Heat Transfer Performance Factors in a Vertical Ground Heat Exchanger for a Geothermal Heat Pump System

Abstract

Ground heat pump systems (GHPSs) are esteemed for their high efficiency within renewable energy technologies, providing effective solutions for heating and cooling requirements. These GHPSs operate by utilizing the relatively constant temperature of the Earth’s subsurface as a thermal source or sink. This feature allows them to perform greater energy transfer than traditional heating and cooling systems (i.e., heating, ventilation, and air conditioning (HVAC)). The GHPSs represent a sustainable and cost-effective temperature-regulating solution in diverse applications. The ground heat exchanger (GHE) technology is well known, with extensive research and development conducted in recent decades significantly advancing its applications. Improving GHE performance factors is vital for enhancing heat transfer efficiency and overall GHPS performance. Therefore, this paper provides a comprehensive review of research on various factors affecting GHE performance, such as soil thermal properties, backfill material properties, borehole depth, spacing, U-tube pipe properties, and heat carrier fluid type and velocity. It also discusses their impact on heat transfer efficiency and proposes optimal solutions for improving GHE performance.

1. Introduction

In recent years, geothermal energy has emerged as a powerful and rapidly advancing source of clean energy [1,2]. Geothermal heat pumps (GHPs), commonly referred to as ground source heat pumps (GSHPs) or ground-coupled heat pumps (GCHPs) in North America. This technology was initially developed by Robert Webber in 1940 [3]. Following decades of theoretical and practical research and development, GHPS has progressively grown in popularity around the world. This is particularly evident in colder regions, where it provides significant efficiency and cost savings on utility bills compared to traditional heating and cooling systems powered by diesel or gas. As of 2021, approximately 6.46 million ground source heat pump (GSHP) units were in operation across about 30 countries. In the United States, 1.7 million units—equivalent to about 26.2% of the global total—were active. Among these, 60% were used for commercial purposes, while the remaining 40% were for residential use, according to the International Energy Agency (IEA) 2020 Geothermal Report [4]. Thus, GHPSs, known for their efficiency and environmental benefits, have become a popular alternative to traditional heating and cooling systems, including heating, ventilation, and air conditioning (HVAC) systems, in various commercial and residential buildings [5,6,7]. Furthermore, although a GHPS requires only a small amount of electricity for its water circulation and heat pumps, powering GHPSs with renewable energy sources, rather than traditional electric utilities, would be highly beneficial [5]. GHPSs can reduce cooling bills by 20% to 50% and heating bills by 30% to 60% compared to conventional systems [8,9]. Moreover, GHPSs are more efficient than traditional heating and cooling systems, particularly in winter, since they do not burn fuel to produce heat. Instead, they primarily operate by transferring existing heat from one location to another [5]. For example, GHPSs provide heating efficiency that is 30% to 70% higher than traditional heating systems and 20% to 50% higher than air conditioning systems [10,11]. Additionally, GHPSs are available year-round and are not affected by weather conditions, which stabilizes energy output to meet baseload demand, unlike other renewable energy sources (i.e., wind, solar, and hydro) [1,5,12]. Furthermore, GHPSs can potentially decrease overall greenhouse gas (GHG) emissions by 66% and carbon dioxide (CO₂) emissions by 50% compared to the traditional heating and cooling systems that rely on fossil fuels [13]. Moreover, GHPSs typically have a longer lifespan compared to most conventional heating and cooling systems, with heat pump systems expected to last up to 25 years, and high-density polyethylene (HDPE) pipes that can last up to 50 years [14]. Therefore, GHPSs are regarded as a renewable, clean, dependable, and sustainable energy source that requires minimal maintenance [15]. However, despite the numerous advantages of GHPSs, their installation can be costly. Vertical systems require extensive borehole drilling, while horizontal systems need a large land area. Additionally, the initial capital cost of a GHPS is approximately 30–50% higher than that of traditional heating and cooling systems [16,17].

Table 1. Advantages and disadvantages of geothermal heat pump systems.

Aspect	Advantages	Disadvantages	Refs
Popularity	GHPSs have grown in popularity worldwide, especially in colder regions, due to their efficiency and cost savings.	—	[5]
Global Adoption	As of 2021, around 6.46 million GSHP units were operational in about 30 countries, with 1.7 million in the U.S.	—	[4]
Energy Consumption	GHPSs can reduce cooling bills by 20% to 50% and heating bills by 30% to 60% compared to conventional systems.	—	[8,9]
Heating Efficiency	GHPSs are more efficient than traditional systems, particularly in winter, as they do not burn fuel and transfer existing heat. GHPSs provide heating efficiency 30% to 70% higher than traditional systems and 20% to 50% higher than air conditioning systems.	—	[5,10,11]
Availability	GHPSs are available year-round and not affected by weather conditions, ensuring stable energy output.	—	[1,5,12]
Environmental Impact	GHPSs can reduce GHG emissions by 66% and CO2 emissions by 50% compared to fossil-fuel-based systems.	—	[13]
Lifespan	GHPSs typically have a longer lifespan compared to conventional systems; HDPE pipes last up to 50 years and heat pump systems up to 25 years.	—	[14]
Maintenance	GHPSs require minimal maintenance.	—	[5]
Installation Cost	—	Installation can be expensive; vertical systems require extensive borehole drilling; and horizontal systems need large land areas. Initial capital cost is approximately 30–50% higher than that of traditional heating and cooling systems.	[16,17]

shows the advantages and disadvantages of geothermal heat pump systems.

Despite their relatively high initial capital costs, GHPSs are rapidly expanding globally and have significant development prospects, emphasizing their increasing significance in the energy sector. The increasing number of annual GHPS installations demonstrates the growing demand for this technology. The installed capacity (MWt) of GHPSs worldwide from 1995 to 2020 is depicted in Figure 1.

The geothermal heat pump system consists of a vertical or horizontal closed-loop ground heat exchanger (GHE), a water circulating pump and control unit, a heat pump, and a distribution unit system, as shown in Figure 2. The vertical GHE is a suitable alternative when space is limited and the ground surface consists of rocky terrain. Additionally, vertical GHEs provide greater performance stability compared to horizontal systems because they are installed at deeper depths, where ground temperatures remain consistent and are unaffected by seasonal weather variations or surface climate conditions. This deeper placement ensures more reliable and efficient heat exchange throughout the year. Although the installation of vertical GHEs is significantly more expensive than horizontal systems, they are more commonly installed due to their performance advantages [19,20,21]. The GHE is a critical component of the GHPS due to its direct contact with the Earth, which effectively harnesses its thermal energy. The GHE’s performance significantly impacts the overall system efficiency. The maximum benefits from the Earth’s thermal energy can be reached when the GHE is optimally designed with careful consideration of configuration, installation location, borehole heat exchanger capacity, and performance factors [22]. Therefore, improvements should emphasize optimizing the characteristics of the GHE, as enhancements in these aspects are likely to improve the overall efficiency of the GHPS [5,7,23].

In this paper, we provide a detailed review of research on factors affecting ground heat exchanger (GHE) performance, such as soil thermal properties, backfill material properties, borehole depth, spacing, U-tube pipe properties, and heat carrier fluid type and velocity. Furthermore, we discuss their impact on heat transfer efficiency and propose optimal solutions for improving GHE performance. The remainder of the paper is organized as follows: Section 2 discusses the factors influencing ground heat exchanger (GHE) performance, including soil thermal properties, backfill material thermal properties, borehole layout (such as depth, diameter, spacing, and configuration), pipe thermal properties (material and shank spacing), and working fluid properties (including fluid type and velocity). Section 3 presents the discussion and conclusions.

