Energy density is a term used for the amount of
energy stored in a given system or region of space per unit
volume, or per unit
mass, depending on the context. The latter is more formally known as
specific energy. In some cases it is obvious from context which quantity is most useful: for example, in
rocketry, energy per unit mass is the most important parameter, but when studying pressurized gas or
magnetohydrodynamics the energy per unit volume is more appropriate. In a few applications (comparing, for example, the effectiveness of
hydrogen fuel to
gasoline) both figures are appropriate and should be called out explicitly. (Hydrogen has a higher energy density per unit mass than does gasoline, but a much lower energy density per unit volume).
Energy per unit volume has the same physical units as
pressure, and in many circumstances is an exact
synonym: for example, the energy density of the magnetic field may be expressed as (and behaves as) a physical pressure, and the energy required to compress a gas may be determined by multiplying the pressure of the compressed gas by its change in volume.
Energy density in energy storage and in fuel and in
Selected Energy Densities Plot
In
energy storage application the energy density relates the
mass of an energy store to its stored energy. The higher the energy density, the more energy may be stored or transported for the same amount of mass. In the context of
fuel selection, that energy density of a fuel is also called the
specific energy of that fuel, though in general an
engine using that fuel will yield less energy due to
inefficiencies and
thermodynamic considerations—hence the
specific fuel consumption of an engine will be greater than the
reciprocal of the specific energy of the fuel.
The highest density sources of energy are
fusion and
fission. Fusion includes energy from the sun which will be available for billions of years (in the form of
sunlight) but humans have not learned to make our own sustained fusion power sources. Fission of U-238 in
nuclear power plants will be available for billions of years because of the vast supply of the element on earth.
Coal and
petroleum are the current primary energy sources in the U.S. but have a much lower energy density. Burning local
biomass fuels supplies household energy needs (
cooking fires,
oil lamps, etc.) worldwide.
Energy density (how much energy you can carry) does not tell you about
energy conversion efficiency (net output per input) or
embodied energy (what the energy output costs to provide, as
harvesting,
refining, distributing, and dealing with
pollution all use energy). Like any process occurring on a large scale, intensive energy use creates environmental impacts: for example,
global warming,
nuclear waste storage, and
deforestation are a few of the consequences of supplying our growing energy demands from fossil fuels, nuclear fission, or biomass.
By dividing by 3.6 the figures for megajoules per kilogram can be converted to kilowatt-hours per kilogram. Unfortunately, the useful energy available by extraction from an energy store is always less than the energy put into the energy store, as explained by the
laws of thermodynamics. No single energy storage method boasts the best in
specific power,
specific energy, and energy density.
Peukert's Law describes how the amount of energy we get out depends how quickly we pull it out.
Gravimetric and volumetric energy density of some fuels and storage technologies (modified from the
Gasoline article):
Note: Some values may not be precise because of
isomers or other irregularities. See
Heating value for a comprehensive table of specific energies of important fuels.
True energy densities
This table gives the energy density of a complete system, including all required external components, such as oxidisers or heat sources. One
MJ ≈ 278
Wh ≈ 0.32
HPh.
Energy densities ignoring external components
This table lists energy densities of systems that require external components, such as oxidisers or a heat sink or source. These figures do not take into account the mass and volume of the required components as they are assumed to be freely available and present in the atmosphere. Such systems cannot be compared with self-contained systems.
Energy density of electric and magnetic fields
Electric and
magnetic fields store energy. In a vacuum, the (volumetric) energy density (in SI units) is given by
,
where
E is the
electric field and
B is the
magnetic field. In the context of
magnetohydrodynamics, the physics of conductive fluids, the magnetic energy density behaves like an additional
pressure that adds to the
gas pressure of a
plasma.
In normal (linear) substances, the energy density (in SI units) is
,
where
D is the
electric displacement field and
H is the
magnetizing field.
Energy density of empty space
In
physics, "
vacuum energy" or "
zero-point energy" is the volumetric energy density of empty space. More recent developments have expounded on the concept of energy in empty space.
