Performance enhancement, economic analysis, and futuristic insight of single-well medium-deep and deep geothermal systems

Abstract

Geothermal energy is clean and renewable, derived from the heat stored within accessible depths of the Earth’s crust. The adoption of a single-well system for medium-deep and deep geothermal energy extraction has attracted significant interest from the scientific and industrial communities because it effectively circumvents issues such as downhole inter-well connections and induced seismicity. However, the low heat transfer capacity in geothermal formations limits the heat extraction performance of single-well systems and hinders their commercial deployment. This review covers various enhancement concepts for optimizing the heat transfer within single-well systems, emphasizing critical parameters such as heat transfer area, heat transfer coefficient, and temperature difference. Additionally, it presents the thermo-economic evaluation of different configurations of single-well borehole heat exchangers and super-long gravity heat pipes (SLGHPs). The SLHGP, utilizing phase-change heat transfer, is recognized as a highly effective and continuously productive technology, capable of extracting over 1 MW of heat. Its pumpless operation and ease of installation in abandoned wells make it cost-effective, offering a promising economic advantage over traditional geothermal systems. It also highlights the challenges and potential research opportunities that can help identify gaps in research to enhance the performance of single-well geothermal systems.

Graphical abstract

Keywords

borehole heat exchanger / coaxial downhole heat exchangers / geothermal energy exploitation / single-well system / super-long gravity heat pipe

1 Introduction

Geothermal resources are characterized by their renewability, sustainability, ample reserves, and independence from seasonal and climatic conditions. Geothermal energy has a capacity factor exceeding 73%, which is 3–4 times greater than wind energy (20%) and 4–5 times greater than solar energy (14%) [1]. Positioned as a viable alternative to the unsustainable utilization of fossil fuels, geothermal energy offers stability, continuity, and local accessibility [2] in response to the pressing need for global carbon neutrality and climate change mitigation [3,4]. Additionally, geothermal energy has an estimated worldwide technical potential of 120 to 1100 EJ per year, capable of meeting around 5% of global heating and cooling needs and 3% of electricity requirements by the year 2050 [5,6].

In general, geothermal resources are classified by depth into shallow (less than 0.2 km), medium-deep (0.2–3 km), and deep (greater than 3 km) [7]. Conventional geothermal resources, such as hydrothermal systems with hot springs located near the Earth’s surface, have demonstrated maturity and commercial viability [8]. However, these hydrothermal resources confront certain limitations regarding geographic distribution and capacity for large-scale electricity generation [9,10]. The operation of hydrothermal systems relies on the presence of natural hot geothermal water reservoirs, which are highly dependent on hydrogeological conditions [11,12]. Moreover, the energy extracted from shallow depths by heat pumps can only be used for small to medium-scale direct application (heating and cooling) [13]. To facilitate the widespread adoption of geothermal energy applications and large-scale electricity generation with greater sustainability, exploration of higher temperature geothermal reserves in the deep earth is imperative. Hot dry rock (HDR), typically found at depths of 3–10 km with temperatures generally ranging between 150 and 650 °C [14], has emerged as a critical focus for current and near-future investigations [15,16].

The enhanced geothermal system (EGS) was proposed for the exploration of HDR resources and requires drilling at least two wells, one designated for production and the other for injection. Rock-fracturing techniques, such as hydraulic stimulation, are used to create an artificial fracture reservoir that connects the injection and production wells. Currently, however, the utilization of EGS techniques to extract HDR energy from deep geothermal resources is notably less mature for successful marketing, primarily due to challenges associated with high drilling costs, difficulties in artificial reservoir stimulation, and working fluid loss [10,17]. Additionally, EGS also faces the potential hazard of inducing earthquakes; the microseisms triggered by high-pressure fluids may result in the generation of large quakes [9,18,19].

In comparison to the EGS system, the single-well closed system stands out by eliminating the need for additional wells to re-inject geothermal fluid to maintain the subsurface water table. This system offers several advantages over dual- and multi-well systems, including reduced exploration risk and cost, no working fluid loss, land conservation, minimal ecological repercussions, and a conspicuous absence of seismic activity [20].

Single-well systems can be classified as open and closed systems based on the circulation of working fluid in the borehole reservoir. An open single-well system depends on distinct well components (such as casing and tubing) connecting to injection layers or production layers that are isolated from one another [21–23]. However, the practical application of the open geothermal extraction technique is hindered by challenges associated with geofluid disposal, scale deposition, corrosion [21,24–27], and reservoir creation. In contrast, in closed models, working fluids are circulated within the borehole heat exchanger surrounded by geothermal formations. Since there is no direct fluid-rock contact, this approach is more appealing from both geological and reservoir risk perspectives.

The downhole heat exchanger (DHE) is a traditional closed-loop single-well system that typically consists of coaxial pipes through which the working fluid is pumped to extract heat [28]. DHEs have been widely implemented for the direct utilization of geothermal resources, including space heating and power generation via an Organic Rankine Cycle machine [24,29–31]. Coaxial downhole heat exchanger (CDHE) consists of an inner steel tube that is covered by an annulus pipe as casing. The working fluid is injected down through the annulus space, heated by surrounding hot rock, and then returns back to the surface through inner pipe, as depicted in Fig.1(a), Compared with U-shaped DHEs, CDHEs are able to decrease pressure drop by about 65% across a range of flow rates (from 0.1 to 0.7 L/s) [32] and can operate with higher flowrates [33]. Therefore, CDHEs have garnered research interest for deep energy extraction, due to their ability to facilitate increased borehole heat exchange with reduced pumping power [34,35].

Despite these advantages, heat transfer in CDHE relies on forced single-phase convection of working fluid. To achieve high heat extraction, the flow rate of the working fluid must be significantly increased by pumping. Wang et al. [36] conducted a field test of a 1800 m CDHE for space heating near Bejing, China, and found that the long-term heat extraction rate of CDHE ranged between 237.24–256.54 kW at a flow rate of 20–25 m³/h. Numerical research conducted by Dai et al. [37] indicated that the heat extraction rate of the CDHE system increased from 25.1 to 386.6 kW as flow rate increased from 1 to 23 m³/h. Gascuel et al. [38,39] proposed utilizing a heat pump coupled with a CDHE, enabling a reduction in the injected water temperature while enhancing heat extraction from the CDHE. As the single-phase working fluid absorbs heat while descending into the well, its temperature progressively increases. At greater depths, although the reservoir temperature is higher, it becomes challenging to maintain a sufficient temperature difference between working fluid and the geothermal reservoir. This reduced temperature difference limits the heat extraction rate of the system, as the process is primarily driven by this temperature difference. Generally, the heat extraction of CDHE is below 1 MW even under sufficient pumping power. Therefore, the heat extraction capability of the CDHE single-well geothermal system is inherently restricted by its reliance on single-phase convection and the resultant decrease in temperature difference, limiting its efficiency and requiring higher flow rates to maintain performance.

In contrast, the recently proposed super-long gravity heat pipe (SLGHP) offers an innovative approach to significantly enhance the heat extraction rate in single-well geothermal systems, facilitating the exploitation of deep geothermal heat [40–42]. As shown in Fig.1(b), the SLGHP leverages the phase transition of the working fluid and vapor-liquid two-phase spontaneous flow to harness deep geothermal energy. By utilizing gravity to return the condensed working fluid, the system eliminates the need for a wick and enables long-distance heat transport without requiring pumping. The boiling heat transfer coefficient within the heat pipe is generally around 10000 W/(m²∙K), approximately 10 times greater than that of the CDHE, which reaches approximately 1000 W/(m²∙K) under sufficient pump power [43,44]. Moreover, SLGHP maintains a higher temperature difference between its deep end and the surrounding HDR. The working fluid absorbs heat in the form of latent heat, preventing a significant increase in temperature along the single well. The temperature gradient of the SLGHP against depth is around 1.1 K/km [45], in contrast to the CDHE, which usually ranges from 5 to 10 K/km even at very high flow rates [36].

The SLGHP was first implemented in a field test at Matouying Uplift, Tangshan, China. With a length of 3000 m, the SLGHP achieved a heat extraction rate (Q) of 190 kW over 30-day heat mining period. Based on the difference between the average temperature of the formation and the condensation temperature of the output vapor, the thermal resistance of the SLGHP (Z) is calculated as Z = ΔT/Q = 1.45 × 10⁻⁴ K/W. After extensive research on refining the working fluid [46], optimizing the two-phase flow pattern [47] and enhancing the inner structure design [48], a recent SLGHP system in Xiong’an, Hebei province, China has demonstrated the ability for continuous heat output exceeding 1 MW. The thermal resistance of SLGHP has been decreased to approximately 10⁻⁵ to 10⁻⁶ K/W [48]. This advancement has markedly improved the heat extraction capability within the well. However, Chen et al. [49] reported that in the SLGHP geothermal systems, the thermal resistance of HDR is about 10⁻⁴ K/W, which is an order of magnitude higher than that of SLGHP. Additionally, compared with doublet geothermal system, which more effectively utilize both conduction and advection mechanisms, single-well systems often extract less heat per meter of drilled length. Ensuring sufficient thermal contact between the pipe and the surrounding ground in deep single-well systems is particularly challenging. The low thermal conductivity of HDR formations surrounding the single well, along with the difficulty in achieving effective thermal contact between the well and the surrounding ground emerges as the principal constraint to further improving the heat transfer performance in single-well geothermal systems.

Enhancing heat transfer in single-well geothermal systems not only improves the efficiency of the system but also reduces operational costs and increases the lifespan of geothermal installations by maintaining optimal thermal conditions. Given that the SLGHP has effectively minimized the inner thermal resistance and enhanced the heat extraction rate within the well, improving heat transfer in the geological formation outside the well becomes essential to enhance overall system efficiency.

Economic analysis is also crucial for single-well geothermal systems, as it helps evaluate commercial viability and cost management, optimizes operational efficiency, guides decision-making and financial planning, and ensures long-term sustainability. Additionally, economic analysis encourages technological advancements, leading to cost reductions and efficiency improvements. These factors collectively enhance the feasibility and attractiveness of geothermal energy projects, promoting their wider adoption and commercial success.

The primary objective of this study is to comprehensively explore the current state and developments in the performance enhancement of single-well systems employed for the extraction of medium-deep and deep geothermal energy. This exploration is conducted through a detailed heat transfer enhancement analysis, with a particular focus on regions outside the well. The framework for this investigation is established in three sections. Section 2 provides a systematic analysis of enhancement concepts, focusing on characteristics such as heat transfer area, heat transfer coefficient, and temperature difference. Section 3 delves into the thermo-economic analysis of various configurations of single-well systems to establish the correlation between cost and enhancement concepts of heat transfer. Section 4 presents an overview of future research scope and challenges, providing guidance for the scientific community and stakeholders in selecting research directions and making informed decisions regarding financial investments.

2 Heat transfer enhancement methods for single-well geothermal systems

Based on the outer casing tube, which acts as a barrier between the reservoir and the circulated working fluid, the resistance of a single-well system can be roughly divided into two distinct categories: interior thermal resistance occurring within the casing tube and exterior thermal resistance occurring outside the casing tube.

