Geothermal Energy Production in Italy: An LCA Approach for Environmental Performance Optimization

Abstract

Geothermal energy is a resource of natural and renewable energy and its exploitation in the Italian Tuscany Region contributes significantly to the regional share of electricity generation from renewable sources, with a value that has grown to about 35% in 2015. The energy produced by geothermal source, such as that produced by other energy sources, generates non-negligible impacts on the environment, closely related to the site-specificity of the source itself. This preliminary study analyzes the operational phase of seven geothermal plants located in the three main Tuscan geothermal areas, with a specific focus on the impacts generated by emissions into the atmosphere. The aim is the assessment of the geothermal power plants environmental performances in relation to the geomorphological characteristics of the sites and the technologies used to exploit the resource.

1 Introduction

In 2015, geothermal energy contributed to about 1% of the global electric power generation in the world, through the activity of 613 geothermal power plants for a total installed geothermal power generation capacity of 12,640 MWe (Bertani 2015). In this context, European power plants account for 2,133 MWe of the world installed capacity while Italian ones for 916 MWe, basically all located in Tuscany, in the area of Larderello-Travale (795 MWe) and of Monte Amiata (121 MWe).

In this Italian region, where historically geothermal energy started to be used for electricity production, it contributes significantly to the regional electrical and energy needs (Bertani 2015; Bravi and Basosi 2015). In Italy, there are two main geothermal areas at high enthalpy that are currently exploited, both located in southern Tuscany, namely the boraciferous zone in the Southeast of Pisa and West of Siena and the area of Monte Amiata in the Northeast of Grosseto and Southeast of Siena.

Stated that the only clean energy is that one which is not consumed (i.e., saved), geothermal energy can be considered a sustainable and renewable energy only if employed in a suitable environmental manner. In fact, geothermal sources properties are deeply connected to geo-mineralogical phenomena and to highly site-specific factors that have allowed the formation, store, and conservation of the reservoirs.

As any energy source, the use of geothermal energy produces impacts on the environment that are, in turn, very site-specific because of the nature of the resource and its characteristics that change according to the geological age and reservoir depth. Moreover, additional impacts are connected with the particular technology employed for power generation from geothermal energy.

In the context of the environmental impacts generated by anthropic activities, the debate concerning whether or not the geothermal power plants contribute to the global climate change issue has assumed a certain importance in Italy, and Tuscany in particular.

To date, in the analysis of the CO₂ and other greenhouse gases emissions, neither the Kyoto Protocol nor the Intergovernmental Panel on Climate Change (IPCC 2008) have considered the release of greenhouse gases of geothermal origin quantitatively significant as compared to global emissions. This approach has been based on the concept that natural CO₂ emissions from geothermal areas are comparable with those connected with the exploitation of the same area for energy purposes, thus neglecting the temporal variable altogether. Such stance could be considered controversial as the assumption regarding the comparability between greenhouse gases emissions released by a geothermal power plant in all its service life (about 25–30 years) and natural greenhouse gases emissions generated in hundreds of thousands of years might appear in contrast with all the initiatives launched to meet the 2 °C global temperature target (Paris Agreement, COP21 2015 and COP22 2016) in 2030. Indeed, natural CO₂ degassing phenomena have been identified and studied in Italy suggesting that they are strongly affected by the geological and hydrogeological settings of a particular region (Chiodini et al. 1999, 2005). Sometimes, rarely, there are areas without active power plants where CO₂-rich gas emission values are comparable or even higher than those typical of geothermal power generation activity (Chiodini et al. 2007; Frondini et al. 2009).

In addition to the climate change concerns, power generation from geothermal energy can also be significantly responsible for impact on other environmental categories on a local and regional territory scale.

Stated that geothermal energy is definitely a valuable resource with a large potential (it is estimated that only a small part of the available heat is exploited to date) and that, when available, it should always be exploited, it is necessary to implement appropriate technologies that would be able to minimize as much as possible the pressure on the environment of this activity.

Moreover, concerning the use of geothermal energy sources, the economic and energy sustainability could be improved by linking the direct use of cascading heat to electricity production. Indeed, the potential offered by multiple uses, even thermal ones, would allow for a further optimization of the geothermal energy use that nowadays seems to be excessively aimed at the most valuable vector, namely the electric vector.

To address this issue, life cycle assessment offers a powerful methodological approach for the evaluation of the environmental performances of existing geothermal power plants and for the investigation of potential impacts associated with new projects prior their construction in order to define the best strategies and implement suitable methods for environmental emissions mitigation or annihilation.

Given such framework, this study has been developed for the assessment of the environmental performances of selected Italian geothermal power plants for electricity production from an LCA perspective.

