Difference between revisions of "Radioisotope Thermoelectric Generators: Advantages and Disadvantages"

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[[File:RTGs OpEnv.png|thumb|Operational envelope for different power conversion technologies ]]
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Spacecraft have three main options for power generation: chemical, solar, and nuclear. To the general public, the last of these sources may conjure images of reactors using fission processes, and many probes (particularly those launched by Russia<ref name=":0">Karacalıoğlu, G. (2014, January 16). ''Energy Resources for Space Missions –''. Space Safety Magazine. <nowiki>http://www.spacesafetymagazine.com/aerospace-engineering/nuclear-propulsion/energy-resources-space-missions/</nowiki></ref>) have successfully employed such systems. Most nuclear-powered probes traveling beyond Earth orbit, however, have instead utilized radioisotope thermoelectric generators (RTGs), which harness the heat produced by radioactive decay rather than a nuclear chain reaction. RTGs offer an alternative to the more typical solar power when conducting missions where sunlight is scarce, as occurs when traveling to the outer solar system or the dusty atmosphere of Mars.
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== How RTGs Work ==
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Although RTGs use radioactive fuel to generate electricity, they should not be confused with nuclear reactors. The latter harness the energy produced by controlled fission or fusion processes, but no chain reaction takes place in RTGs.
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[[File:RTGs Thermocouple.png|thumb|Electricity production in an RTG. Radioactive decay heats one side of a thermocouple (1). An electric current is produced on the opposite, cooler side (2), then tapped from terminals connected to the thermocouple (3), producing power (4). ]]
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Instead, unstable radioactive materials known as radioisotopes produce heat as a by-product of their radioactive decay as emitted particles transfer their energy into surrounding atoms.<ref>Decay heat. (2020). In ''Wikipedia''. <nowiki>https://en.wikipedia.org/w/index.php?title=Decay_heat&oldid=951406419</nowiki></ref> This decay takes place within a shell of semiconductors which generates an electric current when each end is exposed to differing temperatures due to the [[w:Thermoelectric_effect|thermoelectric effect]]. Simply put, charge carriers diffuse away from a heat source and build up at the cold end of a material; in semiconductors, these charge carriers can be electrons (“[[w:Extrinsic_semiconductor#N-type_semiconductors|n-type semiconductors]]”) or electron holes (i.e. a position where an electron could exist; these semiconductors are called “[[w:Extrinsic_semiconductor#P-type_semiconductors|p-type]]”). By connecting n-type and p-type semiconductors with a metallic strip, electrons flow between the two once heat is applied, generating an electric current. This connection of n- and p-type semiconductors is called a thermocouple.<ref>Alfred. (2018, October 25). ''How Thermoelectric Generators Work''. Applied Thermoelectric Solutions LLC. <nowiki>https://thermoelectricsolutions.com/how-thermoelectric-generators-work/</nowiki></ref><ref>James, L., & Granath, E. (2020, February 19). ''Understanding the difference between n- and p-type semiconductors''. Power & Beyond. <nowiki>https://www.power-and-beyond.com/understanding-the-difference-between-n-and-p-type-semiconductors-a-905805/</nowiki></ref>
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In RTGs, the thermocouple’s heat source comes from the radioactive decay of the RTG’s fuel source heating the interior of its thermoelectric shell, while the exterior is kept cool by the surrounding atmosphere or vacuum. As long as a constant temperature gradient is maintained, electricity will be produced.<ref name=":0" /><ref>''U.S. Plutonium Stockpile Good for Two More Nuclear Batteries after Mars 2020''. (n.d.). Retrieved April 19, 2020, from <nowiki>https://spacenews.com/u-s-plutonium-stockpile-good-for-two-more-nuclear-batteries-after-mars-2020/</nowiki></ref><ref>Nerlich, S. (2010, October 9). Astronomy Without A Telescope—Solar Or RTG? ''Universe Today''. <nowiki>https://www.universetoday.com/74755/astronomy-without-a-telescope-solar-or-rtg/</nowiki></ref><ref>''Nuclear Reactors for Space—World Nuclear Association''. (2020, April). <nowiki>https://www.world-nuclear.org/information-library/non-power-nuclear-applications/transport/nuclear-reactors-for-space.aspx</nowiki></ref>

