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Sublimation engine for Mars

Generate power on Mars using empty space and CO2
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Inspired by "Mercury vapor nuclear engine for Mars," and taken from my comment there.

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Pressurize a working fluid, like hydrogen or helium gas, and keep it highly insulated.

At night, pump the fluid through an IR-transparent tube inside a gas-evacuated parabolic reflector trough that is aimed towards the sky. The trough is insulated on all sides, apart from the top, where it is covered in an IR-transparent pane.

The fluid will cool to substantially below atmospheric temperatures as it radiates its heat into empty space, hopefully until it's cold enough to freeze CO2 from the atmosphere.

Freeze and collect atmospheric CO2 ice.

Use this ice in the sublimation engine. It can be heated by the martian atmosphere, or even concentrated sunlight. It could be used to cool a Stirling engine (solar/atmosphere heated on the hot side), and the expanding gas used to power a secondary engine. Whatever works best. The trough could even serve as a solar collector during the day.

TIB, Jul 22 2015

Simultaneous solar heating and blue-sky cooling http://ac.els-cdn.c...a6070ea92a383245b57
[MaxwellBuchanan, Jul 22 2015]

Carnot cycle https://en.wikipedi...A9onard_Sadi_Carnot
Fundamental. [8th of 7, Jul 22 2015]

[link]






       // The fluid will cool to substantially below atmospheric temperatures as it radiates its heat into empty space, hopefully until it's cold enough to freeze CO2 from the atmosphere. //   

       Since the Laws of Thermodynamics have been suspended in this instance, constructing an over-unity powerplant shoul present no problem whatsoever.
8th of 7, Jul 22 2015
  

       [8th of 7]: as odd as it seems, it's even possible to do something like this on Earth, but it would work far better on Mars due to the thin dry atmosphere. Just think about how satellites dump heat from their electronics, or how a planet cools down for that matter (radiative cooling). Space is big, and cold, and therefore a great place to dump some unwanted heat.
TIB, Jul 22 2015
  

       //as odd as it seems, it's even possible// I too still have doubts about the physics of this. If I aim a parabolic dish at a block of ice, in a room with an air temperature of 25°C, will something at the focal point of the dish actually cool towards 0°C?   

       <later> Hmmm. OK, apparently it is possible, or at least possible enough to get past referees. Paper in second link shows the use of parabolic reflectors to (a) heat an object on which sunlight is focussed and simultaneously (b) cool an object on which the [cold] sky is focussed.
MaxwellBuchanan, Jul 22 2015
  

       Sublime.   

       //as odd as it seems, it's even possible//   

       [marked-for-tagline]   

       This Idea would work better if water was boiled to steam under concentrated sunlight, and allowed to re-condense while shielded from sunlight. That's because many substances like CO2 have roughly a 1:1000 expansion ratio between the condensed state and the gaseous state, while the ratio for water is 1:1600 or so.
Vernon, Jul 22 2015
  

       It takes advantage of an the fact that the fluid in question isn't in a closed system. The fluid, at equilibrium in atmosphere is receiving radiation from everything around it at the same rate it is radiating energy out. Isolate it such that it is receiving less radiation in than it is giving off, and it will cool off.   

       It's not simple to isolate your fluid sufficiently that it doesn't still receive local radiation, as well as convection and conduction energy, but it is at least theoretically possible. Easy if you're working in an isolated vacuum, which is how space craft do it.
MechE, Jul 22 2015
  

       There's no heat pump, so in the energy recovery phase the temperature difference across the heat engine won't be that big ... any guesses as to the Carnot efficiency ?
8th of 7, Jul 22 2015
  

       I tried a sublemon once, but it was a bit sour.
RayfordSteele, Jul 22 2015
  

       Yesss! A tagline submission, and from the great [Vernon] no less. Made my day!   

       [MechE]: Correct. It's not so difficult to isolate though, from what I've read. On cold -- but not freezing -- nights here on Earth, ice will often form on small puddles, roads, plants, tents etc., due to radiative cooling. A purpose built device, like an insulated bowl with IR transmissive plastic covering it, will be far more effective at radiating in one direction than a puddle, so freezing at even higher ambient temperatures is possible.   

       [8th of 7]: The sublimation engine relies on CO2's expansion as it changes phase, so Carnot efficiency is not a concern. If a Stirling engine was used in conjunction with the sublimation engine, the CO2 ice would hold its cold side at a constant temperature, and heat could be gathered by various means for the hot side. I'd suggest using the troughs to gather solar radiation very effectively. In this case the heat differential would be substantial and Carnot efficiency high.
TIB, Jul 22 2015
  

       Given that Mars is quite a close planetary neighbour, couldn't we just run an extension lead?
MaxwellBuchanan, Jul 22 2015
  

       It would need to be well-insulated as we're sometimes on the wrong side of the sun.
RayfordSteele, Jul 23 2015
  

       A similar concept is in the book The Martian which is about a manned mission to Mars there's a device gathers atmosphere and through a chemical process (I believe harvesting nitrogen and more from the air and condensing it gathers enough propellant over the course of 4 years. It is sent ahead of time so the next manned mission would be able to leave Mars' atmosphere and fly back to Earth with a fully fueled rocket.
Duck Lagrange, Jul 25 2015
  

       )
MaxwellBuchanan, Jul 25 2015
  

       So.. I decided to check with NASA to see if this might work, and they said it would, depending on the region and time of year. They said one problem is that although Mars has a thin atmosphere, the dust and CO2 block a large amount of IR, meaning that it could be more difficult to equilibrate with the sky temperature.   

       Now I have a new project to build in my backyard once I get a place on Mars. It may or may not work, and the uncertainty is entertaining!
TIB, Jul 28 2015
  

       //I decided to check with NASA// Kudos!
MaxwellBuchanan, Jul 28 2015
  

       // a device gathers atmosphere and through a chemical process (I believe harvesting nitrogen and more from the air and condensing it gathers enough propellant over the course of 4 years. It is sent ahead of time so the next manned mission would be able to leave Mars' atmosphere and fly back to Earth with a fully fueled rocket. //   

       This is called ISPP (in-situ propellant production), a type of ISRU (in-situ resource utilization). I first learned of it in The Case for Mars by Robert Zubrin (a nonfiction book where he advocates for his mission plan Mars Direct). Mars's atmosphere is nearly 100% carbon dioxide, so the machine is shipped with a bunch of hydrogen from Earth, and combines that with carbon from the atmosphere to make methane and oxygen, which can be used in a rocket. (That's why SpaceX is developing methane/oxygen rockets—they want to be ready for launching from Mars.)
notexactly, Apr 28 2019
  

       If part of the crust of the Yellowstone Caldera could be replaced with a transparent lens, underlying an eye-of-Sauron tower with a rotatable top, then we could wink focused radiant energy in the direction of Mars at certain times of year.
pertinax, Apr 29 2019
  
      
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