Life cycle assessment of shallow to medium-depth geothermal heating and cooling networks in the State of Geneva

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Article

Reference

Life cycle assessment of shallow to medium-depth geothermal heating and cooling networks in the State of Geneva

PRATIWI, Astu Sam, TRUTNEVYTE, Evelina

Abstract

Geothermal heating and cooling from a depth of 10–3′500 meters have considerable decarbonization potential but understudied life-cycle environmental impacts. We quantified the life-cycle impacts of six heating and cooling configurations from shallow to medium-depth geothermal wells with connected, decentralized heat pumps and district heating and cooling in the State of Geneva. Shallower systems with connected heat pumps have better environmental performance than systems with district heating, whereas shallow systems with free cooling perform best for cooling. These environmental impacts are lower than those of fossil fuels, except for mineral resource scarcity, especially with decentralized heat pumps and free cooling.

PRATIWI, Astu Sam, TRUTNEVYTE, Evelina. Life cycle assessment of shallow to

medium-depth geothermal heating and cooling networks in the State of Geneva. Geothermics , 2021, vol. 90, no. 101988

DOI : 10.1016/j.geothermics.2020.101988

Available at:

http://archive-ouverte.unige.ch/unige:146185

Disclaimer: layout of this document may differ from the published version.

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Geothermics 90 (2021) 101988

Available online 5 December 2020

Life cycle assessment of shallow to medium-depth geothermal heating and cooling networks in the State of Geneva

Astu Sam Pratiwi *, Evelina Trutnevyte

Renewable Energy Systems, Institute for Environmental Sciences (ISE), Section of Earth and Environmental Sciences, University of Geneva, Switzerland

A R T I C L E I N F O Keywords:

Geothermal Heat pumps Life-cycle assessment District heating Cooling Environment

A B S T R A C T

Geothermal heating and cooling from a depth of 10–3^′500 meters have considerable decarbonization potential but understudied life-cycle environmental impacts. We quantified the life-cycle impacts of six heating and cooling configurations from shallow to medium-depth geothermal wells with connected, decentralized heat pumps and district heating and cooling in the State of Geneva. Shallower systems with connected heat pumps have better environmental performance than systems with district heating, whereas shallow systems with free cooling perform best for cooling. These environmental impacts are lower than those of fossil fuels, except for mineral resource scarcity, especially with decentralized heat pumps and free cooling.

1. Introduction

Decarbonization efforts need to include heating and cooling in the building sector, as it represented 40% of the total energy demand in the European Union in 2015 (European Commission, 2018). In Switzerland, the share of non-renewable energy used for heating purposes in buildings needs to be reduced from 70% in 2016 to 10–15% by 2050 (Con- fédération Suisse, 2017; Narula et al., 2019). The State of Geneva in Switzerland adopted an objective to reduce its CO2 emissions in its building sector by 45% in 2030, as compared to 2012 (République et Canton de Genève, 2018), in line with its ambition to reduce its greenhouse gas emissions by 60% in 2030, as compared to 1990 (République et canton de Genève, 2019). These objectives have driven increased interest in renewable energy in buildings, including geothermal heating and cooling. For example, geothermal heat is projected to amount to around 900 GWh per year in 2035, provided that it can be extracted from shallow to medium depths (Quiquerez, 2017). Geothermal systems could also play an important role in meeting cooling demand through free and active cooling (Saner et al., 2010; Inayat and Raza, 2019; Esen et al., 2006, 2017), especially now that cooling demand in European countries is growing due to climate change (Frank, 2005; Spinoni et al., 2015) and the urban heat island phenomenon (Ward et al., 2016).

Geothermal heating and cooling would benefit from economies of scale when used in district heating and cooling networks, where a larger geothermal installation would have higher efficiency and cost less per energy unit. Depending on the geothermal extraction temperature and

the existence of other heat sources (Sayegh et al., 2018), the networks could be configured in a traditional centralized or decentralized manner (Molyneaux et al., 2010).

Comprehensively assessing the environmental impacts of geothermal heating and cooling systems from a life-cycle perspective is critical to acknowledge and manage the associated environmental consequences.

Life-cycle assessment (LCA) methodology has been widely used for this purpose because it facilitates standardized evaluations of a range of environmental impacts (Bartolozzi et al., 2017). In May 2019, we sys- tematically identified 36 scientific publications on the environmental assessment of geothermal heating and cooling, 25 of which were based on LCA (Pratiwi and Trutnevyte, 2020). Fig. 1 shows that 20 of these publications investigated the applications of ground-source heat pumps, and only a few of them concerned geothermal systems involving groundwater extractions from low and medium enthalpy—systems representing most of the geothermal district heating in Europe (EGEC, 2018). When compared to the number of installations in Europe (Fig. 1), this shows that these geothermal systems are still significantly under- represented in terms of LCA studies. Therefore, more systematic LCAs on shallow to medium-depth geothermal heating and cooling are needed.

The reviewed literature not only proved to be disproportionate in terms of the types of systems addressed but also lacked breadth and depth in investigating the overall impacts of geothermal heating and cooling networks. Concerning the impact categories, a large portion of the studies limited their work only to impacts concerning CO2 emissions, assessing whether geothermal is a suitable option to decarbonize the

* Corresponding author at: Uni Carl Vogt, Boulevard Carl Vogt 66, CH-1211, Geneva 4, Switzerland.

E-mail addresses: [email protected] (A.S. Pratiwi), [email protected] (E. Trutnevyte).