In the pursuit of improving the heat extraction performance of single-well systems, incorporating a promoter pipe within the casing of a shallow borehole aquifer system was proposed in the early phases to reduce internal thermal resistance [50–53]. The implementation of this technique establishes a convection cell within the borehole [54]. The extent of the improvement in heat output is contingent upon the diameter and length of the promoter pipe, with potential enhancements ranging from 60% to 120% [51,55,56]. The presence of perforations below the water level facilitates the generation of buoyancy forces and promotes isothermal conditions as the mean temperature of the downhole exchanger increases [27]. The bottom perforated with the “pumping and dumping” technique method was also proposed [56]; this suction pumping system extracts fluid with heat from the well, propelling the fluid and establishing a heat transfer mechanism similar to the convection cell. Injecting air into the geothermal fluid below the water level within the wellbore elevates the pressure head of the reservoir, a process termed air lifting technology; this method yields a substantial 125% increase in heat output. However, there is also a notable concurrent increase in corrosion rates when air is discharged into the well.

In recent decades, the incorporation of metal and metal oxide nanoparticles into working fluids has demonstrated a noteworthy outcome in borehole geothermal systems [21,57–70]. Often regarded as enhancement strategies, these nanoparticles have demonstrated an increased ability to improve heat transfer in fluids (Tab.1). Additionally, the occurrence of heat dissipation in the inner tube is unavoidable and can contribute to as much as 30% of the overall power consumption [71–74]. Therefore, it is recommended to use an inner tube with lower thermal conductivity in order to minimize the heat flux in CDHE systems [75].

The external heat transfer enhancement principles pertain to variables related to the exterior of the casing tube, such as an increased surface area, an elevated temperature difference between the borehole formation and working fluid within the single-well system, and an enhanced heat transfer coefficient. This section conducts a comprehensive exploration of the thermal performance of geothermal-based single-well systems, facilitated by the previously mentioned external heat transfer enhancement features.

2.1 Heat transfer area

2.1.1 Open single-well system

The heat transfer area is a significant parameter that plays a crucial role in single-well geothermal systems. In an open system, the working fluid passes directly through the natural or artificial reservoir to facilitate heat transfer, similar to EGS. In a study conducted by Wang et al. [25], the single-well is connected to the fracture both with and without packer, indicating that the circulation of working fluid in the reservoir increases the thermosyphon effect (Fig.2(a)). The assessment reveals a notable disparity in heat transfer between the scenarios with and without the fracture network, for a 30-year operational period. The evaluation measured the heat transfer at 530 kW with the fracture network and 180 kW without it. Liang et al. [22] produced a remarkable increase of 10.64 times by utilizing multi-lateral well system and injecting working fluid into the reservoir (Fig.2(b)).

Using an inerratic fracture network model, Cheng et al. [76] proposed a power generation methodology in the abandoned oil field by directly injecting cryogenic fluid into the thermally fractured reservoir well (Fig.2(c)). The evaluation identifies key parameters, including the depth of the thermal reservoir, the rate of fluid injection, and porosity, as crucial factors influencing heat production. Additionally, the results indicate that the thermal reservoir with a length of 300 m and a porosity of 0.02 exhibits maximum power production, increasing heat extraction by fourfold compared to the case without the thermal reservoir. Gedzius and Teodoriu [77] proposed a borehole-based system that consists of horizontal lateral boreholes produced by casing and open bottom tubing of appropriate diameter and length. In cases where a borehole lacks a fracture network, it is possible to create artificial hydraulic network, as shown in Fig.2(d). Song et al. [78] performed a comparative analysis between conventional dual-well EGS and single-drill multilateral-well EGS by the direct injection of working fluid into the fracture network, as depicted in Fig.2(e). The study covered four cases over 30 years of operation by employing a 3D fluid flow COSMOL solver: double-well system (conventional double-well EGS); base case (single drill multilateral-well with the injection well at the top and the production well at the bottom); Case 1 (single drill multilateral-well with the injection well at the bottom and the production well at the top); and Case 2 (single drill multilateral-well with the injection well at the top and the production well at the bottom rotated by 45°). The results indicated that the multilateral well exhibits a significant improvement in heat extraction ratio, with a 45.33% increase compared to the conventional double-well EGS system. Erol et al. [23] conducted an investigation on a multilateral drilled single well, as depicted in Fig.2(f), considering both isotropic and anisotropic permeable reservoirs. The utilization of multi-lateral wells proved to yield a significant thermal output, particularly in cases where the permeability is isotropic and exceeds 5 × 10⁻¹⁵ m².

Although the numerical analysis of the direct injection open system shows significant improvement with an increased heat transfer area, the practical implementation by the GeneSys-Project [79,80] (illustrated in Fig.2(g)), revealed unfavorable outcomes due to the salt deposition and challenges in reservoir engineering. This case study has demonstrated to the geothermal research community the proactive necessity of addressing flow management. Mitigating issues arising from the chemical interaction between the injected fluid and the surrounding rock [81,82] and overcoming the significant challenges in reservoir engineering are essential to the overall success of open single-well projects.

2.1.2 Closed single-well system

Fig.3 depicts the closed single-well system proposed by various researchers. In a study conducted by Cui et al. [83], it was observed that increasing the length of the horizontal segment of the L-shaped well to 5000 m resulted in heat extraction ranging from 2.2 to 2.8 MW. In contrast, a vertical well with a length of 1000 m extracted only 1 to 1.5 MW of heat from the hot dry rock over a period of 10 years. Wang et al. [30] conducted a comparative analysis, revealing that the multilateral well exhibits a thermal power increase of 263.68% and 87.92% compared to the vertical and horizontal wells, respectively. This notable performance-enhancement is attributed to the larger heat exchange area and longer circulation time within the borehole. When analyzing the use of the CO₂ working fluid in the abandoned oil well in a horizontal configuration, Sun et al. [84] found that a higher mass flow rate combined with a lower injection temperature result in a high extraction rate of heat.

In addition, the connection between heat transfer and tube configurations in geothermal systems, such as U-tube, 2U, 3U, and branches of 12 and 16 tubes, has been observed to exhibit a direct correlation with both length and diameter [86–96]. Similarly, proposed approaches involve the examination of heat transfer processes with various shapes [30,83,84,97–99]; the objective is to enhance the contact area between the well and the subsurface.

In the context of augmenting fluid flow surface area through horizontal and multilateral wells, it is observed that the maximum temperature of the fluid attainable is directly proportional to the length of the formation. This correlation arises due to the simultaneous increase in the heat exchange surface area and the temperature difference between the well and the surrounding rock as the well segment length increases. The scientific community has recently expressed interest in the concept of doublet deep borehole heat exchangers [85,100] (Fig.3(e)) for retrofitting abandoned wells, which produce higher temperature and output power than single-well CDHEs.

2.2 Heat transfer coefficient

In the 1990s, Büchi [101] patented a method involving the injection of highly conductive material into the surrounding area of the bottom hole. This process aims to create an improved conductive environment, as depicted in Fig.4(a). After drilling a deep hole, the utilization of blasting or hydraulic fracturing techniques generates passages in the bottom zone, extending outwards. Re-drilling and flushing out soft rocks using chemicals create a bottom zone with a larger diameter and an enhanced penetrating passage. The heat-conducting materials, which include water, cement, siliceous gel, and finely divided metal powder, are injected to fill the passages along the exterior of the casing. Hara [102] patented a method involving the use of filler material, primarily graphite, placed in the gap between the casing and the borehole (see Fig.4(b)) to improve the heat transfer within the borehole. An analysis of the two methods mentioned above was conducted by Renaud et al. [103] utilizing a coupled reservoir and well-bore simulator. The assessment concluded that Hara’s patent is superior in both efficiency and technical aspects, and demonstrated also that the thermal recovery is augmented by 21% in comparison to traditional CDHE systems through the implementation of higher thermal conductivity material, i.e., graphite, along the wellbore.

Dahi Taleghani [98] proposed a method to improve heat transfer in an artificial conductive reservoir, as shown in Fig.4(c). The objective was to facilitate the introduction of a high-conductivity slurry proppant into a hydraulic fracture rock reservoir with lower porosity and permeability. By incorporating filler shortcut pathways, the introduction of a high conductivity slurry proppant substantially increased the contact area within the hydraulic fracture rock reservoir, achieving a remarkable increase of one thousand. This approach not only enhances heat transfer, but also mitigates the risk of temperature-induced compaction, particularly when proppant is injected into a horizontal well configuration characterized by numerous fractures.

In Li et al.’s investigation [43], with respect to the SLGHP geothermal system, the reservoir region was filled with molten phase change material to create an artificially enhanced reservoir region (Fig.4(d)). The material of choice is high-thermal conductivity composites, specifically of graphene or carbon nanotubes (CNTs). The evaluation indicated a positive correlation between heat transfer and the thermal conductivity of the enhanced region. Notably, the heat transmission of the heat pipe was found to be a few times higher than that of the original case with natural reservoir, as shown in Fig.5(a). Fig.5(b) and Fig.5(c) illustrate the influences of enhanced region geometry’s size on heat transfer rate for the heat pipe geothermal system over a span of 30 years. Recommendations suggested an enhanced length of approximately 2000 m and an enhanced region of around 30 m radius surrounding the SLGHP for achieving optimal results. Moreover, studies on the filler material for geothermal use indicated that the thermal conductivity of the grouting material impacts the efficiency of heat transfer in borehole heat exchangers [105–109].

In downhole heat exchanging process, to reduce internal thermal resistance, the artificial geyser concept is applied with boiler and flash evaporator for forced convection of working fluid [110,111]. Similarly, in order to optimize external heat transfer near the borehole, the utilization of an artificial convective reservoir is more effective than relying on conduction mode [28,112], where thermal energy efficiently exchanges between HDR and adjacent fluids. This specific category includes open single-well geothermal systems, which employ a method of injecting working fluid directly into the reservoir [21–23,77–79]. Moreover, Chen et al. [45] demonstrated that the interlayer crossflow of groundwater significantly enhances the heat extraction rate of the single-well geothermal system. They conducted two-well SLGHP geothermal system (2020 and 2180 m respectively) for space heating in Taiyuan, Shanxi province, China. The longer SLGHP, which utilizes convection due to interlayer groundwater crossflow in the reservoir, extracts 70% more heat than the shorter SLGHP, which relied solely on conduction for heat transfer. Similar enhancement strategy involving interlayer crossflow in thermal reservoir has been studied in detail by Li et al. [113].

In a heat pipe reservoir system (as shown in Fig.6(a)), Huang et al. [114] proposed to inject CO₂ into the reservoir flow channel outside the borehole formation. CO₂ was employed for its unique rheological and thermodynamic properties, which give rise to the thermosyphon effect. This effect is characterized by a high coefficient of thermal expansion, resulting in a significant variation in density if the temperature changes. The generation of a strong buoyancy force enhances circulation and improves heat exchange performance. The sensitivity analysis of heat extraction was assessed by introducing water and CO₂ into the reservoir. The evaluation revealed that the heat extraction achieved by artificial CO₂ reservoir system is twice as efficient as that of water (Fig.6(b)). The permeability of the reservoir flow channel and the volume increase of the artificial reservoir emerge as significant factors influencing the heat extraction rate.