Modern physics is commonly classified into two fundamental theories:
quantum field theory and
general relativity. Quantum field theory takes
quantum mechanics and
special relativity into account, and it's a theory of all the forces and particles except
gravity. General relativity is a theory of gravity, but it is incompatible with quantum mechanics. Currently these two theories have not yet been reconciled into one unified description, though research into "
quantum gravity" seeks to bridge this divide.
In
general relativity, the
cosmological constant is proportional to the energy density of empty space, and can be measured by the curvature of space.
Quantum field theory considers the vacuum ground state not to be completely empty, but to consist of a seething mass of
virtual particles and
fields. These fields are quantified as probabilities—that is, the likelihood of manifestation based on conditions. Since these fields do not have a permanent existence, they are called vacuum fluctuations. In the
Casimir effect, two metal plates can cause a change in the vacuum energy density between them which generates a measurable force.
Some believe that vacuum energy might be the "
dark energy" (also called
Quintessence) associated with the cosmological constant in general relativity, thought to be similar to a negative force of gravity (or
antigravity). Observations that the expanding universe appears to be accelerating seem to support the
cosmic inflation theory—first proposed by
Alan Guth in 1981—in which the nascent universe passed through a phase of exponential expansion driven by a negative vacuum energy density (positive vacuum pressure).
Energy density of food
Energy density is the amount of energy (
kilojoules or
calories) per amount of food, with food amount being measured in grams or milliliters of food. Energy density is thus expressed in cal/g, kcal/g, J/g, kJ/g, cal/mL, kcal/mL, J/mL, or kJ/mL. What is popularly referred to as the number of "Calories" in a portion of food (invariably spelled with a capital C to show that it is the food Calorie) is therefore the number of
kilocalories (
thousands of calories with the small c) in the portion.
Energy density measures the energy released when the food is metabolised by a healthy organism when it ingests the food (see
food energy for calculation) and the food is
metabolized with oxygen, into waste products such as
carbon dioxide and water.
Carbohydrates,
fats, and
proteins are the only sources of energy for an individual abstaining from
alcohol, and they make up ninety percent of the dry weight of food. Therefore,
water content is the most important factor in energy density. Carbohydrates and proteins provide four calories per gram, whereas fat provides nine calories per gram, 2.25 times as much energy. Foods that derive most of their energy from fat have a much higher energy density than those that derive most of their energy from carbohydrates or proteins, even if the water content is the same. Nutrients with a lower absorption, such as
fiber or
sugar alcohols, lower the energy density of foods as well. A moderate energy density would be 1.6 to 3 calories per gram; salmon, lean meat, and bread would fall in this category. High-energy foods would have more than three calories per gram and include crackers, cheese, dark chocolate, and peanuts. The energy density of a food can be determined from the label by dividing the calories or joules by the weight in grams.
Miscellaneous
- Potential energy with respect to gravity, close to earth, per unit mass: ca. 9.8 h J/kg, with h the height in m.
See also
External references
Zero point energy
- Eric Weisstein's world of physics: energy density
- Baez physics: Is there a nonzero cosmological constant?
- Introductory review of cosmic inflation
- An exposition to inflationary cosmology
Density data
- "Aircraft Fuels." Energy, Technology and the Environment Ed. Attilio Bisio. Vol. 1. New York: John Wiley and Sons, Inc., 1995. 257–259
- “” - Dr. James J. Eberhardt - Energy Efficiency and Renewable Energy, U.S. Department of Energy - 2002 Diesel Engine Emissions Reduction (DEER) Workshop San Diego, California - August 25 - 29, 2002
Energy storage
Books
- The Inflationary Universe: The Quest for a New Theory of Cosmic Origins by Alan H. Guth (1998) ISBN 0-201-32840-2
- Cosmological Inflation and Large-Scale Structure by Andrew R. Liddle, David H. Lyth (2000) ISBN 0-521-57598-2
- Richard Becker, "Electromagnetic Fields and Interactions", Dover Publications Inc., 1964
Footnotes
Category:Fundamental physics conceptsCategory:Energy storageCategory:DensityCategory:Energy densityar:كثافة الطاقةde:Energiedichteel:Θερμική αξίαfr:Densité d'énergieit:Densità energeticanl:Energiedichtheidpl:Gęstość energiipt:Densidade de energiasl:Gostota energijskega tokafi:Energiatiheyszh:能量密度
