gap energy of 0.70 electron Volts (eV). This corresponds to
a 1700 nm wavelength photon to produce 0.70eV. The emitter used in this research is a
cylindrical silicon carbide (SiC) rod. It is a grey body emitter that has an emissivity of
0.80. This is not a selective emitter, but rather a broadband emitter that emits photons
with wavelengths throughout the whole spectrum.
Problem
Thermophotovoltaics is the conversion of thermal energy to electrical energy by
using a low band gap solar cell. PSU’s TPV system currently consists of a GaSb
photocell and a 35cm by 20cm by 8cm chrome plated rectangular box that has an ellipse
milled into it. The emitter used in the past, however, has been a halogen lamp powered
by electricity. Overall, the system has a unique design that is more than adequate in all
areas aside from the present emitter. This poses a problem because, in principle,
Thermophotovoltaics convert heat into DC electricity by burning a fuel, not by heating a
substance with electricity. Emitter materials, operating temperature, and fuel sources
need to be investigated.
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4. Objectives
These are the main objectives of the research. By fulfilling these objectives, other
smaller objectives will be met.
i. Research and Determine an Emitter material
ii. Determine the heating fuel for the emitter.
iii. To establish an apparatus to stabilize the emitter that can
withstand temperatures of at least 1200C and hold the emitter
firmly in place so test can be ran.
iv. A thermal profile of the SiC emitter.
v. Test the efficiency of the system using the emitter heated by a
fossil fuel.
Literature Review
Literature Reviews
1. Australian Energy News. "Radiant Thermophotovoltaics: Photovoltaics that Convert
Heat to Electricity." Australian Energy News. 11 March 1999. Australian Government.
4 Jun. 2002. <http://www.isr.gov.au/resources/netenergy/aen/aen11/11thermo.html>.
This article describes the basics behind Thermophotovoltaics. It also gives a
description of the parts and a few of their properties. It also describes a variety of
different approaches.
2. Barlow, Douglas Alderman. "Thermophotovoltaics." Thesis Paper Pittsburg State
University, 1997.
This paper described the advances and support for the choices made on the
design, optical coating, and photocell for PSU's TPV system. It gave results and data
from previous tests run on the system. It also discussed areas of improvement and gave
recommendations that could improve the efficiency of the system.
3. Green, Martin A. Solar Cells: Operating Principles, Technology, and System
Applications. Englewood Cliffs, New Jersey: Prentice-Hall, 1982.
This book discusses silicon-based solar cells and their properties with and without
thin films applied. It also covers how to construct and connect solar cell arrays in an
efficient manner. Applications and uses are also covered.
4
5. 4. Uniweld Products Inc. "Play it Safe in the Work Place." Uniweld Products
Operation Procedures. 2001. Uniweld Products Inc. 4 Jun. 2002.
<http://www.uniweld.com/catalog/operation_procedures.htm>.
This site provided general information and procedure on how to use and operate
an oxyacetylene torch. It also had several important safety instructions.
5. Zweibel, Kenneth. Basic Photovoltaic Principles and Method. New York: Van
Nostrand Reinhold Company Inc., 1984.
This book discusses the principles by which solar cells operate and the theories
behind them. Principles and properties of semiconductors are also discussed in detail.
Specifically, gallium antimonide's optical properties and background information is
given.
Problem and Significance
Thermophotovoltaics is the conversion of thermal energy to electrical energy by
using a low band gap solar cell. PSU’s TPV system currently consists of a gallium
antimonide (GaSb) photocell and a 35cm by 20cm by 8cm chrome plated rectangular box
that has an ellipse milled into it. The emitter used in the past, however, has been a
halogen lamp powered by electricity. Overall the system has a unique design that is more
than adequate in all areas aside from the present emitter. This poses a problem because
the principle is to convert heat into DC electricity by burning a fuel, not by heating a
substance with electricity. Therefore to get a “true” Thermophotovoltaic system going,
an emitter and fuel must be chosen and tested. By first setting up and testing the TPV
system with a fossil fuel burning emitter, the next step can endeavor to produce selective
emissions.
