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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.
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.
I wonder over the costs of energy transport..
Let's say we have an industry on Mars, that is powered by dry ice evaporation turbines.
In the middle latitudes, dry ice is unstable on the marian surface. It sublimes, and turns into gas. This means that ambient temperatures there are able to turn the ice into useful energy.
Now, if these power plants shipped energy, in the form of electricity on power lines (burried, probably) to the polar region where dry ice can be efficiently mined, what is the feasibility in terms of energy cost for extraction and transport?
I think it would be easier to set up a bunch of solar panels at the middle latitudes. Or go nuclear.
"High temperature superconductor" research has yeilded superconductive materials that would operate in those ranges as well.
Mars being bitching cold might actually be BENEFICIAL.
less practical. Insolation is a tiny fraction of that of the earth. Conversely, the amount of expansion (and pressure) that heated dry ice turning into gas produces is very high, enabling high efficiency power generation.
The question is if the costs of harvesting and transporting the dry ice are sufficiently low to enable this as a viable solution.
"High Temperature" superconductors exist now that would be superconductive at the polar latitudes.
Indeed, but as a "mature" energy infrastructure, it has many benefits that straight solar or nuclear simply dont have.
1) It's pretty damned low tech, meaning you need need the same amount of energy hungry industrial infrastructure to maintain or build it out.
2) Approx 40% of polar ice on mars is actually water ice, according to spectroscopic analysis from orbit. This means that the turbine generation process would leave behind pretty damned pure water ice in the turbine pressure generators. Useful for a colony.
3) The temperature difference between the polar region and the equitorial region is astounding. In the summer months, mars equator can reach up to 70F in the daytime. Conversely, the pole is -200F. There is also powerful day/night temperature variation at the equator that a heat-engine could capitalize on. Even in the summer, when the daylight surface temp can possibly reach 70F, the night time temperature drops to -150F rapidly. This means that simple mirror concentrators and molten salt tech could be used to drive INSANELY efficient stirling power generators at night.
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.
Serious? Seriousness is well above my pay grade.
The viscous hydrodynamic model is nice (I haven't checked all the mathematics but it looks fine), but what these guys have effectively done is created a combination of a radial-flow turbine and a fluid bearing - not unlike what has been used in compressed-air a dentist's drill for some time. I'm sure it has some use, perhaps in micromechanical devices, but I'm not convinced that this is particularly useful for power generation, martian or not. For a start, FTA:
"Harvesting thermal energy using sublimation as a phase-change mechanism via the Leidenfrost effect is an attractive concept, as it offers the key advantage of a virtually friction-free bearing provided by the vapour layer."
If you look at bearing catalogs, the friction of roller bearings is pretty low - one manufacturer of roller bearings gives a rough estmate of a thousanth of a percent (!) of the power being transmitted. No big win there, especially since these bearings are mounted on a small diameter shaft, thus the resistance torque caused by friction is much lower than when it is applied across the entire surface of the rotor. In any case, fluid bearings already exist and are commonly used in applications where friction must be minimized.
Then there's the fact that this turbine operates well within the creeping flow regime (again FTA: "Using h~H, we then find Re0.2. Therefore, the flow within the vapour layer is dominated by viscous friction.") What that means is that you are dissipating loads of energy in the working fluid through viscous work (some of which, to be fair, is being used to drive the turbine, but it is hardly the best way to do so - your rotor velocity is then limited to the gas velocity, unlike in conventional axial flow turbines.) I would have liked to see a proper comparison of turbine losses for the proposed design against a conventional axial flow turbine included in the paper - it could have been obtained relatively easily from the derived model.
Then, there is the purely practical problem of continuous supply of power during refueling. Once your cylindrical cake of dry ice has been expended, it has to be removed and a new one inserted (presumably with a crane for a large power-generation device). Compare this with a conventional rankine-cycle, where fuel and working fluid (or solid dry ice for a CO2-based cycle - why not?) can be permanently supplied by pumping for fluid and conveyer belts for solids - as is done with dirty old coal-fired rankine power-stations.
But still, it is nice to see people trying to look for novel applications for interesting observed phenomena.
A nice Nuke power plant will be a far better solution.
you get heat, electricity, and a good source of radiation to open up the portal to hell.
Do not look at laser with remaining good eye.
Mars gets 40% earth insolation. 40% isn't a tiny fraction. It's like the difference between California and Germany. Hardly a show stopper.
We used to have a successful, efficient ice harvesting industry with water ice here on Earth. As on Earth, nearby mountains may prove more fruitful than the poles as well. It's a lot easier than shipping all the materials you need to build solar panels from Earth to Mars in sufficient quantities to power a significant colony. Now, if you're just trying to keep a team of 5 scientists alive it's probably better to use solar panels.
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We used to have a successful, efficient ice harvesting industry with water ice here on Earth.
We had existing transportation infrastructure. There are no roads or train tracks on Mars, and no open water where ships can travel. It will take a huge amount of resources to build all of that.
It's a lot easier than shipping all the materials you need to build solar panels from Earth to Mars in sufficient quantities to power a significant colony.
No, you'd start by sending finished solar panels, and/or nuclear reactors, of course. You can't do anything else until you have plenty of energy.
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.