Energy density

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Energy density is the amount of energy stored in a given system or region of space per unit volume or mass, though the latter is more accurately termed specific energy. Often only the useful or extractable energy is measured, which is to say that chemically inaccessible energy such as rest mass energy is ignored.[1]

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 a potential to perform work on the surroundings by converting enthalpy until equilibrium is reached.

Introduction to energy density[edit]

Energy can be stored in many different types of material, and there are several types of reactions that release energy. In order of the typical magnitude of the energy released, these types of reactions are: nuclear, chemical, electrochemical, and electrical.

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[edit]

The following is a list of the combustion energy densities 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.

The following unit conversions may be helpful when considering the data in the table: 1 MJ ≈ 0.28 kWh ≈ 0.37 HPh.

Storage materialEnergy typeSpecific energy (MJ/kg)Energy density (MJ/L)Direct uses
Uranium-235Nuclear fission83 140 0001 546 000 000Electric power plants (nuclear reactors)
Hydrogen (compressed at 70 MPa)Chemical1235.6Experimental automotive engines
Gasoline (petrol) / DieselChemical~46~36Automotive engines
Propane (including LPG)Chemical46.426Cooking, home heating, automotive engines
Fat (animal/vegetable)Chemical37Human/animal nutrition
CoalChemical24Electric power plants, home heating
Lithium-air battery (theoretical)Electrochemical18.7[2]Electronic devices, vehicles
Carbohydrates (including sugars)Chemical17Human/animal nutrition
ProteinChemical16.8Human/animal nutrition
WoodChemical16.2Heating, outdoor cooking
Lithium battery (non-rechargeable)Electrochemical1.84.32Portable electronic devices, flashlights
Lithium-ion batteryElectrochemical0.36[3]–0.8750.9–2.63Laptop computers, mobile devices, some modern electric vehicles
Alkaline batteryElectrochemical0.671.8Portable electronic devices, flashlights
Nickel-metal hydride batteryElectrochemical0.2880.504–1.08Portable electronic devices, flashlights
Lead-acid batteryElectrochemical0.170.34Automotive engine ignition
SupercapacitorElectrical0.018Electronic circuits
Electrostatic capacitorElectrical0.000036Electronic circuits
Energy capacities of common storage forms
Storage deviceEnergy typeEnergy content (MJ)Typical massW × H × D (mm)Uses
Automotive lead-acid batteryElectrochemical2.615 kg230 × 180 × 185Automotive starter motor and accessories
Sandwich (Subway 6 inch club)Chemical1.3[4]240 grams150 × ? × ?Human nutrition
Alkaline AA batteryElectrochemical0.015423 g14.5 × 50.5 × 14.5Portable electronic equipment, flashlights
Lithium-ion battery
(Nokia BL-5C)
Electrochemical0.012918.5 g54.2 × 33.8 × 5.8Mobile phones

Energy density in energy storage and in fuel[edit]

Selected energy densities plot

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.

The greatest energy source by far consists of 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 (.1%), nuclear fusion (1%),[citation needed] 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, doesn't have an equivalent anti-particle form.

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 (2011), sustained fusion power production continues to be elusive. Fission of uranium and thorium in nuclear power plants will be available for a long time due to the vast supply of the element on earth,[citation needed] 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.[5] Coal, gas, and petroleum are the current primary energy sources in the U.S.[6] 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 impacts the world. For example, climate change, nuclear waste storage, and deforestation may be some of the consequences of supplying our growing energy demands from carbohydrate fuels, nuclear fission, or biomass.

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.

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.
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 densities ignoring external components[edit]

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. These values may not be computed at the same reference conditions. Most of them seem to be higher heating value (HHV).

Energy densities of energy media
Storage typeSpecific energy (MJ/kg)Energy density (MJ/L)Peak recovery efficiency %Practical recovery efficiency %
Planck energy density8.99e104.633016e104
Hydrogen, liquid[7]141.868.491
Hydrogen, at 690 bar and 15°C[7]141.864.5
Hydrogen, gas[7]141.860.01005
Lithium borohydride65.243.4
Methane (1.013 bar, 15°C)55.60.0378
Natural gas53.6[10]0.0364
LNG (NG at −160°C)53.6[10]22.2
CNG (NG compressed to 250 bar/~3,600 psi)53.6[10]9
LPG propane[11]49.625.3
LPG butane[11]49.127.7
Gasoline (petrol)[11]46.434.2
Polypropylene plastic46.4[12]41.7
Polyethylene plastic46.3[12]42.6
Crude oil (according to the definition of ton of oil equivalent)46.337[10]
Diesel fuel/residential heating oil [11]46.237.3
100LL Avgas44.0[13]31.59
Gasohol E10 (10% ethanol 90% gasoline by volume)43.5433.18
Jet A aviation fuel[14]/kerosene42.833
Biodiesel oil (vegetable oil)42.2033
DMF (2,5-dimethylfuran)[clarification needed]42[15]37.8
Polystyrene plastic41.4[12]43.5
Body fat metabolism383522[16]
Gasohol E85 (85% ethanol 15% gasoline by volume)33.125.65
Coal, anthracite[17]32.572.4[dubious ]36
Silicon [18]32.275.1
Polyester plastic26.0 [12]35.6
Coal, bituminous[17]2420
PET plastic23.5 (impure)[19]
Hydrazine (toxic) combusted to N2+H2O19.519.3
Liquid ammonia (combusted to N2+H2O)18.611.5
PVC plastic (improper combustion toxic)[clarification needed]18.0[12]25.2
Peat briquette [21]17.7
Sugars, carbohydrates, and protein metabolism[citation needed]1726.2(dextrose)2222[22]
Calcium[citation needed]15.924.6
Dry cow dung and cameldung15.5[23]
Coal, lignite[citation needed]14.0
Sodium (burned to wet sodium hydroxide)13.312.8
Sod peat12.8
Sulfur (burned to sulfur dioxide)[24]9.2319.11
Sodium (burned to dry sodium oxide)9.18.8
Battery, lithium-air rechargeable9.0[25]
Household waste8.0[26]
Iron (burned to iron(III) oxide)5.240.68
Teflon plastic (combustion toxic, but flame retardant)5.111.2
Iron (burned to iron(II) oxide)4.938.2
Battery, zinc-air[27]1.596.02
Liquid nitrogen[clarification needed]0.77[28]0.62
Compressed air at 300 bar (potential energy)0.50.2>50%[citation needed]
Latent heat of fusion of ice[citation needed] (thermal)0.3350.335
Water at 100 m dam height (potential energy)0.0010.0018585-90%[citation needed]
Storage typeEnergy density by mass (MJ/kg)Energy density by volume (MJ/L)Peak recovery efficiency %Practical recovery efficiency %

