Can mass be viewed as dense energy

Energy density

Lexicon> Letter E> Energy Density

Definition: an amount of energy per volume or unit of mass

English: energy density

Categories: Energy Storage, Basic Terms, Physical Basics

Formula symbol: w

Unit: MJ / m3, MJ / kg, kWh / m3, kWh / kg

Author: Dr. Rüdiger Paschotta

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Original creation: 25.09.2010; last change: 15.05.2021


The term Energy density is not uniformly defined. The following ways of using it are common:

  • The volumetric energy density is a z. B. amount of energy stored per volume. It is to be given z. B. in joules per cubic meter (J / m3) or in megajoules per liter (MJ / l). This value can e.g. B. for different types of rechargeable batteries (accumulators) and limits the amount of energy that can be carried for an electric car for a given maximum space requirement of the batteries. Correspondingly, the volumetric energy density of fuels relates to their calorific value or calorific value per volume. Liquid and solid fuels achieve far higher values ​​here than gaseous fuels.
  • Sometimes the mass (also known as Weight called) more important than the volume. Then you look at them gravimetric energy density in J / kg. Gaseous fuels also achieve very high values ​​here. However, if the energy density of an entire storage facility (e.g. a pressurized gas cylinder and not just the natural gas enclosed in it) is taken into account, the energy density values ​​can again be significantly reduced.

Sometimes the term Energy density also very fuzzy instead Power density used. For example, the low power density of solar radiation (of the order of 1 kW / m2) the high space requirements of systems for the use of solar energy. In the case of rechargeable batteries, a high energy density does not necessarily mean a high power density, and vice versa.

Importance of energy density

In the case of accumulators for electric cars and other vehicles (more generally in the case of mobile energy storage devices), a high gravimetric energy density is very desirable, since this is the only way to achieve a good range without excessive weight. At the same time, this tends to reduce the energy required for production (gray energy) and the resulting environmental pollution.

In general, for energy carriers with a low energy density, the transport effort increases for an amount that corresponds to a certain amount of energy. Therefore z. For example, hard coal is more likely to be transported over long distances than lignite, and biowaste for a biogas plant must be recycled closer to the place of origin than biodiesel. The transport of biomass to be used energetically could be optimized by processes that produce decentralized bio-raw materials with higher energy density (e.g. a “bio-slurry”, “biosyncrude” in the bioliq concept).

Nuclear fuels like uranium have a much higher energy density than z. B. all fossil fuels. Therefore, the energy expenditure for their transport to the place of use is negligible. The power density that can be achieved with them exceeds that of chemical energy carriers even more. Both aspects together enable the incomparable effect of nuclear weapons (atomic bombs).

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See also: energy, power, power density
as well as other articles in the categories of energy storage, basic concepts, physical fundamentals