2. GHE Performance Factors

Figure 3 illustrates the single U-tube vertical ground heat exchanger. The main objective of the vertical ground heat exchanger (GHE) is to efficiently transfer heat between the ground and the heat pump system through an underground loop pipe filled with a heat carrier fluid (i.e., antifreeze solution), installed in vertical boreholes to maximize the contact surface area with the ground, thereby enhancing heat absorption or dissipation. Therefore, many factors can influence the performance of vertical GHEs, including soil thermal properties, backfill material thermal properties, borehole depth, spacing between boreholes, U-tube pipe thermal properties, spacing between the legs of the U-tube pipes, and U-tube pipe configuration, as well as the type and velocity of the heat carrier fluid [7,22]. The vertical GHE’s thermal performance factors are shown in Figure 4.

2.1. Soil Thermal Properties

Each soil type’s thermal properties differ according to its thermal conductivity and heat capacity, which are determined by the level of soil saturation. When the soil material’s thermal conductivity is high, it results in a lower thermal resistance, thereby enabling more efficient and rapid heat transfer through the soil layers. Conversely, low thermal conductivity increases thermal resistance, thereby impeding heat flow. Soil with a high sand content generally exhibits higher thermal conductivity than clay but is lower compared to rock and sandstone. The elevated quartz content in sandstone enhances its thermal conductivity [5,24]. Agrawal et al. [25] concluded that increasing soil water content enhances thermal conductivity and reduces thermal resistance. Sand content, dry density, and degree of saturation are three essential parameters that influence soil’s thermo-physical properties. Furthermore, moisture content, density, mineralogy, and particle size all influence thermal conductivity [26]. Malek et al. [27] indicated that soil water content influences thermal conductivity the most. Dry rocks’ thermal conductivity increases as their density increases [26]. The incorporation of additives such as quartz, bentonite, and metal particles can significantly increase soil thermal conductivity [25]. The GHPS operates optimally only when the GHE loop pipes are installed in soil with high thermal conductivity and heat conduction capacity. Therefore, it is crucial to select a geological site with good soil thermal properties before initiating the GHPS design process [5].

Table 2. Soil thermal properties.

Parameters	Density (kg/m3)	Specific Heat Capacity (J/kg·K)	Thermal Conductivity (W/m·K)
Clay	1700	1800	1.2
Sand	1500	1710	1.26
Sandy-silt	1845	1200	1.3
Sandy-clay	1960	1200	2.1

presents some soil thermal properties.

Wang et al. [28] evaluated the thermal properties of loam, clay, and sand on unsaturated soil using a coupled moisture diffusion and heat transfer model. Their results indicated that sand was more effective than loam and clay in heat migration, whereas clay and sand outperformed loam in moisture migration. Tang and Nowamooz [29] indicated that a ground heat exchanger installed in sand achieved an 8% higher performance compared to one installed in clay. Furthermore, saturated soil conditions can enhance the mean heat exchange rate by up to 40% compared to unsaturated soil conditions, as indicated by Choi et al. [30]. Furthermore, Abu-Hamdeh et al. [31] agreed with Wang et al. [28] and Tang and Nowamooz [29] that sand has higher thermal conductivity values compared to clay. Aizzuddin et al. [32] concluded that soil with a thermal conductivity ranging from 1.5 to 5 W/m·K can significantly contribute to optimal ground heat exchanger (GHE) thermal performance.

2.2. Backfill Material Thermal Properties

Backfill material fills the space between the ground and the geothermal pipe and can be either pure soil or some admixture of new material to improve the connection between the ground heat exchanger (GHE) and the surrounding ground. In addition, reducing borehole thermal resistance and keeping the borehole wall from collapsing can extend the system’s life [33]. It is essential to fully grout the space between the GHE’s pipe and the borehole wall to avoid air gaps in the grout, which could result in thermal discontinuity (i.e., contact resistance) [7,22]. Utilizing backfill material with a high thermal conductivity can enhance the GHE’s heat transfer efficiency.

Numerous researchers have examined how the backfill material affects a GHE’s heat transfer performance. For example, Smith and Perry [34] assessed the impact of the backfill material on the heat transfer rate. According to their results, increasing thermal conductivity can improve the heat transfer rate performance between the water inside the pipe and the surrounding ground and raise the GHE’s heat capacity. In addition, Smith and Perry [34] concluded that increasing the backfill material’s thermal conductivity can avoid a reduction in system efficiency. Furthermore, Dehkordi and Schincariol [35] assessed the impact of backfill material on heat extraction. Experimental results showed that increasing the backfill material’s thermal conductivity up to 3 W/m·K can increase heat extraction by more than 10% when compared to low thermal conductivity (1 W/m·K). Increasing the backfill material’s thermal conductivity could reduce the borehole’s heat gradient, as stated by Dehkordi and Schincariol [35]. Alberti et al. [36] found that by increasing the backfill material’s thermal conductivity from 0.7 W/m·K to 2.3 W/m·K, borehole thermal resistance can be lowered from 0.135 m·K/W to 0.054 m·K/W, respectively. Saeidi et al. [37] increased the thermal conductivity for both the ground and backfill materials from 0.5 to 2 W/m·K. The experimental results showed that the heat transfer rate increased by 40% and 48%, respectively. Carlson [38] concluded that backfilling the space between the pipe and the surrounding ground with bentonite-based materials could reduce the borehole’s overall length by 10%. Furthermore, Jahanbin [39] concluded that utilizing highly thermally conductive backfill material can increase the temperature of the borehole’s surface and decrease its thermal resistance. Increasing the thermal conductivity of the backfill material can lower the gradient of the borehole heat exchanger (BHE) (i.e., between the pipe and the surrounding ground), which improves the heat exchange rate’s efficiency [35].

Moreover, Marita and Aristodimos [40] concluded that using a backfill material with high thermal conductivity can create a strong connection between the pipe wall and the surrounding ground. The heat transfer rate increases as the backfill material’s thermal conductivity increases, and vice versa, as stated by Song et al. [41]. Furthermore, Zang et al. [42] examined the impact of the backfill material on the U-tube pipe’s length using four different diameters and configurations. According to the experimental results, increasing the backfill material’s thermal conductivity can appropriately reduce the GHE pipe length; however, the reduction was biggest in double U-tube pipe with an outer diameter of 32 mm, then, in decreasing order, a double U-tube pipes with an outer diameter of 25 mm, a single U-tube with an outer diameter of 32 mm, and a single U-tube with an outer diameter of 25 mm. Chang and Kim [43] evaluated the borehole thermal resistance components. Experimental results showed that the thermal resistance of the backfill material (grout) represents more than 65% of the total borehole thermal resistance. The backfill material has a significant impact on the heat transfer rate between the pipe and the surrounding ground. Thus, the backfill material’s thermal conductivity needs to be either higher or equal to that of the surrounding soil and ultimately increase the heat transfer’s efficiency in two directions [7,22].

2.2.1. Pure Material

The pure backfill material’s thermal conductivity ranges from 0.8 W/m·K to 2.4 W/m·K [44]. Pure backfill materials that are commonly used include sand, clay, silt, quartz sand, cement, bentonite, and coarse/fine gravel. Each type of these materials has different thermal properties that are based on the characteristics of its physical components. For instance, Wang et al. [28] used the improved “de V-1” model to assess the thermal conductivity of various soil types, such as loam, clay, and sand. The results showed that sand had higher thermal conductivity and heat diffusion than loam and clay soils. Cao et al. [45] evaluated the thermal conductivity of organic soil, silt and clay, sand, and organic soil. Experimental results revealed that sand had a higher thermal conductivity than other compared materials. Furthermore, Wang et al. [28] agreed with Fujiao and Nowamooz [29] that sand outperformed clay. Cementitious and sand materials perform better than other pure materials regarding thermal characteristics and geological location [46,47]. Utilizing cement as the backfill material is preferred in dry soils due to its high desiccation and shrinkage rates, as stated by Lee et al. [48]. Moreover, Pahud and Matthey [49] concluded that filling the space between the pipe and the surrounding ground with quartz sand rather than bentonite could lower borehole thermal resistance by 30%. As thermal resistance increases, the heat transfer rate decreases, as shown in Figure 5.