Projected Contributions
The author projects to contribute various data sets along with thermal profiles of
the silicon carbide emitter at different temperatures. Also power outputs and efficiencies
at varying temperatures. From this data one could conclude how to position the emitter
and good idea of the operating temperature. This knowledge is essential to get a “true”
fossil fuel burning TPV system operational.
Thermophotovoltaics
The Emitter
The emitter for a Thermophotovoltaic system is the source of all emissions. The
emitter must be able to handle temperatures of at least 1200C for a prolonged period of
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6. time. Durability is also an issue because it will operate at high temperatures repeatedly in
its lifetime.
The emitter chose for our purposes is a silicon carbide (SiC) rod. It was chosen
because of its ability to handle very high temperatures and heavy thermal cycling. It is a
fairly inexpensive product that oxidizes at a high temperature and can be micro-textured
to produce selective emission.
The downside to choosing SiC is that as an emitter it is a grey body emitter with
an emissivity of 0.80. With greatly varying wavelength emissions, excess heat is another
problem that will need to be addressed. An ideal emitter would have selective photon
emissions whose energies are equivalent to that of the band gap of the photocell.
Fuels for heating the Emitter
A fuel for heating the emitter must be able to burn efficiently in a confined space
at very high temperatures. Being in a confined space poses a problem for having enough
oxygen to burn for an extended period of time. This means that oxygen must be
introduced with a fuel gas so that combustion will occur. Having to introduce oxygen
with the fuel gas eliminates propane from the choices, but focuses attention on
oxyacetylene. Commonly used for cutting metals, this fuel burns at temperatures up to
2800C. Due to its abundance, convenience and fitting the requirements, oxyacetylene
was chosen.
The Gallium Antimonide Photocell
PSU’s Thermophotovoltaic system uses a gallium antimonide (GaSb) photocell.
It has been used previously. It is a water-cooled rectangular array that is approximately
5cm by 1cm with five 1cm by 1cm photocells. GaSb has a band gap of 0.70 eV. This
band gap energy corresponds to the energy of a 1700 nanometer wavelength photon.
Methodology
Equipment
Four digital multimeters were used to measure input and output voltage and
current. The two BK Precision 2831C were AC powered, and the other two Fluke 37
were DC powered. An Amprobe was used to measure amperes over 20 amps due to the
capacity of the digital multimeters. The Physics Department’s electrical transformer
provided the high amperage output needed. High amperage clamps and wire were used
with the transformer. An Infrared Thermometer made by OMEGA, model number
OS3708, was used to make a majority of the high temperature measurements. Its
temperature range is 600C to 5432C. An OMEGA 2176A Digital Thermometer took the
lower temperature measurements with thermocouples. An oxyacetylene torch was used
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7. as a heating source equipped with a Purox W-201 torch body and a Purox #15 torch tip.
Two adjustable platforms and two stands with clamps were also used. A major piece of
equipment used was the PSU TPV system box. It is a 35cm by 20cm by 8cm rectangular
shaped box with a 13cm by 30.4cm Ellipse milled in it. The equation for the Ellipse is
(x^2)/231.04 + (y^2)/42.25 = 1.
Materials
A 25cm long cylindrical silicon carbide rod, with 8mm inner radius and 16mm
outer radius, was chosen for the emitter. Compressed Oxygen and Acetylene were used
as the heating fuel for the torch.
Procedure
Measurement of electricity used to heat the rod
1. Place one end of the SiC rod in one of the high amperage
clamps, and the other end in the other clamp.
2. Double wire both of the leads coming from the voltmeter (due
to the high amperage). Attach clamps to both of the leads, and
then attach to the high voltage clamps.
3. Place the Amprobe amperage meter around one of the leads
going to the high amperage clamps. Turn to the 15 Amp max
scale.
4. Wrap the barren end of one of the thermocouples from the
OMEGA 2176A digital thermometer around the rod
approximately 2cm from either of the high amperage clamps.
5. Attach the lead from one of the high amperage clamps to the
negative terminal on the transformer. Attach the lead from the
other high amperage clamp to the varying positive terminal.
