Hydrogen fuel cells

How much energy is needed to make each kilogram of gasoline?  This may sound like an unusual question, because we tend to think of gasoline as a source of energy.  When one kilogram of gasoline is burned it releases about 45 megajoules (MJ) of energy in the form of heat, that can then be converted into other forms of useful energy.  However, 9 MJ are needed to produce a kilogram of fuel.  Though this number is non-zero, the important point is that less energy goes into the fuel than is recovered from it.  This feature makes petroleum good as a fuel source, among other properties.

In a previous post, I wrote about the energy requirements of producing hydrogen.  Every kilogram of hydrogen contains roughly three times as much energy as a kilogram of petroleum—about 140 MJ.  Steam methane reformation requires 135 MJ per kilogram of hydrogen; this represents a net gain in energy, but a very small net gain.  Furthermore, it relies on the same non-renewable resources that it is meant to replace.  However, even if we rely on electrolysis to manufacture hydrogen, the fuel cells extract energy by performing the electrolysis reaction in the opposite direction.  That is to say, the energy recovered by fuel cells is the same as the energy used in manufacturing the fuel.  At best, this only allows electricity to be converted to a more portable form.

There are difficulties in making hydrogen portable, as well.  Because gasoline is relatively dense, it can carry a large amount of energy in a compact space.  Hydrogen pressurized to 5,000 psi, would occupy over eight times as much space as the equivalent amount of gasoline, at room temperature.  Instead, hydrogen is typically cooled to about −400 degrees Fahrenheit.  At this temperature, the same hydrogen would occupy a similar amount of space as gasoline, but this temperature would need to be maintained.  Gasoline can sit for years before being used, but hydrogen would likely need to be used shortly after it is brought to temperature.