2.2.2. Mixed Material

Modifying a pure material by adding one or more additional mediums is another way to enhance the GHE’s thermal performance. The mixed components include pure silica, acrylic latex, quartz graphite, and other compounds. Bentonite is considered a viable backfill material additive owing to its high water absorption capacity and low hydraulic conductivity [50,51,52]. In the late 1980s, granular bentonite mixed with various additives was utilized as backfill material for GHE installations to promote heat transfer between the water inside the pipe and the surrounding ground [53]. However, the thermal conductivity of these mixed materials is low, ranging from 0.65 W/m·K to 0.9 W/m·K. Therefore, their thermal conductivity should be improved.

Many experiments have been conducted to increase the backfill material’s thermal conductivity by mixing pure materials with one or more additional mediums. For instance, Li et al. [54] increased the thermal conductivity of a sand/bentonite blend by adding quartzite of graphite. Furthermore, Al-Ameen et al. [55] mixed grout materials with metallic parts and found that the modified backfill material can increase GHE thermal performance by up to 77% when compared to pure sand. In addition, Remund et al. [56] concluded that adding quartzite sand to bentonite-based grout can improve the backfill material’s thermal conductivity. Moreover, Lee et al. [57] improved the backfill material’s thermal conductivity by adding quartzite sand to bentonite-based grout. Additionally, Wang et al. [58] pointed out that increasing the percentage of bentonite to sand can improve the backfill material’s thermal conductivity. Liang et al. [59] found that incorporating sand/kaolin can reduce moisture migration and improve heat diffusion. Furthermore, Zhang et al. [42] improved sand’s thermal conductivity by adding bentonite. Additionally, Lee et al. [60] mixed cement with silica sand and graphite and observed that the backfill material’s thermal conductivity increased by 2.6 W/m·K. Better yet, Lee et al. [57] added silica sand and graphite to bentonite grouts. According to the experimental results, the thermal conductivity of bentonite grouts increased to up to 3.5 W/m·K, and their viscosity increased. Even better, Delaleux, et al. [61] added graphite flakes to bentonite and discovered that the backfill thermal conductivity increased to 5 W/m·K.

Choi and Ooka [33] compared between two backfill materials: gravel-backfilled and cement-grouted. They found that gravel-backfilled materials reduced the borehole thermal resistance (BTR) more effectively than cement-backfilled materials, and the heat transfer rate almost doubled. Furthermore, Wang et al. [58] found that utilizing a sand–bentonite mixture rather than standard sand–clay material can increase the heat transfer rate by up to 31%. Agrawal et al. [62] concluded that using wet sand–bentonite as a backfilling material can reduce the required pipe length by 20% compared to using dry sand–bentonite.

Table 3. The conventional backfill materials and additives for ground heat exchangers.

Refs.	Material or Additions	Results
[33]	Gravel and cement	Gravel reduced borehole thermal resistance more effectively The heat transfer rate almost doubled The BTR of gravel-backfilled and cement-grouted BHEs decreased by 9.8% and 8.7%, respectively
[34,39,40,42]	Backfill material with high thermal conductivity	Improved heat transfer rate and raised GHE heat capacity Increased borehole surface temperature; reduced thermal resistance Strong connection between pipe wall and surrounding ground Reduced GHE pipe length, especially for double U-tube pipes
[35]	Backfill material (3 W/m·K)	Increased heat extraction by more than 10%
[36]	Backfill material (0.7 W/m·K to 2.3 W/m·K)	Lowered borehole thermal resistance from 0.135 to 0.054 m·K/W
[37]	Backfill material (0.5 to 2 W/m·K)	Increased heat transfer rate by 40–48%
[38]	Bentonite-based materials	Reduced borehole length by 10%
[49]	Quartz sand instead of bentonite	Lowered borehole thermal resistance by 30%
[59]	Sand/kaolin mixture	Reduced moisture migration and improved heat diffusion
[57]	Silica sand and graphite in bentonite grout	Increased thermal conductivity up to 3.5 W/m·K and increased viscosity
[61]	Graphite flakes in bentonite	Increased backfill material thermal conductivity to 5 W/m·K
[60]	Cement with silica sand and graphite	Increased backfill material thermal conductivity to 2.6 W/m·K
[43]	Graphite with silica sand and bentonite	Increased viscosity and enhanced backfill material thermal conductivity
[54,56,58]	Quartzite or graphite in sand/bentonite Quartzite sand in bentonite-based grout Bentonite to sand mixture	Improved thermal conductivity of sand/bentonite blend
[55]	Metallic parts in grout	Increased GHE thermal performance by up to 77%
[63]	Kaolin/sand	Enhanced the GHE thermal performance
[62]	Wet sand–bentonite	Reduced required pipe length by 20%
[64]	20% Bentonite 30% Bentonite Cement mortar Concrete (50% quartz sand)	Increased the backfill material thermal conductivity to up to 0.73 W/m·K Increased the backfill material thermal conductivity to up to 0.74 W/m·K Increased the backfill material thermal conductivity to up to 0.78 W/m·K Increased the backfill material thermal conductivity to up to 1.9 W/m·K

illustrates the conventional backfill materials and additives for ground heat exchangers.

2.3. Borehole Layout

2.3.1. Borehole Depth

As borehole depth increases, heat transfer improves. However, beyond a certain depth, the borehole starts to lose more heat than it gains. Furthermore, increasing the borehole depth extends the water circulation time and lengthens the pipe, leading to higher energy requirements for water circulation inside the underground loop and increasing initial capital costs. Some suggestions recommend limiting borehole depth to minimize heat loss. For instance, Song et al. [65] suggested that the borehole depth for a single U-tube would be approximately 80 m. Furthermore, Zhou et al. [16] concluded that the borehole depth for a double U-tube with a 25 mm diameter ranges from 80 to 100 m, while for a double U-tube with a 32 mm diameter, it ranges from 90 to 110 m. Zhongjian and Zheng [66] pointed out that the depth of borehole heat exchangers typically ranges from 30 to 90 m, with diameters generally between 76 and 127 mm. Chen et al. [67] investigated the heat transfer capabilities of borehole heat exchangers (BHE) at depths ranging from 60 to 100 m to determine the optimal depth for vertical ground heat exchangers (VGHE). The simulation results demonstrated that the highest heat exchange rate per unit depth was attained at a depth of 70 m, making this the optimal depth.

Rybach and Sanner [39,68,69] indicated that borehole depths generally range from 40 to 200 m, with diameters of 75 to 150 mm. Furthermore, borehole depth normally ranges from 30 to 120 m according to the Energy Saver published by the Department of Energy [21,70,71]. Vertical borehole depths for ground source heat pump systems (GHPS) typically range from 15 to 400 m, with most studies reporting depths between 20 and 200 m and diameters of 100 to 200 mm [72,73,74,75,76,77,78,79,80]. Furthermore, to function effectively, vertical ground heat exchanger (VGHE) systems typically require depths of around 120 m, generally not exceeding 250 m, and diameters of 150 mm [5,81]. Sliwa et al. [82] indicated that the borehole heat exchangers typically do not exceed depths of 200 m, although some, known as deep borehole heat exchangers (DBHE), can extend up to nearly 3000 m.

