Technological Advancements and Challenges of Geothermal Energy Systems: A Comprehensive Review

Abstract

Geothermal is a renewable energy source, but this is not as often seen as other renewable energy sources, such as solar, wind, hydro, etc. Several applications could be implemented through geothermal energy, and heating & cooling systems are one of them. Because of the limits of technology, it is hard to improve cooling systems as an application. To address long-term sustainable space heating and cooling, it is imperative to develop geothermal technology. It is known as the oldest, most flexible, most adaptable, and most prevalent approach toward using renewable energy. Therefore, this review has reviewed the global development and challenges of geothermal energy for cooling systems. There are large reserves of geothermal energy available around the world, and numerous scholars have emphasized its importance, but due to a lack of knowledge, no operational work has been done in using these systems for cooling up to this point. This review paper examines globally available geothermal energy sources and technologies for environmentally friendly and sustainable cooling supplies. Finally, the benefits and challenges of geothermal systems for cooling are outlined to promote local, regional, and global investment in utilizing these resources for cooling.

1. Introduction

The sun and wind are sources of renewable energy, with global capacities of 623 GW and 586 GW in 2019, respectively. This expansion is predicted to continue on the same upward trajectory [1]. However, electric-grid-connected space conditioning in a building contributes to about 40–50% of the total energy consumed in buildings and has an adverse impact on the environment and human health [2]. While thermal rooftop solar systems might generate directly heated air naturally, sustainable energy produced by onshore and offshore wind turbines and solar installations can be converted into heat using heat pumps. In addition, such intermittent energy production from solar and wind can only be solved by energy storage. Additionally, they often produce electricity rather than heat. But geothermal energy conversion, including both heating and cooling, is available as a non-intermittent and possibly endless source [3].

Some countries have used geothermal energy for thousands of years for heating and cooking. People have used geothermal energy in natural hot springs for many things, like bathing and washing, since the beginning of human history [4]. It is simply energy produced by the heat of the Earth [5]. The rocks and liquids below the crust of the Earth contain this thermal energy [6]. Nearly 4000 miles below the surface of the inner crust, geothermal energy is generated. A solid iron core surrounds molten rock in the Earth’s double-layered core. The Earth’s rocks contain radioactive elements that are deteriorating over time [7]. Thus, the continual generation of a very high temperature inside the Earth leads to the production of geothermal energy [8]. Geothermal energy is one of the sustainable energy sources available because the substantial organic heat energy reserve is located within the earth’s crust [7,9]. Geothermal energy is thought by many to have a huge amount of potential to meet our growing energy needs [10]. Over the last few decades, it has successfully satisfied the energy needs of many global locations for both home and commercial purposes [11]. Two different types of geothermal energy sources exist in the world: (1) high enthalpy and (2) low enthalpy geothermal energy reservoirs [12]. Geological characteristics and local temperature are used to classify geothermal reservoirs [13]. Low enthalpy geothermal storage tanks are located at around 1000 m depth and often with temperatures not exceeding 150 °C. Large geothermal reservoirs are located at a depth of 1000 m, with temperatures over 200 °C. There are many countries involved in the investigation and development of geothermal energy, such as Italy, Kenya, Mexico, Iceland, New Zealand, and the Philippines [14]. However, a wide variety of temperatures, from 60 °C to 350 °C, are accessible for the geothermal energy source [15]. On the other hand, due to technological limitations, geothermal energy generation remains uneconomical at temperatures below 80 °C and with low overall system effectiveness [16,17].

