Wood, diesel fuel, natural gas, and other fuels have played a vital role in human energy history by providing essential energy services like cooking, thermal comfort, and illumination. Changes over time in the types of fuel used by society are driven by technological, engineering, economic, political, and environmental forces. Energy density is among the most important factors.
The volumetric energy density of a fuel is the amount of energy (Btu, joules) stored per unit volume (gallon, liter) of a substance (gas, solid, liquid). The mass or gravimetric energy density of a fuel is the amount of energy stored per unit mass (ton, kilogram) of a substance.
Liquid fuels derived from oil occupy a uniquely advantageous position: a combination of high mass and volumetric energy density. Those attributes are the principal reasons why motor gasoline and diesel fuel replaced solid fuels such as wood and coal in applications such as transportation. Solid fuels require more space devoted to storage in a factory, ship, or locomotive. That raises the cost of solid fuels and leaves less room for machinery, people, or goods in the case of transportation.
In 1911, then First Lord of the Admiralty Winston Churchill led the rapid transformation of British navy vessels from coal to oil. The higher mass density of oil meant that boilers could be smaller and that ships could travel twice as far.1 The higher volumetric energy density of oil freed substantial storage space for personnel, munitions, and other cargo. Refueling a vessel with oil was faster and less arduous compared to coal. As Churchill observed, “the advantages conferred by liquid fuel were inestimable.”2
The British Navy story provides another important lesson: changes in fuel type go together with changes in the type of devices that convert fuel into useful energy services. In this case, the shift to oil went hand-in-hand with the shift to the internal combustion engine (ICE) that replaced the steam engine. ICEs have numerous advantages compared to the steam engine: lower weight-to-power ratio, faster start-up, greater fuel efficiency, smaller size and footprint, and lower maintenance requirements.
Energy density helps explain the end uses of different types of coal. Lignite has roughly 40 percent less energy per unit mass and volume compared to bituminous coal. It also has a higher moisture content. This makes lignite expensive ($/joule) to transport by train. In many countries, this limits lignite to use in mine-mouth electric generation facilities that avoid long-distance transport.
Elemental (gaseous) hydrogen has three times the mass density of gasoline and nearly 15 times the mass density of wood. So why isn’t hydrogen widely used in transportation? One reason is hydrogen’s extremely low volumetric energy density (about 1/3500 that of gasoline) which translates to expensive storage vessels and/or large cost to convert it to a liquid state with a higher volumetric energy density.
Note the position of enriched uranium fuel. The energy from the nuclear fission of uranium-235 is about 79 million MJ, which is more than two million times denser than any fossil or biomass fuel. The chemical energy of oil or wood is no match for the enormous binding energy that holds a nucleus that is released in a fission event.
1 Dahl, Erik J., “Naval Innovation. From Coal to Oil,” Joint Force Quarterly, Winter 2000-01, https://apps.dtic.mil/sti/pdfs/ADA524799.pdf
2 The Rt. Hon. Winston S. Churchill, “The World Crisis,” Charles Scribner’s Sons, 1923, produced and released by the Project Gutenberg, June 23, 2019, https://www.gutenberg.org/cache/epub/59794/pg59794-images.html