2.3 Temperature difference

The decrease in temperature of the working fluid and the corresponding increase in temperature at the borehole reservoir casing contribute to an enhancement in thermal energy transfer. In recent years, there has been a significant trend in the adoption of low boiling-point fluids [21,57–62,115] for heat transfer enhancement in geothermal systems. The main factor propelling this transition is the inherent property of low boiling-point fluids to maintain lower temperatures when absorbing heat. These fluids have the ability to absorb latent heat and evaporate at lower temperatures within the borehole, making them a viable option.

There are few documented strategies [116–118], as shown in Fig.7, to increase the temperature of a deep borehole by collecting and inducing heat through various methods, which can augment the heat transfer process. In 2011, GThem, a United States-based company [116], unveiled the HeatNest single-well enhanced geothermal system. This novel technique was intended to improve heat extraction from a reservoir without the requirement for hydraulic stimulation or direct injection of working fluid, as shown in Fig.7(a). In this conceptual framework, secondary heat highways are intricately linked to a vertical primary bore. These heat highways include a heat pipe that incorporates a grouted material with exceptionally high thermal conductivity. These heat pipes [117] efficiently exchange and collect heat within the primary borehole, directing it toward the HeatNest. The working fluid flows toward the HeatNest during the process of heat transfer and power generation, undergoing a heat exchange that results in the transfer of thermal energy. The transfer of thermal energy subsequently drives a turbine linked to a generator, enabling the generation of electrical power [118].

Zhu et al. [119] introduced a novel technique using the COSMOL Multiphysics model for in situ combustion within an oil reservoir (Fig.7(b)), wherein the process does not involve direct contact with the formation fluid. This approach ensures minimal interference with oil production while generating a substantial quantity of heat. The implementation of waste heat harvesting through in situ combustion by air injection in the abandoned oil reservoir has the potential to enhance the temperature of the borehole formation surrounding a geothermal well. The findings indicate a substantial increase in outlet temperature for in situ combustion, reaching approximately 150 °C.

In addition, certain proposals have been proposed to introduce heat at the bottom of the wellbore region using a combination of a thermo-electric generator and a boiler [110,111]. This arrangement facilitates the creation of a binary cycle, ultimately leading to the generation of steam. Additionally, the circulation of hot geofluids around the casing tube achieved a 29% improvement in the performance of the CDHE system [58].

2.4 Other enhancement techniques

The efficiency of geothermal systems is also enhanced by integrating them with other systems [120–124]. Fig.8(a) illustrates the integration of borehole geothermal technology with a liquefied storage tank. This combination offers dual benefits, including the reduction of temperature in the geothermal working fluid and the mitigation of handling issues associated with the storage tank in cold conditions [125].

Introducing the heat pipe into the production well of the existing EGS system, as depicted in Fig.8(b) [126], enables a more uniform and stable temperature distribution within the thermal storage circulation process. This uniformity reduces the precipitation of carbonate salts and prevents wellbore scaling, thereby, mitigating scaling issues and enhancing heat transfer efficiency in EGS systems. Moreover, some geothermal sites contain valuable minerals such as lithium, high-grade silica, and zinc [129]. In such cases, the collection of reservoir minerals along with heat extraction is achievable using heat pipe-based concepts [126,130]. The strategic placement of the heat pipe within the borehole, anchored by the filler material in the adiabatic segment, facilitates the recovery of geofluids through the annulus (outer casing and heat pipe wall).

The renewable solar power system is integrated with the CDHE and SLGHP systems in a synergistic manner, as illustrated in Fig.8(c) and Fig.8(d) [127,128]. Specifically, this integration significantly boosts the heat output of the CDHE when coupled with an artificial reservoir, reaching an impressive 1970 kW, compared to the output of 309 kW of the standalone CDHE system [127].

Tab.2 exhibits an overview of enhancement concepts of borehole single-well geothermal systems in experimental and numerical studies, as well as their inferences. Within this realm, various parameters such as heat transfer area, thermal conductivity, temperature in the deep borehole, and heat transfer coefficient have emerged as significant factors upon careful evaluation.

However, there are presently few single-well systems implemented and available for field test projects, as shown in Tab.3. In Weggis, Switzerland, a borehole heat exchanger generated 230 MWh of heat annually for direct heating and heat pump applications [131,132]. The coaxial borehole project in Weissbad, Switzerland [133] achieved an output temperature of approximately 10 °C, accompanied by a temperature differential of 35 °C. The heating and cooling borehole heat exchange initiative in Aachen, Germany, encountered technical difficulties in the interior tube of the heat exchanger [134,135]. A maximal output power of 363 kW was obtained using a pump input power of 7 kW, according to a field test conducted in UK [136]. A total of 170 kW of heat harvesting was achieved with the aid of a vacuum pump in an experimental evaluation of a gravity heat pipe [137]. A field test of a 3000 m long SLGHP in Tangshan, China, showed that geothermal heat was extracted at a rate of 190 kW [138]. In Xiong’an, Hebei province, China, a 4150 m-long SLGHP using ammonia as the working fluid was tested recently [139]; the results revealed that the system can extract more than 1 MW of heat from the geothermal formations. Furthermore, numerous single-well initiatives are currently underway [136,140–142], signifying the ongoing efforts and emphasizing the need to address the technical challenges associated with heat enhancement concepts in boreholes despite their promising results.

3 Economic analysis of single-well systems

The assessment of the economic characteristics of various geothermal systems holds significant importance for evaluating heat transfer enhancement strategies and their commercial viability. The geothermal investigation encompasses the comprehensive formulation of the total cost, which includes the expenses associated with drilling, plant construction, operation, and maintenance for single-well systems. It is noteworthy that horizontal well drilling costs exceed vertical well drilling expenses by 50 percent [83]. The thermal power of the single-well system utilized determines the plant construction cost for geothermal energy extraction. The quantification of power consumption associated with water injection and circulation, coupled with the conversion of electrical energy, emerges as a pivotal determinant in operational expenditure. In a borehole system, this estimation is predicated on the operational efficiency of the pump. Furthermore, the maintenance cost of a power plant is contingent upon the drilling and construction expenses associated with the establishment of the facility, considering the plant location.

In a comparative study conducted by Wang et al. [30], the performance of closed single-wells with vertical, horizontal, and multilateral configurations was examined. Tab.4 displays the equations representing different costs, accompanied by the corresponding total revenue generated. The assumed maintenance cost is equal to 0.5% of the drilling cost plus 5% of the construction cost. The economic analysis revealed that drilling costs play a significant role in determining capital investment. It is notable that the costs associated with horizontal wells are more than four times higher compared to both vertical and multilateral wells. Fig.9 presents the total cost, revenue, and economic efficiency (defined as the ratio of the total revenue to the cost) associated with these three distinct well configurations: vertical, horizontal, and multilateral. Notably, the revenue generation of horizontal wells failed to meet the criteria for commercial viability, primarily attributed to the pump’s elevated energy consumption resulting from the combined effects of gravity and flow rate.

In the case of horizontal wells, it is crucial to maintain a higher injection pressure to facilitate the proper circulation of working fluid. The fluid flow efficiency in horizontal wells is significantly lower (2.989 (m³/(h∙MPa)) when compared to vertical wells (13.899 (m³/(h∙MPa)) and multilateral wells (16.971 (m³/(h∙MPa)). The utilization of a multilateral well configuration has been found to yield significantly higher net revenue when compared to vertical wells, indicating an increase of 11.76 times. Additionally, the multilateral well provides superior economic efficiency compared to both the vertical and horizontal wells. This is evident from its efficiency ratio of total revenue to cost being 2.15, which is 1.63 and 2.34 times higher than that of the vertical and horizontal wells, respectively. These findings highlight multilateral well as a highly advantageous option in terms of economic viability.

In an economic analysis referencing the Soultz-en-Foret project, Cui et al. [83] evaluated three types of reservoirs: vertical well (fracture reservoirs), horizontal well (without fracture reservoirs), and two single-wells with four horizontal segments each (without fracture reservoirs). The annual expenditure for well maintenance was obtained at a rate of 2% relative to the overall expenses incurred during the drilling and completion process. Similarly, the annual expense for maintaining the ground power was determined to be 10% of the initial capital investment. The estimated cost of generating geothermal power using a single horizontal well is $0.122/kWh (for a well that is 3000 m long). This cost is reduced to $0.084/kWh when using two single-wells with multi-branch segments, where each well consists of four horizontal segments of 3000 m in length. The cost is $0.093/kWh for the fractured vertical well, which is similar to the Soultz-en-Foret project. Furthermore, the power generated by the combination of two multi-branch horizontal wells (1.44 MW) and the reservoir fracture vertical well (1.5 MW) is of similar magnitude.

Xiao et al. [144] used the net present value (NPV) method to compare the costs of deep HDR energy extraction using an L-shaped CDHE and a U-shaped doublet geothermal system in the Gonghe Basin, Qinghai, China. The NPV, defined as the difference between the discounted value of future cash flow and the project investment cost of a project, offers high practical utility in evaluating the economic feasibility of investment schemes. According to the evaluation, the U-shaped doublet system extracts more heat than the L-shaped CDHE, but is 1.3 times more costly due to the drilling and construction costs. They also revealed that increasing the one-time government subsidy rate to 30%, raising the heat price by 30% and reducing drilling costs by 30% can render such a system profitable within a 50-year production period.

Huang et al. [145] conducted an economic analysis to evaluate the performance of three distinct geothermal systems: EGS, CDHE, and SLGHP, over a 30-year operational period. The levelized cost of energy (L_{\mathrm{c}\mathrm{o}\mathrm{e}}) used in this analysis is a crucial metric that plays a significant role in assessing project efficiency and determining project feasibility (expressed in Eq. (1)). L_{\mathrm{c}\mathrm{o}\mathrm{e}} serves as a valuable tool in decision-making and financial planning processes commonly employed in economic analysis to assess and compare different approaches to energy generation [8].

(1)

Tab.5 presents an overview of the costs associated with different systems [145]. Notably, the EGS system stands out with the highest total investment cost, amounting to $19.8 million. In contrast, the SLGHP incurs a cost of $3.7 million, while the CDHE requires an investment of $2.4 million. Based on the assessment, the EGS system demonstrates the most cost-effective electricity production, with a rate of 0.154 $/kWh. In comparison, the SLGHP has a rate of 0.163 $/kWh, while the CDHE system has the highest cost of 0.374 $/kWh. Utilizing abandoned oil and gas wells significantly decreases the overall investment required for single-well systems. Therefore, the SLGHP and CDHE have reduced their operational cost to 0.093 $/kWh and 0.109 $/kWh, respectively. The authors also conducted a detailed analysis of the parameter sensitivity of power generation costs of SLGHP system. In the cost structure of a gravity heat pipe geothermal system, drilling expenses have the most significant impact on the system’s power generation cost. When drilling costs fluctuate between −50% and +50%, the corresponding power generation cost ranges from 0.128 $/kWh to 0.178 $/kWh. In comparison, the cost of thermal reservoir modification affects the power generation cost to a lesser extent, with a variation range of approximately 0.15 to 0.177 $/kWh under the same −50% to + 50% change in parameters. Ma et al. [146] conducted a comparative analysis to evaluate the performance of the CDHE and SLGHP. The results of this analysis revealed that the L_{\mathrm{c}\mathrm{o}\mathrm{e}} associated with the SLGHP system were found to be significantly lower when compared to those of the CDHE, across various production temperature ranges.

