**Preface to "New Trends in Enhanced, Hybrid and Integrated Geothermal Systems"**

The most important challenge for the global energy sector is to rapidly transform the entire system to one less dependent on fossil fuels and so reduce the harmful effects to climate. In sharp contrast, the global energy demand, mainly being met by fossil fuel resources, is continuing to grow, primarily in response to population and economic growth. Hence, finding, developing, and employing effective, economical, and practical solutions to the ongoing and emerging challenges is essential. Meeting global energy demand and simultaneously minimizing the negative consequences of climate change and the threat of global warming requires a transition to energy systems largely based on non-carbon renewable energy sources (e.g., solar, wind, geothermal, and hydro).

Geothermal energy installations—shallow, deep, or a combination of both—provide sustainable and environmentally friendly energy. Exploiting this source consists of extracting and/or storing the Earth's thermal energy for use in meeting electricity and heating/cooling needs for a variety of applications, including to heat and cool dwellings and greenhouses, provide warm and/or cold water for agricultural products in greenhouses, and even to de-ice roadways and parking areas.

The main goal of this Special Issue has been to address the existing knowledge gaps and help advance the deployment of geothermal energy projects worldwide. Of the twelve articles submitted, eight were accepted for publication after the peer-review process, an acceptance rate of 67 percent. The published articles cover a range of topics and applications central to geothermal energy. Although submission to this Special Issue is now closed, the need for further in-depth research and development related to geothermal technologies and systems (stand-alone or hybrid) remains. Due to the nature/specifications of geothermal energy, such as a local, robust, climate-independent, potentially constant, generally available, resilient, almost greenhouse gas-free, and long-lived energy source, it is anticipated that this renewable and sustainable source of energy will play a more prominent role in the future global energy supply mix.

In the end, we would like to take this opportunity to express our most profound appreciation to the MDPI Book staff, the editorial team of Applied Sciences journal, especially Ms. Lexie Gang, the assistant editor of this Special Issue, talented authors, and hardworking and professional reviewers.

> **Alireza Dehghani-Sanij, Jatin Nathwani** *Editors*

## *Editorial* **Special Issue: New Trends in Enhanced, Hybrid and Integrated Geothermal Systems**

**Alireza Dehghani-Sanij 1,\* and Jatin Nathwani 1,2**


#### **1. Introduction**

The most important challenge for the global energy sector is to rapidly transform the entire system to one less dependent on fossil fuels and so reduce the harmful effects on the climate. In sharp contrast, the global energy demand, mainly being met by fossil fuel resources, is continuing to grow, primarily in response to population and economic growth [1]. Therefore, finding, developing, and utilizing effective, economical, and practical solutions to the ongoing and emerging challenges is essential. Meeting the global energy demand and simultaneously minimizing the negative consequences of climate change and the threat of global warming requires a transition to energy systems largely based on noncarbon renewable energy sources (e.g., solar, wind, geothermal, and hydro) [2]. Geothermal energy installations—shallow, deep, or a combination of both—provide sustainable and environmentally friendly energy. Exploiting this source consists of extracting and/or storing the Earth's thermal energy for use in meeting electricity and heating/cooling needs for a variety of applications, including to heat and cool dwellings and greenhouses, provide warm and/or cold water for agricultural products in greenhouses, and even to de-ice roadways and parking areas [3].

The main goal of this Special Issue has been to address the existing knowledge gaps and help advance the deployment of geothermal energy projects worldwide. Of the twelve articles submitted, eight were accepted for publication after the peer-review process, an acceptance rate of ~67 percent. The published articles, briefly described in the following section, cover a range of topics and applications central to geothermal energy.

#### **2. Summary of Published Articles**

In the first article, Akbari Kordlar et al. [4] proposed a novel and adjustable trigeneration system—driven by geothermal heat sources—to meet the heating and cooling demands of a particular district network in Germany based on ambient air temperature. Their tri-generation system integrates a modified absorption refrigeration cycle and a Kalina cycle, utilizing a mixture of ammonia–water as the working fluid for the entire system. To examine the impact of various operational parameters on the system's performance before optimization, they conducted a sensitive analysis in off-design conditions, taking into account mass, energy, and exergy balances, and also off-design model equations for each component as a control volume. Their simulation results reveal that the proposed tri-generation system can meet the required heating and cooling demands. The authors also optimized the proposed system based on two criteria: the maximum exergy efficiency (the first scenario) and minimum total exergy destruction rate (the second scenario). The optimization results indicate that (1) a maximum mean exergy efficiency in the first scenario is obtained as 44.67 percent at the expense of a 14.52 percent increase in the total exergy destruction rate compared to the second scenario; (2) a minimum mean total exergy

**Citation:** Dehghani-Sanij, A.; Nathwani, J. Special Issue: New Trends in Enhanced, Hybrid and Integrated Geothermal Systems. *Appl. Sci.* **2021**, *11*, 3765. https://doi.org/ 10.3390/app11093765

Received: 12 April 2021 Accepted: 20 April 2021 Published: 22 April 2021


**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

destruction rate in the second scenario is calculated as 2980 kW at the expense of an 8.32 percent reduction in the exergy efficiency in the first scenario; and (3) taking into account both the exergy efficiency and total exergy destruction rate in off-design conditions, the best enhancement via the optimization process is achieved on a typical cloudy workday in winter, with 53.21 percent exergy efficiency and a 2570 kW total exergy destruction rate. Comparing the optimization results for both scenarios shows that the system's performance is more improved in the second scenario.

Kim et al. [5] experimentally analyzed a smart geothermal heating and cooling heat pump system consisting of an open loop—meaning the use of underground water from an aquifer—by means of a standing column well (SCW) and cross-mixing balancing well heat exchangers. To do so, they modified the present operational technique of two adjacent SCW geothermal heat exchangers, each with a single well. The authors employed this technology in order to improve the coefficient of performance (COP) of the geothermal system, basically through preventing underground water discharge and sustaining a constant temperature in the underground heat exchanger. The two balancing wells were utilized in the cross-mixing methods to restrain the bleed water discharge. According to the authors' findings, the mean amounts of COP measured from the balanced well cross-heat exchange system were, respectively, 3.76 and 3.27 throughout the cooling and heating operations. In other words, they achieved COP enhancements of 23 and 12 percent, respectively, during the cooling and heating operations compared to the present SCW method of the heat exchange system. Moreover, while utilizing a balancing well cross-mixed heat exchange system, the initial underground temperature was kept constant, with only a small standard deviation of 0.08–0.12 ◦C over 3–5 days of continuous operation, meaning that a relatively stable heat source supply is possible.

Maleki Zanjani et al. [6] numerically investigated the effect of utilizing fins on both the interior and exterior U-tube surfaces of a ground heat exchanger (GHE) under cooling mode, plus axial speed and temperature contours due to the tube curvature for an internally finned U-tube at different speeds. They then compared the results with results obtained from a finless U-tube GHE under the same physical conditions. The authors also simulated both dynamic and static behaviour of simple and finned (both internally and externally) U-tube GHEs to determine the impacts of longitudinal fins on the thermal performance of borehole heat exchangers (BHEs) and the heat transfer rate between circulating fluid and soil around the tubes. The dynamic simulations contained short timescale and frequency response tests. In fact, the authors' goal in performing the dynamic and static simulations was to find the best thermal efficiency. The simulation results illustrate that interior fins enhance the rate of heat transfer more effectively, so that the maximum fluid temperature variation was ~2.9 percent in the tube with exterior fins, and ~11.3 percent in the tube with interior fins, instead of the simple tube. Additionally, increasing the inlet fluid speed can make the differences more substantial in temperature profiles and changes, so changing the inlet fluid speed in the range of 0.02–0.06 m/s enhances the fluid temperature variation in the internally finned U-tube by 5–11.3 percent, in comparison with the finless U-tube. In contrast, the fluid temperature variation is only increased by 0.5–2.9 percent when the external fins are utilized. As for the dynamic behaviour, the priority of the interior fins is also apparent. Generally, the research performed by Maleki Zanjani et al. [6] confirms that the use of fins positively influences fluid temperature variations within the tube as well as the rate of heat transfer between the fluid and the borehole wall. The use of fins, especially interior fins, also enhances the thermal efficiency of a ground source heat pump system and reduces the length of the tube required and the initial expenses of the system.

Huang et al. [7] evaluated and reported on a location for a planned geothermal project, called the Alberta No. 1 project, which would produce power and heat (for direct use) at a commercial scale. The project is situated within the Municipal District of Greenview, south of the city of Grande Prairie, Alberta, Canada, and the evaluations demonstrate that there is a high likelihood of fluids up to 120 ◦C at depths of approximately 4000 m. According to the assessments conducted by the Alberta No. 1 project team, the target formations for

this planned geothermal project consist of dolomitized carbonate units of the Devonian age from the Beaverhill Lake Group to the top of the Precambrian Basement. Additionally, Permeable Devonian-aged sandstone units such as the Granite Wash Formation are also targets. The authors' findings suggest that elevations at the top of the Beaverhill Lake Group range from 3104 m to 4094 m, and temperatures at the top of the formation range from 87 ◦C to 123 ◦C in the project area.

Several potential challenges regarding the performance of sedimentary geothermal well-doublet systems, including heat conductivity, geochemical, and geomechanical conditions and issues, have been comprehensively examined and discussed by Mahbaz et al. [8]. These challenges may occur in the processes inside the reservoir itself, such as channelling, mineral precipitation and flow, and in the access and energy systems such as wells, heat exchangers, pumps, and surface tubing insulation. Some of their findings include that: (1) Chemical reactions can result in changes of flow-path, plus alterations of the rate of heat exchange between rock and water, and, consequently, changes in the reservoir's porositypermeability and heat capacity; (2) Corrosion and scaling damage in the well-doublet systems diminish their performance and also influence their life-span; (3) Factors such as energy discharge rate and strategy, rate of injection, temperature and heat management, and well spacing contribute substantially to the heat recovery and life-span of a geothermal well-doublet system; (4) The injection of geothermal fluid such as water, carbon dioxide, and air into a well-doublet system can induce stress variations at a scale that will increase the likelihood of fault/fracture reactivation and induced seismicity. Therefore, realizing the magnitude of these events and their recurrence in time is important; (5) Factors including channeling, short-circuiting, leaking, heterogeneity, and permeability impairment can influence project viability, so they must be carefully evaluated during site assessment. Recognizing and understanding these challenges and issues will assist designers/engineers in better designing, implementing, and operating sedimentary geothermal well-doublet systems, leading to improved performance and increased efficiency of these types of systems, in addition to lessening their associated expenses and risks.

Gao et al. [9], by means of a hydro-thermal coupling model, numerically examined the impact of fluid flow direction on the heat transfer performance and specifications in a granite single fracture and validated the accuracy of the numerical modeling results experimentally. Their findings indicate a consistently robust relationship between the distribution of the local heat transfer coefficient and the fracture profile, independent of the axis. An alteration in the direction of the fluid flow is likely to change the amount of the heat transfer coefficient, but does not influence the distribution specifications along the flow pathway. An increase in the rate of injection fluid flow has an enhanced influence. Although the heat transfer capacity in the fractured rises with the rate of fluid flow, a sharp reduction in the rate of heat extraction and the total heat transfer coefficient is also seen. In addition, the model with the smooth fracture surface in the flow direction reveals a greater heat transfer capacity in comparison with that of the fracture model with differing roughness. This finding is attributed to the existence of fluid deflection and dominant channels.

