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'''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]].<ref> http://physics.nist.gov/Pubs/SP811/sec04.html </ref>{{Clarify |date=October 2008}}<!-- more clarification needed, maybe in a footnote as to what is actually meant by "energy stored" here. Some mention of how the energy is released, eg. unbinding an atom, combusting a fuel with an oxidant, etc. --> 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 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]].<ref> http://physics.nist.gov/Pubs/SP811/sec04.html </ref>{{Clarify |date=October 2008}}<!-- more clarification needed, maybe in a footnote as to what is actually meant by "energy stored" here. Some mention of how the energy is released, eg. unbinding an atom, combusting a fuel with an oxidant, etc. --> 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 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.

Revision as of 10:39, 27 October 2009

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.[1][clarification needed] 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

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.[clarification needed]

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.[2] Coal and petroleum are the current primary energy sources in the U.S. [citation needed] 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 Table
Storage type Energy density by mass (MJ/kg) Energy density by volume (MJ/L) Peak recovery efficiency % Practical recovery efficiency %
Mass-energy equivalence[3] 89,876,000,000
Nuclear fusion of hydrogen (energy from the sun) 645,000,000
Deuterium-Tritium fusion 337,000,000
Nuclear fission (of 100% U-235) (Used in nuclear bombs) 88,250,000 1,500,000,000
Natural uranium (99.3% U-238, 0.7% U-235) in fast breeder reactor[4] 24,000,000 50%
Pu-238 a-decay 15,500,000
Enriched uranium (3.5% U235) in light water reactor 3,456,000 30%
Hf-178m2 isomer 1,326,000 17,649,060
Natural uranium (0.7% U235) in light water reactor 443,000 30%
Ta-180m isomer 41,340 689,964
Specific orbital energy of Low Earth orbit (approximate) 33
Dinitroacetylene explosive - computed[citation needed] 9.8
Octanitrocubane explosive - computed[citation needed] 8.5 17
Tetranitrotetrahedrane explosive - computed[citation needed] 8.3
Heptanitrocubane explosive - computed[citation needed] 8.2
Sodium (reacted with chlorine)[citation needed] 7.0349
Tetranitrocubane explosive - computed[citation needed] 6.95
Ammonal (Al+NH4NO3 oxidizer)[citation needed] 6.9 12.7
Tetranitromethane + hydrazine bipropellant - computed[citation needed] 6.6
Hexanitrobenzene explosive - computed[citation needed] 6.5
Octogen explosive - computed[citation needed] 6.3
ANFO-ANNM[citation needed] 6.26
TNT [Kinney, G.F. (1985). Explosive shocks in air. Springer-Verlag. ISBN 3-540-15147-8. {{cite book}}: Cite has empty unknown parameter: |month= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)][citation needed] 4.610 6.92
Copper Thermite (Al + CuO as oxidizer)[citation needed] 4.13 20.9
Thermite (powder Al + Fe2O3 as oxidizer) 4.00 18.4
Hydrogen peroxide decomposition (as monopropellant) 2.7 3.8
battery, Lithium ion nanowire 2.54 29 95%[clarification needed][5]
battery, Lithium Thionyl Chloride (LiSOCl2)[6] 2.5
Water 220.64 bar, 373.8°C[citation needed][clarification needed] 1.968 0.708
Kinetic energy penetrator [clarification needed] 1.9 30
battery, Fluoride ion [citation needed] 1.7 2.8
battery, Hydrogen closed cycle H fuel cell[7] Template:Smn 1.62
Hydrazine(toxic) decomposition (as monopropellant) 1.6 1.6
Ammonium nitrate decomposition (as monopropellant) 1.4 2.5
Capacitor by EEStor (claimed prototype capacity)[8] 1.2 5.7 99% 99%
Thermal Energy Capacity of Molten Salt 1[citation needed] 98%[9]
Molecular spring approximate[citation needed] 1
battery, Sodium Sulfur .72[10] 1.23[citation needed] 85%[11]
battery, Lithium-manganese[12][13] 0.83-1.01 1.98-2.09
battery, Lithium ion[14][15] 0.46-0.72 0.83-0.9 95%[16]
battery, Lithium Sulphur[17] 1.26 1.26
battery (Sodium Nickel Chloride), High Temperature 0.56
battery, Silver-oxide[12] 0.47 1.8
Flywheel 0.36-0.5[18][19]
5.56 × 45 mm NATO bullet[clarification needed] 0.4 3.2
battery, Nickel metal hydride (NiMH), low power design as used in consumer batteries[20] 0.4 1.55
battery, Zinc-manganese (alkaline), long life design[12][14] 0.4-0.59 1.15-1.43
battery, Zinc Bromine flow (ZnBr)[21] 0.27
battery, Nickel metal hydride (NiMH), High Power design as used in cars[22] 0.250 0.493
battery, Nickel cadmium (NiCd)[14] 0.14 1.08 80%[16]
battery, Zinc-Carbon[14] 0.13 0.331
battery, Lead acid[14] 0.14 0.36
battery, Vanadium redox 0.09[citation needed] 0.1188 7070-75%
battery, Vanadium Bromide redox 0.18 0.252 80%–90%[23]
Capacitor Ultracapacitor 0.0199[24] 0.050[citation needed]
Capacitor Supercapacitor 0.01[citation needed] 80%–98.5%[25] 39%–70%[26]
Superconducting magnetic energy storage 0.008[27] >95%
Capacitor 0.002[28]
Spring power (clock spring), torsion spring 0.0003[29] 0.0006
Storage type Energy density by mass (MJ/kg) Energy density by volume (MJ/L) Peak recovery efficiency % Practical recovery efficiency %

