Energy presents itself in many aspects and has the peculiar characteristic of moving from one to another while keeping its total quantity constant: what we perceive or measure when studying the energy involved in a productive process are simply different forms of energy, as any process, whatever it may be, cannot create or destroy energy but only convert it according to the laws of thermodynamics.
When we talk about matter, we tend to think of something tangible, whereas when we talk about energy, we must think of something that transforms in different ways and appears in multiple forms: through the exchange of heat, work, or electromagnetic energy, it produces the forms of matter and other types of energy that we are accustomed to recognize.
Energy is conserved, but as it flows through systems, its ability to induce transformations decreases, as clarified by the Second Law of Thermodynamics. It can also be demonstrated that, in a given environmental context, given a finite amount of energy resources, only a part can be used in processes while the remaining portion flows back to the outside. Exergy is defined as the portion of energy resource that can be used in a thermodynamically reversible (ideal) process. Since it can be shown that all mechanical energy is usable, exergy is also referred to as the portion of energy ideally transformable into work. The relationship linking the quantities involved is as follows:
energy = exergy + anergy
where anergy represents the non-useful part of energy.
Exergy is therefore a parameter of thermodynamic quality capable of determining the points where process irreversibilities occur, suggesting to the designer what measures to adopt to increase the system's efficiency. In practice, exergy analysis represents the next step beyond energy analysis to obtain more precise and specific information.
At this point, it is possible to introduce the concept of energy efficiency: roughly speaking, it can be defined as the ratio between the useful output of a process and the energy input. It is a generic term, and there is no unique measure for it: generally, this indicator is used to refer to the use of a smaller amount of energy to produce the same quantity of useful services or products. Besides the type of energy, the quality of the energy is also considered, meaning that a distinction can be made between high-quality energy resources (more useful and productive) and low-quality ones, which are less useful.
Given what has been said, it seems possible to use the energetic or exergetic point of view to approach an LCA analysis. In fact, the first step is to understand what functions each of the two types of analysis can perform during the construction of analogical models of the industrial processes under investigation, and then determine under which circumstances one should be used over the other.
While energy cannot be destroyed since it is always possible to identify all the dispersed flows at the output of any process, exergy is instead destroyed in every irreversible process, providing a more detailed basis for computing the amount of energy allocated to the intended function. The decision to use energy or exergy in the life cycle analysis is therefore closely related to the analysis's own objectives but above all to the type of system in question. Complex systems are typically where exergy analyses can be carried out. By reconstructing the successive steps of a hypothetical production process according to the two approaches of energy and exergy, the differences between these two quantities can be observed, highlighting the operations with the lowest exergetic efficiencies and therefore causing the greatest degradation from exergy to anergy.
Despite these advantages, LCA analyses are normally set up using energy as a fundamental quantity: this is because one of the main objectives of LCA is to identify and quantify in energy units the primary energy sources used by the system for its final uses, in addition to the fact, not insignificant, that the data to be used are almost always available in this form.
