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Like why use RTGs (Radioisotope thermoelectric generator) to power small spacecraft instead of just placing batteries inside them to power stuff?

dotancohen
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max
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    Batteries were and are still used… but not for long-duration missions where you can't use alternative power sources like solar arrays. – DarkDust May 15 '17 at 15:03
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    See also List of spacecraft powered by non-rechargeable batteries – DarkDust May 15 '17 at 15:04
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    Many spacecraft in low Earth orbit that use solar arrays but regularly have the Sun eclipsed by the Earth use batteries so they can operate during those eclipse periods. On the other hand, recharging is not an option for vehicles that go beyond Jupiter. – David Hammen May 15 '17 at 15:51
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    I think it would be helpful to clarify that a RTG is a power source, whereas batteries are for power storage. For satellites, batteries are used to store excess solar-generated electricity for time when solar power is not available. RTGs are power sources used when reliance on solar-generated power is not feasible. – JS. May 15 '17 at 17:36
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    Even some (most?) RTG-powered spacecraft also use batteries, because the power consumption has a peak-to-average power ratio significantly greater than 1. The battery supplies peak loads and is gradually recharged by the RTG. – pericynthion May 15 '17 at 17:58
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    They use RTGs to power a spacecraft for one or more decades. To do it with batteries would require much more mass if the batteries would be funtional for decades anyway. – Uwe May 15 '17 at 19:11
  • There are rechargeable batteries, but there are non rechargeable ones too. – Uwe May 15 '17 at 19:13
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    In what way would you expect a chemical battery to do better than a nuclear battery/power cell? Are you expecting chemical batteries to be cheaper, less massive, more efficient...? The reasons we don't use nuclear batteries on Earth (with the exception of things like emergency lighting) has a lot to do with scaling issues and security/safety issues that aren't really a problem for deep space probes, where RTGs are used. – Luaan May 16 '17 at 15:22
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    @JS. Primary batteries are power sources too. "Primary" often is used as a synonym for "disposable", but that's not quite its original meaning. Back when folks first talked about primary and secondary cells, a primary cell was one that was ready-to-use as soon as it was assembled, whereas a secondary cell was one that had to be assembled and then charged before it could be used. – Solomon Slow May 16 '17 at 20:10
  • @jameslarge: Agreed. I should have followed DarkDust's lead and clarified that my description was for long-duration missions, not short-duration missions (which can get away with primary batteries). – JS. May 16 '17 at 21:16
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    Obligatory xkcd: https://xkcd.com/1162/ – MichaelK May 17 '17 at 07:53
  • Long story short: battery technology still sucks. The main reason we do not have an Iron Man suit is because we lack high density energy schemes that do not involve nuclear reactors. – JamieB May 17 '17 at 15:43
  • Isn't an RTG storage too? It stores the energy in an isotopic rather than chemical state... – rackandboneman May 17 '17 at 19:20
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    Suggestion: if you haven't done much research, it's much more diplomatic to write "What power sources are used in space and why are they preferred over batteries," rather than posing an invalid question. – Carl Witthoft May 17 '17 at 19:48
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    I misread this question in the sidebar as "Why arent butterflies used in space?" lol :) – Kartik May 18 '17 at 12:08
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    @rackandboneman Maybe with plutonium RTGs, you could make that argument - they are man-made as primary batteries, more or less. But don't follow that road too far - or you'd end up saying that every possible energy source is a battery, which isn't too useful (using the same reasoning, the Sun is also just a battery - it has a lot of hydrogen that's slowly being converted into light and heavier elements; it's true, maybe, but is it really useful?). – Luaan May 18 '17 at 14:14
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    @Luaan isn't the sun the non-rechargeable, hard to replace battery this whole show runs on? – rackandboneman May 18 '17 at 15:33
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    The very first satellites from the fifties of last century were all primary battery powered I suspect. Some of the satellites started in the early sixties also. – Uwe May 22 '17 at 20:37

4 Answers4

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Energy density for:

NiMH C battery -> 237,073 Joules per Kilogram.

Plutonium 238 (used in RTGs) -> 2,239,000,000,000 Joules per Kilogram.

Even if we assume that only 10% of a RTG weight is actually Plutonium, then we still get about 9,400,000 times as much energy available as heat from an RTG as from the same mass of batteries.

In most deep-space missions, landers, and rovers, heat generation is essential to maintain spacecraft function. However, for electrical power, the best conversion efficiency from RTG thermal to electrical power is about 7%, making it "only" 661,000 times as much energy available as electrical power from a RTG as from the same mass of batteries. That's still a pretty huge difference!

Source: Energy Density, wikipedia

Mark Rogers
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m.fuss
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RTGs are used in a very small number of spacecraft. They are used only when there are no other options, i.e. for long missions too far away from the Sun to make solar panels feasible.
Those missions have requirements for a few hundred W of power continuously over a decade or more. If you were to use chemical batteries to supply that much power, your spacecraft would become too heavy to launch.

Hobbes
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Nuclear decay is just simply the most energy dense fuel there is. This is enough to overcome huge inefficiencies in power conversion. We can even ignore the inefficiencies of alternate storage methods, and still conclude that fissile material will store more energy per unit mass.

RTG

For RTGs I'll refer to Wikipedia's article about RTGs that have previously been or are in service. By their nature, the output and efficiency of an RTG is complicated to compute, so I'll refer to the actual measured power output, and assume the output declines along with the nuclear decay (half life of 87 years for 238Pu).

