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Mars Power

Dry-Ice Heat Engines For Martian Colonists 91

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.
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Dry-Ice Heat Engines For Martian Colonists

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  • by wierd_w ( 1375923 ) on Wednesday March 11, 2015 @02:42AM (#49231493)

    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?

    • Also gotta wonder how much CO2 this outputs per Joule of energy. (Not that this would be a problem on Mars, just the opposite in fact)

    • There would be very few costs associated with orbital mirrors made of mylar to boost the amount of sunlite in local polar power generation plants. The exhaust would re-condense to complete the cycle. Populate the poles and leave the equator alone.
    • by Solandri ( 704621 ) on Wednesday March 11, 2015 @02:08PM (#49235447)
      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.
      • Uhm-- YES. I know this. The energy comes from sunlight. I never suggested otherwise. That's why I stated that the power plant is at the equator, where insolation is greatest, and thus, harvestable energy is greatest.

        Because there is significantly less energy at the poles, the atmosphere freezes into ice. The amount of energy needed to reconstitute this gas from the ice is significantly less than the energy needed to boil water, meaning you dont need the same intense energy sources.

        Most of the energy in a s

  • Energy (Score:2, Interesting)

    by itzly ( 3699663 )

    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 ?

    • I suggested using the different ambient temperature at the middle latitudes, where dry ice is naturally unstable (which is why there isnt lots of dry ice there) to drive the turbines, which produce electricity.

      The electricity is sent to the polar region to drive electric motors to extract dry ice, and to ship it via electric rail to the power plants.

      I was wondering what the econmy of that kind of arrangement would be.

      • by itzly ( 3699663 )

        I think it would be easier to set up a bunch of solar panels at the middle latitudes. Or go nuclear.

        • Re: (Score:3, Interesting)

          by wierd_w ( 1375923 )

          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.

          • by itzly ( 3699663 )

            The question is if the costs of harvesting and transporting the dry ice are sufficiently low to enable this as a viable solution.

            The killer will be the huge investments to set up the transporting infrastructure. You need energy first.

            • Re:Energy (Score:5, Interesting)

              by wierd_w ( 1375923 ) on Wednesday March 11, 2015 @03:36AM (#49231649)

              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 'ambient temperature' is mainly solar energy as the air pressure is low, that brings us back to solar energy. In those conditions it is also easy to create dry ice locally by radiation cooling in the shadow.
            • A miror is easier to produce than a solar pannel.

              For purposes of thermal expansion based turbines, a mirror is quite sufficient as an energy source improvement.

              For purposes of solar energy to electricity, a mirror is not what you need-- you need much more energy expensive materials and processing.

              The former is more sustainable in place than the latter.

              • by itzly ( 3699663 )

                A miror is easier to produce than a solar pannel.

                Build a bunch of mirrors and concentrate the sunlight on a solar panel.

                • Still have the costs of producing the solar panels, VS the costs of building turbines. Turbines are made of metal (Or even plastic, at these temps!), VS solar panels, which are made of refined, heavily processed rare earth metals and silicon.

                  Solar panels are very expensive, energy wise, to produce. They are also more fragile, and prone to breaking. The mirrors here could just be polished metal plates, and be very durable against sand/dust storms.

                  • by itzly ( 3699663 )

                    A transport infrastructure to move dry ice from the polar regions to the equator is going to cost even more, and the efficiency will be low due to transport losses.

                    • 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.

                    • Re:Energy (Score:4, Insightful)

                      by itzly ( 3699663 ) on Wednesday March 11, 2015 @07:26AM (#49232245)

                      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.

                    • Off road vehicles will work on Mars before you even bother to make a dirt road -- the Apollo moon buggies fully demonstrated the practicality of that already.

                    • I've heard some moon landing hoax stories before, but I don't think anyone claims that Apollo landed on Mars instead of the Moon.

              • Not sure about the ease. They are starting to talk about spray on solar panels, never heard about spray on mirrors without flat glass (I know the old method of window making is using just two metal tubes and a ton of dexterity breath and strength)
                • Mirrors are made out of aluminum metal, deposited onto glass, usually.

                  Silver mirror is made of silver metal deposited onto glass.

                  Both require exotic materials, as far as martian soil mineral is concerned. Polished steel plates have sufficient reflectivity, and could be manufactured cheaply on mars. They are also more resistant to being broken or blown around by martian wind/dust/sand storms.

