|In SI base units||kg·m−1s−2|
|U = E/V|
Energy density is the amount of energy stored in a given system or region of space per unit volume. Colloquially it may also be used for energy per unit mass, though the accurate term for this is specific energy. Often only the useful or extractable energy is measured, which is to say that inaccessible energy (such as rest mass energy) is ignored. In cosmological and other general relativistic contexts, however, the energy densities considered are those that correspond to the elements of the stress–energy tensor and therefore do include mass energy as well as energy densities associated with the pressures described in the next paragraph.
Energy per unit volume has the same physical units as pressure, and in many circumstances is a synonym: for example, the energy density of a magnetic field may be expressed as (and behaves as) a physical pressure, and the energy required to compress a compressed gas a little more may be determined by multiplying the difference between the gas pressure and the external pressure by the change in volume. In short, pressure is a measure of the enthalpy per unit volume of a system. A pressure gradient has the potential to perform work on the surroundings by converting enthalpy to work until equilibrium is reached.
Introduction to energy density
There are many different types of energy stored in materials, and it takes a particular type of reaction to release each type of energy. In order of the typical magnitude of the energy released, these types of reactions are: nuclear, chemical, electrochemical, and electrical.
Nuclear reactions are used by stars and nuclear power plants, both of which derive energy from the binding energy of nuclei. Chemical reactions are used by animals to derive energy from food, and by automobiles to derive energy from gasoline. Electrochemical reactions are used by most mobile devices such as laptop computers and mobile phones to release the energy from batteries.
Energy densities of common energy storage materials
The following is a list of the thermal energy densities (that is to say: the amount of heat energy that can be extracted) of commonly used or well-known energy storage materials; it doesn't include uncommon or experimental materials. Note that this list does not consider the mass of reactants commonly available such as the oxygen required for combustion or the energy efficiency in use. An extended version of this table is found at Energy density#Extended Reference Table.
|Storage material||Energy type||Specific energy
|Deuterium (in Fusion reactor)||Nuclear fusion||87,900,000||3,930,000||Experimental|
|Uranium (in breeder)||Nuclear fission||80,620,000||1,539,842,000||Electric power plants|
|Thorium (in breeder)||Nuclear fission||79,420,000||929,214,000||Experimental|
|Plutonium 238||Nuclear decay||2,239,000||43,277,631||RTGs|
|Hydrogen (compressed at 700 bar)||Chemical||142||9.17||Rocket engines|
|Methane or Liquefied natural gas(compressed)||Chemical||55.5||22.2||Cooking, home heating, electric power plants|
|Diesel||Chemical||48||35.8||Automotive engines, electric power plants|
|LPG (including Propane / Butane)||Chemical||46.4||26||Cooking, home heating, automotive engines, lighter fluid|
|Gasoline (petrol)||Chemical||46.4||34.2||Automotive engines, electric power plants|
|Jet fuel (Kerosene)||Chemical||42.8||37.4||Aircraft engines|
|Fat (animal/vegetable)||Chemical||37||34||Human and animal nutrition|
|Coal (anthracite or bituminous)||Chemical||~30||~38||Electric power plants, home heating|
|Carbohydrates (including sugars)||Chemical||17||Human and animal nutrition|
|Protein||Chemical||16.8||Human and animal nutrition|
|Wood||Chemical||16.2||13||Home heating, cooking|
|Lithium metal battery (Li-Po, Li-Hv)||Electrochemical||1.8||4.32||Portable electronic devices, flashlights, RC vehicles|
|Lithium-ion battery||Electrochemical||0.36–0.875||0.9–2.63||Automotive motors, portable electronic devices, flashlights|
|Flywheel||Mechanical||0.36–0.5||Power plants, Gyrobusses|
|Alkaline battery||Electrochemical||0.5||1.3||Portable electronic devices, flashlights|
|Nickel-metal hydride battery||Electrochemical||0.288||0.504–1.08||Portable electronic devices, flashlights|
|Lead-acid battery||Electrochemical||0.17||0.56||Automotive engine ignition|
|Supercapacitor (EDLC)||Electrical (electrostatic)||0.01–0.036||0.05–0.06||Electronic circuits|
|Electrolytic capacitor||Electrical (electrostatic)||0.00001–0.0002||0.00001–0.001||Electronic circuits|
|Storage device||Energy content
|Energy type||Typical mass
(width × height × depth in mm)
|Alkaline AA battery||9360||Electrochemical||24||14.2 × 50|
|Alkaline C battery||34,416||Electrochemical||65||26 × 46|