4 Future research opportunities and challenges

There are substantial research gaps and opportunities for future studies aimed at addressing the challenges and enhancing the capabilities of single-well borehole-closed energy extraction processes. While the closed system has garnered significant attention regarding environmental concerns and cost, the operational cost of the CDHE increases with pump operation. Moreover, many concepts in the literature and patents focus on generating in situ vapor in the deep borehole [101,102,147–150], which eliminates the need for a flash separator and reduces pump workload. Therefore, it is essential to consider strategies for elevating the temperature within the deep hole and minimizing resistance in both the borehole reservoir and surrounding formation, as the current heat transmission rate remains inadequate.

The corrosion rate of the casing pipe material beneath the subsurface is influenced by various factors, such as pH, chloride and sulfate ions, hydrogen sulphide, dissolved carbon dioxide and oxygen, ammonia, and suspended solid deposits [151]. Selecting appropriate materials for geothermal applications is of utmost importance to prevent corrosion and enhance heat transfer [152]. The material used for tubing direct affects thermal conductivity, even under constant operational parameters [153]. For instance, copper (387.6 W/(m·K)) demonstrates a 16% improvement in heat extraction rate over HDPE pipe (0.461 W/(m·K)) [152]. Similarly, stainless steel (14.9 W/(m·K)) outperforms polybutylene polybutylene (0.38 W/(m·K)) [154] by 9%, and steel (16.27 W/(m·K)) is 36% more effective than polyethylene (0.4 W/(m·K)) [153]. Additionally, steel (50 W/(m·K)) shows a 25%–40% advantage over polyamide (0.25 W/(m·K)) [155] in geothermal heat extraction. In comparison to a bare tube, finned pipes enhance heat transmission and increase the contact area with the heated subsurface [156–162], which reduces temperature differences along the tube surface [163–169] and decreases thermal resistance by 29.2% [108]. The utilization of annular corrugated structural heat pipes [170,171] in geothermal energy extraction has been shown to further decrease thermal resistance and improve thermal energy stability in comparison to bare tubes.

The thermal conductivity and thermal resistance of the borehole formation [172] are key design parameters for single-well systems [173]. Thermal conductivity is influenced by the thermal properties of borehole components, particularly the grouting material and its physical arrangement. The thermal conductivity of the grout or filler material injected between the formations and borehole plays a crucial role in facilitating heat transfer. Since typical grouting materials, such as cement and bentonite, have low thermal conductivity, there is a push for using grouting materials with high thermal conductivity, mechanical strength, and phase-changing capabilities [174]. However, these materials can also have considerable drawbacks, such as susceptibility to freezing, potential damage to the pipe, and thermal imbalances. To mitigate these issues, conducting pre-durability tests on permeability and optimizing these materials can lead to improved heat exchange and reduced temperature fluctuation within the single-well system [174,175].

The enhancement of convective heat transfer is contingent upon the utilization of active, passive, and hybrid methods, which combine elements of both active and passive strategies. Passive methods include surface modifications and the incorporation of nanofluids, while active methods utilize external fields such as magnetic, electric, and acoustic forces to influence fluid behavior. Hybrid approaches can be particularly effective in deep aquifer systems, allowing for controllable heat transfer. However, it is essential to conduct bench tests to assess the feasibility and viability of these methods.

Injecting low-boiling-point fluids (e.g., refrigerants) into deep borehole reservoirs can significantly enhance convective heat transfer [114]. Studies [176–180] have shown that the use of nano-refrigerant—where nanoparticles are dispersed in refrigerant—improves performance in heat transfer devices compared to pure refrigerant. The implementation of nano-refrigerants in geothermal systems is expected to yield positive results. Besides their technical feasibility, it is imperative to emphasize the safety and environmental implications [181] associated with these refrigerants. When recovering heat using SLGHP in a deep aquifer reservoir with direct contact between the evaporator section of the heat pipe and the geofluid, enhancements can be achieved through either coating the outer surface of the tube material or by increasing the thermal conductivity of the geofluid, allowing for more efficient heat recovery.

Coating techniques [182–195,196] on metal surfaces have been shown to significantly improve both single- and two-phase heat transfer. The advantages of these coatings include increased surface area [168,182,197], enhanced critical heat flux [197,198], reduced corrosion [199,200], and improved thermal conductivity of pipes [201,202]. Additionally, they can enhance the heat transfer mechanisms such as bubble frequency, wettability, and hydrophilic roughness [203–206]. However, there are challenges in adapting these techniques for geothermal applications, particularly regarding the long-term stability, adaptability, and wettability of coated pipes. Incorporating nanoparticles into fluids also demonstrates a positive impact on convective heat transfer. It is also clear that the augmentation happens even when the fluid flow is laminar or turbulent [207–212] and for different sections such as circular tubes [209,210,213] and heat exchangers [208,211,212,214–216]. It is crucial to conduct long-term heat transfer analyses to understand the corrosion chemistry [217] between the pipe material and the impact of nanoparticles in subsurface conditions prior to applying these methods to deep aquifer geothermal reservoirs.

In deep borehole systems, the use of heat exchangers, boilers, and thermoelectric generators at shallower depths is a notable practice. This approach aims to efficiently convert liquid into vapor within the borehole, facilitating the direct conversion of thermal energy into electricity for diverse applications [149,218–222]. It is evident that this concept aids in minimizing heat loss, decreasing pipe costs, and reducing site occupancy [30,83]. Therefore, future developments are encouraged to pursue these studies, which incorporate value engineering and reduce environmental impact. Exploring innovative materials and designs, as well as optimizing operational parameters, will be essential for improving the sustainability and viability of these systems.

An enclosed heat recovery mechanism is employed by the SLGHP, with the evaporator section immersed in the reservoir housed within the casing tube. It can be operated in tandem with the indirect interaction of the geofluid with the perforations in the casing tube, facilitating indirect heat transfer through the enclosure with minimal environmental impact. Recent advancements in heat pipe technology have made it possible to enhance heat transfer efficiently and cost-effective, as illustrated in Tab.6. However, as temperature gradients vary with depth, future research should focus on developing a numerical model to analyze the effect of the length-to-diameter ratio. By optimizing both the working fluid and geometrical characteristics of the SLGHP with regard to depth and available heat, substantial advancements can be made in its application across diverse geothermal sites. This optimization will not only enhance efficiency but also broaden the potential for sustainable geothermal energy utilization.

In addition to enhancing heat transfer, the integration of geothermal energy systems presents significant opportunities for broader applications. For example, extracting lithium from deep geothermal lithium brine sources in regions such as the US, Italy, Canada, and the UK has garnered attention as a promising potential low-carbon technology, potentially contributing to the energy transition [229]. In the future, the concept of trigeneration, where power generation, heat production, and geothermal resource extraction occur from a single well, could be exceptionally effective. This integrated approach not only maximizes resource utilization but also enhances overall system efficiency, making it a compelling solution for sustainable energy production. Such advancements could play a pivotal role in addressing energy demands while minimizing environmental impacts.

Based on the assessment, it is evident that less than 10% of the studies have focused on real-time analysis, while the majority of single-well enhancement concepts have relied on numerical computational analysis. There is a pressing need for further research to investigate the practicability and feasibility of geothermal augmentation schemas that facilitate real-time prototype evaluation, which would enhance their implementation. In real-time scenarios, the single-well system undergoes a thermal loss of 15–30 °C throughout the heat extraction processes from the bottom borehole [131,132,134,136,230–232]. To improve system performance and demonstrate sustainability, it is essential to conduct trial experiments aimed at minimizing these heat losses. Future work should prioritize these practical evaluations to ensure the effectiveness and viability of geothermal energy systems in real-world applications.

5 Summary and outlook

This review offers a comprehensive analysis of heat transfer enhancement techniques for intermediate and deep geothermal energy extraction, particularly focusing on indirect heat extraction single-well systems such as the CDHE and SLGHP systems. Conventional hydrothermal systems are geographically limited, facing challenges in scalability and standardization. On the other hand, EGS, which typically employ dual-well setups, are confronted with significant challenges such as high operating costs, scaling, corrosion, and environmental concerns, including the risk of induced seismicity. By exploring advanced techniques and materials for heat transfer enhancement, this review highlights potential pathways for improving the efficiency and feasibility of geothermal energy extraction, addressing both the technical and environmental challenges associated with current systems.

Single-well systems offer a promising alternative for geothermal energy extraction, yet they often face challenges related to suboptimal heat transfer efficiency, which limits their potential for commercial viability. Enhancing heat transfer involves optimizing factors such as heat transfer area, thermal conductivity and heat transfer coefficient of reservoir and borehole formation and temperature differential.

Augmentation of the heat transfer area in single-well systems can be achieved by increasing the bore diameter and length, which significantly improves heat transmission. Currently, the prevailing method for this purpose is horizontal extension drilling. By implementing high thermal conductivity appendages within the borehole formation, injecting low-heating working fluid into the reservoir and utilizing the interlayer groundwater crossflow, the heat transfer coefficient is enhanced. Proposals such as establish bypass paths for heat accumulation from multiple locations and indirect combustion have also been suggested to elevate the borehole temperature. However, further research is essential to fully understand the mechanism pertaining to borehole formation in the context of multiphase flow, parameter optimization, and the gravitational effect in subsurface heat transfer. Exploring these aspects will contribute to the development of more efficient geothermal systems.

Through direct contact between the working fluid and the reservoirs, the open system enhances heat transfer in comparison to the closed system. However, it faces challenges such as salt deposition, engineering complexities, elevated costs, and various environmental and safety concerns. Future research should prioritize real-time prototype evaluations to assess performance under practical conditions. Additionally, material optimization is crucial to enhance durability and heat transfer efficiency. Integration of advanced technologies, such as chemical dissolution methods to manage deposits and acoustic-magnetic treatments to improve fluid dynamics, could significantly mitigate deposition and further enhance heat transfer capabilities. This multi-faceted approach will help address current limitations and improve the viability of open geothermal systems.

Economic analysis reveals that increasing borehole length can significantly affect pump power and overall operational costs. Among single-well systems, the SLGHP stands out as a cost-effective solution, primarily due to its pumpless operation and suitability for installation in abandoned wells, reducing both initial investment and maintenance costs. Future developments should emphasize sustainable engineering practices that prioritize cost-effectiveness, while minimizing environmental impact. Additionally, the integration of geothermal systems with other renewable energy sources, such as solar or wind, can enhance energy efficiency and reliability, creating a more robust and versatile energy infrastructure. This holistic approach will not only improve economic viability but also support the broader transition to renewable energy solutions.

Finally, the growing demand for energy efficiency and the push for low-carbon technologies highlight the significant potential for expanding geothermal energy research. Improving heat transfer enhancement strategies is crucial for boosting the efficiency and commercial viability of single-well geothermal systems. The development of trigeneration systems, where power generation, heating, and cooling occur simultaneously, can greatly enhance the utility of geothermal resources, integrating geothermal energy with other renewable sources. Furthermore, the integration of geothermal energy with other renewable sources will not only diversify energy portfolios but also improve reliability and sustainability. Additionally, employing innovative methodologies and conducting comprehensive economic analyses will optimize the performance utilization of single-well systems. This approach will support sustainable growth in the geothermal sector, positioning it as a key player in the transition to a low-carbon energy future. Overall, the pathway forward is promising, and continued research will be vital to realizing the full potential of geothermal energy.

