Exergy, anergy and entropy

[l-emele]

Description of the issue

In some definitions, we have phrases like "useful" or "usable" energy/heat. The more stringent term here would probably be exergy. If there is usable energy, then there is also unusable energy, this is often called anergy. Related and probably needed to define anergy and exergy is entropy.

See also: https://en.wikipedia.org/wiki/Exergy

Ideas of solution

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Workflow checklist

• [ ] I discussed the issue with someone else than me before working on a solution
• [ ] I already read the latest version of the workflow for this repository
• [ ] The goal of this ontology is clear to me

I am aware that

• [ ] every entry in the ontology should have a definition
• [ ] classes should arise from concepts rather than from words

[l-emele]

Some definitions and explanations from other sources as start of discussion:

exergy:

• Exergy is defined as the potential work that can be extracted from a system by reversible processes as the system equilibrates with its surroundings. (Source: Ayres (1998): Eco-thermodynamics: economics and the second law)
• In thermodynamics, the exergy of a system is the maximum useful work possible during a process that brings the system into equilibrium with a heat reservoir, reaching maximum entropy. (Source: Wikipedia)
• The maximum fraction of an energy form which (in a reversible process) can be transformed into work is called exergy. (Source: Hohnerkamp (2002): Statistical physics)

anergy:

• Als Anergie wird der Bestandteil einer Energie bezeichnet, der in einem Prozess keine Arbeit verrichten kann. (Anergy is the component of energy that cannot do any work in a process. Source: Wikipedia-de)
• [E]xergy is always destroyed when a process is irreversible, for example loss of heat to the environment [...]. This destruction is proportional to the entropy increase of the system together with its surroundings (see Entropy production). The destroyed exergy has been called anergy. (Source: Wikipedia)
• ... The remaining part is called anergy, and this corresponds to the waste heat. (Source: Hohnerkamp (2002): Statistical physics)

[l-emele]

Btw, we also already have a class waste thermal energy (alt. waste heat): Waste thermal energy is thermal energy that is the physical output of an energy transformation process and that is released into the environment unused. This might be equivalent or is at least related to anergy.

[robbiemorrison]

It is a while since I worked with exergy. But it divides into chemical exergy and physical exergy. The former interacts with the chemical dead state of the environment and the latter with the thermal dead state of the environment and at rest. The two can be added but that is not usually necessary. The chemical/physical observation explains the divergence in the definitions given above. Also anergy is thermodynamically useless whereas waste heat may only be economically or technically useless. Waste heat often possesses low grade exergy: grade or energy quality being the ratio between exergy and anergy. Work is 100% by definition, solar irradiation about 93% (from memory), and waste heat just a few percent. Chemical work is also a construct in this scheme.

I think anergy and exergy are concepts. And waste thermal energy a label (without diving into the recent EU taxonomy debacle which classified gas turbines as sustainable). And both strands need supporting.

Note that the chemical dead state is a fiction or at least an approximation: the environment is not usually in full technical chemical equilibrium.