# Energy Density Comparison

Updated on July 20, 2011

Almost always, people misunderstand the notions of energy density. For example, people often ask me why cars still use gasoline and not lithium ion batteries for example. The answer to this is straightforward. Batteries simply don't have the energy density of gasoline. They're not even close. To carry enough batteries on board the vehicle, to obtain the same range of driving as a conventional gasoline car, would take up too much space and weigh so much that it would be completely impractical, let alone the extreme cost of tens of thousands of dollars for the initial investment.

## Units of Measurement

There are in fact different ways of calculating energy density, which often is a source of confusion. The units of measurement must be paid attention to. Joules per kilogram (J/kg), is a fairly common unit. But so are Joules / Liter and KWh / gallon. Always look for the units. If they're not explicitly defined, look for reason to believe that a certain system of units is preferred.

## The Energy in a Pound of Human Fat

A pound of fat is roughly 3500 Calories. Dieters and exercisers often know this relationship. For every 3500 Calories burned from running (about 23 miles), one pound of fat is consumed. Where does the fat go? Through your sweat? Think again.

The act of running increases your metabolism. The human body is like a car, to perform faster requires more fuel. To burn more fuel requires more air intake (oxygen). Here's a hint. What happens to the gasoline after it's burned? It is exhausted in the form of carbon dioxide, water, and a whole mess of other chemicals out of the exhaust pipe. And what is the human analog of an exhaust pipe?.... think higher. It's your mouth. Every breath you exhale takes a bit of burned fat vapors along with it. This is why you breathe harder when you run faster.

The energy density of a pound of fat can be stated as 3,500 Calories / Pound. However it would be more useful to engineers in the SI form J / kg, which translates to 14,664,000 J / kg, or 14.664 MJ / kg (that's mega Joules).

Let's compare this with gasoline, which is 44.4 MJ / kg, meaning gasoline is about 3 times more energy dense, per mass, than human fat. That's not bad! Plus fat serves other functions like keeping us warm and providing structure to cells, and all kinds of stuff. Gasoline would not serve us well here, but... if we COULD drink gasoline instead of eating food, we'd only have to drink about a fifth-gallon a day (assuming a 2,000 Calorie per day diet). This would be far cheaper than food, yet we complain about gasoline prices.

So, is there anything better than gasoline?

## Comparison of Energy Densities

Material
MJ / kg

Clock spring (steel)
0.0003

Capacitor
0.002

0.14

AA battery
0.13

Battery, nickel metal hydride (NiMH)
0.25

Flywheel (rotating mass)
0.36 - 0.5

Lithium Ion Battery
0.46 - 0.72

Molecular spring (torsion of molecules)
1

Ammonium nitrate (as monopropellant)
1.4

Lithium Ion Nanowire Battery
2.54

TNT
4.61

Nitroglycerine
6.38

Mass in low earth orbit
33

Pure uranium
88250000

Deuterium-tritium fusion
576000000

Antimatter
89,876,000,000

For convenience, all of the following energy densities are expressed in MJ / kg.

## Analysis

According to this chart, taken and reduced from Wikipedia, gasoline is more energy dense than TNT and nitroglycerine, yet we think of these substances as being very powerful. Well, that's true, they are powerful. They can put out all of this energy is extremely short amounts of times, therefore they make better explosives than gasoline does.

Only the nuclear reactions can provide unimaginably high amounts of energy. A uranium fission reactor produces 2005682 times more energy per kilogram than gasoline. This means a gas tank filled with uranium would power your car for about 601,704,545 miles or about 1,000 trips to the moon and back.

Antimatter, on the other hand, a thousand times farther. It's the kind of energy necessary to power ion engines to reach nearly the speed of light.

Since antimatter is very rare in nature, and it takes energy to make it in the laboratory, it's only function to humanity will serve as the ultimate energy storage device. Not as a fuel it itself.

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• Dan R Sandberg

5 years ago from San Francisco, California

Hi Ben,

My question comes from wondering what the highest possibly energy storage mechanism (that is rechargeable from electricity) for cars or electronic devices would be assuming (unreasonably) that biological fat is the best that can be done.

Sounds like you're saying that fat yields about 3 MJ/kg after muscle inefficiency is considered. If so, this is still 3 times higher than Lithium-Ion batteries can do, so there's still lots of headroom for battery improvement.

• Ben Goldberg

5 years ago

Mr Sandberg:

According to wikipedia, human muscles have an efficiency between 18% and 26%.

In contrast, automobiles use internal combustion engines, where are typically about 30% efficient.

Thus, humans are no worse at converting body fat into mechanical energy than cars are at converting gasoline into mechanical energy.

As for lithium-ion batteries (which are 80 to 90 percent efficient), don't forget that the energy needed to charge them in the first place doesn't come out nowhere.

• Dan R Sandberg

6 years ago from San Francisco, California

Great article! I've been looking for a comparison of human fat with lithium-ion batteries for a while. I fear the energy density of fat is measured in a way that leads to an inaccurate understand. Imagining the fat is "burnt" leads to fat having a much higher energy content than if the energy is biologically converted to energy through muscles. Is there a figure for the energy density of FAT ("usable") compared to the energy density of lithium-ion (where nearly 100% is usable)?

• jrsearam

7 years ago from San Juan, PR

Interesting, straightforward and refreshingly clear... Thanks for the info. JR

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