Exergy is the maximum amount of work that can be done by a subsystem as it approaches thermodynamic equilibrium with its surroundings by a sequence of reversible processes. The term was coined by Zoran Rant (1904-1972) in 1953 from the Greek words ex (external) and ergos (work). Exergy measurements are made relative to an equilibrium state in which there are no gradients of any kind. This implies uniformity of temperature, pressure, density, chemical composition as well as uniform gravitational and electro-magnetic fields. Thus, the exergy of a subsystem is a measure of its 'distance' from equilibrium. Mechanical exergy is known as kinetic energy, while thermal exergy is more familiarly known as heat. These concepts are important in the design of energy-efficient machines. Chemical exergy is used in chemical engineering and thermoeconomics for process optimization, but also in economics and environmental science.

Figure 1. Global exergy flows. (Source: Exergy.org)

Fuel combustion is the spontaneous recombination of hydrocarbons or carbohydrates with atmospheric oxygen, resulting in their mutual chemical equilibrium state. Thus, the heat of combustion (enthalpy) of a fuel is roughly equivalent to its exergy content. For non-fuels, chemical exergy is a measure of distinguishability from the surroundings. A high-grade ore has more embodied exergy than a low-grade ore, and thus needs more energy to be upgraded. Exergy also is an important concept in understanding life processes. Seemingly dead structures in space convert into organized, self-reproducing structures as life and life forms by means of converting and partly destroying exergy. Nature creates a state far from thermodynamic equilibrium on Earth by an everlasting redesign of the environment mainly powered by the exergy of the sunlight.

There is a substantial amount of exergy that flows into an within the Earth in addition to the non-renewable exergy reservoirs on which our energy system currently depends. Figure 1 details these fluxes and reservoirs. Exergy flux is represented by paths ending in shaded areas of natural exergy destruction or arrows representing anthropogenic destruction for energy services. Unlabeled dashed circles along these paths represent natural exergy destruction processes such as precipitation and plant decay that are small compared to the natural destruction modes of solar exergy. The ovals illustrate accumulations of exergy.

Further reading

• Szargut, J., D. Morris, F. Steward, 1998. Exergy Analysis of Thermal, Chemical, and Metallurgical Processes.' Hemisphere Publishing Corporation, New York.
• Wall, Göran and M. Gong, 2001. On Exergy and Sustainable Development, Part I: Conditions and Concepts. Exergy, An International Journal, Vol. 1, No. 3.