                  • Maybe mirrors are cheaper, even then the mirrors must be oriented to something and maybe even that is cheaper. With Mars atmosphere being "about 0.6% of Earth's mean sea level pressure", Mars storms are paper tigers.
                • by khallow ( 566160 )
                  Spray a liquid stream of molten aluminum onto a shaped frame. Lot easier than a spray on solar panel IMHO.
          • by radl33t ( 900691 )
            Mars gets 40% earth insolation. 40% isn't a tiny fraction. It's like the difference between California and Germany. Hardly a show stopper.
            • As I recall, Germany's solar infrastructure consistently delivers a fraction of what its expected output is, year after year. Might have something to do with how far north they are.

              You have to remember that solar already sits around 20-40% efficiency, chopping another 60% out of that is a pretty serious hit.

              • The issue with solar efficiency is having enough space for enough panels, it scales easily. The biggest issue for Mars wouldn't be space or efficiency, but cleaning the dust from the panels.

                • By the time we're colonising Mars, microwave solar power from orbit will already be old news. The idea of cutting ice, transporting it from the poles to the equator and then popping it into an engine is cute but really only a scientific curiousity.

              • by radl33t ( 900691 )
                AC output of solar energy is typically more like 12-16%. No it isn't a serious hit, it is more than sufficient, for example, to power non heating and cooling loads for most people on earth based on population density.
        • I think it would be easier to set up a bunch of solar panels at the middle latitudes. Or go nuclear.

          You had me at nuclear. That is what is necessary, along with exploiting the environment's flow, to establish stations on bodies.

      • Set up a sun shade to make dry ice, have a solar concentrator to make it gas. Use the heat engine to capture the energy. No long power wires to the poles required.
    • by Pikoro ( 844299 )

      So use solar sails attached to transport ships carrying some other materials to mars. Once the ships enter orbit, detach the solar sails and manuver the sail material (probably mylar) to Mars's L1. Use the solar wind to keep them in place, and balance that with the gravity of mars and use that to refract the solar radiation hitting Mars until it equals that of earth. That would increase insolation and generally supply more solar radiation to the surface of mars. Once the extra heat is added, it will su

  • by Anonymous Coward

    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 [capstoneturbine.com]

    • "High temperature superconductor" research has yeilded superconductive materials that would operate in those ranges as well.

      Mars being bitching cold might actually be BENEFICIAL.

  • 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.
    • Re:Go nuclear (Score:5, Informative)

      by JWSmythe ( 446288 ) <jwsmythe@@@jwsmythe...com> on Wednesday March 11, 2015 @05:31AM (#49231953) Homepage Journal

      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.

      • by itzly ( 3699663 )

        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.

        That's why the whole idea of setting up a Mars colony is fantasy.

        • If you don't fund Columbus..... how will we ever conquer Mars?

        • There's no good reason we can't set up a Mars colony. Ideas on how to do it have been floated since the late 1940s. Feasible plans have been around since the 1960s. The only thing holding us back is the fact that governments prefer to fund killing people more efficiently, than to extend the reach of the human race.

          The way we're going, the human race will die with this planet. We're trying hard to make sure that happens.

      • Some designs of Project Orion have payloads as high as 6,100 t to LEO.

      • As an alternative, JAXA is already working on a microwave solar power satellite.

      • by Anonymous Coward

        The differences here are misleading. The few Watt designs are RTG - Radio-Thermal Generators. That's pretty much nuclear decay heat. A nuclear reactor works fundamentally different, by exploiting a chain reaction. Simply put: when uranium captures a neutron, it decays and in the process produces 3 more neutrons which can be captured again (I'm ignoring things like neutron energies here, details).

        As you can see from nuclear weapons, there are two main issues here. First off, you want to keep this reaction un

        • There's nothing misleading in what I wrote. He asked about those, so I answered those.

          On the submarine reactors, I would have preferred to only give the weight on the reactor portion, but I couldn't find any numbers. It's almost like the DoD doesn't want you to know. :) I have seen pictures of decommissioned submarine reactor compartments. They just slice out the whole compartment and bury it.

          I'm sure they could make something a bit more portable, but chasing down test or theoretical reactors that w

      • by rayd75 ( 258138 )

        Nuclear satellites and probes use tiny reactors only capable of watts of output.

