Life cycle assessment of geothermal power generation technologies: An updated review

Accepted Manuscript Research Paper Life cycle assessment of geothermal power generation technologies: An updated review C. Tomasini-Montenegro, E. Santoyo-Castelazo, H. Gujba, R.J. Romero, E. Santoyo PII: DOI: Reference:

S1359-4311(16)32346-8 http://dx.doi.org/10.1016/j.applthermaleng.2016.10.074 ATE 9276

To appear in:

Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

6 March 2016 31 August 2016 10 October 2016

Please cite this article as: C. Tomasini-Montenegro, E. Santoyo-Castelazo, H. Gujba, R.J. Romero, E. Santoyo, Life cycle assessment of geothermal power generation technologies: An updated review, Applied Thermal Engineering (2016), doi: http://dx.doi.org/10.1016/j.applthermaleng.2016.10.074

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LIFE CYCLE ASSESSMENT OF GEOTHERMAL POWER GENERATION TECHNOLOGIES: AN UPDATED REVIEW C. Tomasini-Montenegro1, E. Santoyo-Castelazo2, H. Gujba3, R.J. Romero4 and E. Santoyo1,*

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Departamento de Sistemas Energéticos, Instituto de Energías Renovables, Universidad Nacional Autónoma de México (UNAM), Priv. Xochicalco s/n, Col. Centro, 62580 Temixco, Morelos, Mexico; 2Secretaría de Energía, Insurgentes Sur 890, Col. Del Valle, Delegación Benito Juárez, 03100, México, D.F.; 3African Climate Policy Centre (ACPC), United Nations Economic Commission for Africa (UNECA); 4Centro de Investigación en Ingeniería y Ciencias Aplicadas, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, Chamilpa, Cuernavaca 62100, Morelos, México.

*

Corresponding author: [email protected]

Special Issue of the International Symposium on Renewable Energy & Sustainability (ISRES’2015) Applied Thermal Engineering Journal Submitted in March (2016); Revised in August (2016)

Abstract The market of energy production coming from geothermal energy is growing around the world. However, in some countries little attention is given to the life cycle environmental impacts of this industrial sector. In addition to the available work presented in previous reviews about this matter, this paper provides an updated review of life cycle environmental studies for the geothermal power generation. For the first time, these results have been compiled by energy conversion technology: dry steam, binary cycle, single flash, and double flash, including the generation pilot projects of enhanced geothermal systems. The new analysis of the review shows that the environmental impacts are evaluated by considering from 1 to 18 impact categories commonly used in life cycle assessment (LCA). These impacts are mainly affected by: (i) reservoir characteristics; (ii) geothermal fluid chemistry; (iii) power generation technology; (iv) type of emissions of the life cycle inventory; and (v) data availability. Most of the LCA studies reported global warming, which is mostly caused by the fuel consumption in the construction and operation stages. A deeper analysis of the life cycle environmental impacts to promote an environmental sustainable management of geothermal power generation is proposed, which still represents a big challenge for this industrial sector.

Keywords: geothermal energy, life cycle assessment, environmental impacts, sustainability, greenhouse gas emissions, environmental legislation.

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1 Introduction Power generation is one of the biggest technological commitments for our society to meet energy needs and to reduce inequities of a growing population, which is expected to be over 9 billion by 2050 [1]. To meet these needs as well as in response to the growing concern in the implementation of environmental legislation designed to control global warming, the exploitation of renewable energy resources arises as an attractive response for the power generation market. These resources represent approximately 28% of the power generating capacity in the world [2], creating an option to find not only innovative, but also sustainable solutions to meet the future population´s energy requirements. Among these renewable resources, this study is focused on geothermal energy, which is considered a reliable energy source because of its independence of seasonal, climatic and geographical conditions [3]. Over more than 100 years, the commercial exploitation of geothermal energy resources has provided base-load electricity in the world. Although nowadays, total installed capacity from worldwide geothermal power plants is currently around 13 GWe, an exponential forecasting of geothermal power production indicates a probable increase up to 21 GWe by the year 2020 [4]. Eventhough reaching the amount projected depends on different economic, environmental and political factors; this forecasting is a promising scenario that establishes the potential growth of geothermal energy in the near future. Commercial generation projects under development are concentrated in five regions: Asia Pacific (4.81 GW), followed by North America (3.45 GW), Europe (2.13 GW), Latin America (1.64 GW) and Africa (0.6 GW) [4,5]. Within these regions about 90% of the total geothermal power generation is concentrated in the following eight countries: U.S. (3,567 MW), Philippines (1,930 MW), Indonesia (1,375 MW), Mexico (1,069 MW), New Zealand (973 MW), Italy (944 MW), Iceland (665 MW), and Japan (533 MW) [5]. In addition to these countries, where there are well-established markets, Bertani (2016) points out that other countries from emerging markets such as Turkey (637 MW) and Kenya (607 MW) are starting to play an important role in the potential growth for the geothermal power production industry [4,5]. Considering the potential increase in the geothermal power production industry, the concern about the environmental impacts coming from this important industrial sector has attracted the attention of stakeholders (i.e., decision-makers, industrial sector, and scientific sector) [6-9]. Hence, that research to develop a framework to specifically address the sustainability evaluation for geothermal energy projects has been conducted [10,11]. 3

In the same context, sustainability indicators have been proposed to evaluate the performance of geothermal energy compared to other renewable energy options to produce electricity [12-15]. However, in relation to the environmental sustainability little information is available [16, 17]. Like any other industrial process, geothermal power production leads to direct and indirect environmental impacts. According to Bayer et al. (2013) direct environmental impacts encompass land use, geological hazards, waste heat, atmospheric emissions, solid waste, water consumption, impact on biodiversity and noise [6]. In addition to the analysis of these impacts, information related to indirect (i.e. impacts coming due to materials and energy required over the life cycle of the power generation process) environmental impacts is required in order to fully understand the environmental implications of geothermal power production [6]. Such evaluation should be conducted on a product system approach, which allows an assessment within the supply chains in the life cycle of this industrial sector. Based on this approach, information about the environmental impacts refers to the whole system instead of a single process, thus avoiding environmental burden shifting among the life cycle stages that comprise a geothermal power generation system [18]. Consequently, the development of mechanisms that allow an environmentally sustainable management of this sector along its life cycle is strengthened. Such an evaluation can be conducted using Life Cycle Assessment (LCA), which according to the ISO norms (ISO 14040, 2006a; ISO 14044, 2006b) is defined as the quantitative estimation of environmental impacts, based on the analysis of energy and material inputs and emissions outputs along the life cycle of a product/process [19,20]. Although the results from this study show that available information about the geothermal power production´s life cycle environmental impacts is scarce, these results also show that there is a concern in relation to both the sustainability evaluation and the environmental evaluation of this industrial sector. According to Bayer et al. (2013), results from life cycle environmental impacts from geothermal power plants are scarce and diverse, varying based on the size and kind of technology under analysis [6]. Available information is usually gathered and presented for the whole geothermal energy production sector, addressing general aspects of this sector (i.e., such as water consumption [21,22]); however each conversion technology faces different challenges in managing their environmental impacts. Some of these challenges include lack of life cycle inventories because these databases are either kept under confidential conditions or probably data exist, but may be they are not structured in an exploitable way.

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Additionally, the environmental impacts obtained under certain assumptions from a particular geothermal plant will be only valid for a specific site where the power plant is installed, and therefore, these impacts can hardly be extrapolated to other places because the geological conditions, the geothermal reservoir, and the geofluids are unique [6]. There are other technical reasons that make difficult to perform the environmental sustainability evaluation of geothermal technologies such as the quantification of actual steam and gas emissions produced prior (natural background or baseline) and after exploitation, the effects of power plants on the water aquifers, the use of new cogeneration plants and hybrid technologies, the energy conversion used by different power plants, as well as the economic and social issues [6]. In the present study, therefore, a compilation of the life cycle environmental impacts per type of technology has been conducted. In order to consider the differences between plant types, the results from these studies are presented per type of conversion technology under analysis. Based on the ISO norms (ISO 14040 2006a [19], ISO 14044 2006b [20]), a direct comparison of these results is not possible. It is because the considered studies utilize different functional units, system boundaries, system products, and impact assessment methods, as well as methodological choices. Thus, rather than a quantitative evaluation the objective of this work is to provide a qualitative analysis of the life cycle environmental impacts of electricity production generated from geothermal energy. It is worth mentioning that the difference among the functional units lies in the use of net electricity, gross electricity and different assumptions such as the life span considered in each study. By reaching this objective, the present study differentiates from previously mentioned reviews that have been conducted by Bayer et al. (2013), Amponash et al. (2014), Asdrubali et al. (2015) [6,16,23]. Although these reviews provide a structured point of view on the life cycle environmental impacts (through the identification of the main pollutant emissions, energy and resource usage, and some social aspects associated with the power plant operation), to the authors´ knowledge the comprehensive analysis conducted in this study has not been done before. The results analyzed in this review are presented by type of technology used for geothermal power generation. An updated summary about the geothermal generation power technologies has been outlined, which is followed by a section of legal frameworks commonly required for the implementation of environmental assessments, then the analysis of the results and a set of conclusions and recommendations are given.

