Energy Density (Fuel)

Energy density is the usable energy a fuel stores per unit mass (gravimetric, MJ/kg) or per unit volume (volumetric, MJ/L), and it sets how much tank space and weight a given range demands. Marine fuels are usually rated by lower heating value: VLSFO and MGO sit near 40 to 43 MJ/kg, methanol about 19.9, ammonia about 18.6, and liquid hydrogen about 120 MJ/kg by mass but only about 8.5 MJ/L by volume. Low-density alternative fuels need two to four times the tank volume of oil for equal range, which cuts cargo capacity and drives bunkering frequency. Volumetric density usually governs ship design more than gravimetric.