As a renewable resource, geothermal energy has several benefits. It can heat or cool buildings and generate power without putting out any potentially dangerous pollutants. It is capable of continuously generating thermal energy [9]. However, occasionally, it could release harmful gases from deep inside the ground. There are no negative effects if geothermal energy is used properly. For a few reasons, a geothermal system is very effective at cooling a space. The heat pump, which is the first component of the system, moves heat and humidity from one place to another before dispersing it into the ground, where the loop is situated. A constant, low-level air distribution provides cooling, maintaining a comfortable, uniform temperature throughout the spaces. Second, compared to conventional air conditioners, geothermal systems typically dehumidify the air up to 30% more, which naturally balances the moisture levels in any space [18]. In contrast to conventional power generation plants, geothermal power plants utilize locally available renewable resources. On average, geothermal power facilities have very little influence on the environment. Since there is no need for fuel to produce electricity from geothermal heat, geothermal plants’ operating costs are reduced [11]. Additionally, it can provide base-load electricity continuously throughout the day, something that other renewable energies cannot do. For example, the only time solar energy may be generated is during the day, or it is reduced on a cloudy day. Windmills, however, are dependent on airspeed, which is always fluctuating [19]. On the other hand, the geothermal energy supply is stable, and the power stations are thought to release only 13 to 380 g of carbon dioxide per kWh of electricity. Comparatively, power stations that use natural gas emit roughly 453 g of CO₂, compared to 1042 g for coal-fired power plants and 906 g for oil-fired [20]. There have been several surveys and reviews conducted around the world regarding World Geothermal Congresses in places like Italy, Japan, Turkey, Indonesia, India, and Australia, and the scholars found that the breakdown of thermal energy usage is around 58.8% for geothermal heat pumps, 18.0% for bathing and swimming, 16.0% for space heating, 3.5% for greenhouse heaters, 1.6% for industrial uses, 1.3% for farming pond and raceway heating, 0.4% for farmland drying, 0.2% for melted snow and chilling, and 0.2% for many other purposes. Total energy saves about 596 million barrels of oil (81.0 million tonnes) every year. This keeps about 252.6 million tonnes of greenhouse gases and 78.1 million tonnes of carbon from going into the atmosphere [21].

Furthermore, an innovative cascade power generation technology includes a trans-cortical carbon dioxide compression refrigeration system (CCRS) with either a LiBr–H₂O after-cooler or a proton-exchange membrane electrolyzer (PEME) to make up a tri-generation arrangement. A tri-generation system may be installed anywhere on the globe and has great potential for harnessing geothermal energy to efficiently transform low-to-medium-grade clean energy sources into electricity, cooling, and hydrogen. That system’s effectiveness was evaluated by altering the efficient input data from energy, economic, and exergy standpoints [22]. Tri-generation technologies that can generate electricity through an Organic Rankine Cycle (ORC) and offer chilling, warming, or other functions will be highly sought-after in the upcoming year. Four alternative tri-generation facility layouts are examined in two serial concepts, one parallel concept, and one serial–parallel concept. The important results of this study, as summarized, are the COP that a geothermal-driven absorption can achieve, which is the highest yearly net power production. The use of a dual-effectiveness chiller is not advantageous since it necessitates a temperature that greatly limits the amount of heat transfer that is accessible for the ORC operation [23]. In order to produce electricity, distilled water, or hydrogen, correspondingly, a group of authors introduced as well as assessed the viability of a unique hybridization of the improved Kalina cycle, water filtration desalination, and small-temperature water electrolysis. The proposed system is evaluated using a comprehensive evaluation and genetic algorithm-based optimization. In accordance with parametric analysis, changing the flash tank pressure does have the greatest impact on how sensitively the system in its entirety is affected by changing the performance requirements [24]. A single-stage organic Rankine cycle of 10 kW, a single-stage absorption chiller of 15 kW, and a drying chamber of 20 kW, respectively, were utilized to assess cascading aerosol multigenerational using a novel description of such energy, exergy, socioeconomic, and environmental models (3, 4E-Chaiyat models). The findings of this study revealed that first-law efficiency is superior to second-law efficiency. Therefore, the aerosol is the appropriate conditioning method for something like the cascade combined heat and power system employed in San Kamphaeng, Thailand [25]. The examination of a geothermal polygeneration program’s energy consumption and environmental impacts that fulfill the thermal requirement for the energy industry’s cooling, residential hot water, and electrical needs on Ischia Island was conducted. This geothermal-triggered organic Rankin cycle module serves as the system’s foundation for electric space heaters. These simulation and analysis findings were used to contrast the proposed system with the conventional one that supplies the energy community’s need for electricity by drawing power from the national electricity supply. Accordingly, compared to the existing system, the proposed system enables the avoidance of 29.9 t/year of CO₂ emissions and the saving of 119.9 MWh/year of power production [26].