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2 Review methodology To achieve the objective of this review, the following tasks were carried out:

(i)

To update information available in literature about the life cycle environmental impacts coming from geothermal power generation;

(ii)

To identify life cycle environmental impacts per type of energy conversion technology based on the available publications;

(iii)

To identify system boundaries within the life cycle environmental assessment that has been conducted so far;

(iv)

To identify information gaps related to the environmental assessment of geothermal power production by considering a life cycle approach;

(v)

To summarize information about the hot spots (technically used in LCA as the key processes to address in order to control the identified environmental impacts) within the considered environmental impacts, gathering data based on the type of geothermal power generation plants (conventional and emerging technologies);

(vi)

To summarize information about the technical characteristics of the considered options in order to gather this information in a succinct document; and

(vii)

To provide conclusions and recommendations based on the findings obtained from the literature review.

The results are presented by type of technology used for geothermal power generation, which includes well-established or conventional technologies: (1) dry steam; (2) single-flash; (3) double-flash power plants; and (4) binary cycle. Moreover a fifth category has been included: enhanced geothermal systems (EGS), which, as described further below, represents an emerging option in the market of the exploitation of low temperature reservoirs [3]. Although the implementation of triple-flash power plants is already taking place in some countries of North America, Asia and Oceania [4], up to our knowledge there is not information available regarding to the analysis of the life cycle environmental impacts on this new technology.

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In addition to power generation, geothermal energy is also exploited for other direct uses (e.g., geothermal heating systems). However, since the aim of this study is only focused on the analysis of life cycle impacts coming from geothermal power production, information about the life cycle environmental impacts from heat geothermal production is not included [24]. A comprehensive review on this important sector of the geothermal energy use will be carried out in the future.

3 Overview of geothermal power generation technologies Most of the power generation technologies currently available in the geothermal industry have been designed for exploiting the conventional convective geothermal systems (also referred as hydrothermal systems). The selection process of the most suitable geothermal power generation technology essentially depends on the properties of the geothermal resource (fluid and reservoir) that require to be exploited (i.e., geological, chemical, physical and thermodynamic properties) [3]. According to Lund (2007), geothermal resources suitable for power generation can be categorized in three major groups: (1) vapor dominated systems with temperatures >240 °C; (2) liquid (or hot water) dominated systems with temperatures up to 350°C; and (3) petrothermal or hot dry rock resources with temperatures up to 650°C [25]. Groups (1) and (2) are related to the convective hydrothermal systems which are commercially exploited in the world, whereas group (3) is referred to the exploitation project of the hot dry rock (HDR) or enhanced geothermal systems (EGS). The energy conversion technology used for exploiting the geothermal systems depends on the reservoir properties (e.g., geological, geophysical, geochemical, physicochemical, thermodynamic, among others). Three types of mature technologies have been commercially and successfully used for the exploitation of geothermal resources, namely dry steam, flash (single, double and triple), and binary cycle power plants. A brief overview of these technologies is given as follows.

3.1 Dry Steam Technology There are privileged places, such as The Geysers in California and Larderello in Italy, where the earth´s gradient temperature leads to reservoirs with high temperature (>200°C). The vapor extracted from these reservoirs is transported to a steam turbine that converts thermal energy into mechanical energy, which is then sent to a generator from where electricity is produced and distributed into the grid (Fig. 1A). 7

This conversion technology is known as dry steam, and due to its plant set up it is the cheapest geothermal generation process [3]. This energy conversion technology currently contributes with 22% of the worldwide installed capacity with 63 dry steam geothermal power plants, which are mainly concentrated in the U.S., Italy, Indonesia and Japan [4]. Furthermore, based on the steam´s chemical composition, which is generally characterized by water steam (>90% wt. of steam) and non-condensable gases (NCG) (<10% wt. of steam) [26], the plant set up can also have a gas extraction system. This system can include vacuum pumps or streamjet ejectors, which are designed to remove NCG that among other gases include CO2, H2S, NH3 and some trace gases (e.g., He, H2, Ar, N2, CH4, and CO) [26]. The presence of the NCG in the steam stream has represented a challenge in the market of electricity production by geothermal means, since due to their potential corrosive effects different modifications sometimes are required to avoid a reduction in the turbine’s efficiency [27]. This is because of two factors that decrease the power production rate, which might lead to a reduction in the plant´s profit [28]. Firstly, NCG could be accumulated in the condenser, decreasing its efficiency; secondly, the presence of H2S sometimes has an acidification effect on the steam stream, which turns this stream into a corrosive agent affecting the turbine operation as well as the composition of the emissions sent to the atmosphere. According to DiPippo (2015), environmental effects from dry steam power plants are minor due to the control of NCG either by standard means or abatement techniques [27]. However, in spite of the implementation of abatement techniques, the results displayed in section 5.1 show that the environmental effects of NCG should be carefully analyzed.

3.2

Single and Multi-stage Flash Technologies (Double and Triple)

If the geothermal fluid in the reservoir is a liquid-vapor mixture, then a separation process commonly known as flash is used for the power generation. As explained below, based on the thermodynamic mixture´s characteristics, the separation process can include one, two or three stages, namely single-, double-, and triple-flash systems, respectively [31]. When the mixture temperature is over 210 °C, a single-flash set up is generally used (see dotted lines in Fig. 1B). In this case, the geothermal fluid is extracted from the production well and sent to a cyclonic separator (Webre type) where the liquid and vapor phases of the mixture are efficiently separated due to a difference in densities [31]. 8

The primary vapor passes from the separator to an expansion steam turbine and finally to a generator to complete the process. The remaining liquid phase (also known as brine) obtained from the separator is sent to a reinjection well, which in turns receives cooling water from a condensation process that is designed to treat steam coming from the expansion turbine. In order to increase the efficiency of this process a second separation stage (known as doubleflash) is added (see solid lines in Fig. 1B). This process is used to separate low-pressure steam coming from the brine leaving the single flash cycle. The secondary low-pressure steam is led to either a lowpressure turbine or a suitable stage of the main turbine (with dual-pressure and dual-admission specifications). Although this is a general description of the process, it should be noted that based on the chemical composition of the geothermal fluid, an integration of a NCG abatement equipment could be also required if the amount of the NCG is high. The double-flash power plants are recommended to increase both the efficiency of the process generally by 35% and the power generation by 20% in relation to the single-flash set up [31]. In this context, a third separation stage could be integrated in the plant set up, which is known as triple-flash power plants (see Fig. 1C). This process is designed to utilize energy available in the brine coming from the double flash cycle, as well as to decrease the non-condensable gas (NCG) content of the geothermal fluid [28]. This technology is currently used in some of geothermal fields of U.S., New Zealand, and Turkey [4]. The use of single and double flash conversion technology contributes to 63% of the world’s geothermal power installed capacity, and an additional 2% is provided by triple flash power plants [4]. This information is in contrast to the scarcity of information about the life cycle environmental impacts of this energy conversion technology.

3.3 Binary Cycle Technology In liquid-dominated reservoirs with temperatures lower than 200 °C, a binary cycle system is used for power generation, which according to Bertani (2016) represents 12% of the worldwide installed capacity [4].

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In this system, the geofluid cannot be used directly as in other power generation technologies previously described. This is due to the low temperature of the geofluid, which leads to a poor vapor production. However, the thermal energy available in the geofluid can be used to vaporize a working fluid (which has a lower boiling point, e.g., n-isobutane, n-isopentane and pentane), by using a thermodynamic organic Rankine cycle (ORC) or Kalina cycle to produce electricity [31]. The heat transfer process occurs in a heat exchanger from where an organic vapor is produced and sent to a turbo generation system for producing electricity (see Fig. 1D). Remaining steam coming from the turbine is sent to a condenser whose brine is conducted to the heat exchanger, thus closing the thermodynamic cycle.

3.4 Engineered or Enhanced Geothermal Systems (EGS) The power generation process theoretically proposed for the exploitation of enhanced geothermal systems (EGS) is generally the same as the one described for binary cycle plants. Therefore, the reader is referred to references [32-36] in order to see a full description of the EGS technology. These systems are aimed to exploit widely available deep underground reservoirs (namely hot dry rock, hot wet rock and hot fractured rock resources), where insufficient water exists and/or the rock-formation permeability is low [37]. In order to exploit such geothermal systems, an enhanced process in the rock permeability is required either by opening pre-existing fractures in the rock or by forming new ones to create an artificial reservoir. The thermal energy is generally exploited by injecting water, or another appropriate fluid (e.g., CO2) into the hot fractured rock (or artificial reservoir) to stimulate an intense heat exchange, and to extract most of the energy available in the rock. Sometimes, there is circulation of the fluid already present in the rock formation, which acts as a geothermal fluid loop [37]. The hot fluid is extracted from production wells and pumped to a power plant installed on the surface to generate electricity [37]. In spite of the potential use of the EGS systems reported in the M.I.T. study [36], the implementation of these systems in the commercial market is not widespread. This is explained because the learning curve of this technology is at an early stage [38]. Nowadays, there are technological advances with the installation of some pilot projects in Australia, U.S., Italy, France, Germany, Switzerland, Japan, and El Salvador, which have demonstrated the feasibility of exploiting these systems at depths between 3 km and 10 km [37, 39-45].

