Energy Quality

Not only does quantity matter, but quality as well. The value of any resource systems is also a function of its intrinsic qualities, which determine how it can be used. Of particular interest are the energy density, the power density, and the related dispatchability of the resource.

Energy Density

“Energy density,” also known as specific energy, relates primarily to intensity. It can describe either how much energy is carried per unit mass or per unit volume, typically expressed in units of mega joules per kilogram (MJ/kg). It basically asks the question: How much mass (or volume) do you need to provide a given amount of energy? This is an important characteristic for the ability to move energy to where it is needed.

Consider raw biomass that has not been refined by natural or human processes. Firewood can be burned directly, but in biomass-dependent regions charcoal is preferred. Charcoal is woody material that has been processed by heat to drive out most of the water. It burns hotter and cleaner, with a much higher energy content per pound. That makes it easier to transport. The fact that it readily breaks up into small chunks also makes it easier to select the appropriate amount for a given task. People pay a premium for the increased energy intensity and combustion performance for charcoal rather than firewood. Wood must still be harvested in a laborious process, but the resulting charcoal product can be moved readily to urban markets, where it is a prized commodity.

Note that “coal” is part of the word “charcoal.” It is not a coincidence. Coal has also undergone processing, which drives off water, increasing the energy density of the fuel. It delivers more energy to the task than unprocessed biomass. Crude oil is even more processed by nature, and natural gas the most. Of naturally occurring combustion fuels, natural gas has the greatest energy content by mass (and the cleanest burn). It is so light, though, that it must be compressed under high pressure to hold nearly as much energy per unit volume as the liquid or solid fuels.

The only natural energy source with greater intensity than natural gas is nuclear fuel. While uranium is rare, and the naturally fissile isotope (U-235) represents less than 1% of uranium, it takes very little fuel to yield a great deal of energy. The limits on nuclear fuel use currently have less to do with the abundance of the fuel, and more to do with social factors stemming from anxieties about waste, accidents, or misuse.

Another fuel worthy of consideration is hydrogen gas. Hydrogen has the highest theoretical chemical energy density of any nonnuclear fuel (see Table 2.1) and is the most abundant element in the universe. It burns

Table 2.1 Specific energy of various fuels.


Specific Energy (MJ/kg)

Uranium (0.2% in ore)


Hydrogen (derived)


Natural gas


Diesel, gasoline


Anthracite coal


Charcoal (derived)




even cleaner than natural gas. However, hydrogen is not naturally available, but exists in the chemical bonds of countless compounds. Thus, it must be unstuck before being useful, which reduces the net energy density of the fuel.

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