This paper reviews geothermal energy sites and developments in using geothermal energy for cooling. The key terms relevant to optimization have been searched as ‘geothermal systems for cooling’, ‘geothermal systems for tri-generation’, and ‘global advancements and challenges of geothermal systems’. Due to the technical limitations and low advancement level, geothermal energy could be a significant finding through this review. However, the scope of this paper is limited to geothermal energy reservoirs and geothermal systems for cooling purposes. The keywords for this literature review were found using well-known search engines such as IEEE Explore, Web of Knowledge, Google Scholar, and MDPI. The timeline for the search has been limited to almost fifteen years, and journals and conference papers have been encapsulated in the review. The entire literature review has been presented in a chronologically descending order to encapsulate the recent advancements in the cited issue. This study’s main concerns are global problems for geothermal energy sites and developments in using geothermal energy for cooling. The various geothermal energy reservoirs and harvesting techniques are covered in this article. Examining the technological advancements related to geothermal energy along with its advantages and disadvantages, there are six sections in this study. In the Section 2, we talk about geothermal energy reservoirs. In the Section 3, we talk about geothermal energy technologies. In the Section 4, we will talk in more depth about advances in geothermal energy technologies. In the Section 5, we talk about the pros and cons of using geothermal cooling systems. The Section 6 is the conclusion.

2. Geothermal Energy Reservoirs

Geothermal energy is generated by radioactive substance depletion, and the primordial heat produced during the creation of the Earth are the sources of geothermal heat. The typical heat transfer across the surfaces of that same Earth is 82 mW/cm², while the overall production of the world exceeds 4 × 10¹³ W [27]. When the temperature of the earth rises above ambient levels, thermal energy is transferred between the host rock that makes up the planet and the natural fluid that is present in its pores and fissures. This fluid, which primarily consists of water with different levels of dissolved salts, is normally present in an in situ liquid phase. However, it may occasionally be a saturated liquid, liquid vapor mixture, or superheated vapor.

The position and temperature of the resource play a major role in how geothermal energy is used, whether for producing electricity or for other purposes. Even though low temperatures (below 90 °C) or middle temperatures (between 90 °C and 150 °C) seem ideal for clear benefits like warming and cooling space and processes, marine culture, and fish farming, geothermal resources seem ideal for clear benefits like power generation [28]. Utilizing high-temperature geothermal resources for many purposes will improve the system’s effectiveness and save costs [29,30].

Throughout geothermal areas, the temperature of the stones increases with depth. This increase has a typical slope of 30 °C/km. However, there are regions of the earth beneath that can be drilled into where the gradient is much higher than usual. This occurs when a magma mass that is still fluid or is hardening while producing heat is also being cooled just below the layer (a few kilometers below the surface). Specific crustal geological characteristics in some places where magmatic action is absent will promote heat accumulation, resulting in unusually elevated geothermal gradient values [31,32]. The geothermal energy recovery of the reservoir, the recharge region, and the connecting pathways via which cold superficial fluid enters the reservoir and, in most instances, exits back to the surface make up a typical hydrothermal energy system, as shown in Figure 1 [31,33].

The component in this is made up of landfill gas. For instance, heat typically moves from deep to subsurface regions first by conduction and then by convection. These substances are simply rainwater that has seeped into the crust of the earth from recharge places, heated up upon exposure to the heated stones, as well as kept in aquifers, sometimes at hot temperatures and increased pressure (up to over 300 °C). The majority of geothermal areas depend heavily on these aquifers. The reservoir is often covered with impermeable rocks, which prevent heated fluids from rising to the surface and maintain pressure in these reservoirs. Geothermal fields, unlike hydrocarbon fields, usually have a constant flow of heat and fluid. The fluid enters the reservoir through the recharge zones and leaves through the discharge zones (hot springs, wells). A viable geothermal resource must satisfy a number of requirements. Accessibility is the first requirement. This is usually done by drilling to the desired depths, frequently with traditional techniques similar to those used to extract oil and gas from subsurface reserves [34]. Sufficient reservoir production is the second prerequisite. To assure long-term production at levels that are acceptable commercially, hydrothermal systems typically require a substantial volume of warm, organic fluids to be trapped in an aquifer that has a significant rock formation pore structure [6,35]. A reinjection plan is required to sustain output rates whenever the hydrothermal systems do not get enough natural recharge. Integrated transportation methods (sandstone portions with porous or cracked surfaces will transfer heat to different parts of the network, and also the rock itself will conduct heat) take thermal energy first from the reservoir [34]. The in situ hydrologic, lithological, and geologic restrictions that currently exist were taken into consideration while designing the heat extraction method. To finish the process, all byproducts must be carefully handled and disposed of. Geothermal heat production shares many characteristics with the oil, gas, coal, and mining sectors. Due to these parallels and the usage of tools, methods, and terminology that have been adopted or taken from the oil and gas industry to be used in geothermal development, the development of geothermal resources has in some ways been sped up.

