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American Institute of Aeronautics and Astronautics

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Advanced Thermophotovoltaic Cells Modeling, Optimized

for Use in Radioisotope Thermoelectric Generators (RTGs)

for Mars and Deep Space MissionsB. Davenport

*and S. Michael

Naval Post Graduate School, Monterey, California, 93943

Thermophotovoltaic cells are a good candidate for use in high efficiency RTG power

devices for deep space missions. This paper examines the use of Silvaco Virtual Wafer

Fabrication Software as a tool for designing and optimizing TPV cells for different possible

spectrums. It gives results for a cell optimized to the AM0 spectrum which closely matches

published data as well as a hypothetical cell optimized to the spectrum of a 1300 K

blackbody.

Nomenclature

AM0 = spectrum from the sun measured outside earths atmosphere

RTG = Radioisotope Thermoelectric GeneratorTPV = Thermophotovoltaic

I. Introduction

s space missions take man-made vehicles and satellites farther and farther from the sun the need for a better

means of power generation becomes more and more important. The intensity of light from the sun reduces with

the square of the distance from it meaning that massive solar arrays would be needed in order to produce minimal

power on deep space missions.

An alternative to these massive arrays is to bring a fuel supply that will be capable of providing adequate power

over the entire lifespan of the vehicle. It is not feasible to use just batteries because the number of batteries required

for such a long duration would be far too heavy. Conventional fuels are also not viable options due to the poor mass

to power ratios.

The best alternative is to use a radioisotope thermoelectric generator (RTG). Such a generator consists of a fuel

supply, Pu-238, that produces heat along with a means to convert this heat into electrical energy. Pu-238 has a half-life of 87 years, meaning that after 20 years of operation it will still produce 80% of the power it produced when

new. This isotope, when pure, has a relatively large power to mass ratio of 540 W/kg and can produce surface

temperatures over 1300 K.

Conventional RTGs use thermocouples to convert heat into electricity. A thermocouple produces power using

what is known as the Seebeck Effect. Seebeck discovered that when two materials from the thermoelectric series

were connected, a potential could be measured across them if the two materials were at different temperatures. In

addition, if the two materials were connected with a wire so as to close the circuit, a small current was forced to

flow. In an RTG, a SiGe thermocouple is placed in the vicinity of the heat source with one side exposed to the heat

and the other side to a heat sink. The electrical power produced is proportional to the difference in temperature

between the hot and cold junctions. These devices do a relatively poor job of power conversion, however, only

achieving efficiencies of 6-8% in optimum working conditions.

This paper will explore the thermophotovoltaic cell as a candidate for replacing the thermocouple in RTGs.

Such a cell operates in a manner very similar to how a solar cell works, the difference being the band of theelectromagnetic spectrum the cell is designed to respond to. Where a solar cell responds to the visible band of the

spectrum, thermophotovoltaic cells respond to the infrared region. Currently there is a significant amount of research

being done in the area of thermophotovoltaics, but it is very difficult to predict the output of a prospective cell

*Ensign, United States Navy, Department of Electrical and Computer Engineering, Naval Postgraduate School,

Monterey, California 93943.

Associate Professor, Department of Electrical and Computer Engineering, Naval Postgraduate School, Monterey,

California 93943.

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22nd AIAA International Communications Satellite Systems Conference & Exhibit 20049 - 12 May 2004, Monterey, California

AIAA 2004-327

This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.