Environmental and Socio-Economic Impact of Deep Geothermal Energy, an Upper Rhine Graben Perspective

Abstract

The Upper Rhine Graben is a region renowned in Europe for the exploitation and development of geothermal energy with projects in France, Germany and Switzerland. In the last 20 years, numerous seismic events have been felt by local population triggering social concerns that have been addressed at different levels (state regulation, technical adaptation of projects and communication). Indeed, geothermal projects need a high level of acceptance by inhabitants in the surrounding area. In this regard, the local socio-economic impact is a crucial factor in social acceptance. Nevertheless, this energy resource has many advantages such as competitive heat prices and low environmental impacts, quantified by Life Cycle Analysis. This approach is also completed by continuous environmental monitoring. Moreover, additional valorization of geothermal water through its use for low temperature heating or recovery of mineral resources are ways of providing additional benefits to the local community. This chapter is dedicated to present the environmental and socio-economic impacts of two operational EGS projects (Soultz-sous-Forêts and Rittershoffen) located in Northern Alsace (France) producing geothermal electricity and heat in a rural area.

1. Introduction

Geothermal development in the Upper Rhine Graben (URG) involves a geothermal doublet system consisting in a production well with a down-hole pump and an injection well which reinjects cold water into the geothermal reservoir. Thus, they consist of two deviated wells that crosscut a local permeable normal fault or fracture zone in which geothermal brines are circulating by thermal convection [1] (Figure 1). Typical production and injection temperatures in the URG range from 150–170°C to 60–80°C.

Figure 1.

Schematic of a generic deep geothermal project in the URG showing a doublet structure (production and reinjection wells) in a naturally permeable faulted reservoir.

Over the last 20 years, several deep geothermal energy projects in Europe experimented with enhancing initially low reservoir permeability based on various stimulation techniques. Those projects known as Enhanced or Engineered Geothermal System (EGS) are mainly located in the Upper Rhine Graben in France (Soultz-sous-Forêts, Rittershoffen, Vendenheim, Illkirch), Germany (Landau, Insheim, Bruchsal) and Switzerland (Basel, Riehen) (see Figure 2). The Riehen project located in the Eastern part of the URG, is not considered as a EGS project but as a hydrothermal project [1].

Figure 2.

Map of deep geothermal projects in the upper Rhine graben, modified after [2]; project abbreviations: Cr.: Cronenbourg; Eck.: Eckbolsheim; GN: Graben-Neudorf; hurt.: Hurtigheim; Illk.: Illkirch; Ritt.: Rittershoffen; Ven.: Vendenheim; and Wiss.: Wissembourg.

This chapter presents the environmental and territorial impacts of these geothermal projects. It focuses on induced seismicity and how the operators and mining authorities have introduced operational limitations to mitigate the seismic risk. Additionally, it reviews the other risks to the local environment and their mitigation practices in comparison to the environmental benefits of energy resource. Furthermore, it highlights the economic impact of the development of geothermal energy and mineral extraction on the local area. This chapter is mostly illustrated with the French experience of EGS in the URG but also includes German and Swiss examples where relevant.

2. The upper Rhine graben, a transborder region where deep geothermal energy is commonly associated with seismic risk

In the URG, natural water infiltrates and circulates through convection loops up to the interface between sediments and the basement due to a natural network of faults and fractures [3]. To obtain an economically viable flow rate in the production and injection wells, various techniques may be applied to enhance the well connection to the fractured reservoir, through thermal, chemical and/or hydraulic stimulation techniques which qualify them as EGS [4]. Induced microseismicity can occur in the vicinity of the well due to the reinjection of the water in the fractured reservoir: due to a hydromechanical mechanism [5], but also a thermal effect [6].

The examples listed below focused on deep geothermal energy and the associated induced seismicity which occurs during hydraulic stimulation phases on average after only a few days of technical operation. Geothermal sites in the exploitation phase that represent more than at least two decades of continuous operations are also presented below. The drilling phases and related potential nuisances are not considered here given that there are generally, no seismic events during drilling operations.

2.1 Impact of massive injection during hydraulic stimulation

2.1.1 Soultz-sous-Forêts site

Even though the Soultz (Soultz-sous-Forêts) wells were stimulated hydraulically and chemically several times from the 1990s onwards [7] only a few examples of felt seismicity have occurred during hydraulic stimulations at Soultz in 2000 and 2003. The most striking episode corresponds to a massive hydraulic stimulation carried out in the Soultz fractured granite reservoir in 2003 at a depth of 5 km with a wellhead overpressure of around 170 bar, when an induced seismic event was felt with a magnitude MD of 2.9 (MD—Magnitude Duration) [8]. In that specific case, experts from the site owners and the local population’s insurers were mobilized, but were unable to prove any concrete structural damage to housing. Therefore, no damage was actually caused by this event, but in the minds of local residents, acceptability became a real issue. Thus, the site’s operators are continuing to develop this site whilst minimizing hydraulic stimulation and explaining and communicating in-depth with local stakeholders such as politicians or associations.

2.1.2 Basel geothermal site

A deep geothermal well drilled at Basel in a fractured granite reservoir at 5 km depth in the southern part of the URG was hydraulically stimulated by massive injection with a well-head over pressure of 300 bar. This event caused structural damages and led to the permanent shutdown of this project [9]. The Basel project is considered as a counter reference for EGS development because an earthquake of magnitude ML > 3.4 (ML—Magnitude Local) was felt during a hydraulic stimulation in 2006 [10].

