6 ms·

This would be interesting for powering spacecraft because it doesn't have any moving parts. For the most part, spacecraft are not serviced ever, so it's best to

by gene-h 5y ago

This would be interesting for powering spacecraft because it doesn't have any moving parts. For the most part, spacecraft are not serviced ever, so it's best to minimize things that can break. Nuclear powered spacecraft have been hard to develop because of the need for moving parts. NASA's cancelled Advanced Stirling Radioisotope Generator, which was supposed to be a more efficient radioisotope powered generator than the thermoelectric generators previously used, had trouble because the moving part based generator wasn't very reliable.[0] In addition to eliminating moving parts this is also interesting for nuclear powered spacecraft because it may be possible to pass light through a radiation shield to prevent damage to the converter. The problem is that the reactor would need operate at extremely high temperatures. While this probably isn't high enough to melt the fuel, the fuel might not be structurally stable. Although liquid uranium nuclear rockets are being considered[1]. [0]https://en.wikipedia.org/wiki/Advanced_Stirling_radioisotope_generator#Development https://en.wikipedia.org/wiki/Advanced_Stirling_radioisotope... [1]https://www.uah.edu/news/items/bubble-through-nuclear-engine-might-be-a-future-nasa-workhorse https://www.uah.edu/news/items/bubble-through-nuclear-engine...

beambot 5y ago
I was under the impression that NASA RTGs relied on solid-state Peltier / Seebeck thermocouples that had no moving parts: https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...
- deleted 5y ago
  [deleted]
- blamazon 5y ago
  Those thermocouples are less than 10% efficient, usually less than 5%. This means you have to dissipate more than 10 times more energy as heat than the energy you actually want to use as electricity. Dissipating heat in space is not easy because unlike earth there is no surrounding fluid to dissipate heat into through convection. This means you have to spend precious mass budget on huge (compared to your energy budget) direct heat radiation systems that cannot leverage convection efficiencies. Thus, a significant energy efficiency increase would be a big deal for RTG powered spacecraft design. It is curious that the article above does not consider this. Bonus thought: consider what this lack of convection problem means for "Hyperloop" type vehicle systems that operate in a vacuum tunnel, as most conventional trains dissipate excess braking energy through convection from resistive heating elements atop the roof.
  - caf 5y ago
    Conventional trains that run under wires can also dump excess braking energy into the overhead. That seems feasible even if you had to have your resistive heating elements somewhere outside the tunnel system.
    - deleted 5y ago
      [deleted]
- foxyv 5y ago
  That is correct, I think the RPG design was to improve the efficiency of the RTG.
- tomxor 5y ago
  Yup, Voyager 1 and 2 are still running off RTGs right now since 1977. However the thermoelectric converter used in those RTGs exploit a different effect, and appear to have quite different properties as a result... What I can tell from a quick dig: - Thermocouples ("Seebeck effect") operate on a temperature difference, and are more commonly used as temperature sensors. They have the advantage of working at a larger range of temperatures, but the disadvantage of needing to maintain a temperature gradient for power production... Any inherent efficiency is likely negated by the requirement for constant cooling. [0] - Themal Diodes ("Thermophotovoltaic") is more like a solar panel for a different wavelength (infrared). The principle they operate on suggests no temperature gradient is required, not sure about cooling for other reasons though, but the clear disadvantage is the requirement for a high operating temperature for effective use in power production. [1] Historically thermal diodes don't appear to have been particularly efficient compared to thermocouples either, obviously this particular one changes that. [0] https://en.wikipedia.org/wiki/Thermocouple#Power_production https://en.wikipedia.org/wiki/Thermocouple#Power_production [1] https://en.wikipedia.org/wiki/Thermophotovoltaic https://en.wikipedia.org/wiki/Thermophotovoltaic
  - the8472 5y ago
    > The principle they operate on suggests no temperature gradient is required A temperature gradient is absolutely required. Ignoring materials problems (such as melting) a photovoltaic cell ultimately does work off a temperature differential because it has to absorb photons. It cannot do that if its temperature is the same as the black body temperature of the lightsource illuminating it, otherwise they would be at an emission/absorption equilibrium. This is also why PV cells could theoretically work in reverse mode at night, emitting IR into space. They'd just do that paltry power ratings because ΔT(earth, cosmic background) is much smaller than ΔT(photosphere, earth). And they'd have to be made of a material with a much smaller bandgap.
    - tomxor 5y ago
      You're right, I kinda knew it needed cooling, but I guess your point is even though the effect being exploited doesn't require a temperature gradient, in practice the heat dissipation requirements are the same if the TVP is to survive - or are they different... surely for the purposes of satisfying the first law the temperature dissipation requirements are dependent on the efficiency of conversion? Which would make the cooling requirements of this TVP lower (relative to input) due to it's higher efficiency? but still substantial.