Revision as of 01:23, 19 April 2020

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Operational envelope for different power conversion technologies

Spacecraft have three main options for power generation: chemical, solar, and nuclear. To the general public, the last of these sources may conjure images of reactors using fission processes, and many probes (particularly those launched by Russia[1]) have successfully employed such systems. Most nuclear-powered probes traveling beyond Earth orbit, however, have instead utilized radioisotope thermoelectric generators (RTGs), which harness the heat produced by radioactive decay rather than a nuclear chain reaction. RTGs offer an alternative to the more typical solar power when conducting missions where sunlight is scarce, as occurs when traveling to the outer solar system or the dusty atmosphere of Mars.

How RTGs Work

Although RTGs use radioactive fuel to generate electricity, they should not be confused with nuclear reactors. The latter harness the energy produced by controlled fission or fusion processes, but no chain reaction takes place in RTGs.

Electricity production in an RTG. Radioactive decay heats one side of a thermocouple (1). An electric current is produced on the opposite, cooler side (2), then tapped from terminals connected to the thermocouple (3), producing power (4).

Instead, unstable radioactive materials known as radioisotopes produce heat as a by-product of their radioactive decay as emitted particles transfer their energy into surrounding atoms.[2] This decay takes place within a shell of semiconductors which generates an electric current when each end is exposed to differing temperatures due to the thermoelectric effect. Simply put, charge carriers diffuse away from a heat source and build up at the cold end of a material; in semiconductors, these charge carriers can be electrons (“n-type semiconductors”) or electron holes (i.e. a position where an electron could exist; these semiconductors are called “p-type”). By connecting n-type and p-type semiconductors with a metallic strip, electrons flow between the two once heat is applied, generating an electric current. This connection of n- and p-type semiconductors is called a thermocouple.[3][4]

In RTGs, the thermocouple’s heat source comes from the radioactive decay of the RTG’s fuel source heating the interior of its thermoelectric shell, while the exterior is kept cool by the surrounding atmosphere or vacuum. As long as a constant temperature gradient is maintained, electricity will be produced.[1][5][6][7]

  1. ↑ 1.0 1.1 Karacalıoğlu, G. (2014, January 16). Energy Resources for Space Missions –. Space Safety Magazine. http://www.spacesafetymagazine.com/aerospace-engineering/nuclear-propulsion/energy-resources-space-missions/
  2. ↑ Decay heat. (2020). In Wikipedia. https://en.wikipedia.org/w/index.php?title=Decay_heat&oldid=951406419
  3. ↑ Alfred. (2018, October 25). How Thermoelectric Generators Work. Applied Thermoelectric Solutions LLC. https://thermoelectricsolutions.com/how-thermoelectric-generators-work/
  4. ↑ James, L., & Granath, E. (2020, February 19). Understanding the difference between n- and p-type semiconductors. Power & Beyond. https://www.power-and-beyond.com/understanding-the-difference-between-n-and-p-type-semiconductors-a-905805/
  5. ↑ U.S. Plutonium Stockpile Good for Two More Nuclear Batteries after Mars 2020. (n.d.). Retrieved April 19, 2020, from https://spacenews.com/u-s-plutonium-stockpile-good-for-two-more-nuclear-batteries-after-mars-2020/
  6. ↑ Nerlich, S. (2010, October 9). Astronomy Without A Telescope—Solar Or RTG? Universe Today. https://www.universetoday.com/74755/astronomy-without-a-telescope-solar-or-rtg/
  7. ↑ Nuclear Reactors for Space—World Nuclear Association. (2020, April). https://www.world-nuclear.org/information-library/non-power-nuclear-applications/transport/nuclear-reactors-for-space.aspx
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