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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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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

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.

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.

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”.

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?

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.

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.

> 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?

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.

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.

You can buy little coffee-cup-powered Stirling engine tchotchkes.

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

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.

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...

Except the ideal one doesn't exist and multistage stream turbines are more efficient in practice.

But I suppose that having a multistage steam turbine in mars in not something will see at first

And more reliable too. The Stirling does have one huge advantage which is that it is entirely self contained.

Definitely not at that scale. Small turbines have low efficiency because tip speed increases quadratically as they get smaller.

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.

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.

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

No moving parts is the key advantage.

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).

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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