References

[1] Huang Y, Zhang Y, Hu Z. . Economic analysis of heating for an enhanced geothermal system based on a simplified model in Yitong Basin, China. Energy Science & Engineering, 2019, 7(6): 2658–2674

[2] Zhu J, Hu K, Lu X. . A review of geothermal energy resources, development, and applications in China: Current status and prospects. Energy, 2015, 93: 466–483

[3] Srimuang W, Amatachaya P. A review of the applications of heat pipe heat exchangers for heat recovery. Renewable & Sustainable Energy Reviews, 2012, 16(6): 4303–4315

[4] Moya D, Aldás C, Kaparaju P. Geothermal energy: Power plant technology and direct heat applications. Renewable & Sustainable Energy Reviews, 2018, 94: 889–901

[5] BeerepootM. Technology roadmap—Geothermal heat and power. International Energy Agency 44 Report, 2011

[6] Tester J W, Beckers K F, Hawkins A J. . The evolving role of geothermal energy for decarbonizing the United States. Energy & Environmental Science, 2021, 14(12): 6211–6241

[7] Huang Y, Zhang Y, Xie Y. . Long-term thermal performance analysis of deep coaxial borehole heat exchanger based on field test. Journal of Cleaner Production, 2021, 278: 123396

[8] International Renewable Energy Agency (IREA). Renewable Power Generation Costs in 2021. 2022

[9] Giardini D. Geothermal quake risks must be faced. Nature, 2009, 462(7275): 848–849

[10] Olasolo P, Juárez M C, Morales M P. . Enhanced geothermal systems (EGS): A review. Renewable & Sustainable Energy Reviews, 2016, 56: 133–144

[11] Tester J W, Anderson B J, Batchelor A S. . Impact of enhanced geothermal systems on US energy supply in the twenty-first century. Philosophical Transactions—Royal Society. Mathematical, Physical, and Engineering Sciences, 2007, 365(1853): 1057–1094

[12] Liao Y, Sun X, Sun B. . Geothermal exploitation and electricity generation from multibranch U-shaped well-enhanced geothermal system. Renewable Energy, 2021, 163: 2178–2189

[13] Zhu N, Hu P, Xu L. . Recent research and applications of ground source heat pump integrated with thermal energy storage systems: A review. Applied Thermal Engineering, 2014, 71(1): 142–151

[14] Haraden J. The status of hot dry rock as an energy source. Energy, 1992, 17(8): 777–786

[15] Huang Y, Zhang Y, Yu Z. . Experimental investigation of seepage and heat transfer in rough fractures for enhanced geothermal systems. Renewable Energy, 2019, 135: 846–855

[16] Huang W, Cao W, Jiang F. Heat extraction performance of EGS with heterogeneous reservoir: A numerical evaluation. International Journal of Heat and Mass Transfer, 2017, 108: 645–657

[17] Lu S M. A global review of enhanced geothermal system (EGS). Renewable & Sustainable Energy Reviews, 2018, 81: 2902–2921

[18] Cao W, Huang W, Jiang F. Numerical study on variable thermophysical properties of heat transfer fluid affecting EGS heat extraction. International Journal of Heat and Mass Transfer, 2016, 92: 1205–1217

[19] Kim K, Ree J, Kim Y. . Assessing whether the 2017 MW 5.4 Pohang earthquake in South Korea was an induced event. Science, 1979, 2018(1009): 1007–1009

[20] CollinsM A, Law R. The development and deployment of geothermal single well (DGSW) technology in the United Kingdom. European Geologist, 2017: 63–68

[21] Wang Z, McClure M W, Horne R N. Modeling studies of single-well EGS configurations. Transactions—Geothermal Resources Council, 2009, 33: 243–247

[22] Liang H, Guo X, Gao T. . Performance analysis of single-well enhanced geothermal system for building heating. The Scientific World Journal, 2020, 2020: 1–8

[23] Erol S, Harcouët-Menou V, Laenen B. . Efficiency of single-well geothermal systems with multi-lateral drills. Geothermics, 2021, 89: 101928

[24] Alimonti C, Soldo E, Bocchetti D. . The wellbore heat exchangers: A technical review. Renewable Energy, 2018, 123: 353–381

[25] WangZ, Mcclure M W, HorneR N. A single-well EGS configuration using a thermosiphon. In: 34th Workshop on Geothermal Reservoir Engineering, 2009

[26] Lyu Z, Song X, Li G. . Numerical analysis of characteristics of a single U-tube downhole heat exchanger in a geothermal well. Geothermics, 2018, 72: 15–23

[27] Lund J W. The use of downhole heat exchangers. Geothermics, 2003, 32(4-6): 535–543

[28] Nalla G, Shook G M, Mines G L. . Parametric sensitivity study of operating and design variables in wellbore heat exchangers. Geothermics, 2005, 34(3): 330–346

[29] Ozgener O, Hepbasli A. Performance analysis of a solar-assisted ground-source heat pump system for greenhouse heating: An experimental study. Building and Environment, 2005, 40(8): 1040–1050

[30] Wang G, Song X, Shi Y. . Comparison of production characteristics of various coaxial closed-loop geothermal systems. Energy Conversion and Management, 2020, 225: 113437

[31] Shi Y, Song X, Li G. . Numerical investigation on heat extraction performance of a downhole heat exchanger geothermal system. Applied Thermal Engineering, 2018, 134: 513–526

[32] AcuñaJ. Improvements of U-pipe borehole heat exchangers. Dissertation for the Doctoral Degree. Stockholm: KTH Royal Institute of Technology, 2010

[33] Yildirim N, Parmanto S, Akkurt G G. Thermodynamic assessment of downhole heat exchangers for geothermal power generation. Renewable Energy, 2019, 141: 1080–1091

[34] AcuñaJ. Distributed thermal response tests new insights on U-pipe and coaxial heat exchangers in groundwater-filled boreholes. Dissertation for the Doctoral Degree. Stockholm: KTH Royal Institute of Technology, 2013

[35] Wang J. Comparative heat transfer efficiency study of coaxial and U-loop boreholes. GSTF Journal of Engineering Technology, 2014, 2: 1–6

[36] Wang H J, Xu Y S, Sun Y K. . Heat extraction by deep coaxial borehole heat exchanger for clean space heating near Beijing, China: Field test, model comparison and operation pattern evaluation. Renewable Energy, 2022, 199: 803–815

[37] Dai J, Li J, Wang T. . Thermal performance analysis of coaxial borehole heat exchanger using liquid ammonia. Energy, 2023, 263: 125986

[38] Gascuel V, Raymond J, Rivard C. . Design and optimization of deep coaxial borehole heat exchangers for cold sedimentary basins. Geothermics, 2022, 105: 102504

[39] Gascuel V, Rivard C, Raymond J. Deep geothermal doublets versus deep borehole heat exchangers: A comparative study for cold sedimentary basins. Applied Energy, 2024, 361: 122826

[40] Jiang F, Huang W, Cao W. Mining hot dry rock geothermal energy by heat pipe: conceptual design and technical feasibility study. Advances in New and Renewable Energy, 2017, 5: 426–434

[41] Huang W, Chen J, Cen J. . Heat extraction from hot dry rock by super-long gravity heat pipe: Effect of key parameters. Energy, 2022, 248: 123527

[42] Anand R S, Li A, Huang W. . Super-long gravity heat pipe for geothermal energy exploitation—A comprehensive review. Renewable & Sustainable Energy Reviews, 2024, 193: 114286

[43] Li Z, Huang W, Chen J. . An enhanced super-long gravity heat pipe geothermal system: Conceptual design and numerical study. Energy, 2023, 267: 126524

[44] Huang Y, Zhang Y, Xie Y. . Field test and numerical investigation on deep coaxial borehole heat exchanger based on distributed optical fiber temperature sensor. Energy, 2020, 210: 118643

[45] Chen J, Li Z, Huang W. . Super-long gravity heat pipe geothermal space heating system: A practical case in Taiyuan, China. Energy, 2024, 299: 131521

[46] Chen J, Huang W, Cen J. . Heat extraction from hot dry rock by super-long gravity heat pipe: Selection of working fluid. Energy, 2022, 255: 124531

[47] Chen J, Cen J, Huang W. . Multiphase flow and heat transfer characteristics of an extra-long gravity-assisted heat pipe: An experimental study. International Journal of Heat and Mass Transfer, 2021, 164: 120564

[48] Huang W, Chen J, Ma Q. . Developing kilometers-long gravity heat pipe for geothermal energy exploitation. Energy & Environmental Science, 2024, 17(13): 4508–4518

[49] Chen J, Huang W, Cen J. . Operational characteristics of the super-long gravity heat pipe for geothermal energy exploitation. Applied Thermal Engineering, 2024, 236: 121530

[50] Lei H, Liu X, Dai C. Experiment of heat transfer in downhole heat exchangers with a promoter pipe. Transactions—Geothermal Resources Council, 2012, 36(1): 255–258

[51] AllisR G, James R. Natural-convection promoter for geothermal wells. In: Proceedings of Geothermal Resource Council annual meeting, Salt Lake City, UT, USA, 1980

[52] Carotenuto A, Casarosa C, Dell’isola M. . An aquifer-well thermal and fluid dynamic model for downhole heat exchangers with a natural convection promoter. International Journal of Heat and Mass Transfer, 1997, 40(18): 4461–4472

[53] Carotenuto A, Casarosa C. A lumped parameter model of the operating limits of one-well geothermal plant with down hole heat exchangers. International Journal of Heat and Mass Transfer, 2000, 43(16): 2931–2948

[54] Freeston D H, Pan H. The application and design of downhole heat exchanger. Geothermics, 1985, 14(2‒3): 343–351

[55] CulverJ W, Lund G. Downhole heat exchangers. Geo-Heat Center Quarterly Bulletin, 1999

[56] BoydT T. Design of a convection cell for a downhole heat exchanger system in klamath falls, Oregon. In: Proceedings of 34th Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, USA, 2009

[57] Yu C, Zhang Y, Tan Y. . Performance comprehensive evaluation of downhole heat exchanger geothermal systems with various configurations and working fluids. Applied Thermal Engineering, 2022, 213: 118745

[58] Feng Y, Tyagi M, White C D. A downhole heat exchanger for horizontal wells in low-enthalpy geopressured geothermal brine reservoirs. Geothermics, 2015, 53: 368–378

[59] Zhang Y, Yu C, Li G. . Performance analysis of a downhole coaxial heat exchanger geothermal system with various working fluids. Applied Thermal Engineering, 2019, 163: 114317

[60] Cheng W L, Li T T, Le Nian Y. . Evaluation of working fluids for geothermal power generation from abandoned oil wells. Applied Energy, 2014, 118: 238–245

[61] Hu Z, Xu T, Feng B. . Thermal and fluid processes in a closed-loop geothermal system using CO₂ as a working fluid. Renewable Energy, 2020, 154: 351–367

