Energy density is the metric by which we compare a wide range of technologies. It is often used as the basis of comparison for battery technology and the ability to store electrical charge. The problem is those pesky electrons have no mass, so storing them in comparison to a fuel like gasoline is hard to do. It’s not a level comparison, however. The conversion to mechanical work in a car, for example, is very inefficient with gasoline. Most of the energy is converted to heat and lost. Electric propulsion is very efficient making a comparison very difficult.
Energy density is not limited to the comparison of fuels for combustion of battery storage technology. It is literally the energy stored in a given material. Energy density can be measured by weight and by volume. This distinction is very important because sources like hydrogen and methane are energy dense when compressed or in liquid form, but as gases, obviously not so much. In the realm of battery storage both parameters of merit are needed for a successful electric vehicle.
Wikipedia has an interesting table of energy density of many major power sources at http://en.wikipedia.org/wiki/Energy_density that is worth taking a long, contemplative look at. The highest energy density material is Uranium. Since nuclear power involves no combustion, there are no byproducts of combustion to pollute the atmosphere. Clearly this would seem the way forward. There is also Thorium which many believe is a safer alternative fuel and newer reactor designs that are smaller and safer to use. In order to make a proper evaluation we need to consider the complete life cycle cost of the energy source, processing the fuel, and the cost of building a power plant or battery. It’s amazing that the utility industry can process rail car loads of coal every day, burn it somewhat cleanly and deliver the power to your house at pennies per kilowatt/hour. That is an incredible deal. I know that nuclear power plants have been engineered at roughly the same cost as coal, but I don’t know how expensive it is to process the fuel.
Notably the Wiki table shows hydrogen with almost 3 times the energy density per kilogram of gasoline. One minor problem, hydrogen is a gas at standard temperature and pressure. Hydrocarbons like gasoline are efficient because they are energy dense at room temperature. How much energy is required to convert hydrogen to a liquid? How safe is it to store at 5000 psi or higher? This question is really difficult to solve and the DOE continues to throw millions at the research. So far the only economical approach to hydrogen as a fuel is either to strip it from methanol or buy it from an industrial gas supplier.
In the final analysis it’s about what is cost effective for the consumer. So far, all the “old school” solutions are still the most cost effective. We can do better, but it has to be based on what helps people keep their cost of living low, not what is on someone’s wish list.