The study conducted by Gizzi et al. [10] deals with the reuse of abandoned or disused deep hydrocarbon wells in Italian oilfields to extract the associated geothermal resources. The authors utilized existing information on Italian oil and gas wells and on-field temperatures to employ a simplified closed-loop coaxial wellbore heat exchanger (WBHE) model at three hydrocarbon wells positioned in different Italian oilfields, namely, the Villafortuna–Trecate, Val d'Agri-Tempa Rossa, and Gela fields. The goal was, in fact, to appropriately examine the heat transfer mechanisms of the three, focusing on the fact that the potential to extract thermal energy can alter based on the geological and sedimentary context. The results demonstrate considerable difference between the potential quantities of thermal energy to be extracted from the three wells studied. Taking into account the maximum extracted working fluid temperature of 100 ◦C and hypothesizing a cascading exploitation mode of the heat accumulated, it was only possible for the Villafortuna 1

WBHE to consider a multi-purpose and comprehensive use of the resource, based on the use of existing infrastructure, available technologies, and current knowledge.

In the last article, Cukovi´ ´ c Ignjatovi´c et al. [11] evaluated an approach for the cascade utilization of geothermal energy potential for space heating and cooling and balneological treatments in the most south-eastern region of the Pannonian basin, in Central Europe. For this purpose, they selected two specific sites with various geothermal resources (meaning different temperatures and chemical compositions) and located within different urban contexts—one in a natural environment called Ljuba and the other inside a small settlement called Banatsko Veliko Selo—situated in the province of Vojvodina, northern Serbia. At the Ljuba site, a geothermal spring with a temperature of 20.5 ◦C was characterized to employ a geothermal heat pump (GHP) for the production of heat, whereas at the Banatsko Veliko Selo site, a geothermal well with a temperature of 54 ◦C was appropriate for direct use. The authors calculated the available thermal power from the geothermal resource according to geothermal conditions for both sites, ranging from 300 kW (GHP) to 950 kW (direct use), along with the cascade method of utilization. At the same time, they proposed a development concept with an architectural design, matched with energy availability based on the modularity concept, to enable sustainable energy-efficient development of wellness and spa/medical facilities that can be supported by local authorities. The resulting energy heating demands for different scenarios were 16–105 kW, which can fully be supplied by geothermal energy.

#### **3. Future Research Need**

Although submission to this Special Issue is now closed, the need for further in-depth research and development related to geothermal technologies and systems (stand-alone or hybrid) remains. Due to the specifications of geothermal energy, such as a local, potentially constant, robust, generally available, resilient, almost greenhouse gas-free, and long-lived energy source, it is anticipated that this renewable and sustainable source of energy will play a more prominent role in the future global energy supply mix.

**Acknowledgments:** This Special Issue would not have been possible without the contributions of various talented authors, hardworking and professional reviewers, and dedicated editorial team members of the Applied Sciences journal. Congratulations to all authors who submitted articles—no matter what the final decisions of the editors were. Feedback, comments, and suggestions have been offered by the peer reviewers and editors to help the authors improve their articles. Finally, we would like to take this opportunity to express our gratitude to the editorial team of Applied Sciences, and special thanks to Lexie Gang, Assistant Editor of this Special Issue.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Abbreviations**


#### **References**


## *Article* **Sustainable Modularity Approach to Facilities Development Based on Geothermal Energy Potential**

**Nataša Cukovi´ ´ c Ignjatovi´c 1,\*, Ana Vranješ 2,\*, Dušan Ignjatovi´c 1, Dejan Mileni´c <sup>2</sup> and Olivera Kruni´c <sup>2</sup>**


**\*** Correspondence: natasa@arh.bg.ac.rs (N.C.I.); ana.vranjes@rgf.bg.ac.rs (A.V.) ´

**Abstract:** The study presented in this paper assessed the multidisciplinary approach of geothermal potential in the area of the most southeastern part of the Pannonian basin, focused on resources utilization. This study aims to present a method for the cascade use of geothermal energy as a source of thermal energy for space heating and cooling and as a resource for balneological purposes. Two particular sites were selected—one in a natural environment; the other within a small settlement. Geothermal resources come from different types of reservoirs having different temperatures and chemical compositions. At the first site, a geothermal spring with a temperature of 20.5 ◦C is considered for heat pump utilization, while at the second site, a geothermal well with a temperature of 54 ◦C is suitable for direct use. The calculated thermal power, which can be obtained from geothermal energy is in the range of 300 to 950 kW. The development concept was proposed with an architectural design to enable sustainable energy efficient development of wellness and spa/medical facilities that can be supported by local authorities. The resulting energy heating needs for different scenarios were 16–105 kW, which can be met in full by the use of geothermal energy.

**Keywords:** geothermal energy; Pannonian basin; geothermal cascade use; energy efficiency; wellness and spa facilities; balneology; bioclimatic architecture; passive design strategies; modular building

#### **1. Introduction**

Geothermal energy is recognized as a valuable resource having a variety of uses: from electricity generation (through geothermal power plants or through cogeneration systems) [1] and space heating and cooling (using the heat pumps) to direct use in a wide range of applications in areas such as balneotherapy [2,3], agriculture, industry, swimming pool heating, and in individual and district heating systems [4,5]. While harnessing geothermal energy for electricity production is mainly related to specific tectonic regions, its direct use is more common. Coupling the energy potential with the healing properties of water's temperature and chemical composition, is recognized as a versatile and efficient way of exploiting this geothermal resources [6–9].

Hot water springs have been used for healing and medical recovery since ancient times. Today, however, balneology is undergoing a transition both concerning its place in formal medical science and as an asset for the development of tourism, local and state economies and general popular well-being. Contemporary definitions of medical tourism encompass medical travel, recreational travel and traveling for other purposes [10]. All of the stated aspects are highly relevant for balneotherapy, which, by its nature, can successfully meet a variety of their needs if provided with adequate infrastructure. Serbia is particularly rich in mineral, thermal and thermo-mineral waters with sites that have been in use ever since the Roman Empire [11]. The same hydrogeological resources show great potential as a convenient source of hydrogeothermal energy [12] that could be used to achieve high levels of energy efficiency in wellness and spa facilities. The Serbian province of Vojvodina, despite its abundance of various resources, is undergoing constant depopulation and even

**Citation:** Cukovi´ ´ c Ignjatovi´c, N.; Vranješ, A.; Ignjatovi´c, D.; Mileni´c, D.; Kruni´c, O. Sustainable Modularity Approach to Facilities Development Based on Geothermal Energy Potential. *Appl. Sci.* **2021**, *11*, 2691. https://doi.org/10.3390/app11062691


Academic Editor: Jatin Nathwani

Received: 9 January 2021 Accepted: 8 March 2021 Published: 17 March 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

impoverishment in certain agricultural areas. Although current national and regional planning documents barely consider geothermal energy to be a strategic resource [13,14], its great potential should be used as a tool for sustainable development, especially in areas where the abundance of this resource may help address the economic challenges and a decades-long trend of depopulation.

In recent times, comprehensive research has been carried out in northern Serbia— Vojvodina province—with the aim of considering and estimating the geothermal potential. The defined potential of geothermal resources represents a strategic foundation for planning sustainable economic development, since geothermal energy is an abundant source of renewable energy that requires low-carbon and non-intermediary technology [15,16]. A multidisciplinary research concept was applied, which tended to display, in a single place, the potential for geothermal resources to provide sustainable models for development supported by local municipalities. The areas for the application of geothermal energy were analyzed through an integrated approach of economic-commercial factors based on the main assumption of cascade use [17]. In that respect, among other things, an analysis of increasing the share of geothermal energy within the field of balneology was performed. Geothermal resources were observed both as a healing factor due to temperature and chemical composition and also as an energy source for heating and cooling facilities where geothermal waters are already used for medical and wellness purposes.

The studied terrain represents a typical basin structure genetically correlated to southeastern parts of the Pannonian basin, which extends over the territory that includes presentday Hungary, Croatia, Slovenia, Romania and Serbia, where it occupies terrain to the north of the Sava and Danube rivers (Figure 1) [18]. Generally speaking, the Pannonian basin represents an area that has an expressed geothermal anomaly and extraordinary geothermal potential [19,20]. The basin's highest heat flow values, above 100 mW/m2, have been registered in the northeastern part and in the central part with distribution towards the southern peripheral parts, that is, towards the Serbian basins and the Vardar zone [21,22]. The values of geothermal gradients in the area of Vojvodina province range from 4.0 to 7.5 ◦C/100 m [23,24]. The dominant way of geothermal exploitation is via wells. The average geothermal outlet temperature ranges from 25 to 75 ◦C. In Vojvodina province, 50 new balneological facilities were proposed for development [25–27]. When defining prospective locations for them, wellness and spa, sports and recreation, and medical programs were all taken into account as was the urban-development potential for these facilities. These were also analyzed from the point of view of energy efficiency, different building material, energy independence, energy self-sustainability, and carbon footprint minimization.

The architectural programs, derived from the multidisciplinary research and conceived as illustrative proposals for potential developments on the site, range from modest drinking fountains to complex resorts with variety of facilities (Figure 2). Since 20 out of 50 proposals refer to wellness and spa facilities, they have remained in the spotlight of further investigations. Throughout the process, a series of model units was developed and tailored to match medical and therapeutical requirements bearing in mind the imperatives of environmental consciousness, sustainability and architectural resilience suited to the local municipalities' needs and resources. The two sites, Ljuba and Banatsko Veliko Selo, were chosen to further explore the proposed model in two different settings regarding the built-up environment and geothermal capacities. The study presented in the paper aims to point out the general design guidelines for development of wellness and spa/medical facilities that can be supported by local municipalities. The resulting proposals should help them gradually develop facilities for public or commercial use by relying primarily on their own resources rather than waiting for developers to exploit the balneological potential.

**Figure 1.** (**a**) Main tectonic units of the Alpine Fold Belt and Alpine–Carpathian–Dinaric Mountains (modified after [28]); (**b**) Geological map of the Vojvodina part of the Pannonian basin basement (modified after [29]).

**Figure 2.** Proposed programs for new balneological sites in Vojvodina.

Since the design proposals referred to on-site use of the balneological resource, the character of the sites also varied greatly, from facilities placed within urban or rural settlements to sometimes rather remote places with poor or no infrastructure.

#### **2. Materials and Methods**

This paper shows two sites where geothermal resources were formed within different types of reservoirs, as well as with different temperatures and diverse chemical compositions. According to the features of these resources and spatial-urban conditions of the terrain, the manner of exploiting the resources and an architectural concept was suggested. The first site, Ljuba, is characterized by low-temperature geothermal water (<30 ◦C), which requires a geothermal heat pump (GHP) for heat production. The geothermal source is in the form of a spring, which is very rare for the basins. The reservoir is formed within Triassic limestone, and the water belongs to the HCO3–Ca type as expected with this kind of reservoir. The uniqueness of this geothermal water may be seen from the chemicalspecific composition of its microelements. From a water sample, a high concentration of arsenic was detected. The second location presented typical geothermal resources formed in basin sediments. The reservoir was formed within the sandy layers of the Pliocene epoch, with temperatures that ensure direct use (54 ◦C). The geothermal waters, produced by the geothermal well, belong to the HCO3–Na type, and the main balneological factor is temperature. The available thermal power from the geothermal resource was calculated for both sites, along with setting the cascade system concept. Simultaneously, the architectural concept was developed, matching energy availability according to the concept of modularity.