|NiMH AA battery||9072||Electrochemical||26||14.2 × 50|
|NiMH C battery||19,440||Electrochemical||82||26 × 46|
|Lithium-ion 18650 battery||28,800–46,800||Electrochemical||44–49||18 × 65|
|Potato Chip||41,900||Chemical||1.89||60 × 40 × 1|
|Ham and Cheese Sandwich||1,470,000||Chemical||145||100 × 100 × 28|
Energy density in energy storage and in fuel
In energy storage applications the energy density relates the mass of an energy store to the volume of the storage facility, e.g. the fuel tank. The higher the energy density of the fuel, the more energy may be stored or transported for the same amount of volume. The energy density of a fuel per unit mass is called the specific energy of that fuel. In general an engine using that fuel will generate less kinetic energy due to inefficiencies and thermodynamic considerations—hence the specific fuel consumption of an engine will always be greater than its rate of production of the kinetic energy of motion.
Nuclear energy sources
The greatest energy source by far is mass itself. This energy, E = mc2, where m = ρV, ρ is the mass per unit volume, V is the volume of the mass itself and c is the speed of light. This energy, however, can be released only by the processes of nuclear fission (0.1%), nuclear fusion (1%), or the annihilation of some or all of the matter in the volume V by matter-antimatter collisions (100%). Nuclear reactions cannot be realized by chemical reactions such as combustion. Although greater matter densities can be achieved, the density of a neutron star would approximate the most dense system capable of matter-antimatter annihilation possible. A black hole, although denser than a neutron star, does not have an equivalent anti-particle form, but would offer the same 100% conversion rate of mass to energy in the form of Hawking radiation. In the case of relatively small black holes (smaller than astronomical objects) the power output would be tremendous.
The highest density sources of energy aside from antimatter are fusion and fission. Fusion includes energy from the sun which will be available for billions of years (in the form of sunlight) but so far (2016), sustained fusion power production continues to be elusive. Power from fission of uranium and thorium in nuclear power plants will be available for many decades or even centuries because of the plentiful supply of the elements on earth, though the full potential of this source can only be realised through breeder reactors, which are, apart from the BN-600 reactor, not yet used commercially. Coal, gas, 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 differs from energy conversion efficiency (net output per input) or embodied energy (the energy output costs to provide, as harvesting, refining, distributing, and dealing with pollution all use energy). Large scale, intensive energy use impacts and is impacted by climate, waste storage, and environmental consequences.
No single energy storage method boasts the best in specific power, specific energy, and energy density. Peukert's Law describes how the amount of useful energy that can be obtained (for a lead-acid cell) depends on how quickly we pull it out. To maximize both specific energy and energy density, one can compute the specific energy density of a substance by multiplying the two values together, where the higher the number, the better the substance is at storing energy efficiently.
- 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.
- Note: Also it is important to realise that generally the density values for chemical fuels do not include the weight of oxygen required for combustion. This is typically two oxygen atoms per carbon atom, and one per two hydrogen atoms. The atomic weight of carbon and oxygen are similar, while hydrogen is much lighter than oxygen. Figures are presented this way for those fuels where in practice air would only be drawn in locally to the burner. This explains the apparently lower energy density of materials that already include their own oxidiser (such as gunpowder and TNT), where the mass of the oxidiser in effect adds dead weight, and absorbs some of the energy of combustion to dissociate and liberate oxygen to continue the reaction. This also explains some apparent anomalies, such as the energy density of a sandwich appearing to be higher than that of a stick of dynamite.
Energy density of electric and magnetic fields
where E is the electric field and B is the magnetic field. The solution will be (in SI units) in Joules per cubic metre. 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 and nondispersive) substances, the energy density (in SI units) is
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