2 Environmental Impacts of the Geothermal Resource

In the literature, there are several studies dealing with the impacts generated by geothermal power plants (Hagedoorn 2006), some of which approaching this topic in a life cycle perspective (Sullivan et al. 2010; Bayer et al. 2013; Manzella et al. 2018) while others propose simulations for the production of energy in a sustainable way through management models of geothermal sources (Axelsson and Stefansson 2003).

As already discussed, geothermal energy is an energy source that generates impacts in the environment, some of which are highly site-specific, such as land use and effects on biodiversity, subsidence phenomena, heat dispersion in the surrounding environment in relation with the technology used; water consumption in the drilling and operating phases of the plant (which increases in the presence of fluid return systems in the geothermal reservoir), radon emissions; soil emissions related to the contaminants present in the volume of geothermal fluid extracted and the resulting waste, particularly for liquid-dominant systems.

The emissions into the atmosphere connected with geothermal activities are probably the environmental aspect most discussed at present and for this reason, they are the focus of the analysis carried out in this study.

Gases are naturally present in the geothermal fluids, dissolved in the liquid phase or free in the vapor phase depending on the pressure and the temperature of the tank. Gases commonly found in geothermal fluid are carbon dioxide (CO₂), hydrogen sulfide (H₂S), hydrogen (H₂), nitrogen (N₂), methane (CH₄), ammonia (NH₃), argon (Ar), and radon (Rn) (Fridriksson et al. 2016).

These gases are called non-condensable gases (NCGs) since they do not condense in the same conditions as water vapor, usually present in the gaseous phase in higher quantities, but remain in the gaseous phase generating several problems from the point of view of both productivity and efficiency of the plant and from the point of view of the environmental sustainability of energy generation. For this reason, they must be removed from the condensers and heat exchangers.

However, the removal of NCGs has both an economic and an energy impact, as the elimination process requires additional costs and the use of part of the energy produced by the plant itself. Therefore, studying the chemical composition of the tank can be useful during the development of the plant, as it allows to set up the system with technologies suitable for the management of the process depending on the composition of the geothermal fluid.

3 Methodological Issues

3.1 Goal and Scope of the Study

The aim of the research is the calculation and evaluation of the environmental performances of selected Italian geothermal power plants for electricity production from an LCA perspective in order to assess and recommend solution for the minimization of the environmental impacts in the exploitation of geothermal energy. In particular, the study is focused on the atmospheric emissions of NCGs contained in geothermal fluids produced during the operational phase. The geothermal plants considered in this study are located in the two main Tuscan geothermal areas:

• The boraciferous zone in the Southeast of Pisa and West of Siena which includes the geothermal fields of Larderello and Travale-Radicondoli

• The area of Monte Amiata in the Northeast of Grosseto and Southeast of Siena, where the geothermal fields of Bagnore and Piancastagnaio are located.

A map of the Tuscany geothermal areas showing the geothermal fields exploited for electricity generation is reported in Fig. 3.1.

Fig. 3.1

Location of geothermal areas for electricity production in Tuscany

To date, in Tuscany, there are 34 geothermal power plants managed by Enel Green Power, located in the four territorial areas of Larderello, Radicondoli, Lago and Piancastagnaio belonging to the provinces of Pisa, Siena and Grosseto, with an electricity generation in 2016 equal to about 2% of the national electricity production and 35.6% of the total regional production (TERNA 2016).

The geothermal reservoirs exploited in the Larderello-Travale/Radicondoli and Monte Amiata areas are two: a shallow reservoir contained in the cataclastic levels of carbonate rocks that produce superheated steam, and a deeper and much more diffused reservoir characterized by a crystalline (metamorphic and granitic) rock system located at a depth greater than 2 km.

In the deeper steam-dominated reservoir of the Larderello-Travale/Radicondoli geothermal field, there are values of 20 MPa and 300–350 °C at about 3 km of depth. The percentage of steam phase is between 92 and 98% and the geothermal fluid is characterized by NCGs content ranging between 1 and 20% in weight, with an average value of about 11%. The fluid composition in weight is about 97% of CO₂, 0.3% of H₂S, 0.02% H₂, 1.5% of CH₄, 1.2% of N₂, 0.2% of NH₃, and 0.02% of H₃BO₃.