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4 Environmental Legislation and the Role of Life Cycle Assessment Due to the reliability of the geothermal power plants resulting from the fact that they are not intermittent, delivering a steady output over 24 h, this technology could increase its contribution to a portfolio of clean energy production [46-47]. However, in order for geothermal energy share growth to continue, different factors have to be considered. Among these factors are policy-making, technical expertise, and geothermal energy availability [5,7]. In regard to policy-making, it is aimed to secure a sustainable management of geothermal energy for playing a determinant role in the future promotion of geothermal power projects around the world. Little information is available with regards to environmental legislation related to geothermal power generation. If this kind of legislation is no available, it is unclear if an environmental assessment of the geothermal power generation section would have been conducted. The progress in the creation of a regulatory framework specific to geothermal energy is diverse, being registered mainly in countries of the developed world. For example, in the U.S., in addition to federal initiatives (i.e. Energy Policy Act), a great deal of effort is concentrated at the state level, passing 24 projects related to geothermal power and heat production in thirteen states in 2013 [47]. Moreover, in the European Union, where the target of supplying 20% of the total energy consumption from renewable sources by 2020 has been set, a global legislative framework has been established through the promotion of both the electricity produced from renewable energy resources in the internal electricity market, and the use of energy from renewable energy sources for the electricity, transport, heating and cooling sectors [48]. In addition to this global effort, there is legislation specifically addressing the field of geothermal energy on a national level for member countries of the European Union (EU). According to Šušteršić et al. (2010), these countries are Austria, France, Germany, Greece, Hungary, Italy, Romania and Poland [48]. Other EU candidate countries, which have geothermal power potential, are working in order to develop legislation to manage their geothermal energy resources. Turkey is an example of these countries, progressing in the establishment of a regulatory framework that promotes a sustainable exploitation of this kind of energy [49].

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Other regions with worldwide geothermal potential, such as China, are also making progress with regard to support policies related to exploiting its geothermal resources. In this regard, according to Zhao and Wan (2014), China´s efforts have been focused on policies related to supporting two sectors: geothermal heating and ground sound heat pumps, with future work to address other areas [50]. Furthermore, the creation of regulatory frameworks is also being conducted in some countries of Latin America, which based on a study conducted by the International Renewable Energy Agency (IRENA), encompasses legal supportive schemes to promote the use of renewable energy [51]. The current situation of these schemes is analyzed considering eight major themes for twenty countries as shown in Table 1. Among these countries, Mexico stands out with a great progress on the establishment of a new legislation on the use of geothermal energy, such as, the Law of Geothermal Energy (in Spanish: “Ley de Energía Geotérmica”) [52], the new Law for the Energy Transition, that comes from the former law LAFAERTE (in Spanish “Ley para el Aprovechamiento de Energías Renovables y el Financiamiento de la Transición Energética”[53]). Although the overall summary does not represent a comprehensive review of the available legislation of geothermal energy, it has been included in this section to illustrate the previous work carried out in the world for developing legislative instruments as a key element for the transition to a mix energy portfolio. These instruments trigger the environmental evaluation of geothermal projects, since this environmentally sound legislation establishes the guidelines that have to be met prior to the implementation of commercial generation projects. Based on these guidelines, there are different tools for identifying the environmental impacts of using geothermal energy on an industrial basis. Within these tools, a widely accepted instrument within the EU and the U.S. is environmental impact assessment (EIA), which is a methodology used to identify on a local basis a project´s environmental impacts prior to the implementation phase [7]. Another tool extensively used for the environmental assessment of products or process is LCA [6,7,54]. Unlike EIA, LCA is based on a system product approach of the geothermal energy sector, thus avoiding the burden transfer from one life cycle stage to another that is commonly consider under the evaluation of a single life cycle stage (i.e. operation stage) [55]. Although due to the lack of life cycle inventory data this information is limited, in this study the available data has been aggregated, analyzed and presented by type of technology.

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5 Life cycle environmental impacts for geothermal power plant technologies 5.1 Dry steam The number of studies related to the life cycle environmental assessment of dry steam plants is restricted to only two studies, considering different goals and scopes for each study. Firstly, Sullivan and Wang (2013) analyzed fossil energy consumption and greenhouse gas (GHG) emissions, by using a life cycle inventory analysis (LCI) and considering emissions of CO2, CH4 and N2O in a U.S. context [56]. Secondly, Bravi and Basosi (2014) carried out the estimation of environmental impacts (including global warming potential, acidification potential, and human toxicity potential) due to the emissions of non-condensable gases (CO2, H2S, NH3, CH4, and Hg) produced during the operation stage [57]. It is worth mentioning that the emissions quantified, by Bravi and Basosi (2014) correspond to the emissions of non-condensable gases that are produced only during the operation stage. In this study, neither drilling nor operation of the wells or maintenance have been considered by the authors, since they assume that: (1) the emissions coming from these stages have a small contribution of the total background (i.e., as secondary or auxiliary LCA system), and foreground (i.e., as the main LCA system) emissions; and (2) the environmental impacts of the plant´s construction are attenuated or diluted over 25 years, which is considered as life span [57]. Moreover, it also should be mentioned that the quantified emissions are measured with the use of a gas abatement technology (abatement of mercury and hydrogen sulfide technology (AMIS) system) for removing part of the gases produced. Such an improvement might affect the estimated effects if the plant under analysis does not use this technology. A summary of the technical characteristics of the plants considered in both studies is presented in Table 2. In relation to the system boundaries, they are different across the studies analyzed in this section. In the first case the system boundary encompasses the plant´s construction and operation are considered, [56] whereas for the second case, the emission of non-condensable gases during the operation stage is studied [57]. These elements are used to provide the system boundaries for the dry steam plants under analysis (see Fig. 2). The environmental impacts, the impact assessment method, and the functional unit considered for each dry steam study together with other power plant technologies analyzed within this section have been included in Table 3.

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In order to provide a summary of the LCA results for dry steam power plants an analysis per environmental impact is given in the following section. 5.1.1 Global warming potential The estimated global warming potential (GWP) value estimated by Bravi and Basosi (2014) and Sullivan and Wang (2013) are shown in Fig. 3, where the impact of the non-condensable gases emissions produced during the operation is shown by the atypical high GWP value calculated (670 kg CO2 eq/MWh) by Bravi and Basosi (2014) for the geothermal Mount Amiata area (located in Italy) [57]. This high GWP estimate should be taken with caution, and cannot be generalized for other dry steam power plants because highly anomalous emissions of gases are mainly due to the volcanic nature of this specific site. High emissions of CO2 (ranging from 413 to 779 kg/MWh), CH4 (ranging from 2.31 to 12.2 kg/MWh), H2S (ranging from 0.0397 to 11.14 kg/MWh), and NH3 (ranging from 0.0859 to 7.83 kg/MWh) are considered by the authors as the main cause of the overestimated GWP value. A range of GWP values is actually estimated by Bravi and Basosi (2014), which corresponds to the range of emissions previously reported. As seen in Fig. 3, the average GWP is shown in the bar, where the error bars represent the high and low estimated GWP (henceforth error bars represent data ranges instead of standard deviation). These results highlight the argument to include operational emissions within the system boundaries for a better and reliable estimation of the environmental impacts. However, this decision should be analyzed based on the goal and scope of the LCA conducted in order to evaluate the environmental impacts of a given project. On the other hand, Sullivan and Wang (2013) reported a GWP value that on average amounts to 74 g CO2 eq/kWh, which according to these authors is estimated considering the plant´s construction and operation without detailed information about the activities that lead to this result [56]. 5.1.2 Acidification potential and human toxicity potential The environmental assessment of the production of geothermal electricity should include information about other environmental impacts in addition to global warming. In this sense, Bravi and Basosi, (2014) present information about the acidification and human impact by their estimation of acidification potential (AP) and human toxicity potential (HTP), respectively [57].

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As seen in Fig. 4, on average the AP amounts to 5.18 g SO2 eq/kWh, whereas for HTP amounts to 8.40 g 1.4 DB eq/kWh (where DB is referred to 1.4 dichlorobenzene (DB) as a reference unit). As in the case of GWP, these values are representative of the specific conditions prevailing in the geothermal Mount Amiata volcanic area, Italy.