Contents lists available at ScienceDirect

Geothermics

journal homepage: www.elsevier.com/locate/geothermics

https://doi.org/10.1016/j.geothermics.2020.101988

Received 28 May 2020; Received in revised form 7 September 2020; Accepted 7 October 2020

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energy sector (Lo Russo et al., 2009; Bayer et al., 2012; Blum et al., 2010). Other environmental concerns that could arise were overlooked by these studies. Concerning cooling applications, the impacts of free cooling supply using ground-source heat pumps have been investigated (Saner et al., 2010; Blum et al., 2010; Bonamente and Aquino, 2017;

Hong et al., 2016), but only Saner et al. (2010) separately assessed the environmental advantages achieved from providing free cooling and reported a considerable avoidance of greenhouse gas emissions from supplying cooling on top of heating, respectively, compared to gas furnace heating. The other studies neither differentiated nor discussed such implications; instead, they reported the environmental impacts of heating and cooling supply as aggregated results. Further, these studies dealt with free cooling but not with other geothermal cooling technologies, such as the refrigeration cycle (reversed heat pumps) or absorption chillers. The use of these technologies is imperative when the geothermal temperature is higher than the ambient temperature (Wang et al., 2013). Regarding the network, a number of researchers considered district heating and cooling networks in their work and unani- mously highlighted the contribution of the district heating infrastructure to environmental impacts, attributable to the use of steel (Bartolozzi et al., 2017; Kaltschmitt, 2000; Karlsdottir et al., 2014; Ghafghazi et al., 2011). However, these studies dealt only with centralized heating networks, and an understanding of the decentralized networks is still lacking. Finally, although the authors considered a mix of heat sources in their work, only one study (Kaltschmitt, 2000) presented and discussed the implications of the heating mix composition. In addition to all these limitations of the existing literature, no LCA exists for geothermal heating and cooling in conditions similar to those found in Geneva.

The objectives of our work are to address these research gaps by quantifying the environmental impacts of various configurations of shallow and medium-depth geothermal heating and cooling networks, and thereby answering the following questions:

•What are the life-cycle environmental impacts of shallow to medium- depth geothermal heating and cooling networks in the State of Geneva, including parameter uncertainties?

•How do these impacts change when geothermal heating is used in combination with other heating sources?

•Where does the environmental performance of shallow to medium- depth geothermal heating and cooling stand when compared to that of other heating and cooling sources used in the State of Geneva?

In this work, we built six hypothetical configurations representing the practical options of geothermal heating and cooling systems in the State of Geneva and used data from existing and planned geothermal wells and installations. The life-cycle environmental impacts of the heating and cooling supply were quantified following the LCA

methodology outlined in ISO 14040 and 14044. The uncertainty ranges of these impacts were then appraised using bounding analysis, and we analyzed the impacts of combining geothermal supply with other heat sources. Finally, the environmental impacts of geothermal heating and cooling systems were compared with the impacts of other heating and cooling sources to evaluate the advantages and disadvantages of geothermal systems.

2. Methods

There are several ways to implement geothermal heating and cooling systems in the State of Geneva, and thus, the environmental impacts may vary. To encompass the key options, we first defined six different configurations supplying space heating, hot water, and space cooling to multi-family houses and defined the reference values for the main parameters (Section 2.1). Second, data from existing installations in Switzerland and France were collected for a life-cycle inventory (Section 2.2). Then, a bottom-up model was built to calculate eight environmental impacts (Section 2.3), and bounding analysis was conducted to assess parameter uncertainties (Section 2.4). We further analyzed the scenarios in which geothermal district heating is also supplied by other heat sources (Section 2.4). Lastly, to have a broader comparison, we compared the environmental impacts of the geothermal systems with those of other heating and cooling alterna- tives in Geneva, such as individual oil boilers, waste incineration and natural gas district heating, individual ground-source heat pumps, solar thermal collectors, individual biomass furnaces, and electric heating.

2.1. Six configurations of geothermal heating and cooling systems To analyze environmental impacts across possible options of geothermal heating and cooling systems, we defined six configurations by coupling four categories of well depths with three types of the network on the ground (Fig. 2A) and delineated the reference values of the main geothermal parameters. The four categories of well depths were named ‘very shallow,’ ‘shallow,’ ‘medium depth,’ and ‘medium-large depth,’ to which specific ranges of production temperature and flow rate were assigned. The capacity of the geothermal resource was calculated using three critical parameters:

production and injection temperatures and flow rate. For the first two parameters, the reference values were defined as the middle values of the assigned ranges. The values of injection temperatures for heating applications were adopted from projects in Switzerland and France (Fig. 2). In particular, the injection temperature of 55 ^◦C was chosen for Configuration 6, considering the return temperature of the district heating network. For cooling applications, the values were adopted from the relevant Swiss or French regulations due to the lack of data. The parameters chosen as reference values are presented in Fig. 2B.

Three types of heating and cooling networks were identified: connected decentralized heat pumps (Henchoz et al., 2016), district heating and cooling with a heat pump, and district heating and cooling without a heat pump, all aiming to supply heating at 75 ^◦C and cooling at 14 ^◦C.

The first two types differ in the placement of heat pumps (Fig. 3A and Fig. 3B). An example of the first type can be found in the district heating of La Tour-de-Peilz (Henchoz et al., 2015), while the latter is used in a project in Concorde (Owsianicki, 2016). The third type is employed only when the geothermal temperature is higher than the supply temperature of district heating during the winter (Fig. 3C). During the summer, the means to supply cooling were determined based on the geothermal production temperature, as shown in Fig. 2B. The heating and cooling peak to be supplied by each system was scaled to equal the capacity of the heating and cooling system, while the theoretical annual supply was determined according to the annual heating and cooling load profiles.

These profiles were calculated based on the heating and cooling degree Fig. 1. Comparison of the number of LCA studies in the scientific literature

worldwide and the number of installations in Europe by geothermal system category.

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days, considering the mean daily temperatures of the State of Geneva from 2006 to 2016 (Federal Office of Meteorology and Climatology Meteosuisse, 2019) and assuming the base temperatures as 12 ^◦C and 18 ^◦C for heating and cooling, respectively (SIA, 1982; ASHRAE, 2001;

Christenson et al., 2006; Zubler et al., 2014).

