Introducing energy density

Well, as everyone knows, here in the U.S. we drive our cars everywhere. When I sit down and think about it, what really amazes me is how powerful gasoline really is. (I am using "powerful" here in a loose sense, so don't pester me about the difference between energy and power...that'll be the subject of another post). I mean, I can go outside, get in a 4000 pound metal box, hurl myself (and said box) down the road at 60 miles per hour...and it only takes about half a cup of gasoline to travel one mile. Yep, you can move 4000+ pounds a distance of one mile, in a time of one minute, on half of a cup of gasoline. I don't know if that amazes you, but it amazes me. It makes me wonder about energy density.

Most of us, if we think about density at all, think of it simply as "how heavy is a given volume of stuff." And that's certainly correct, as far as it goes: that's mass density, and we usually talk about it in units of grams/cubic centimeter. Water has a density of 1 gram/cubic centimeter (abbreviated hereafter as g/cc)...things that float in water have a density of less than 1 g/cc; gases have densities much less than 1 g/cc. Very dense materials are metals like lead (11.3 g/cc) and the very dense metal osmium (22.6 g/cc). Oh, while I'm talking about it, sometimes people will talk about "specific gravity" rather than density. That's basically just the ratio of a material's density relative to that of water. Since water's density is 1 g/cc, the specific density of a material is, essentially, just the value of the material's density (when expressed in g/cc), although because it's a ratio of two quantities with the same units, the ratio is unitless. So it's correct to say "a specific gravity of 5.5," while it's also correct to say "a density of 5.5 g/cc." Back to energy density. You can generalize the concept of density from the commonly-understood mass density to really just mean generically "how much something per unit of something." So, for energy density, we're asking about how much energy per unit of volume (i.e. on a volumetric basis) a material has, or how much energy per unit of mass (i.e. on a mass basis) a material has.

So I thought it might be interesting to put a table together showing the energy density, on volume and mass bases, for some common fuels. Also shown is the mass density of the material. For many of the materials, the values actually fall in ranges, rather than having constant values. Natural gas, for example, has a varying composition...while it's always mostly methane plus other short-chain hydrocarbons, the exact composition varies from place to place, and with refining, I'd imagine.

Another point worth mentioning before getting into the data in the table is that energy density certainly doesn't tell the whole story in terms of suitability as a vehicle fuel. We'd also have to consider the efficiency of combustion for the fuel. This is something that I know varies, though I don't know much about this yet. Maybe I'll research this and post on it in the future.

Take a look at the table below! I know it's small to read like this, so I suggest right-clicking on the table and opening it up in a new window, where it'll be larger.



I've listed data for gasoline, and a number of other fuels, some of which are possible gasoline replacement fuels, and some of which are not, but are still fuels nonetheless in that they are used to provide heat or perform mechanical work. The unit abbreviations mean the following..."MJ" stands for megajoules, or million joules; kg is kilogram; and we already covered g/cc. In another post I'll talk about the units of energy, and the important but often misunderstood distinction between power and energy.

Note that I've also presented two columns of energy density data normalized to gasoline--that is to say, I defined the value for gasoline to be 100%, then compared the other fuels to gasoline on this basis. Just an easier way to compare the fuels to gasoline.

What did I find interesting about the table? Quite a few things.

The first is that ethanol doesn't stack up that well compared to gasoline. This is pretty well known, people are always talking about it on the web. It's only got about 2/3 the energy content of gasoline, on either a mass or volume basis. And ethanol has other concerns too, in terms of compatibility with distribution infrastructure.

Natural gas and propane look pretty decent compared to gasoline, at least as liquids. Natural gas used in vehicles, though, is compressed at around 200 bar, not liquid. It's a significantly poorer performer as CNG than LNG, in energy density terms. Liquid propane as a home-heating or cooking fuel, though, is reasonably energy dense.

Hydrogen is an interesting case. Note first of all that the energy/mass is the same for liquid or compressed...as it should be; the energy/mass of a fuel should be an intrinsic property. And, energy/mass of hydrogen is awesome compared to gasoline, 305%. But, if we look at the energy/volume, liquid hydrogen looks pretty poor, only 29% of the energy/volume of gasoline--and compressed hydrogen at 350 bar looks worse yet, 9% relative to gasoline. And as an automotive fuel, it gets even worse; you'd probably be using compressed hydrogen, and not only is that 9% as energy-dense (per volume) as gasoline, the storage system for the hydrogen would add very significant weight to the car. I think this tells us that significant strides need to be made to get decent range out of compressed hydrogen in internal combustion cars.

Regarding coal, I put down data for the main three classifications of coal. Be aware that the values for a type of coal actually range a bit, more or less average values are given. Anthracite is the good stuff, mostly gone now I think; bituminous is common and not as good; and lignite is the worst grade of coal. Note that the energy/mass of all three are less than gasoline, and decrease as you go from best to worst grade. Interestingly though, on a volume basis anthracite and lignite look better than gasoline, if you could cram it into your gas tank and get the car to run, because they're about three times as dense (mass density, this time) as gasoline. Bituminous has a fairly high energy/mass, but because it has a low mass density, the energy/volume is low.

And lastly, firewood. Apparently, all dry firewood has about the same energy content on a mass basis, around 16 MJ/kg...they differ in their volumetric energy density due to the difference in mass density. The data I gave is for oak.

Welcome to Energy Musing!

I believe the availability of energy is one of the critical issues facing the world today. I hope to put down my thoughts and share what knowledge I have of the issue. I hope to learn a lot along the way, and I hope you'll help. Welcome!