2.1.3 Vendenheim geothermal site

In the Strasbourg area, two deviated wells were drilled to a depth of about 5 km in a fractured granite reservoir in an urban area. A series of man-made earthquakes was felt between 2019 and 2021 in the Strasbourg area and on the German side induced by a complex sequence of hydraulic injection involving both high well-head overpressure and high cumulative massive volume [11]. The second largest event (ML 3.6), induced on 4 December 2020, led to the project being permanent shut down, but further events continued to be felt later, in 2021, with the largest one reaching a maximum magnitude of ML > 3.9 felt on the surface on 26 June 2021 [12]. As a result of the structural damage observed on many houses, the Prefect of Strasbourg decided to suspend all geothermal activity in this urban area.

2.2 Impact of geothermal exploitation on induced seismicity

2.2.1 Landau geothermal site

The Landau geothermal plant is made up of two deviated wells drilled to a depth of about 3.5 km in fractured granite. Hydraulic and chemical stimulations were successfully conducted to improve permeability without any felt seismicity [13]. However, during geothermal exploitation, an induced event was felt on 15 August 2009 with MD = 2.7. Moreover, from 2013 to 2014, a technical incident occurred in the injection well inducing an uplift of several centimeters in the geothermal platform [14]. Thus, some damage potentially caused by the uplift was observed. The injection well was repaired and the geothermal exploitation restarted some years later.

2.2.2 Rittershoffen, Soultz-sous-Forêts and Insheim sites

Some other geothermal projects, such as Rittershoffen are considered as geothermal success story because no induced earthquakes were felt during their development phase (thermal, chemical or hydraulic stimulation) [4], or during geothermal exploitation [15]. The maximum local magnitude recorded during the on-site development phase on was 1.7 [15]. This plant has now been operating for 6 years and has a capacity of more than 24 MW of heat at highest flow rate in the URG (i.e. more than 80 L/s).

At Soultz, no induced seismic event have been felt after more than 6 years of exploitation [16]. The same observations are made for the Insheim geothermal power plant in Germany [17].

2.3 Impact of felt seismic events on deep geothermal energy development

2.3.1 Differing risk perception towards new projects

Following the shutting down of the Vendenheim project in 2020, the Illkirch project located in the southern part of Strasbourg was also suspended by the French mining authorities even though no induced event related to this site had been felt. Therefore, only one deviated well was drilled in fractured granite to a depth of 3.3 km, and this geothermal site is now on standby awaiting a decision from the mining authorities for it to go ahead. The Prefect of Strasbourg mandated a group of scientific experts to provide a better appreciation of the seismic behavior of the deep reservoir and to find out why such sequence of felt events took place. Nevertheless, it is clear that the series of seismic events at Vendenheim drastically affected the perception of geothermal energy by the local population in the Strasbourg’s area.

Despite these seismic events, some projects have continued their development such as the Riehen geothermal heat plant. This plant extracts geothermal water at a depth of 1500 m in fractured Triassic limestones to deliver 20 L/s of water at 65°C. It has been providing thermal energy to 8500 residents since 1994. Despite its very close proximity to Basel, the two city centers being just a few kilometers apart, this plant has not been affected by the overall distrust shown towards geothermal energy. Indeed, in Autumn 2020, in a timeframe coinciding with seismic events in Strasbourg, a plan to expand this geothermal plant was put to a local referendum and accepted by the population. To enhance social acceptance, the Riehen and Basel local utility companies, developing this project, set up transparent communication mediated by an independent third party, the Risiko-Dialog foundation (https://www.risiko-dialog.ch/en/geothermie-im-dialog/, July 2022).

On the German side of the Upper Rhine Graben, projects developed after 2015 changed the drilling target from the deep fractured granite basement to the shallower fractured Triassic sediments (mostly sandstones) overlying the basement. This change in target was accompanied by a communication drive to highlight the lower seismic risk associated with such reservoirs. Currently, in 2022, these projects are still in development, and further observations will be necessary in the upcoming years to assess this strategy. On the French side, a similar approach has been developed since 2020 with projects targeting a relatively shallower depth (down to max. 3500 m) compared to that of Vendenheim or Basel (around 5000 m). Furthermore, the mining authorities are requesting more detailed seismic risk analyses as part of the process of examining applications for authorizations to drill geothermal wells.

These examples highlight the fact that, in spite of the various counter examples cited above, the public perception of the seismic risk associated with deep geothermal energy in the URG is dependent on many factors.

2.3.2 How local population could contribute as a stakeholder to geothermal development in Alsace?

The first acceptability survey conducted in Alsace was carried out in 2012 among the local populations living close to the Soultz power plant, including the two villages of Soultz-sous-Forêts and Kutzenhausen [18]. This study, which involved the mayors of the two villages, demonstrated that the Soultz plant was well accepted by the local population even if there were some complaints about some technical nuisances such as the noise generated by the geothermal plant or reservoir management drawbacks such as induced seismicity.

At that time, geothermal energy was perceived as a favorable technology by the local population, even if there were some people who were reticent. In conclusion, the risks related to geothermal exploitation were reasonably well accepted by residents in the surrounding area.

More recently, as part of the EU DESTRESS project, social scientists have conducted an acceptability study comparing the public perception of geothermal energy in a rural area like Northern Alsace, where geothermal energy is accepted, and in an urban area like Strasbourg, where geothermal projects have raised some issues [19]. It turned out that locally “anchored” projects implemented by companies with local roots are much better perceived by the public than “unbound” or “off-ground” projects managed by non-Alsatian companies with no cultural connections or history in those territories.