2.2. Scope of life-cycle assessment and life-cycle inventory analysis Our LCA analysis encompassed activities involved in drilling geothermal wells, constructing the heating and cooling network, oper- ating and maintaining the network, and decommissioning. For the life- Fig. 2. Illustration of the six configurations of geothermal heating and cooling systems and reference values of the main parameters. The data refer to (1) injection temperature of Marmande project in France, (2) injection temperature of Riehen project in Switzerland, (3) return temperature of district heating, (4) the diameter of Concorde well in Switzerland, (5) diameter of Lully well in Switzerland, (6) 3 K temperature difference based on Swiss regulation Le Conseil fédéral suisse (1998), (7) 11 K temperature difference based on French regulation (H¨ahnlein et al., 2013), (8) the output temperature specified by the absorption chiller.

Fig. 3. Illustration of the locations of heat exchangers and heat pumps for a network of connected decentralized heat pumps (A), district heating and cooling with a heat pump (B), and district heating and cooling without a heat pump (C). Section (D) illustrates the heating load profile for Configurations 1 and 3 supplied by 100%, 90%, and 50% geothermal resource.

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cycle inventory of these activities, a database of material and energy requirements was created using information from projects in Switzerland and France, as well as the equipment’s technical datasheets (Table A.1). All data were cross-validated with expert consultations.

For the geothermal wells, we began by considering the drilling of shallow exploration wells used for geothermal resource assessment (Garg and Combs, 1997) and, later, the main wells. Production and injection wells were required because we assumed a 100% injection rate, and they were of the same depth, determined using a temperature gradient. For ‘very shallow’ wells, a temperature gradient between 36 ^◦C/km and 44 ^◦C/km was used, and for deeper wells, it was between 25 ^◦C/km and 30 ^◦C/km (Chelle-Michou et al., 2017). After drilling, the wells are cased with cemented steel tubes, and a production test is carried out from 30 to 90 days (Baujard et al., 2017) to obtain the steady flowrate value. To reach the desired flowrate, downhole pumps are installed in production wells, assuming a productivity index of 25 m³/h/bar and a well-head pressure of 11 bar based on geothermal wells in Villejuif and a static level of 40 m based on the well in Concorde.

Injection pumps were not considered in our study.

For the heating and cooling network, the equipment installed near geothermal wells (hereafter referred to as the geothermal plant), the substations, and the network pipes were considered, but not the equipment in individual residences. The geothermal plant and substations consist of stainless-steel heat exchangers and, sometimes, heat pumps and an absorption chiller. For simplification, the water consumption for cooling process of the absorption chiller’s solution is fixed to 6 m³per MWh of cooling (Primas, 2007). The setup of the heat plant for Configurations 1 and 3 adopted that of La Tour-de-Peilz, and the setup of Configurations 2 and 4 adopted that of Concorde. Meanwhile, the setup for Configuration 5 adopted that of Riehen, owing to the temperature limit of the available heat pumps. The schemas of the configuration are shown in Fig. A.1. The length of network pipes was determined based on the annual heat delivered, assuming a linear heat density of 1.8 MW h/m/year as the minimum threshold for profitable district heating (Quiquerez, 2017; Nussbaumer and Thalmann, 2016).

The diameter was determined from the flow rate of the water as the media to carry heat in district heating and by maintaining a pressure loss of under 100 Pa/m (ASHRAE, 2016). The material was pre-insulated high-density polyethylene for a working temperature of less than 55 ^◦C; otherwise, it was pre-insulated steel (Owsianicki, 2016; Fangar- eggi and Bertucelli, 2012).

For the operation and maintenance, we considered electricity consumption, refrigerant leakage, and the replacement of equipment, over a lifetime of 30 years (H¨ahnlein et al., 2013). Electricity for the downhole, circulation, and heat pumps was included. Geneva’s electricity mix was assumed to be dominated by hydropower (Table A.2). For heat pumps, the coefficient of performance (COP) was calculated based on geothermal production and heating supply temperatures, and the ratio of real to ideal COP was set to 0.55 (Harvey, 2006). R1234ze was

selected as the refrigerant with an annual leakage rate of 3.5% (Eunomia Research & Consulting Ltd and London Southbank University, 2014).

This refrigerant is suitable for high-temperature heat pump and has a relatively low GWP100 of 6 (Fukuda et al., 2014). Within its lifetime, several overhaul maintenances were assumed, during which many parts would be replaced following the scenario in Table A.3. For decommissioning, we considered the installation of cement bridges (Treyer et al., 2015) during well abandonment, transport of all the equipment to the recycling station (except for the network pipes), and end-of-life treatment of heat pumps and refrigerant.

2.3. The computational framework of life-cycle assessment

Our computational framework of LCA is a bottom-up model comprised of three main parts. The system design part (Fig. 4A) calculates the sizes of wells and equipment based on geothermal parameters, the choice of heating and cooling network, and the desired share of geothermal heat in the network. Next, the generation of life-cycle in- ventory (Fig. 4B) part translates these sizes into energy and material flows using the collected databases. Flows referring to the production of heating and cooling were allocated using exergy allocation (Zhou and Gong, 2013; Rosen, 2008). Then, a life-cycle inventory was generated in terms of elementary flows using OpenLCA and the Ecoinvent 3.5 database (Wernet et al., 2016). The LCA database from WEEE-LCI (ecosystem, 2020) was used for the end-of-life treatment of heat pumps.

Finally, the Quantification of impacts part (Fig. 4C) calculates the environmental impacts per MWh of heating or cooling received by users at the substations. We selected the impact categories by referring to the most frequent indicators in the review by Dorning et al. (Dorning et al., 2019) and quantified them using LCIA Methodology of Cumulative En- ergy Demand and Recipe midpoint 2016 H (Huijbregts et al., 2017):

• Global warming impacts are evaluated as the greenhouse gas emissions in kg CO_{2 eq}.

• Particulate matter is measured as the emissions of particulate matter with a diameter of less than 2.5 mm, including aerosols from the emission of SO2, NH3, and NOx, which contribute to respiratory morbidity, measured in kg PM2.5 eq.

• Terrestrial acidification impacts express the deposition of inorganic substances that change soil acidity, measured in kg SO2 eq.