5.2 Single-flash geothermal power generation plants According to Bertani (2016), 41% of the energy produced from the exploitation of geothermal energy is generated through single flash plants [4]. In spite of the importance of this conversion technology, little attention has been given to the evaluation of its environmental impacts under a life cycle perspective. The results compiled in this section are based on the analysis conducted by Bravi and Basosi (2014), which up to our knowledge is the unique published study that integrates the environmental assessment of a single flash geothermal power plant [57]. These results correspond to the geothermal plant labeled Bagnore 3 which is located in the geothermal Amiata Mountain, Italy, and whose main technical characteristics are summarized in Table 4. As mentioned in Table 3, the goal of the study carried out by Bravi and Basosi (2014) is to evaluate the impact on GWP, AP and HTP due to the emissions of NCG generated during the operation stage [57]. As shown in Fig. 5, the system boundaries encompass the operation stage with special emphasis on the previously mentioned emissions. 5.2.1 Global warming potential As shown in Fig. 3 and Fig. 6, average results reported for the considered impacts are 690 g CO2 eq/kWh, 22.30 g SO2 eq/kWh, and 8.40 g 1,4 DB eq/kWh for GWP, AP, and HTP respectively. According to Bravi and Basosi (2014), these results are site specific and can be attributed to the geological characteristics of the Amiata Mountain Area. The presence of active or open fractures, which might be created as a result of the well construction, is probably responsible of the high GWP values estimated because these slips stimulate unusual larger flows of geothermal fluid (steam and gases) towards the surface [57]. Thus, the evaluation obtained for the Bagnore 3 power plant should not be generalized because the installation of the same technology in different geological areas may produce different environmental impacts.

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5.2.2 Human toxicity potential and acidification potential With regards to AP, Bravi and Basosi (2014) attributed the estimated values to the large ammonia emissions coming from the Bagnore 3 plant (which ranged from 4.48 to 289 kg/MWh), whereas the natural emissions of Hg, H2SO4, H3BO3, As and Sb are identified by the authors as the main cause of the HTP estimated values [57]. As previously mentioned in section 5.1 the results reported by Bravi and Basosi (2014) corresponds to the emissions measured during the operation stage.

5.3 Double-flash power plants About 20% of the 12.6 GWe of today´s worldwide installed capacity is produced from 68 plants working with double flash conversion technology [4]. Information about their life cycle environmental is scarce, encompassing six analyses: Hondo (2005) [59]; Karlsdottir et al. (2010a, 2010b) [60,61]; Sullivan et al. (2010) [12]; Marchand et al. (2015) [62]; and Atilgan and Azapagic (2016) [63]. These studies are conducted considering different geographical areas: Japan, Iceland, U.S., France and Turkey, respectively. Independent goals and scopes for the environmental assessment of these studies, which are technically described in Table 5, are established. Hondo (2005) defined the comparison of a double flash geothermal power plant in addition to other eight technologies, considering the aggregated global warming potential of CO 2 and CH4 as greenhouse gases as the result of his environmental assessment of power generation [59]. In addition to GWP Karlsdottir et al. (2010a) also estimated energy efficiency in order to meet their defined goal, which is set as the estimation of indicators for energy efficiency and CO 2 emissions from geothermal power production [60]. In a companion paper Karlsdottir et al. (2010b) estimated GWP from the Hellisheidi power plant including heat generation, which leads to a necessary allocation process for establishing the estimation of values corresponding to power generation [61]. Furthermore, Karlsdottir et al. (2010a) reported the evaluation of other nine environmental impacts by using the LCA model, and GWP and primary energy consumption for the case analyzed in Iceland [60]. Sullivan et al. (2010) set the goal of their study as the comparison geothermal power production and 8 power generation technologies, which includes both conventional and renewable options [12].

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The authors of this study conducted this comparison based on two indicators that includes emissions of greenhouse gases (CO2, CH4 and N2O, which are aggregated based on their global warming potentials and the energy ratio estimated as the relationship between the energy requirements and the energy produced by the power generation system. Marchand et al. (2015) defined a twofold goal. On one hand their objective was to measure the environmental impacts of geothermal power production produced in a geothermal power plant located in France [62]. On the other hand, Marchand et al. (2015) also analyzed the effect on the estimated environmental impacts due to two potential changes in the plant set up configuration, which involves a reinjection strategy and two different alternatives to the current plant´s cooling system [62]. Finally, Atilgan and Azapagic (2016) defined the identification of life cycle environmental impacts of electricity generation from renewable energy sources in Turkey, which includes the exploitation of geothermal energy [63]. The system boundaries for the six case studies included exploration, construction and operation (including maintenance) and decommissioning phases (See Fig. 7). According to Hondo (2005) and Marchand et al. (2015) maintenance aspects considers the construction of additional wells and maintenance of both subsurface [59,62], surface equipment and premises. The results for each study, along with the impact assessment method, the functional unit, and software used within the impact assessment phase are reported in Table 3. 5.3.1 Global warming potential According to Hondo (2005), the aggregated GWP value amounts to 15 g CO2 eq/kWh net power, from which 64.7% (9.7 g CO2 eq/kWh) corresponds to the operation stage, specifically due to the equipment changes (30%), drilling of additional wells (19.6%) and general maintenance of the existing facilities (15.1%) [59]. The remaining 35.3% of the estimated value (5.3 g CO2 eq/kWh) comes from the construction phase, and according to Hondo (2005) are mainly due to machinery use (21.2%), building activities (13.2%), and exploration (0.9 %) [59]. In relation to the GWP estimated by Karlsdottir et al. (2010a), these authors report a range rather than an unique value, which is based on four different definitions about the energy content in the geofluid that is used through the energy conversion system. These definitions include: (i) energy based 17

on enthalpy; (ii) energy based on enthalpy minus energy content in the reinjected fluid; (iii) energy based on exergy according to a dead state temperature of 15°C; and (iv) energy based on exergy minus exergy content according to a dead state temperature of 15°C in the reinjected fluid. Thus, the estimated GWP ranges from 35 to 45 g CO2 eq/kWh electricity produced at the power plant [60]. According to these authors about 76% of this estimate is due to the geofluids, however, although information about the causes of this contribution is not given, it might be attributed to the chemical composition of the geofluid. While the remaining 24% is a consequence of the construction phase, mostly provided by the drilling operations [60]. As previously mentioned, Karlsdottir et al. (2010b) also estimate the GWP corresponding to a combined heat and power production in Hellisheidi power plant [61]. Their estimated value, which is allocated for the electricity production, is 29 g CO2 eq/kWh. Based on their analysis 87.5% of this value is caused due to the contribution from the geothermal fluid. Even though there is not an explanation about the origin of this contribution, based on our experience the geofluid chemistry might be also the cause. The remaining 8% and 4% of their estimated value are estimated as a consequence of drilling wells and the power plant and its components (4%) [58]. The GWP estimated by Sullivan et al. (2010) amounts to 103 g CO 2 eq/kWh about 96% of this value corresponds to the fuel use during the operation stage, in spite of the lack of information about the causes of this contribution it might be attributed to the use of reinjection pumps or the geofluid chemistry. The remaining 4% is accounted for the fuel consumption within the construction stage [12]. Furthermore, the value corresponding to the energy ratio amounts to 0.0116. According to the analysis conducted by the authors about 91% of this value is due to the contribution from cement and steel used in both wells and well to plant connections of the geothermal power plant. In relation to the GWP value estimated by Marchand et al. (2015), it amounts to 47 g CO2 eq/kWh net electricity deliver to the network. Based on the authors’ analysis, about 89% of this value comes due to the emissions of CO 2 and CH4 released during the operation stage [62]. As shown in Table 3, Marchand et al. (2015) estimated other 13 environmental impacts, whose main contribution comes from the construction stage. Overall this contribution ranges from 47% for ecotoxicity to 207% for human toxicity potential [62]. The authors have credited the system accounting for material recycled from the decommissioning stage, which based on their analysis explains that there are values higher than 100% in the contribution analysis. Due to the lack of data inventory sources for the Turkish power plant conditions, the GWP result estimated by Atilgan and Azapagic (2016) is reported as an aggregated value, which amounts to 63 g CO2 eq/kWh [63]. 18

5.3.2 Cumulative energy demand and other environmental impacts In addition to GWP, on one hand Karlsdottir et al. 2010a estimated other nine impacts, namely abiotic depletion (AD), acidification potential (AP), eutrophication potential (EP), human toxicity potential (HTP), photochemical oxidation potential (POP), ozone layer depletion potential (ODP), freshwater aquatic eco-toxicity (FAETP), marine aquatic ecotoxicity potential (MAETP), and terrestrial ecotoxicity potential (TETP) [61]. Although the absolute values of these impacts are not reported, information about the main contributions is provided, showing that the contribution from power plant unit is the main share for all impact categories (ranging from 90% for POP to 97% for EP), except for HTP whose share amounts to 64%. With regards to the geofluid contribution, it has contributions in all the impact categories with values less than 10 %, whereas the geothermal well construction only contributes to HTP and POP [61]. On the other hand, Karlsdottir et al (2010a) estimated the primary energy efficiency as cumulative energy demand (CED), which is also based on the four definitions of the energy content in the geofluid that were previously mentioned on section 5.3.1 [60]. Based on these definitions the authors reported that CED ranges from 2.7 to 9 MWh primary energy/MWh of electricity produced [60]. When the combined production of heat and electricity is analyzed, Karlsdottir et al (2010b) established their CED estimation on the geofluid energy content (that is defined based on the geothermal fluid´s enthalpy). The allocated value corresponding to electricity production without considering the reinjection of waste streams amounts to 6.33 MWh primary energy per MWh of energy produced [61]. Atilgan and Azapagic (2016) also estimated other 10 environmental impacts, which include: abiotic depletion potential elements (ADP elements), abiotic depletion potential fossil (ADP fossil), AP, EP, FAETP, HTP, MAETP, ODP, POCP, and TETP. The estimated impacts ranged from -4x10-6 µg of refrigerant R11 eq/kWh for ODP to 0.5 kg DCB eq/kWh for MAETP [63].