[62] Chen J, Huang W, Cen J. . Heat extraction from hot dry rock by super-long gravity heat pipe: Selection of working fluid. Energy, 2022, 255: 124531

[63] Diglio G, Roselli C, Sasso M. . Borehole heat exchanger with nanofluids as heat carrier. Geothermics, 2018, 72: 112–123

[64] Daneshipour M, Rafee R. Nanofluids as the circuit fluids of the geothermal borehole heat exchangers. International Communications in Heat and Mass Transfer, 2017, 81: 34–41

[65] PengY, Wang Y, DuR. The effect of nanofluid to vertical single U-tube ground heat exchanger. In: Proceedings of BSO 4th Building Simulation and Optimization Conference, Cambridge, UK, 2018

[66] Sun X H, Yan H, Massoudi M. . Numerical simulation of nanofluid suspensions in a geothermal heat exchanger. Energies, 2018, 11(4): 1–18

[67] Javadi H, Urchueguia J F, Mousavi Ajarostaghi S S M. . Impact of employing hybrid nanofluids as heat carrier fluid on the thermal performance of a borehole heat exchanger. Energies, 2021, 14(10): 2892

[68] Jahanbin A, Semprini G, Pulvirenti B. Nanofluid suspensions as heat carrier fluids in single U-tube borehole heat exchangers. Journal of Physics: Conference Series, 2021, 2116: 0–4

[69] Jahanbin A, Semprini G, Pulvirenti B. Performance evaluation of U-tube borehole heat exchangers employing nanofluids as the heat carrier fluid. Applied Thermal Engineering, 2022, 212: 118625

[70] Chappidi S, Kumar A, Singh J. Geothermal energy extraction from abandoned oil and gas wells using mono and hybrid nanofluids. Geothermics, 2023, 114: 102794

[71] Liu J, Wang F, Cai W. . Numerical study on the effects of design parameters on the heat transfer performance of coaxial deep borehole heat exchanger. International Journal of Energy Research, 2019, 43(12): 6337–6352

[72] Hu X, Banks J, Wu L. . Numerical modeling of a coaxial borehole heat exchanger to exploit geothermal energy from abandoned petroleum wells in Hinton, Alberta. Renewable Energy, 2020, 148: 1110–1123

[73] Śliwa T, Kruszewski M, Zare A. . Potential application of vacuum insulated tubing for deep borehole heat exchangers. Geothermics, 2018, 75: 58–67

[74] Kujawa T, Nowak W, Stachel A A. Utilization of existing deep geological wells for acquisitions of geothermal energy. Energy, 2006, 31(5): 650–664

[75] Li G, Yang J, Zhu X. . Numerical study on the heat transfer performance of coaxial shallow borehole heat exchanger. Energy and Built Environment, 2021, 2(4): 445–455

[76] Cheng W L, Liu J, Le Nian Y. . Enhancing geothermal power generation from abandoned oil wells with thermal reservoirs. Energy, 2016, 109: 537–545

[77] GedziusI, Teodoriu C. Innovative development concepts for hot-dry-rock project in the North German Basin. In: DGMK/ÖGEWSpring Conference, Department of Execution and Production, Kassel, Germany, 2011

[78] Song X, Shi Y, Li G. . Numerical simulation of heat extraction performance in enhanced geothermal system with multilateral wells. Applied Energy, 2018, 218: 325–337

[79] TischnerT, Evers H, HauswirthH, et al. New concepts for extracting geothermal energy from one well: The gene sys-project. In: Proceedings of World Geothermal Congress, Bali, Indonesia, 2010

[80] Jung R, Orzol J, Kehrer P, et al.Collaborative project: Genesys – Feasibility study—Testing the waterfrac technique and the single well two-layer process for direct heat use from low-permeability sedimentary rocks, Final Report, 2006

[81] Breede K, Dzebisashvili K, Liu X. . A systematic review of enhanced (or engineered) geothermal systems: Past, present and future. Geothermal Energy, 2013, 1(1): 4

[82] Falcone G, Liu X, Okech R R. . Assessment of deep geothermal energy exploitation methods: The need for novel single-well solutions. Energy, 2018, 160: 54–63

[83] Cui G, Ren S, Zhang L. . Geothermal exploitation from hot dry rocks via recycling heat transmission fluid in a horizontal well. Energy, 2017, 128: 366–377

[84] Sun F, Yao Y, Li G. . Performance of geothermal energy extraction in a horizontal well by using CO₂ as the working fluid. Energy Conversion and Management, 2018, 171: 1529–1539

[85] Hu X, Banks J, Guo Y. . Retrofitting abandoned petroleum wells as doublet deep borehole heat exchangers for geothermal energy production—A numerical investigation. Renewable Energy, 2021, 176: 115–134

[86] Jalaluddin A, Miyara K. . Experimental study of several types of ground heat exchanger using a steel pile foundation. Renewable Energy, 2011, 36(2): 764–771

[87] Xia C, Sun M, Zhang G. . Experimental study on geothermal heat exchangers buried in diaphragm walls. Energy and Building, 2012, 52: 50–55

[88] Park H, Lee S R, Yoon S. . Evaluation of thermal response and performance of PHC energy pile: Field experiments and numerical simulation. Applied Energy, 2013, 103: 12–24

[89] Song X, Shi Y, Li G. . Heat extraction performance simulation for various configurations of a downhole heat exchanger geothermal system. Energy, 2017, 141: 1489–1503

[90] Liu Y, Zhang Y, Gong S. . Analysis on the performance of ground heat exchangers in ground source heat pump systems based on heat transfer enhancements. Procedia Engineering, 2015, 121: 19–26

[91] Liu X, Xiao Y, Inthavong K. . Experimental and numerical investigation on a new type of heat exchanger in ground source heat pump system. Energy Efficiency, 2015, 8(5): 845–857

[92] Serageldin A A, Sakata Y, Katsura T. . Thermo-hydraulic performance of the U-tube borehole heat exchanger with a novel oval cross-section: Numerical approach. Energy Conversion and Management, 2018, 177: 406–415

[93] Choi H K, Yoo G J, Pak J H. . Numerical study on heat transfer characteristics in branch tube type ground heat exchanger. Renewable Energy, 2018, 115: 585–599

[94] Luo J, Zhao H, Gui S. . Thermo-economic analysis of four different types of ground heat exchangers in energy piles. Applied Thermal Engineering, 2016, 108: 11–19

[95] Mehrizi A A, Porkhial S, Bezyan B. . Energy pile foundation simulation for different configurations of ground source heat exchanger. International Communications in Heat and Mass Transfer, 2016, 70: 105–114

[96] Sáez Blázquez C, Farfán Martín A, Martín Nieto I. . Efficiency analysis of the main components of a vertical closed-loop system in a borehole heat exchanger. Energies, 2017, 10(2): 1–15

[97] Westphal D, Weijermars R. Economic appraisal and scoping of geothermal energy extraction projects using depleted hydrocarbon wells. Energy Strategy Reviews, 2018, 22: 348–364

[98] Dahi Taleghani A. An improved closed-loop heat extraction method from geothermal resources. Journal of Energy Resources Technology, 2013, 135(4): 1–7

[99] Sharma P, Al Saedi A Q, Kabir C S. Geothermal energy extraction with wellbore heat exchanger: Analytical model and parameter evaluation to optimize heat recovery. Renewable Energy, 2020, 166: 1–8

[100] Gascuel V, Rivard C, Raymond J. Deep geothermal doublets versus deep borehole heat exchangers: A comparative study for cold sedimentary basins. Applied Energy, 2024, 361: 122826

[101] BüchiH F. Method and apparatus for extracting and utilizing geothermal energy. US Patent No. 4912941 A, 1990

[102] HaraH. Geothermal well using graphite as solid conductor. US Patent No. 20110232858 A1, 2011

[103] RenaudT, Verdin P, FalconeG, et al. Heat transfer modelling of an unconventional, closed-loop geothermal well. In: Proceedings of World Geothermal Congress 2020+1, Reykjavik, Iceland, 2021

[104] Li Z, Huang W, Chen J. . An enhanced super-long gravity heat pipe geothermal system: Conceptual design and numerical study. Energy, 2023, 267: 126524

[105] Sliwa T, Rosen M A. Efficiency analysis of borehole heat exchangers as grout varies via thermal response test simulations. Geothermics, 2017, 69: 132–138

[106] Chen F, Mao J, Chen S. . Efficiency analysis of utilizing phase change materials as grout for a vertical U-tube heat exchanger coupled ground source heat pump system. Applied Thermal Engineering, 2018, 130: 698–709

[107] Oh K, Lee S, Park S. . Field experiment on heat exchange performance of various coaxial-type ground heat exchangers considering construction conditions. Renewable Energy, 2019, 144: 84–96

[108] EswiasiA. Novel pipe configuration for enhanced efficiency of vertical ground heat exchanger (VGHE). Dissertation for the Doctoral Degree. Victoria: University of Victoria, 2021

[109] Song X, Zheng R, Li G. . Heat extraction performance of a downhole coaxial heat exchanger geothermal system by considering fluid flow in the reservoir. Geothermics, 2018, 76: 190–200

[110] HellerK, Teodoriu C, FalconeG. A new deep geothermal concept based on the geyser principle. In: Proceedings of 39th Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, USA, 2014

[111] TeodoriuC, Falcone G, BarcelosJ. The artificial geyser concept—New insights. In: Proceedings of 14th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, USA, 2015

[112] He Y L, Tao W Q. Convective heat transfer enhancement: Mechanisms, techniques, and performance evaluation. In: Advances in Heat Transfer, 2014, 46: 87–186

[113] Li Z, Guo J, Huang W. . Heat extraction performance of the super-long gravity heat pipe applied to geothermal reservoirs of multi-aquifers. International Journal of Heat and Mass Transfer, 2024, 225: 125440

[114] Huang W, Cao W, Jiang F. A novel single-well geothermal system for hot dry rock geothermal energy exploitation. Energy, 2018, 162: 630–644

[115] Yildirim N, Parmanto S, Akkurt G G. Thermodynamic assessment of downhole heat exchangers for geothermal power generation. Renewable Energy, 2019, 141: 1080–1091

[116] EPRI. Single-well enhanced geothermal system front-end engineering and design, optimization of a renewable geothermal system for harvesting heat from hot, dry rock. 2011 Technical Report-Electric Power Research Institute, 2011

[117] ParrellaM J. Heat pipes for a single well engineered geothermal system. US Patent No.0248345,2017

[118] MontalvoF E. Current status of geothermal resources development in central America. In: Short Course on Geothermal Development and Geothermal Wells, Santa Tecla, El Salvador, 2012

[119] Zhu Y, Li K, Liu C. . Geothermal power production from abandoned oil reservoirs using in situ combustion technology. Energies, 2019, 12(23): 4476

[120] Tozlu A, Gençaslan B, Özcan H. Thermoeconomic analysis of a hybrid cogeneration plant with use of near-surface geothermal sources in Turkey. Renewable Energy, 2021, 176: 237–250

[121] Cao Y, Dhahad H A, Togun H. . Exergetic and financial parametric analyses and multi-objective optimization of a novel geothermal-driven cogeneration plant; adopting a modified dual binary technique. Sustainable Energy Technologies and Assessments, 2021, 48: 101442

