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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?
You still need energy to heat up the CO2. And if the energy is available in electric form, which is most likely, why not simply drive an electric motor instead ?
Hmm. Very interesting.
So one of the toughest problems of gas generator design is the thermal limits of turbine material. Making hotter, more efficient combustion is easy. The problem is the turbine melts.
If your working gas and fuel is cryogenic then you're starting out at a temperature much further away from the material limit. The greater difference in temperature has to translate to greater efficiency, as it does in all heat engines.
Mechanically there should be no problems; we're already running air bearing turbine generators in garbage trucks today. A friction-less gas generator + power turbine + electric generator train should operate at cryogenic temperatures just fine.
Colonial history suggests that Martian colonists would inevitably rebel against the iron fist of Terran rule The Revolution will be crushed now before it starts. No human shall ever leave Earth, under penalty of torture.
Could not compact nuclear engines (eg. similar to those on submarines or on earlier probes like Voyager) not solve the energy problem for Mars? Are we so superstitious of nuclear power that we'd give up a perfectly good, long-term, powerful source without even considering it as an option? Nuclear power can and has been launched into space before, and as long as the risks of launch failure are mitigated (eg. launch over open ocean) then dry ice sublimation engines are not needed.
... should be great if your heat sink temperature is well below zero degrees (you can pick Celsius or Fahrenheit here).
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.
Fill its atmosphere with CO2. Yes I'm kidding.
If you don't fund Columbus..... how will we ever conquer Mars?
I only look human.
My mother is a halfling and my dad is an ogre, so that makes me an Ogreling
At least in space there are ways to create a space station to simulate artificial gravity. All these ideas about colonizing Mars are pointless until there is some scientific breakthrough to simulate earth gravity on the Mars colony.
You REALLY want to just piss off martians, don't you?
but it would appear the martians have plenty of frosty piss for us to use for fuel???
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.
Originally, it was intended for the moon. And at 10 MWe, 30 year lifetime without refueling, and less than 20 tonnes, it is ideal.
I prefer the "u" in honour as it seems to be missing these days.