In the Monte Amiata area, the deeper reservoir is liquid-dominated and there are values of 20 MPa and 300–350 °C between 2.5 and 4 km of depth. In this area, the shallow steam-dominated reservoir is characterized by NCGs content ranging between 1 and 20% in weight (average value 11%), while in the deeper reservoir, the chlorine-alkaline geothermal fluid has a high content of ammonium and boric acid and presents NCGs content ranging between 5 and 10% in weight (average value 8%, della Terra 2008). The relative percentage values that characterize the fluid composition in weight are about 97.3% of CO₂, 0.1% of H₂S, 0.05% H₂, 0.9% of CH₄, 0.1% of N₂, 1.5% of NH₃, and 3.7% of H₃BO₃.

In order to evaluate the potential impact associated with geothermal power plants production of electricity, for this analysis, the system boundaries are set to account only for the operational phase of the geothermal power plants, as shown in Fig. 3.2.

Fig. 3.2

System boundaries for the LCA of geothermal power plants; in this study, only the electric energy production phase has been considered

The consumption of resources associated with the drilling, construction, and operation of the wells and the additional materials needed for the construction and operating of geothermal plants have not been included. This is because the impact of plant construction is diluted over the assumed 25 years of plant operation and only accounts for a small amount of total foreground and background emissions (2% of yearly CO₂ emissions, 1% of yearly fossil energy use, 1% of annual matter flows, according to Ulgiati and Brown (Brown and Ulgiati 2002). The functional unit employed is the electricity (1 MWh) produced by the various plants through the conversion of the geothermal energy.

The reference timeframe for the analysis of the plants operational phase is the 2010–2014 historical series.

3.2 Life Cycle Inventory Analysis

The Tuscan geothermal power plants investigated in this study are seven and are located in the territorial areas of Piancastagnaio, Larderello, and Radicondoli. In details, power plants are:

• Monte Amiata: Bagnore 3, Piancastagnaio 5

• Larderello: Farinello, Sesta 1 and Nuova Larderello

• Travale-Radicondoli: Nuova Radicondoli 1 and Nuova Radicondoli 2

In Table 3.1, general information about the analyzed power plants are reported.

Table 3.1 Characteristics of the Tuscan geothermal power plants selected for the study: all power plants are equipped with the AMIS (abatement of mercury and hydrogen sulfide) technology in the historical series 2010–2014

Data concerning the air emissions generated by these power plants have been collected from the geothermal areas monitoring annual reports published by ARPAT (Tuscany Regional Agency for Environmental Protection) during the period 2010–2014 (ARPAT 2010–2014). ARPAT measurement data are based on sampling of the emission materials from the geothermal power plants chimneys in a defined period of the year. We assume that this sample values correspond to an average constant value during the whole year considered.

This study is focused on the potential environmental impacts associated with the emissions of those NCG that are found in greater concentration in the geothermal fluid (CO₂, CH₄, NH₃, H₂S) and that give an appreciable contribution on the selected impact categories, in addition to mercury (Hg) emissions, other metals (lead, arsenic, selenium, chromium, cadmium, nickel, copper, manganese, vanadium), metal compounds (arsenic, antimony and mercury compounds), and boric acid (H₃BO₃).

All data regarding these chemical species have been normalized with respect to the functional unit using the values of power plants inventoried from ARPAT during the tests. Sampling was performed with the following temporal frequency: Piancastagnaio 5 years 2010, 2011, 2013 (no CO₂), 2014; Bagnore 3 years 2010, 2011, 2012, 2013, 2014; Farinello years 2010, 2011, 2012, 2013; Sesta 1 years 2010, 2011, 2013; Nuova Larderello years 2010, 2011, 2012, 2014; Nuova Radicondoli 1 years 2010, 2011, 2012, 2014; Nuova Radicondoli 2 years 2010, 2011, 2012 (no CO₂), 2013, 2014.

Starting from data regarding the composition of the geothermal fluid characteristic for each geothermal field analyzed and from the production capacities based on the MWh effectively generated in the reference period, an estimate of the geothermal fluid mass flow entering each power plants for every year of the historical series was performed. Thus, it has been possible to calculate the average NCGs emission factors for each investigated power plant as the ratio of mass flows (kg/h) on the average load of the power plants measured in MWe/h.

3.3 Life Cycle Impact Assessment

The impact categories that have been selected for this study are those associated with the global warming, soil acidification, and human health issues.

For reasons of continuity and comparability of the analysis carried out in this study with previous studies published in the literature (Bravi and Basosi 2014), we chose environmental indicators defined in the impact assessment method CML 2001 V2.05: the Global Warming Potential on a temporal window of 100 years (GWP100, expressed in kilogram of carbon dioxide equivalent, kg CO₂ eq/MWh), the Acidification Potential (ACP, expressed in kilogram of sulfur dioxide equivalent, kg SO₂ eq/MWh), and Human Toxicity Potential (HTP100, expressed in kilogram of 1,4 dichlorobenzene, kg 1,4-DB eq/MWh).