[122] Chen H, Wang Y, Li J. . Thermodynamic analysis and economic assessment of an improved geothermal power system integrated with a biomass-fired cogeneration plant. Energy, 2022, 240: 122477

[123] Wang L, Huang X, Babaei M. . Full-scale utilization of geothermal energy: A high-efficiency CO₂ hybrid cogeneration system with low-temperature waste heat. Journal of Cleaner Production, 2023, 403: 136866

[124] Wang D, Ali M A, Alizadeh A. . Thermoeconomic appraisal of a novel power and hydrogen cogeneration plant with integration of biomass and geothermal energies. International Journal of Hydrogen Energy, 2024, 52: 385–400

[125] Xiao F, Zhao Z, Yang L. Innovative measures for thermal performance enhancement of single well-based deep geothermal systems: Existing solutions and some viable options. Geomechanics and Geophysics for Geo-energy and Geo-resources, 2022, 8(5): 1–19

[126] 126. An enhanced geothermal system combining gravity heat pipes for integrated geothermal development and utilization. China Patent No. CN202111114527.3, 2021 (in Chinese)

[127] Bu X, Jiang K, Guo Z. Storing high temperature solar thermal energy in shallow depth artificial reservoir for space heating. Scientific Reports, 2022, 12(1): 1–13

[128] JiangF, Ma Q, Huang W, et al. A solar-thermal integrated super-long gravity heat pipe geothermal power generation system. China Patent No. CN 219638904U, 2023 (in Chinese)

[129] GoldsteinB, Hiriart G, BertaniR, et al. Chapter 4—Geothermal energy: Geothermal energy in IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation. 2011

[130] Jiang F, Chen J, Huang W, et al. An integrated geothermal system with heat pipe heat extraction for simultaneous cooling, power generation and heating applications. China Patent No. CN113847745A, 2020 (in Chinese)

[131] Kohl T, Brenni R, Eugster W. System performance of a deep borehole heat exchanger. Geothermics, 2002, 31(6): 687–708

[132] Berger E, Grünenfelder T, Keller B. . Geothermal drilling in Lucerne. Bulletin of the Swiss Association of Petroleum Geologists and Engineers, 1994, 61: 17–43

[133] KohlT, Salton M, RybachL. Data analysis of the deep borehole heat exchanger plant Weissbad (Switzerland). In: Proceedings of World Geothermal Congress 2000, Kyushu, Japan, 2000

[134] Gaschnitz R, Preuße A. Deep geothermal heat exchangers—Technical and commercial aspects. Technical Report, RWTH Aachen University, 2001,

[135] Dijkshoorn L, Speer S, Pechnig R. Measurements and design calculations for a deep coaxial borehole heat exchanger in Aachen, Germany. International Journal of Geophysics, 2013, 2013: 1–14

[136] CollinsM A, Law R. The development and deployment of deep geothermal single well (DGSW) technology in the United Kingdom. European Geologist Journal—Geothermal–The Energy of the Future, 2017

[137] ZhangL, Wu Z, DengB. Experimental analysis and modification of an super-long gravity heat pipe. Energy Conservation, 2015, 397: 77–80 (in Chinese)

[138] Huang W, Cen J, Chen J. . Heat extraction from hot dry rock by super-long gravity heat pipe: A field test. Energy, 2022, 247: 123492

[139] MaQ, ChenJ, HuangW, et al. Effect of working fluid on super-long gravity heat pipe geothermal power generation system. In: Proceedings of 7th International Conference on Smart Grid and Smart Cities, Lanzhou, China, 2023

[140] Geothermal Engineering Ltd, Arup, and St Andrews University. Feasibility Report for the Deep Geothermal Single Well (DGSW) at Aberdeen Exhibition and Conference Centre. 2016

[141] Bu X, Ran Y, Zhang D. Experimental and simulation studies of geothermal single well for building heating. Renewable Energy, 2019, 143: 1902–1909

[142] Jia G S, Ma Z D, Xia Z H. . A finite-volume method for full-scale simulations of coaxial borehole heat exchangers with different structural parameters, geological and operating conditions. Renewable Energy, 2022, 182: 296–313

[143] Ma Q, Chen J, Huang W. . Power generation analysis of super-long gravity heat pipe geothermal systems. Applied Thermal Engineering, 2024, 242: 122533

[144] Xiao D, Liu M, Li L. . Model for economic evaluation of closed-loop geothermal systems based on net present value. Applied Thermal Engineering, 2023, 231: 121008

[145] Huang W, Cao W, Li T, Jiang F. Numerical study and economic analysis of gravity heat pipe hot dry rock geothermal system. CIESC Journal, 2021, 72: 1302–1313

[146] Ma Q, Huang W, Chen J. . Thermodynamic and economic performance of super-long gravity heat pipe geothermal power plant. Applied Thermal Engineering, 2024, 248: 123115

[147] Akbarzadeh A, Johnson P, Nguyen T. . Formulation and analysis of the heat pipe turbine for production of power from renewable sources. Applied Thermal Engineering, 2001, 21(15): 1551–1563

[148] Franco A, Vaccaro M. On the use of heat pipe principle for the exploitation of medium-low temperature geothermal resources. Applied Thermal Engineering, 2013, 59(1-2): 189–199

[149] LakicN. Self-contained in-ground geothermal genera tor and heat exchanger with in-line pump. US Patent No. 20130055714 A1, 2018

[150] HuisenV. Geothermal exchange system. US Patent No. 3470943 A, 1969

[151] KayaT, Hoshan P. Corrosion and materials selection for geothermal systems. In: Proceedings of World Geothermal Congress 2005, Antalya, Turkey, 2005

[152] Selamat S, Miyara A, Kariya K. Numerical study of horizontal ground heat exchangers for design optimization. Renewable Energy, 2016, 95: 561–573

[153] Cao S J, Kong X R, Deng Y. . Investigation on thermal performance of steel heat exchanger for ground source heat pump systems using full-scale experiments and numerical simulations. Applied Thermal Engineering, 2017, 115: 91–98

[154] Kim W J, Yoon S, Kim M J. . Thermal performance evaluation of stainless steel pipe as a ground heat exchanger. In: Proceedings of 15th Asian Regional Conference on Soil Mechanics and Geotechnical Engineering, Fukuoka, Japan, 2015,

[155] Hantsch A, Gross U. Numerical investigation of partially-wetted geothermal heat pipe performance. Geothermics, 2013, 47: 97–103

[156] Demir H, Koyun A, AtayilmazŞ Ö. . Investigation of the effects of geometrical parameters on heat transfer from buried finned pipes. Journal of Mechanical Science and Technology, 2013, 27(8): 2497–2506

[157] Bouhacina B, Saim R, Oztop H F. Numerical investigation of a novel tube design for the geothermal borehole heat exchanger. Applied Thermal Engineering, 2015, 79: 153–162

[158] Saeidi R, Noorollahi Y, Esfahanian V. Numerical simulation of a novel spiral type ground heat exchanger for enhancing heat transfer performance of geothermal heat pump. Energy Conversion and Management, 2018, 168: 296–307

[159] Jamshidi N, Mosaffa A. Investigating the effects of geometric parameters on finned conical helical geothermal heat exchanger and its energy extraction capability. Geothermics, 2018, 76: 177–189

[160] Huang Y, Li W, Xu D. . Spatiotemporal rule of heat transfer on a soil/finned tube interface. Sensors, 2019, 19(5): 1159

[161] Saeidi R, Noorollahi Y, Chang S, Yousefi H. A comprehensive study of Fin-assisted horizontal ground heat exchanger for enhancing the heat transfer performance. Energy Conversion and Management: X, 2023, 18: 100359

[162] Ma P, Mo L, Zhang J. . Enhanced heat transfer performance of a new horizontal buried tube heat exchanger. Thermal Science, 2023, 27(2 Part B): 1507–1516

[163] Alizadeh M, Ganji D D. Heat transfer characteristics and optimization of the efficiency and thermal resistance of a finned thermosyphon. Applied Thermal Engineering, 2021, 183: 116136

[164] Maleki Zanjani A, Gharali K, Al-Haq A. . Dynamic and static investigation of ground heat exchangers equipped with internal and external fins. Applied Sciences, 2020, 10(23): 1–19

[165] Naresh Y, Balaji C. Experimental investigations of heat transfer from an internally finned two phase closed thermosyphon. Applied Thermal Engineering, 2017, 112: 1658–1666

[166] Hakim I I, Putra N, Gunawan Y. Experimental study for heat pipe applications on low enthalpy geothermal energy utilization for agricultural products dryers. Journal of Advanced Research in Fluid Mechanics and Thermal Sciences, 2019, 56: 68–77

[167] Kim Y, Kim J S, Shin D H. . Enhanced thermal performance of a thermosyphon for waste heat recovery: Microporous coating at evaporator and hydrophobic coating at condenser. Applied Thermal Engineering, 2020, 175: 115332

[168] Ng V O, Yu H, Wu H A. . Thermal performance enhancement and optimization of two-phase closed thermosyphon with graphene-nanoplatelets coatings. Energy Conversion and Management, 2021, 236: 114039

[169] Hosseinzadeh K, Ganji D D, Ommi F. Effect of SiO₂ super-hydrophobic coating and self-rewetting fluid on two phase closed thermosyphon heat transfer characteristics: An experimental and numerical study. Journal of Molecular Liquids, 2020, 315: 113748

[170] Liu H, Gan W, Yao H. . Visualization and heat transfer comparative analysis of two phase closed thermosyphon. Applied Thermal Engineering, 2022, 217: 119172

[171] Liu H, Wang X, Zheng L. . Temperature response and thermal performance analysis of a super-long flexible thermosyphon for shallow geothermal utilization: Field test and numerical simulation. International Journal of Heat and Mass Transfer, 2022, 192: 122915

[172] Javed S, Spitler J. Accuracy of borehole thermal resistance calculation methods for grouted single U-tube ground heat exchangers. Applied Energy, 2017, 187: 790–806

[173] ASHRAE. Geothermal Energy. In: ASHRAE Handbook—HVAC Applications (SI). 2015

[174] Mahmoud M, Ramadan M, Pullen K. . A review of grout materials in geothermal energy applications. International Journal of Thermofluids, 2021, 10: 100070

[175] Lyne Y, Paksoy H, Farid M. Laboratory investigation on the use of thermally enhanced phase change material to improve the performance of borehole heat exchangers for ground source heat pumps. International Journal of Energy Research, 2019, 43(9): 4148–4156

[176] Maheshwary P B, Handa C C, Nemade K R. Effect of shape on thermophysical and heat transfer properties of ZnO/R-134a nanorefrigerant. Materials Today: Proceedings, 2018, 5(1): 1635–1639

[177] Kasaeian A, Hosseini S M, Sheikhpour M. . Applications of eco-friendly refrigerants and nanorefrigerants: A review. Renewable & Sustainable Energy Reviews, 2018, 96: 91–99

[178] Jiang F, Zhu J, Xin G. Experimental investigation on Al₂O₃–R123 nanorefrigerant heat transfer performances in evaporator based on organic Rankine cycle. International Journal of Heat and Mass Transfer, 2018, 127: 145–153