The most recent, ongoing study dealing with public perceptions of deep geothermal energy in Alsace is related to participatory science. This can be defined as forms of scientific knowledge production in which civil society actively participates. Such projects promote dialog between science and society. For instance, based on a scientific approach, scientists contribute to exchanges with citizens on growing concerns about induced seismicity related to deep geothermal energy in Alsace (https://anr.fr/Projet-ANR-21-CE05-0033). The University of Strasbourg has launched a new research project that involves an innovative way of testing a new collaborative geohazard monitoring paradigm in an urban context. The basic principle is to deploy a large number of cheap seismological sensors, working closely with mining authorities and citizens to get a dense seismological dataset. This research project was proposed by scientists after the seismic events which took place in Strasbourg in December 2020. It aims at evaluating the seismic risks induced by the geothermal operations and how they are perceived by the population. The results will be co-analyzed by scientists and non-seismological experts. Based on such a collaborative approach, we can measure public involvement and how induced seismicity related to industrial operations is perceived and represented in the URG.

On the French side of the URG, local populations are also invited to participate in public inquiries when geothermal projects are close to the operational stage. Organized by the local Prefecture, public consultations consist in making available technical documents that are in most cases difficult for citizens not familiar with science and technology to understand [20]. These consultations are seen as a way of measuring acceptability more than active participation by citizens. For instance, low participation by the public in such inquiries is interpreted as a high level of acceptance of the project (low level of protest). The main criticism voiced by inhabitants concerns the potential impacts of drilling operations on their environment, such as induced seismicity, groundwater pollution and radioactivity issues [20]. Moreover, the technology used to produce electricity (Organic Rankine Cycle) is also liable to raise some major concerns. The risk of explosion due to the presence of isobutane in the power plant was a major argument against one project in the Strasbourg area, where the site had pre-existing industrial risks (Seveso zone). A consensus between inhabitants, residents’ associations and local politicians against this urban geothermal project led the operator to give up on this site.

3. A look-back at the evolution of seismic monitoring

As mentioned in the previous section, it has been shown that stimulation operations (even moderate ones) and exploitation of the geothermal loop are likely to generate induced micro-seismic activity, temporarily or continuously [15, 21], which can occasionally be felt by local population [22, 23]. In extreme cases, the occurrence of induced seismicity may lead geothermal operations to be shut down, as was the case of the geothermal project in Basel [24] or Vendenheim [11]. As a result, it became common in deep geothermal energy projects, which target naturally fractured reservoirs, to monitor geothermal activities with high-sensitivity through seismological networks. Initially, on the European EGS pilot site at Soultz-sous-Forêts, the seismic monitoring was dedicated to understand the fracture network activated by hydraulic stimulation, and in order to get an image of the reservoir development. More recently, the objective of the seismic monitoring then switched to the necessity to minimize the seismic risk [25].

These networks are generally designed to accurately assess the state of the natural seismicity before any operation, but also to detect any emergence of induced micro-seismic activity attributable to the geothermal operations.

Until 2015, there was no regulatory framework in France to supervise the environmental monitoring of geothermal plants. The deployment of such monitoring networks was left up to the operators. The experience acquired at the Soultz-sous-Forêts and Rittershoffen geothermal plants, together with the growing number of operators involved in deep geothermal energy, highlighted the need to monitor these systems. For this purpose, the French mining authorities established clear rules to regulate the construction, development and exploitation of geothermal plants. The same happened in Germany, where the mining authorities of regions (e.g., the Palatinate) developed their own regulatory framework. In Switzerland, the canton recommends good practices but does not impose a clear regulatory framework.

Table 1 gives a comparison of the different regulation on seismic monitoring in France, Germany and Switzerland.

	France/Alsace	Germany/Palatinate	Swiss
National or regional framework	Regional	Regional	Cantonal
Authority	Local mining authority	Local mining authority	Local mining authority
Regulation	Prefectural decree (legifrance.gouv.fr)	DIN 4150 (beuth.de/de/norm/din-4150-3)	ETH Good Practice Guide [26]
Seismic network Mandatory	Yes	Yes	Recommended, category III for traffic light system
Seismic network technical requirement	Real-time data monitoring 4 short period velocimeters 1 “multi-sensor” station including a broad-band seismometer, an accelerometer, a GNSS receiver and a corner-coin reflector	One network designed with four velocimeters according to the DIN 4150 (on buildings) One real-time network, that can use both accelerometers or velocimeters; Number (generally 4 stations) and kind of stations of this network needs to be coordinated with the mining authority	Real-time data monitoring Shallow borehole (80–150 m depth) At least 3 stations around the project One station in the center of the network equipped with an accelerometer 3 orthogonal components for all stations
Seismic network installation	6 months before start of drilling	3 months before or at the latest when drilling starts	6 months before stimulation
Public data	All data from the “multisensory” station	All data from the velocimeters used to monitor DIN 4150 compliance	Possibly all data
Organization collecting public data	ReNaSS (French seismic monitoring network)	The local (normally state operated) seismic observation network	SED (Swiss Seismological Service)
Definition of threshold	Yes	Yes	Yes
Physical value used for defining thresholds	PGV	PGV	ML and PGV
Number of thresholds	3 thresholds (measured on 2 stations): 0.5 mm/s, close monitoring 1.0 mm/s, short term reduction in flow 1.5 mm/s, stop operation	5 thresholds (measured on 1 station only): 0.2 mm/s, daily reporting 0.5 mm/s, short term reduction in flow 1 mm/s, long term reduction in flow 5 mm/s, operate at minimum flow 10 mm/s, stop operation	To be defined with local mining authority
Reporting to the mining authority	Before drilling: report on natural seismicity; Drilling: monthly; Testing: daily; Operation: monthly	Before drilling: report on natural seismicity and a seismic hazard analysis; Testing: daily; Operation: monthly	To be defined with local mining authority

Table 1.