Yang et al. [83] concluded that increasing the depth of the vertical ground heat exchanger borehole from 80 to 140 m resulted in a reduction of heat transfer per unit by approximately 9.72%. The optimal depth for the VGHE borehole is typically between 100 and 120 m [83]. More precisely, the optimal borehole depth is approximately 100 m [84]. At a depth of 100 m, the temperature typically remains at about 12 °C, maintaining a relatively constant value throughout the year that is unaffected by seasonal changes [85]. Ahmad [86] concluded that in some cases, using a mix of borehole depths can lead to a reduction of up to 10% in borehole depth compared to utilizing boreholes with the same depth. Li et al. [87] indicated that thermal interference diminishes as the borehole depth and diameter increase. Moreover, increasing the borehole depth from 18 to 24 m increases the heat exchange rate by 30% [88,89]. According to [88,89,90], the difference between the inlet and outlet temperatures increases as borehole depth increases, although this relationship is not linear [88,90]. For instance, Sandler et al. [90] concluded that increasing the borehole depth by 200% results in only a 36% increase in the difference between the input and output temperatures. Furthermore, increasing the borehole depth exacerbates thermal shunting [90], and increases outlet temperatures in heating mode, but lowers performance and pressure in cooling mode [91,92]. Therefore, increasing borehole depth is not a cost-effective solution due to the elevated investment costs [73,90]. It is essential to identify the optimal borehole length to minimize the total costs.

Borehole depth depends on the geological site’s ground characteristics; when the ground temperature is high, there is no need to drill deep. For example, Salhein [5] conducted a comparative study of the efficiency of three geothermal heat pump systems (GHPSs) installed at Oklahoma State University (USA), Universitat Politècnica de València (Spain), and Oakland University (USA). The results revealed that the GHPS at Oklahoma State University was the most effective, achieving the highest temperatures with a borehole depth of just 20 m. It was followed by the system at Universitat Politècnica de València, with a borehole depth of 50 m, and the system at Oakland University, which had the least effectiveness with a borehole depth of 98 m. This is due to the higher ground temperatures in Oklahoma (22.1 °C) compared to those in Valencia (18.5 °C) and Michigan (11.6 °C). Furthermore, the highest coefficient of performance (COP) for geothermal heat pump systems was 4.9 at Oklahoma State University, followed by 4.5 at the University Politècnica de València, and 4.1 at Oakland University, as shown in Figure 6. According to the results [5], installing GHPSs at sites with good thermal properties (i.e., high ground temperature) is crucial, as it can reduce both initial capital and operating costs.

2.3.2. Borehole Diameter

When the borehole diameter is large, heat exchange between the two pipes decreases, leading to an increase in borehole thermal resistance [88,93]. A small borehole diameter requires less boring and grouting volume, which makes the installation process both more cost effective and faster. For instance, Dehkordi et al. [94] concluded that minimizing the space between the outer pipe walls and the borehole wall, thereby adopting a more compact borehole design, can achieve acceptable thermal performance while also optimizing the volumes needed for drilling and grouting. However, a larger diameter slightly improves the GHE’s performance [95]. Luo et al. [96] conducted an experimental study of borehole heat exchangers in a ground source heat pump (GSHP) system in Nuremberg, Germany, involving 18 boreholes divided into three blocks with diameters of 121 mm, 165 mm, and 180 mm, monitored from March 2009 to October 2012. They found that larger borehole diameters slightly improved thermal performance. Specifically, thermal loads for 165 mm and 180 mm diameters were 1.64% and 3.45% higher than those for the 121 mm diameter, respectively. Focaccia and Tinti [78] recommended that the diameter of borehole heat exchangers should be 127 mm for a single U-tube, 152 mm for a double U-tube, and 200 mm for a double U-tube with a tube diameter of 40 mm.

Research studies have recommended various borehole diameters. Yang et al. [97] indicated that the typical borehole heat exchanger diameter generally ranges from 100 mm to 200 mm. Ahmad and Chiasson [84] pointed out that the borehole radius is approximately 75 mm. Furthermore, borehole diameters reported in various studies include approximately 120 mm [98], and 150 mm [39,76,81], and ranges from 76 and 127 mm [66], from 75 mm to 150 mm [68,99], from 110 to 200 [88], from 180 to 205 mm [100], from 150 to 250 [83,87], from 112 mm to 300 mm [101], and 300 mm to 900 mm [74]. Dehkordi et al. [94] found that the thermal resistance for a borehole with a 100 mm diameter is only 1.5 times higher than that for a borehole with a 175 mm diameter. According to their study, borehole wall temperatures are more affected by changes in borehole diameter. Therefore, it is worth noting that the most common borehole diameter ranges from 100 to 200 mm, depending on the borehole configuration (e.g., single U-tube or double U-tube).

2.3.3. Borehole Distance

It is important to note that the space between two boreholes significantly affects the GHE’s borehole thermal resistance. Adequate spacing between boreholes is essential to prevent thermal interference, which can compromise the performance of the ground heat exchanger (GHE). Proper distance ensures optimal efficiency and effectiveness of the system by avoiding negative impacts on heat transfer. If the distance between boreholes is less than the recommended specifications for the GHE pipe installation, increased thermal interference may occur, adversely affecting system performance. Conversely, if the distance exceeds optimal levels, a larger land area is required for installation, necessitating an expanded pipe network to connect the boreholes, which in turn increases initial capital costs. Consequently, it is essential to balance this distance to avoid both excessive thermal interference and unnecessary increases in installation costs [7,22,102]. Many researchers have suggested maintaining a specific distance between boreholes to achieve the most effective design of vertical ground heat exchanger (VGHE) systems. For instance, Gultekin et al. [20] and Stuart J. et al. [103] determined that the spacing between boreholes should be maintained within a range of 4.5 to 6 m to prevent thermal interactions and achieve optimal performance. Furthermore, Sagia et al. [104] concluded that a spacing of 5.5 m between boreholes effectively prevents thermal interference. Increasing the distance beyond this point does not significantly reduce borehole thermal resistance, whereas decreasing the distance, particularly below 5 m, results in a proportional increase in thermal resistance [104]. Furthermore, Signorelli et al. [105] pointed out that the minimum distance between borehole heat exchangers should not be less than 8 m to ensure sustainable performance. More precisely, the space between the VGHE boreholes should be at least 5 to 6 m, as indicated by [70,71]. Han et al. [106] found that spacing borehole heat exchangers 4 to 6 m apart has a minimal impact on the design accuracy of the ASHRAE method. Desideri et al. [107] developed the pipe group model using borehole spacings of 4 m, 8 m, and 12 m, and conducted numerical simulations using TRNSYS 16 to evaluate its performance. The results revealed that with closer borehole spacing, the temperature fluctuations in both the soil and the heat exchange medium became excessively high. Moreover, Sailer et al. [108] found that the spacing between borehole heat exchangers typically runs from 5 to 8 m; however, increasing the distance beyond 8 m does not affect system design, therefore 6 m is optimal. Matteo and Adriana [109] concluded that the distance between VGHE boreholes typically ranges from 3 to 7 m, and the optimal spacing is around 7 m. Gultekin et al. [20] agreed with Stuart J at el. [103], Sagia et al. [104], Zheng et al. [110], and Guo et al. [111] that the optimal distance between borehole heat exchangers is 5 m.

2.3.4. Borehole Configuration

There are several borehole heat exchanger (BHE) configurations, such as single U-tube, double U-tube, triple U-tube, and multi-U-tube designs, each offering different performance characteristics, which influence efficiency and application applicability. For example, Zeng et al. [112] evaluated the thermal resistances for both single U-tube and double U-tube borehole configurations. The findings demonstrated that the double U-tube borehole configuration outperformed the single U-tube configuration, with its thermal resistance reduced by 30–90% compared to that of the single U-tube borehole’s thermal resistance. At a constant borehole depth, a double U-tube reduces borehole thermal resistance by nearly 70% compared to a single U-tube and increases thermal conductivity by 8–10% [113]. Additionally, connecting the double U-tube configuration in parallel, rather than in series, significantly enhances the heat transfer efficiency of the ground heat exchanger (GHE) [112]. Qi et al. [114] established a mathematical model to examine the heating thermal performance of a U-tube ground heat exchanger (GHE) in both parallel and series configurations. The results revealed that the parallel configuration significantly outperformed the series configuration in terms of heating performance. Furthermore, Desideri et al. [107] concluded that a parallel BHE configuration results in higher average ground temperatures. Better yet, Florides, et al. [115] assessed the heat transfer performance of single and double U-tube borehole heat exchanger configurations, examining their efficiency in series and parallel connections. According to their results, the double U-tube BHE configuration demonstrated superior performance compared to the single U-tube BHE configuration in both series and parallel connections. Even better, the multi-U-tube BHE configuration outperforms both the double-U-tube and single-U-tube BHE configurations, as indicated by Zarrella et al. [116]. However, there are instances where the double U-tube configuration performs better than the others. For example, Miyara et al. [117] conducted an experimental study to assess the performance of double tube, single tube, and multi-tube BHE configurations installed in a steel pile foundation. The results demonstrated average heat exchange rates of 49.6 W/m for the double tube, 34.8 W/m for the multi-tube, and 30.4 W/m for the single U-tube. This indicates that the double tube BHE configuration provided the highest heat exchange rate, followed by the multi-tube configuration, while the single U-tube GHE exhibited the lowest rate.

