Revised 24 May 2019

Exergy is available energy, the maximum fraction of an energy form which, in a reversible process, can be transformed into work. Exergy analysis quantitatively evaluates and detects thermodynamically inefficient processes of energy production and transformations.  Diluted or dispersed minerals in the ground are more difficult to mine, collect, and refine than more concentrated sources.  A measure which takes the concentration of minerals into account is its chemical potential or chemical exergy. For example, the chemical potential of pure iron is much higher than the chemical potential of an iron ore diluted by other rocks.

A standard reference point of exergy is needed for different natural minerals in the ground, and hence the concept of ‘Thanatia’ which is a hypothetical version of our planet Earth where all mineral deposits have been exploited and their materials have been dispersed throughout the crust. With Thanatia as a reference level, the exergy of different minerals in the ground can be estimated and compared. By adding up all the exergy expenditures of mining and refining, the rarity of minerals and their embodiment in final products can be assessed. 

There can be no production of goods and services without exergy destruction. Unlike energy, exergy is not subject to the law of conservation. Loss of natural resources is an objective physical reality which cannot be fully expressed using subjective monetary valuations. Loss of natural resources also cannot be expressed in terms of weight or energy alone, as these measures do not fully capture quality and value. The ‘exergy cost’, the embodied exergy of any material which takes the concentration of resources into account measured with reference to the ‘dead state’ of Thanatia is a measure of the loss of natural resources. An exergy destruction footprint can be established for same-purpose products which use different resources and processes during manufacturing and the full life cycle of maintenance, replacement, and recycling. By using exergy analysis, better choices of resources and processes can be rationalised before production begins.