by gene-h 5y ago

beambot 5y ago

I was under the impression that NASA RTGs relied on solid-state Peltier / Seebeck thermocouples that had no moving parts: https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...

deleted 5y ago
[deleted]
blamazon 5y ago
Those thermocouples are less than 10% efficient, usually less than 5%. This means you have to dissipate more than 10 times more energy as heat than the energy you actually want to use as electricity. Dissipating heat in space is not easy because unlike earth there is no surrounding fluid to dissipate heat into through convection. This means you have to spend precious mass budget on huge (compared to your energy budget) direct heat radiation systems that cannot leverage convection efficiencies. Thus, a significant energy efficiency increase would be a big deal for RTG powered spacecraft design. It is curious that the article above does not consider this. Bonus thought: consider what this lack of convection problem means for "Hyperloop" type vehicle systems that operate in a vacuum tunnel, as most conventional trains dissipate excess braking energy through convection from resistive heating elements atop the roof.
- caf 5y ago
  Conventional trains that run under wires can also dump excess braking energy into the overhead. That seems feasible even if you had to have your resistive heating elements somewhere outside the tunnel system.
  - deleted 5y ago
    [deleted]
foxyv 5y ago
That is correct, I think the RPG design was to improve the efficiency of the RTG.
tomxor 5y ago
Yup, Voyager 1 and 2 are still running off RTGs right now since 1977. However the thermoelectric converter used in those RTGs exploit a different effect, and appear to have quite different properties as a result... What I can tell from a quick dig: - Thermocouples ("Seebeck effect") operate on a temperature difference, and are more commonly used as temperature sensors. They have the advantage of working at a larger range of temperatures, but the disadvantage of needing to maintain a temperature gradient for power production... Any inherent efficiency is likely negated by the requirement for constant cooling. [0] - Themal Diodes ("Thermophotovoltaic") is more like a solar panel for a different wavelength (infrared). The principle they operate on suggests no temperature gradient is required, not sure about cooling for other reasons though, but the clear disadvantage is the requirement for a high operating temperature for effective use in power production. [1] Historically thermal diodes don't appear to have been particularly efficient compared to thermocouples either, obviously this particular one changes that. [0] https://en.wikipedia.org/wiki/Thermocouple#Power_production https://en.wikipedia.org/wiki/Thermocouple#Power_production [1] https://en.wikipedia.org/wiki/Thermophotovoltaic https://en.wikipedia.org/wiki/Thermophotovoltaic
- the8472 5y ago
  > The principle they operate on suggests no temperature gradient is required A temperature gradient is absolutely required. Ignoring materials problems (such as melting) a photovoltaic cell ultimately does work off a temperature differential because it has to absorb photons. It cannot do that if its temperature is the same as the black body temperature of the lightsource illuminating it, otherwise they would be at an emission/absorption equilibrium. This is also why PV cells could theoretically work in reverse mode at night, emitting IR into space. They'd just do that paltry power ratings because ΔT(earth, cosmic background) is much smaller than ΔT(photosphere, earth). And they'd have to be made of a material with a much smaller bandgap.
  - tomxor 5y ago
    You're right, I kinda knew it needed cooling, but I guess your point is even though the effect being exploited doesn't require a temperature gradient, in practice the heat dissipation requirements are the same if the TVP is to survive - or are they different... surely for the purposes of satisfying the first law the temperature dissipation requirements are dependent on the efficiency of conversion? Which would make the cooling requirements of this TVP lower (relative to input) due to it's higher efficiency? but still substantial.