5.4 Binary cycle power plants Literature information available in relation to the results of the life cycle environmental impacts of binary cycle geothermal power plants (whose technical characteristics are given in Table 6), has been summarized in this section [12,64-65]. The goal and scope of these studies are established on a similar basis comparing conventional and renewable energy options (See Table 3).

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On one hand, Rule et al. (2009) compared geothermal electricity technologies among other renewable electricity technologies [64], whereas on the other hand Gamboa et al. (2015) and Sullivan et al. (2010) included conventional power generation systems within the environmental evaluation [65,12]. Regarding to the system boundaries, under different assumptions the construction, operation and decommissioning stages are considered for each study. As a general basis, fuel and electricity production as well as the construction of the production and reinjection wells are considered as part of the construction stage. In particular, Rule et al. (2009) defined as cut-off criteria the step-up to 11kV, where the point of link to the national grid is found [64], whereas Sullivan et al. (2010) considered both the plant construction and operation without taking into account equipment production and fuel and power required for the construction site [12]. In relation to the operation stage, the difference lies in the contribution from maintenance and the working fluid losses. The former is included by Rule et al. (2009), assuming component replacement and the construction of new production well as part of the maintenance activities [64], while the latter is considered by Gamboa et al. (2015) in order to understand the effect of considered the working fluid losses, which in that study is assumed as HCFC-124 [65]. As to decommissioning, it is considered by Rule et al. (2009), assuming that 100% of materials that cannot be recycled such as plastic, concrete and fiberglass are sent to landfill, conversely recyclable materials such as steel and aluminum are assumed to be 50% recycled and 50% landfilled [64]. Additionally, Gamboa et al. (2015) considered the effects of drilling and sludge that are produced as a consequence of the drilling fluid treatment [65]. Based on this information, the system boundaries are drawn for a geothermal binary cycle power plant as shown in Figure 8. In relation to environmental impacts analyzed for each study, all of them considered both energy requirements and global warming as key parameters for the LCA. Moreover, Gamboa et al. (2015) included other five environmental impacts as described below [65]. The environmental impacts are estimated by using different assessment methods, functional units, and LCA software, which have been described for each case in Table 3. A description of each environmental impact is briefly described in the following section.

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5.4.1 Global warming potential Global warming potential (GWP) values considered across the studies are shown in Fig. 3. As seen in this figure, the variation in the GWP values estimated by the authors is small with a statistical estimated median of 5.7 g CO2eq/fu. The contribution of the life cycle stages is schematically represented in Fig. 9. Due the assumptions about the plant´s lifespan of each study, the main cause of this impact varied between maintenance (80%) and construction (20%) phases for Rule et al. (2009) [64] with 100 years, whereas for Gamboa et al. (2015) and Sullivan et al. (2010) who assumed a plant lifespan of 30 years [12,65], the construction phase is mainly responsible for the estimated GWP value (see Fig. 9). Rule et al. (2009) quantified the maintenance contribution as 80% of the GWP value (5.6 g CO2eq/fu) [64]. Within this percentage about 50% comes from maintenance activities due to the manufacture of the piping system (water and steam transporting pipes, and the well production liners), whereas another 25% comes from the fossil fuel consumption for the construction of new production wells in the lifespan period of 100 years. As seen in Fig. 9, the remaining 20% of the total CO2 equivalent emissions comes due to the initial construction phase (i.e., piping system and the well construction). As for the GWP estimated (5.7 g CO2eq/fu) by Sullivan et al. (2010) [12], it is 100% attributed to the construction stage, which is defined as the plant life cycle. This is explained due to the diesel consumption occurring during the well drilling and completion. These findings are paralleled by Gamboa et al (2015), who also identifies the construction stage as the main hot spot, contributing to 67% of the GWP estimated value (5.79 g CO2eq/fu) [65]. This is due to the diesel requirements, specifically due to the diesel production that accounts for 40% of the given percentage. 5.4.2 Energy ratios Different indicators are used to analyze energy efficiency over the plant’s life cycle. Sullivan et al. (2010) defined an energy ratio parameter, which illustrates the relationship between energy required for the production and assembly of the construction materials (E pc) and energy output (Eout) over the 30 years defined as lifespan, finding that 2% of the energy output is devoted to energy requirements [12].

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In the same context, Rule et al. (2009) identified embodied energy as an indicator of the energy consumed over the life cycle of the plant, quantifying a value of 94.6 kJ/kWh (or 0.026 kWh/kWh) [64]. About 95% of this value comes from maintenance and construction activities. Maintenance activities are assumed for the manufacture of the pipes and production wells, which contribute to 60% and 15% respectively of the given percentage. According to Rule et al. (2009), this is because over a plant lifespan of 100 years, the production wells require to be replaced at least 6 times (assuming a life time of 17 years), which implies that maintenance requirements represent the main contribution to embodied energy estimated [64]. The remaining 20% of the contribution comes from construction activities, which consume energy for manufacturing of the well piping system (15%) and the construction of the wells (5%) [64]. On the other hand, Gamboa et al. (2015) utilizes CED as an indicator of the energy consumed within the production and injection wells, which is considered as part of the plant life cycle, quantifying a total amount of 50 MJ/MWh [65]. From this amount, 65% and 35% come from the production and injection wells, respectively. For the production site percentage, 45% are coming from the production and consumption of diesel and 10 % from the steel manufacture, whereas for the injection percentage, 20% and 5 % are estimated for the same processes [65]. 5.4.3 Other environmental impacts In addition to life cycle CO2 equivalent emissions and total primary energy, Gamboa et al. (2015) analyzed other environmental impacts, including abiotic depletion potential (ADP), ozone layer depletion potential (ODP), photochemical oxidation formation (POF), acidification potential (AP) and eutrophication potential (EP) [65]. The corresponding values for these impact categories are included in Table 7. According to the plant life cycle defined by these authors, ADP, POP, AP and EP results are attributed to the construction of the production site (amounting to 65%-67%), whereas the remaining contribution is due to the construction of the injection site, which on average has 32% of the estimated values for the categories already mentioned. With regards to ODP, the total value reported in Table 7 comes from the plant operation, mainly due to the working fluid loss to air (75%) and its manufacture process (25%).

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Another important impact is related to the sludge treatment of the drilling muds. During the operation stage, drilling mud is normally recovered from the well annulus circulation, separated from the drilling cuttings, and stabilized before it is sent to a landfill. Gamboa et al. (2015) found, for the first time that due to these processes there is an eutrophication potential, which is quantified as 0.304 kg PO43-/MWh [65].

5.5 Enhanced geothermal systems (EGS) Although electricity production from enhanced geothermal systems (EGS) is still found in an introductory phase to the market because of its initial stage in the learning curve [38], it has been analyzed in the present work due to its promissory potential for future’s energy supply, and its capacity to produce electricity from low-to-medium temperature reservoirs throughout the world [71]. As in any process, electricity generation through EGS systems leads to environmental impacts, which should be analyzed in parallel to the growing implementation of this technology. However, information in this regard is scarce mainly due to the lack of both life cycle inventories and actual production projects, because most of the information reported is coming from pilot exploitation projects [12]. Available information is mostly concentrated in Europe and the U.S. A description of the systems analyzed in these regions is given in Table 8. In the U.S., research based on the environmental assessment of EGS projects has been conducted by Argonne National Laboratory [12], whereas in Europe, similar projects has been conducted in Germany [7,38,72], Switzerland [73,74] and France [75]. According to the goal and scope of these LCA studies, they have been grouped in three categories. In the first category, comparative studies of the environmental impacts produced by EGS and those generated by conventional energy systems for the production of electricity are grouped [12, 72,74]. For the second category, a comprehensive evaluation of environmental impacts of EGS by considering a sensitivity analysis and different configurations of binary power plants is grouped [38,75]. This comprehensive sensitivity analysis applied to some pilot projects of EGS has enabled to identify relationships among the main power generation parameters: reservoir, geothermal fluid and binary cycle.