Comparison of the regulation on seismic monitoring in France, Germany and Switzerland (regulations in force in 2021).

Along with this clarification of the regulations, the operators have developed their workflows to clarify the decision-making process in event of defined thresholds being exceeded (see Figure 3). Up to now, by following this procedure French operator Électricité de Strasbourg has managed to keep the seismic risk under control since no induced seismicity has been felt by local population around the exploitation of Soultz and Rittershoffen geothermal plants since 2016 or during the drilling of GIL-1 well in Illkirch [16].

Figure 3.

Decisional chart designed by Électricité de Strasbourg in case of occurrence of induced micro-seismic activity, based on French mining authority regulation.

In the future, it can be expected that new ways of managing seismic risk on site will appear, with the use of predictive tools to anticipate the occurrence of events that could be felt by the population [26].

4. Environmentally friendly energy source

4.1 Environmental impacts

Apart from induced seismicity, which is presented in the previous section of this chapter, geothermal energy can induce other disturbances in the environment and for residents. Five kinds of impacts are identified, minimized when possible and monitored [27]. They correspond to slow surface deformations, shallow groundwater protection, contamination of soils by geothermal brine leakages, precipitation of radioactive scales in surface infrastructures, and noise from the geothermal plant. Their main impacts and mitigation schemes are presented below:

Slow surface deformations (subsidence, uplift) has been identified as a potential major impact of the plants on their surrounding areas since it was reported for the Landau power plant in the German part of the URG [14]. To identify such slow vertical ground motion before it reaches significant deformation, a telemetered GNSS (Global Navigation Satellite System) receiver is installed on each geothermal plant platform. Additionally, one corner reflector is installed to measure surface deformations through satellite radar interferometry (InSAR technique). Results of the GNSS monitoring is reported on a monthly basis to the mining authorities. To date (July 2022), no significant ground motion has been observed at the different geothermal sites in the French part of the URG.

The protection of shallow groundwater resources is a major concern in areas where the Rhine aquifer is present. This regional aquifer represents 35,000 millions of cubic meters, making it one of the biggest freshwater resources in Europe. It is extremely important for the region’s economic development and its drinking water supply. However, this groundwater resource is very vulnerable. More than a third of its surface is already undrinkable without treatment, due to various human activities (https://www.ermes-rhin.eu/FR/documents-et-publications/copy-of-acc%C3%A8s-libre.html). Since 2014 new projects have been in development in the Strasbourg area (Illkirch, Vendenheim), where this aquifer can reach a thickness of 150 m. For these projects, the design of the geothermal wells involves isolating the geothermal water from the aquifer by three cemented casings. Over the lifetime of a plant, the casings should be inspected on a 3-year basis for the injection well and 6-year basis for a production well. All these inspections must be reported to the mining authorities. In addition to these mechanical barriers, a piezometric monitoring network has been deployed. The monitoring starts 3 months prior to the drilling and continues after the drilling and well testing phases. For instance, during all the geothermal activities at the Illkirch site, physico-chemical parameters such as pH, Eh (redox potential), conductivity, temperature and water level were continuously monitored and were available remotely in real time, to quickly detect any possible leakage. The Rhine water was sampled before and during the drilling to perform detailed chemical analyses and monitor possible contamination due to the geothermal activities. In Illkirch, an important result was that the Rhine aquifer water remained drinkable and unpolluted during all of the geothermal activities [16].

The leakage from the geothermal loop, mostly scale formed in the plant’s piping and geothermal water could lead to the contamination of soil or surface water in the vicinity of the geothermal plant. Indeed, geothermal water is highly saline (over 3 times more than seawater [28]) and the scales are currently mostly made of galena (PbS) which contains heavy metal and radionuclides [29]. This risk is managed at different levels. A fundamental parameter to assess, to prevent leakage, is corrosion in the geothermal loop. Corrosion is a major issue that must be taken into account in plant design. To prevent corrosion issues, the most strategic parts of the geothermal loop (heat exchanger, filters, some valves and pumps) are made of a specific grade of steel which shows high resistance to corrosion from the URG geothermal water [30]. Less critical parts are usually made of carbon steel including an over thickness to ensure their longevity. During the operation, corrosion inhibitors are used and corrosion monitoring is performed through either coupon testing or corrosion probe. Additionally, several parts of the geothermal loop are inspected on a regular basis to check the current corrosion situation and anticipate part replacement if needed. In the design of the plant, a specific water management system is set up to keep the different types of water (geothermal water, wastewater and rainwater) apart and prevent the geothermal water contaminating the environment. The rainwater at the power plant is collected in a tank by gravity and treated with a sand filter together with a scrubber and an oil separator. Before releasing the rainwater into the environment, an operator checks its conductivity. If this measurement indicates contamination of the rainwater by the geothermal water, it is pumped to the geothermal storage pool and then reinjected into the geothermal reservoir.