• Fossil resource scarcity expresses the depletion of fossil fuels available for future generations and assuming a marginal cost increase, measured in kg Oil eq.

• Mineral resource scarcity expresses the depletion of mineral resources, measured in kg Cu eq.

• Water consumption impacts are measured as water consumed by the process in m³, excluding extracted water that is eventually returned to the water body.

Fig. 4.The LCA computational framework used to calculate environmental impacts.

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•Land use impacts represent species loss due to the change in land use, measured in m²⋅year crop _eq.

•Total energy demand quantifies the total energy used, including geothermal energy, in kWh.

2.4. Base-case scenario, uncertainty analysis, and multi-source scenarios We first analyzed the six configurations under a base-case scenario, meaning that the reference values were used (Fig. 2), assuming one production and two injection wells, and that 100% of the heating and cooling load was supplied by geothermal systems. The peak of the heating supply was set to be equal to the nominal capacity of the geothermal system. Owing to heterogeneous geological conditions (Moscariello et al., 2020), the output of future geothermal projects in Geneva is highly uncertain. To assess this uncertainty, we carried out a bounding analysis, which is a method to estimate the lower and upper bounds of the quantity of interest by defining the plausible range of minimum and maximum input parameters (Casman et al., 1999;

Schweizer and Morgan, 2016). We established eight bounding scenarios to which a combination of the lowest and highest values of temperature, flow rate, and 35 other parameters related to material and energy intensity were assigned as presented in Table 2 (the bounding scenarios are illustrated in Fig. A.2). The parameters of the material and energy intensity determine the amount of material or energy flow needed to realize a specific process or activity and are listed in Tables A.3 – A.6.

We further investigated the impacts of heating networks that are supplied not just by geothermal heat, which represent most geothermal district heating installations. To investigate changes in the environmental impacts in such cases, we created two multi-source scenarios, namely 90% geothermal and 50% geothermal, representing a geothermal share of 90% and 50%, respectively. The shares were achieved by increasing the total heating supply of the multi-source systems while maintaining the geothermal installed capacity represented by the reference values, as illustrated in Fig. 3D. The capacity factors of the geothermal systems were computed accordingly. The selected supple- mental heating technologies are electric heating and waste incineration and natural gas district heating, as detailed in Table 1. The data for these technologies were taken from Ecoinvent 3.5 and adjusted for applications in Geneva. We applied the electricity mix of Geneva for electric heating, added network pipes based on the installations of natural gas and waste incineration district heating in Geneva (Services industriels de Gen`eve, 2015). We applied economic allocation of impacts using the information on the revenue streams of a waste incineration facility to only include the emissions linked to heat production and not those linked to waste treatment and electricity production (Services

industriels de Gen`eve, 2018). In multi-source scenarios, no cooling production was considered.

3. Results

3.1. Environmental impacts under the base-case scenario

In this section, the characteristics and environmental impacts of the six configurations in the base-case scenario are presented. Table 2 shows the technical details of the configurations in this scenario (more information is available in Table A.7), in which the capacity factor was calculated to be 34% (Fig. 3D). Here, the capacities of cooling are shown to be less than capacities of heating, primarily due to the limitation of the injection temperature during the summer. During the cooling period, the production of hot water consumes 11% of the geothermal flowrate, thereby further reducing the cooling capacity. The number of cooling days is one-third of the heating days, resulting in cooling supplies that are even smaller than heating. This discrepancy in supplies and the difference between the exergy qualities of heating and cooling resulted in the allocation factors being heavily weighted on heating.

Fig. 5 shows the values of eight environmental impacts of the configurations in a base-case scenario per MWh of heating and cooling.

Complete information regarding all the environmental impact categories is presented in Table A.8–A.9. For heating, except for total energy demand, the impacts of Configurations 1 and 3 are lower than those of Configurations 2 and 4, respectively, whereas those of Configuration 3 are the lowest. This indicates that, for a given geothermal resource, connected decentralized heat pumps have lower impacts than district heating, and combining ‘shallow’ wells with connected decentralized heat pumps, lowering the impacts even further. For cooling, except for

Table 1

The main parameters assigned to the bounding scenarios for bounding analysis and multi-source scenarios. The other 33 parameters concerning material and energy intensity are presented in Table A.3 – A.6 Note: * supplied by district heating.

Configuration

1 2 3 4 5 6

Lowest/highest values of the parameters related to geothermal production

Geothermal temperature (^◦C) 10/14 10/14 20/55 20/55 60/75 78/120

Geothermal flowrate (l/s) 10/50 10/50 20/80 20/80 20/80 20/80

Lowest/highest values of the parameters for material and energy intensity

Diesel for drilling (GJ/m) 4.6/6.8 4.6/6.8 1.4/1.76 1.4/1.76 2.9/3.47 2.9/3.47

Efficiency of the circulation pumps 0.85/0.7 0.85/0.7 0.85/0.7 0.85/0.7 0.85/0.7 0.85/0.7

Thermal gradient (^◦C/km) 44/36 44/36 30/25 30/25 30/25 30/25

Multi-source scenarios

Share/type of supplementary source for 90%

geothermal 10%/electric

heating 5%/natural gas*

5%/waste incineration*

10%/electric

5%/natural gas*

Share/type of supplementary source for 50%

geothermal 50%/electric

50%/electric

25%/natural gas*

Table 2

Calculated characteristics of the geothermal system configurations in the base- case scenario.

Configurations

1 and 2 3 and 4 5 6

Depth (m) 50 1^′282 2082 3464

Heating capacity (kW) 590 7’576 7’205 10’241

Annual heat supply (GWh) 1.6 20.3 19.3 27.4

Length of heat network (km) 0.8 10 0.7 13.7

Cooling capacity (kW) 331 1.473 1.606 1.287

Annual cooling supply (MWh) 308 3270 3319 3382

COPheating 2.76 5.8 9.67 n/a

COPcooling n/a 2.8 2 n/a

Heating allocation factor 92% 93% 93% 95%

Cooling allocation factor 8% 7% 7% 5%

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mineral resource scarcity and land use impacts, the impacts of Config- urations 1 and 2 are lower than those of Configurations 3 and 4, which indicates that free cooling has lower impacts than the other cooling technologies. Interestingly, Configuration 5 has high values in almost all impact categories, for both heating and cooling. For heating, this is explained by supply being constrained by the injection temperature, whereas for cooling, this is explained by the small cooling COP due to high geothermal temperatures.