2.4. Pipe Thermal Properties

2.4.1. Pipe Material

The pipe is a heat transfer interface between the working fluid inside the pipe (e.g., pure water or an antifreeze solution) and the surrounding soil or backfill material. Since heat transfer performance is influenced by the pipe material, the material should have high thermal conductivity to enhance heat transfer in either direction. Furthermore, the pipe should be resilient, adaptable, long lasting, and resistant to wear, leaks, and damage; easy to install; and cost effective [5,7,22,118]. High-density polyethylene (HDPE) pipe is commonly used in geothermal heat pump system projects due to its flexibility, durability, affordability, and long lifespan, even though its thermal conductivity is lower than that of metal pipes. HDPE pipes can last for up to 50 years [14]. The cost of HDPE pipe typically ranges from USD 1 to USD 30 per linear foot, depending on several factors, including pipe diameter, wall thickness, length, regional pricing, and specifications [119]. On the other hand, despite the many benefits of HDPE, its thermal conductivity is relatively low, typically ranging from 0.35 W/m·K to 0.49 W/m·K [120,121,122]. It can be observed that the thermal conductivity of HDPE pipe material is lower than that of the liquid and the surrounding ground. This results in reduced heat transfer in both directions, increased borehole thermal resistance, and ultimately reduced GHE [5]. Therefore, improving the thermal conductivity of HDPE pipe material is essential to enhance the heat transfer process.

Numerous advancements in HDPE pipe material manufacturing have been implemented to enhance its thermal conductivity. For example, Bassiouny et al. [123] mixed 2 mm and 3 mm aluminum wires with HDPE pipe material and found that the thermal conductivity increased by 25% and 150%, respectively, compared to the original HDPE pipe. Furthermore, Raymond et al. [124] mixed some additives to the polymer resin during the initial HDPE pipe extrusion. The results indicated that the new pipe material’s thermal conductivity increased by 75% (from 0.4 to 0.7 W/m·K) and reduced borehole thermal resistance by 24%. Moreover, Benamar et al. [125] incorporated longitudinal fins onto the inner surface of the original HDPE pipe. They concluded that the new pipe material increased heat extraction by about 7% compared to the pure HDPE pipe. Narei et al. [126] added graphite to the original HDPE pipe material. The new graphite/HDPE composite reduced borehole length by 12.76% compared to the original HDPE pipe. Guo et al. [127] incorporated 40% boron nitride (BN) sheets and 7 wt.% carbon nanotubes (CNTs) into ultra-high-molecular-weight polyethylene composites, achieving a thermal conductivity of 2.38 W/m·K.

Covalent functionalization of boron nitride nanotubes (BNNTs) with short polyethylene chains can increase thermal conductivity by up to 250% compared to the pure HDPE matrix, as concluded by Quiles-Díaz et al. [128]. Travaš et al. [122] added metallic and non-metallic additives to pure HDPE pipe, significantly improving its thermal conductivity. Tests on HDPE composites with expanded graphite (EG) and boron nitride (BN) particles—both micro and nano—demonstrated enhanced thermal and tensile properties. The primary drawback of these composites is a reduced yield strain, decreasing from 8.5% in pure HDPE to 1.8%, depending on additive ratios and BN particle size [122]. Moreover, Li and Xiaolong [129] fabricated HDPE/CNTs porous scaffolds using a sacrificial template method, melt blending, and solvent etching. These scaffolds prevented PW leakage and increased the thermal conductivity of HDPE/CNTs/PW-3:7 by 2.94 times and electrical conductivity by 13 orders of magnitude compared to HDPE/PW-3:7, respectively. Enhancing the thermal conductivity of the pipe material can lower borehole thermal resistance, reduce the required pipe length, and improve heat extraction from the ground, all of which can contribute to reduced initial installation costs [7,22,122].

Table 4. Improvements in thermal properties of HDPE pipes.

Refs	Additives	Results
[123]	2 mm and 3 mm aluminum wires	Thermal conductivity increased by 25% with 2 mm wires and 150% with 3 mm wires compared to the original HDPE pipe
[124]	Additives mixed into polymer resin during extrusion	Thermal conductivity increased by 75% (from 0.4 to 0.7 W/m·K); borehole thermal resistance reduced by 24%
[125]	Longitudinal fins on the pipe’s inner surface	Heat extraction increased by about 7% compared to pure HDPE pipe
[126]	Graphite	The borehole length reduced by 12.76% compared to the original HDPE pipe
[127]	40% boron nitride sheets and 7 wt.% carbon nanotubes	Achieved a thermal conductivity of 2.38 W/m·K
[128]	Covalent functionalization of BNNTs with short polyethylene chains	Increased thermal conductivity by up to 250% compared to pure HDPE matrix
[122]	Metallic and non-metallic additives, including expanded graphite (EG) and boron nitride (BN) particles	Enhanced thermal and tensile properties; reduced yield strain from 8.5% (pure HDPE) to 1.8%, depending on additives and BN particle size
[129]	Carbon nanotubes (CNTs) in porous scaffolds	Increased thermal conductivity of HDPE/CNTs/PW-3:7 by 2.94 times and electrical conductivity by 13 orders of magnitude compared to HDPE/PW-3:7
[130]	50 wt.% EG	Achieved a thermal conductivity of 2.18 W/m·K; tensile strength decreased by 40% compared to pure HDPE.
[122]	15 wt.% EG and 25 wt.% mBN	Achieved a thermal conductivity of 3 W/m·K; tensile strength 24.37 MPa
[131]	55 wt.% EG	Achieved a thermal conductivity of 1.97 W/m·K
[132]	50 wt.% micro-sized hexagonal BN	Thermal conductivity: 2.08 W/m·K (non-treated); 0.9 W/m·K (titanate-treated)
[133]	5 wt.% BN nanosheets	Thermal conductivity: 0.96 W/m·K (with silane coupling agent)
[134]	BN treated with PE-g-MAH	Thermal conductivity: 2.6 W/m·K (treated) vs. 2.2 W/mK (non-treated)
[135]	Boron Nitride (BN) filler in HDPE	Achieved a maximum thermal conductivity of 1.26 W/m·K with 35 vol% BN loading and a tensile strength of 40 MPa
[136,137,138]	5 wt.% graphene nanoplatelets (GNPs)	Increased thermal conductivity from 0.45 to 0.65 W/m·K
[139]	30 wt.% Boron Nitride (BN)	Increased HDPE thermal conductivity to 0.93 W/m·K
[140,141,142]	5-30 wt.% Boron Nitride (BN)	Enhanced thermal conductivity of polymers; increased thermal conductivity from 0.479 to 0.93 W/m·K

summarizes the developments in the thermal properties of HDPE pipes.