As mentioned above, scale formed from the geothermal water can contain radionuclides [31], mostly ²²⁶Ra and ²¹⁰Pb and their respective daughter elements. Therefore, it is important to keep the dose rate as low as reasonably acceptable for both, the workers and the public. Periodic radioactivity and dose rate measurements are performed on site to evaluate the risk within the installation, identifying zones with restricted access (where the public is not allowed) and where wearing a dosimeter is mandatory. In addition to these monitoring, an important research and development effort has been made since 2009 to reduce mineral precipitation in the power plant. This project led to the elimination of barite which was the radium bearing mineral [32, 33]. Whereas ²²⁶Ra has a long half-life and generates important gamma-ray emissions that can propagate trough the pipes, ²¹⁰Pb is a short half-life radionuclide (less than 30 years) emitting mostly alpha and beta rays that cannot propagate through the pipes. Thus, the main remaining risk is currently inhalation and ingestion. Since 2020, an annual dust and radon measurement campaign has been performed to assess this risk for workers and in the area around the plants. These measurements are not significantly higher than the reference points outside of the plant [34].

Noise near the geothermal plant has also an impact on local residents. Indeed, in 2012 when an acceptability survey of the Soultz power plant was performed, collecting the perceptions of the inhabitants of Soultz-sous-Forêts and Kutzenhausen, the most cited impact was noise from the plant [18]. However, at that time, the air cooling system had a defect, which was temporarily generating an abnormal level of noise, but this was quickly corrected.

In France, geothermal plants must meet maximum noise emission levels at the boundary of the facility, i.e. noise must not exceed 70 dB(A) during the day and 60 dB(A) at night, except if the ambient noise is above these limits. Additionally, it should not increase the ambient noise in surrounding regulated noise area—such as a residential area—more than the values presented in Table 2.

Ambient noise in regulated area, including noise emission of the project	Acceptable emergence values between 7 am and 10 pm, excepted Sundays or public holidays	Acceptable emergence values between 10 pm and 7 am, and on Sundays or public holidays
Between 35 dB(A) and 45 dB(A)	6 dB(A)	4 dB(A)
Above 45 dB(A)	5 dB(A)	3 dB(A)

Table 2.

Acceptable sound emergence values in the regulated areas of the geothermal projects in northern Alsace.

During the plant design phase, a noise impact study is performed for the selection of low-noise emission equipment, such as the air condenser, but also for proper positioning of the equipment on the power plant platform. It can provide recommendations for sound insulation (for instance, anti-noise wall around the plant) to respect the noise regulations.

4.2 Life cycle assessments

As a complement to the site-specific analysis of environmental impacts and their respective mitigation schemes, on a general level, the environmental impacts of geothermal energy can be assessed using the Life Cycle Assessment (LCA) methodology. LCA is a widely accepted and standardized methodology which translates the resources, materials and energy flows necessary for the entire life cycle of a system into a series of potential environmental impacts [35, 36]. To make sure that geothermal energy does not involve additional environmental impacts, LCA can provide very valuable information to ease decision-making processes and make comparisons with other energy sources, even if some potentially relevant environmental impacts are still missing from current LCA methodology, such as noise or seismicity.

The first LCA of a geothermal plant in the URG was published Lacirignola et al. in 2013 [37]. This comprehensive study presents the environmental performances per kWh of electricity of the Soultz geothermal plant considering different design options. Greenhouse gas (GHG) emissions contributing to global warming were estimated at about 36.7 gCO₂eq/kW. The GHG emissions from power generation of the Soultz geothermal plant appeared to be lower than the average GHG emissions of the French electricity mix, which are about 58 gCO₂eq/kW or German electricity, 349 gCO₂eq/kW (https://www.statista.com/statistics/1291750/carbon-intensity-power-sector-eu-country/). This LCA was then used to propose a simplified model for the estimation of greenhouse gases emitted by an enhanced geothermal system for power generation [38].

The GHG emissions of the Rittershoffen geothermal heat plant were first assessed by [39]. This preliminary work was then completed in line with the methodological guidelines developed as part of this European H2020 GEOENVI research project which provided recommendations to harmonize methodological choices in each of the four steps of LCA [40]. Environmental performances per kWh of heat of the Rittershoffen geothermal plant were published by [41] and compared to at the production of 1kWh of heat with natural gas. Results of this study are presented in Figure 4. A parameterized model for enhanced geothermal system for heat production was established based on the Rittershoffen geothermal plant LCA to assess seven environmental criteria [42].

Figure 4.

Comparison between the production of 1 kWh of heat from natural gas and from the Rittershoffen geothermal plant for different impact categories: Acidification (ACI), climate change (CC), freshwater ecotoxicity (Eto), marine eutrophication (Eum), freshwater eutrophication (Euf), terrestrial eutrophication (Eut), human toxicity-cancer (HTC), human toxicity-non-cancer (HTN), ionizing radiation-human health (IR), land use (Lnd), ozone depletion (ODP), particulate matter (PM), photochemical ozone formation-human health (POz), resource use-fossils (REn), resource use-minerals and metals (RMi), and water use (wat). From ref. [41].

According to Figure 4, the potential impacts of the Rittershoffen geothermal heat plant are similar to or lower than those of natural gas in most impact categories. In particular, the potential impact on Climate change is estimated at 3.7 gCO₂eq/kWh for the Rittershoffen geothermal heat plant. This impact is 67 times lower than that of heat from natural gas. The only exception is the Human Health—Ionizing radiation impact category, for which the environmental impact is higher for the Rittershoffen geothermal plant. This impact is indirect, due to the electricity consumption during operation of the geothermal heat plant provided by the French electricity mix, which is 75% nuclear. A mix with a higher share of renewable energy, or a heat plant with on-site self-power generation, would automatically reduce the impact in this category.

The Soultz and Rittershoffen LCAs confirm that geothermal energy generated under URG conditions has very low environmental impacts. This energy appears to be a promising renewable energy source for the decarbonization of power generation, district heating and heat used in industrial processes in Europe.