#### *2.1. Architectural Model—Design Premises*

Design premises were in line with UN sustainability goals [30], where sustainable and resilient architecture for healthcare and spa facilities was conceived as the driver for a circular economy and improved well-being of the local population. The design approach was tailored with regard to the urban environment, the facility's current development status, need for (and availability of) accommodation, and seasonal sensitivity. The study of such a size was also seen as an opportunity to explore options for "branding" the wellness and spa/medical facilities of Vojvodina through recognizable design features.

The urban context was classified as "town" for sites within small urban settlements, "village" for those within rural settlements, and "remote" for those the farthest away from existing settlements (Figure 3). The fact that remote sites have no infrastructure and the ones in the villages are often poorly equipped stressed the necessity of providing a high level of infrastructure independence.

**Figure 3.** Number of proposed wellness and spa/medical facilities regarding urban/rural context.

The majority of proposed facilities (13 out of 20, Figure 4) were supposed to be completely new developments, while 7 cases were supposed to offer additional content mainly to partly developed outdoor facilities (usually pools, baths or ponds). In almost all cases, small scale structures were more suitable than conventional buildings that had all utilities in a single volume (Figure 5).

**Figure 4.** Number of proposed new developments and complementary contents that are adjacent to already existing wellness and spa or leisure facilities.

**Figure 5.** (**a**) Ljuba—currently an undeveloped site although used in Roman era; (**b**), Banatsko Veliko Selo—paths to outdoor pools (**c**,**d**) Banatsko Veliko Selo—outdoor swimming pool (existing facility).

The need to provide accommodation directly associated with the exploitation of balneological resource varied (Figure 6a). In 7 cases there was already accommodation in the vicinity (usually within walking distance); in 7 cases accommodation was not necessary due to the location and the nature of the balneological resource; and in 6 cases some accommodation had to be provided within the new development. This led the team to explore flexible design concepts where accommodation would be an optional feature rather than placing all functions within a single volume. While allowing gradual development in pace with the local municipality's strategies and investment capability, this approach also provided more options for public use, which is of great importance to the community, giving the locals easier access to the wellness and spa/medical facilities.

**Figure 6.** Number of proposed wellness and spa/medical facilities regarding (**a**) the need for and availability of accommodation, and (**b**) seasonal sensitivity.

The nature of the resource along with the site and the expected modes of use reflect the facility's "seasonal sensitivity" (Figure 6b). Most developments show medium seasonal sensitivity, which means that they can operate year-round in but with greater variation in capacity, while the ones with low seasonal sensitivity show less oscillation in seasonal use. In both cases, more demand on weekends than on weekdays can be expected. The modular approach presented in this paper addresses the issues of flexibility in the use of certain features and mitigation of operating costs for periods of lower demand. High seasonal sensitivity implies that the facility may operate only during the summer, so providing thermal comfort and natural ventilation on warmer days while staying in direct contact with the environment was dominant.

Climate in Vojvodina is classified as warm temperate, predominantly Cfa with areas of Cfb according to the Köppen–Geiger system. Month-to-month weather data for Vojvodina (Table 1) indicates that for 6 to 7 months some additional heating is needed while cooling might be needed for certain periods during the summer.


**Table 1.** Weather data for Vojvodina [31].

The issues of sustainability and resilience were addressed on several levels:


• local production—supported by the choice of proposed building technology and materials.

While resource efficiency in material use with reference to the 3R (reduce, reuse, recycle) concept remained as a general design goal, specific design strategies were proposed for energy efficiency, stressing the impact of architectural design on minimizing demand, thereby enabling coverage from on-site (renewable) energy sources. Program-specific energy efficiency design strategies focused on a wide range of passive design measures that integrated the technology related to necessary active systems from the conceptual design phase.

Water management in these facilities is very dependent on water composition, flow and temperature yet needs to be treated with extreme delicacy case be case to preserve the unique hydrogeological characteristics. All proposals were designed with extreme care to maximize porous surfaces, minimize potential contamination of surface water, and to make recommendations for rainwater collection and use where appropriate.

Design goals were addressed through a variety of design strategies, which were applied to 20 sites with wellness and spa facilities that ranged in size from small mobile units for balneotherapy to complex multifunctional developments. The main design strategies, developed mainly based on bioclimatic architecture, [32] included


The general concept was developed with south-facing wellness and spa/medical units and corresponding utility spaces in the centre with optional additions of hospitality and medical facilities (Figure 7). However, adjacent medical and hospitality facilities might be planned with program-specific design strategies, so they were not further discussed in detail at this stage of the research.

#### *2.2. Architectural Model—Boundary Conditions and Calculation Methodology*

Following the principles of modular design, modules of the same footprint (5.5 × 10 m) were used for both wellness and spa/medical facilities and utility spaces (heated and unheated). To enhance functionality and energy efficiency of the study model, balneotherapy units were grouped in pairs with a small joint space to access interior and exterior facilities. Heated auxiliary spaces were designed as combinations of 2–4 modules, while unheated modules may be placed along the northern side of the connecting corridor in a manner that enables efficient service and easy access. Figure 8 presents three development stages.

**Figure 8.** Three development stages for balneotherapy facility: (**a**) Stage 1 with 4 balneotherapy units, (**b**) Stage 2 with 6 balneotherapy units, and (**c**) Stage 3 with 8 balneotherapy units.


Stage 1 (Figure 8a) comprises of 4 balneotherapy units *Mb1–Mb4* with 3 additional utility modules *Mu1–Mu3* for reception, changing rooms, office, examination room and café, and an unheated module *Mt1* as the mechanical room. Stage 2 (Figure 8b) is conceived as an expansion of Stage 1 with 2 added balneotherapy units *Mb5–Mb6*, an additional utility module *Mu4* and an additional technical module *Mt2*. The third stage (Figure 8c) presents a further extension with balneotherapy modules *Mb7–Mb8*, additional utility module *Mu5* and technical module *Mt3*. An overview of the basic data for all three stages is given in Table 2.


**Table 2.** Development stages—an overview of basic data.

In terms of energy performance, two different approaches to building the thermal envelope were explored:


#### The thermal envelope components and their *U*-values are presented in Table 3.


#### **Table 3.** Thermal envelope components.

<sup>1</sup> Values defined for new construction in national Rulebook on energy efficiency of buildings [33]. <sup>2</sup> Values defined for cool-temperate climate in Criteria for the Passive House, EnerPHit and PHI Low Energy Building Standard [34].

For all six scenarios, energy needs were calculated in accordance with national legislation [33,35], which is based on the methodology defined by EN-ISO13790. All calculations regarding the annual energy needed for heating were done using the software *KnaufTerm2s v.27.20* [36] using the following calculation parameters:


Maximum values for annual energy needs for heating for relevant building categories are shown in Table 4.

**Table 4.** Maximum values for annual energy needs for heating for mandatory EPC class C [35].


Energy demands were compared to the resource capacity for two sites in Vojvodina: Ljuba, a remote location with no available infrastructure, and Banatsko Veliko Selo, a settlement with a population of 2525 [37]. Here, the proposed structure should be placed adjacent to an existing outdoor swimming pool.

Complementary structures for hospitality and healthcare services were not considered in the study since the focus was on investigating innovative design approaches for wellness and spa/medical facilities, such as wellness and spa and public baths. In regard to estimating overall potential use of geothermal energy for heating, an approximation was made using current national regulations (estimated energy needs according to the EPC mandatory class C).

#### *2.3. Geothermal Potential—Geological and Geothermal Conditions*

Generally speaking, in the lithological structure of the Pannonian basin, one can clearly observe two massive lithological complexes: (1) The basin's Palaeozoic–Mesozoic bedrock and (2) the Neogene–Quaternary complex and the related two main types of geothermal reservoirs: fractured and karstified basement reservoirs, and sand/sandy-clay basin-fill reservoirs. The bedrock of the basin in a wider study area is represented by all types of rock: metamorphic, igneous and sedimentary, whereas the Neogene-Quaternary complex is represented by conglomerates, sandstones, clay stones, marl, alevrolites, clay sediments, sands and pebbles (Figure 1). The rocks of the basin bedrock were discovered on the surface of the terrain in the zone of the Fruska Gora horst.

The site of Ljuba is located in the southwestern part of Vojvodina province on the southern slopes of the Fruska Gora horst. The horst structure, extending east–west on its southern and northern sides, is controlled by gravity faults, along which blocks were downthrown in the Neogene period [38]. On Fruska Gora three outcropping domains can be distinguished: (1) a metamorphic core from the Palaeozoic era, (2) a clastic–carbonate sequence with intercalated ophiolites, ophiolitic mélange, and volcanics of the Upper Permian–Paleogene period, and (3) Miocene–Quaternary sediments [39]. Palaeozoic metamorphites are in tectonic contact with low- and middle-Triassic rock [40].

In terms of its geothermal features, this part of the terrain is characterized by high geothermal gradients, particularly on the edges of Fruska Gora. These values range from 6 to 7.5 ◦C/100 m. Geothermal basement reservoir discharge is carried out in two ways: predominantly via deep geothermal wells or in a natural way over the springs. The natural discharge is connected only to the terrain of Fruska Gora, where it takes place along fault-line structures and at the contact point of water-permeable and water-resistant rocks. The outlet temperatures of geothermal waters formed within this type of reservoir in the vicinity of Fruska Gora vary from 15 to 60 ◦C. At the study site, geothermal resources were formed within the dolomite and partially silicified Triassic limestone.

The site of Banatsko Veliko Selo is located in the northeastern part of Vojvodina province and is known for largest depths to the basin bedrock, which has the largest thickness of Neogene sediments. The terrain belongs to the area of intensive sinking (Great Hungarian Depression) from the Neogene period. This area is characterized by block structure and gravitational faults. In this part of the terrain, there is a dominant exploitation of geothermal waters formed within the basin-fill reservoirs. The Geothermal reservoir is represented by sands and sandstones of the Pliocene and Upper Pontian age and this kind of reservoir can be followed continuously over the whole area.

In this part of the terrain, relatively low of geothermal gradients were registered. The reason is the presence of very thick Neogene sediment (up to 3.5 km), overlaying magmatite and metamorphic rock. The correlation between low geothermal gradients and geological settings of this area can also be found in the existence of marly and silty layers at depths greater than 1000 m. Geothermal gradients measured in Pliocene sediment displayed very low values from 2.5 ◦C/100 m to 3 ◦C/100 m. Basin-fill reservoir discharge is performed exclusively via geothermal wells, whereas outlet temperatures do not exceed 60 ◦C. Geothermal waters, from the Pliocene and Upper Pontian belong to the HCO3– Na–Cl water type, respectively to HCO3–Na type and are characterized by relatively low mineralization (up to ≈8 g/L). As for gases, the dominant one is methane with 84–88 mol%, followed by nitrogen and CO2.