Overall energy output ratio withdrawal using a reservoir is governed by a variety of resource-related factors, including temperature gradient, inherent porosities of the stone characteristics, water accumulated in the rock, tensions inside the rock, and even earthquake risk. A well in a good hydrothermal reservoir typically generates 5 MW or more of output electric power using a mixture of high fluid and temperature flow rates. For example, to provide around 4.7 MW of total electrical energy for the system, a well in a deep hydrothermal reservoir generating water at 150 °C must flow at roughly 125 kg/s [6,29,36]. A high saturation point in the reservoir is required for high rates of flow, while resistivity, which comprises the merge which the fluid travels past on its journey up towards the well, may be changed by well structure and permeability. In contrast to oil and gas reservoirs, where measured transmissivity is frequently about 100 mD, geothermal systems typically have very high transmissivity (more than 100 D) [36]. Exceptionally pressured rocks that collapse under shear and movement caused by stimulation should result in cracks that remain open and permit fluid flow. When there are many breaches with tiny fracture ports, both a high discharge rate and modest pressure drop are possible. A linked circulation system is more likely to form stimulation in rocks that have a minimum linked susceptibility through faults or pores [36]. Additionally, the fracture system needs to provide injected cool water for enough time to interact with hot rock so that when it emerges from producing wells, the temperature is close to that of the formation. The greater the reservoir’s life is, the simpler the economics are under the current flow circumstances [36].

3. Geothermal Energy Technologies

Geothermal energy technologies are divided into four major classes: conventional hydrothermal, low-temperature geothermal system, enhanced geothermal system (EGS), and direct use, including geothermal heat pumps (GHPs). The first three classifications produce energy, while the fourth is typically utilized to produce heat and cold water [20]. The following succinctly lists the qualities of each of the four technologies.

3.1. Hydrothermal Energy

• A geothermal structure that occurs naturally.
• Seldom need drilling deeper than 3 km, and the maximum depth that can be reached with the current drilling technique is 10 km [36].
• At Chena warm springs in Alaska, successful electricity production took place in 2006 at a temperature of 74 °C [37].

3.2. Low Temperature Energy Source

• Hot air is produced by a geothermal field at 150 °C or below, which is frequently applied in practical applications.
• Lower-temperature geothermal supplies frequently use Kalina or Organic Rankine Cycle (ORC) [38,39].
• Overall efficiency necessitates huge power machines and equipment (turbine, condenser, pump, and boiler), which might be prohibitively expensive.

3.3. Enhanced Geothermal System (EGS)

• This is a reservoir that is built by humans in an area with hot rock, but little to no natural solubility or saturation. In the EGS system, stimulation is used to increase natural permeability or manufacture it where it is lacking.
• Two wells are drilled in the EGS, which are the production well and the injection well [36].
• In Europe and the US, EGS has been fully implemented on a pilot scale.

3.4. Closed-Loop System

• Through buried or submerged plastic pipes, a closed-loop geothermal system continuously circulates a solution of water and non-toxic antifreeze. An indoor heat pump is connected to these underground pipes [40].
• Closed-loop geothermal systems are efficient for both small and large properties, whether there is a water source nearby or not [40].
• The prototype of a closed-loop geothermal power plant to supply energy in Alberta, Canada was finished by Eavor, a clean energy start-up established in 2017 [41].

The direct use of geothermal energy is summarized as follows:

• For very little money, it is possible to greatly enhance the performance of air-conditioning and heating systems by drilling up to 305 m.
• The heat that is taken from homes by the majority of residential air conditioners is released into the outside air, which is inherently warmer than the inside air.
• A basic heat cooling system has lower running costs than a heat exchange system, but it costs more to build. Both methods require the use of heat pumps, which may transfer heat or absorb it.
• Figure 2 [42] illustrates the differences between systems that interchange heat also with the ground (Figure 2B) or with air (Figure 2A) and contrasts them.
• Presently, installing an air-cooling system that uses ground transference and heating systems is roughly three times more costly than installing a ventilation system.

A ground source heat pump is another possible energy source for a district energy system, which typically has a coefficient of performance (COP) of about 4. A ground source heat pump transfers heat into the ground in the summer and extracts heat from the ground in the winter [43]. Figure 3 illustrates a ground source heat pump for a district heating system consisting of a long (∼50 m) U-shaped ground heat exchanger, a refrigeration cycle to extract heat from the circulating fluid, and a radiator and fan to provide heat to a building [44].

