dnmun said:
how many moles of hydrogen and how many moles of CH4 fit in the same container?
how much energy is released by combustion of CH4 to CO2 and 2 H2O vs combustion of H2 to H2O only.
For high speed travel, it is frontal area that matters most. It doesn't matter, to some extent, how large the tank is, but how much frontal area it projects. Google "energy density of liquified hydrogen vs. LNG". There is a reason the space shuttle flies (flew) on liquid hydrogen and solid boosters and it has nothing to do with how "green" they might or might not have been.
LNG is a fuel source, but hydrogen does not exist as an element in any appreciable amount. You can gouge the earth for methane but you have to create hydrogen from another source of energy. Hydrogen is only as "green" as the technology used to create it. Creating hydrogen from reforming natural gas will always create CO2, so that process is as "green" as the CO2 it creates. It does not involve an appreciable use of coal-created electricity; it is fueled by the methane. Hydrogen created by electrolysis is not a cost-effective way to create it, although that point is not universally agreed on. You can read technical journal articles all day and still come away confused on what is the "greenest" technology for fueling transportation. At this point, all articles involve a good deal of speculation about future technologies. So anyone who tells you they have "the answer" to future transportation is full of it, in my opinion. You can always find another, equally competent, expert that disagrees with you. Not enough is known yet. There are several possible technologies but none is unanimously the clear winner. Then you get industries with a "dog in the fight" to chime in and the opinions really start flying. My advice; keep an open mind and seriously pursue multiple technologies. It is a known fact that we will deplete the reserves of petroleum to the point where recovery will be too expensive to pursue for general transportation needs. What is not known is when; it is a moving target. There is no transportation silver bullet, in the air, land, or sea. Unless you want to walk, bike, or sail everywhere (actually, that sounds good to me!)
EDIT: moles per volume of liquid H2 is around 0.034 mol/cm3, for LNG it is around 0.028 mol/cm3, if I calculated correctly. So liquid H2 has a slightly higher molar density. The volumetric energy density of liquid hydrogen is about 8.5 MJ/l and for LNG it is around 21 MJ/l. Since the molecular weight of hydrogen is so much smaller and the molar density is about the same, the energy mass density is high. So, energy per unit volume, LNG wins. Energy per unit mass, hydrogen wins. When you are flying, it is all about beating gravity and drag, so a light fuel in a tank with a low frontal area is the result. If you are driving a car, the volume of the car matters more, so burning hydrogen is not such a great idea. Use a fuel cell, however, and you are in a different part of the map (no max. efficiency of a heat engine business)