2.4.2. Shank Spacing

The spacing between the two legs of the U-tube pipes, also known as shank spacing, affects heat transfer performance [5,143]. When the shank spacing is too narrow, it can cause a thermal short circuit phenomenon and reduce heat transfer efficiency. Therefore, thermal shunting increases as the pipe spacing decreases [90,144]. To prevent a thermal short circuit and improve heat transfer performance in the vertical ground heat exchanger (GHE), the U-tube pipe must be centered in the borehole with proper spacing around it. Cui et al. [145] studied the impact of different shank spacings on thermal heat transfer rates, specifically at 10 mm, 20 mm, 40 mm, 60 mm, 80 mm, and 100 mm. The results indicated that as the shank spacing increased, both the output temperature and the heat transfer rate improved. Therefore, the spacing between the U-tube pipes should be maintained between 60 mm and 100 mm. Furthermore, a spacing between U-tube pipes of less than 60 mm can lead to a thermal short circuit, reducing heat transfer efficiency [146]. However, Yuanlong et al. [145] indicated that shank spacing greater than 60 mm effectively prevents thermal short circuits. For example, in the geothermal heat pump system (GHPS) described in [147], the shank spacing was 70 mm, and no thermal short circuit was observed. Similarly, the GHPS system in [148] used a shank spacing of 85 mm, and no thermal short circuit occurred. Better yet, according to Tang and Hossein [29], stability in heat transfer rates can be achieved if the shank spacing exceeds 90 mm. Furthermore, Zheng et al. [110] and Yang et al. [83] found that the optimal spacing between two U-tube pipes ranges from 100 mm to 200 mm. Figure 7 shows the shank space within a U-tube pipe in a vertical ground heat exchanger.

Vella et al. [149] utilized a 3D steady-state computational fluid dynamics (CFD) model to evaluate the effect of shank spacing on the thermal performance of two shallow U-tube ground heat exchangers, with depths of 20 m and 40 m. The results showed that while temperature drop varies with shank spacing, performance gains decrease as spacing increases, with pipe length having a greater impact than shank spacing for the vertical U-tubes.

2.5. Working Fluid Properties

2.5.1. Fluid Type

The working fluid, which transfers heat between the ground and the heat pump system, should have good thermal properties—high heat capacity, high thermal conductivity, and low viscosity—since these characteristics affect the convective thermal resistance between the fluid inside the pipe and the surrounding ground [7,22,150]. Dada and Benchatti [151] assessed the thermal properties of water, gasoline, and glycol as fluid carriers. The result showed that the gasoline achieved higher temperatures, especially in cold weather, while water stored and recovered more heat than gasoline or glycol. Pure water has been widely used as a heat-carrying fluid in ground source heat pump (GSHP) systems due to its high heat absorption and storage capacity, availability, as well as its low cost. However, its use is limited by its poor antifreeze properties. In cold climates, adding antifreeze solution to pure water can prevent freezing and enhance the heat transfer performance of ground heat exchangers (GHE). When mixing antifreeze solution with the working fluid, it is crucial to use the precise volume concentration to prevent freezing, ensure optimal viscosity, and enhance the GHE’s thermal performance while keeping operating costs low [29,152]. However, incorrect proportions can affect convective thermal resistance (i.e., between the fluid inside the pipe and the inner pipe wall) and consequently reduce the GHE’s performance. For example, Neuberger et al. [153] indicated that mixing 67% pure water with 33% ethanol raises kinematic viscosity, reduces thermal conductivity, and raises convective thermal resistance, which affects heat exchange. However, reducing ethanol to 20% improves the fluid’s properties and enhances heat transfer between water inside the pipe and the surrounding ground. Adding 24% ethanol to the working fluid was more effective than adding 20% CaCl2, 25% propylene glycol, or 33% propylene glycol, as indicated by Fujiao and Hossein [29]. Additionally, Casasso et al. [73] found that a 20% calcium chloride solution is more effective than 25% and 33% propylene glycol solutions and a 24% ethanol solution. Giuseppe et al. [154] pointed out that pure water produces a lower fluid temperature in the GHE than a 25% ethylene glycol and water mixture in a mild climate.

Recently, engineers have increasingly utilized nanofluids as the working fluid in ground source heat pump (GSHP) systems, owing to their significant enhancement in the convective heat transfer coefficient [155]. Nanofluids are advanced fluids containing nano-sized particles (1–100 nm) suspended in a base fluid, typically metal or metal oxide, which enhance both heat conduction and convection to improve heat transfer efficiency [156,157]. Diglio et al. [158] numerically studied the use of nanofluids as an alternative to traditional ethylene glycol/water mixtures for use as heat carriers in borehole heat exchangers (BHE), analyzing the effects of different nanofluids with nanoparticle concentrations ranging from 0.1% to 1% on thermal resistance and pressure drop. The findings indicated that copper nanofluid achieved the most significant reduction in thermal resistance, reaching up to 3.8% at a 1% concentration, though it also led to a significant increase in pressure drop. However, copper-based nanofluid costs around EUR 10 per meter and represents about 12% of the total cost of the BHE.

Furthermore, Asadi [159] developed a three-step guideline for selecting effective nanofluids for heat transfer. This included preparing nanofluid samples with varying concentrations of MWCNT-ZnO nanoparticles, evaluating their stability, and measuring thermal conductivity at different temperatures. The results revealed that the nanofluid increased convective heat transfer coefficients by 42% and recommended its use, especially as a coolant, for enhancing heat transfer performance in various applications. Moreover, Hamid et al. [160] investigated the use of Al₂O₃/water nanofluid for reducing borehole length in vertical ground source heat pumps, although the optimized nanofluid achieved less than a 1.3% reduction in borehole length compared to pure water. They highlighted that backfill material has a more significant effect on reducing borehole length than either the heat transfer fluid or the pipes. Alim et al. [161] theoretically investigated how nanoparticles (Al₂O₃, CuO, SiO₂, TiO₂) suspended in pure water influence entropy generation, heat transfer improvement, and pressure drop within a flat plate solar collector. The results demonstrated that water/CuO nanofluid reduces entropy generation by 4.34% and increases heat transfer by 22.15% compared to pure water, with only a slight 1.58% increase in pumping power. Duangthongsuk and Wongwises [155] experimentally investigated the heat transfer coefficient and friction factor of TiO₂-water nanofluids, with 21 nm TiO₂ nanoparticles at volume concentrations of 0.2% to 2%, in a horizontal double-tube counter-flow heat exchanger under turbulent flow conditions. They found that TiO₂-water nanofluids improved the heat transfer coefficient by approximately 26% compared to the base fluid at lower concentrations. However, at a 2% volume concentration, the heat transfer coefficient was approximately 14% lower than that of the base fluid.

Furthermore, Tsai et al. [162] experimentally investigated gold-DI water nanofluids in a conventional heat pipe and observed that the use of these nanofluids significantly reduced the heat pipe’s thermal resistance compared to DI water at the same concentrations. Moreover, Dongsheng and Yulong [163] investigated the convective heat transfer coefficient of Al₂O₃-DI water in a copper tube under constant heat flux, specifically targeting the entrance region in laminar flow. The results demonstrated that the local heat transfer coefficient was enhanced with rising Reynolds numbers and higher nanofluid volume concentrations. Bock Choon and Young [164] experimentally examined the heat transfer performance of γ-Al₂O₃ and TiO₂ nanoparticles dispersed in water flowing through a horizontal circular tube under turbulent flow conditions and constant heat flux. The finding revealed that the Nusselt number of the nanofluids increased with both Reynolds number and volume concentration. Despite this, the convective heat transfer coefficient of nanofluids with 3 vol.% nanoparticles was still 12% lower than that of pure water under the same conditions. Heris et al. [165,166] experimentally examined the heat transfer performance of Al₂O₃-water and CuO-water nanofluids in an annular concentric tube under a constant wall temperature for laminar flow. According to their results, the heat transfer coefficient improved with higher Peclet numbers and greater particle volume concentrations, with Al₂O₃-water nanofluids achieving a higher heat transfer enhancement than CuO-water nanofluids. Li et al. [167] experimentally evaluated the heat transfer and friction factors of Cu-water nanofluid in a tube, measuring both laminar and turbulent flow conditions. The results revealed that nanoparticles significantly increased the heat transfer coefficient—by about 60% for 2.0 vol% Cu nanoparticles—while having little effect on the friction factor at low concentrations.