The mild temperature heat from geothermal sources is generally squandered, since it cannot be transformed into electrical power using standard power generation techniques. For the transformation of minimal heat sources into electricity, the trilateral flash cycle, supercritical Rankine cycle, Kalina cycle, Goswami cycle, and ORC have been suggested [45,46,47]. The second law evaluation [48] revealed just a 3% advantage for the Kalina cycle in terms of performance when compared to the Rankine cycle. The ORC, however, requires significantly less upkeep and is less complicated. The ORC employs the same idea as the steam Rankine cycle but recovers heat from heating sources that are at lower temperatures by using organic vapor compression with a low boiling point. When superheat is required, the cycle is set up with a superheater, pumps, boilers, and condensers, as well as an expansion turbine [49,50]. The Kalina cycle is a new notion for power production and heat recuperation. Water and ammonia are combined to form the solvent for the cycle. To boost thermodynamic reproducibility and the overall thermodynamic performance, the proportion of any of certain elements is changed in various areas of the apparatus. The Kalina cycle is designed for temperatures below 170 °C and has a simpler layout than other systems. The Kalina cycle is developed as a utilization of cooling resources, such as geothermal heat resources (refer to Figure 4) [51]. The fundamental factor behind the improvements is that, with the exception of steam, the boiling of an ammonia–water combination takes place across a wide temperature range, increasing the maximum energy content that can be taken back from the gas stream. In support of the Rankine cycle, where minimum temperature limits the condenser, it returns pressure as well as the system power output, and ammonia–water condensation that sometimes happens across a wide range of temperatures typically varies, allowing for further recuperation of heat in the condensing systems. In comparison to Kalina systems, current ORC systems can operate at temperatures above 170 °C while maintaining the same or even higher efficiency (see Figure 4). The Rankine cycle clearly outperforms both the ORC and the Kalina system at 350 °C. Since geothermal fluids have a temperature range, there seem to be inherent restrictions on converting geothermal energy into electricity. Because of the second law of thermodynamics, lower energy source temperatures have a negative impact on both the highest work-producing potential as a result of the fluid’s availability and exergetic and heat-to-power efficiency. For a specific temperature, density, and pressure, the degree of accessibility indicates the most electrical energy that might be generated for a specific geofluid flow [51].

4. Advancements in Technology for Geothermal Resources

Even though geothermal resource techniques currently account for a small portion of district temperature regulation systems, they may play an important role in sustainable growing techniques. Although multiple subsurface levels can be targeted, there is no single description or categorization for shallow, intermediate, and deeper geothermal systems [6,52]. The categorization is challenging because of the wide range of potentially relevant variables, such as digging depth, drilling machine type, liquid temperature, and usage of heating pumps [53]. For such a project, always choose a geothermal resource based on the temperature of the hydrothermal fluid. The applications of geothermal energy based on the temperature are shown in

Table 1. Geothermal Energy Classifications [54].

Geothermal Techniques	Temperatures of the Wellhead Fluids	Production for DHC	Apparatus for Supplying a Structure’s Systems
			Space Heating	Domestic Hot Water	Cooling
Superficial	<25 °C	5GDHC (prevailing temperature)	Heat pump	Heat pump	Electric chiller/direct
Intermediate	25–90 °C	4GDH (low temperature)	Heat pump/Heat exchanger/direct	Heat pump/Heat exchanger/direct	Absorption chiller
Deep	>90 °C	3GDH and 2GDH (high temperature)	Heat exchanger/direct	Heat exchanger/direct	Absorption chiller

2GDH—second generation of district heating; 3GDH—third generation of district heating; 4GDH—fourth generation of district heating; 5GDHC—fifth generation of district heating and cooling.

[54].

Geothermal energy is a renewable resource with a nearly inexhaustible supply. More than 3.8 gigawatts (GW) of electricity-generating capacity are currently installed in the United States [55,56]. According to the Department of Energy’s (DOE) most recent Energy Earthshot, the price of enhanced geothermal systems (EGS) will be lowered to $45/MWh by 2035, an ambitious target that would reduce current EGS costs by 90%. EGS are highly engineered geothermal reservoirs that require drilling deep wells beneath the earth’s surface in hot rock that is between 175 °C and 300 °C in temperature but has little to no natural permeability or fluid saturation [57]. A geothermal heat pump and an air-source heat pump are combined to create a dual-source heat pump. The most effective elements of both systems are combined in these appliances. While geothermal units are more efficient than dual-source heat pumps, air-source units have higher efficiency ratings. The main benefit of dual-source systems is that they are much less expensive to install than a single geothermal unit while performing nearly as well. However, in the United States, about 50,000 geothermal heat pumps are installed annually [58].

