Most of us understand the need to diversify our energy portfolio by adding large amounts of alternative and sustainable energy. Economics, scarcity and fossil fuel pollution will drive us relentlessly. However, there are people who believe that we can go beyond diversity and can achieve energy independence by erecting windmills, adding solar panels to our homes, cultivating crops for use as biofuels, along with smaller doses of geothermal, tidal and hydroelectric sources.

As comforting as that notion is, there are two physical concepts that suggest that energy independence may not come that easily: energy density and power density. In this article we’ll talk about energy density.

The last major change in our energy portfolio occurred as a result of the Industrial Revolution.

The search for a solution to Britain’s energy problem was a major cause of industrialization. From the beginning of history until the 1780’s the world’s major energy sources were biofuels, mostly wood. Humans and animals did all the work. By the 1780’s, though, most of Britain’s wood supplies were gone. The answers that emerged to solve the wood problem were the fossil fuels: coal, natural gas, and oil. Nuclear power rounded out our current base energy portfolio.

Coal had been used for heat before 1700, but it wasn’t until James Watt’s more efficient steam engine that it was used for mechanical energy.

In 1821 William Hart successfully drilled for natural gas and established the Fredonia Gas Light Company. Edwin Drake’s 1859 oil discovery added oil to the mix.

Finally, the world’s first nuclear power plant became operational in Obninsk, Russia in 1954.

The history of these discoveries represents a progression from less energy dense to more energy dense fuels. Energy density is measured in megaJoules per kilogram (MJ/kw). Please see the accompanying chart for the energy density of various fuels. Energy Density is especially important for transportation fuels. The less fuel you have to carry around, the further you can go on a unit of energy. Let’s take a look at the difference between gasoline (a fossil fuel) and ethanol (a biofuel).

Think of a car that gets 27 mpg. A gallon of gasoline weighs about six pounds. We can drive about 350 miles on 13 gallons. Thirteen gallons of gasoline weighs approximately 78 lbs or a little over 35 kg. Automotive gasoline has an energy density of 45.8 MJ/kg, so the full tank we need to go 350 miles contains 1600 MJ of energy.

Now let’s take a look at ethanol. Ethanol contains 29.7 MJ/kg. In order to drive 350 miles on ethanol, we need 1600 MJ/29.7 MJ/kg, or nearly 54 kg or 119 pounds of ethanol.

If we want to maintain our fuel weight at 78 pounds, instead of 119 pounds, and still have the same range, we have to increase the fuel economy. A gallon of ethanol weighs 6.59 pounds, so 78 pounds of ethanol is about 12 gallons. To drive 350 miles on 12 gallons we need a fuel economy of 29 miles per gallon, but the extra fuel weight makes it hard to achieve higher mileage.

We could also settle for decreased range. If we maintained the fuel economy at 27 miles per gallon, our range would decrease to 324 miles.

Today, we are moving away from the progression of ever denser fuels into less dense ones. That fact gets particularly troublesome when we look at battery-powered electric cars. A preview of coming attractions is the Chevy Volt.

A lithium ion battery for the electric Chevy Volt is likely to have an energy density somewhere between 0.46 and 0.72 MJ/kg. On electricity alone, it is expected to go 40 miles on a 175 kg fully charged battery pack. Converting, that’s 385 pounds of battery pack weight. Remember that we went 350 miles on 78 pounds of gasoline.

Certainly technology will improve provided we remain committed to diversification, but no technologies currently exist to displace gasoline or diesel without greatly reducing vehicle range.

So when you hear people tell you that we can now do without oil from the middle east, without oil from Canada, and without oil from the Gulf of Mexico, we can—but only if we don’t want to go anywhere.

According to Edmunds, driving on E85 (85% ethanol and 15% gasoline can decrease your fuel mileage 7%. Replacing E85 with the usual 90% gasoline—10% ethanol mix yields the equivalent of driving 2.87 miles less every day.