Energy Density and Batteries versus Fossil Fuels

There's a simple reason mankind uses gasoline and diesel for moving so many of its creations around: there are very few materials that can match the energy stored in fossil fuels. Case in point: energy density, a measure of how much energy is in one kilogram of material. The energy density of these liquid fuels is astronomical, which is very important in something like a car, where you have to carry the weight of your fuel with you—and even more important in a plane, where you have to carry the weight of your fuel with you and also 30,000 feet up into the air.

Diesel fuel has almost 45.5 MJ of energy per kilogram inside it—that's 45,500,000 J, which is enough energy to accelerate a large dog to Mach 5 (or enough to move your Camry about 35 miles). Crude oil's about the same, and gasoline is slightly higher at 47 MJ/kg. Even running a car on steam generated by burning coal would create 27 MJ per kilogram of coal.

Electric carmakers would obviously love to compete with this, but that's hard to do when a lithium battery has an energy density of 2 MJ/kg. A gasoline car isn't quite 20 times more efficient at converting its fuel into energy (you can't get 100% of the energy in a liquid fuel; some of it escapes as heat, noise, etc.), but it's close.

But fear not. There's hope for greenhouse gas-free cars yet. The energy density of gasoline and oil is fixed and is never going to increase. Batteries, on the other hand, are constantly getting better storage densities. While they may never be the equal of gasoline, they're definitely set to be a viable alternative in the near future.

Better energy storage devices, such as energy-dense batteries, are also a boon for renewable energy generation. One of the biggest challenges facing solar and wind farms is the intermittent nature of the technology—sometimes the wind blows, sometimes the sun shines, and sometimes they don't.

The times when the sun is out or the wind is particularly blustery don't necessarily line up with the times people turn on their TVs, and so renewables suffer from a problem fossil fuels don't have—it's possible to have too much energy available, yet also definitely possible not to have enough. We can always throw more coal on the fire, but we can't ask the sun to shine just a little bit brighter for an hour. Being able to store energy from renewable sources cheaply and densely makes intermittent power a lot less intermittent.