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 Densities Table
Storage type Energy density by mass (MJ/kg) Energy density by volume (MJ/L) Peak recovery efficiency % Practical recovery efficiency %
Hydrogen, liquid (burned in air) 143 10.1
Hydrogen, compressed at 700 bar (burned in air)[30] 143 5.6
Hydrogen, gas (burned in air) 143 0.01079
Beryllium (toxic) (burned in air) 67.6 125.1
Lithium borohydride (burned in air) 65.2 43.4
Boron[31] (burned in air) 58.9 137.8[citation needed]
Methane (1.013bar, 15°C) (burned in air) 55.6 0.0378
Natural gas (burned in air) 53.6[32] 0.0364
CNG (NG compressed to 250 bar ~3600 psi) 53.6[32] 9
LPG propane (burned in air)[33] 49.6 25.3
LPG butane (burned in air)[33] 49.1 27.7
Gasoline[33] 46.4 34.2
Diesel fuel/residential heating oil (burned in air)[33] 46.2 37.3
Polyethylene plastic 46.3[34] 42.6
Polypropylene plastic 46.4[34] 41.7
Gasohol E10 (10% ethanol 90% gasoline by volume) 43.54 33.72
Gasohol E85 (85% ethanol 10% gasoline by volume) 33.1 25.65
Lithium (burned in air) 43.1 23.0
Jet A aviation fuel[35] / kerosene 42.8 33
Biodiesel oil (vegetable oil) 42.20 33
DMF (2,5-dimethylfuran)[clarification needed] 42[36] 37.8
Crude oil (according to the definition of ton of oil equivalent)[clarification needed] 46.3 37[32]
Polystyrene plastic 41.4[34] 43.5
Body fat metabolism 38 35 22[37]
Butanol 36.6 29.2
Graphite (burned in air) 32.7 72.9
coal, Anthracite[38] 32.5 72.4 36
Silicon (burned in air)[39] 32.2 75.1
Aluminum (burned in air) 31.0 83.8
Ethanol 30 24
Polyester plastic 26.0 [34] 35.6
Magnesium (burned in air) 24.7 43.0
coal, Bituminous[40] 24 20
PET plastic 23.5 (impure)[41]
Methanol 19.7 15.6
Hydrazine (toxic) combusted to N2+H2O 19.5 19.3
Liquid ammonia (combusted to N2+H2O) 18.6 11.5
PVC plastic (improper combustion toxic)[clarification needed] 18.0[34] 25.2
Peat briquette [42] 17.7
Sugars, carbohydrates & protein metabolism[citation needed] 17 26.2(dextrose) 2222[43]
coal, Lignite[citation needed] 14.0
Calcium (burned in air)[citation needed] 15.9 24.6
Glucose 15.55 23.9
Dry cowdung and cameldung 15.5[44]
Wood[citation needed] 6.0
Sodium (burned to wet sodium hydroxide) 13.3 12.8
Household waste 8.0[45]
Sod peat 12.8
Sodium (burned to dry sodium oxide) 9.1 8.8
Zinc (burned in air) 5.3 38.0
Teflon plastic (combustion toxic, but flame retardant) 5.1 11.2
iron (burned to iron(III) oxide) 5.2 40.68
iron (burned to iron(II) oxide) 4.9 38.2
battery, Zinc air[46] 1.59 6.02
Liquid nitrogen[clarification needed] 0.77[47] 0.62
Latent heat of fusion of Ice[citation needed] 0.335 0.335
Water at 100 m dam height[clarification needed] 0.001 0.001 8585-90%[citation needed]
Storage type Energy density by mass (MJ/kg) Energy density by volume (MJ/L) Peak recovery efficiency % Practical recovery efficiency %

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.[48] 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.[49] The energy density of a food can be determined from the label by dividing the calories or joules by the weight in grams.

Miscellaneous

See also

Template:EnergyPortal

External references

Zero point energy

  1. Eric Weisstein's world of physics: energy density[50]
  2. Baez physics: Is there a nonzero cosmological constant?[51]
  3. Introductory review of cosmic inflation[52]
  4. An exposition to inflationary cosmology[53]

Density data

  • ^ "Aircraft Fuels." Energy, Technology and the Environment Ed. Attilio Bisio. Vol. 1. New York: John Wiley and Sons, Inc., 1995. 257–259
  • “Fuels of the Future for Cars and Trucks” - 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

