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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.
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.
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.