Here on Earth we are in the first part of the industrial grand transformation, and so know a bit about basic chemistry and physics, so we should be able to shed some light on fluid fuels, as they might exist in alien civilizations. If we can list the functions that energy in general must perform in an alien civilization, we might figure out what would be the likely class of fuel used there.
Energy functions in providing temperature control, transportation, communication, power for robotics, preparation of nutrition, surveillance, research, construction, recycling, maintenance and repairs, information transmission and storage, mining and resource extraction, and health. While there may be others, this wide a spectrum probably covers the fuel requirements. Electricity can be used for most of these, and it can be generated at a power plant or locally from a fluid fuel. Power plant generation implies some means of moving the energy from the power plant to the cities where the aliens live, which can mean direct transmission in a network of wiring, or transportation of fluid fuel.
The trade-off between wired electricity and locally generated electricity might be done on the basis of efficiency, or its equivalent, cost in whatever accounting system the alien planet uses. Efficiency depends on how much storage is needed for the energy, as electricity is harder to store than chemical energy. Power plants might work continuously except for maintenance breaks, seasonally if dependent on some seasonal source of power, diurnally if dependent on some solar source, or with another periodicity, such as that related to a large satellite producing tides. There could be a large random contribution or none at all.
Likewise, consumption can have temporal fluctuations in it as well, diurnal, seasonal, or related to other cycles of the civilization. Arrangements can be made to modulate the fluctuations in both production and consumption, but there will likely be some residual mismatch, implying storage of energy is necessary. Storage of hard-to-store-directly electrical energy can be done by transforming it into thermal energy, gravitational energy, mechanical energy or chemical energy, and each transformation, one-way or round trip, has an efficiency cost, which might be of the order of 50%, as opposed to 5%. This does not include the inefficiencies in producing or collecting the energy, which might also be other order of 50%, to use an order of magnitude. The overall result is that most of the energy produced or collected is turned into heat at the production site, as opposed to at the consumption site. This is inevitable in our Earth society or in any alien civilization.
For most of the functional uses of energy in an alien civilization, electrical energy works fine, whether transmitted over long distances or whether generated locally. The utility advantage breaks down in transportation, as energy must be stored and then transported on the vehicle, except for transportation corridors which have electric transmission systems built into them and can couple energy into the mobile vehicles, either kinetic energy or electrical energy.
Inside an arcology, transportation could easily be done electrically, and if the large majority of movement occurred there, there would be no need for fluid fuels for transportation on the inside. On the outside, if there was still some substantial amount of transportation taking place, either gaseous or liquid chemical fuel could be used. Hydrogen is one obvious candidate for a gaseous fuel, but the energy content per volume is low, meaning that tankage needs to be large, and consume considerable volume and weight. The trade-off is between the efficiency of converting hydrogen to some other, more dense fuel, such as an alkane, and carrying around the tank weight. If the whole point is simply to transport energy to an arcology from a distant power source, then the tradeoff is between the production of electricity and then a network of conductors to bring it to the arcology, and the production of hydrogen or a gaseous alkane and then a pipeline or vehicle ensemble to bring it there.
If the energy at the power plant is initially used to create electricity, then it would seem that there is little to be gained from converting to a fluid fuel and then transporting the fuel to the arcology, where it would be converted back to electricity. Our limited knowledge of electrical transmission versus pipelines would indicate there is no way to made the latter more efficient, given the round trip inefficiencies of conversion. Thus, the only use for fluid energy sources would possibly be in transportation outside of the arcologies, in situations where it would not be most efficient to have a wired corridor, such as between two arcologies.
Thus, storage of large quantities of energy, such as at the power plant, and mobile vehicles using significant energy on non-corridor transportation, would be the only two possible uses of fluid chemical energy sources in an alien planet, once they are past the stage where there was abundant fossil fuels. Would the energy storage at a power plant be done with hydrogen or with alkanes? Chemical energy is one of the most dense forms, and so one of these would be the likely choice. The tradeoff of tankage versus chemical form might be based on the duration of the storage, which relates to the amount of material needed to be stored. Diurnal storage might be with hydrogen, as the amount of tankage is not so large, and seasonal storage might be with alkanes, for the opposite reason.
This route of technology would mean that all three of the energy modes, electricity, hydrogen, and alkanes, would be available at a power plant site, and the other use of chemical energy sources, irregular transportation, might avail themselves of either fuel, both originating from the same source. Since alkanes have a significantly lower tankage cost, and have about the same available energy per mass, assuming atmospheric oxygen is used for combustion, then it is likely that an alien civilization would, perhaps unexpectedly, have vehicles analogous to those on Earth. This would be a strange case of interstellar convergence, when the physics, chemistry and other laws of nature push all civilizations in the same direction.
The last use of energy for transportation is for interplanetary travel, or simply to travel into orbit around the alien planet. Here, energy per mass and energy per volume are dominant. To keep tankage costs down on the heaviest stage, solid fuel propellants are used by most Earth launchers with liquid alkanes being the alternative, with liquid hydrogen and oxygen used for higher stages, where energy per mass counts more than energy per volume. This optimum has been well explored, and might be also expected to occur on exo-planets that do not barren areas on their planet where nuclear thrust options might be employed.
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