TEGs

The basic TEG is a simple device. It is based on an effect discovered by the German scientist Thomas Johann Seebeck in 1821. He found that when two dissimilar wires are connected at two junctions, and if one junction is kept hot while the other is cold, an electric current will flow in the circuit. Such a pair of junctions is called a thermocouple or thermoelectric couple.

The heat can be supplied from an isotope as in an RTG. The conversion of the heat is static. The device has no moving parts and is, therefore, very reliable and continues for as long as the radioisotope source produces a useful level of energy. The heat production is, of course, continually decaying but the radioi-sotope is custom selected to fit the intended use of the electricity and for its planned mission duration.

Figure 3 shows a hot shoe, through which radioisotopic heat is introduced, connecting the positive and negative legs. Some excess heat is rejected at the bottom and an electric current is generated.

A comparison between the predicted performance of a 150 W RTG over 12 years and its actual performance during that time is shown in Fig. 4.

RTGs have been used in 26 US and many Russian missions over the past forty years, as well as in the later French missions. They were originally installed in long term remote navigational and meteorological satellites, but RTGs have since been used in a variety of lunar and planetary missions. An RTG is a very versatile unit that can be custom designed for very specific appli-cations.

An example of an RTG SNAP is shown in Fig. 5. This is the SNAP-27 and Fig. 6 shows it being removed from the Lunar Excursion Module by astronaut Gordon Bean during the Apollo 12 mission to the moon in 1969. Five of these units were used to power experimental packages on the lunar surface. They were an ideal choice for long missions that required the supply of continuous power during both the lunar day and night. Each unit produced 63 W at the end of a year of service.

The US designed general purpose heat source (GPHS) comprises ²³⁸Pu fuel pellets encased in iridium shells (4 pellets each weighing 151 g) and 572 multiply redundant thermocouples made of silicon–germanium (see Fig. 7).

Each thermocouple can produce more than half a watt. However, for other missions, different fuel and different thermocouple materials can be used.

Moreover, RTGs can be used as modules of a total space auxiliary power

system for both redundancy and for total power output. For the Galileo and Ulysses space missions, which had much higher power requirements than the lunar experiments, the GPHS–RTG was designed to provide 300 W of electrical power with a nominal fuel loading of 4.4 kW. It used 18 heat source modules.

Another design, the lightweight radioisotope heater unit (RHU), is shown in Fig. 8. These units provide temperature control for sensitive electrical components. Each includes a 2.68 g ²³⁸Pu dioxide fuel pellet producing 1 W, clad in platinum–rhodium and encased in a graphite capsule for protection in the event of an accident. The Galileo spacecraft had 120 of these lightweight units in addition to its GPHS. The Galileo spacecraft was launched on FIG. 3. Operating principle of the thermoelectric converter. Source: Rockwell International.

TIME (Years)

POWER OUTPUT (W)

FIG. 4. Comparison between the predicted and actual performance of a 150 W RTG over a 12 year period.

HEAT REJECTION FINS

OUTER CASE

HERMETIC SEAL

THERMOPILE HOT FRAME

COLD FRAME

MOUNTING LEGS

HERMETIC SEAL

FUEL CAPSULE LATCH PLATE

RADIOISOTOPE FUEL CAPSULE

FIG. 5. The SNAP-27 system. Source: NASA.

18 October 1989 and arrived at Jupiter on 7 December 1995. The mission was extended through 1999 to allow it to fly past Europa, Callisto and Io. These dates and the invaluable information fed back indicate the reliability of its on-board sources of thermal control and electricity generation.

Appendix II shows a listing of US and Russian spacecraft that have used RTGs (or radioisotope powered TEGs in the Russian Federation), the numbers of RTG systems and the reasons for those missions. Appendix III lists the successes of programmes supported by those power systems. These successes, with requirements for the supply of steady and reliable power for up FIG. 6. Removal of SNAP-27 from the Lunar Excursion Module by astronaut Gordon Bean during the Apollo 12 mission to the moon in 1969. Source: NASA.

to 14 years in locations well beyond those which would allow the use of solar power, would not have been possible without RTGs.

The international² Cassini mission to Jupiter and Saturn was equipped with 3 RTGs (see Appendix IV) which produced 885 W at the beginning of the mission and 633 W at the end. Cassini also had 82 small RHUs and there were 35 more on the Huygens probe, each producing 1 W of heat to keep nearby electronics warm. These contained a total of about 0.32 kg of ²³⁸Pu.

2 Partners: The National Aeronautics and Space Administration (NASA), the European Space Agency (ESA), the Italian Space Agency (Agenzia Spaziale Italiana – ASI) and there were a total of 17 countries involved.

FUEL CLAD GRAPHITE

AEROSHELL CAP

GRAPHITE IMPACT SHELL CBCF

DISC LOCK

MEMBER AEROSHELL

LOCK SCREW

FLOATING MEMBRANE

FUEL PELLET

GRAPHITE IMPACT SHELL

CBCF DISC

CBCF SLEEVE

FIG. 7. GPHS module assembly. Source: US Department of Energy/General Electric Co.

For the future, a new advanced radioisotopic power system has been designed. It uses alkali metal thermal to electric conversion (AMTEC) technology to convert the heat produced by its plutonium heat source. The AMTEC cell (Appendix V) is made up of eight beta alumina solid electrolyte tubes connected in series. The end of the cell with the tubes is adjacent to the hot end of the heat source. At this end, liquid sodium is heated to a vapour state and the sodium atoms in the vapour are driven through the walls of the tubes and in so doing are stripped of an electron, thus creating positively charged sodium ions. The vapour is cooled and collected in a condenser at the cold end of the cell and the cycle is repeated as the sodium flows through the

‘artery’ towards the hot surface at the other end of the cell. The cell uses thermal shields in its upper section to reduce radiative bypass heat losses from the hot side components to the cold side condenser.

HEAT SHIELD END CAP

CAPSULE

INSULATOR MIDDLE TUBE

INSULATOR PLUG

HEAT SHIELD INSULATOR OUTER TUBE

INSULATOR INNER TUBE INSULATOR

PLUG

FIG. 8. Lightweight RHU.

Leads are taken from the first and eighth tubes in series as the positive and negative leads for the cell. An explanatory cutaway diagram of the AMTEC system is shown in Appendix V.

This is an area of space research and development in which the latest ideas can be beneficial to various ongoing international innovative reactor technology research and development initiatives for terrestrial applications, particularly because older versions of these devices have already been used to provide power in remote situations, e.g. lighthouses and in the Arctic.