Let’s consider the question of energy storage, which has practical applications in a variety of contexts. There are so many reasons to store energy and ways of doing so that some sort of classification system is worthwhile to sort through the mess. An important sorting criterion for energy storage solutions is by mobile vs stationary applications, with the former including most (but not all) transportation equipment, and the latter including everything else. Another way of sorting energy storage is by storage technique, i.e. according to the physics & chemistry of the medium. Many energy storage applications need to be concerned with *energy density* and *specific energy*. *Energy density* is defined as the energy per unit volume, while *specific energy* is the energy per unit mass.

A look at statistics on attainable energ density [1] and specific energy [2] shows that of the three main categories of energy storage – nuclear, combustion chemical, and electricity storage – nuclear energy offers by far the highest energy densities and specific energies at > 40,000 MJ/kg and > 600,000 MJ/litre. Combustible fuels manage between 3 and 70 MJ/kg with energy densities on the same order of magnitude. Experimental electric batteries manage as much as 2.5 MJ/kg, but realistic mass-producible batteries deliver below 1 MJ/kg, with lead-acid technology at 0.14 MJ/kg for example. Ultracapacitors and capacitors do even worse, at < 0.02 MJ/kg. There are some energy storage technologies that don’t fall into these main categories – flywheels and pumped water storage for example. Flywheels may be capable of 0.5 MJ/kg. In the case of pumped water storage, energy density is a question of how high one is willing to lift the water: the gravitational potential energy is about 9.8 J/kg/m (note that is in joules, not megajoules).

We will consider the energy storage needs of motor vehicles, and one stationary application namely peak leveling for electricity grids.

[1] http://en.wikipedia.org/wiki/Energy_density, http://en.wikipedia.org/wiki/File:Energy_density.svg

[2] http://en.wikipedia.org/wiki/Specific_energy