Energy densities for various fuels and forms of energy. desc=

Although all forms of energy can be measured by their heat content, the usefulness of a heat unit of energy varies greatly among energy types. The world uses about 500 exajoules (500×10¹⁸ joules) of energy each year.

This total is generated by adding together the heat content, measured in joules, of many forms of energy: crude oil, natural gas, coal, geothermal, biomass, and electricity from wind, solar, nuclear, and other sources. The advantage of the content approach is that it uses a simple and well-defined accounting system based on the conservation of energy, and the fact that thermal equivalents are easily and uncontroversially measured. This approach underlies most methods of energy accounting in economics and ecology.

Despite its widespread use, summing different energy types by their heat units embodies a serious flaw: it ignores qualitative differences among forms of energy. Simply put, not all joules are equal in terms of their potential to meet human needs. Energy quality refers to differences in the ability of a unit of energy to produce goods and services for people. Energy quality is determined by a complex combination of physical, chemical, technical, economic, environmental and social attributes that are unique to each form of energy. These attributes include gravimetric and volumetric energy density, power density, emissions, cost and efficiency of conversion, financial risk, amenability to energy storage, risk to human health, spatial distribution, intermittency, and ease of energy transport.
Average solar insolation, or rate of incoming sunlight at the Earth's surface, over the entire globe for the months of January and April. The colors correspond to values (kilowatt hours per square meter per day). Credit: NASA

For example, the substitution of coal for biomass and then petroleum for coal were shifts to more concentrated forms of energy. Solid and liquid fossil fuels have much larger mass densities than biomass fuels, and an even greater advantage in terms of volumetric densities. The preeminent position of liquid fuels derived from crude oil in terms of its combined densities is one reason why it transformed the availability, nature and impact of personal and commercial transport in society. The lower energy density of biomass (12-15 MJ/kg) compared to crude oil (42 MJ/kg) means that replacing the latter with the former will require a significantly larger energy infrastructure (labor, capital, materials, energy) to produce an equivalent quantity of energy.

Accounting for energy quality is essential to the understanding of the relation between energy and human well-being. For example, the decline in the energy/gross domestic product ratio in the major industrial nations over the past several decades is due in large part to the shift from coal to oil, natural gas, and primary electricity. Similarly, nations that use high quality energy sources such as primary electricity (Japan, Sweden) tend to use fewer joules to produce a dollar's worth of GDP than nations that rely heavily on solid and liquid fuels (Russia and Poland).

Sources

• Cleveland, Cutler (Lead Author); Dagmar Budikova (Topic Editor). 2007. Energy quality. In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [First published in the Encyclopedia of Earth February 22, 2007; Last revised February 23, 2007; Accessed March 24, 2008]. 
• Cleveland, Cutler J., David I. Stern, and Robert K. Kaufmann.  Aggregation and the Role of Energy in the Economy, Ecological Economics, 32: 301-317 (2000).