Pittsburg State University / National Science Foundation -Research Experience for Undergraduates and Teachers

Summer 2002

Initial test on Pittsburg State University'sFossil Fuel burning Thermophotovoltaic system

Prepared by:

Curtis E. WilliamsDr. Bob Backes, Advisor

Dr. Chuck Blatchley, AdvisorDepartment of Physics

Submitted to:

Dr. Christopher IbehPSU/NSF-REU/RET Director

Plastics Engineering TechnologyPittsburg State University

Pittsburg, KS 66762

August 1, 2002

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Table of Contents

I. Abstract……………………………………………………………..……………..3

II. Introduction………………………………………………………………………..3A. Definitions…………………………………………………………………3B. Problem…………………………………………………………………....3C. Objectives………………………………………………………………....4

III. Literature Review…………………………………………..…………………..…4A. Literature Reviews………………………………………………..…....….4B. Problem and Significance……………………………...………...…….….5C. Projected Contributions……………………………………………….…..5

IV. Thermophotovoltaics……………………..…………………….…………………5A. The Emitter………………………………………….…………………….5B. Fuels for Heating the Emitter………………………………………..……6C. The gallium antimonide Photocell……………..…………………………6

V. Methodology………………………………………………………………………6A. Equipment…………………………………………………………………6B. Materials…………………………………………………………………..7C. Procedure………………………………………………………………….7

VI. Evidence…………………………………………………………………………..9A. Temperature vs. Length…………………………………………………...9B. Power vs. Temperature………………………………………..….……...10C. Temperature vs. Efficiency………………………………………….…..10

VII. Results…………………………….……………………………………………...11A. Temperature vs. Length………………………………………………….11B. Power vs. Temperature…………………………………………………..11C. Efficiency vs. Temperature………………………………………………11

VIII. Conclusions………………………………………………………………………11

IX. Recommendations………………………………………………………………..11

X. References………………………………………………………………………..13

XI. Appendix…………………………………………………………………………14A. Temperature vs. Length………………………………………………….14B. Power vs. Temperature…………………………………………………..17C. Pictures…………………………………………………………………..20

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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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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 materialii. 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.

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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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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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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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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 1\2 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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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.

This is an average of all tests in which the SiC emitter was heated with oxyacetylene. Individual tests can be seen in the appendix.

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Power vs. Temperature

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

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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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

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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.

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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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Appendix

Temperature vs. Length

The following three graphs are individual test results from heating the emitter with oxyacetylene.

Length (cm)

Temp (C)

0 1303.01 1280.02 1212.03 1152.04 1075.05 1023.06 1006.07 943.08 920.09 865.010 841.011 803.012 775.0

Length (cm)

Temp (C)

13 731.014 703.015 689.016 649.017 630.018 619.019 597.020 579.021 561.022 558.023 551.024 529.025 460.0

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Length (cm)

Temp (C)

0 1286.01 1190.02 1100.03 1025.04 958.05 916.06 877.07 828.08 755.09 703.010 686.011 628.012 609.0

Length (cm)

Temp (C)

13 590.014 569.015 547.016 508.017 481.018 452.019 435.020 418.021 400.022 386.023 374.024 366.025 351

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Length (cm)

Temp (C)

0 1184.01 1090.02 980.03 920.04 860.05 745.06 686.07 647.08 619.09 585.010 558.011 531.012 508.0

Length (cm)

Temp (C)

13 474.014 461.015 449.016 420.017 399.018 374.019 348.020 329.021 313.022 301.023 298.024 276.025 245

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Power vs. Temperature

Temp (C) Power (W)

20 025 3.343 12104 36179 76250 125317 165383 210445 260508 308

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Temp (C) Power (W)

20 027 452 1598 36187 72293 135361 176415 222465 260519 301

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Temp (C) Power (W)

20 028 452 16107 37.5202 76295 140355 176414 222460 260512 301

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Pictures

This is a picture of the TPV chamber

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This is a picture Of the TPV System setup.

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August 1, 2002

Dr. Christopher IbehPSU/NSF-REU/RET DirectorPlastic Engineering TechnologyPittsburg State UniversityPittsburg, 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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