The Department of Energy (DOE) has funded Utah FORGE, a specialized underground field laboratory, to develop, test, and accelerate innovations in Enhanced Geothermal Systems technologies to promote the use of geothermal resources around the globe. Located on the western side of the Mineral Mountains, in Beaver County, Utah, close to the community of Milford, they are perfecting the drilling, stimulation, injection production, and subsurface imaging technologies necessary to set up and maintain continuous fluid flow and energy transfer from an EGS reservoir as short-term goals [59].

There are many different conceivable technological configurations for shallow geothermal systems. They may use the heat from the ground or groundwater (open- or closed-loop systems). Depending on how a ground heat exchanger (GHE) is installed, the latter can be classified into horizontal systems (1.5–2 m deep) versus vertical structures (often 50 to 150 m down) (collectors or boreholes, respectively). U-tubes plus coaxial tubes seem to be the two most typical designs for boreholes [60]. It has been shown [21,61] that the thermal pumps that use groundwater (modest geothermal systems with loops), which are presently the dominant component of the direct exploitation of geothermal energy, are gaining popularity around the globe. The area of interest in open-loop systems compared to closed-loop networks is due to the reduced climate dangers and even simpler licensing requirements for closed-loop systems [62].

An examination of the economy and environment [63] shows that a centralized heating system relies on shallower geothermal techniques, which is preferable to the current hydrocarbon structure for a university campus in Spain. One more study [64] showed that, unlike subsidies, horizontal as well as horizontal moderate geothermal facilities inside the UK are not financially competitive. Similarly, for the purposes of replacing the residential house stock in an Italian area, underground heating systems need to have better advantages than those of gas boilers [65]. Large-scale geothermal collectors connected to 5GDHC networks have been the subject of the research reported in Germany [66]. Accordingly, although moderate geothermal structures have not been proven consistently favorable, several pilot projects are being created. Although shallow geothermal technology is frequently one level above even moderate-deep geothermal, contemporary technological, as well as scientific problems, describe the best design and novel materials to boost shallow geothermal systems’ efficiency. The goal of upcoming shallow geothermal energy studies and technological advancement is to raise such numbers. Regarding horizontal systems, computational fluid dynamics (CFD) has been applied [67] in order to compare systems using slinky, straight, and helical (spiral) networks. These findings show that horizontal helical systems outperform the other two in terms of energy efficiency. Efforts have been focused on the linear loop, slinky coil, and spiral coil critical for which mathematical methods of the horizontal GHE have also been conducted [68], as well as other concerns about the effectiveness of subsurface heat exchangers [69]. A heat pump is required for a shallow geothermal system to be effective. GHE functionality has an impact on heat pump systems, but so do the heat pump’s design as well as its constituent parts (condenser, evaporator, compressor, and throttle valve) [70], the active substance [71] and operating systems. Considering work schedules, 32 various heat pump systems providing space heating have undergone testing procedures [72]. As little more than a result, the author’s emphasis is that when building a system with heat pumps, part-load circumstances should be taken into account. Use variable-speed drives or more small-capacity heat pumps to make the system more reliable when it is only partially loaded.

