Bibliometric Analysis and Review of Low and Medium Enthalpy Geothermal Energy: Environmental, Economic, and Strategic Insights

The escalating global demand for electrical power, primarily driven by industrialization and urbanization, has underscored the burgeoning significance of renewable energy within the global energy sector [1]. Traditional energy resources, such as coal, oil, and natural gas, necessitate sufficient capacity to sustain a balance between their availability and energy demand, as emphasized by Rosales-Calderon and Arantes [2]. Moreover, existing energy consumption patterns could potentially exhaust these nonrenewable resources in the ensuing decades [3]. Compelling interest in alterations to our energy paradigm also stems from issues relating to climate change, global warming, and energy security [4], [5]. Consequently, the global community is increasingly tilting toward renewable energy, viewed as a reliable energy supply with diminished environmental impact. Evaluating natural resources, such as biodiversity [6], [7], along with mining and energy resources [8], [9], has emerged as a primary strategy in various countries to ensure their future availability.

Projections by the International Energy Agency (IEA) suggest that the contribution of renewable energy to the total global energy capacity will experience a 50% upsurge between 2019 and 2024 [10]. Renewable energies, including wind, solar, biomass, and geothermal energy, underpin this growth. However, the global energy system has thus far only marginally adopted geothermal energy [11]. This form of energy originates from subterranean heat generated by rock temperatures, which increase with depth [12], [13]. The relationship between temperature and depth, defined by the geothermal gradient, dictates the feasibility and utility of this resource-a higher geothermal gradient implies greater potential for exploitation [14]. Depending on these conditions, geothermal resources can be harnessed for diverse applications such as electricity generation, heating, cooling, drying, dehydration, food processing, and balneology [15].

From 1995 to 2019, the number of countries utilizing geothermal energy escalated from 28 to 88, with a 52% increase in geothermal usage reported between 2015 and 2019 [16]. This growing interest in geothermal energy can be attributed to its non-reliance on weather conditions, granting it a distinct advantage over other renewable energy sources. Despite geographical constraints, geothermal energy can deliver consistent energy with a higher heat capacity than other renewable sources. Its efficiency hinges on enthalpy criteria and atmospheric conditions [17]. Geothermal energy resources are categorized into convective or hydrothermal systems, conductive systems, and deep aquifers, providing high, medium, and low enthalpy resources based on temperature. Resources with high enthalpy surpass 150℃, while medium-enthalpy resources range between 100℃ and 150℃, and low-enthalpy resources are below 100℃ [18], [19].

High-enthalpy geothermal energy, which constitutes 60% of the global installed capacity, has yielded remarkable results from economic, environmental, and energy efficiency perspectives [20], [21]. Although medium-enthalpy resources have been underutilized due to the technological limitations of the 20th century, contemporary technologies such as the Organic Rankine Cycle (ORC), Kalina Cycle (KC), and direct applications, facilitate the exploitation of these resources through hybrid renewable energy systems, providing efficient and cost-effective energy [22]. In recent years, low-enthalpy processes, primarily used in cooling due to climate change impacts, have seen significant development [23].

Low and medium-enthalpy geothermal energy have diverse applications, including heating-cooling, electricity, and water supply. As a response to socioeconomic growth and global climate variation, there is a rising need for refrigeration in the industrial, commercial, and residential sectors, particularly in Africa, Asia and Europe [24]. Clean cooling/heating resources are internationally interested in transforming renewable energy within the industrial sector, as the Renewable Energy Policy Network for the 21st Century’s (REN21) Renewables 2018 Global Status Report indicates. According to a study by Gong et al. [25], “the geothermal industry is experiencing positive outside attention to its development activities”

The cooling mechanism involves extracting geothermal heat from the subsoil to the cold refrigerant to evaporate it and convert it into low-pressure steam. The compressor receives this steam to increase the pressure and temperature and finally transports it to the condenser to convert the hot vapour into a liquid state of high pressure and temperature [26].

The vital role of geothermal energy necessitates a comprehensive literature review of the contributions of the scientific community and its applications in the public and industrial sectors [15], [27]. Bibliometric and literature reviews provide a profound and detailed insight into this field of study, utilizing systematic tools or techniques to extract bibliographic information from databases such as Scopus and Web of Science (WoS) [28], [29].

