2.1. Life Cycle Analysis (LCA)
LCA is a method to evaluate the environmental load associated with a product, process, or activity. LCA allows quantifying the used amount of energy and materials and of emissions and waste released in the environment, allowing for the evaluation of the associated potential impacts. The assessment is performed over the entire life cycle of the product, process, or activity covering extraction and processing of raw materials; manufacturing, transportation, and distribution; use, re-use, maintenance; recycling, and final disposal. The results of LCA can be expressed via a large number of environmental indicators and, generally, several impact categories are used to circumspectly detect the full range of ecological burdens associated with the investigated process or activity over the three environmental compartments (atmosphere, soil, and water), thus aiming at avoiding burden shifting. The LCA methodology is regulated according to the general guidelines described in the International Standard Organization (ISO) series 14040 [19
] and consists of four phases:
Goal and scope definition: In this phase, the goal of the study, the system boundaries, the quality requisites of the data sources are described, and the functional unit of the analysis is specified.
Life cycle inventory analysis (LCI): The purpose of this phase is to collect the input/output data pertinent to the system studied; generally robust and reliable LCIs are built on primary data, that is to say specific data that highly characterize the system under study.
Life cycle impact assessment (LCIA): This phase evaluates the significant potential environmental impacts using the LCI results; the process involves associating inventory data with specific environmental impact categories and the calculation of indicator values using accepted characterization factors.
Life cycle interpretation: It is the final phase of an LCA study in which the results of the LCI and LCIA steps are presented and discussed; interpretation includes conclusions and recommendations adapted to the goal and scope of the study.
LCA was born as a detailed and quantitative approach for the evaluation of environmental sustainability [21
]. The regulatory approach described in the ISO standards and the more completely elaborated ILCD Handbook guidelines [22
] claims for the development of an LCA study until the characterization of the environmental impacts at a midpoint level. With this approach, the LCIA method looks at the impact earlier along the cause–effect chain of the environmental mechanism, and can refer to a relevant number of impact categories characterized by a low uncertainty but, on the other hand, is difficult to interpret. In principle, it represents a good approach for the characterization of the eco-profile of the product or activity under exam to use several wide-scope LCIA methods and check if findings are consistent in all of them. If so, it is possible to claim that findings appear robust. But when this is not the case, the LCA practitioner might have to delve into the particularities of the LCIA methods and find out why the results are dissimilar, which can be a good learning experience about the characteristics of the applied LCIA methods.
The environmental evaluation at the endpoint level is a non-mandatory part of LCA, which includes normalization and weighting steps that allow expressing the results referring to a limited number of damage categories, typically resources availability, human health, and ecosystem quality. Endpoint results provide insight on the environmental impact at the end of this cause–effect chain of the environmental mechanism, thus with larger uncertainty. If interpretation at this level provides a more limited amount of details, it is recognized that it is more suitable for the presentation of results to non-technical audiences. The various LCIA methods apply different impact category grouping, normalization, and weighting factors, thus it is necessarily recommended to refer to the same methodology when comparing different technologies dealing with the same product or process.
Energy conversion and utilization is one of the most famous and important fields of application of LCA calculations. LCA indeed offers a powerful approach to analyze systems overarching the complete life cycle of a system (from cradle to grave) which is necessary when considering the substitution of fossil fuels with renewables. When applying LCA to energy conversion systems, for fossil fuel-based technologies it is common to find high impacts connected to the use of fossil resources in the operational phase [23
]; on the other hand, RES, which minimizes the use of consumables such as fossil fuel, entail a consistent use of materials because of the diffuse nature of renewable energy, some of which are rare, or whose extraction and/or production entails direct or indirect negative effects on the environment. In general, RES scores better environmental performance than fossil fuel systems in most impact categories. However, these outcomes should be evaluated, validated, and compared among different RES.
Several LCA studies are available on solar PV energy conversion systems [26
]; in general, the results indicate that a significant impact is coming from the manufacture of the PV modules, with the current silicon technology performing better than cadmium telluride, notwithstanding substantial advantages for thin-film manufacturing [36
]. A significant fraction contribution to the overall eco-profile (20–30%) comes from the structural materials and glazing. The environmental footprint is lower than the best fossil fuel-based technologies in most categories, with a weighted score typically 4–8 times smaller. The relatively standard production process has led to the development of accepted guidelines [37
], which have determined an improved homogeneity in the results and better comparability of the studies. Wind energy has also attracted several LCA studies [38
]: In comparison with fossil fuel-based systems, the environmental footprint is very limited, and only a restricted number of categories is usually involved (global warming potential, GWP; acidification potential, AP; eutrophication potential, EP; cumulative energy demand, CED). In the field of wind energy, no specific LCA guidelines are available; however, significant studies have been published by leading manufacturers such as Vestas [42
]; the results have been cross-checked by researchers and substantial agreement is documented [43
Geothermal power plants have raised the attention of local and national policymakers in terms of their environmental performances and sustainability, and the comparison with other RES has then become necessary. Several studies on the application of LCA to geothermal power systems are available in the literature [47
]; however, most studies are only focused on GWP, and there is a considerable spread in the results.
Examples of comparison of RES options are documented in the technical literature [59
]; however, they mostly rely on previously published LCA studies and, in most cases, on the use of literature data. There is a substantial lack of primary data (produced by the plant owner or operator), which stand as more reliable as the source of the information can be completely tracked. Utilities such as Enel Green Power have a good opportunity to access these primary data (often gathered with the purpose of economic analyses, or in case of the commitment of construction works or trusting of maintenance services), and to use them to document the environmental quality of their product (electricity). This represents a key passage in the environmental evaluation, both in terms of company, services, and products (possibly leading to an ECO-Label), and is also a motivation behind the present study.
The case studies described in the following section were analyzed using the OpenLCA 1.10 software package [63
]; for secondary data, the Ecoinvent database 3.6 was adopted [64
]. The system modeling approach employed is cradle to grave. The functional unit was set as 1 kWh of electricity delivered to the grid, assuming a lifetime of 30 years for all the power plant solutions investigated. The system boundaries were defined to include the whole life cycle of the system. Operation and maintenance (including replacement of major equipment) were modeled following the experience of Enel Green Power as power plant manager.
The assessment was performed comparing the ReCiPe 2016 [66
] and the ILCD 2011 Midpoint+ [67
] LCIA methods both employed for the characterization of potential impact at midpoint level. Normalization and weighting, applying an hierarchist (H) cultural perspective, were applied to the ReCiPe 2016 results to determine the systems eco-profiles at the endpoint level (with weighted results expressed in Eco-points).
All data gathered in the LCI are primary data provided by Enel Green Power resulting from checked information about materials employed for construction. Secondary data were used for common materials (e.g., steel, concrete, copper, plastics, etc.) and upstream processes (e.g., transport). The LCI also reports data for operation and maintenance, including replacement of equipment, consumables, etc.