One sort of vertical geothermal system utilized at shorter depths (10–30 m) is called an “energy pile,” and it has a bigger bridge than a traditional vertical system. Energy piles serve dual structural and energy purposes, and they offer a wide range of possible applications in the 5GDHC and NZEB [73]. Four plans for geothermal facilities with energy heaps [74], along with a number of numerical and analytical methods appropriate for simulations of energy heaps, have been discussed. Analysis of the performance of various GHE setups of energy heaps has been done [75], demonstrating that for similar energy requirements, winding needs 20–30% less than W- as well as 3U, although it is three times more costly. Therefore, since the cost of energy piles represents less than 1% of the whole construction process expenses [75], it implies that energy stacks’ energy performance rather than their cost might be given priority. In Sapporo, Japan, several subsurface heat exchanger designs were tested as energy heaps. Temperature expulsion rates were 53.81, 54.76, and 68.71 W/m for the double-U-shaped indirect and U-shaped twin-piping designs, respectively [76]. Zurich Airport’s terminal is effectively heated and cooled using an energy piling system [77]. The outcomes demonstrated that the solution meets 1.2 out of 1.2 GWh of the tunnel’s cold requirement and 2.2 out of 3 GWh of its heat requirement. Despite being a desirable alternative for heating and air-conditioning buildings, energy piles also contribute to the mechanical and thermal load, which should be taken into consideration when designing energy heaps. Energy pile design techniques were evaluated, and temperature impacts on the interactions between the pile and the earth were taken into account [78]. Although it is not a widely used technology, shallow geothermal air circulation is another potential shallow geothermal energy utilization. These systems, which use subterranean horizontal (in 90% of cases) or perpendicular piping to warm or chill incoming air, can be used in conjunction with or in place of traditional ventilation systems. For cooling, it has a very pronounced energy-saving impact [79].

A superficial geothermal energy system’s heat production is insufficient for particular uses; more boreholes must be sunk, boosting the system’s necessary contact; alternatively, the drilling needs to be deepest in the region. That last one introduces the concept of a moderate geothermal network, which might be useful in metropolitan settings with a limited amount of open space [80]. Through the utilization of field experiments, a thorough investigation of numerous shallow but also medium-deep geothermal systems has been carried out throughout China [81]. The results show that medium-deep technology is significantly more effective, but it is important to consider the higher drilling costs associated with these systems. As such an illustration, the effectiveness of a building’s heating and cooling systems at the RWTH-Aachen University’s 2500 m deep coaxial drilling heat exchanger (BHE) [82], using this type of network is practical from an economic standpoint. There has been substantial research on the environmental effects of intermediate as well as moderate-deep geothermal heat even coolant systems [83]. It demonstrates that decentralized heat pumps linked to geothermal structures along a varied depth of wells (350 to 1600 m) have a smaller impact on the environment compared with the other geothermal solutions under consideration.

The storage of thermal energy is frequently discussed during medium-deep BHE [84]. There have been several simulations concerning heat energy storage at medium-depth drilling featuring coaxial BHE [85], With the aim of improving its design, several elements affecting the effectiveness and performance of moderate-down BTES are now being studied. These findings demonstrate that when BHE numbers and depths rise, holding efficiency also rises. Moreover, 5 m was determined to be the ideal BHE spacing. A number of heat supply alternatives, along with a moderate-down BTES of 10 GWh/a, were assessed in terms of their economic as well as environmental implications [86]. Their analysis demonstrates that because a heat supply system with modest CHP units, a huge solar thermal collector field, and medium-deep BTES may be a practical substitute for boiler- or CHP-based systems, thermoelectric energy stored in large aquifers (HT-ATES) is taken into account during medium-deep open-loop geothermal technologies. Whenever creating such a system, drilling (licensing the discovery and use of resources) as well as liquid regulations (permission) are mediatory [87]. Several research groups are presently concentrating on medium-geothermal systems because they might be a desirable option for heat generation and storage. Furthermore, because not enough research has been performed yet, and because drilling expenses are still high, medium-deep BHE is still not widely used across the world [88]. Additionally, the creation of medium-deep ATES systems is challenging because of certain hydrogeological, environmental, and regulatory constraints; as a result, there are currently few if any systems in existence [89].

One of the very popular applications for deep geothermal energy includes the direct production of heat and power using flash or binary cycles (Organic Rankine Cycle and Kalina Cycle) [90] or generating either heat or power. Many studies [91,92] prove that worldwide primary energy utilization does not even come close to the enormous potential of deep geothermal energy. Meanwhile, the production of geothermal power worldwide in 2020 was just 95.1 TWh/a [93], or only 45.3 TWh/a of geothermal heat was generated globally per year for space heating [21]. For context, the amount of electricity, as well as heat produced globally each year in 2019, was 27,044 TWh (of which 0.35 percent came from geothermal energy) and 4297 TWh (of which one percent came from geothermal energy), respectively [94]. Focusing on regions with certain geological characteristics, such as those near the edges of the tectonic plates, has traditionally been a major factor contributing to the resource’s present restricted utilization [95]. New methods are being invented to take advantage of less appropriate subsurface circumstances and utilize the vast potential of deep geothermal power, as most locations on Earth do not possess any good underlying circumstances for deep geothermal energy extraction.

