ENERGY RESOURCE EXTRACTION SITES: GEOTHERMAL ENERGY

Each year the Earth releases energy equal to about 40TW, three times the amount needed for human energy demands. Only a portion of this energy is usable, and only a small part of that is actually harnessed. In most cases, the geothermal energy utilized is that associated with hydrothermal systems (conventional geothermal energy). In these cases, a hot magmatic body present at depth heats an aquifer, and the circulating water becomes the energy carrier (in liquid or gaseous phase) for heat transport to the surface, either spontaneously (fumaroles, steam vents, geysers, etc.) or through wells. Due to lithospheric dynamics, the highest values of terrestrial heat flow—and consequently conventional geothermal areas—are located where hot masses are closer to the surface, such as along plate margins, which are also associated with intense volcanism (Figures 1 and 2). In 2022, 22 countries worldwide generated about 100 billion kWh of electricity from geothermal energy (Figure 3).

For many years, the United States has led the ranking, partially exploiting the huge potential of the western coast, home to the largest and most famous geothermal field in the world, The Geysers. After being the cradle of geothermal energy production, with the world’s first plant built in Larderello in 1913 and world leader until the mid-20th century, Italy is currently ranked seventh in geothermal electricity production. Given its geological structure, Italy is a country with high geothermal potential. Geothermal fluids at sufficiently high temperatures for electricity production (medium and high enthalpy) are located in areas with high heat flow, corresponding to buried magmatic bodies and extinct or active volcanic systems, such as the coastal Tuscan-Lazio-Campania belt, the volcanic islands of the Tyrrhenian Sea, and the Etna area (Figure 4). Locally, as in Larderello and Monte Amiata, the heat flow reaches very high values. Conversely, medium-low enthalpy resources, suitable for direct uses (building heating, bathing, spa use, greenhouse farming, aquaculture, etc.) are found in many other areas of the national territory. With geothermal heat pumps, even low-temperature resources present almost everywhere and at shallow depths can be exploited. Geothermal resources covered by mining titles are concentrated in the Tuscan-Lazio area. Out of a national total of 36 concessions and research titles, 25 fall within Tuscany and 6 in Lazio (Figure 5, Table 1). Geothermal electricity production is exclusive to 10 concessions located in south-central Tuscany and concentrated in the Larderello and Monte Amiata zones (Figure 6). The concessions in Ferrara and Vicenza are related to direct use of geothermal resources (heating), while the concession in Valentano (Lazio) is not currently productive, nor is that of Poggio Montone (GR). Within the 10 Tuscan concessions, there are 34 plants (with 37 production sections) (Figure 6), 29 of which are "steam-dominated" in the Larderello and Radicondoli geothermal fields and 5 "water-dominated" in the Monte Amiata area, where the Bagnore3 plant houses the only currently operational binary group (Table 2). The total nominal capacity is 915.8 MW (Table 2), with a gross efficient capacity of 817.1 MW corresponding to a 2023 production of 5,692 GWh (Figures 7,8). Installed capacity has been stable since 2014 (Figure 7). Geothermal energy accounts for only 2.15% of national electricity production, but in Tuscany it represents about 36% of regional electricity production and meets more than 31% of regional needs (Table 3). Considering electricity production from RES, geothermal accounts for about 5% nationally but reaches 70% of Tuscany's production (Figure 9, Table 3). More widespread across the national territory are the direct uses of geothermal heat, whose main application is building climate control, including via geothermal heat pumps and district heating networks (Table 5). According to the EEA, building climate control represents 50% of European energy consumption, highlighting the significant contribution geothermal could make to reducing fossil fuel use. The use of geothermal sources for urban district heating networks is under development but, despite great potential, still represents only a small portion of energy sources used in Italian networks. The extensive use of geothermal heat pumps in Northern European countries clearly demonstrates the potential of this technology, which could also significantly contribute to residential heating in Italy, thereby reducing the use of fossil fuels and biomass. Although the number of geothermal heat pumps in Italy is steadily increasing, with about 20,000 units, it is still far from the figures seen in Northern Europe (Table 4). This figure is useful to define the order of magnitude of domestic systems but should be considered uncertified, pending the establishment of an official registry of geothermal heat pumps by the regions. Environmental impacts are limited to geothermal electricity uses but can be controlled through specific mitigation measures even in areas that, due to their geological nature, naturally release pollutants into the environment (e.g., through steam vents, fumaroles, geysers). Greenhouse gas emissions (carbon dioxide and methane) are not generated by the plants, which simply channel the gases present in geothermal fluids—gases that would otherwise be diffusely released into the atmosphere via the soil—into single emission points. Since there is no combustion, no fine particles or nitrogen oxides are emitted, while the channeled emissions of hydrogen sulfide and mercury are mitigated by AMIS abatement systems. Subsidence caused by fluid extraction is mitigated by water reinjection, while landscape impact can be limited by specific landscape architecture measures. Land use is significantly lower than other RES, especially photovoltaics. Induced seismicity, linked to high-pressure water injection to fracture rock, is characteristic of Enhanced Geothermal Systems (EGS), which are not in operation in Italy. Epidemiological surveys by ARS Toscana have not identified significant health impacts on populations from geothermal electricity production; however, in the Amiata area, further investigation is needed on the increase in blood and urinary concentrations of mercury, thallium, and arsenic, carefully considering the natural background levels in water and soil. Geothermal fluids can contain high quantities of elements essential for the entire green economy industry, such as lithium and rare earths, which can be suitably recovered. In particular, geothermal fluids in the peri-Tyrrhenian volcanic-geothermal zones (Tuscany-Lazio-Campania) show lithium concentrations up to 480 mg/l (Figure 10). These values are about double those found in the brines of California's Salton Sea geothermal field, considered by Americans as the source that will allow the U.S. to achieve independence from foreign lithium markets. Currently, 6 research permits have been issued in the Lake Bracciano area to assess the feasibility of the operation.