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A comment on NASA completes full-power tests of small, portable nuclear reactor.

Re:NASA completes full-power tests of small, portable nuclea... by syntaxing on Thursday May 03, 2018 @08:40AM (#16985409)
For those curious, this uses a nuclear reactor to power a Stirling engine.Stirling engines are a good choice because there is no mass transfer to create the work (i.e. no fuel needed, just heat). It only needs a temperature differential and sealed gas/liquid to work. I highly recommend to read more about Stirling engines since they are of one the most efficient form of work generation with just a temperature differential.
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Re:NASA completes full-power tests of small, portable nuclea... by larkeith on Thursday May 03, 2018 @09:16PM (#16991466)
A Stirling engine supplied by sodium heat pipes [1][2], even! It's a fascinating design of closed-cycle engineering, with minimal moving parts.

[1] https://en.m.wikipedia.org/wiki/KRUSTY

[2] https://en.m.wikipedia.org/wiki/Heat_pipe

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Re:NASA completes full-power tests of small, portable nuclea... by InclinedPlane on Thursday May 03, 2018 @04:08PM (#16989436)
Stirling engines aren't that efficient but they are very reliable and capable of running for many years with zero maintenance. They compete with bimetallic solid state thermoelectric generators which also require no maintenance but are less efficient than Stirling engines and also degrade slowly over time.
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Re:NASA completes full-power tests of small, portable nuclea... by nerdponx on Thursday May 03, 2018 @12:55PM (#16987623)
Funny this should come up today. Just last night I was wondering why we don't have better technology for learning how to turn heat into electricity.
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Re:NASA completes full-power tests of small, portable nuclea... by nojvek on Thursday May 03, 2018 @08:50PM (#16991343)
If our civilization figures out how to turn heat into electricity with a high efficiency and a small solid state device, it changes the economies of so many things.

But it’s a hard problem, mostly because of “entropy”.

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Re:NASA completes full-power tests of small, portable nuclea... by jcims on Thursday May 03, 2018 @05:49PM (#16990286)
Seems like this would be limited in space applications by how much you could radiate away. Wouldn't you need to get rid of ~2x the desired output power? So to get 1kW you'd need to dissipate ~2kW?
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Re:NASA completes full-power tests of small, portable nuclea... by inamberclad on Thursday May 03, 2018 @09:54PM (#16991638)
The upper limit of any heat engine (turning heat into useful work) is called the Carnot efficiency and is limited by the temperature differential. It's usually not more than 70%, though for a Stirling engine, a few percent would be considered excellent efficiency.

A Stirling engine easily needs to radiate 20 times as much heat as it produces as power.

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Re:NASA completes full-power tests of small, portable nuclea... by taejo on Friday May 04, 2018 @05:22AM (#16993248)
> for a Stirling engine, a few percent would be considered excellent efficiency.

Wikipedia says "Stirling engines have a high efficiency compared to internal combustion engines, being able to reach 50% efficiency," which seems to contradict that. What's the discrepancy?

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Re:NASA completes full-power tests of small, portable nuclea... by hatsunearu on Friday May 04, 2018 @05:49AM (#16993333)
Yeah, even thermoelectric piles (Peltier effect devices) can easily get ~~double digit efficiencies. Not sure what that guy is talking about~~.

Thermoelectric piles are basically the worst form of generating electric power with heat (it has 0 moving parts, so it's used in various places), and it's still not that bad.

edit: Never mind, apparently they are worse than what I thought they were, at 5~8%. But they are still the least efficient way to turn heat into electricity, and yet they can achieve what that guy said.

