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| Fig. 1: Basic diagram of radioisotope thermoelectric generator design. (Courtesy of NASA) |
The radioisotope thermoelectric generator (RTG) is a power generation device that harvests the energy released by the spontaneous decays of atomic nuclei. This design takes advantage of the electrical energy yielded by spatial gradients in temperature to convert the heat release from nuclear decays into electric energy. In the following sections, the basic physical principle of operation will be explained, and the thermodynamic performance of the RTG will be analyzed.
RTGs yield usable energy by means of thermal conversion. As radioactive substances decay, they emit alpha particles, beta particles, and gamma rays, the former of which subsequently deposit their energy into neighboring atoms in the form of heat. This thermal energy is converted to electrical energy by a phenomenon called the Seebeck effect; when a temperature differential is established in an electrically conducting material, free electrons are driven in the direction of decreasing temperature. This flow of electrons builds a charge imbalance, inducing an electric field that yields a voltage across the conducting material. The electrical energy per unit of charge injected into the material in this way is called an electromotive force (EMF), and can subsequently be used elsewhere in an electrical circuit. RTGs take advantage of this phenomenon by coupling one junction of a thermocouple array (essentially a large collection of thermally conducting legs) to the radioactive material, while the other junction is cooled (often by radiation through thermal fins). Thus, a steady-state temperature gradient is built between the two junctions, resulting in a usable EMF. A diagram of this scheme is depicted in Fig. 1.
The power density and thermal efficiency of the RTG is quite low. Key historical NASA RTGs have achieved specific powers (i.e., total power output per unit system mass) in the range of 3-5 W/kg. [1] More recent engineering advancements have increased this number to as high as 9.5 W/kg. [2] In comparison, historical solar arrays achieve specific powers on the order of tens to hundreds of W/kg. [3] The thermal efficiency of representative NASA RTGs, defined as the useful energy output divided by the energy input (in this case, from the radioisotope heat release), lands around 6.3%. [1] This lackluster performance is driven largely by the necessity of massive storage and cooling apparatuses, as only 30% - 45% of the RTG mass is accounted for by the heat source modules (i.e., the radioisotope fuels), the rest being associated with housing, power conversion subsystems, and components regulating heat distribution (e.g., thermal insulation, radiative cooling fins). [1]
The chief advantage that RTGs possess over other systems of power generation is the superior energy density of their fuels. In general, radioisotopes have energy densities on the order of millions of MJ/kg. The most common isotope used in RTGs, Plutonium-238, has an energy density of 2.2e6 MJ/kg. [4] Assuming a 5% thermal efficiency, a deep-space mission requiring an average power draw of 100W (a reasonable historical estimate), this implies that one kilogram of Pu-238 fuel contains nearly 35 years worth of energy. [5] Thus, although the power output per kilogram of RTG is comparatively low, very little mass is necessary to achieve long-lifetime generators.
Despite their low efficiency and specific power output capability, the longevity of radioisotope thermoelectric generators, driven by their extremely high energy densities, make them ideal candidates for power generation in long-lifetime, low-power draw systems, especially those for which human supervision is impossible or otherwise undesirable. In particular, for deep space exploration, planetary rovers, &c., RTGs prove to be a natural choice of power system.
© Ben Nicastro. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] L. S. Mason, "Realistic Specific Power Expectations for Advanced Radioisotope Power Systems," J. Propuls. Power 23, 1075 (2007).
[2] D. F. Woerner, "Next-Generation Radioisotope Thermoelectric Generator Study Final Report," U.S. National Aeronautics and Space Administration, JPL-D-99657, June 2017.
[3] R. Verduci et. al., "Solar Energy in Space Applications: Review and Technology Perspectives," Adv. Energy Mater. 12, 2200125 (2022).
[4] M. B. Naseem, J. Lee, and S. I. In, "Radioisotope Thermoelectric Generators (RTGs): A Review of Current Challenges and Future Applications," Chem. Commun. 60, 14155 (2024).
[5] H. Oman, "Deep Space Travel Energy Sources," IEEE 1183867, IEEE Aerosp. Electron. Syst. Mag. 18, No. 2, 28 (2003)