Fuel Cells for Space Science Applications

NASA/TM--2003-212730

AIAA?2003?5938

Fuel Cells for Space Science Applications

Kenneth A. Burke Glenn Research Center, Cleveland, Ohio

November 2003

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NASA/TM--2003-212730

AIAA?2003?5938

Fuel Cells for Space Science Applications

Kenneth A. Burke Glenn Research Center, Cleveland, Ohio

Prepared for the First International Energy Conversion Engineering Conference sponsored by the American Institute of Aeronautics and Astronautics Portsmouth, Virginia, August 17?21, 2003

National Aeronautics and Space Administration Glenn Research Center

November 2003

Acknowledgments

The author wishes to thank the Office of Space Science Energy Storage Review Committee for the use of battery data presented at the September 26, 2002 review meeting held at the Goddard Space Flight Center.

NASA Center for Aerospace Information 7121 Standard Drive Hanover, MD 21076

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Fuel Cells For Space Science Applications

Kenneth A. Burke

National Aeronautics and Space Administration Glenn Research Center Cleveland, Ohio 44135

ABSTRACT

Fuel cell technology has been receiving more attention recently as a possible alternative to the internal combustion engine for our automobile. Improvements in fuel cell designs as well as improvements in lightweight high-pressure gas storage tank technology make fuel cell technology worth a look to see if fuel cells can play a more expanded role in space missions. This study looks at the specific weight density and specific volume density of potential fuel cell systems as an alternative to primary and secondary batteries that have traditionally been used for space missions. This preliminary study indicates that fuel cell systems have the potential for energy densities of >500 W-hr/kg, >500W/kg and >400 W-hr/liter, >200 W/liter. This level of performance makes fuel cells attractive as high-power density, high-energy density sources for space science probes, planetary rovers and other payloads. The power requirements for these space missions are, in general, much lower than the power levels where fuel cells have been used in the past. Adaptation of fuel cells for space science missions will require "down-sizing" the fuel cell stack and making the fuel cell operate without significant amounts of ancillary equipment.

INTRODUCTION

NASA's fuel cell usage to date has consisted of Proton Exchange Membrane Fuel Cells (PEMFC) and Alkaline Fuel Cell (AFC) technology. Currently NASA is funding the development of only PEMFC and Direct Methanol Fuel Cell (DMFC) technology for space applications. No further development of AFC technology is envisioned. This paper will address only the PEMFC technology.

Both the PEMFC and AFC technologies that have flown in space have used hydrogen and oxygen reactants, which were delivered from external cryogenic tanks. NASA has used fuel cells instead of primary batteries for energy storage on almost all manned missions. Manned missions have required primary energy storage with long discharge times and generally higher power levels than unmanned missions. These requirements, as will be shown later in this paper, favor fuel cells rather than batteries because fuel cells are a lighter weight alternative. Space science missions, on the other hand, have generally

involved shorter durations and lower power levels, which favor batteries rather than fuel cells. Consequently, NASA has used batteries of different types instead of fuel cells for energy storage on all unmanned space science missions.

The purpose of this study was to examine what "added value" fuel cells might provide (if any) to NASA as an alternative energy storage and power generation technology that would provide space science mission planners new or enhanced mission capabilities over existing and projected energy storage capabilities. Besides addressing "added value" niches, this study also was directed toward identifying fuel cell technology gaps that would need to be filled in order to meet the science mission requirements.

In order to meet the objectives of this study it was necessary to analyze the fuel cell system in terms of its weight, volume, power, efficiency and stored energy. These parameters were then compared to available and projected battery technologies to determine added value areas and what technology issues NASA would need to address to develop fuel cells to meet these areas.

BACKGROUND

The heart of a fuel cell is an electrochemical "cell" that combines a fuel and an oxidizing agent, and converts the chemical energy directly into electrical power. A "stack" of cells is usually employed in applications. For this paper, fuel cells will be described as either primary (not rechargeable) or secondary (rechargeable). Primary fuel cells for space use tanks of fuel and oxidant, which are gradually discharged and not replenished. Secondary fuel cells (also referred to as regenerative fuel cells) use hydrogen and oxygen and produce water and electrical power. An external power source is used to electrolyze the water to replenish the hydrogen and oxygen.

The amount of energy stored in the fuel and oxidant per unit mass is large compared to the energy stored in a typical battery. Unlike batteries, fuel cells generally do not store their fuel and oxidizer within the cell stack, but instead fuel and oxidizing agent are stored externally to the stack. Because of this characteristic, the energy capacity of a fuel cell power

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