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What transportation energy “currency” (methane, H2, electricity) would be most suitable for mobile machinery on the Martian surface?

Photovoltaic (PV) cells or nuclear energy will likely be the only energy sources on Mars, but both are low mobility. Electricity needs storage before it can be used for the transportation and heavy construction proposed for Mars colonization.

Direct storage of PV electricity in batteries or super capacitors is very attractive, but they have much lower energy density than chemical fuels which lowers the endurance of mobile machinery.

Current plans project large scale ISRU liquid Methane and Oxygen production for rocket propellant, so it is an obvious candidate for transportation and heavy machinery.

A practical problem with chemical fuels which use molecular Oxygen as the oxidizer is the high temperatures produced when they are mixed stoichiometrically. Methane has a 1 bar flame temperature of 1880 °C in air and 2750 °C in oxygen https://www.sciencedirect.com/topics/engineering/flame-temperature . A Carnot cycle engine (ICE, turbine) will have significantly higher pressures and temperatures, exceeding the melting point of many materials.

Could ICE engines (piston or turbine) be run with multistage combustion (like a Raptor) for net stoichiometric combustion with “conventional” temperatures? By running fuel/oxidizer mixture at non-1.0 equivalence, temperatures are significantly lower. The uncombusted fuel (or oxidizer) could be utilized in the next combustion step with the inverse equivalence ratio. https://en.wikipedia.org/wiki/Adiabatic_flame_temperature

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Another option would be an ECE such as a Sterling Engine. NASA has a isotope-powered free-piston Sterling alternator which has been running at full power continuously since 2003 https://www.spaceflightinsider.com/space-centers/glenn-research-center/it-keeps-going-and-going-stirling-engine-test-sets-long-duration-record-at-nasa-glenn . Since it outputs electricity, it would pair nicely with PV and battery storage.

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Perhaps the Holy Grail is a low-temperature Methane Fuel Cell. https://cen.acs.org/articles/93/i43/Best-Effort-Yet-Make-Direct.html

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    $\begingroup$ Depending on what gaseous waste products are generated during the production of oxygen, methane & whatever other "chemical factories" might produce on Mars, it may be possible to dilute the amount of oxygen by including some of the wastes gases thereby reducing the temperature of combustion within ICE, ECE or turbines. $\endgroup$
    – Fred
    Nov 3, 2021 at 1:25
  • $\begingroup$ Good point. I think the most readily available non-combustible waste gas is the engine's own exhaust (CO2 + H2O) . ICE's sometimes use exhaust gas recirculation for reduction of pre-ignition. $\endgroup$
    – Woody
    Nov 3, 2021 at 2:15
  • $\begingroup$ Topic of interest in Earthly CO2 capture is Oxycombustion for power generation. Intake air is processed to remove N2 so combustion takes place with only hydrocarbon and Oxygen. The exhaust is pure H2O and CO2, which makes CO2 capture dead easy. Conventional exhaust is mostly N2 which needs to be concentrated out of the hot, wet exhaust. It is easier to remove N2 from the cold, dry intake air. This process suffers from the same problem as this discussion: how to burn Methane and Oxygen without melting the engine. ieaghg.org/docs/General_Docs/Reports/… $\endgroup$
    – Woody
    Nov 3, 2021 at 2:31
  • $\begingroup$ As a second thought. A technological solution might be to periodically supply measured amounts of oxygen to the engines via a yet to be devised oxygen injection system. $\endgroup$
    – Fred
    Nov 3, 2021 at 9:46
  • $\begingroup$ Good point. The strategy would not apply to piston engines (a batch process) but could be used in a turbine to provide a continuous burn as the fuel gas passes through the power turbine. Research into the technology would have immediate application here on Earth for oxycombustion power generation. High speed generators are used on F-1 car turbocharger shafts without reduction gearing. A similar setup for the Mars Buggy would provide a light, compact, reliable power source. $\endgroup$
    – Woody
    Nov 3, 2021 at 14:30

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The best transportation energy "currency" is probably the kilowatt-hour. Power on Mars is likely to the bottle neck for all activities as it will be needed for so many things. For heat, light, communications, growing food, making propellants for return trips to Earth and for charging all vehicles or making the chemical fuels and oxidizers that they will use.

Solar and nuclear are the best current contenders, nuclear for Mars has political issues and/or technical readiness issues and solar has sizing issues and dust storm issues. None of these are show stoppers, but will probably lead to a slowly evolving mixture of power sources, storage and back up systems. The common denominator cost will probably be how much power was needed.

On this basis the most efficient option would be battery electric and that would probably be used where ever possible on relatively short range rovers and lighter utility vehicles. For very heavy duty or long range batteries are less suitable so might be backed up with other options.

Methanol is relatively easy to synthesize from carbon monoxide and hydrogen, is a very useful chemical intermediate in its own right and would be a good candidate for use in a fuel cell.

If combustion engines are required then methane and oxygen would probably be easiest to use as they are already needed, but some form of diluting gas would be useful to reduce combustion temperature. Compressed carbon dioxide from the Martian atmosphere might be used for this purpose.

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  • $\begingroup$ The question specifies "for mobile machinery". In other words, after the end of the extension chord. Solar and nuclear are low mobility for the surface machinery which resource extraction will depend on. $\endgroup$
    – Woody
    Nov 4, 2021 at 18:14
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    $\begingroup$ Yes I understand, the solar and nuclear points were a digression sorry. My point was aimed at what "currency" could be used to compare different means of transportation. Given a battery powered rover and a fuel cell powered rover I would ask how much power was taken to charge the battery and how much power was used to provide reactants for the fuel cell. The one that requires the least power is the cheapest. In some cases batteries are not practical so a more "expensive" solution must be found but the comparison is by kWh or perhaps kWh per km of travel or similar. $\endgroup$
    – Slarty
    Nov 5, 2021 at 1:19
  • $\begingroup$ Good point. From a physics point of view, there is a reversible conversion equivalence between different forms of energy. X btu of oil = Y kilowatt hours of electricity, and visa versa. In the real world, equivalency depends on use. Oil has zero kw-hr of electricity for running my laptop. Electricity has zero value for my vehicle without a storage device. By asking about energy “currency”, I was referring to universal utility as opposed to equivalency. Like the way people speak of the “hydrogen economy” rather than the “oil economy”. Battery uses electricity, fuel cell uses H2. $\endgroup$
    – Woody
    Nov 5, 2021 at 18:49
  • $\begingroup$ Body of question edited to clarify. Thank you. $\endgroup$
    – Woody
    Nov 5, 2021 at 18:51
  • $\begingroup$ Fair enough. I suggest that there need not be just one universal "currency". Electric vehicles for lighter duty shorter range and some other fuel for long range or very heavy duty applications. fuels cells using methanol or Methalox engine using CO2 diluent. $\endgroup$
    – Slarty
    Nov 5, 2021 at 21:26

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