Dry-Ice Heat Engines For Martian Colonists

LeadSongDog writes: Heat engines using the "Leidenfrost effect" can exploit the gas expansion as CO2 sublimates to drive turbines. "The technique has exciting implications for working in extreme and alien environments, such as outer space, where it could be used to make long-term exploration and colonisation sustainable by using naturally occurring solid carbon dioxide as a resource rather than a waste product. If this could be realised, then future missions to Mars, such as those in the news recently, may not need to be ‘one-way’ after all.

Dry ice may not be abundant on Earth, but increasing evidence from NASA’s Mars Reconnaissance Orbiter (MRO) suggests it may be a naturally occurring resource on Mars as suggested by the seasonal appearance of gullies on the surface of the red planet. If utilised in a Leidenfrost-based engine dry-ice deposits could provide the means to create future power stations on the surface of Mars. " The research was published in Nature Communications, and one of the researchers published an explanatory article at The Conversation.

Comments

Re:Go nuclear

[JWSmythe]

Nuclear satellites and probes use tiny reactors only capable of watts of output. Voyager 1's has 3 MHW-RTG weighing 37.7 kg, and making 147w each.

The S5G reactor compartment weighed 650 tons.
The S9G reactor compartment weighed 1,400 tons and measures 31 ft in diameter, 37 feet deep.

We (anyone on Earth) don't have anything that will lift a submarine reactor to LEO. To the best of my knowledge, nothing like that has even been designed.

For comparison, the ISS is about 460 tons, and it wasn't delivered in one shot. I believe most of what's there was delivered in 31 flights.

Also, nuclear reactors don't last forever. From what I could find, the S9G is designed to be refueled at about 30 years.

Re:Energy costs of transport

[LeadSongDog]

CO2 ice boils for 758 J/g H2O ice boils for 2594 J/g We only use water for heat engines because it's so damn cheap on Earth. Otherwise it's a pain to work with, mostly because it's a polar molecule.

This isn't an energy source

[Solandri]

Forget the energy cost of transport. The energy cost of using this device exceeds the energy it can produce. The summary and the first TFA completely misrepresent what the researchers are proposing. They are not saying we can "harvest energy" from the CO2.

You can do the exact same thing by boiling water. When water boils, it expands into a more voluminous gas. The energy from that volume change can be harnessed to do work. Free energy! Right? Well as we all know (or should know), that energy isn't free. You have to put in that energy when you boil the water. The phase change from liquid to gas takes a lot more energy than merely heating up the liquid. Exactly as much energy as needed to cause the volume change as it expands into gas (net zero energy gain). Except the engine extracting energy from the volume change (aka steam engine) is never 100% efficienct, so you end up putting more energy into it than you get out.

All they've done is replaced boiling water with sublimating solid CO2. The thermodynamic and energy principles behind it are the same. And thus this will never produce as much energy as you put into it. The only exception is when you have waste heat (e.g. a generator running outside). Then, like any heat engine, you could use this to convert some of that waste heat into usable energy (the energy you're "putting in" to it is energy that you would've lost anyway). But it's never gonna be usable as a primary energy source, because it's not an energy source.

The summary and first TFA have heralded this as some new energy source on Mars. It's not. If you read the direct words from the authors in the last TFA, they're merely proposing this as an alternative to water and steam engines. See, water is exceedingly rare on Mars. It's only popular here on Earth to convert heat energy into mechanical energy (via a steam engine, like in nuclear plants) because of its abundance. We can just slurp some up from a local river or ocean, run it through the steam cycle, and dump the steam back into the environment. The ecosystem will take care of converting it back into liquid water for us, and returning it to the river or ocean for future reuse.

Not so on Mars. There's precious little water, and you'd be a fool to dump waste steam into the environment when your colonists need it to survive. What these researchers have proposed is a "CO2 engine" which uses sublimating CO2 to convert (not extract) heat energy from another energy source into mechanical energy for doing work.

For the same reason, this won't work in space. You lose the CO2 gas to space, and your engine stops working. Just like if you used a steam engine in space and vented out the resulting steam. You either need a constant supply of new, solid CO2 (like on Mars). Or you need the whole thing to operate in a closed loop (where you also handling the cooling phase which converts the coolant back into a liquid or solid), in which case water or ammonia (freezes at -78 C) is probably a better choice because closed loops work a lot better with a liquid heat exchange medium.