Evaluating the RTGs used in aerospace applications, the absolute worst performer was the SNAP-3B generator with a specific power of 1.3 W/kg (at launch). This was used on the Transit4B satellite, which was operational for roughly 1 year (accidentally destroyed by nuclear test detonation). During this time its 2.1kg RTG produced roughly 23.558 kWh of electricity. This gives a specific storage of 11.2 kWh/kg

Li-Ion

Typical quoted values of Lithium ion specific power capacity are usually around 100 - 200 Wh/kg, however this post (linked article no longer accessable; see wikipedia) from the electronics stackexchange explores the performance of lithium-air batteries (currently have the highest specific energy of any chemistry) with a value of 1.7kWh/kg for a lithium air battery (Li - O2)

As you can see the absolute worst performing RTG is still several times more energy dense that the best performing chemical battery.

Fuel Cell

As far as chemical energy generation goes, fuel cells are much better, as you can more or less ignore the weight of the fuel cell and only consider the fuel (making the assumption that the mass of fuel is much larger than the cell). Hydrogen fuel cells can reach near 85 - 90% theoretical efficiency from the reaction 2H2 + O2 -> 2H2O (40 - 60% in practice). Even ignoring the efficiency loss (because it's small and I don't want to add the calculation step) we can calculate the specific energy density to be 3.73kWh/kg using the enthalpy of formation of water (the absolute theoretical amount of energy released when hydrogen and oxygen combine)

Even fuel cells at above theoretical maximum performance are not quite as good as one of the worst RTGs (keep in mind we're only considering space applications. RTGs for land use have lots of extra radiation shielding and are very heavy)

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Aaron
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  • I think alkaline primary cell would answer "why not a battery". Lithium primary would be nice, too, as it's the best primary we have. – Agent_L May 15 '17 at 19:16
  • I didn't discriminate between primary and rechargeable, nor do I think it's very interesting, as both mentioned are less power dense than Li-air. Li-primary are better than rechargable (~450 Wh/kg) but not compared to Li-air (~1700 Wh/kg) – Aaron May 15 '17 at 19:26
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    It is a problem to store hydrogen and oxygen for a fuel cell in space for many years. Using liquid hydrogen and oxygen is possible only if the boiloff rate is as small as the medium consumption by the fuel cell. – Uwe May 15 '17 at 19:27
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    @Uwe great point. Fuel cells were used in the apollo missions, but are not a great candidate for satellites. Another big concern is hydrogen embrittlement over time. – Aaron May 15 '17 at 19:28
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    Li-air cells would help only if there is air for free. Do the ~1700 Wh/kg include the storage of the necessary oxygen? – Uwe May 15 '17 at 19:30
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    As an aside, English plurals do not carry an apostoph. – Peter - Reinstate Monica May 15 '17 at 19:32
  • @Uwe I cannot find the reference in wikipedia for this value, so i'm not sure. The theoretical limit excluding the mass of the oxygen is around 12000 Wh/kg, so it's anybody's guess.. – Aaron May 15 '17 at 19:33
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    @PeterA.Schneider not according to The Times. (which references the Chicago Manual of Style) – Aaron May 15 '17 at 19:35
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    @Aaron OMG they are so wrong! ;-) (Actually, I respect the Times a lot in many respects and consider them authoritative also regarding style, so I stand corrected.) As an aside to an aside, they want apostrophes only for abbreviations with periods; e.g. M.D.'s, but DVDs. – Peter - Reinstate Monica May 15 '17 at 19:43
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    @PeterA.Schneider fair enough. although if you search it, there are many examples of both. languages are living things. what's "right" and "wrong" are simple what's most commonly used. – Aaron May 15 '17 at 19:44
  • A value for the energy density of Li-air cells given for the use on earth and in air would not include the weight of the oxygen, oxygen tank and pressure controller. – Uwe May 15 '17 at 20:07
  • Can someone give my fuel cell calculation a sanity check? It feels too low.. – Aaron May 15 '17 at 20:25
  • Aaron, I think the fuel cell number feels low because the Li-air battery doesn't include the required oxygen (which is abundant on Earth, but the probe would need to carry it as fuel). A self-contained battery is still about ten times less energy dense than that; fuel cells are much better. – Luaan May 16 '17 at 15:27
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Simply put, RTGs last a long time. Space probes need a reliable, long-lasting power source, since we can't just change the batteries when they run out. An RTG can run for decades with relatively little reduction in power output, unlike a traditional chemical battery.

Nuclear Hoagie
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  • Does traditional chemical battery lose power with time? – max May 15 '17 at 14:19
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    @max of course it does. That's what happens when a battery dies. – Tristan May 15 '17 at 15:12
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    @Tristan: I think max meant self-discharge, not just normal discharge (from usage i.e. drawing current). Some chemistries have very low self-discharge and it drops with temperature. So I guess it’s a manageable problem. – Michael May 15 '17 at 17:37
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    Low temperature would decrease self-discharge but also normal discharge. If a battery is too cold, you could not get the nominal power and energy defined at the nominal temperature. The peak current is also decreased with low temperature. – Uwe May 15 '17 at 19:21
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    @max Adding to what Uwe said in the previous comment, you can experience that effect perfectly well right here on Earth. You need two identical, reasonably sturdy flashlights and two sets of batteries, ideally identical. Load one set of batteries into each flashlight, turn both on, put one in the freezer and the other out on the kitchen table. Return after an hour and observe the difference in light output between the two. Measure the battery voltage with a multimeter if you are so inclined. You will likely see a marked difference. – user May 16 '17 at 16:36