#### *2.4. Geothermal Potential—Calculation Methodology*

Available thermal power from the geothermal resource is calculated based on geothermal conditions at the sites along with the cascade method of usage. The geothermal resource is considered as a source of energy for providing heat for facilities and balenological purposes. This principle led to the three limiting parameters for the system:


The possibility of geothermal energy application depends on fluid temperature. The two selected locations covering the whole range of geothermal heating purposes. The Ljuba site comes with hydrogeothermal resources accumulated in the groundwater of a temperature scope to 30 ◦C which is conditioned by the application of geothermal heat pumps [41], while Veliko Banatsko Selo comes with hydrogeothermal resources accumulated in the groundwater of temperature above 30 ◦C, which means direct use is possible. Therefore, the heating operating system on the Ljuba site is considered to be an open loop GHP system contrary to the passive geothermal system with heat exchanger at the Banatsko Veliko Selo site.

The thermal power availability for the both geothermal sites can be calculated as in Equation (1)

$$\mathbf{E} = \mathbf{H} \times \mathbf{Q} \times \boldsymbol{\Delta T} \tag{1}$$

where E is the nominal available power quantity (kW); H the specific heat of water (constant, 4.2 kJ/kg/◦C); Q the yield of the source (kg/s, for water the same as l/s); and ΔT denotes the temperature difference between the entering water and the leaving water.

Geothermal systems on the both sites perform heating operations and must be able to cover the maximum thermal load demands. The physical and chemical characteristics have been analyzed simultaneously to define the method for balneological use (drinking, swimming, inhalation). Furthermore, the concept of the geothermal heating system is conditioned by the method of balneological application. For example, drinking does not consider temperature, but swimming does, which means a projected geothermal system must satisfy these conditions.

In order to provide relevant data for calculations, a geological/hydrogeological reconnaissance was carried out, followed by groundwater parameters regime monitoring (temperature, chemical composition, flow capacity). For the purpose of geothermal water and reservoir characterization, the D'Amore method and diagram was applied [42]. The method is based on the determination of six main genetic parameters, mainly based on cation–anion ratios. Calculated genetic parameters are in line with geothermal reservoir geology and are applied for both sites, Ljuba and Banatsko Veliko Selo. Geothermal investigations and monitoring last for 12 months.

#### **3. Results and Discussion**

The concept of a multidisciplinary approach to geothermal energy utilization has been justified by building modular wellness facilities. The calculated thermal power that can be obtained from geothermal energy is in the range of 300 kW (GHP) to 950 kW (direct use). The size of wellness facilities can be predicted by matching developed scenarios based on different standards for envelope U values and outside temperature in modular facilities with available geothermal energy. The results of the investigation on geothermal waters showed that resources can be used as a balneological resource as well. Two factors determine multipurpose usage: chemical composition, in the case of the first site, and increased temperature, in the case of the second location. The direct benefit is increased efficiency of the geothermal cascade systems.

#### *3.1. Available Thermal Power and Potential Balneological Utilization*

#### 3.1.1. The Site of Ljuba

The yield of geothermal waters at the discharge site is changeable, and its average value equals Q = 4.5 L/s. Geothermal resources at the site of Ljuba can be classified as subgeothermal [43] since the temperature is 20.5 ◦C. According to their chemical composition, groundwater belongs to the HCO3–Ca water type with low mineralization.

Table 5 shows basic physical–chemical values of the analyzed geothermal phenomenon. Taking into account the complex geological composition of the terrain and its active tectonics, what was carried out is the classification of geothermal water using the method of D'Amore's genetic parameters. According to the calculations of genetic parameters, geothermal water on the Ljuba site belong to the carbonated type, and this confirmed that the primary geothermal reservoir was formed in the Triassic rocks of the basin bedrock (Figure 9).


**Table 5.** Physical-chemical features of geothermal waters on the site of Ljuba.

Legend for Figure 9a: A—indicates that geothermal waters had circulated through a calcareous terrains rather than an evaporative one; B—indicates that the geothermal water is enriched by sodium and may come from sedimentary terrains; C—is low, which excludes water deriving from a flysch, volcanite or schistose basement; D—indicates water circulation through dolomite; E indicates that geothermal water had circulated in a carbonated reservoir rather than in a sulfated one; F—no indication in growth of potassium content, which excludes possible circulation in granitic field.

**Figure 9.** (**a**) D'Amore parameters diagram of chemical analyses; (**b**) α: evaporative terrains; β circulation in limestone; γ: circulation through a crystalline basement; δ: circulation in argillaceous rocks.

Available thermal power from the geothermal resource at the Ljuba site is calculated for the following scenario:


In accordance with Equation (1) the available thermal power was:

$$\mathbf{E} = 4.2 \text{ kJ/kg} / ^\circ \text{C} \\ 4.5 \text{ l/s} \times 16^\circ \text{C}$$

which generates around 300 kW of thermal power available for use.

Geothermal waters on the Ljuba site, apart from their energy significance, also possess a healing importance, so they can be used for balneological purposes as well. On the basis of their physical–chemical characteristics, these are characterized as natural, hypothermal, arsenic oligomineral waters, which can be used for health purposes under medical surveillance.

3.1.2. The Site of Banatsko Veliko Selo

At the site of Banatsko Veliko Selo, two geothermal wells were drilled: VS-1/H (925 m) and VS-2/H (895 m). The maximum measured outlet temperature was 43 ◦C at VS-1/H and 54 ◦C at VS-2/H. The yield of the VS-1/H well equaled 10 l/s, whereas the VS-2/H well had a somewhat higher yield of 12 L/s. According to the temperature values, geothermal resources at Banatsko Veliko Selo could be categorized as geothermal resources with low enthalpy, and according to their chemical composition belong to the HCO3–Na water type. Table 6 shows the basic physical–chemical values of geothermal waters sampled from the borehole VS-2/H. Applying the method of D'Amore's genetic parameters, geothermal waters at Banatsko Veliko Selo belong to the clastic water type, and this confirmed that the primary geothermal reservoir was formed within the sands of Pliocene epoch (Figure 10).

**Table 6.** Physical-chemical features of geothermal waters VS-2/H.


Legend for Figure 10a: A—indicates that geothermal waters had circulated through calcareous terrain rather than an evaporative one; B—indicates that this geothermal water, highly rich in sodium, may have come from sedimentary terrain; C—indicates that the geothermal water may have circulated through flysch sediments; D—excludes water circulations through dolomite; E indicates the geothermal water had circulated in a sulfated reservoir rather than a carbonated reservoir; F—indicates no in growth of potassium content, which excludes possible circulation in a granitic field

**Figure 10.** (**a**) D'Amore parameters diagram of chemical analyses from Table 3; (**b**) α: evaporative terrains; β circulation in limestone; γ: circulation through a crystalline basement; δ: circulation in argillaceous rocks.

Available thermal power from the geothermal resource at B.V. Selo (VS-2/H) site is calculated for the following scenario:


The calculation of disposable thermal energy from the geothermal borehole VS-2/H was performed according to the Equation (1).

$$\mathbf{E} = 4.2 \text{ kJ/kg} / ^\circ \text{C} \times 12 \text{ l/s} \times 19^\circ \text{C}$$

which brought around 950 kW of thermal power.

The first cascade level of exploitation provided energy for facilities heating, and the second level was provided a warm bath for balneological purposes. After geothermal waters pass through the heat exchanger, what remained disposable was water at 12 L/s at 35 ◦C, which is the most frequent temperature of healing waters used for curative purposes.

#### *3.2. Architectural Design*

Architectural design (general layout, massing, orientation, thermal zoning and materialization) was developed through a holistic approach and the consistent application of the design principles described in "Methodology". The result was a model for developing wellness and spa facilities, as well as public baths that were applicable at various sites and satisfied site conditions, sustainability goals and development scenarios.

#### 3.2.1. Modularity

Structuring the program into modular units proved to be a very powerful and effective design tool for accomplishing the several design goals set for this study. While easily enabling programmatic diversification and flexibility, the modular approach also opened the door for local production of prefabricated elements by small and medium enterprises.

Carefully planned and positioned modules allowed for phase execution, which significantly reduced initial construction costs. This is even more important when the local community is funding or co-funding the facility since the median value of overall costs for 20 proposals included in the study (excluding the land, which is already publicly owned) is estimated to be approximately 1,500,000 euros. Only two of them were designed as single multifunctional buildings (total area 300 m<sup>2</sup> and 1000 m2) due to the specific site and programmatic conditions.

Interconnected modules, which can be individually included or excluded from the facility's current operation, allow for very economical operation in reduced capacity as well, which is very important, having in mind that 15 out 20 facilities were characterized as being significantly seasonal, meaning that with this concept all of them may remain open to the public year round with optimized operational costs.

The modular approach, which visually and functionally brings together nearly isolated open-door and enclosed spaces (Figure 11), provides a layout with a high level of privacy, which minimizes contact among users of different treatments while allowing optimal hygiene and maintenance intervals during operating hours.

**Figure 11.** Visualization of proposed model for wellness and spa facilities.

The modular design also permits capacity expansion beyond the initial plans while retaining original design features or remaining at a reduced number of constructed modules for long periods in case of unfavourable economic, social or climate conditions. Expansions can be anticipated mainly along the east–west axes in two basic scenarios.


Simple layout and structure of individual modules makes them very flexible, allowing for partial adaptations, changes in use, or even regaining functionality after an extreme catastrophic event and severe material damage.

#### 3.2.2. Sizing and Positioning of Open-Air and Indoor Features

The general placement and interconnection of open-air features with built-up structures was proposed through a holistic design approach, taking into consideration functional demands, passive design principles [32,44], and the advantages of the modular approach as described in the previous section. The design strategies presented were developed bearing in mind the specific nature and demands of the wellness and spa features that incorporate balneotherapy as well as the geothermal potential of those sites.

Open-air and indoor features were purposely sized and organized in a way that allowed a shared use of auxiliary spaces like reception, changing rooms, lockers, and service rooms (Figure 7). The therapy functions are distributed throughout the open-air and enclosed spaces in a way that facilitates compatibility and provides a continuity of various treatments in different weather scenarios. This enables extended operation time, mitigating the sensitivity to seasonal or annual variations in climate while optimizing running costs.

Most open-air features are rather small, following the general concept of modularity and phase execution and operation. The width *i* of intermediate open-air spaces *O* may vary in regard to the height of the neighbouring modules *M* (Figure 7) for functional and privacy requirements. Additional screens may be added for enhanced privacy and additional sun protection of intermediate open spaces.

The modest capacities of both enclosed and open-air spaces allow the local production of most common types and enable less demanding maintenance, which may be one of the preconditions for sustainability of such features in small communities.

#### 3.2.3. Passive Design Strategies

Striving for high level of energy efficiency and climate-responsive design, a series of passive design strategies were proposed to support the overall design goals.

Placement and orientation: Placement of all open-air features and enclosed spaces was done in a way that provided optimal daylight, solar exposure and shading. Northfacing sections were used for technical rooms like storage and sanitary spaces. Circulation corridors connecting the modules were also placed on the northern side, leaving the southfacing open-air spaces between the modules to be sheltered and functionally supported by the side volumes. Longitudinal circulation space can be open or enclosed with glazed south-facing walls to serve as a buffer space for modules.

Massing, solar gains and shading: Modules are compact to provide a good shape factor for each unit. The same principles were applied to all public and utility spaces except for the longitudinal circulation corridor. Modular units and the corridor were mainly opaque on the north façade for improved thermal performance and almost fully glazed on the South façade to maximize solar gain. Adequate shading was provided by various design features tailored to provide unobstructed views under various weather

conditions (Figure 12) and additionally supported by an adequate choice of glazing and flexible interior shading.

