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Comment by mrfusion | Hacker News Reader

It could still be really useful for remote sensors and who knows what else. A lot of places a little power is all you need.

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One of the batteries that Dad worked on back in the 1950's was for the Union Pacific railroad. It was intended to be loaded into an underground vault along the tracks in remote areas, and would provide power to run signals through most of a winter. If modern LED power draw is low enough, and the new thermocouple is efficient enough, they could run the signals off the temperature difference between the mountain air and the temperature of the earth about 10 meters down.

Yeah -- interstellar space, we're talking 67.5% Carnot efficiency.

I don't think heat engines would work in space generally due to the low mass of stuff around you that can absorb heat.

They work great as long as you have a source of heat and a good radiator. You don't need mass as a heat sink.

Interesting, I wasn't thinking about that. Thanks

At a practical level, your initial instinct was right. In space, nobody can feel you convect. Essentially all of the waste heat has to be disposed of via radiation, and this is orders of magnitude less effective than convection at the temperatures that any known material can withstand. Worse, you don't have to radiate "just" the waste heat. Eventually, all of your electric energy becomes heat, too. In other words, all of the energy you extract from your decay source will end up as heat that you have to get rid of via radiation. (Well, you could wind a giant spring, I suppose. But, a spring that large would be a better radiator than energy storage device.)

>In space, nobody can feel you convect.

I know I'm on HN, as opposed to one of the political sites I read, when I run across that sort of remark.

All that being said (and true!), you also have a very cold sink to reject waste heat to, whether you want to or not. The end result is that heating is as much an issue as cooling. Thus spacecraft usually wind up with blankets and radiators, both. :)

Here's a detailed look at the thermal control system for the ISS, for the curious: https://www.nasa.gov/pdf/473486main_iss_atcs_overview.pdf

https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...

From there:

“RTGs use thermoelectric generators to convert heat from the radioactive material into electricity. Thermoelectric modules, though very reliable and long-lasting, are very inefficient; efficiencies above 10% have never been achieved and most RTGs have efficiencies between 3–7%”

Also interesting, the price of RTGs:

"Plutonium 238, which is used to power Radioactive ThermoGenerators such as the ones that power the Voyagers, Galileo, and Cassini probes has not been produced in the US for 25 years, but NASA and DOE have budgeted $50 million to restart production of about 2 Kg per year for 5 years…an average cost of about $5 million per Kg. That is expensive stuff!"

One SNAP-27 unit, the type used in Apollo missions, providing only around 70 W, had around 4 Kg, which means its Pu price alone would be at least around 20 million, if it would be produced now.

So, a kind of thermal solar panel that could possibly scale down/cheap better than a solar panel?

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