5. Contribution of geothermal energy to local sustainable development

5.1 Impact on the local economy

The impact of deep geothermal energy on the local economy is unfortunately not well documented. This impact can be assessed using Life Cycle Cost Analysis (LCCA). LLCA is a tool mainly used to determine the most cost-effective option for an object or process among different alternatives and over its entire lifetime. LLCA also provides information about the origin of purchase, supplier typology or the creation of added value. That information can be used to assess the impact on the local economy.

Perez et al. published a first study assessing the environmental and economic impact of the Rittershoffen geothermal plant [43]. An LLCA was performed for the entire project lifetime. The life cycle costs of the Rittershoffen geothermal plant (Figure 5) were detailed for different levels (local, i.e., Department of Bas-Rhin, national, i.e., France, and international) and for 4 project stages: (1) Exploration and drilling, (2) Geothermal plant construction, (3) Geothermal plant operation, and (4) End of life.

Figure 5.

Life cycle costs of the Rittershoffen geothermal plant.

This first study clearly confirmed the benefits of the deep geothermal project in the URG for the local economy. Indeed, about 45% of expenditure over the lifetime of the project benefits the local economy and about 87% the national economy. The construction, operation and end-of-life phases have stronger relative impacts on the local economy: respectively 48, 60, and 57% of the total costs compared to exploration and drilling, which only contributes 19% of the total costs. Indeed, drilling and service companies are mostly part of the upstream oil and gas industry, which is located outside the department of Bas-Rhin or even outside France.

5.2 Impact on local employment

Impacts on the local economy can also be evaluated according to the impact on local employment. The study in [43] also assessed this aspect, based on the Rittershoffen geothermal plant LCCA. In this study, direct employment was defined as workers who are employed by companies involved in the different stages of the life cycle of the Rittershoffen project and indirect employment as job creation in the local economy due to demand created by the project and its direct employees. Indirect employment is unfortunately very difficult to assess, and as a result this study focused on direct employment within the boundaries of France. The unit used in this employment assessment study is full-time equivalents (= 1607 h/year) (FTEs).

Input data used for this study were the plant owner’s accounting and other data on its suppliers such as legal structure, location, business identification, activity sector and economic data (turnover, added value and average number of permanent staff). These data were enriched with national economic statistics extracted from data produced by INSEE (National Institute of Statistics and Economic Studies), which collects, analyses and disseminates information on the French economy and society. Associating costs with the location of the companies involved in the life cycle costs of the Rittershoffen geothermal plant has made it possible to identify where the direct jobs are located: either at local level in the French Department of Bas-Rhin or at national level (Figure 6).

Figure 6.

Direct employment in France during the life-cycle of the geothermal plant at Rittershoffen.

Exploration, drilling and plant construction, from early 2012 to mid-2016, occupied about 124 FTEs. Annually, maintenance and operation of the geothermal plant has involved about 4 local FTEs and 1 in the rest of France, which amounts to about FTEs over 25 years of operation. End-of-life jobs are estimated at about 12 FTEs over a period of 4 to 6 months for well cementing and plant dismantling.

This study shows that the geographical distribution of the direct employment within France during the life cycle is like that of the life cycle costs. Direct employment is more important at local level than at national level during the operation and construction phases. Conversely, direct employment is stronger at national level during the exploration and drilling phases. It is the local economy that has mainly benefited from direct employment resulting from the Rittershoffen project, with 63% over the entire lifetime of the project and 82% during the 25 years of operation.

Further investigations are nevertheless required to assess the impact on local indirect employment.

5.3 Impact on local attractivity

Geothermal energy is a local, non-intermittent, renewable and decarbonized energy source. In the context of global warming, the taxonomy and the high variability of natural gas, this energy source is an opportunity for the economic attractivity of a region, especially in terms of heat production. Indeed, according to a study published by ADEME, the French agency for the ecological transition, in 2020, deep geothermal and waste heat are the most economical energy sources for industry and the residential sector [44]. The cost of heat from deep geothermal energy was in a range of €15 to 55/MWh, while the cost of heat from natural gas was €51–85/MWh [44] before the Ukrainian crisis and the rise of the natural gas price. Deep geothermal energy can really boost the economic development of a territory by attracting energy consumers or reducing the carbon footprint of existing facility.

Since 2019, over 60% of the heating needs of Bruchsal police headquarters have been supplied by the nearby geothermal power plant. The reduction in GHG emissions has reached 700 ton/year. Thus, the Bruchsal geothermal power plant demonstrates that in the URG geothermal energy can supply a climate-friendly alternative to fossil fuel heating locally, safely and reliably.

6. Towards additional valorization to increase local benefits

6.1 Geothermal power production and use of the residual heat in the URG

The deep geothermal power plants in the URG produce saline water at temperatures above 150°C by pumping up fluid from deep geothermal reservoirs. The hot fluid produced is used to generate electricity by means of an Organic Rankine Cycle (ORC) or to supply heat to industrial users, following which the brine is reinjected back into the fractured granite reservoir at around 70°C. Reinjection at a lower temperature was deemed non-feasible due to the build-up of scale in the heat exchanger below 60°C. However, recent studies carried out as part of the European MEET project suggest that no new scaling was found at lower temperatures at Soultz [29, 45, 46] and that operators could reinject at 40°C, valorizing 20–45% of residual thermal energy.

Thus, a generic research study has been conducted to identify low-temperature industrial processes that could use the residual heat produced from ORC-based power plants in this region [47]. As the ORC cooling system is usually using ambient air, an energy analysis was carried out for the Soultz-sous-Forêts power plant to establish a relationship for the residual thermal power with the outside air temperature. Based on that relationship, a residual thermal energy profile was produced for a theoretical ORC power plant with a brine production rate of 200 m³/h. This resulted in a thermal power of 6–8 MW depending on the time of year.