By examining the impacts per process (Table A.10), we found that the primary sources of environmental impacts across all configurations are metal products, hydropower plants, electricity transmission and distribution lines, diesel, and cement and concrete. In terms of metal products, the key sources of impacts are well casings, network pipes, and heating equipment in the heating plant that in particular add to mineral resource scarcity, water consumption, global warming, fossil fuel scarcity, land use, particulate matter, and terrestrial acidification impacts. In terms of hydropower plants and the electricity transmission and distribution lines, the electricity consumption during the operation phase mainly adds to terrestrial acidification, mineral resource scarcity, particulate matter, land use, and water consumption. Diesel that is consumed for well drilling increases the impacts of particulate matter emissions, fossil resource scarcity, and global warming. Cement and concrete that are needed for well casings, well chambers, and drilling platform contribute to global warming, fossil resource scarcity, and water consumption. Finally, the impacts of transport activities are only

visible in the impacts on land use and they are attributable to road construction.

Because of these differing proportions, certain life-cycle activities contribute to impacts more than others, as demonstrated in Fig. 6. The contributions of geothermal wells to impacts are higher for wells with either a large diameter or a ‘medium’ to ‘medium-large’ depth. This is because ‘shallow’ wells are drilled with mobile drilling machines, which are the least energy-intensive of the drilling techniques considered in this work. This trend is observed for both heating and cooling because the environmental burden of the wells is allocated to both. For Config- urations 1 and 2, the contribution of wells is dominant in the cooling systems, amounting to more than 50% of total impacts across all impact categories, except for mineral resource scarcity. This is because the impacts of electricity and additional equipment here are minimum. The contributions of the network pipes to environmental impacts are evident in district heating applications, owing to the steel used in the pipes to carry high-temperature water. By contrast, the network pipes in decentralized systems are made of high-density polyethylene because they work at a lower temperature, resulting in considerably lower impacts compared to steel-based pipes. This type of pipe is also used for cooling supply; hence, the small contribution of network pipes to the environmental impacts of cooling is similarly observed. In fact, this is the main reason why the environmental impacts of cooling are lower than those of heating. Supplying cooling, along with heating, using a decentralized network allows even lower impacts as no additional set of Fig. 5. Environmental impacts of geothermal heating and cooling systems in the base-case scenario. The numbers in circles refer to the configurations as described in Fig. 2.

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Fig. 6. Contribution analysis of the environmental impacts of geothermal heating and cooling networks in the base-case scenario. GW: Global warming (in kg CO2 eq), PM: Fine particulate matter formation (in g PM2.5 eq), TA: Terrestrial acidification (in g SO2 eq), FS: Fossil resource scarcity (in kg Oil eq), MS: Mineral resource scarcity (in g Cu eq), LU: Land use (in m²⋅year crop eq), and TED: Total energy demand (in kWh). The numbers in figures refer to the absolute values of the environmental impacts.

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network pipes is required, in contrast to the district heating system that needs additional cooling pipes to allow a simultaneous supply of heating and cooling. The contributions of electricity consumption are more noticeable for systems using heat pumps, indicating that the electricity consumed by the downhole pump, network pipes, and absorption chiller is negligible. From the total energy demand illustrated in Fig. 6, the electricity consumption appears to decrease with increasing geothermal production temperatures, whereas for cooling, the opposite is found.

This is expected as it is in line with the calculated COP. Large contributions of the heat plant are observed for cooling in Configuration 6.

This is due to the absorption chiller installed in the heat plant, which is comprised mostly of steel, aluminum, and copper. The cooling supply of Configuration 6 also entails additional water consumption on site by the absorption chiller during the cooling process of the absorption chiller’s solution. However, this water consumption is much lower as compared to the indirect water consumption caused by the electricity needed for heat pumps in Configurations 3, 4, and 5. Finally, free cooling configurations have the lowest impacts due to negligible electricity consumption and no additional equipment.

3.2. Uncertainty analysis

In this section, we present the maximum and minimum values of the environmental impacts of all configurations resulting from our bounding analysis (Fig. 7). We find that for each configuration, the maximum environmental impacts mainly correspond to the scenarios with the lowest flowrate and the highest material and energy intensity. In contrast, the minimum environmental impacts correspond to scenarios with the highest flow rate and the lowest material and energy intensity, except for the impacts of total energy demand (Table A.11). Contrary to the intuition that high temperature will lead to more heat output and thus lower impacts per MWh of heat produced, our results do not show this trend. The temperature is closely related to depth through a temperature gradient and hence achieving higher temperature also means reaching greater depth and causing more impacts. Therefore, high temperature of geothermal resource does not necessarily bring down the impacts per MWh. From Fig. 7, it is evident that the environmental impacts of configurations in the base-case scenario are close to the minimum values, indicating that within the boundaries of our work, these configurations represent environmentally favorable systems. In fact, the environmental impacts of a large portion of bounding scenarios for heating lie in the lower range (Fig. A.3), while the upper range corresponds only to several scenarios. The same trend is observed for cooling, although the spread is more even than the one observed for heating. The uncertainty spans for the impacts of global warming, particulate matter, terrestrial acidification, and fossil resource scarcity appeared to be highly dependent on the contribution of wells, indicating that geothermal wells have an important influence on these impacts.

Consistent with the previous observations in the base-case scenario, the impacts per MWh of cooling in all bounding scenarios are lower than those per MWh of heating. For heating, the maximum and minimum values of the environmental impacts of Configurations 1 and 3 are lower than those of Configurations 2 and 4, respectively, and this result is consistent with the previous observation. Configuration 3 remains the configuration with the lowest impacts compared to other configurations.