**Figure 12.** Solar exposure and shading: all glazed surfaces of wellness modules and circulation volume are fully shaded during the summer to prevent overheating (**upper row**) while remaining almost completely exposed during winter (**lower row**), enabling direct passive solar gain.

> Natural ventilation and cooling: Glazed surfaces are designed to maximize the share of operable elements with cross-ventilation that are provided for all spaces. The basic layout, as previously described in this chapter, provides very good preconditions for natural ventilation and effective application of night cooling, which are further enhanced by the green areas and water features. Under favourable weather conditions, the glazed façade of the long corridor can be open along the intermediate semi-atrium spaces.

> Thermal mass and materials: The modules are designed as lightweight structures where a low thermal mass optimizes comfort and energy efficiency in intermittently used spaces. Enhanced thermal insulation and a high-performance glazed element could significantly contribute to further mitigation of energy demands for cooling and heating. The façade and roof finishing also affect the energy efficiency, but they should be considered case by case. In exposed sites, with peak occupancy in summer season, reflective surfaces or green roofs should be prioritized. In more secluded sites, where structures are surrounded by deciduous trees (naturally shaded in summer and exposed in winter), reflectivity is not so much of an advantage.

#### *3.3. Energy Demands*

Energy demands were calculated for 6 cases—3 development stages with different operational capacities and two approaches to thermal envelope materialization. The basic climate data, as defined by the Rulebook on Energy Efficiency [33] for both sites are given in Table 7. The data for Banatsko Veliko Selo are somewhat less favourable (lower temperatures and higher HDD values) and were used as input to calculate the energy needed for heating.


**Table 7.** Basic climate data for Ljuba and Banatsko Veliko Selo.

<sup>1</sup> Average temperature during the hating season. <sup>2</sup> Design temperature for sizing the heating system, HDD— Heating degree-days.

The calculated annual energy needs for heating are presented in Table 8. The potential reduction of energy demand resulting from an improved thermal envelope are 40.35%, 35.74% and 35.98% for stages 1–3, respectively. While the enhanced model shows notably better energy performance, the results still remain within the EPC class C (50–100 kWh/m2a).


**Table 8.** Annual energy needed for heating.

The monthly heating energy demands for all six scenarios is shown in Figure 13. It can be observed that the values of "enhanced case" for one stage almost correspond to the values of "base case" for a smaller stage. Stage 3 enhanced case is lower than Stage 2 base case and Stage 2 enhanced case is just slightly above Stage 1 base case (lower in March). These findings indicate that the heating system and materialization should be chosen to correspond to seasonal sensitivity and the anticipated capacity of the final stage to achieve a balance between the initial costs and the effects throughout the facility's lifespan.

**Figure 13.** Energy needed for heating.

Cooling energy is not calculated since the model was designed to enable natural ventilation whenever necessary. In July and August, the average temperature is 23.9 ◦C and maximums are 28.7 and 28.9 ◦C, respectively (Table 1), which means that the cooling systems (if any) should be designed primarily to meet the specific medical and therapyrelated demands of a particular facility or specific modules within the facility.

The power demands *P* for heating were calculated for two different outside temperature settings:


The power demands *Pd* for the heating with regard to the outside design temperature were calculated based on *Pa*, using the following formula

$$Pd = \frac{Pa \times (t\_i - t\_d)}{(t\_i - t\_a)}\tag{2}$$

where *ti* is the designed interior temperature, *td* is designed exterior temperature and *ta* is the average exterior temperature.

The resulting power demands for the study cases are shown in Figure 14.

**Figure 14.** Power needed for heating.

During initial research, after developing the modularity-based approach, the modules were formed as single-purpose entities (one module for one type of balneological treatment—Figures 7, 11 and 12). This approach was rather successful in many cases, where it provided more options for interaction with the natural environment and versatility of outdoor balneological treatments on semi-private or private patios. However, extending such a concept to more units would be rather demanding because land occupancy, daily operation, supervision and other issues would have to be considered on case-by-case basis. The model used for the calculations is somewhat more compact with paired wellness modules (Figure 8). The initial module would obviously show higher energy demands for heating, but it is expected to be used in facilities that operate only during the summer. In both cases, special attention is dedicated to passive measures for thermal comfort during the summer, thereby minimizing mechanical ventilation and air conditioning while addressing the well-being of users, in addition to enhancing hygiene standards, new occupancy protocols and energy efficiency issues.

Exploring energy performance of three development stages in two types of materialization provided six scenarios where some additional observations can be made regarding thermal envelope components:


**Figure 15.** Distribution of thermal envelope components: (**a**) per surface area and (**b**) per contribution to heat transmission losses.

Regardless of the capacity of the location and the materialization variant, all development models consumed significantly less energy for heating than the geothermal potential provides. At Ljuba, the site with the lower capacity, development model 3 (maximum size) in the basic code-compliant state of materialization, and taking into account extreme external temperatures, yielded 35% of geothermal capacity; at Banatsko Veliko Selo 2, it was only 11%. In the improved variants, these needs are reduced to 13% and 4.7%, respectively.

Developed modules reached a significant level of energy efficiency, not only in heating demands but also in the sustainable use of groundwater. More than 50% of flow capacity was reduced in all three module stages between the base average exterior temperature and the enhanced average exterior temperature (Figure 16). The efficiency calculation was done for the Banatsko Veliko Selo site, where a cascade system with a constant ΔT value (19 ◦C) was adopted. The lower flow capacity may lead to less power consumption by circulation pumps and a low operational cost. In addition, these beneficial effects reflect resource preservation and sustainable exploitation.

Additionally, the results indicated the possibility of the further use of the investigated geothermal potentials, primarily to provide energy for other facilities (healthcare, hospitality) or to heat complementary facilities, even larger pools or other aquatic venues.

#### **4. Conclusions**

Balneological usage presents one of the driving forces for the activation of geothermal resources in modern medical and rehabilitation practices. This paper addressed two sites chosen from previous interdisciplinary research conducted in the Vojvodina region that have different geothermal resources and placed within different urban contexts. Coupling the identified and analyzed geothermal potential with a purposely formulated architectural model for balneological application based on sustainable and green design strategies, the paper illustrated a possible development concept that could serve as an implementation tool for activation at various locations.

The developed architectural model showed good potential both for phase development and for effective and efficient use with adaptable capacities during the low season. The modular approach in placing indoor and outdoor balneotherapy as well as ancillary spaces enables a high level of flexibility with minimized operational costs. The base model was designed as a structure of approximately 500 m<sup>2</sup> of gross area, encompassing all basic wellness and spa activities with proper support spaces (e.g., reception, examination room, changing rooms and technical room). It was a modest start, but rather feasible for the local municipality that met all contemporary standards for this type of program. The largest structure was some 1000 m2 as the result of preliminary design research, which indicated that for larger sites a different design approach should be explored to overcome certain functional shortcomings.

As far as materialization of thermal envelope was concerned, practically all variations remained within the boundaries of EPC class C, but enhanced envelope proposals were very close to EPC class B (for the smallest, initial case it was just below the threshold). Having in mind the abundance of geothermal energy at both sites, the pursuit of enhanced

energy efficiency might be questionable unless the NZEB (nearly zero-energy buildings) standard is targeted. However, Serbia has not yet established any formal NZEB definition.

The abundance of geothermal resources broadens the scope of the wellness and balneotherapies that can be offered within the facility. In this context, further feasibility studies should be conducted to explore more development options for health tourism in the province of Vojvodina.

On the other hand, the focus of further research regarding the architecture of wellness and spa/medical facilities in the Vojvodina region should be on enhanced comfort (while optimizing the construction and operation costs), building technologies that are based on local production, and flexibility of development and operation. The proposed model also includes a set of structured open spaces that were not developed in detail in this study. Some future research should address options for the multifunctional use of such spaces and the potential for their enclosure to provide extended use throughout the year.

Depending on the business model, we can consider this starting model to be not economically demanding and able to serve as a demonstration model to attracting further investment and multiplication in other locations.

**Author Contributions:** Conceptualization, N.C.I. and A.V.; methodology, D.M. and A.V. (hydrogeol- ´ ogy and energy resources); N.C.I. and D.I. (architecture and energy needs), validation, D.I. (energy ´ efficiency); investigation, D.M. and A.V.; resources, A.V. and O.K.; writing—original draft preparation, N.C.I. (architecture and energy needs), and A.V. (hydrogeology and energy resources); writing— ´ review and editing, D.I.; visualization, N.C.I. (architecture and energy efficiency); supervision, D.M. ´ and N.C.I. All authors have read and agreed to the published version of the manuscript. ´

**Funding:** This study was partly funded by the Ministry of Education, Science and Technological Development of Republic of Serbia through project TR 33053 "Research and development of renewable subgeothermal groundwater resources in the concept of increasing energy efficiency in buildings". The previous research—"Geothermal potential of the territory of AP Vojvodina—Resources research, multiparameter valorisation and development projections" was funded by The Provincial Government of the AP Vojvodina—Provincial Secretariat for Energy; "Balneology potential of the territory of AP Vojvodina—Resources research, multiparameter valorisation and development projections" was funded by The Provincial Government of the AP Vojvodina—Provincial Secretariat for Economy and Tourism. The paper was prepared within research laboratory SaRA (Sustainable and Resilient Architecture), University of Belgrade—Faculty of Architecture and within Laboratory for Geothermal Energy and Energy Efficiency, University of Belgrade—Faculty of Mining and Geology.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** Data preparation and technical support for energy modelling: Nikola Mileti´c, PhD student, University of Belgrade—Faculty of Architecture.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **References**


## *Article* **Reuse of Decommissioned Hydrocarbon Wells in Italian Oilfields by Means of a Closed-Loop Geothermal System**

**Martina Gizzi, Glenda Taddia \* and Stefano Lo Russo**

Department of Environment, Land and Infrastructure Engineering, Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy; martina.gizzi@polito.it (M.G.); stefano.lorusso@polito.it (S.L.R.) **\*** Correspondence: glenda.taddia@polito.it

**Abstract:** Geological and geophysical exploration campaigns have ascertained the coexistence of low to medium-temperature geothermal energy resources in the deepest regions of Italian sedimentary basins. As such, energy production based on the exploitation of available geothermal resources associated with disused deep oil and gas wells in Italian oilfields could represent a considerable source of renewable energy. This study used information available on Italian hydrocarbon wells and on-field temperatures to apply a simplified closed-loop coaxial Wellbore Heat Exchanger (WBHE) model to three different hydrocarbon wells located in different Italian oilfields (Villafortuna-Trecate, Val d'Agri field, Gela fields). From this study, the authors have highlighted the differences in the quantity of potentially extracted thermal energy from different analysed wells. Considering the maximum extracted working fluid temperature of 100 ◦C and imagining a cascading exploitation mode of the heat accumulated, for Villafortuna 1 WBHE was it possible to hypothesise a multi-variant and comprehensive use of the resource. This could be done using existing infrastructure, available technologies, and current knowledge.