Therefore, there are various low-temperature industrial applications that could harness this residual heat to reduce the reinjection temperature and increase economic and energy efficiency.

Based on the residual thermal power available and a maximum temperature constraint of 70°C, extensive market research has been conducted to identify the industrial processes that might use low-temperature heat [47]. Several aspects were taken into consideration such as the application’s current availability, projected development, availability of companies, environmental constraints, distribution of industrial units and local actors. The operators of the activities and applications identified were consulted to identify the most promising processes for residual heat valorization, and an implantation study and economic assessment were carried out in order to determine the levelized cost of geothermal heating (LCOH). The main applications are drying processes (sludge from wastewater treatment, brewery waste recovery, wood sector), aquaculture (fish farming, spirulina production, aquaponics), agriculture, insect protein production, biogas production and district heating. The various applications considered in the market study are outlined in Figure 7. The applications painted in red were explored further to determine their feasibility in the techno-economic evaluation [47]. The results of the techno-economic study provided key insight for geothermal companies with regard to the applications that can be used to valorize residual heat and the production capacity required for economic feasibility.

Figure 7.

Temperature ranges for various industrial applications.

In conclusion, geothermal energy can significantly improve the sustainability of the food sector by providing heat to new innovative technologies such as insect protein production and farming methods such as aquaponics, greenhouses, fish farming and spirulina production. Geothermal power plants can really increase local benefits by supplying the residual heat at low to marginal cost, creating an economic ecosystem in the surrounding area and indirect jobs.

6.2 Geothermal power production, critical raw materials, and lithium extraction in the URG

Harvesting geothermal power (heat and electricity) from subsurface reservoirs is a process that can be used as a renewable energy source by the local population and industries, thanks to the geothermal power plants. However, profits associated with a geothermal power can be hard to maintain (heat and electricity sales), and strategies must be developed to ensure profitable growth. To improve the economics of geothermal power plants, numerous investigations have been carried out to find ways of coupling the production of geothermal energy with the extraction of minerals and metals dissolved in the fluid [48, 49]. Numerous chemical elements in these dissolved solids accumulated in the solution due to weathering of the rocks are a potential source of valuable metals and minerals: critical raw materials (CRMs). CRMs are defined as raw materials which are economically and strategically important for the world economy, but which have a high risk associated with their supply [50]. They are essential to the functioning of a wide range of industrial and public activities (consumer electronics, health, steelmaking, space exploration, etc.). It is well known that such CRMs are contained in geothermal water, and recovery methods are developed where the process is considered to be economically profitable now or in the future [49]. Silica, zinc, lithium, manganese, potassium and a number of rare earth elements are among the most studied elements due to their high concentrations in geothermal fluids [51, 52]. Although CRMs concentrations in the geothermal fluids are lower than what is measured in mineral ores (e.g., ppm in brines vs. % in ores for lithium [53, 54] the costs associated with CRMs extraction are potentially low for the following reasons set out by [49]:

The associated costs will be divided between power and mineral production. Mineral extraction would be developed at geothermal power plants that already exist where the technical staff on site already have a good knowledge of the surface installations.

There are no costs associated with the beneficiation of the minerals/metals, which normally include disaggregation, physical separation (gravity and magnetic separation), and chemical separation (leaching, froth flotation).

In spite of a lower concentration than in ores, the geothermal process involves large quantities of water (e.g., around 300 m³/h produced at Rittershoffen power plant) and therefore a high quantity of CRMs could be extracted.

Concentrations of these metals in the fluid depend on several parameters that affect the chemical composition of the water during its underground circulation: 1. Chemical composition and properties of the rocks (e.g., mineral content and porosity); 2. Initial composition of the fluid (e.g., rainwater, pH); 3. Duration of interaction with the rocks; 4. Temperature and pressure during fluid/rock interaction; 5. Fluid/rock ratio in terms of volume. 6. Possible anthropogenic influence. Geothermal fluid compositions are therefore dependent on their location in the world (e.g. subsurface geology), and metal recovery technologies must be developed to adapt to the different properties of the fluids. In the URG, these fluids have high concentrations of dissolved solids (~100 g/L), mostly Cl, Ca, and Na [28, 34, 55] due to the water movement through the different in-situ geological units (sedimentary and granitic rocks).

In the URG, it is possible to identify several CRMs with interesting concentrations that can be profitable in the future [34]. Table 3 summarizes the valuable CRMs found in Rittershoffen and Soultz-sous-Forêts geothermal fluids.

Geothermal power plant	CRMs	Mean concentration in the brine (mg/L)	Market interest	Prices in 2022 (€/ton)	Possible income per year (€)
Soultz-sous-Forêts	Mg	136	Magnesium powder, Mg alloy and Mg ingot	5000	5.42E+05
	Sr	431	Strontium metal	14,135	4.84E+06
	Li	168	Lithium carbonate, Li hydroxide monohydrate, Li chloride	60,000 for LCE	4.27E+07
	Si	89	Silicon metal, Si powder	3000	2.12E+05
	Zn	2.7	Zinc ingot, Zn oxide, Zn powder	3600	7.81E+03
			Possible Income (€/L)		0.044
			Possible income (€/year)		4.83E+07
Rittershoffen	Mg	123	Magnesium powder, Mg alloy and Mg ingot	5000	1.05E+06
	Sr	459	Strontium metal	14,135	1.11E+07
	Li	181	Lithium carbonate, Li hydroxide monohydrate, Li chloride	60,000 for LCE	9.86E+07
	Si	98	Silicon metal, Si powder	3000	5.00E+05
	Zn	3.3	Zinc ingot, Zn oxide, Zn powder	3600	2.04E+04
			Possible Income (€/L)		0.047
			Possible income (€/year)		1.11E+08

Table 3.