Numerous literature reviews on low and medium enthalpy geothermal energy have been conducted globally, discussing topics like geothermal resource storage [30], [31], low-enthalpy geothermal exploitation mine-oriented enthalpy [32], the current understanding of the use of low-enthalpy geothermal energy cells for enhancing the energy efficiency of buildings [33], and the legal framework of superficial geothermal energy [34]. However, a comprehensive assessment of the economic and environmental aspects considered in this field remains elusive.

The relationship between the importance of geothermal energy and the literature review allows the generation of research questions such as: What research areas exist in the scientific production of low and medium enthalpy geothermal energy? What scientific contributions exist within geothermal energy’s environmental and economic aspects with low and medium enthalpy?

This work aims to analyze the importance of low and medium enthalpy geothermal energy, through bibliometric analysis and review of existing literature published in Scopus and WoS, for a strategic vision of strengthening and importance of renewable systems and an analysis of its application's environmental and economic aspects.

The methodology consisted of three stages: i) configuration and systematic processing of the information obtained from the Scopus and WoS databases; ii) generation of bibliometric analysis of production, contribution by country, and scientific distribution; iii) a literature review of the field analysed considering the environmental, technical, and economic aspects; and iv) analysis of Strengths, Weaknesses, Opportunities and Threats (SWOT) for the proposal of promotion strategies and strengthening of the field (Figure 1).

The study considered the Scopus and Web of Science (WoS) databases to obtain publication records. These databases were selected because of the many high-quality standard journals that index publications from various scientific disciplines, which is crucial for longitudinal and review analyses [35], [36]. This information contemplates studies conducted by academics and industrial entities focusing on energy development through Boolean term operators.

In the search, it was defined that the algorithm reflects information based on the titles, abstracts and author keywords. The search equation used the following topics and conditions: (“geothermal” or “geothermal energy” or “geothermal power”) AND (“renewable energy” or “alternative energy” or “green energy”) AND (“low enthalpy” or “medium enthalpy”).

Once the search was completed, the data were downloaded in Comma-Separated Values (CSV) and Bibtex formats. These records allowed bibliographic processing and generation of graphs to visualise the field’s behaviour in phase two.

The bibliometric processing carried out in this study contemplated three types of analysis: i) scientific production of the field investigated over time, ii) analysis of the contribution of studies by country, and iii) scientific distribution through an analysis of keywords that allow determining the research areas within the investigated field for the identification of the cognitive structure and intellectual relationships through the Bibliometrix software (version 4.0) [37] for the unification of databases and processing of the analyzed fields (e.g., citations, authors, countries, keywords ) and VOSviewer [38], [39] for the bibliometric maps visualisation.

The literature review process contemplated research with environmental and economic considerations; for this, the full content of the publications were read and analysed, identifying the documents that answered the research question. According to the applied process, the analysed database was reduced to 13 scientific publications and fully reviewed. This phase allowed us to understand the environmental and political considerations within low and medium enthalpy geothermal energy.

The final stage of the work involved a SWOT analysis [40], [41] in determining strategies for promoting and strengthening research aimed at implementing low and medium-enthalpy geothermal energy, as well as its main benefits and limitations. The tool used for the analysis consisted of a focus group [42] composed of experts from the energy sector and experts from this study, considering unanimous opinions.

The experts included six members with academic and professional experience in energy resources, territorial planning, and sustainable management of natural resources. The analysis was conducted using a questionnaire divided into four axes: i) geothermal energy and the environment, ii) geothermal energy and technological development, iii) geothermal energy and legal framework, and iv) geothermal energy and economic viability. The different axes analysed included a series of questions aimed at jointly defining the experts' opinions on the topic under investigation. The criteria used to guarantee that the opinions were unanimous consisted of two rounds of the questionnaire for analysis at the national and global levels.

Research related to medium and low enthalpy geothermal energy began in 1984, with the study by Ungemach [43] which reflects the need for adequate techniques for exploring and evaluating geothermal resources, as well as the barriers associated with low enthalpy energy sources.