**Keywords:** renewable energy; geothermal energy; mature oilfield; abandoned hydrocarbon well; wellbore heat exchanger

#### **1. Introduction**

The policy visions of the 2030 UN Agenda for Sustainable Development and the Paris Agreement on Climate Change were both approved by the member states of the UN in 2015 and represent two fundamental contributions that guide the transition towards an economic model that aims not only for profitability but also for social progress and environmental protection. To achieve increased energy efficiency, all nations must change the ways in which they produce and manage natural energy resources in order to create more sustainable and environmentally resilient communities.

The current Italian urban energy paradigm relies heavily on fossil fuels; given the air pollution and resource depletion caused by fossil-fuel use, this is unsustainable. Public and private investments in energy must be increased, especially for innovative technological models that transform the energy system, integrating renewable energy into end-use applications in buildings, transportation and industry.

Geoscience offers solutions to this issue through the development of various options that could encourage decarbonisation and the transition to renewable energy sources at local and regional scales: electricity production from renewable sources, domestic heating/cooling using low-enthalpy geothermal energy resources and larger-scale technologies that target harmful emissions, such as bioenergy production and carbon capture and storage [1].

Geothermal energy is a weather-independent, environmentally friendly and currently available renewable resource; it represents an effective solution for power generation, heating and cooling, and direct-use applications. Specifically, energy production from available low- to medium-temperature geothermal resources associated with disused deep

**Citation:** Gizzi, M.; Taddia, G.; Lo Russo, S. Reuse of Decommissioned Hydrocarbon Wells in Italian Oilfields by Means of a Closed-Loop Geothermal System. *Appl. Sci.* **2021**, *11*, 2411. https://doi.org/10.3390/ app11052411


Academic Editor: Woochul Kim

Received: 29 January 2021 Accepted: 3 March 2021 Published: 9 March 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

oil and gas wells in Italian oilfields has considerable potential. As a renewable energy source, it could solve problems associated with suspended wells near municipalities and allow for longer-term use of hydrocarbon wells, even at the end of their production cycle, which would benefit industry, civil and agriculture districts. According to [2], the variety of possible direct applications of geothermal resources in production districts, together with the corresponding temperature demand, is wide. It includes space heating, industrial uses, swimming pools, horticulture (especially greenhouses) and aquaculture.

Studies have shown that hydrocarbon reservoirs and geothermal energy resources can coexist in sedimentary basin contexts because both have similar reservoir conditions [3–5]. Hydrocarbon resources are generated under specific temperature and pressure conditions in the source rocks, and groundwater is always involved in both the primary migration of oil from the source rock and the secondary migration of oil and gas to the reservoir. Consequently, oil and gas reservoirs in hydrocarbon basins act as geothermal reservoirs.

Sedimentary basins have been explored for both oil and gas extraction purposes, so deep hydrocarbon wells are located in this geological context. Well logs, temperature distribution profiles and reservoir properties, such as depth to basement and geological formation thickness, are generally well known.

Since 1985, 7246 wells have been drilled for hydrocarbon extraction in Italy, 898 of which are located onshore with varying operational statuses [6]. A variety of oil and gas reservoirs have been identified; geological and geophysical exploration campaigns into the deepest regions of such geological contexts have ascertained the coexistence of hydrocarbons and the low- to medium-temperature geothermal energy resources [7,8].

Recent investigations have attempted to assess geothermal potentials, exploring deep geothermal resources in different regions and reconstructing heat flow maps at different depths [9,10].

Additionally, in order to determine heat conductivity values at the regional and local scales, the thermal conductivity of 200 rock samples collected from four different regions of southern Italy (Calabria, Campania, Apulia and Sicily) was investigated by [11], measuring its value in both dry and wet conditions. Morover, in reference [12] it was utilised the framework of the MIUR−2008 project "Geothermal resources of the Mesozoic basement of the Po Basin: groundwater flow and heat transport" to accurately estimate the thermophysical properties of a wide variety of sedimentary and intrasedimentary volcanic rocks from the Po Basin through laboratory measurements of density and porosity.

Since it is stored in subsurface geological formations and associated with hydrocarbons, geothermal energy must be extracted before it can be used. Decommissioned or disused oil and gas wells, especially those in mature oilfields, are good candidates for geothermal heat exploitation and may provide access to subsurface energy resources.

Considering the large number of existing oil wells dismissed every year in Italian fields, various studies have proposed new engineering tools to evaluate the possible use of such thermal resources. In reference [13], it was developed an effective method for doing so through the use of a closed-loop system with associated Wellbore Heat Exchanger (WBHE) technology. Unlike in conventional open-loop geothermal systems, heat carrier fluids in closed-loop systems circulate inside WBHEs, with no ground fluids extracted from surrounding rocks.

Despite the recent success of theoretical oilfield geothermal closed-loop system experiments, certain challenges remain for understanding the possibility of large-scale access to geothermal resources in oilfields using closed-loop technologies. Additionally, due to the continuous spatial variability of geological formations associated with deep oil and gas wells in oilfields, the thermophysical parameters of geological strata surrounding the well, as well as the depth and thickness of the strata, must be considered to achieve accurate and realistic estimates of heat exchanger performance.

Given the above considerations, this study aims to contribute to the discussion, encouraging a reflection on the potential benefits and limitations of using low- to medium-

temperature geothermal energy resources associated with dismissed Italian hydrocarbon wells as a renewable source of energy.

Using the information on Italian hydrocarbon wells and on-field temperatures available from both the National Mining Office of the Italian Ministry for Economic Development (MISE) and the Italian National Geothermal Database, the simplified heat exchange model (coaxial WBHE) described in [13] was applied to three hydrocarbon wells. The main purpose was to properly analyse heat exchange mechanisms in three different Italian oil and gas fields, emphasising that the quantity of the potentially extracted thermal energy can change based on geological and depositional context. Wells from the Villafortuna–Trecate field, the Val d'Agri field and the Gela field were selected.

The assumption that the thermophysical parameters (thermal conductivity, volumetric heat capacity and rock density) are constant values, as described in [13], has been overcome by applying the elaborated model to the detailed stratigraphic data of each case study.

The final use of the potentially extracted heat would be in possible direct applications by means of a cascade plant system, which provides specific thermal energy amounts to production cycles in manufacturing, agricultural and recreational districts near the oilfields. Considering specifically the examples of disused hydrocarbon wells analysed, located in the Italian territory, it is possible to state that they are located far from inhabited areas or buildings: it is therefore not generally possible to hypothesize the use of the extracted thermal energy for heating purposes.

#### **2. Materials and Methods**

#### *2.1. Oilfields in Italian Sedimentary Basins*

To introduce the analysis of the selected case studies, this paper provides an overview of the main sedimentary systems and associated hydrocarbon fields.

The Italian landscape is dominated by the Southern Alps and the Apennines mountain chains. The two chains are bordered in their outer margins by well-developed sedimentary foreland basins, especially along the Adriatic sectors, and by relatively wide foreland areas (e.g., the Po Plain, Adriatic Sea and Hyblean Basin). Due to Italy's complex geological and sedimentary history, a variety of petroleum systems have been identified. Authors provided overviews of the Italian peninsula's geological evolution and described how Italian hydrocarbon occurrences can be classified by their association with three main tectono-stratigraphic systems (Figure 1) [14–16]: (1) carbonate Mesozoic substratum of the foredeep/foreland area and of the external thrust belts; (2) thrusted terrigenous Oligo-Miocene foredeep wedges (Southern Alps, Northern Apennines, Calabria and Sicily); and (3) terrigenous Pliocene-Pleistocene successions of the late foredeep basins of the Apennines, in both the central and northern Adriatic Sea and the Po Plain.

According to [17], at least five important source rocks have been recognised, with ages ranging from the Mesozoic through the Pleistocene. Three of the source rocks were deposited during Mesozoic crustal extension and are mainly oil-prone. Hydrocarbon occurrences associated with these sources are usually found in complex carbonate structures along the Apennines thrust-and-fold belt and in the foreland; the Villafortuna–Trecate (Po Plain), Val d'Agri-Tempa Rossa (southern Apennines) and Gela (Sicily) fields represent the largest oil accumulations pertaining to these systems.

The two other source rocks were deposited in the foredeep terrigenous units of the foreland basins, which formed during the Cenozoic orogenesis. The older of the two sources is thermogenic gas-prone and is found in the highly tectonised Oligo-Miocene foredeep wedges; gas occurrences associated with this source are mainly concentrated along the northern Apennines margin (e.g., the Cortemaggiore field), in Calabria (e.g., the Luna field) and in Sicily (e.g., the Gagliano field). The younger source is biogenic gas-prone and is situated in the outer Plio-Pleistocene foredeeps.

Reference [17] clearly showed the geographic limitation of the oil-prone Villafortuna– Trecate Middle Triassic and Val d'Agri-Tempa Rossa Cretaceous systems, as opposed to the

wide distribution of the Late Triassic–Early Jurassic system and of the biogenic gas-prone Plio-Pleistocene systems (Figure 1).

**Figure 1.** Stratigraphic and geographic location of the Italian petroleum systems [17].

This study focuses on the analysis of geothermal energy potential associated with disused hydrocarbon wells of the different petroleum systems of the Mesozoic carbonate succession. Three deep hydrocarbon wells located inside of the Villafortuna–Trecate oilfield, the Tempa Rossa Field and the Gela Field are discussed in detail.

Detailed information related to the litho-stratigraphic units and temperature data visualisation can be found on The Italian National Geothermal Database (BDNG), the largest collection of Italian geothermal data. BDNG was established in the 1980s and implemented by the Institute of Geosciences and Earth Resources (IGG) of the National Research Council (CNR) of Italy [9].

Information regarding productive and dismissed oil and gas wells in Italy was also provided by the National Mining Office of the Italian Ministry for Economic Development (MISE) and by the website of the VIGOR, promoted by the MISE-DGRME (Direzione Generale Risorse Minerarie ed Energetiche), the Italian Geological Society and the Assomineraria Association.

#### 2.1.1. The Villafortuna–Trecate Field

First, data were analysed for the geothermal potential associated with the disused Villafortuna 1 hydrocarbon well, located within the Villafortuna–Trecate oilfield (Figure 1). The Villafortuna–Trecate system represents one of the largest oil accumulations of the Italian Middle Triassic carbonate petroleum system. The petroleum system is wholly developed inside the Triassic succession and consists of two main reservoirs, composed of dolomitised carbonate platform rocks and a set of source rocks deposited in the adjacent anoxic intra-platform basins.

Generally, the depth of the main reservoir associated with the Villafortuna–Trecate field is between 5800 m and 6100 m deep, with temperatures of approximately 160–170 ◦C [18]. Because of its depth, the main reservoir can be pursued only in the outer sector of the foredeeps and in foreland regions (the Piedmont area); areas along the thrust belt (the Po Plain) are generally too deep.

As demonstrated by information related to the litho-stratigraphic units and temperature data visualisation reported in Table 1 and Figure 2, the stratigraphic succession is mainly composed of clastic sedimentary and carbonate rocks. The analysed well has a maximum depth of 6202 m and temperatures reaching 165 ◦C.

**Table 1.** Villafortuna 1 hydrocarbon well-lithostratigraphic profile (Villafortuna Trecate Oilfield, Western Po Plain).