6. Turn both variax dials counterclockwise to 0.
7. Plug in the transformer and turn on the digital multimeter to
AC voltage.
8. Turn on digital thermometer to proper thermocouple.
9. Use the large dial as the coarse adjustment and the small dial as
fine adjustment.
10. Adjust dials to get desired power or temperature. DO NOT
EXCEED 50 AMPS!!
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8. Measuring the thermal displacement of the SiC rod
1. Set up a stand with an adjustable clamp close to the bottom to
hold the torch body.
2. Set up another stand with a clamp that holds a 30cm rod
parallel to the table. Attach a clamp on the end of the 30cm
rod (this will hold the SiC rod).
3. Make sure the Oxygen and Acetylene tanks are turned off.
4. Place the Purox W-201 torch body in the adjustable clamp and
point the Purox #15torch tip straight up.
5. Place the OMEGA OS3708 infrared thermometer on an
adjustable stand 1 meter away from the torch tip.
6. Screw the adjustable stand all the way down. Place the
adjustable stand on blocks so you can see the torch tip in the
crosshairs of the Infrared thermometer.
7. Turn the IR thermometer on. With the arrows on the right, set
the emissive to 0.80. The read out should display “LO”.
8. Place the SiC rod in the clamp on the end of the 30cm rod that
is parallel to the table.
9. Adjust the rod so that it is straight above the torch tip with just
enough clearance between the rod and the torch tip so it can be
pulled away without readjusting. The flame of the torch will
burn inside of the rod.
10. Pull the apparatus aside.
11. Starting at the top, wrap the thermocouples from the OMEGA
2176A Digital Thermometer around the SiC rod in 1cm
increments going down. Leave enough slack so it can be
moved directly over the torch tip.
12. Make sure the valves on the torch body are closed.
13. Turn the Oxygen and Acetylene bottles on at the top of the
canisters.
14. Adjust the regulator for the Oxygen so that the pressure output
is 30psi. Then adjust the Acetylene regulator so that the output
pressure is 5 psi.
15. Turn on the acetylene valve on the torch body 12 a turn and
light with the striker.
16. Slowly open the oxygen valve and close the acetylene slightly.
A hissing noise will be heard. If the flame pops and goes out
shut both valves off and repeat 1steps 14 and 15.
17. Adjust the valves until there is a small blue-white conical flame
about 1cm tall coming out of the tip. The total length of the
flame should be no taller than 10cm to 13cm.
18. Once the flame is adjusted slide the SiC rod directly over the
top of the flame, so that it burns inside the rod. Be careful not
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9. to smother out the flame by letting the SiC rod cover the very
tip of the torch (where the flame comes out).
19. Take temperature measurements by raising the adjustable stand
under the IR thermometer 1cm at a time, keeping the SiC rod
in the crosshairs at all times.
Evidence
Temperature vs. Length
This data came from heating the SiC emitter with an oxyacetylene torch. The
results show a significant drop in temperature as the distance from the heat source is
increased.
Temperature Vs Length
0.0
200.0
400.0
600.0
800.0
1000.0
1200.0
1400.0
0 5 10 15 20 25 30
Length (cm)
Temperature(C)
This is an average of all tests in which the SiC emitter was heated with
oxyacetylene. Individual tests can be seen in the appendix.
9
10. Power vs. Temperature
Power vs Temperature
0
100
200
300
400
0 100 200 300 400 500 600
Temperature (C)
Power(W)
This data was taken from running high power through the SiC emitter. As seen
the efficiency increases as the power does. This data is an average of all test of the
emitter when heated with electrical current. Individual tests can be seen in the appendix.
Temperature vs. Efficiency
Temperature vs Efficiency
0
0.05
0.1
0.15
0.2
0.25
1100 1150 1200 1250 1300 1350 1400
Temperature (C)
Efficiency(%)
This is the efficiency of the TPV system using the SiC rod as an emitter. Higher
temperatures will have a greater efficiency.
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11. Results
The temperature deviation was +/- 1.7%. The deviation for power was +/- 3.3%.
This is a negligible amount for both power and temperature.