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The work conducted by Huenges (2010) has been included in a third category, where EGS systems have been analyzed in order to identify the relationships between the design of the plant and its environmental impacts [7]. It is worth mentioning a parameterized model developed by Lacirignola et al. (2014) for the estimation of life-cycle greenhouse gases emissions of EGS [76]. This model is based on the identification of key operation parameters related to GHG emissions produced from EGS systems, to establish a comparison with GHG literature values. Therefore, since it is based on a modified LCA approach rather than on the LCA methodology, results from the study conducted by these authors have not been included for this review. With regards to the system boundaries, they are consistent across the studies encompassing the construction, operation, maintenance and decommissioning stages. Recycling process was only considered by Pehnt (2006), who assumed that part of the primary materials might be replaced by recycled materials [72]. Based on available information from these studies, a representation for the system boundaries of EGS is shown in Fig. 10. Since the results of the LCA include different impact categories as well as different impact assessment methods, a direct comparison of the results was not carried out. In addition to the GWP impact, LCA studies considered the evaluation of up to 17 impact categories. One of these categories is the induced seismicity impact that should be carefully analyzed during the planning stage of an EGS project. A description per impact category is given below, while the functional unit, impact assessment method and goal and scope considered across the studies is shown in Table 9. It is important to note that both Frick et al. (2010) and Lacirignola and Blanc (2013) have conducted an extensive sensitivity analysis, however the results presented in this section correspond to the base case scenario to ensure a fair comparison between the rest of the LCA studies analyzed in this section [38, 75]. 5.5.1 Global warming potential and climate change In spite of the differences between the approach considered in the life cycle impact assessment methodologies (midpoint and endpoint), global warming (which refers to the increase of the Earth´s temperature due to the effect of greenhouse emissions) and climate change (which refers to the side effects in climate due to the emissions of greenhouse gases), these impact categories are grouped just to illustrate their corresponding values in Fig. 11. 24

The analysis of these results shows that the main contribution to these environmental impacts comes during the construction stage, specifically due to the drilling activities that are required to dig wells. During the drilling process combustion of diesel takes place, which is why this activity is mainly responsible for the effect on global warming. The contribution from drilling clearly dominates the estimated global warming impacts, ranging on one hand from 60% to 95% of the corresponding value estimated by Frick et al. (2010) and Hirschberg and Wiemer (2015) [38, 74], and on the other hand from 76% to 89% of the values estimated by Huenges (2010) [7]. These values are shown in Table 10. As to the value estimated by Bauer et al. (2008) for climate change, it amounts to 27 g CO2 eq/fu, without information about the hot spots [73]. From the analysis of these impacts, it can be seen that in order to reduce the impact on global warming from EGS, low-emitting energy sources should be considered as an option to reduce the effect created by diesel consumption. In this regard, Lacirignola and Blanc (2013) proposed the change in the environmental impacts already described by means of the replacement of diesel as power supply by the French electricity grid [72]. The analysis of this change leads to a reduction in the environmental impacts shown in Table 10, since the contribution from the drilling phase is reduced. However, the authors pointed out that a further analysis is required for evaluating the actual impact of different electricity mixes as a viable substitute of diesel to meet energy requirements. 5.5.2 Resource consumption As shown in Table 10, different indicators are used across the LCA studies to evaluate the consumption of either energy or resource consumption. The obtained results range from 0.54 MJ/kWh to 0.87 MJ/kWh based on the estimations reported by Frick et al. (2010), Pehnt (2006) and Huenges (2010) [39,69,7]. In addition to these results, Hirschberg et al. (2015) reported a value of 8.35 g Feeq/kWh for metal depletion and 8.15 g oil eq/kWh for fossil depletion [71]. The well drilling process is the main cause of these results due to the energy consumed during these activities, amounting on average to 80% of the given values. Based on the results reported by Frick et al. (2010), casing and cementation are the second and third sources for the primary role of the well drilling activities within energy/resource consumption [39].

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5.5.3 Acidification and eutrophication potential The results obtained for the acidification and eutrophication potential impacts are also included in Table 10. Based on the information from Frick et al. (2010) [38] and Pehnt (2006) [72], the acidification and eutrophication potential vary in the intervals from 190 mg SO2 eq/kWh to 400 mg SO2 eq/kWh, and from 25 mg PO43- eq/kWh to 55 mg PO43- eq/kWh, respectively. Additionally, based on the results from Huenges (2010) [7], both AP and EP range from 453 to 471 mg SO2 eq/kWh, and from 62 to 64.1 mg PO43- eq/kWh. These impacts are caused by the emissions produced during the construction of the wells, specifically during the drilling process. According to Frick et al. (2010) [38] and Hirschberg et al. (2015) [74], the contribution of this process varies between 70% and 80% of the estimated values, whereas according to Huenges (2010) this contribution ranges from 88% to 94% of the estimated values [7].

5.5.4 Toxicity, land use and seismicity impacts In addition to the environmental impacts above mentioned, both Hirschberg et al. (2015) and Lacirignola Blanc (2013) report other indicators to evaluate the toxicity along the geothermal plant life cycle on the human health, ecosystem, freshwater and land use

[71,72]. These results are also

summarized in Table 10. According to these authors, the energy consumed during the well drilling phase is responsible for about 80-90% of the reported values. Lacirignola and Blanc (2013) pointed out that emission of particulates and NO X clearly dominate the impact on human health, while NO x, aluminum and zinc emissions are mainly responsible for the impact on ecosystem quality [72]. Land use is another impact that is rarely accounted for within the environmental assessment of geothermal power generation, which according to Bayer et al. (2013) is hard to be evaluated because it varies in relation to location, type of geothermal plant, timing, and life cycle stage [6]. Therefore, information about this impact is scarce, and results concerning this impact are only reported by Hirschberg et al. (2015) [71]. These authors consider three indicators to evaluate the impact on land use: agricultural land occupation, urban land occupation, and nature land transformation (see Table 10). These impacts are mainly attributed to diesel consumption during drilling activities, which are obviously due to construction lifecycle stage.

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In addition to land use, induced seismicity is another impact that is barely analyzed during the life cycle environmental assessment of EGS systems. In this context Hirschberg et al. (2015) [74], reported the impact on induced seismicity, identifying that plant operation conditions, specifically reinjection flow rate, play a key role in this impact. In this regard, Lacirignola and Blanc (2013) identified a direct relationship between reinjection flow rate and the potential risk of induced seismicity, pointing out that an appropriate balance between the installed capacity and the risk of seismicity should be defined at the early design stage [75]. This information is strictly required, since otherwise high induced seismicity could lead to the cancelation of the project, as occurred in Basel, Switzerland and Landau, Germany [75].

6

Conclusions

Geothermal energy currently represents a potential option to meet energy needs of the population on a sustainable basis, forecasting an installed capacity for 2020 of ~21 GWe. To achieve such a power generation challenge, stakeholders involved in the implementation of new sustainable geothermal power projects would require comprehensive information about the environmental, economic, and social impacts. Consequently, the development of sustainability indicators that allow such an evaluation should grow in parallel with the future increase in world geothermal electricity production. Regarding to environmental indicators estimated as life cycle environmental impacts, information is still scarce as shown in the present study. Available information was compiled and classified, for the first time, based on the type of energy conversion technology used for geothermal power generation (i.e., dry-steam, single- and double-flash, binary cycle, and EGS). The results of this review show that most of the LCA studies are focused on the evaluation of global warming impact specific to the type of technology under analysis, as well as in a comparison to other conventional energy systems. Regardless of the type of technology, the analyses show that diesel consumption, which is required for the construction stages (well drilling and completion: drilling fluid and cement pumping, and casing due to steel production; and well and fluid transport piping), is the main factor responsible for related impact on global warming. Furthermore, diesel requirements are also responsible for the energy consumption along the life cycle of the geothermal power plants.

