Energitætheder

Energitætheder, or energy density, describes how much energy is stored in a material or system per unit of mass or per unit of volume. It is a central factor in evaluating energy storage options, affecting how far a vehicle can travel, how much equipment is carried, or how much energy a food provides. Energy density is typically discussed in two forms: gravimetric energy density (per mass, often expressed as MJ/kg or Wh/kg) and volumetric energy density (per volume, often expressed as MJ/L or Wh/L).

Gravimetric energy density measures how much energy a substance can release for each kilogram of it. Volumetric energy density measures how much energy fits into a given volume. The two can diverge significantly; a substance may store a lot of energy per kilogram but take up a large volume, or vice versa. This distinction is important in transport, portable electronics, and gridsized storage.

Common examples illustrate the contrast. Liquid fuels such as gasoline have high volumetric energy density (roughly 34 MJ/L) and high gravimetric density (about 46 MJ/kg), enabling long ranges in vehicles. Hydrogen has a very high gravimetric density (about 120–140 MJ/kg) but a relatively low volumetric density when stored as gas at ambient pressure, though compression or liquefaction increases its volumetric density. Batteries, notably lithium‑ion chemistries, offer moderate gravimetric energy density (roughly 0.5–1.0 MJ/kg, or 140–250 Wh/kg) and lower volumetric density than liquids. In nutrition, energy density describes calories per gram of food, typically ranging from about 0.5 to 9 kcal/g depending on composition.

Energy density is influenced by temperature, pressure, and state of charge, and it represents the amount of usable energy under practical conditions rather than a simple theoretical maximum.