Table 5. Experimental results on enhancements in flow properties of nanofluids.

Refs	Additives	Results
[73]	20% calcium chloride, 25% and 33% propylene glycol, 24% ethanol	20% calcium chloride solution is more effective than the other solutions
[154]	Pure water, 25% ethylene glycol/water mixture	Pure water produces a lower fluid temperature in a ground heat exchanger compared to the ethylene glycol mixture
[155]	TiO2-water nanofluid (21 nm, 0.2% to 2% concentration)	Improved heat transfer coefficient by 26% at lower concentrations, but decreased by 14% at 2% concentration
[158]	Copper nanofluid (0.1% to 1% concentration)	Copper nanofluid reduced thermal resistance by up to 3.8% at 1% concentration but increased pressure drop significantly
[159]	MWCNT-ZnO nanoparticles	Increased convective heat transfer coefficients by 42%, recommended for enhancing heat transfer performance
[160]	Al2O3/water nanofluid	Achieved less than 1.3% reduction in borehole length compared to pure water; backfill material has a greater impact
[161]	Al2O3, CuO, SiO2, TiO2 nanoparticles in water	Water/CuO nanofluid reduced entropy generation by 4.34% and increased heat transfer by 22.15% with a slight increase in pumping power
[162]	Gold-DI water nanofluids	Significant reduction in thermal resistance compared to DI water
[164]	γ-Al2O3, TiO2 nanoparticles dispersed in water	Nusselt number increased with Reynolds number and particle concentration; convective heat transfer coefficient with 3 vol.% nanoparticles was 12% lower than pure water
[165,166]	Al2O3-water, CuO-water nanofluids	Heat transfer coefficient increased with Peclet number and particle concentration; CuO-water showed less enhancement than Al2O3-water
[167,168]	Cu-water nanofluids	Enhanced heat transfer performance; friction factor matched water; proposed new correlations for laminar and turbulent flow
[163]	Al2O3-DI water nanofluids	Local heat transfer coefficient increased with Reynolds number and particle concentration
[169]	Graphite nanoparticles in two liquids	Heat transfer coefficient increased with Reynolds number and particle concentration; different sources of nanoparticles showed varying results
[170]	CNT-distilled water nanofluids	Local heat transfer coefficient significantly higher than pure water; enhancement depended on flow conditions, CNT concentration, and pH value; aspect ratio is important
[171]	TiO2-distilled water nanofluids	Local heat transfer coefficient increased with nanoparticle concentration; pressure drops similar to the base fluid
[172]	Al2O3 nanoparticles in water	Higher heat transfer coefficient compared to base liquid; 36 nm particles performed better than 47 nm particles
[173]	CuO-water nanofluids	Greater energy absorption at low flow rates; no added benefit at high flow rates
[174,175]	0.2 vol.% TiO2 nanoparticles in water	Thermophysical models had minimal effect on predicted Nusselt number; heat transfer coefficient slightly greater than water (6–11%); minimal penalty in pressure drop

summarizes the experimental results on improvements in nanofluids.

2.5.2. Fluid Velocity

Numerous studies have investigated how varying fluid velocity in the pipe influences both the exit temperature and the overall heat transfer efficiency in vertical ground heat exchangers (VGHEs). For instance, Zhou et al. [16] investigated how altering the water velocity in vertical U-tube pipes with different diameters and configurations influenced heat transfer performance. The findings revealed that the optimal water velocity ranges for improving heat transfer performance were 0.4 to 0.6 m/s for a single U-tube with a 32 mm outer diameter, 0.4 to 0.5 m/s for a double U-tube with a 25 mm outer diameter, and 0.3 to 0.4 m/s for a double U-tube with a 32 mm outer diameter. Furthermore, Yong et al. [88] proposed that, to achieve maximum efficiency, the water velocity in a single U-tube should be maintained between 0.4 m/s and 0.7 m/s. Additionally, Salhein [5] recommended a water velocity range of 0.28 to 0.38 m/s for a single U-tube with a 32 mm outer diameter at a depth of 50 m, and a range of 0.17 to 0.22 m/s for the same diameter U-tube at a depth of 20 m. Similarly, Salhein et al. [5,7,22] concluded that to maximize the benefit of the Earth’s temperature, the optimal water velocity for vertical single U-tube pipes at a depth of 98 m should be between 0.33 and 0.43 m/s for a 25 mm outer diameter, 0.35 and 0.45 m/s for a 32 mm outer diameter, and 0.38 and 0.48 m/s for a 40 mm outer diameter. As the pipe length increases, the water velocity also increases, as illustrated in Figure 8.

Zhihua et al. [176] used a numerical model to investigate the impact of water velocity on the heat transfer efficiency of deep borehole heat exchangers (DBHEs) at a depth of approximately 2000 m. Their study, which examined water velocities from 0.04 m/s to 1.3 m/s, revealed that the best heat exchanger performance and highest output temperature were achieved with water velocities between 0.3 m/s and 0.7 m/s. Moreover, Shuang et al. [177] investigated the impact of circulating water velocity on the heat exchanger rate by using three different velocities: 0.26 m/s, 0.51 m/s, and 1.02 m/s. Throughout the operational test, the water temperature inside the pipe was consistently maintained at 35 °C. The findings showed that the heat exchanger rates were 84 W/m, 116 W/m, and 94 W/m for water velocities of 0.26 m/s, 0.51 m/s, and 1.02 m/s, respectively. The highest heat exchanger rate was observed at a water velocity of 0.51 m/s, but the rate declined when the water velocity increased to 1.02 m/s. Thus, the optimal water velocity was determined to be between 0.5 m/s and 0.6 m/s. This conclusion indicates that the heat exchanger rate per meter increases with rising water velocity up to a certain threshold; however, once this allowable range is exceeded, the heat exchanger rate begins to decline. Miyara et al. [117] assessed the performance of double-tube, U-tube, and multi-tube ground heat exchangers (GHEs) in a steel pile foundation. The double-tube GHE showed the highest heat exchange rate, with significant performance improvements observed as the flow rate increased from 2 to 4 L/min, but only slight improvements from 4 to 8 L/min. Yang et al. [83] examined the effect of fluid velocity on the heat exchange rate in a vertical U-tube underground heat exchanger based on fluid–structure coupled simulations. The results indicated that increasing the flow rate from 0.4 m/s to 1.0 m/s improved heat transfer per buried depth by 123.34%. Furthermore, Bouhacina et al. [178] studied the impact of water velocity on heat transfer performance in a vertical U-tube with a 32 mm outer diameter. Their findings indicated that a water velocity between 0.3 and 0.4 m/s is optimal for maximizing heat transfer efficiency. Optimizing water velocity can decrease the borehole’s thermal resistance, thereby maximizing heat transfer efficiency, as noted in [179].

Zhu et al. [180] developed a heat transfer model to assess the performance and temperature distribution of a vertical double U-tube borehole heat exchanger (BHE) across a range of flow velocities from 0.1 to 0.5 m/s. The results demonstrated that the heat exchanger rate increased as the water velocity rose from 0.1 to 0.3 m/s but began to decrease when the flow velocity exceeded 0.3 m/s up to 0.5 m/s. The optimal flow velocity was found to be 0.3 m/s. Furthermore, Cui and Zhu [145] established a 3D transient heat transfer model to study the effects of the fluid flow rate on the heat exchanger rate and output fluid temperature, applying it to flow rates between 0.1 m³/h and 1.2 m³/h in a U-tube with a 32 mm diameter. The results indicated that the output fluid temperature increased as the liquid flow rate rose from 0.1 m³/h to 0.7 m³/h, but then decreased as the flow rate further increased beyond 0.7 m³/h to 1.2 m³/h. The optimal flow rate was determined to be between 0.57 m³/h and 0.76 m³/h, which corresponds to 0.3 to 0.4 m/s. Moreover, Salhein et al. [5,7,22] developed a mathematical model of heat transfer behavior between water inside vertical underground pipes and the surrounding ground, to assess how water velocity influences the GHE’s output water temperature. The results showed that the GHE’s output water temperature approached the ground temperature when the water velocity was reduced enough to allow complete heat exchange between the water in the pipe and the surrounding ground (see Figure 9 and Figure 10).

