*3.2. Life Cycle Assessment (LCA)*

Life cycle assessment (LCA) is a de-facto standard method for the evaluation of the environmental performance of a wide range of industrial processes and technologies, and it mainly owes its wide acceptance to its comprehensiveness (in terms of considering all supply chain stages, from extraction of raw materials through transportation and manufacturing, to use phase and end-of-life). It also benefits from a high degree of standardization [66,67], and from the availability of extensive, industry-vetted inventory databases, among which a prominent role is played by Ecoinvent [38].

Various life-cycle impact assessment methods have been developed, which enable the calculation of dedicated impact indicators for a wide range of impact categories. Among the latter, the focus of this paper is on global warming potential (GWP), estimated using IPCC-derived characterization factors with a time horizon of 100 years (in units of kg of CO2-equivalent) for all gaseous emissions, excluding biogenic CO2.

Additionally, two life-cycle energy metrics are also calculated here, namely the cumulative energy demand (CED) and the non-renewable cumulative energy demand (nr-CED), respectively quantifying the total amount of primary energy directly and indirectly harvested from the environment per unit of electricity output, and the non-renewable share thereof (in both cases the results are expressed in MJ of oil-equivalent) [68].

Based on the definition above, it is also self-evident that the life-cycle primary-to-electric energy conversion efficiency of the grid mix taken as a whole (ηG) can be conveniently calculated as the reciprocal of its CED (Equation (1)):

$$
\Delta \eta\_{\rm G} = 1/\text{CED}\_{\rm G} \tag{1}
$$

#### *3.3. Net Energy Analysis (NEA)*

Net energy analysis (NEA) [69] provides an alternative viewpoint on the energy metabolism of energy harvesting and conversion technologies, whereby the primary energy resource(s) that are directly exploited and converted to useful energy carriers (e.g., the natural gas that is extracted, conveyed by pipeline and then burnt in a power plant to produce electricity; or the solar energy that is harvested and converted to electricity by PV panels) are deliberately excluded from the accounting, and instead the focus is put solely on how much previously-available commercial energy needs to be "invested" in order to operate the energy supply chains (e.g., the energy needed to extract the gas from the ground, build the pipeline, pump the gas through the pipeline, and build the gas turbine; or the energy needed to manufacture the PV panels and their balance-of-system).

When put in rather blunt but arguably vivid terms, it can therefore be said that instead of being concerned with the overall thermodynamic efficiency of a process, NEA aims to quantify the energy "bang for the buck" from the point of view of the end user. Fittingly, its main indicator is the energy return on (energy) investment [70] (defined as per Equation (2)):

$$\text{EROI} = \text{Out/Inv} \tag{2}$$

However, the NEA literature has historically been characterized by a much lower degree of standardization than the LCA one, which has led to many inconsistent comparisons [71,72].

In this study, in order to integrate the LCA and NEA viewpoints, and to maximize the consistency of the calculations, both internally and externally with some of the more recent literature [10,12,13,15,17], when calculating the EROI of electricity (either produced by a specific technology, or by the grid mix as a whole), all energy investments at the denominator are always accounted for in terms of their respective life-cycle CED (and are thus quantified in units of oil-equivalent).

Then, when the EROI numerator is simply measured as the amount of electricity delivered (i.e., not converted to some form of "equivalent" primary or thermal energy), a subscript "el" is appended to the resulting indicator (i.e., EROIel). Alternatively, when the EROI numerator is expressed as "primary energy equivalent" (on the basis of the life-cycle primary-to-electric energy conversion efficiency of the grid mix in the current year), a subscript "PE-eq" is used (i.e., EROIPE-eq).
