Any canon information to be had on radio-isotope thermoelectric generators? These are the devices powering some of our deep space hardware, heat from a highly radioactive element (a version of plutonium is mentioned, and radioactive cesium) is converted directly to electricity rather than using the more conventional mechanical approach - don't ask me how. Seems to me they'd have been a useful addition to MegaTraveller, but I don't find them there.
Radio-isotope thermoelectric generators
- Thread starter Carlobrand
- Start date
none that I've seen. Current generation real world are pretty low power; not even kW per kL
Timerover51
SOC-14 5K
The Plutonium isotope used is Plutonium-238, with a half-life of about 30 years, decays with alpha particle emission. The Cesium is Cesium-137. The electricity is produced by the heat difference in the thermocouples surrounding the isotope casing.
See following Wikipedia article.
http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator
Looks like a warm and glowing hail-Mary method of power generation best left to unmanned operations to me.
Memory informs me it won't even power basic life support on a merchant ship, according to my MegaTraveller rules.
MT's life support power costs appear to be a brute force method - the power draw is sufficient for electrolytic cracking of CO2...
GT, via the usefulness of GURPS Vehicles , includes such things. I built a portable generator using those rules. They are, as aramis said, not very powerful, but have the advantage of running for years to decades without requiring refueling.
If you can assume the magical level of radiation shielding that exists for starships, it is trivial to build an safe RTG.
Timerover51
SOC-14 5K
I have been trying to design something like an RTG using a blend of Plutonium-238, Pu-239, and Pu-240. The mass of the Plutonium Oxide fuel cubes would be less than a critical mass of the Pu-239/240 mixture, while the spontaneous fissioning of the Pu-240 would produce Pu-239 from the Pu-238, and fission some of the Pu-239, along with the energy from the spontaneous fissioning of the Pu-240, increasing the total energy from the generator without requiring a complex means of generating the electricity, still allowing the use of thermocouples. In theory, it will work, but the shielding is a problem because of the fission byproducts of the Pu-239/240.
The nice thing about the Pu-238 is that the thermocouple (with radiators) and thermal insulation systems are pretty much all the shielding needed (the 75mm of thermal insulation is comparable to the 25mm of lead needed), and the critical mass is exceptionally high; the distance on Voyager and Pioneer was more about thermal contamination. (Not entirely, but mostly.) Which leads me to ask - how much more energetic is the fission of Pu-239 and Pu-240?
Timerover51
SOC-14 5K
Correction on Pu-238 half-life, it is 87.7 years.
The energy comparison is approximately 200 Million electron volts for fission, verses approximately 5.6 MeV for alpha particle decay of Pu-238. So, a factor of about 35.8, plus energy from fission products, particularly Strontium-90.
The energy comparison is approximately 200 Million electron volts for fission, verses approximately 5.6 MeV for alpha particle decay of Pu-238. So, a factor of about 35.8, plus energy from fission products, particularly Strontium-90.
I'd seen the Wiki article. I was hoping for some canon usage to save me having to do the hard work.
What I see in the Wiki article are versions ranging from the SNAP-19, generating 40 watts electricity for about a kilo of plutonium in a 13.6 kilo construct, to the GPHS-RTG, generating 300 watts electricity for 7.8 kilos of plutonium in a 56-58 kilo construct. Efficiency varies quite a bit but is quite low, somewhere between 5% and 10%. That is certainly something amenable to TL improvement - for example, figuring out ways to make photovoltaic cells that are efficient at that wavelength of the spectrum.
Three of those larger models is almost a kilowatt for about 170 kilograms of construct. Bulky but useful for something that can deliver a serviceable level of charge for years without refueling. There are drawbacks: I don't know if it's shielded enough for use around humans, the charge is very gradually diminishing over the years, and gods help the poor folk if it's damaged and the fuel got out. Still, it'd make a good way to keep an emergency low berth powered for many years, and there we have an existing model at TL7 to pattern after. Timerover's TL9 version seems to be a greatly improved version, generating 2 Kw at about 6% of the mass, presumably applying greatly improved efficiency so it can use a much smaller mass of fuel.
(Incidentally, it's also useful for that MT Imperial Encyclopedia nav satellite, which claims to use a nuclear plant though the smallest plant allowed by Referee's Manual is 17 times larger than the satellite.)
What I see in the Wiki article are versions ranging from the SNAP-19, generating 40 watts electricity for about a kilo of plutonium in a 13.6 kilo construct, to the GPHS-RTG, generating 300 watts electricity for 7.8 kilos of plutonium in a 56-58 kilo construct. Efficiency varies quite a bit but is quite low, somewhere between 5% and 10%. That is certainly something amenable to TL improvement - for example, figuring out ways to make photovoltaic cells that are efficient at that wavelength of the spectrum.
Three of those larger models is almost a kilowatt for about 170 kilograms of construct. Bulky but useful for something that can deliver a serviceable level of charge for years without refueling. There are drawbacks: I don't know if it's shielded enough for use around humans, the charge is very gradually diminishing over the years, and gods help the poor folk if it's damaged and the fuel got out. Still, it'd make a good way to keep an emergency low berth powered for many years, and there we have an existing model at TL7 to pattern after. Timerover's TL9 version seems to be a greatly improved version, generating 2 Kw at about 6% of the mass, presumably applying greatly improved efficiency so it can use a much smaller mass of fuel.
(Incidentally, it's also useful for that MT Imperial Encyclopedia nav satellite, which claims to use a nuclear plant though the smallest plant allowed by Referee's Manual is 17 times larger than the satellite.)
Almost any Pu-238 is shielded well enough to be human safe - but they don't function inside.
Specifically, they use the migration of the heat through the thermocouple to generate electricity. That heat has to go as short as possible to radiators to maximize the current. And if those radiators aren't radiating to space, they have additional energy losses to collect and pump that heat further.
The mass isn't the big issue - much of the actual displacement is insulation, and lightweight insulation at that - but the installation volume is MUCH higher, and mostly empty space... space through which the radiators radiate. And space which must be exterior, and clear of other structure besides the big radiator sinks.
Specifically, they use the migration of the heat through the thermocouple to generate electricity. That heat has to go as short as possible to radiators to maximize the current. And if those radiators aren't radiating to space, they have additional energy losses to collect and pump that heat further.
The mass isn't the big issue - much of the actual displacement is insulation, and lightweight insulation at that - but the installation volume is MUCH higher, and mostly empty space... space through which the radiators radiate. And space which must be exterior, and clear of other structure besides the big radiator sinks.
What about thermal output?
Could it be used to boil water at a higher power output?
(I always had this crazy 'steampunk' idea for a train that used nuclear fuel pellets to make steam.)
In theory, yes. But, unlike the thermocouples, that runs a lot of risks of radiation escape due to water's corrosivity to metals. Plus, it's not very efficient.
1J = 0.239005 cal or so.
1 cal is 1g up 1°
1 W is 1J/sec... or 0.239005 cal/sec.
1kg water (≅1L) requires about 4.184 kJ to raise 1°C, and another 2261 kJ to transition to steam. Assuming a typical 20°C for water, to boil 1kg you need 2.595 Mj... a 4 kW system will boil off about 1.75 CC per second (which expands to about 50cc of LP steam. Assuming you limit the water flow to what can boil rather than overflow. In a simple immersion, such a unit is going to be slow. Also, the water needs to be insulated itself or you get massive losses out the tank sides.
It's great for a coffeemaker, tho'. Or running a calculator. Because the steam recapture is going to be 33-40% or so.