  1. ^ http://physics.nist.gov/Pubs/SP811/sec04.html
  2. ^ http://www-formal.stanford.edu/jmc/progress/cohen.html
  3. ^ The energy density per kg of anti-matter is twice this amount.
  4. ^ http://petroleum.berkeley.edu/patzek/ce24/Spring2003/heatvalues.htm
  5. ^ http://news-service.stanford.edu/news/2008/january9/nanowire-010908.html
  6. ^ http://www.nexergy.com/lithium-thionyl-chloride.htm
  7. ^ http://www.llnl.gov/str/Mitlit.html
  8. ^ http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=7033406.PN.&OS=PN/7033406&RS=PN/7033406
  9. ^ http://www.solar-reserve.com/technology.html
  10. ^ http://www.heraldextra.com/news/article_b0372fd8-3f3c-11de-ac77-001cc4c002e0.html
  11. ^ http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=5960185
  12. ^ a b c "ProCell Lithium battery chemistry". Duracell. Retrieved 2009-04-21. Cite error: The named reference "duracell-Ag2O" was defined multiple times with different content (see the help page).
  13. ^ "Properties of non-rechargeable lithium batteries". corrosion-doctors.org. Retrieved 2009-04-21.
  14. ^ a b c d e "Battery energy storage in various battery types". AllAboutBatteries.com. Retrieved 2009-04-21.
  15. ^ A typically available lithium ion cell with an Energy Density of 201 wh/kg [1]
  16. ^ a b Justin Lemire-Elmore (2004-04-13). "The Energy Cost of Electric and Human-Powered Bicycles" (PDF). p. 7. Retrieved 2009-02-26. Table 3: Input and Output Energy from Batteries
  17. ^ "Lithium Sulfur Rechargeable Battery Data Sheet" (PDF). Sion Power, Inc. 2005-09-28.
  18. ^ Storage Technology Report, ST6 Flywheel
  19. ^ "Next-gen Of Flywheel Energy Storage". Product Design & Development. Retrieved 2009-05-21.
  20. ^ Advanced Materials for Next Generation NiMH Batteries, Ovonic, 2008
  21. ^ "ZBB Energy Corp". Archived from the original on 2007-10-15. 75 to 85 watt-hours per kilogram
  22. ^ High Energy Metal Hydride Battery
  23. ^ http://www.vfuel.com.au/infosheet.pdf
  24. ^ http://maxwell.com/ultracapacitors/products/large-cell/bcap3000.asp
  25. ^ http://www2.fs.cvut.cz/web/fileadmin/documents/12241-BOZEK/publikace/2004/Sup-Cap-Energy-Storage.pdf
  26. ^ http://www2.fs.cvut.cz/web/fileadmin/documents/12241-BOZEK/publikace/2004/Sup-Cap-Energy-Storage.pdf
  27. ^ http://www.accel.de/pages/2_mj_superconducting_magnetic_energy_storage_smes.html
  28. ^ http://www.doc.ic.ac.uk/~mpj01/ise2grp/energystorage_report/node9.html
  29. ^ http://garagedoor.org/residential/torsion-springs.php
  30. ^ http://www.gov.pe.ca/photos/original/dev_solutions.pdf
  31. ^ http://www.eagle.ca/~gcowan/boron_blast.html#TOC
  32. ^ a b c Envestra Limited. Natural Gas. Retrieved 2008-10-05
  33. ^ a b c d IOR Energy. List of common conversion factors (Engineering conversion factors). Retrieved 2008-10-05
  34. ^ a b c d e http://www.aquafoam.com/papers/selection.pdf
  35. ^ http://hypertextbook.com/facts/2003/EvelynGofman.shtml
  36. ^ http://www.nature.com/nature/journal/v447/n7147/abs/nature05923.html
  37. ^ Justin Lemire-Elmore (2004-04-13). "The Energy Cost of Electric and Human-Powered Bicycles" (PDF). p. 5. Retrieved 2009-02-26. properly trained athlete will have efficiencies of 22 to 26%
  38. ^ Fisher, Juliya (2003). "Energy Density of Coal". The Physics Factbook. Retrieved 2006-08-25.
  39. ^ http://dbresearch.com/PROD/DBR_INTERNET_EN-PROD/PROD0000000000079095.pdf
  40. ^ http://hypertextbook.com/facts/2003/JuliyaFisher.shtml
  41. ^ http://www.payne-worldwide.com/documents/cms/Elite_bloc_msds.pdf
  42. ^ Bord na Mona, Peat for Energy
  43. ^ http://www.ebikes.ca/sustainability/Ebike_Energy.pdf
  44. ^ http://home.hccnet.nl/david.dirkse/math/energy.html
  45. ^ David E. Dirkse. energy buffers. "household waste 8..11 MJ/kg"
  46. ^ "Technical bulletin on Zinc-air batteries". Duracell. Retrieved 2009-04-21.
  47. ^ 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.
  48. ^ Carbohydrates, Proteins, and Fats: Overview of Nutrition. The Merck Manual.
  49. ^ The Okinawa Diet: Caloric Density Pyramid.
  50. ^ http://scienceworld.wolfram.com/physics/EnergyDensity.html
  51. ^ What's the Energy Density of the Vacuum?
  52. ^ http://arxiv.org/abs/hep-ph/0304257
  53. ^ http://arxiv.org/abs/astro-ph/0005003
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