The first two decades (23 years until 2006) of research in the field are scarce, representing only 7.21% of the total scientific production. However, between 2007 and 2023, 92.79% of the investigations had an exponential growth trend with R2 equal to 0.8455, in which 2023 represents a decrease in production since it is the year in the course (Figure 2).

The initial period (1984–2006) highlights research oriented toward planning and evaluating low-enthalpy geothermal energy projects [44] and their potential for electricity generation [45] and alternative uses [46], [47]. However, since 2007, over the last 26 years, scientific production has been increasing on issues related to the importance of geothermal energy in the transition towards sustainable district heating [48], [49], the design and evaluation of low-enthalpy geothermal systems [50], [51], [52], and assessments of the positive and negative aspects of geothermal energy from an environmental point of view [53], [54], [55].

According to the bibliographic coupling of countries with at least one medium and low enthalpy geothermal energy study, 42 countries contributed to research within the field (Figure 3). Generally, three countries stand out according to their scientific production: Italy (44 documents), Spain (22 documents), and Germany (17 documents). It is important to highlight Italy, Germany, and Greece as countries with the most cited studies globally (Table 1).

Figure 3 shows the bibliographic coupling analysis by country represented by nodes with variable sizes according to the number of documents registered. The links established in the graph reflect the existing collaboration between countries, in which thickness shows the strength of cooperation. Generally, bibliographic processing indicates 42 countries and 457 links with a relationship strength of 5684 grouped into seven clusters of different colours.

Table 2 shows the country that produces the most by cluster and the top three countries it collaborates with. For example, the first cluster (red) contains 14 countries, of which Italy is the leader in production, and collaborates mainly with Australia, Germany, and the United States.

According to the co-occurrence analysis of author words, connections and construction of a domain structure of 35 keywords were established. Table 3 shows the top 10 words with the highest frequency in the studied area, highlighting “low enthalpy”, “heat pump”, and “numerical simulation” as the three most frequent keywords.

The generated bibliometric map grouped the different keywords into five clusters, representing the research areas of low and medium enthalpy geothermal energy (Figure 4). The variation in the size of the nodes indicates the number of occurrences of the keywords, and the links reflect the relationships with other themes.

Cluster 1 (red), called “Geothermal energy and its contribution to sustainable development”, contemplates the understanding of the importance of low and medium enthalpy geothermal energy in global sustainable development and represents the most studied area within the field [56], [57]. Studies have analysed the role of geothermal heat sources in district heating and cooling to supply the global energy demand [48], [58] and evaluate its contribution as an environmentally friendly technique [53], [54].

The area called "cascade systems: geothermal polygeneration" includes cluster 2 (green) with research that promotes the use of geothermal resources at cascade levels, characterized by sequential use of subsoil heat for the combined production of electricity, heating or refrigeration, drying and dehydration processes, as well as recreational uses [15], [59]. This type of cascading energy use has been evaluated from technical and economic points of view, as well as it’s potential as a rational and sustainable use of geothermal resources. On the other hand, there is evidence of studies on using Organic Rankine Cycle (ORC) technology for low enthalpy geothermal systems as profitable and efficient alternatives within this cluster [60], [61].

On the other hand, the area "geothermal heat pumps (GHP) as a tool for geothermal use" is identified, which includes cluster 3 (blue), in which the use of heat pumps is exposed as an alternative for the use of geothermal energy from low enthalpy, which allows the reduction of primary energy consumption and greenhouse gas emissions [62], [63]. Research in this area mainly contemplates the design and implementation of GHP as viable techniques from a technical and economic point of view [64], [65], considering the evaluation of impacts on the four damage criteria for each phase of the entire life cycle (production, installation, operation, and end of life) [66].

Likewise, it is possible to observe a trend of studies related to “numerical simulation for the design of geothermal systems” within cluster 4 (yellow) with research related to modelling or numerical simulation of geothermal systems under different physical-chemical conditions, existing lithology data, and soil temperature [67], [68], [69]. These studies evaluated different scenarios to identify the potential and limitations of geothermal systems [49], [70], [71].