In order to perform a more significant evaluation, the results of this analysis have been compared with two other power generation systems of comparable power, namely coal, and natural gas.

The environmental impact potential connected with electricity production from these two kinds of fossil fuels has been taken from the Ecoinvent database v. 2.0 (Emmenegger et al. 2007; Roder et al. 2007), where five life cycle phases are considered: before construction, construction, transportation, operation and maintenance, and demolition of power plants.

This discrepancy among the system boundaries defined in the Ecoinvent processes built for power generation plants and the system boundaries chosen for the analysis developed here would not affect the validity of the comparison proposed in the present study.

In fact, for the electricity produced by coal-fired and natural gas power plants, GWP, ACP, and HTP impact categories are predominantly due to direct emissions during the operation of the power plant. In particular, the operation phase accounts for 95% of GWP in coal and 83% in gas plants and 87 and 40% of ACP and 79 and 64% of HPT, respectively (Emmenegger et al. 2007; Roder et al. 2007).

The Ecoinvent data set employed to calculate the coal-fired and natural gas power plants emissions gave values reported in Table 3.2.

Table 3.2 Potential environmental impacts associated to the electric power generation from coal-fired and natural gas power plants for the climate change, acidification and human toxicity categories

To perform the analysis, the software Simapro v 8.0 has been employed.

4 Results and Discussion

In general, geothermal wells reaching over 3000 m of depth are realized through earth’s crust drilling that increases the permeability of both geothermal fluids and NCGs. The amount of gases and metals contained in geothermal fluids and releases to the environment depends on several factors: depth and location of the geothermal reservoir; characteristics of the electricity generation (flash, binary, or combined cycle) and the abatement systems.

The outputs of the analysis performed in this study are reported in Fig. 3.3 for the three selected impact categories. Looking at the indicators’ trend along the reference historical series, it is evident that the environmental impacts connected with the air emissions associated to the NCGs releases are quite significant, at least, for two impact categories when compared to the potential impact connected with electric energy generation through coal-fired and natural gas power plants.

Fig. 3.3

Environmental impacts for GWP, ACP and HTP indicators of the geothermal power plants analyzed in the historical series 2010–2014 (average values for coal-fired and natural gas power plants are in dark gray and light gray, respectively)

The GWP computed values depend on the high quantities of CO₂ contained in the fluid that characterize all the analyzed geothermal areas, due to the presence of carbonate rocks in the reservoirs that release gaseous CO₂ in the fluids at high temperatures.

In the Monte Amiata area, the average value of GWP for Bagnore 3 is 771 kg CO₂ eq/MWh, while for Piancastagnaio 5, it has been calculated an average value of 646 kg CO₂ eq/MWh. Such dissimilarity is due to the different conversion technology employed, but most of all it depends on the different geochemistry of the reservoir from which the geothermal fluid is extracted: the elevated values for Bagnore 3 are due to the deeper reservoir exploited by the power plant that extends into metamorphic rock system with consequent presence of high concentrations of CH₄ in the geothermal fluid. In this context, even if CH₄ occurs at lower concentration than CO₂, it has an emission factor 34 times higher than CO₂ and thus it gives a significant contribution to the GWP value. Results for Piancastagnaio 5 reflect the difference in the geomorphological characteristics of the reservoir from which the geothermal fluid is extracted. In fact, the Piancastagnaio reservoir is less deep and it extends into a carbonate rocks system that is characterized by a considerable CO₂ concentration but a minor CH₄ presence compared to the Bagnore reservoir.

In the Travale-Radicondoli area, differences between the two geothermal power plants are mostly due to the installed capacity as both exploit the same geothermal reservoir. The average values are 383 kg CO₂ eq/MWh for Radicondoli 1 and 550 kg CO₂ eq/MWh for Radicondoli 2.

In the Larderello area, lower GWP values were obtained with differences that depend on the effective working capacities of the power plants. The average values are 259 kg CO₂ eq/MWh for Farinello, 196 kg CO₂ eq/MWh for Nuova Larderello and 109 kg CO₂ eq/MWh for Sesta 1.

Concerning the ACP values, for Monte Amiata area, we calculated notable impacts with average values equal to 11.35 kg SO₂ eq/MWh for Bagnore 3 and to 1.94 kg SO₂ eq/MWh for Piancastagnaio 5. In the former case, the higher acidification values depend on the anomalous content of NH₃ in the geothermal fluid extracted from the Bagnore reservoir.