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Re:NASA completes full-power tests of small, portable nuclea... by ajross on Thursday May 03, 2018 @07:50PM (#16991034)
Yes. Though cooling sails are relatively feasible. Assume you can cool to ~250K: the back of my envelope and a Stefan-Boltzmann constant cribbed blindly from wikipedia tells me that such a sail would radiate ~200W/m^2, which is very doable for a spacecraft with a 1kW power budget.
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Re:NASA completes full-power tests of small, portable nuclea... by jgamman on Friday May 04, 2018 @12:40AM (#16992240)
Ormat does large Stirling engines for geothermal power. The binary units mean you don't run the risk of taking too much of the reservoir fluid. I wouldn't be surprised if tech like this ramps up for the same reason battery tech went nuts when people wanted longer life electronics - except the driver is climate change and carbon pricing.
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Re:NASA completes full-power tests of small, portable nuclea... by caf on Friday May 04, 2018 @12:01AM (#16992094)
You can buy little coffee-cup-powered Stirling engine tchotchkes.

https://www.youtube.com/watch?v=CSgo9NSyFPs

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Re:NASA completes full-power tests of small, portable nuclea... by cathhhhji on Thursday May 03, 2018 @10:17AM (#16986238)
Stirling engines are not efficient. Using that heat to make steam to turn a turbine is much more efficient. But it would be harder to make that portable.
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Re:NASA completes full-power tests of small, portable nuclea... by telchar on Thursday May 03, 2018 @10:38AM (#16986410)
Yes they are. An ideal stirling engine has the Carnot level efficiency, i.e. maximum theoretical efficiency for a heat engine.

https://blog.mide.com/thermodynamic-theory-of-the-ideal-stir...

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Re:NASA completes full-power tests of small, portable nuclea... by coryrc on Thursday May 03, 2018 @12:20PM (#16987308)
Except the ideal one doesn't exist and multistage stream turbines are more efficient in practice.
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Re:NASA completes full-power tests of small, portable nuclea... by hwillis on Thursday May 03, 2018 @03:54PM (#16989314)
Definitely not at that scale. Small turbines have low efficiency because tip speed increases quadratically as they get smaller.
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Re:NASA completes full-power tests of small, portable nuclea... by jacquesm on Thursday May 03, 2018 @01:36PM (#16987970)
And more reliable too. The Stirling does have one huge advantage which is that it is entirely self contained.
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Re:NASA completes full-power tests of small, portable nuclea... by Faaak on Thursday May 03, 2018 @12:24PM (#16987356)
But I suppose that having a multistage steam turbine in mars in not something will see at first
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Re:NASA completes full-power tests of small, portable nuclea... by rfrey on Thursday May 03, 2018 @02:24PM (#16988423)
You may be interested in reading Allan Organ’s books. He claims that aspirations to Carnot efficiency are a red herring and claims (with Much Math) that the peak efficiency is closer to 50% of The Carnot cycle.
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Re:NASA completes full-power tests of small, portable nuclea... by ttul on Thursday May 03, 2018 @12:26PM (#16987374)
Hmm. Why not talk to Etalim: https://www.etalim.com/

They invented and are very close to commercializing a direct heat-to-electricity converter that uses acoustic waves. No moving parts. Close to 30% efficiency.

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Re:NASA completes full-power tests of small, portable nuclea... by baybal2 on Thursday May 03, 2018 @02:26PM (#16988432)
It is effectively another stirling, just with membrane instead of piston, and very high frequency of operation.

Sound == heat.

An AMTEC converter may be an another "fully self-contained" solution

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Re:NASA completes full-power tests of small, portable nuclea... by ttul on Saturday May 05, 2018 @12:36PM (#17002520)
No moving parts is the key advantage.
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Re:NASA completes full-power tests of small, portable nuclea... by pjc50 on Thursday May 03, 2018 @01:47PM (#16988091)
Turbines are only efficient at large size (see failed gas turbine cars), while Stirling engines are very dependent on construction details for efficiency.
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Re:NASA completes full-power tests of small, portable nuclea... by anovikov on Thursday May 03, 2018 @12:37PM (#16987476)
Nope. It is easy to make an efficient heat engine if temperature differential is so huge that it is sufficient for a steam turbine. And efficiency of a turbine will be nowhere like Carnot efficiency. Stirling engines work on much lower temperature difference, providing efficiencies dead close to Carnot levels (which is still, on that temperature difference, quite low).
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