These findings affirm that connected decentralized heat pumps have lower impacts than district heating. For cooling, we observed that the minimum values of the environmental impacts of Configurations 1 and 2 are lower than those of Configurations 3, 4, and 5, as found in the base case scenario, but this is not the case for the maximum values. The maximum values are mostly represented by a bounding scenario with the lowest geothermal flow rate, highest material and energy intensity, and highest geothermal temperature. The latter corresponds to deeper wells that require additional diesel consumption, where the amount per meter of drilling is higher for Configurations 1 and 2 compared to the other configurations due to their large well diameter and the type of

drilling machine. This high consumption leads to higher environmental impacts of wells that dominate the overall environmental impacts of Configuration 1 and 2. Therefore, the statement that geothermal free cooling has the lowest impacts cannot be generalized.

3.3. Analysis of multi-source heating scenarios

We analyzed the implications of incorporating supplementary heating sources in multi-source scenarios, namely 90% geothermal and 50%

geothermal, as described in Table 1. Fig. 8 shows the changes in environmental impacts and the contributions of different supplementary heating sources. The values of the impacts are available in Table A.12.

For Configurations 1 and 3, which are complemented by electric heating, the impacts of global warming and fossil fuel scarcity decrease with the decreasing geothermal share in the district heating. In 50%

geothermal scenario, the global warming impacts for Configurations 1 and 3 are 38% and 17% of those in 100% geothermal scenario, respectively. For the fossil resource scarcity, they are 44% and 23%, respectively. In contrast, for Configurations 2, 4, 5, and 6, which are complemented by a mix of waste incineration and natural gas district heating, these impacts increase importantly, reaching global warming impacts of 124 kg CO2 eq/MWh and fossil resource scarcity of 35 kg Oil

eq/MWh, represented by Configurations 2 and 5 in 50% geothermal scenario, respectively. These findings illustrate the advantage of electric heating, which is mostly based on hydropower in the case of Geneva, over a mix of waste incineration and natural gas district heating. The impacts of total energy demand decrease for all configurations, although the decrease caused by natural gas and waste incineration is greater than that caused by electric heating because the energy coming from the feedstock of waste incineration is considered burden-free. For Configu- rations 2, 4, 5, and 6, the contributions of natural gas to the impacts are the most evident, except for the impacts of mineral resource scarcity, where the contributions of the additional network exceed those of natural gas. Being decentralized systems, Configurations 1 and 2 do not suffer additional impacts from district network construction.

We also observed that the impacts attributed to geothermal systems (indicated by the blue color in Fig. 8) decrease. Under the 50%

geothermal scenario, across all configurations, the values are nearly half of the values under the base-case scenario. This is because multi-source scenarios allow more geothermal energy production with the same nominal capacity due to an increase in the geothermal capacity factor from 34% in the base-case scenario to 66% (see Fig. 3D). Thanks to Geneva’s electricity mix, which is based on hydropower, the environmental impacts are sufficiently low during the operation phase, allowing the increase in production to reduce the impacts per MWh of geothermal heat. In this perspective, the multi-source scenarios are advantageous.

3.4. Comparison with other heating and cooling sources

In Fig. 9, we present eight environmental impacts of all geothermal heating and cooling configurations analyzed under all scenarios and compare them with other heating and cooling technologies applicable in the State of Geneva, based on both fossil fuels and renewable sources.

Complete information on all the environmental impact categories is presented in Table A.13. When compared to fossil fuels, we observe that geothermal heating systems under all scenarios are environmentally preferable, except for the impact on mineral resource scarcity, which is attributable to electricity consumption (due to the use of copper in the transmission and distribution lines and cement during the construction) and the heat network. This comparison proves that geothermal heating is not only beneficial for decarbonization in Geneva, as suggested in the literature (Lo Russo et al., 2009; Bayer et al., 2012; Blum et al., 2010). It also lowers other environmental impacts, whereas the impact on mineral resource scarcity needs to be managed and minimized (Sovacool et al., 2020). We also identified three cases in which geothermal heat could have larger impacts than fossil fuels in terms of terrestrial

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Fig. 7.Ranges of environmental impacts for all configurations, calculated using bounding analysis.

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acidification, particulate matter, water consumption, and land use.

These cases are for Configurations 1, 2, and 6, which have a combination of a minimum flowrate and high material and energy intensity (i.e., the cases with larger material and energy consumption but lower heat production).

In comparison to renewable energy technologies, the environmental impacts of geothermal heating systems in the base-case scenario are lower than those of biomass, except for total energy demand, water consumption, and mineral resource scarcity. The impacts of geothermal systems are in the same order of magnitude as those of the individual ground-source heat pump, solar thermal collectors, and electric heating (assuming the renewable electricity mix of the State of Geneva), and they are slightly higher in terms of global warming, water consumption, fossil resource scarcity, and total energy demand impacts. In contrast, the impacts of geothermal heating are higher than those of waste incineration, except for global warming. Indeed, waste incineration has the lowest impacts of all the heat sources across all impact categories, except for global warming. Looking at the 50% geothermal scenario, where district heating is supplied by a mix of geothermal heat, waste incineration, and a natural gas plant (Figure A.4), we noticed a large difference between the impacts of this scenario and those of fossil fuels, which signifies the potential of avoiding impacts should such a scenario be deployed to replace natural gas-based district heating or individual oil boilers. This avoidance applies for all impacts, except for mineral resource scarcity, and persists even when the share of waste incineration is substituted with natural gas-based district heating (Fig. A.4).

The impacts of geothermal cooling are lower than those of a natural gas absorption chiller (Fig. 9), except for the water consumption impacts. Geothermal absorption chiller has higher water consumption than natural gas absorption chiller. Both systems have similar direct water

consumption for the cooling process of the absorption chiller’s solution.