        THAT would be cool. The reality is a little more boring but a lot more safe and practical. Radioisotope thermoelectric generators are nothing more than a high-efficiency version of those pots that charge a cell phone from the heat of your camp fire. They use what most here would recognize as Peltier coolers, though optimized for operation in reverse. (generating electricity from a heat differential.) There is no nuclear reaction taking place, only the natural decay of radioisotopes. And that, only for a fai

      • Make the reactor is pieces small enough to launch. Refuel it a bit at a time just like civilian reactors do on earth. build a smaller reactor, as you only need a few megawatts of power. The cooling of the reactor will be an issue, no oceans/lakes or viable air cooling.
      • by fizzup ( 788545 )

        Including water, a SLOWPOKE reactor weighs on the order of 100 tonnes. If you could harvest water on Mars, you could probably land one on the surface with existing vehicles.

        Even with water, it could be in play to get that reactor to low earth orbit in one launch and later attach a transfer vehicle in orbit. That reactor is in the ~1 MWe range.

    • by mbone ( 558574 )

      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.

      The SNAP 10A [wikipedia.org] reactor weighed 300 kg, was flown in 1965, and produced 30 kW of heat power. In the Apollo days, there were serious plans to power lunar bases through the long lunar night with CANDLE [blogspot.com] type reactors, also known as traveling wave reactors, which have no moving parts and could be just stuck in a hole in the ground to provide 10's or 100's of kW for decades. These reactors are not particularly "hot" before they are turned on, and shouldn't produce an unusual launch risk (as they would not be launch

    • Wouldn't work. Those Little Green Martians would all say "Not In My Back Martian Yard".

      This CO2 turbine idea sounds interesting. It's too bad that we don't have enough CO2 on Earth. Otherwise, we could switch from fossil fuels to CO2 turbines, and stop global warming!

      Maybe.

    • Originally, it was intended for the moon. And at 10 MWe, 30 year lifetime without refueling, and less than 20 tonnes, it is ideal.
  • ... should be great if your heat sink temperature is well below zero degrees (you can pick Celsius or Fahrenheit here).
  • A few criticisms (Score:5, Insightful)

    by Anonymous Coward on Wednesday March 11, 2015 @06:28AM (#49232095)

    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.

    • by Viol8 ( 599362 )

      "your rotor velocity is then limited to the gas velocity,"

      Well duh. There's no combustion happening inside the turbine so of course its going to be limited by the velocity of gas flowing into it.

      • by Anonymous Coward

        "your rotor velocity is then limited to the gas velocity,"

        Well duh. There's no combustion happening inside the turbine so of course its going to be limited by the velocity of gas flowing into it.

        Of course the amount of energy (hence velocity) in the working fluid places a hard limit on the amount of energy you can extract - but that wasn't my point.
        What I was saying is that using only viscous forces to transfer momentum from the working fluid to the turbine is a poor way of doing it.
        For a simple friction driven turbine, the tip-speed ratio can't be more than 1. For a properly designed turbine, it can be considerably greater than 1: http://en.wikipedia.org/wiki/Tip-speed_ratio
        For the same reason, a

      • by dj245 ( 732906 )

        "your rotor velocity is then limited to the gas velocity,"

        Well duh. There's no combustion happening inside the turbine so of course its going to be limited by the velocity of gas flowing into it.

        As you approach a 1:1 ratio of tangential velocity and gas velocity, the efficiency falls off dramatically. At a 1:1 velocity ratio, that turbine stage is 0% efficient and not helping at all- the gas is no longer pushing it. For the most efficient design the tangential rotor velocity should be limited to 50-75% of the gas velocity.

  • by Lumpy ( 12016 ) on Wednesday March 11, 2015 @06:40AM (#49232127) Homepage

    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.

  • 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.
    • Mars gravity is close enough that it shouldn't have any health or safety risks, and most of our technology should still work correctly.
    • What method works in space but wouldn't work on mars? You can straight line accelerate but that takes unrealistic energy in either case. Centrifuges would work just fine in either case, on mars you would simply have your 'down' located at some angle to Martian gravity. Given the thin atmosphere on Mars you wouldn't need very much energy to overcome the drag.
      I'm not aware of any studies of long term 1/3 gravity on humans. Most are for the microgravity you encounter in orbit. There may or may not be

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