Finally, cluster 5 (purple) contemplates the area called “groundwater in the use of geothermal energy”, in which hydrogeological and thermal characterisation studies of shallow aquifers for use in heating and cooling systems stand out, as well as the evaluation of the environmental impact in the different stages of construction of this type of system [72], [73], [74], [75], [76], [77], [78], [79], [80], [81], [82], [83], [84], [85], [86].

The review considered 13 studies that analysed the environmental or economic aspects of low and medium enthalpy geothermal systems (Table 4). The information analysis reflects that the contribution is focused on European countries, which evaluate the operation of low enthalpy geothermal systems for cooling or heating buildings, highlighting their environmental, social and economic importance. Additionally, studies have been conducted at a global and continental level to promote geothermal energy as a sustainable alternative.

Geothermal energy is a tool for solving the energy demand and reducing the environmental impacts of non-renewable energy sources. From an environmental point of view, low and medium enthalpy energy is a resource that reduces greenhouse gas emissions, which cause global warming [53], [76]. These techniques, commonly used in electricity generation, cooling, and heating systems, are widely used in urban and industrial areas with a clear objective of contributing to sustainable energy development.

The reviewed studies mainly reveal the functionality of low and medium enthalpy systems through case studies or global analyses, in which the use of geothermal energy is promoted internationally, complying with the current regulations of each country. According to Milenić et al. [65], shallow aquifers could be a strategic option for energy production for district heating and cooling as a medium or low enthalpy mechanism. These sources can increase the energy capacity of a city, reducing primary energy consumption by up to 60% and 70%, respectively. In Europe, various geothermal projects seek to supply the energy demand sustainably. However, efforts are still being made on other continents to promote this type of energy in urban systems, becoming a new technological challenge for adapting geothermal energy as a sustainable energy system [77], [78]. According to Carotenuto [79], it is possible to recover freezing probes connected to a thermo/cooling plant structured by a heat pump, inertial storage tanks, and circulation pumps. This system reduces the environmental impact and improves energy efficiency, adapting to the political guidelines of world organisations related to the change in the energy matrix and promoting the reuse of existing structures.

Although implementing geothermal systems represents a sustainable alternative, inadequate resource management can contribute to physical properties and soil temperature alterations [58], [80]. Among the environmental effects registered by geothermal energy, most consider the construction phase [81] owing to the diesel consumption by the drilling platform, the steel used for the casing of the wells, and the construction of the power plant [82], [83]. One option to mitigate these environmental effects is to use biodiesel or non-contact drilling technologies [84].

Another alternative for improving and optimising the efficiency of geothermal systems is the cascading use of low and medium enthalpy resources as a sequential technique of geothermal heat, generally for generating electricity, heating or cooling, recreational uses, or drying or dehydration processes [15], [85]. Finally, studies such as Lohse [53] recommend successful exploration to access geothermal deposits with minimal drilling efforts, thereby reducing the environmental impact in the construction stage.

Economic evaluations show that geothermal systems are economically profitable in long- and short-term projects, but they will depend on the objective of the work (e.g., change in the matrix or energy expansion) [62], [82], [86]. A study by Mac-Lean et al. [78] concluded that there were greater savings when cooling capacities were used instead of heating in geothermal mechanisms. Part of the economic investment is the heat pump and execution of the well for pumping and reinjection of the water used at the end of the industrial cycle.

Although the evaluation of the economic and environmental aspects of geothermal resources is rarely addressed in scientific literature, specific studies have presented the advantages and disadvantages of these aspects (e.g., [58], [62], [65], [83]). Implementing a geothermal system requires a novel concept of drilling that does not involve overpricing through an innovative design and intelligent measurement of business associations to reduce the initial investment costs significantly [77]. From an economic point of view, the transition of the energy matrix with a focus on geothermal resources represents a high investment in the construction stage. However, the recovery rate of this type of project is possible within a few years, depending on the magnitude of the implemented system [62], [86].

From the studies reviewed in this research, the authors pointed out three key points for achieving an efficient and profitable project: i) the participation of a group of companies that are interested or dedicated to geothermal exploitation works; ii) the technical use of studies carried out in this area to understand the case study and actual drilling costs; and iii) good communication in the exploration phase between geothermal companies and local communities.