According to Figure 9, the water temperature increased exponentially as water flowed inside the pipe, starting at 2 °C and reaching the ground temperature of 11.6 °C at pipe lengths of 90 m and 95 m when the water velocity was 0.35 m/s and 0.45 m/s, respectively. However, when the water velocity increased to 0.9 and 1.2 m/s, the water temperature did not reach the ground temperature by the pipe’s exit, resulting in significant heat energy loss and reduced efficiency. The water temperature decreased exponentially along the pipe, starting at 22 °C and approaching the ground temperature of 11.6 °C at pipe lengths of 90.5 m and 95.5 m when the water velocity was 0.35 m/s and 0.45 m/s, respectively. However, as shown in Figure 10, when the water velocity increased to 0.9 and 1.2 m/s, the water temperature did not reach the ground temperature.

The aforementioned studies demonstrated the importance of maintaining an optimal water velocity for efficient heat transfer and maximizing output temperature without wasting additional power on the water circulation pump. When the flow velocity is too low, the water reaches the ground temperature too quickly, rendering the rest of the pipe ineffective. Conversely, if the water velocity is too high, more power is needed to increase the flow, and the water does not have enough time to fully exchange heat with the surrounding ground, preventing the output temperature from reaching the ground temperature. Thus, optimal control of water velocity is essential for achieving the desired output temperature [5,7,22].

Table 6. Recommended flow velocity ranges in varying vertical pipe diameters.

Refs	Type of Pipe	Length (m)	Optimal Flow Rate (m/s)
[16]	Single U-tube, Double U-tube	—	0.4–0.6 (32 mm), 0.4–0.5 (25 mm), 0.3–0.4 (32 mm).
[88]	Single U-tube	—	0.4–0.7
[5]	Single U-tube	50, 20	0.28–0.38 (50 m), 0.17–0.22 (20 m).
[5,7,22]	Single U-tube	98	0.33–0.43 (25 mm), 0.35–0.45 (32 mm), 0.38–0.48 (40mm).
[176]	Deep Borehole	2000	0.3–0.7
[177]	U-tube	—	0.5–0.6
[83]	U-tube	—	0.4–1.0
[178]	U-tube	—	0.3–0.4
[180]	Double U-tube	—	0.3
[145]	U-tube	—	0.3–0.4

presents the acceptable ranges of water velocity for various vertical pipe diameters.

3. Discussion and Conclusions

In summary, in this paper we discussed the key factors influencing the performance of vertical ground heat exchangers (GHEs), emphasizing the heat transfer efficiency between the fluid inside the U-tube pipe and the surrounding ground. The first performance factor is the soil’s thermal properties. GHPS is site dependent, performing optimally when the loop pipes are installed in soil with high thermal conductivity and strong heat conduction capacity. Hence, selecting an appropriate geological location with favorable soil conditions, such as moist soil, sand, or sandstone, is crucial for optimal performance. The second performance factor is the backfill material’s thermal properties. The space between the pipe and the surrounding soil must be fully grouted with a high-thermal-conductivity material to ensure a strong thermal connection with the ground, enhancing heat transfer and preventing borehole collapse. Several suggestions have been made to enhance the thermal conductivity of backfill materials by adding components such as graphite flakes, silica, or improved bentonite-based mixtures, combined with minerals like quartz sand, as shown in Table 3. High-thermal-conductivity backfill materials provide significant benefits, such as reducing borehole depth by 10%, reducing borehole thermal resistance by 30%, lowering the thermal gradient by 10%, improving heat exchange rates by 31%, and increasing heat extraction by 10%, all of which contribute to lower initial capital costs [35,38,49,58].

Next, borehole depth is a key factor that impacts system performance. Heat transfer increases as borehole depth grows but eventually reaches a point where the heat loss exceeds the gains. Thus, some recommendations focus on optimizing the depth and configuration of geothermal boreholes to minimize heat losses and maximize the benefits of the earth’s heat. For single and double U-tube pipes, the optimal borehole depth would range between 80 m and 100 m, respectively [5,81,83,84]. When the borehole diameter is large, it leads to an increase in borehole thermal resistance, which reduces the heat exchange rate [88,93]. The borehole diameter should neither be too small nor too large; therefore, the most common diameter typically ranges from 100 to 200 mm, depending on the borehole configuration [74,88,97,100]. In addition, the distance between two boreholes impacts borehole thermal resistance. Sufficient spacing is crucial to avoid thermal interference, which can compromise the GHE’s performance. The optimal distance between borehole heat exchangers is around 5 m [20,103,104,110,111]. Some borehole heat exchangers have better performance than others. For example, the multi-U-tube configuration outperforms triple, double, and single U-tube setups, while the parallel configuration delivers better heating performance than the series configuration [107,112,114,116].

An additional critical performance factor is the pipe material’s thermal conductivity, which should be high to enhance heat transfer between the fluid inside the pipe and the surrounding soil. HDPE pipes are the most commonly used in GHPS design due to their strength, flexibility, and longevity. However, their thermal conductivity is lower than that of the surrounding soil and the fluid inside, highlighting the need to improve their thermal conductivity for better system performance [5,7,14,22,118,119]. Several suggestions to enhance the thermal conductivity of the pipe material are summarized in Table 4. Shank spacing between the legs of U-tube pipes impacts heat transfer efficiency, with narrow spacing leading to thermal short circuits and reduced performance. To prevent this and enhance heat transfer in vertical GHE systems, the U-tube pipes must be properly centered in the borehole with adequate spacing [5,90,143,144]. The optimal spacing between two U-tube pipes ranges from 100 mm to 200 mm [83,110].

Finally, the last key performance factor is the working fluid and its velocity, which are crucial for efficient heat transfer. The working fluid serves as a heat transfer carrier between the ground and the heat pump system, requiring high thermal conductivity, high heat capacity, and low viscosity to prevent convection thermal resistance between the fluid inside the pipe and the surrounding ground and enhance efficiency [7,22,150]. A mixture of water and antifreeze is the most widely utilized heat transfer fluid in ground heat exchanger systems. Improving the thermal properties of the working fluid could offer benefits such as a 1.3% decrease in borehole depth and a 1.14% reduction in the thermal resistance of U-tube ground heat exchanger (GHE) boreholes [154,160]. Several suggestions to enhance the thermal properties of the working fluid are summarized in Table 5. Studies have emphasized that maintaining optimal water velocity is essential for efficient heat transfer and maximizing output temperature without wasting additional power on the water circulation pump. If the flow velocity is too low, the water quickly reaches ground temperature, making the remaining length of the pipe ineffective. On the other hand, if the flow velocity is too high, additional power is required to increase the flow, and the water does not have sufficient time to transfer heat effectively with the surrounding ground, which prevents the output temperature from reaching the ground temperature (refer to Figure 9 and Figure 10). The recommended flow velocity ranges for different pipe diameters are listed in Table 6.

A detailed review of research on factors influencing ground heat exchanger (GHE) performance—including soil thermal properties, backfill material properties, borehole depth, spacing, U-tube pipe properties, and working fluid type and velocity—was presented. Furthermore, we discussed their impact on heat transfer efficiency and proposed optimal solutions for enhancing GHE performance. These suggestions can assist in designing a high-performance vertical ground heat exchanger.