[179] Sharif M Z, Azmi W H, Mamat R. . Mechanism for improvement in refrigeration system performance by using nanorefrigerants and nanolubricants—A review. International Communications in Heat and Mass Transfer, 2018, 92: 56–63

[180] Celen A, Çebi A, Aktas M. . A review of nanorefrigerants: Flow characteristics and applications. International Journal of Refrigeration, 2014, 44: 125–140

[181] Abas N, Kalair A R, Khan N. . Natural and synthetic refrigerants, global warming: A review. Renewable & Sustainable Energy Reviews, 2018, 90: 557–569

[182] Alimoradi H, Shams M, Ashgriz N. Enhancement in the pool boiling heat transfer of copper surface by applying electrophoretic deposited graphene oxide coatings. International Journal of Multiphase Flow, 2023, 159: 104350

[183] Ahmadi V E, Khaksaran M H, Apak A M. . Graphene-coated sintered porous copper surfaces for boiling heat transfer enhancement. Carbon Trends, 2022, 8: 100171

[184] Wu C H, Lee Y A, Lai Y J. . Heat transfer enhancement of a multilayer graphene coating surface. Experimental Thermal and Fluid Science, 2020, 118: 110175

[185] Pecherkin N, Pavlenko A, Volodin O. . Experimental study of heat transfer enhancement in a falling film of R21 on an array of horizontal tubes with MAO coating. International Communications in Heat and Mass Transfer, 2021, 129: 105743

[186] Surtaev A, Kuznetsov D, Serdyukov V. . Structured capillary-porous coatings for enhancement of heat transfer at pool boiling. Applied Thermal Engineering, 2018, 133: 532–542

[187] Joseph A, Mohan S, Sujith Kumar C S. . An experimental investigation on pool boiling heat transfer enhancement using sol-gel derived nano-CuO porous coating. Experimental Thermal and Fluid Science, 2019, 103: 37–50

[188] Xu P, Li Q, Xuan Y. Enhanced boiling heat transfer on composite porous surface. International Journal of Heat and Mass Transfer, 2015, 80: 107–114

[189] Lee S, Seo G H, Lee S. . Layer-by-layer carbon nanotube coatings for enhanced pool boiling heat transfer on metal surfaces. Carbon, 2016, 107: 607–618

[190] Lee J, Kim J, Seo B. . Layer-by-layer solution-processed two-dimensional graphene oxide–polyethylenimine thin-film coatings for enhanced pool boiling heat transfer. International Journal of Heat and Mass Transfer, 2023, 209: 124067

[191] Brusly Solomon A, Mathew A, Ramachandran K. . Thermal performance of anodized two phase closed thermosyphon (TPCT). Experimental Thermal and Fluid Science, 2013, 48: 49–57

[192] Hong S J, Zheng X, Park C W. Enhanced flow boiling heat transfer characteristics of R134a on graphene-Cu nanocomposite coating on copper substrate. International Communications in Heat and Mass Transfer, 2019, 108: 104343

[193] Senthilkumar C, Krishnan A S, Solomon A B. Effect of thin porous copper coating on the performance of wickless heat pipe with R134a as working fluid. Journal of Thermal Analysis and Calorimetry, 2020, 139(2): 963–973

[194] Sarangi S, Weibel J A, Garimella S V. Effect of particle size on surface-coating enhancement of pool boiling heat transfer. International Journal of Heat and Mass Transfer, 2015, 81: 103–113

[195] Li C, Zou Y, Shen Z. . Effect of graphene oxide coating on bubble dynamics and nucleate pool boiling heat transfer. Advanced Powder Technology, 2023, 34:

[196] Raymond J, Mercier S, Nguyen L. Designing coaxial ground heat exchangers with a thermally enhanced outer pipe. Geothermal Energy, 2015, 3(1): 7

[197] Gupta S K, Misra R D. An experimental investigation on flow boiling heat transfer enhancement using Cu-TiO₂ nanocomposite coating on copper substrate. Experimental Thermal and Fluid Science, 2018, 98: 406–419

[198] Choi G, Yun M, Hsu W T. . Enhanced boiling heat transfer by nucleation patterning with self-assembly of reduced graphene oxide coating. International Journal of Heat and Mass Transfer, 2022, 197: 123329

[199] Behera A, Mallick P, Mohapatra S S. Nanocoatings for anticorrosion: An introduction. In: Corrosion Protection at the Nanoscale, 2020, 227–243

[200] Nhiem L T, Oanh D T Y, Hieu N H. Nanocoating toward anti-corrosion: A review. Vietnam Journal of Chemistry, 2023, 61(3): 284–293

[201] Yu W, France D M, Routbort J L. . Review and comparison of nanofluid thermal conductivity and heat transfer enhancements. Heat Transfer Engineering, 2008, 29(5): 432–460

[202] Quintino L. Overview of coating technologies. Surface Modification by Solid State Processing, 2014, 1–24

[203] Wang X Q, Mujumdar A S. Heat transfer characteristics of nanofluids: A review. International Journal of Thermal Sciences, 2007, 46(1): 1–19

[204] Panda A, Pati A R, Kumar A. . Dropwise evaporation of hydrophilic coolant droplets: A methodology for the development of hydrophilic coating and attainment of enhanced heat transfer. International Communications in Heat and Mass Transfer, 2019, 105: 19–27

[205] Li C, Zou Y, Shen Z. . Effect of graphene oxide coating on bubble dynamics and nucleate pool boiling heat transfer. Advanced Powder Technology, 2023, 34(8): 104080

[206] Chen Y, Fu R, Yan Y. Enhanced pool boiling heat transfer on grooved surfaces by wettability modification with nanoparticle coatings. International Communications in Heat and Mass Transfer, 2022, 137: 106298

[207] Gupta M, Singh V, Kumar S. . Experimental analysis of heat transfer behavior of silver, MWCNT and hybrid (silver + MWCNT) nanofluids in a laminar tubular flow. Journal of Thermal Analysis and Calorimetry, 2020, 142(4): 1545–1559

[208] Ahammed N, Asirvatham L G, Wongwises S. Entropy generation analysis of graphene–alumina hybrid nanofluid in multiport minichannel heat exchanger coupled with thermoelectric cooler. International Journal of Heat and Mass Transfer, 2016, 103: 1084–1097

[209] Sundar L S, Singh M K, Sousa A C M. Turbulent heat transfer and friction factor of nanodiamond-nickel hybrid nanofluids flow in a tube: An experimental study. International Journal of Heat and Mass Transfer, 2018, 117: 223–234

[210] Askari S, Koolivand H, Pourkhalil M. . Investigation of Fe₃O₄/graphene nanohybrid heat transfer properties: Experimental approach. International Communications in Heat and Mass Transfer, 2017, 87: 30–39

[211] Hashemzadeh S, Hormozi F. An experimental study on hydraulic and thermal performances of hybrid nanofluids in mini-channel: A new correlation for viscosity of hybrid nanofluids. Journal of Thermal Analysis and Calorimetry, 2020, 140(3): 891–903

[212] Ataei M, Sadegh Moghanlou F, Noorzadeh S. . Heat transfer and flow characteristics of hybrid Al₂O₃/TiO₂–water nanofluid in a minichannel heat sink. Heat and Mass Transfer, 2020, 56: 2757–2567

[213] Megatif L, Ghozatloo A, Arimi A. . Investigation of laminar convective heat transfer of a novel TiO₂-carbon nanotube hybrid water-based nanofluid. Experimental Heat Transfer, 2016, 29(1): 124–138

[214] Bhattad A, Sarkar J, Ghosh P. Experimentation on effect of particle ratio on hydrothermal performance of plate heat exchanger using hybrid nanofluid. Applied Thermal Engineering, 2019, 162: 114309

[215] Allahyar H R, Hormozi F, ZareNezhad B. Experimental investigation on the thermal performance of a coiled heat exchanger using a new hybrid nanofluid. Experimental Thermal and Fluid Science, 2016, 76: 324–329

[216] Bhattad A, Sarkar J, Ghosh P. Heat transfer characteristics of plate heat exchanger using hybrid nanofluids: Effect of nanoparticle mixture ratio. Heat and Mass Transfer, 2020, 56: 2457–2472

[217] Fotowat S, Askar S, Ismail M. . A study on corrosion effects of a water based nanofluid for enhanced thermal energy applications. Sustainable Energy Technologies and Assessments, 2017, 24: 39–44

[218] HuntR D. Method of enhanced heat extraction from a geothermal heat source for the production of electricity thermoelectrically and mechanically via the high-pressure injection of a cryogen into a U-tube or open tube heat exchanger within a geothermal heat source, US Patent: US20010906951, 2001-7-16

[219] BrysonM. Conversion of low grade heat into electricity using the thermosyphon rankine engine and trilateral flash cycle. Dissertation for the Doctoral Degree. Melbourne: RMIT University, 2007

[220] Nguyen T, Johnson P, Akbarzadeh A. . Design, manufacture and testing of a closed cycle thermosyphon rankine engine. Heat Recovery Systems and CHP, 1995, 15(4): 333–346

[221] NguyenT, Mochizuki M, MashikoM, et al. Heat pipe turbine for production of electrical power from renewable sources. In: Proceedings of 5th ASME/JSME Thermal Engineering Joint Conference, San Diego, USA, 1999

[222] Xie H, Wang J, Liao J. . A novel in-situ power generator system for geothermal resources without fluid extraction. In: Proceedings of 55th US Rock Mechanics/Geomechanics Symposium, Virtual, 2021,

[223] Anand R S, Jawahar C P, Brusly Solomon A. . Experimental investigations on modified thermosyphons using R134a/Al₂O₃ and comparative machine learning analysis. Applied Thermal Engineering, 2022, 212: 118554

[224] Kyuil H, Cho D H. A comparison of the heat transfer performance of thermosyphon using a straight groove and helical groove. Journal of Mechanical Science and Technology, 2005, 19(12): 2296–2302

[225] Han K I, Yee S S, Park S H. . A study on the improvement of heat transfer performance in low temperature closed thermosyphon. KSME International Journal, 2002, 16(9): 1102–1111

[226] Alammar A A, Al-Mousawi F N, Al-Dadah R K. . Enhancing thermal performance of a two-phase closed thermosyphon with an internal surface roughness. Journal of Cleaner Production, 2018, 185: 128–136

[227] Joudi K A, Witwit A M. Improvements of gravity assisted wickless heat pipes. Energy Conversion and Management, 2000, 41(18): 2041–2061

[228] Lin L, Groll M, Rösler S. One-dimensional analysis of maximum performance in a closed two-phase thermosyphon with a crossover flow separator. Heat Transfer Engineering, 1996, 17(3): 19–28

[229] Weinand J M, Vandenberg G, Risch S. . Low-carbon lithium extraction makes deep geothermal plants cost-competitive in future energy systems. Advances in Applied Energy, 2023, 11: 100148

[230] Morita K, Bollmeier WS, Mizogami H. An experiment to prove the concept of the downhole coaxial heat exchanger (DCHE) in Hawaii. Transactions—Geothermal Resources Council, 1992, 16: 9–16

[231] Faghri A. Heat pipe science and technology. Fuel and Energy Abstracts, 1995, 36(4): 285

[232] DunnP D, Reay D A. Heat pipes. 4th ed. Oxford: Pergamon, 1994

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