Divide joule metre−3 with 109 to get MJ L−1.

Energy density of electric and magnetic fields[edit]

Electric and magnetic fields store energy. In a vacuum, the (volumetric) energy density (in SI units) is given by

 U = \frac{\varepsilon_0}{2} \mathbf{E}^2 + \frac{1}{2\mu_0} \mathbf{B}^2

where E is the electric field and B is the magnetic field. The solution will be 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) substances, the energy density (in SI units) is

 U = \frac{1}{2} ( \mathbf{E} \cdot \mathbf{D} + \mathbf{H} \cdot \mathbf{B} )

where D is the electric displacement field and H is the magnetizing field.

See also[edit]


  1. ^ "The Two Classes of SI Units and the SI Prefixes". NIST Guide to the SI. Retrieved 2012-01-25. 
  2. ^ "A review of high energy density lithium-air battery technology". Springer. 
  3. ^ "Overview of lithium ion batteries". Panasonic. Jan., 2007. Archived from the original on Nov 7, 2011. 
  4. ^ "Subway Nutritional Information". The Food Information Database. Retrieved 27 July 2013. 
  5. ^ "Facts from Cohen". 2007-01-26. Retrieved 2010-05-07. 
  6. ^ "U.S. Energy Information Administration (EIA) - Annual Energy Review". 2009-06-26. Archived from the original on 2010-05-06. Retrieved 2010-05-07. 
  7. ^ a b c Hydrogen properties Hydrogen Properties. Retrieved 2011-11-30.
  8. ^ Greenwood, Norman N.; Earnshaw, Alan (1997), Chemistry of the Elements (2nd ed) (page 164)
  9. ^ "Boron: A Better Energy Carrier than Hydrogen? (28 February 2009)". Retrieved 2010-05-07. 
  10. ^ a b c d Envestra Limited. Natural Gas. Retrieved 2008-10-05.
  11. ^ a b c d IOR Energy. List of common conversion factors (Engineering conversion factors). Retrieved 2008-10-05.
  12. ^ a b c d e Paul A. Kittle, Ph.D. "ALTERNATE DAILY COVER MATERIALS AND SUBTITLE D - THE SELECTION TECHNIQUE". Retrieved 2012-01-25. 
  13. ^ "537.PDF" (PDF). June 1993. Retrieved 2012-01-25. 
  14. ^ "Energy Density of Aviation Fuel". Retrieved 2010-05-07. 
  15. ^ Nature. "Production of dimethylfuran for liquid fuels from biomass-derived carbohydrates : Abstract". Nature. Retrieved 2010-05-07. 
  16. ^ Justin Lemire-Elmore (2004-04-13). "The Energy Cost of Electric and Human-Powered Bicycles". p. 5. Retrieved 2009-02-26. "properly trained athlete will have efficiencies of 22 to 26%" 
  17. ^ a b Fisher, Juliya (2003). "Energy Density of Coal". The Physics Factbook. Retrieved 2006-08-25. 
  18. ^ Silicon as an intermediary between renewable energy and hydrogen
  19. ^ "Elite_bloc.indd" (PDF). Retrieved 2010-05-07. 
  20. ^ "Biomass Energy Foundation: Fuel Densities". Archived from the original on 2010-01-10. Retrieved 2010-05-07. 
  21. ^ "Bord na Mona, Peat for Energy". Archived from the original on 2007-11-19. Retrieved 2012-01-25. 
  22. ^ Justin Lemire-Elmor (April 13, 2004). "The Energy Cost of Electric and Human-Powered Bicycle". Retrieved 2012-01-25. 
  23. ^ "energy buffers". Retrieved 2010-05-07. 
  24. ^ Anne Wignall and Terry Wales. Chemistry 12 Workbook, page 138. Pearson Education NZ ISBN 978-0-582-54974-6
  25. ^ Mitchell, Robert R.; Betar M. Gallant; Carl V. Thompson; Yang Shao-Horn (2011). "All-carbon-nanofiber electrodes for high-energy rechargeable Li–O2 batteries". Energy & Environmental Science 4: 2952–2958. doi:10.1039/C1EE01496J. 
  26. ^ David E. Dirkse. energy buffers. "household waste 8..11 MJ/kg"
  27. ^ "Technical bulletin on Zinc-air batteries". Duracell. Archived from the original on 2009-01-27. Retrieved 2009-04-21. 
  28. ^ C. Knowlen, A.T. Mattick, A.P. Bruckner and A. Hertzberg, "High Efficiency Conversion Systems for Liquid Nitrogen Automobiles", Society of Automotive Engineers Inc, 1988.

External references[edit]

Density data[edit]

Energy storage[edit]