**1. Introduction**

The European Commission (EC) is promoting the transition of the European Union (EU) to a highly energy efficient and low-carbon economy system [1]. Energy production from renewable energy sources (RES), saving energy and natural resources, as well as reducing carbon dioxide (CO2) emissions while managing wastes, are pivotal actions to enable such a transition [2]. The EC adopted the "2030 climate and energy framework" in 2014, which has been subsequently revised in 2018 to include broader targets and policy objectives on greenhouse gasses (GHG) emission reduction for the period from 2021 to 2030. The targets for RES and energy efficiency are set to at least 40% cuts in greenhouse gas emissions (from 1990 levels), at least a 32% share for renewable energy, and at least a 32.5% improvement in energy efficiency [3]. On November 2018, the EC presented the analytical foundation for the development of an EU Long Term Strategy for climate and energy policy and a political vision for achieving a Net Zero economy by 2050 [4]. In this context, power generation has been identified as one of the sectors with the highest potential to decarbonize. To ensure that the EU

targets are met, EU legislation [5] requires that each Member State drafts a 10-year National Energy and Climate Plan (NECP), setting out how to reach its national targets. The Italian NECP [6], largely built on the 2017 Italian Energy Strategy, broadly meets the requirements set by the Regulation. The draft of the plan has been positively judged by EC, as it includes an extensive list of 101 policies and measures. These would be enough for Italy to meet the above targets, with a particularly important contribution coming from the objective of gradually phasing out coal for electricity generation by 2025. The draft plan qualitatively mentions the interactions with air quality and air emissions policy, specifically in the context of the proposed contribution expressed as 30% share of energy from RES in gross final consumption of energy in 2030. Electric energy production from RES, particularly those not emitting into the atmosphere during the operational phase like solar, wind and hydro, will play a key role in achieving such an ambitious objective. Biomass and geothermal can also play a role in replacing fossils toward a more sustainable development, but they are not exempt from drawbacks concerning CO2 emissions [7]. As geothermal energy has big potential for development [8], it is becoming important to explore the state of the art of the technology in terms of a benefit/cost ratio from an environmental point of view. Among RES, geothermal energy is considered a competitive energy source, because of its independence from seasonal and climatic conditions [9], ensuring reliable performances peculiar to non-renewable sources. Geothermal power plants can provide a stable production output, unaffected by the external environment, resulting in high capacity factors (ranging from 60% to 90%), and making the technology suitable for baseload production [10]. The technologies for power production from geothermal resource exploitation depend on the quality of the geothermal field, which, in general, increases with its enthalpy, typically spanning from liquid-only to steam-only (i.e., dry steam) reservoirs. Naturally occurring geothermal systems, known as hydro-thermal, are characterized by a resource fluid condition that can be considered directly available. By contrast, enhanced geothermal systems (EGS) aim to produce hot water at locations where natural aquifers are not present by developing an "engineered reservoir". This technology has received significant attention, because it allows the exploitation of geothermal energy virtually anywhere. Hydrothermal (mono, double or triple flash and dry steam) plants account for around 85% of the global geothermal power generation. In 2018, this was an estimated 90 TWh, while the cumulative capacity reached 14 GW [11]. Around 14% of the global electricity production is due to a different technology based on binary cycles [12]. This technology often exploits the total re-injection of non-condensable gases (NCGs) with some environmental advantages, despite a significant decrease in efficiency and larger land occupancy [8]. In this context, the concern about the environmental performance of geothermal energy exploitation has been growing in recent years, due to the expected increase of power production from geothermal sources [13].

Life Cycle Assessment (LCA) methodology is one of the most reliable and powerful tools to assess the environmental performance of power generation systems, capable of providing results that cover several environmental aspects, thus approaching the system in a more comprehensive and holistic way [14–16].

Even though LCA has been applied for quite a long time now to energy-producing systems, the field of geothermal energy exploitation still lacks primary data. Only a few studies have been aimed at determining the environmental profile of currently operating geo-thermoelectric installations in Italy [17–19] and in Iceland [20,21].

The relative complexity and high dependency on geomorphological factors of the geothermal energy source also contributes to the scarcity of specific information. Reviews performed by several authors [22–24] underlined the inaccuracy due to the lack of primary data. This trend is even more evident in harmonization [25–27], which needs to deal with very large variability, making the elaboration of reliable eco-profile very difficult [28,29].

The consequence is that papers which analyze geothermal power plants mostly use secondary data, forcing the authors to rely on the general literature data, which are often not adequately representative of the technology and of the investigated system [30,31].

Recently, special attention has been paid to the evaluation of environmental performances of EGS [32,33]. However, at present, hydro-thermal systems dominate current electricity generation in the geothermal sector, and the exploitation of this type of reservoir is predicted to become dominant in the future [11,34]. This picture outlines the importance of assessing the life cycle environmental impacts of conventional geothermal technologies to make sustainable choices in the context of the electric energy production sector. To avoid uncertainties, a reliable and high-quality life cycle inventory of a flash installation is needed. The only current source of data is the one provided in the study by Karlsdottir et al. [35].

The scope of the present work is to provide a high-quality, complete and documented life cycle inventory of a flash power plant, and to perform the LCA of electricity production from geothermal source with a cradle to grave approach, and to evaluate how much uncertainty of data is reflected on the final LCA results. The quality of data was assessed employing a so-called pedigree or uncertainty matrix. The Italian Bagnore power plant was selected as one of the most representative flash-based conversion system power plants. This work has been made possible by the full availability of primary data which, according to our knowledge, is unique in the literature.

The Bagnore power plant system consists of three connected units, namely: Bagnore 3, the binary group of Bagnore 3 and Bagnore 4. To correctly assess the environmental footprint of these plants, it is necessary to consider them as a whole system, namely the Bagnore system. Bagnore power plants integrate two systems for atmospheric emissions abatement, namely the AMIS (i.e., the abatement system for mercury (Hg) and hydrogen sulfide (H2S)) and the ammonia (NH3) abatement system. The adoption of state-of-the-art management strategies by the operator, Enel Green Power (EGP), aims for the best trade-off between production performance and environmental compliance [36].