But the indirect impacts of geothermal absorption chiller are higher than those of natural gas, mainly due to well drilling and more complex equipment. As mentioned in the last paragraph of Section 3.1, the electricity consumption of the reversed heat pumps is the main reason for the high amount of water consumption impacts.

4. Discussion

This work quantified, for the first time in the scientific literature, the life-cycle environmental impacts of various configurations of shallow to medium-depth geothermal heating and cooling systems, combined with connected decentralized heat pumps or traditional district heating and cooling. Geothermal heating systems from shallow wells (350–1600 m) have lower environmental impacts because the impacts attributable to well drilling are low, and those attributable to electricity consumption are lower than those of very shallow wells (0–100 m). This lower electricity consumption has minor implications for environmental impacts.

However, with different electricity mixes elsewhere, this could be a determining factor for environmental impacts. For instance, Greening and Azapagic (2012) argued that considering the United Kingdom’s (UK) electricity mix, the use of heat pumps would help achieve the UK’s climate change targets only marginally, but it would increase the other environmental impacts significantly. Bayer et al. (2012) reported that with a COP of 3, decarbonization could not be achieved in Poland using heat pumps, while the same COP allows CO2 emission savings of over 60% in Switzerland, France, and Belgium. For countries whose electricity mix has high environmental impacts, geothermal resources from deeper wells (2500 m or more) would offer an advantage as they allow considerably smaller amount of electricity consumption.

Fig. 8.Environmental impacts of multi-source scenarios. As the supplementary sources, electric heating is assigned to Configurations 1 and 3 (with stars), and a mix of natural gas and waste incineration district heating is assigned to Configurations 2,4,5, and 6.

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This work also quantified the environmental impacts of geothermal cooling produced simultaneously with heating. In general, cooling production minimizes the environmental impacts of geothermal systems per MWh, as it increases supply without major additional equipment.

Free cooling was shown to have lower environmental impacts, as no energy conversion is needed, leading to its independence from electricity. Cooling supply using an absorption chiller also consumes mini- mal electricity, but this application requires a volume of water for the cooling tower that could amount to three times the water extracted from the well (Ma et al., 2010). Although the overall impacts on water consumption of this configuration are lower than those of reversed heat pumps, 25% of this consumption constitutes accessible water nearby, which could be problematic, especially in water-stressed areas.

In line with the findings on the advantages of shallow wells (350–1600 m), this work also consistently showed the advantage of using connected decentralized heat pumps to supply heat over traditional district heating in terms of life-cycle environmental impacts. This is because the environmental burden of high-density polyethylene pipe is lower than that of steel pipe. However, the decentralized system be- comes challenging if the difference between the supply and return temperatures in the network is limited by the geothermal injection temperature. Smaller temperature differences mean larger pipe di- ameters. For instance, in the case of very shallow wells (10–100 m), the diameter of the heating network of Configuration 1 is twice that of Configuration 2, potentially making it more complex and expensive to install in urban areas (Nussbaumer and Thalmann, 2016). Geothermal heat from very shallow and shallow wells (10–1600 m, respectively) offers flexibility in terms of the types of networks, which could make these resources more favorable when facing spatial constraints. For cooling, the use of a decentralized network carries around 9% lower environmental impacts than a traditional centralized network due to the efficient use of network pipes.

We also showed that, like other types of low-carbon technologies for

decarbonization (Sovacool et al., 2020), geothermal heating and cooling combined with a renewable electricity mix poses a challenge concerning the impacts on mineral resource scarcity, which are higher than those of fossil fuel-based systems. While these impacts do not affect the State of Geneva directly, in the long term, they could contribute to a global increase in the price and vulnerability of the mineral resource supply.

These impacts can be managed by opting for shallow geothermal systems with connected decentralized heat pumps whenever feasible or by exploiting geothermal resources from deeper wells (2500 m or more), thereby limiting electricity consumption during the operation stage. A decrease could also be achieved by installing district heating in densely populated areas to achieve higher linear heat density (Quiquerez, 2017;

Chambers et al., 2019), thus leading to shorter pipe lengths, or by increasing the use of recycled steel and the efficiencies of equipment.

The uncertainty identified using bounding analysis in this work provided the spans of environmental impacts from geothermal heating and cooling applications in the State of Geneva. We compared these spans with two studies in the literature as a validity check Pratiwi et al.

(2018) quantified the life-cycle global warming impacts of an industrial geothermal heat plant with a production temperature of 170 ^◦C from a 3196 m depth. The electricity mix used in that study contained 42 kg CO2 eq/MWhel. and resulted in reported emissions from geothermal heat from 6.97 to 9.15 kg CO2 eq/MWhheat. Karlsdottir et al. (2014) quantified the environmental impacts of geothermal district heating in Iceland with a production temperature of 85 ^◦C and reported climate change impacts of 5.8 kg CO2 eq/MWh and acidification impacts of 29 g SO2 eq/MWh.

Compared to our Configuration 6 (as the plant operates without a heat pump), their emissions are slightly lower than our range of uncertainty.

This could be due to the high exploitable temperature and high capacity factor for industrial sites assumed by Pratiwi et al. (2018). Karlsdottir et al. (2014) studied a district heating system with supply and return temperature of 79 ^◦C and 35 ^◦C, respectively, thus having more output than the district heating in our study. At the same time, the capacity Fig. 9. Top: Environmental impacts of various heating sources in comparison to those of geothermal systems (in grey). Bottom: Environmental impacts of cooling production using a natural gas absorption chiller in comparison to those of geothermal systems. The comparison of geothermal systems is presented in Figs. 5,6, or 7.

