Energy Density Definition

A kilogram of uranium fuel contains roughly 80 trillion joules of energy. A kilogram of gasoline contains about 46 million. A kilogram of lithium-ion battery stores about 0.9 million. Same mass, six orders of magnitude difference in stored energy. That gap governs which fuels work for which applications.

Energy density measures how much energy is packed into a given amount of material. It comes in two flavors. Gravimetric energy density (MJ/kg) measures energy per unit mass: critical for anything that moves (vehicles, aircraft, rockets). Volumetric energy density (MJ/L) measures energy per unit volume: critical for anything with limited space (fuel tanks, battery packs, urban storage).

A fuel can rank high on one metric and low on the other. Natural gas has good gravimetric density (about 54 MJ/kg) but poor volumetric density as a gas (0.036 MJ/L at atmospheric pressure), which is why it must be compressed or liquefied for transport. Diesel has moderate gravimetric density (45 MJ/kg) and excellent volumetric density (36 MJ/L), which is why it dominates long-haul trucking and shipping.

Worked Example

A delivery truck needs 500 MJ of energy for its daily route. Compare diesel fuel versus a lithium-ion battery pack.

• Calculate diesel mass needed. 500 MJ / 45 MJ/kg = 11.1 kg (about 13 liters, or 3.4 gallons).
• Calculate battery mass needed. 500 MJ / 0.9 MJ/kg = 556 kg.

Why is the battery 50 times heavier for the same energy?

Because lithium-ion batteries have roughly 1/50th the gravimetric energy density of diesel. The battery carries its "exhaust products" (discharged chemical states) with it; diesel vents combustion products as gas. This weight penalty is why battery-electric long-haul trucking remains difficult.

Energy density is the reason different energy applications use different fuels, and why "just electrify everything" encounters physics constraints, not just economic ones.

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