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In addition to global warming, information about eutrophication, acidification, resource consumption and land use is presented, which includes from 1 to 18 life cycle indicators, identifying the LCA hot spots for each impact category, subject to data availability. Furthermore, it is possible to conclude that the life cycle environmental impacts vary in relation to two factors: local geological characteristics and methodological choices, such as the inclusion of fugitive emissions within the system boundaries. The former play a crucial role in the performance and maintenance of the geothermal power plants, thus affecting both material and energy requirements. The latter are fundamental to include the goal and scope, the functional units, the system boundaries, and the life span. In relation to the fugitive emissions (specifically from NCG) as part of the system boundaries, there is still a debate about considering these emissions as part of the system, since they would naturally occur even without the installation of the geothermal power plant. Hence, qualitative and quantitative measurements are required in the exploration stages for defining an accurate and reliable baseline of the natural background (or fugitive) emissions for the future geothermal power generation projects. This important geochemical task will help to promote and increase the use of these geoenergy resources. As shown in this review, special emphasis is given to the role that geothermal power generation has in reducing the GHG emissions, as well as to the energy demand along the life cycle of the considered conversion technologies. However, in spite of the attempt to include other environmental impacts (namely acidification, eutrophication or human health and land use) further research is still required to identify the impact from geothermal power production in other environmental impacts such as photochemical ozone creation and eutrophication due to the corresponding sludge management. Depending on the geographical region, the information of life cycle environmental impacts is likely to be required due to the increasing use of regulatory frameworks to ensure a sustainable management of geothermal power production. For example, in the case of Mexico, with the new Law on Energy Transition, there are plans to develop new methodologies to evaluate externalities, where the LCA technique will constitute a suitable technique not only for estimating the externalities, but also to promote the construction of a power generation infrastructure that produce both a systemic benefit and to facilitate the interconnection of clean renewable energy sources into the national system of electricity generation. 28

In spite of the effort performed in the analyzed studies, and in addition to this review, future research is required for identifying the relationship between different power plant design parameters and the life cycle environmental impacts. On one hand, a lack of consistency about the technical information given through the studies was identified in this review. Therefore, an effort should be made to promote the standardization of the technical information given for the future related LCA studies. On the other hand, LCA studies require to be carried out in other geographical regions, especially in developing countries, where exist enormous geothermal power potential such as Mexico, Turkey, Philippines, Indonesia or Kenya because, up to our knowledge, they do not have sufficient information about their life cycle environmental impacts. This can be explained due to the lack of information about life cycle inventories in these regions. Further research is still required to identify the relationship between different power plant parameters and the life cycle environmental impacts. Based on local legislation, the culture to link LCA studies with future commercial geothermal power generation projects should be urgently developed to contribute to the environmental sustainable use of the geothermal potential.

Acknowledgments We want to thank the anonymous reviewers for their careful reading of the earlier version of the paper, and their insightful and helpful comments and suggestions, which greatly improved the final version of the manuscript. The first author wants to thank to DGAPA-PAPIIT Postdoctoral Research Programme (UNAM) for the financial support. Finally, the corresponding author also acknowledges the Consejo Nacional de Ciencia y Tecnología (CONACyT) and the CIICAp-UAEM for the financial and infrastructure support in a sabbatical leave program carried out on 2016.

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35

Figure captions

Fig. 1

Simplified schematic diagrams showing the typical technologies used for geothermal power generation [A: Dry steam]; [B: Single and double flash systems]; [C: Triple-flash], and [D: Binary cycle] based on information collected from the references [29,30].

Fig. 2

System boundaries identified for the LCA studies of dry steam geothermal power plants, based on references [56,57].

Fig. 3

General GWP results taken from LCA studies applied for evaluating all the geothermal power production technologies (See the text for the original literature sources). @To read the original values of GWP, the y-axis value should be multiplied by the factor listed inside the plot. *For the functional units of each case, see their description in Table III.

Fig. 4

Results of AP and HTP identified for a dry steam geothermal power plant (based on Ref. [57]).

Fig. 5

System boundaries identified for the LCA studies of a single-flash geothermal power plant (based on Ref. [57]).

Fig. 6

Results of AP and HTP found for a single-flash power plant (based on Ref. [57]).

Fig. 7

System boundaries identified for double-flash geothermal power plants (based on Refs [5963,12]).

Fig. 8

Representation of the system boundaries of binary cycle geothermal power plants (based on Refs. [12, 64-65]).

Fig. 9

Contribution to the GWP identified for geothermal binary cycle power plants (based on Refs. [12, 64-65]).

Fig. 10 System boundaries for LCA studies of an EGS system (based on Refs [7,38,72-75]). Fig. 11 Results of GWP identified from the evaluation of pilot EGS projects (based on Refs [7,38,72-75]). *The functional unit for each case is given in Table 9.

36

Fig. 1

37

Fig. 2

38

Fig. 3

39

Fig. 4

40

Fig. 5

41

Fig. 6

42

Fig. 7

43

Fig. 8

44

Fig. 9

45

Fig. 10

46

Fig. 11

47

Venezuela

Uruguay

Suriname

Peru

Paraguay

Panama

Nicaragua

Mexico

Honduras

Guyana

Guatemala

El Salvador

Ecuador

Costa Rica

Colombia

Chile

Brasil

Bolivia

Belize

Argentina

Table 1. Renewable energy policies implemented on a national level in Latin America

Theme 1

X

X

2

X

X

X

X

X

X

X

X

3

X

4

X

5

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X X

X

X X

X

X

7

X

X

X

X

X

X

X

X

X

X

6

8

X

X

X X

X

X

X

X

X

X

X

X

X

X X

X

X

1: renewable energy target; 2: renewable energy law/strategy; 3: solar heating law; 4: solar power law; 5: wind power law; 6: geothermal law; 7: biomass law; and 8: biofuels law.

Table 2 Characteristics of the dry steam power plants considered across the LCA studies

Geothermal Power Plant

Bottle Rock Geysers NC1 NC2 PC3

Running capacity (MW)

Well depth (km)

Reservoir temperature (°C)

Life span (y)

Reference

11 725 52 56 20a

NA

NA

30

[56]

2-4

300-350

25

[57]

PC4

20

PC5

20 a

NA: not available; Information collected from Razzano and Cei (2015) [58].

48

X

Table 3 LCA studies for geothermal power plants (dry steam, flash, and binary cycle energy conversion technology) Technology Dry steam and Flash power plants

Dry steam and Single flash

Impact categories/indicatorsa 1,13

1,2,3

Double flash

1

Double flash

1,2,3,4,5,6,7,8,10,11

Double flash

1,13,

Double flash

1,2,4, 15-22j

Double flash

1-8,23-25

Binary cycle

Impact assessment method/approach GHG emissions aggregated based on GWP values b

LCA software GREET

CML, 2002d

Simapro 7.3

LCEe

NA

Functional unit c Power plant´s electricity output along the life span in kWh Power produced at plant in MWh

kWh net power

CML2, 2000f CEDg

Simapro 7

kWh power produced

GHG aggregated based on the GWP potential from IPCC 2007h

GREET v 2.7

kWh power produced

NA

kWh power produced

NA

CML 2001i

GaBi v6

Generation of 1 kWh power

1,13,

GHG aggregated based on the GWP potential from IPCC 2007h

GREET v 2.7

kWh produced

Binary cycle

13,14

CO2 emissions aggregated

Simapro 7

kWh

Binary cycle

1,2,4,5,6,7,11

CML 2001i

Simapro 7

MWh net power at plant

a

Goal and scope

Country

Reference

Comparative evaluation of geothermal power plants, from cradle to grave. Environmental assessment of electricity production from geothermal power plants, from cradle to grave. Comparative evaluation of nine power generation technologies, from cradle to grave. Environmental assessment of geothermal power production, from cradle to grave. Comparative evaluation of geothermal and other electricity generation technologies, from cradle to grave Estimation of the environmental impacts of geothermal power, from cradle to grave

U.S.

[56]

Italy

[57]

Japan

[59]

Comparative evaluation of geothermal and other electricity generation technologies, from cradle to grave Comparative evaluation of geothermal and other electricity generation technologies, from cradle to grave Comparative evaluation of four renewable energy technologies, from cradle to grave Environmental assessment of geothermal power production, from cradle to grave

Iceland

[60,61]

U.S.

[12]

France

[62]

Turkey

[63]

U.S.

[12]

New Zealand

[64]

Spain

[65]

1: global warming potential; 2: acidification potential; 3: human toxicity potential; 4: abiotic depletion potential; 5:eutrophication potential; 6: ozone layer depletion potential; 7: freshwater aquatic eco toxicity potential; 8:marine aquatic eco toxicity potential; 9: terrestrial eco toxicity potential; 10:

49

photochemical oxidation potential; 11: cumulative energy demand (CED); 13: energy ratio (dimensionless); 14: CO2 emissions; 15: freshwater eutrophication; 16: marine eutrophication; 17: terrestrial eutrophication; 18: agricultural and urban occupation; 19: human toxicity cancer; 20: human toxicity no cancer; 21:CED renewable; 22: CED no renewable; 23: ozone layer depletion potential; 24: photochemical oxidant creation potential; 25: terrestrial eco toxicity potential b Global warming potential equivalency factors for CH4 and N2O are taken from Ref [56]; c Based on the definition of net power, gross power and life span each functional unit has been differently defined in each study. d Based on Ref [66]; e Life cycle GHG emission factor is an index developed by Hondo, 2005 in order to estimate an aggregated value of the CO 2 and CH4 emissions based on their GWP; f Based on Ref [67]; g Based on Ref [68];h Based on Ref [69]; i Based on Ref [70]; j Lack of information about the reference corresponding to each impact assessment method.