**Figure 2.** Temperature data visualisation for the Villafortuna 1 hydrocarbon well: Depth (m); Temperature (◦C).

2.1.2. The Val d'Agri-Tempa Rossa Field

The second case study, the Tempa Rossa 1D hydrocarbon well, is located within the Tempa Rossa oilfield (Figure 1). The Tempa Rossa system lies in the Mesozoic carbonate substratum of the foredeep and foreland areas and in the external thrust belt of the southern Apennines. The system bears the largest oil and gas accumulations in Italy, including the Val d'Agri and Tempa Rossa oil fields.

The reservoirs are composed of fractured limestones from the buried Apulia Platform, which extends from the Cretaceous to the Miocene. The majority of the oil column exceeds 1000 m and sometimes exceeds 2000 m. The seal is composed of Lower Pliocene shales. The source rocks, identified in a few deep wells of the area, are mainly Albian–Cenomanian and are marine anoxic carbonates facies containing sulphur [19].

Unlike the Villafortuna 1 hydrocarbon well, the Tempa Rossa 1D well features lithostratigraphic units mainly composed of sandstone and associated shales (Table 2). The maximum depth of the analysed well is 5042 m; temperatures reach 107 ◦C (Figure 3).

**Table 2.** Tempa Rossa 1D hydrocarbon well-lithostratigraphic profile (Tempa Rossa Field, Western Po Plain).


**Figure 3.** Temperature data visualisation for the Tempa Rossa 1D hydrocarbon well: Depth (m); Temperature (◦C).

#### 2.1.3. The Gela Field

The third case study is the Gela 38 hydrocarbon well, located in the Gela oilfield in Sicily. The Gela field is part of the Late Triassic–Early Jurassic petroleum system and is linked to the main phase of the Tethyan rifting. It is the most explored of the three systems, both in the foreland and in the thrust belt, and from Lombardy to Sicily. The source rocks are terrigenous or mixed carbonate-terrigenous and were deposited during the anoxic stage that preceded the extension of the Jurassic basins.

The Ragusa-Gela fields were discovered in the 1950s and have been the largest source of Italian oil for some of the past decades. The reservoir is composed of fractured, massive dolomites of the Upper Triassic Gela Formation. The traps are large-scale, probably polyphase anticlines bounded by high-angle normal faults.

Based on the available lithological and temperature data reported in Table 3 and Figure 4, the area's stratigraphic succession is composed of marl, calcareous marl and clays. The well has a maximum depth of 3446 m and temperatures that reach 85 ◦C.


**Table 3.** Gela 38 hydrocarbon well-lithostratigraphic profile (Gela Field, Sicily).

**Figure 4.** Temperature data visualisation for the Gela 38 hydrocarbon well: Depth (m); Temperature ( ◦C).

#### *2.2. Closed-Loop Geothermal Energy Systems: WBHEs*

Exploiting the geothermal energy resources associated with decommissioned hydrocarbon wells requires that the borehole to be retrofitted with a heat exchanger. In current practice, two main types of closed-loop systems are used to harness geothermal energy resources by taking advantage of disused boreholes in oilfields: U-tube and coaxial doublepipe WBHE technologies [20,21]. Both kinds of systems allow for the extraction of heat from the ground without extracting or re-injecting any geothermal fluids.

In U-tube heat exchangers, fluid is pumped through one tube string and comes out the other, while coaxial heat exchangers are composed of two concentric pipes, as shown in Figure 5 and Table 4. In coaxial WBHEs circulating working fluid is injected into the outer pipe (the injection pipe), flows down to the lower part of the exchanger and is gradually warmed by heat from the rocks. After the fluid reaches the bottom hole of the well, it flows upwards through a thinner pipe, which acts as the inner pipe (the extraction pipe). The gap between internal pipes is filled with an insulating material, and the bottom hole is sealed. Heat exchange occurs between the geological formation and the fluid in the injection pipe and between the fluid in the injection pipe and the fluid in the extraction pipe.

**Figure 5.** Schematic representation of a coaxial Wellbore Heat Exchanger (WBHE).


**Table 4.** Coaxial WBHE—geometric parameters.

Corrosion must be considered when selecting this type of system; it may be ideal for heat exchange, but it may also reduce the system's operational life. Due to its low cost and its heat transfer and storage capacity, water is still one of the most commonly used fluids. The operating parameters of subsurface closed-loop systems, such as fluid flow rate and pipe diameter, should be selected to guarantee transient turbulent flow conditions, since these conditions facilitate heat transfer, and low hydraulic head losses, since this indicates lower energy expenditure on circulation pumping.

Compared to U-tube heat exchangers, coaxial heat exchangers have a higher surface area and a higher volume of the working fluid through which heat exchange occurs. As a result, under the same injection rate conditions (q), the fluid flow velocity in the coaxial pipe system and the hydraulic pressure required for fluid circulation can be lower, resulting in decreased energy consumption from pumping [21]. The coaxial geometry of a doublepipe heat exchanger also has the advantage of reducing thermal resistance between the circulating fluid and the wellbore.

In this paper, in order to evaluate and analyse the temperature profiles of the selected fluid (water) associated with a coaxial double-pipe WBHE technology, the simplified models proposed below were implemented in MATLAB and applied to the selected case studies: the Villafortuna 1, Tempa Rossa 1D and Gela 38 hydrocarbon wells.

For the elaborated model, two main assumptions were considered: the propagation of heat in the reservoir occurs by means of conduction (convection phenomena are neglected), and the propagation of heat inside the wellbore tubes takes place through both conduction and convection.

#### *2.3. Heat Transfer in Coaxial WBHEs*

In coaxial WBHEs, the steel downward tube is cemented to the rock wall and so is in contact with the hole in the well. The energy balance of the fluid in the injection pipe can be expressed with the following equation:

$$\frac{\partial \left( (\rho c)\_f A\_0 T\_{fo} \right)}{\partial \tau} + \frac{\partial \left( (\rho c)\_f A\_0 \upsilon\_f T\_{fo} \right)}{\partial z} = -\frac{dQ}{dz} + \frac{dQ\_{i0}}{dz} \tag{1}$$

where *A*<sup>0</sup> and *vf* are the outer pipe area and fluid velocity, respectively; *Tfo* is the fluid temperature in the outer pipe; and *dQ*/*dz* is the heat extraction from the formation at unit well depth (W/m).

Although insulation is used to prevent heat loss from the inner-pipe fluid, heat is partly transferred between the two pipes, so *dQio*/*dz* represents the heat flux from the inner pipe to the outer pipe. Therefore, the energy equation for the inner pipe can be given as: 

$$\frac{\partial \left( (\rho c)\_f A\_i T\_{fi} \right)}{\partial \tau} + \frac{\partial \left( (\rho c)\_f A\_i v\_f T\_{fi} \right)}{\partial z} = -\frac{dQ\_{i0}}{dz} \tag{2}$$

By assuming steady heat transfer and constant heat flux in wellbore components (e.g., insulation, casing and cement), heat extraction from the formation *dQ*/*dz* can be assumed to be equal to the heat flux through the outside surface of the wellbore (interface of the wellbore and rock formation) to the injected fluid [22]:

$$\frac{d\mathbb{Q}}{dz} = 2\pi r\_w k\_w \left( T\_{f0} - T\_w \right) = \left( T\_{f0} - T\_w \right) / R\_w \tag{3}$$

where *Tw* is the temperature at the interface of the wellbore and the formation, *kw* is the heat transfer coefficient between the outer-pipe fluid and wellbore exterior and *Rw* is the resistance between the outer pipe and surrounding rocks.

At the well bottom, the heated fluid is forced to enter and flow through the internal pipe of the coaxial WBHE. Going up to the wellhead, heat transfer occurs only through the wall of the internal pipe. Thus, *dQi*0/*dz* is determined by considering the temperature difference between the outer-pipe and inner-pipe fluids, as well as the estimated thermal resistance of the insulation:

$$\frac{dQ\_{i0}}{dz} = 2\pi \mathbf{r}\_0 k\_{i0} \left( T\_{fi} - T\_{f0} \right) = \left( T\_{fi} - T\_{f0} \right) / R\_{i0} \tag{4}$$

where *Tfi* is the fluid temperature in the inner pipe, *ki0* is the heat transfer coefficient between the outer pipe and inner pipe and *Ri0* is the thermal resistance between the outer pipe and inner pipe.

2.3.1. Coaxial WBHE: Coefficient of Heat Exchange between Outer-Pipe Fluid and the Wellbore Exterior

In an analysis of the energy balance equation for the fluid in the outer pipe (injection pipe) of a coaxial WBHE, a careful estimate of the parameter *kw* is fundamental for proper evaluation of the heat exchange between the outer-pipe fluid and the drilled geological formations.

For a coaxial WBHE, the heat exchange coefficient for the injection pipe can be expressed as the sum of heat transfer components, expressed in terms of thermal resistance values (*Rw*) [22]:

$$R\_w = \ \ R\_s + \ R\_d + \ \ R\_c \tag{5}$$

where *Rs* is a function of time that represents the thermal resistance due to conductive heat transfer in the rock, *Ra* is the thermal resistance due to convective heat transfer into the pipe and *Rc* is the thermal resistance due to conductive heat transfer through the casings of the well.

In the evaluation of total thermal resistance, the conductive term prevails; consequently, the thermal exchange is directly proportional to the convective transfer coefficient.

Conductive thermal resistance (*Rs*) can be expressed as follows:

$$R\_s = \frac{1}{2\lambda\_s} \ln \frac{2\sqrt{\alpha\_s t}}{r\_w} \tag{6}$$

where *λ<sup>s</sup>* (W/mK) is the thermal conductivity of the rock and *α<sup>s</sup>* (m/s) is the thermal diffusivity of the rock. In Equation (6), the relationship in the numerator of the second term represents the time-dependent radius of the thermal influence of the well (*rs*).

Convective thermal resistance (*Ra*) can be determined by the following equation:

$$R\_d = \frac{1}{2r\_c h\_f} \tag{7}$$

where *rc* is the external radius of the external casing and *hf* is the convective heat transfer coefficient, which was calculated by using the Nusselt number (Nu) and a form of the Dittus-Boelter equation that assumes turbulent flow inside the tubes (Reynolds number ≥ 104) [23]:

$$h\_f = \frac{Nu\lambda\_f}{2r\_c} \tag{8}$$

$$Nu = 0.023 Re^{0.8} Pr^{0.4} \tag{9}$$

with *Pr* <sup>=</sup> *<sup>ρ</sup><sup>c</sup> <sup>f</sup> <sup>μ</sup> λf* and *Re* <sup>=</sup> *<sup>ρ</sup><sup>v</sup> <sup>f</sup>* <sup>2</sup>*rc <sup>μ</sup>* .

Finally, the thermal resistance to heat conduction through the casings of the well can be determined as follows:

$$R\_{\mathcal{E}} = \sum\_{i=1}^{n} R\_{\lambda i} = \frac{1}{2} \sum\_{i=1}^{n} \frac{1}{\lambda\_i} \ln \frac{r\_{c,i+1}}{r\_{c,i}} \tag{10}$$

where *λ<sup>i</sup>* is the thermal conductivity of the rock in correspondence with the different casings of the well. Generally, due to the high thermal conductivity of the steel piping, the total thermal resistance of the casing is negligible compared to the rock thermal resistance [13].

