A.3.1.1 Energy Content

Every fuel can liberate a fixed amount of energy when it reacts completely with oxygen. This energy content is measured experimentally and is quantified by a fuel's higher heating value (HHV) and lower heating value (LHV). The difference between the HHV and the LHV is the "heat of vaporization" and represents the amount of energy required to vaporize a liquid fuel into a gaseous fuel, as well as the energy used to convert water to steam. The higher and lower heating values of comparative fuels (at 25°C and 0.1 MPa) are indicated in Table A-2. Gaseous fuels are already vaporized so no energy is required to convert them to a gas. The water that results from both a combustive reaction and the electrochemical reaction within a fuel cell occurs as steam therefore the lower heating value represents the amount of energy available to do external work.

Both the higher and lower heating values denote the amount of energy (in Joules) for a given weight of fuel (in kilograms). Hydrogen has the highest energy-to-weight ratio of any fuel since hydrogen is the lightest element and has no heavy carbon atoms. It is for this reason that hydrogen has been used extensively in the space program where weight is crucial. Specifically, the amount of energy liberated during the reaction of hydrogen, on a mass basis, is about 2.5 times the heat of combustion of common hydrocarbon fuels (gasoline, diesel, methane, propane, etc.). Therefore, for a given load duty, the mass of hydrogen required is only about a third of the mass of hydrocarbon fuel needed.

The high energy content of hydrogen also implies that the energy of a hydrogen gas explosion is about 2.5 times of that of common hydrocarbon fuels [46]. Thus, on an equal mass basis, hydrogen gas explosions are more destructive and carry further. However, the duration of a deflagration tends to be inversely proportional to the combustive energy, so that hydrogen fires subside much more quickly than hydrocarbon fires.

Table A-2 Energy densities of comparative fuels [46, 227]


Energy Density (LHV, in kJ/m3 )




Gas at 0.1 MPa and 15°C


Gas at 20 MPa and 15°C


Gas at 69 MPa and 15°C





Gas at 0.1 MPa and 15°C


Gas at 20 MPa and 15°C





Gas at 0.1 MPa and 15°C






A.3.1.2 Energy Density

Whereas the energy content denotes the amount of energy for a given weight of fuel, the energy density denotes the amount of energy (in Joules) for a given volume (in m3) of fuel. Thus, the energy density is the product of the energy content (LHV) and the density of a given fuel. The energy density is really a measure of how compactly hydrogen atoms are packed in a fuel. It follows that hydro-carbons of increasing complexity (with more and more hydrogen atoms per molecule) have an increasing energy density. At the same time, hydrocarbons of increasing complexity have more and more carbon atoms in each molecule so that these fuels are heavier and heavier in absolute terms. On this basis, hydrogen's energy density is poor (since it has such low density) although its energy to weight ratio is the best of all fuels (because it is so light). The energy density of comparative fuels, based on the LHV, is indicated in Table A-2.

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