Table 4 General technical characteristics of a single-flash geothermal power plant Running capacity (MW)

Well depth (km)

Number of injection wells

Number of production wells

Reservoir temperature (°C)

Life span (y)

Reference

20

2-4

4

7

300-350

25

[57]

Table 5 General Technical characteristics of double-flash geothermal power plantsa

Running capacity (MW) NA

Maximum gross output* (MW) 55

213 213 15.75

NA 300 NA

Number of injection wells

Number of exploration Wells

Number of production Wells

Life span (y)

Reference

7 wells -1 kmNA NA 0

5 wells -1.5 kmNA NA 3

14 wells -1 kmNA NA 4

30

[59]

30 30 30

[60] [61] [62]

NA: information not available; a Information related to the Turkey double-flash geothermal power plant [63] was not available; The maximum gross output of the plant is also known as installed capacity, whereas the running capacity is the actual gross being produced.

50

Table 6 General technical characteristics of binary cycle geothermal power plants Running Capacity (MW) 151

Maximum Gross Output (MW)

NA

2.9

NA

10

NA

Well depth (km) 0.660a

Life span (y) 100

Reference

pw1b: 0.45 pw2:0.73 iwc:0.60

25

[65]

0.67<2

30

[12]

a: include : production, reinjection, monitoring and exploration well NA: Not available b: pw; production wells c: iw; injection wells

51

[64]

Table 7 Environmental impacts reported for a binary cycle geothermal power plant, based on the results [65]

Impact category

Value

ADPa

2.33 x 10-2 kg Sb eq/MWh net electricity at plant

ODPb

9.39 x 10-5 kg CFC 11 eq/ MWh net electricity at plant

POPc

6.72 x 10-4 kg C2H4 eq/ MWh net electricity at plant

APd

1.20 x 10-2 kg SO2 eq/ MWh net electricity at plant

EPe

4.17x 10-1 kg PO43- eq/ MWh net electricity at plant

a: abiotic depletion potential; b: ozone depletion potential; c: photochemical oxidant potential; d: acidification potential and e: eutrophication potential.

depletion

Table 8 General technical characteristics of the EGS systems considered within the analysis of the environmental evaluation a

Capacity (MW)

Number and kind of wells

1.6 20

2 production 2 production 1 reinjection 1 or 2 (production 1 injection) 1 production 1 injection 6 (2 production 4 reinjection) NA 1 production 2 reinjection

50

1.75 5.5

3 2.28

Well depth (km) 4 4-6

Reservoir temperature (°C) 125-165 150-225

Well replacement

Life span (y) 20 30

Reference

NA 1

Full load hours 7500 NA

4-6

150-225

1

NA

30

[12]

3-8

125

NA

7000

30

[38]

5

190

NA

NA

NA

[71]

5.5 4

NA 165

NA NA

7000 NA

30 25

[73] [75]

a

[7] [12]

Stated values are referred to the base case scenario of the LCA studies, which has been selected as a baseline from each reference. a Information related to the LCA study performed by Pehnt et al (2006) [72] was not available. To optimize space in this review, the reader is referred to the original literature sources for obtaining more details about other different scenarios used by the authors for evaluating the EGS power plants (specifically with the references [38,73]), including the set of assumptions, and the range of the geological and technical parameters.

52

Table 9 LCA studies for pilot EGS geothermal power plants

Impact assessment method/approach Aggregate life cycle inventory flows in 4 environmental indicators: CED,GWP, AP and EP

Functional unit

Software

Goal and scope

Country

Reference

kWh net power

NA

Identification of correlations between EGS

Germany

[7]

Aggregate life cycle inventory flows in 2 environmental indicators: GWPb and energy ratio.

kWh produced

GREET v2.7

U.S.

[12]

Aggregate life cycle inventory flows in 4 environmental indicators: GWP, AP, EP and CEDa

kWh net power

NA

Comparative assessment of renewable and conventional energy systems, from cradle to grave Environmental assessment of EGS systems, from cradle to grave

Germany

[38]

Aggregate life cycle inventory flows in 4 environmental indicators: CED,GWP, AP and EPc

kWh electricity at plant

Umberto

Comparative assessment of conventional and renewable energy systems, from cradle to grave

Germany

[72]

Eco-indicator’ 99d

kWh

NA

Comparative assessment of conventional, renewable energy systems, from cradle to grave Environmental assessment of EGS systems, from cradle to grave

Switzerland

[73]

Switzerland

[74]

ReCiPee

kWh net power

SimaPro 7.3.3

a

Information for the corresponding characterization factors is sourced from Ref [77]; Information for GWP characterization factor is sourced from Ref [69] c Information for the corresponding characterization factors is sourced from Ref [72] d Ref [78] e Ref [79] f Ref [80] b

53

Table 10 Life cycle environmental impacts quantified for EGS systems Impact category Cumulative energy demand Global warming potential Acidification potential Eutrophication potential Global warming potential Acidification potential Eutrophication potential Cumulative energy demand Global warming potential Global warming potential Acidification potential Eutrophication potential Cumulative energy demand Greenhouse gas emissions Climate change Ozone depletion potential Terrestrial acidification Freshwater eutrophication Marine eutrophication Human toxicity Photochemical oxidant formation Particulate matter formation Terrestrial ecotoxicity Freshwater ecotoxicity Marine ecotoxicity Ionizing radiation Agricultural land occupation Urban land occupation Natural land transformation Water depletion Metal depletion Fossil depletion

Value Ref. [7]

Range

NA NA NA NA

831-872 KJ/kWh 54.9-57.5 g/kWh 453-471 mg/kWh 62-64.1 mg/kWh

Ref. [12] 23 g CO2 eq/kWh Ref. [38] 420 mg SO2 eq/kWh 58 mg PO43-eq/kWh 0.8 MJ/kWh 50 g CO2 eq/kWh Ref. [72] 41g CO2 eq/kWh 190 mg SO2 eq/kWh 24.8 mg PO43-eq/kWh 0.54 MJ/kWh Ref. [73] 27 g CO2 eq/kWh Ref. [74] 28.8 g CO2 eq/kWh 0.00257 mg CFC-11 eq/kWh 0.104 g SO2 eq/kWh 0.0176 g P eq/kWh 6.27mg N eq/kWh 17.4 g 1,4 DB eq/kWh 0.0866 g NMVOC/kWh 0.0591 g PM10 eq/kWh 3.47mg 1,4 DB eq/kWh 0.3360 g 1,4 DB eq/kWh 0.3420 g 1,4 DB eq/kWh 64 g U235 eq/kWh 8.18E-04 m2a/kWh 4.17E-04 m2a/kWh 4.71E-06 m2a/kWh 7.97E-04 dm3/kWh 8.35 g Fe eq/kWh 8.15 g oil eq/kWh Ref. [75] 36.7 g CO2eq/kWh 6.78E-08 DALYa/kWh 1.17E-02 PDFb-m2.-t 579 kJ primary/kWh

18.3-31.9 g CO2 eq/kWh 350-520 mg SO2 eq/kWh 48-70 mg PO43-eq/kWh 0.6-0.9 MJ/kWh 42-62 g CO2 eq/kWh NAa NA NA NA NA 7.55-45.6 g CO2 eq/kWh 0.0007-0.00407 mg CFC-11 eq/kWh 0.0279-0.1650 g SO2 eq/ kWh 0.0047-0.0276 g P eq/kWh 1.67-9.95 mg N eq/kWh 4.6-27.4 g 1,4 DB eq/kWh 0.0224-0.1380 g NMVOC/kWh 0.0155-0.0944 g PM10 eq/kWh 0.93-5.54 m g 1,4 DB eq/kWh 0.0905-0.5380 g 1,4 DB eq/kWh 0.0920-0.5470 g 1,4 DB eq/kWh 18.2-101 g U235 eq/kWh 2.44E-04-1.37E-03 m2a/kWh 1.29E-04-8.86E-04 m2a/kWh 1.23E-06-7.52E-06 m2a/kWh 2.20E-04-1.31E-03 dm3/kWh 2.36-14.5 g Fe eq/kWh 2.13-12.9 g oil eq/kWh

Climate change Human health Ecosystem quality Resources NA: Not applicable a: DALY: disability adjusted life year b: PDF: potentially disappeared fraction (of species)

NA NA NA NA

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Abbreviations AD: Abiotic depletion potential AMIS: Abetment of mercury and hydrogen sulfide technology AP: Acidification potential CED: Cumulative energy demand EGS: Engineered geothermal systems EIA: Environmental impact assessment ENEL: Ente Nazionale per l’Energía eLettrica EP: Eutrophication potential FAETP: Freshwater aquatic eco-toxicity potential GHG: Greenhouse gas GWP: Global warming potential HCFC-124: Hydrochlorofluorocarbon 124 HTP: Human toxicity potential LCA: Life cycle assessment LCI: Life cycle inventory MAETP: Marine aquatic ecotoxicity potential NCG: Non condensable gases NOx: Nitrogen oxides ODP: Ozone layer depletion potential ORC: Organic rankin cycle POP: Photochemical oxidation potential RE: Renewable energy resources RES: Renewable energy sources TETP: Terrestrial eco-toxicity potential

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