As a result, the heat exchange coefficient *kw* can be correctly determined as follows:

$$\frac{1}{k\_w} = \frac{2r\_\varepsilon}{2\lambda\_s} \ln \frac{4\sqrt{\alpha\_s t}}{2r\_w} + \frac{1}{h\_f} \tag{11}$$

where *rc = rw* as the thickness of the external tube is negligible.

2.3.2. Coaxial WBHE: Coefficient of the Heat Exchange between the Outer-Pipe Fluid and the Inner Pipe

Unlike in the injection pipe, the total heat flux in the upward pipe (extraction pipe) is determined by a conductive component of the composite pipe and by two convective components, one on the internal wall and one on the external wall of the WBHE.

Consequently, the total heat exchange coefficient *ki0* for the extraction pipe can be calculated as follows:

$$\frac{1}{k\_{i0}} = \frac{r\_0}{r\_{0+d}} \frac{1}{h\_i} + r\_0 \sum\_{i=1}^n \frac{1}{\lambda\_i} \ln\left(\frac{r\_{i+1}}{r\_i}\right) + \frac{1}{h\_0} \tag{12}$$

where *r*<sup>0</sup> is the radius of the inner pipe, *d* is the thicknesses of the pipe exchanger, *h*<sup>0</sup> and *hi* are the coefficients of convective heat transfer to the inner and outer wall, respectively, and *λ<sup>i</sup>* is the thermal conductivity of the pipe material.

#### *2.4. WBHE Model Assumptions*

MATLAB is a software for numerical and statistical calculations that is written in the C programming language. MATLAB R2018b version was used to perform the analysis of WBHEs by implementing the proposed model, with consideration of the following assumptions and approximations.

The reservoir model was built by assuming a single well-positioned at the centre of a circular reservoir. The temperature profile in the radial direction was assumed to be constant. Therefore, there was no temperature gradient in the annulus or in the inner tube. Due to turbulent flow, enhanced mixing phenomena occurred, decreasing the radial gradient. The temperature changed only in the annulus and in the vertical direction of the inner tube, so the temperature profile was vertically unidirectional.

The properties of the heat-carrier fluid were assumed to be constant. As the fluid used in this study was water (100 ◦C, 2 bar), no variations occurred due to pressure or/and temperature gradients.

The model was built under steady-state conditions; there were no temperature variations over time, with each point in the tubes (annulus and inner tube) maintaining the same temperature for the lifecycle of the system. For the elaborations, *rc* (external radius of the external casing) was considered equal to *rw* (radius of outside wellbore).

In addition, the model considered the resistance associated with tube thickness to be negligible. The tube material had very high conductivity, so its resistance could be considered small compared to the other resistances in the system. For estimation of the resistance associated with the rock (see Equation (6)), the time value used was 3 years after the starting of the system. This assumption made the method a conservative estimate: the system in the years before (1–2 years) turns out in fact to work better. As consequence, the heat exchange phenomena are bigger with the possibility of causing overestimations.

The proposed model followed the path of the fluid using a step-by-step approach. It considered intervals of length *dz* in which the inlet and outlet temperatures were calculated by solving the energy balance equation for each volume *dv*. For estimating the energy exchange in the radial direction, the mean value of the temperature in each volume *dv* was used.

#### **3. Results and Discussion**

The temperature profiles associated with the analysed coaxial WBHE system configurations were obtained by making use of the specific ground properties of the selected case studies (the Villafortuna 1, Gela 38 and Tempa Rossa 1D hydrocarbon wells). Values relating to the thermal properties of the different rock formations have been attributed in accordance with references [11,12] (Tables 1–3).

For all cases analysed, the flow rate of the fluid and the inlet temperature were first considered as 3 kg/s and 50 ◦C, respectively. This temperature is a typical value for direct applications like production cycles in manufacturing and agricultural districts [2]. Subsequently, an analysis was conducted on the thermovector fluid temperature at the outlet as the inlet flow rate varied.

The sizing of the inner and outer tubes, as well as the final casing size, were fixed according to the values proposed in Figure 6 and Table 5. The thermal conductivity value of the insulating material was set to 0.025 W/mK.

**Figure 6.** Coaxial WBHE geometry: considered configuration.


**Table 5.** WBHE tube sizing—ID: internal diameter, OD: external diameter.

In the first section of an external pipe of a coaxial WBHE system, the downward fluid is in thermal contact with both the ground on one side and the upward tube on the other.

Because of its thermal properties, the ground in contact with the external piping provides a negative heat contribution, while the inner tube of a coaxial WBHE provides a positive one. As the negative contribution is usually larger, the water temperature (working fluid temperature) decreases slightly. This behaviour can be observed by analysing the obtained temperature profiles associated with the Gela 38 and Tempa Rossa 1D hydrocarbon wells, in which the thermovector fluid was cooled at depths of up to 1800 m and 1200 m, respectively. Unlike in the Gela 38 and Tempa Rossa 1D hydrocarbon wells, the thermovector fluid maintained a constant temperature (50 ◦C) for the first portion of the borehole (1400 m) in the Villafortuna 1 hydrocarbon well (Figures 7a, 8a and 9a). The presence of a thick stratigraphic horizon made up of terrigenous sedimentary deposits (Table 1), which are characterised by very low conductivity and specific heat values, negatively influenced the heat exchange and so limited heat dispersion.

**Figure 7.** (**a**) Temperature profile associated with the coaxial WBHE configuration considering the site-specific stratigraphy of Villafortuna 1 well. (**b**) Wellhead temperature behaviour as the flow rate changes.

**Figure 8.** (**a**). Temperature profile associated with the coaxial WBHE configuration considering the site-specific stratigraphy of Tempa Rossa 1D well. (**b**) Wellhead temperature behaviour as the flow rate changes.

**Figure 9.** (**a**) Temperature profile associated with the coaxial WBHE configuration considering the Scheme 38 well. (**b**) Wellhead temperature behaviour as the flow rate changes.

However, when the water and ground temperatures approached similar values in all three hydrocarbon wells, the negative contribution decreased, and water temperature increased. As the downward water profile line crossed the ground temperature line, the ground's heating process began, and the ground contribution became positive. Due to the presence of the insulating material, the heat exchange coefficient between the annulus and the inner tube was quite low, and the increase in the working fluid temperature could be mainly associated with the contribution of the ground.

Using the fixed inlet working fluid temperature (50 ◦C) and the estimated fluid temperature at the outlet for the three case studies (Figures 7a, 8a and 9a), thermal power values were evaluated for 627.9 KW (100 ◦C, Villafortuna 1), 75.3 KW (56 ◦C, Tempa Rossa 1D) and 100.5 KW (58 ◦C, Gela 38).

Considering a cascading exploitation mode of the heat accumulated by the working fluid water in Villafortuna 1 WBHE, it is correctly possible to hypothesize a multi-variant and comprehensive use of the resource. The outflow temperature of geothermal water at the wellhead is 100 ◦C, which allows it to progressively be used for greenhouse heating (100–80 ◦C), domestic hot water and food industry (80–70 ◦C), animal breeding (60 ◦C), biomass and agricultural culture (50 ◦C). On the contrary, the thermal load accumulated in correspondence with Tempa Rossa 1D and Gela 38 wells turns out to be sufficient neither to justify the costs of plant retrofitting nor to plan a cascading exploitation of the geothermal fluid produced.

As can be seen in Figures 7b, 8b and 9b, the inlet flow rate strongly influenced the temperature of the wellhead thermal fluid. In each case, it is possible to identify an inlet flow rate value (kg/s) in order to obtain a higher fluid temperature at the outlet and optimize the quantity of extracted thermal power. When considering inlet flow rate values between 0.5 and 0.8 kg/s, the output fluid temperature increased up to about 122 ◦C (0.8 kg/s, Villafortuna 1), 65.5 ◦C (0.7 kg/s, Gela 38) and 72 ◦C (0.5 kg/s Tempa Rossa 1D). For higher values, the trend was inverted in all three cases.

However, for coaxial WBHE systems like the one analysed, such low inlet flow rates may not be technically appropriate. The heat-exchange modalities are in fact not the only aspect that must be considered to carry out the correct analysis of the heat transfer mechanisms associated with coaxial WBHE. Pressure loss phenomena need to be analysed as they affect pumping costs and are not negligible in the management of a closed-loop geothermal system. In coaxial WBHE, the inlet flow rate strongly influences the temperature of the wellhead thermal fluid. As pressure losses are proportional to the velocity, an increase in the inlet flow rate values will cause an increase in the required pumping power.

#### **4. Conclusions**

Over time, the petroleum systems in Italian sedimentary basins have been explored for oil and gas extraction. Since 1985, 7246 wells have been drilled for hydrocarbon extraction. Of these, 898 wells are located onshore with various operational statuses. Geological and geophysical exploration campaigns have ascertained the coexistence of low- to medium-temperature geothermal energy resources in the deepest regions of such geological contexts. As such, energy production based on the exploitation of available lowto medium-temperature geothermal resources associated with disused deep oil and gas wells in Italian oilfields could represent a considerable source of renewable energy.

This study used information on Italian hydrocarbon wells and on-field temperatures to apply a simplified coaxial WBHE model to three hydrocarbon wells. The main purpose was to analyse heat exchange mechanisms in three different Italian oil and gas fields (the Villafortuna-Trecate, Val d'Agri and Gela fields), emphasizing differences in the quantity of extracted thermal energy and considering different geological and depositional contexts.

All calculations considered the detailed stratigraphic data and related thermophysical parameters (e.g., thermal conductivity, volumetric heat capacity and rock density) of each case study. The results indicate a substantial difference in the potential amount of extracted thermal energy between analysed sites, located in different Italian sedimentary contexts. With a fixed inlet working fluid flow rate of 3 kg/s and a fixed temperature of 50 ◦C, thermal power values were evaluated at 627.9 KW for Villafortuna 1, 75.3 KW for the Tempa Rossa 1D well and 100.5 KW for the Gela 38 hydrocarbon well. The Villafortuna 1 WBHE recorded a maximum extracted fluid temperature of 100 ◦C, which allows us to hypothesize a cascading exploitation mode of the heat accumulated. Unlike Villafortuna 1, Tempa Rossa and Gela fields had thermal load values that would be of no practical use. As such, the implementation of a coaxial borehole heat exchanger in a such hydrocarbon well may not be energetically or economically worthwhile.

Improving the accuracy of the proposed models by means of future analysis is required: the basic assumption related to the constancy of the properties of the water as working fluid must be overcome by properly analysing the possibility of having phase change (evaporation) in the well, which would change the proposed models. The role in heat transfer and performance of extracting heat from abandoned wells of intraformational flows also needs to be properly considered.

Despite the above considerations, the analysis approach with the associated simplified model proposed in this paper could represent a useful simplified methodological tool to allow the preliminary definition of the possibility of a selected Italian hydrocarbon well to be converted into a geothermal one by means of a coaxial-WBHE technology.

After a preliminary analysis about the presence of industries and agricultural districts, it will be useful to produce a detailed evaluation of the industrial plants available in the

area near the Villafortuna 1 well: a technical feasibility and cost–benefit analysis of the selected configuration in this proposed case study could represent the subject of future research work.

**Author Contributions:** All the authors conceived the research work aim; all the authors contributed in finding materials and analysis tools. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Information related to Italian hydrocarbon wells and temperature data are available at the web page of the National Mining Office of the Italian Ministry for Economic Development (MISE) and the Italian National Geothermal Database.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Abbreviations**


#### **References**