In the Travale-Radicondoli and Larderello areas, we found lower values on the average compared to those computed for the Monte Amiata area that depend on the lower concentration of NH₃. The average values are 1.84 kg SO₂ eq/MWh for Nuova Radicondoli 1, 3.2 kg SO₂ eq/MWh for Nuova Radicondoli 2, 2.96 kg SO₂ eq/MWh for Farinello, 3.11 kg SO₂ eq/MWh for Sesta 1 and 2.07 kg SO₂ eq/MWh for Nuova Larderello. Nevertheless, these ACP values are remarkable when compared to the effects connected with electric energy generation from the two selected fossil resources.

On the other hand, regarding the impacts calculated for the HTP indicator, we found values about 15 times lower than those potentially generated by the use of coal and natural gas for power generation, meaning that at least from the human toxicity point of view the geothermal energy production has a much lower impact.

The effects on this impact category are principally due to the presence of NH₃, H₂S, H₃BO₃, and several metals in the geothermal fluid. Also, in this case, the Monte Amiata area is characterized by more significant results for the sizeable presence of Hg, NH₃, H₂S, and H₃BO₃ in the geothermal fluid in both the reservoir that are exploited by the two power plants: for Bagnore 3 the average value is 0.88 kg 1,4-DB eq/MWh and for Piancastagnaio 5 is equal to 0.5 kg 1,4-DB eq/MWh.

In the area of Travale-Radicondoli, the average computed values are 0.7 and 1.1 kg 1,4-DB eq/MWh for Radicondoli 1 and Radicondoli 2, respectively. Even if mercury has not been detected in 2010, the value of Radicondoli 2 is double compared to the other power plants because of the ammonia and hydrogen sulfide contributions.

In the Larderello area, the computed values are even lower for reasons depending on the geothermal fluid composition and the effective power plants capacity (average values in 1,4-DB eq/MWh: Farinello: 0,33; Sesta 1: 0,361; Nuova Larderello: 0.26). Anyhow, the toxicity potential found in this study show no worrying values. Moreover, in support to our findings, it should be mentioned that a careful monitoring by the ARS (Regional Health Agency) has already been underway for several years and despite the fact that it has detected some critical health problems in the Monte Amiata area compared to other Tuscan contexts, it has not yet shown significant correlations with geothermal activity.

5 Concluding Remarks

The results of this study show that exploitation of the geothermal resource, although desirable to replace abuse of fossil resources, does not produce a zero impact and, in particular, it cannot be considered carbon-free.

By focusing the attention on emissions into the atmosphere, it can be shown that there are various factors responsible for variations in the composition and mass of NCGs and metals that are released from the cooling towers of the various plants: location and depth of the reservoirs, characteristics of the technology used (flash, dry steam, binary cycle, combined cycle, etc.), and the abatement systems adopted.

For all these reasons it is evident that, from the comparison among various power plants located in different regions or countries, it is not possible to perform wide range analyses or forecasts valid for multiple sites nor to collect universal data concerning the geothermal power production.

The assessment of the impacts associated with the exploitation activities of geothermal systems is highly site-specific, exactly like the resource. Such assessment should be carried out starting from inventory data as accurate and complete as possible in order to propose solutions and interventions aimed at optimizing the performance of the plants, with a view that should privilege the minimization of environmental pressure, even to the detriment of economic aspects, if necessary.

In addition to abatement systems that work properly and in continuous, the total reinjection of geothermal fluids into the same sampling basin in controlled conditions is the best way to go in order to make geothermal energy a cleaner and safe source of energy with higher social acceptability.

Only with the development of more advanced geothermal exploitation technologies set up to minimize the pressure on the environment, it will be possible to pursue the use of this natural and renewable resource.

To date, progresses made with the closed-loop technology (generally based on the organic rankine cycle—ORC), which exploits geothermal fluids only to transfer heat to a working fluid in a circuit system that supplies the plant, allow to hypothesize viable solutions for geothermal systems even with high enthalpy fluids and with a high concentration of NCGs like those present in Tuscany.

However, we are aware of technological limitations due to site-specific resource conditions, which make it difficult to apply alternative and less invasive technologies (e.g., binary cycles or mixed flash-track systems). The complete re-injection of NCGs in contexts similar to the Monte Amiata one (geothermal fields with a high percentage of NCGs, high pressure and temperatures), that also takes into account aspects related to the economic sustainability of such solutions, should be one of the main challenges that geothermal technological research will have to face in the next future.

In principle, a technology that takes into account environmental and social issues, rather than exclusively economic/financial aspects should be developed. In addition to reducing the pressure of this activity on the environment, this would also allow to overcome or mitigate the social and political concerns that have hitherto slowed down development in the use of a particularly valuable resource at national level and especially for the territories where it is located.