Temperature vs. Length
The data shows that, as distance from the heat source increases, temperature
decreases steadily. This implies that the heat source needs to be aligned with the
photocell so that the higher temperature can be fully used by the emitter. The higher
temperature will increase the efficiency.
Power vs. Temperature
The data concludes that as the power to the emitter is increased the temperature
rises linearly. Power was used to heat the emitter to calculate a relationship between
power and temperature. This information was used to calculate efficiency.
Efficiency vs. Temperature
Looking at the data, one can see that efficiency increases with rises in
temperature. The higher the temperature the more the black body spectra shift toward the
infrared region. Infrared photons are the proper wavelength for the gallium antimonide
photocell to capture and convert into electricity. Overall, higher temperatures mean a
higher efficiency.
Conclusions
Based on the results, higher temperatures are needed to reach a higher
efficiency. The higher the temperature gets, the more the black body spectra
will be shifted to the infrared region, thus improving efficiency. At lower
temperatures, the black body spectra is mostly in the visible region so the
efficiency is very low. Overall, silicon carbide is a base material for an emitter
that can withstand high temperatures and heavy thermal cycling. This material
is inexpensive and could be fixed to emit selectively or filtered. But silicon
carbide is a grey body emitter that emits over a wide spectrum of wavelengths.
Recommendations
To get to higher temperatures and increase the efficiency, pre-heating the fuel-gas
mixture is required. JX Crystals has proven that with the efficiency of their Midnight
Sun Thermophotovoltaic Co-generator. In order to boost the efficiency, a process of
making SiC a selective emitter also needs to be researched. Areas of applying a thin film
or micro-texturing might prove helpful. Finally, one could filter out the unwanted
11
12. photons with some sort of a filter. Either a selective emitter or a filter would drastically
reduce the heat of the system and increase the efficiency.
12
13. References
Australian Energy News. "Radiant Thermophotovoltaics: Photovoltaics that Convert
Heat to Electricity." Australian Energy News. 11 March 1999. Australian
Government. 4 Jun. 2002.
<http://www.isr.gov.au/resources/netenergy/aen/aen11/11thermo.html>.
Barlow, Douglas Alderman. "Thermophotovoltaics." Thesis Paper Pittsburg State
University, 1997.
Green, Martin A. Solar Cells: Operating Principles, Technology, and System
Applications. Englewood Cliffs, New Jersey: Prentice-Hall, 1982.
Matthew, B. “Photovoltaic Energy Systems.” McGraw-Hill Inc., 1983.
Uniweld Products Inc. "Play it Safe in the Work Place." Uniweld Products Operation
Procedures. 2001. Uniweld Products Inc. 4 Jun. 2002.
<http://www.uniweld.com/catalog/operation_procedures.htm>.
Zweibel, Kenneth. Basic Photovoltaic Principles and Method. New York: Van
Nostrand Reinhold Company Inc., 1984.
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14. Appendix
Temperature vs. Length
The following three graphs are individual test results from heating the emitter
with oxyacetylene.
Temperature vs Length #1
0.0
200.0
400.0
600.0
800.0
1000.0
1200.0
1400.0
0 5 10 15 20 25 30
Length (cm)
Temperature(C)
Length
(cm)
Temp
(C)
0 1303.0
1 1280.0
2 1212.0
3 1152.0
4 1075.0
5 1023.0
6 1006.0
7 943.0
8 920.0
9 865.0
10 841.0
11 803.0
12 775.0
Length
(cm)
Temp
(C)
13 731.0
14 703.0
15 689.0
16 649.0
17 630.0
18 619.0
19 597.0
20 579.0
21 561.0
22 558.0
23 551.0
24 529.0
25 460.0
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22. August 1, 2002
Dr. Christopher Ibeh
PSU/NSF-REU/RET Director
Plastic Engineering Technology
Pittsburg State University
Pittsburg, KS 66762
Re: Initial Test on Pittsburg State University's Fossil Fuel Burning
Thermophotovoltaic System
Dr. Ibeh,
Attached is a copy of my final research report. This program has been a great
experience for me. Thank you for letting me have the opportunity. Enjoy the summer
and thanks again.
Sincerely,
Curtis Williams
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