Source: adapted from C. Ronneau (2004), Energie, pollution de l’air et developpement durable, Louvain-la-Neuve: Presses Universitaires de Louvain.
Different fuels have different energy density levels, which can be measured in terms of equivalent energy released through combustion. Energy density is the amount of energy that can be released by a given mass or volume of fuel. It can be measured in terms of gravimetric energy density (per unit of mass) or volumetric energy density (per unit of volume). Gravimetric energy density is relevant when comparing the energy efficiency of fuels. At the same time, volumetric energy density is relevant when comparing transportation modes as storage space (fuel tank) must be present to carry the fuel propelling a vehicle. The higher the energy density, the higher the fuel quality, which is inversely proportional to its chemical complexity. High-quality fuels are gases, while low-quality fuels are solids, with liquids in between. The highest energy density fuel is hydrogen, which is also the simplest chemical component in existence.
Gasoline, which is derived from refining crude oil, contains much more energy than coal (twice the lower grade bituminous) or wood (three times). Liquid natural gas (LNG) is almost entirely composed of methane, while natural gas has about 85% of its mass accounted for by methane. Jet A-1 is the standard fuel used by commercial jet planes and is mostly composed of kerosene and several additives (antifreeze, antioxidant and antistatic) since the fuel must meet rigorous specifications as it will be exposed to high altitudes and low temperatures. Conversely, Bunker C fuel, which is the primary fuel used for maritime shipping, can be considered one of the lowest quality fuels in liquid form but suitable for the vast ship engines.
Although methane and hydrogen both have higher energy density than gasoline, their gaseous form creates storage difficulties. Furthermore, hydrogen must be synthesized, which requires energy. Compared to a conversion rate of 100%, it would require 100 hours to capture the solar energy equivalent of 1 kg of gasoline on a surface of one square meter. One of the most efficient energy storage devices, the lithium battery, can only hold about the equivalent of 0.5 MJ per kilogram, underlining the challenge of developing electric vehicles.