By John McHale
ANN ARBOR, Mich. - Energy from the sun will not be enough to power future spacecraft reconnaissance missions of Pluto - one of the most outlying planets in Earth`s solar system - so scientists are looking to a few pellets of plutonium instead of solar panels to provide electricity for instrument packages headed to Pluto and beyond.
Engineers at Advanced Modular Power Systems in Ann Arbor, Mich., are developing sodium heat engines, also known as alkali metal thermal-to-electric conversion (AMTEC) cells, as part of a small business initiative research project.
The technology is intended not only for the Pluto Express missions early in the 21st century, but they also are candidates for the different Earth orbits where radiation levels are high. AMTEC cells are inherently resistant to radiation.
The Pluto mission, taking shape at the NASA Jet Propulsion Laboratory (JPL) in Pasadena, Calif., will consist of two spacecraft taking scientific measurements and photographs as they fly past Pluto between 2010 and 2015.
Each spacecraft will weigh about 220 pounds and consume 75 to 100 watts of electric power. The efficiency, reliability, and thermal characteristics of direct thermal power make AMTEC technology attractive for deep-space and high-radiation applications, explains Tom Hunt, president of Advanced Modular Power Systems.
Pluto lies about 29 times farther from the sun than the Earth, so the sunlight reaching Pluto is roughly equal to Earth`s deep twilight. "A solar powered spacecraft would need a solar panel with an area of over 250 square meters to provide 75 watts of power using current thermoelectric technology," Hunt says.
Unlike solar panels, direct power-conversion sodium heat engines convert heat from pellets of radioactive plutonium to electricity, and are useful for applications like the Pluto mission that require lightweight, long-running, high-efficiency power systems, Hunt explains.
Used on earlier NASA missions such as Pioneer, Voyager, and Galileo, solar panels, while reliable, only operate at about five percent efficiency, he says. Predicted cell efficiencies for AMTEC systems range from 18 to 20 percent with cell power densities near 80 watts per kilogram, Hunt notes.
A thermoelectric power system designed for the Pluto Express system would weigh approximately 40 pounds, he says. A radioisotope heat source coupled to a device to convert the heat directly to electric power is much more compatible in size. It would also use less heat source material and reduce the power system`s mass to only about 13 pounds, Hunt says.
At the Air Force Phillips Laboratory in Albuquerque, N.M., engineers are considering a solar-thermal AMTEC power system for low- and middle-earth orbiting spacecraft, Hunt says.
These systems work by storing part of the solar energy they gather when in view of the sun, and using that stored energy during the periods of orbit when the Earth blocks the sun`s rays. They convert only a portion of the solar energy to electric power during the isolation period of the orbit, and store another portion in the solid-liquid phase change of their thermal-energy storage material.
During the eclipse phase of orbit, the systems release energy to the cells, keeping the spacecraft fully powered. The system provides 1 kilowatt of power to the spacecraft.
Researchers at Ford Scientific Laboratories in Dearborn, Mich., first conceived AMTEC technology in 1968 when they identified and patented a sodium heat engine. In the 1980s engineers at JPL looked to the conversion device as a possible power system for spacecraft.
AMTEC is based on the properties of the Beta-Alumina Solid Electrolyte (BASE). This ceramic material conducts sodium ion well, but conducts electricity poorly, Hunt says. Designers place electrodes and current collectors on the inside and outside of the BASE tubes to conduct electrons.
One end of each BASE tube is sealed with a conductor to the inner electrode and the other, open end is attached to the hot side of the AMTEC cell. Low-pressure sodium vapor flows from the outside electrode on the BASE and condenses at the cold condenser end of the cell. It then wicks back through an artery to the hot end where it again evaporates, continuing to fill the inside of the BASE tubes with the sodium vapor, Hunt explains.
The sodium vapor pressure difference across the BASE produced by heating the sodium inside the tubes and cooling the condenser end of the cell produces a sodium activity and voltage difference between the inner and outer electrodes.
The system produces power as it draws electric current from the BASE tubes, which connect in electrical series similar to the cells in a battery, Hunt says. Ionization occurs at the inner surface and recombination at the outer surface of the tube.
Sodium is liquid only in the condenser, artery, and evaporator, he says. It continuously travels the loop at a rate of as much as 20 grams per hour. Sodium ions are conducted through the BASE tube in proportion to the electronic current and pass through the conduction planes that exist in the crystal lattice of the BASE material.
"There are no moving parts, therefore the cells are silent and have long life expectations," Hunt says. "The mission to Pluto will take 20 years and the cells should last longer than that."
Future goals have established 25 to 30 percent efficiencies as a target that could be achieved in about a year, he explains.
Further improvements are possible with advanced materials and higher temperature operation, Hunt adds. Studies show that efficiencies as high as 40 percent with power densities near 500 watts per kilogram are possible, he says.
NASA designers are relying on a sodium heat engine, also known as alkali metal thermal-to-electric conversion, to power a research spacecraft set to fly past Pluto early in the next century.