Possible income associated with the extraction of CRMs at Rittershoffen and Soultz-sous-Forêts (heat and electricity sales not included). The flow rate at Rittershoffen is 75 L/s and 35 L/s at Soultz-sous-Forêts. For the calculation, a mineral extraction of 80% was assumed, and a plant availability of 90%.

Source of average CRM prices: https://ise-metal-quotes.com, July 2022.

Possible annual income from CRM extraction is closely linked to the quantities of lithium in the geothermal fluids (about ~88% of the total income at Soultz-sous-Forêts and Rittershoffen, Table 3). Lithium is a strategic metal, especially for electric vehicle battery manufacturing, for which worldwide demand is constantly increasing. Analysts forecast lithium demand approaching 1 Mt. LCE (Lithium Carbonate Equivalent, Li₂CO₃) by 2026 [53, 56]. Although lithium is found ubiquitously in the environment, the URG geothermal waters are rich in lithium with an average concentration measured between 150 and 210 mg/L [28, 34, 55]. This significant Li concentration (~1000 times more concentrated than in sea water) is therefore of great interest for future exploitation. For instance, one doublet at Rittershoffen (one production and one injection well) could produce more than 1500 tons of LCE per year assuming a plant availability of 90% and a mineral extraction yield of 80%. Given that current worldwide lithium production is concentrated in Australia, Chile, Argentina and China, the production of lithium at geothermal power plants should help the European Union (EU) to reduce its dependency on other countries and to produce more sustainable lithium [53]. Additionally, the operation of a power plant with two doublets producing ~3000 tons of LCE, would create new jobs. In total 72 employees would need to be hired for the effective operation of the lithium production plant including maintenance and lab teams, an operations team, managers and additional staff to cover for holidays and absence.

As part of the EuGeLi (European Geothermal Lithium Brine) project, a collaborative research and innovation project launched in January 2019, direct lithium extraction (DLE) tests were conducted with real brine in geothermal exploitation conditions (80°C and 20 bar) on site. The work managed to recover several kilograms of precipitated battery-grade Li₂CO₃, showing the feasibility of directly extracting Li from geothermal fluids. However, several parameters need to be adjusted to improve and increase the productivity of the DLE process and the overall recovery level. Adjustment of the flow rate in the column, comprehension of the chemical reaction occurring between the brine and the active solid over the long term and management of higher impurities in lithium solutions are among the main parameters to consider in the future to improve the profitability of the project [57]. Given the novelty of the lithium extraction process and the lack of long-term, large-scale operational experience, numerous factors could influence the long-term profitability of DLE such as: a decline over time of the lithium concentration in the production fluids due to poor re-saturation of lithium in the brine after extraction; a drop in lithium prices due to technological shifts or alternative technologies affecting demand for lithium; poor acceptability resulting in less public support through subsidies or permit approvals [58].

Although DLE is a young technology, it should significantly reduce carbon emissions compared to other methods of producing and refining lithium (hard rock mining and evaporation ponds). According to Vulcan Energy and the project Zero Carbon LithiumTM, one ton of LiOH (lithium hydroxide) produced by hard rock mining emits 15,000 kg of CO₂ compared to zero for harvesting geothermal lithium (https://v-er.eu/zero-carbon-lithium/, July 2022).

7. Conclusion

Despite drawbacks due to several felt induced seismic events, deep geothermal energy is still being developed in the Upper Rhine Graben and has a promising future. The operators and the mining authorities have introduced best practices and appropriate rules to mitigate the seismic risk and to minimize other environmental impacts such as slow surface deformation, shallow groundwater protection, contamination of soils by geothermal brine leakages, radioactivity and noise. Life Cycle Assessments highlight the overall low environmental impact of this source of energy. In particular, the potential impact on climate change is estimated at 3.7 gCO₂eq/kWh for the Rittershoffen geothermal heat plant. This impact is 67 times lower than that of heat from natural gas. In terms of socio-economic impact, a Life Cycle Costs Analysis performed on the Rittershoffen plant showed how this industry is well anchored in its territory with around 60% of its costs benefiting local actors during the operating phase. Nevertheless, deep geothermal energy in the URG is still in the early stages of its development and additional valorization such low temperature heating and/or metal extraction from the geothermal water could help the deployment of this industry on a larger scale. This would increase the attractivity of the region by providing heat for agro-industries at reasonable prices and lithium which could be a starting point to develop a new industrial sector in the area.

Acknowledgments

The authors would like to thank several projects whose results have been used in this chapter. The European DESTRESS, GeoEnvi and MEET European projects are mostly funded by the European Commission’s Horizon 2020 Research and Innovation Program under grant agreements No 691728, No 818242, and No 792037 respectively. Some results were partly funded by ADEME (the French Agency for Ecological Transition) as part of the Eranet Geothermica Zodrex project and the EGS Alsace project co-funded by Electricité de Strasbourg. The authors acknowledge the GEIE EMC and ECOGI for providing the data. The authors also acknowledge the EuGeLi project (European Geothermal Lithium brines), co-funded by the European Union, under the European Institute of Technology (EIT) Raw Materials Program.

Conflict of interest

The authors acknowledge that they are employed by ES-Geothermie which is a developer and operator of deep geothermal plants in France.