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factor of the system in their study might be higher than that of Geneva, considering the much colder climate, allowing for more heat production and lower impacts per MWh of heat produced. We further compared our results with those of Liu (2017), who studied the life-cycle impacts of geothermal heating with a temperature of 70 ^◦C from a 1985 m depth using a heat pump. They reported 82.6 kg CO2/MWh, 0.9 kg SO2/MWh, 0.6 kg NOx/MWh, and 0.3 kg dust/MWh, the latter being related to the particulate matter. These values are all outside our uncertainty ranges, mainly due to their assumed electricity mix, with 70% from coal power plants. Finally, as argued in the Introduction, no other study exists on life-cycle impacts of shallow to medium-depth geothermal systems

In terms of integrating geothermal heat in a multi-source system, a combination with electric heating that is based on the Geneva electricity mix will lead to lower impacts in terms of global warming, fossil resource scarcity, and water consumption, while the increase in particulate matter and terrestrial acidification impacts will be insignificant.

The trade-off is that this combination will further increase the impacts on mineral resource scarcity. While the deployment of electric heating could be meaningful in other countries, it is limited in Switzerland (EnDK, 2014), where heat pumps are preferred due to their higher efficiency. Combining geothermal heat with waste incineration and natural gas boilers in district heating would increase the impacts of global warming and fossil resource scarcity as mentioned in the Results section.

However, these impacts are still lower than those of fossil fuels (Figure A.4), and a reduction in environmental impacts could be achieved if such a combination is deployed to reduce the current share of fossil fuels. For instance, deploying Configuration 2 under 50% geothermal to substitute 50% of natural gas in a district heating system will reduce the global warming impacts of the network by 60%, fossil fuel scarcity by 70%, water consumption by 60%, terrestrial acidification by 46%, particulate matter formation by 33%, and land use by 10%. However, this deployment will double the impacts of mineral resource scarcity.

Combining geothermal heat with other sources in a network also allows geothermal system to function at its full capacity for longer hours. These findings demonstrate that the growth of geothermal heat to facilitate decarbonization could be accelerated by connecting geothermal resources to existing fossil fuel-based heating and cooling networks.

However, in cases where the electricity mix is detrimental to the environment, this argument is only relevant for deeper geothermal resources (2^′500 m or more), where less electricity is needed during the operation phase.

The results of our study are the first of their kind and fill an important research gap in the literature. Our analyses are useful for sites that seek to develop various types of geothermal heating and cooling systems, especially those with electricity mix based on hydropower. Neverthe- less, our methodologies have some limitations that should be addressed in future research. First, our analysis only accounted for the conditions in the State of Geneva. Geneva’s electricity mix is dominated by hydropower plants, which is not the case in many countries, where the electricity mixes might have higher environmental impacts. The length of the district heating network was determined using a minimum profitable linear heat density to estimate impacts. However, district heating is usually installed in an urban area with a higher population density to be profitable (Knoblauch and Trutnevyte, 2018), resulting in a shorter network. We also assumed only one heating and cooling load profiles for residential buildings in Geneva, and these profiles are not necessarily representative of other European or global settings. Subject to climate change, these profiles might change in the future, where the cooling load is likely to rise. These factors limit the generalization of our results, and future research shall assess environmental impacts under different conditions and for other types of buildings. Second, owing to the lack of such geothermal projects in Switzerland and the available data, the results were only partially validated. As more data become available, a more comprehensive validation can be achieved. For the same reason, the approach chosen to assess uncertainty was a stylized bounding analysis (Casman et al., 1999) comprising eight scenarios created by

varying the input parameters between optimistic and pessimistic cases.

While this methodology identified the orders of magnitude of the impacts, it only provided a limited understanding of the parameters’ influence and interactions. Improved uncertainty analysis could be achieved by using methods for global sensitivity analysis and scenario discovery in the future (Groen et al., 2017; Jaxa-rozen and Kwakkel, 2018; Kwakkel and Haasnoot, 2019). Third, the sustainability of the geothermal resource itself, such as resource availability and thermal short circuits, as well as the impacts of changing physical properties of groundwater (H¨ahnlein et al., 2013; Pophillat et al., 2020), are not dealt with and remain to be assessed. Finally, our analysis focused on eight environmental impacts separately but did not undertake a more inte- grative approach that would also include value judgments on which impacts are more important than others in the view of citizens and various stakeholders, and this could be done in future research (Volken et al., 2018; Trutnevyte et al., 2011).

5. Conclusions

In this work, we quantified the life-cycle environmental impacts of six configurations of shallow to medium-depth geothermal heating and cooling systems in Geneva, including their impacts on global warming, particulate matter, terrestrial acidification, fossil and mineral resource scarcity, water consumption, land use, and total energy demand. These configurations combined different well depths with corresponding temperatures of geothermal resources and different setups of a connected decentralized heat pump or district heating and cooling networks. Drilling of geothermal wells, district heating network, and electricity consumption were activities found to contribute the most to environmental impacts. Geothermal heating and cooling are suitable options for decarbonizing the building sector in the State of Geneva because, in addition to low greenhouse gas emissions, the other environmental impacts are low, except for impacts on mineral resource scarcity. These environmental impacts are lower if decentralized connected heat pumps are used instead of traditional district heating and cooling, and the equipment’s efficiency can be increased. Combining geothermal heat as a baseload with other heat sources increases the geothermal heat production for the same installed capacity and lowers the geothermal heat-related environmental impacts of heating, especially given Geneva’s electricity mix, which is based on hydropower.

Finally, geothermal heating and cooling systems need to be carefully designed because, however unlikely, it is possible for geothermal systems to have higher impacts on particulate matter, terrestrial acidification, water consumption, land use, and total energy demand than natural gas-based district heating.

Author statement

All persons who meet authorship criteria are listed as authors, and all authors certify that they have participated sufficiently in the work to take public responsibility for the content, including participation in the concept, design, analysis, writing or revision of the manuscript.

Furthermore, each author certifies that this material or similar material has not been and will not be submitted to or published in any other publication before its appearance in Geothermics.

CRediT authorship contribution statement

Astu Sam Pratiwi: Conceptualization, Methodology, Formal anal- ysis, Visualization, Writing - original draft, Writing - review & editing.

Evelina Trutnevyte: Conceptualization, Methodology, Writing - review

& editing, Supervision, Funding acquisition.

Declaration of Competing Interest

The authors declare no competing interests.