An improved geothermal system is EGS [96]. The Heating Dry Rocks (HDR) method implies artificial increases in rock permeabilities, e.g., by creating new cracks in rocks or by opening and/or extending current ones to capture geothermal energy at depths of 3–5 km when sufficient natural permeability is minimal or absent. [97]. Whenever the technology is much more developed and commercially established, a flow rate of 70 L/s is expected from an EGS [98]. It can roughly quadruple an EGS’s typical energy output. EGS has the benefit of often needing less space on the ground in cities with a lot of people.

By operating without any of the requirements, deeper closed-loop geothermal systems are used to stimulate hydraulic, thermal, or even chemical systems as opposed to EGS, which can lessen the danger of seismic activity. Because it doesn’t depend on the reservoir’s conductivity for a specific liquid transfer rate, it is also more regular. In order to determine how well they function in terms of energy and finances, many deep coaxial closed-loop system designs have to be examined [99]. Having undergone successful testing in Canada, a deeper closed-cycle geothermal infrastructure, including multiple horizontal wellbores, is now prepared for scale. These horizontal wellbores lacking casing are sealed using this method, which also makes use of the thermosiphon impact. The second aids in avoiding issues with cementing and covering at extreme temperatures [100]. Based on an analysis of how well this method and absorption cooling work [101], this technology is better than a cooling method that uses an electric chiller.

5. Benefits and Challenges of Geothermal Energy Systems

It is well recognized that geothermal systems for heating and cooling are far more environmentally friendly than conventional ones, or “greener.” As an example, typical systems produce more greenhouse gas emissions than geothermal heating and cooling solutions do. Although a geothermal system removes more than 3 million tonnes of carbon dioxide from the atmosphere annually, its impacts are equivalent to those of removing 650,000 cars off the road. The installation of a geothermal system for cooling and heating is much more costly than establishing a conventional heating and cooling system, according to the U.S. Department of Energy. You should receive your money back in 5 to 10 years, though. Using such a heating and cooling system has various advantages, which may include:

• Environmentally friendly geothermal heating and cooling solutions are acknowledged by the U.S. Environmental Protection Agency as the healthiest heating and cooling methods. This geothermal system not only does not consume any fossil fuels, but requires the least amount of power to transfer the heat energy toward the desired location. Geothermal-energy-based heating and cooling systems have low electrical consumption, which can result in long-term savings.
• Lower utility costs: When compared to traditional systems, geothermal systems merely transport heat from one location to another. Additionally, since the heat beneath is largely consistent, geothermal systems are often the ones that use the lowest energy for cooling. Geothermal heating and air conditioning systems have higher starting expenses, but they have lower ongoing costs, so savings will be evident after a few years, even with the higher starting prices. Geothermal is an excellent option for those looking to save money; in fact, it can save you 20–50% on cooling costs and 30–60% on heating costs.
• Reliability: With scheduled maintenance, the typical time an air conditioning, as well as a heating system lasts, is between 13 and 15 years. Meanwhile, geothermal heating and cooling systems have a life expectancy of 24 years. Even if they are well maintained, they might last 50 years. That refers to its age, which is three times longer than that of conventional systems. When I have used these geothermal air conditioning and heating systems, they require extremely little upkeep and just occasional inspections compared to the maintenance required by conventional and geothermal systems. Inefficient geothermal infrastructure is positioned immediately outside; consequently, these are never vulnerable to climate fluctuations that might harm the system. This is one of the major reasons geothermal has a longer lifetime. Another fan, compressor, and pump make up the indoor component, which is built to survive for many years. The remainder of the infrastructure is outside, underground, and has a multi-generational lifespan. Additionally, the geothermal pipes that make up the subsurface loop often have a 50-year guarantee.
• Protection: Since geothermal systems don’t depend on fossil fuel energy like regular heating and cooling systems do, they are both safe and healthy to use. There shouldn’t be any concerns regarding carbon monoxide poisoning, because geothermal systems employ subterranean renewable energy sources rather than burning anything. Overall, geothermal systems pose no risks such as explosions, poisonous fumes, or products of combustion [102,103].
• Effective cooling: There are a number of factors that contribute to a geothermal system’s high energy efficiency. First off, there are no fossil fuels used at all by the system. Second, the geothermal system’s heat pump uses a negligibly small amount of electricity to operate. Third, a geothermal system utilizes an energy source that is not only perpetually renewable but also constant: the Earth’s constant, steady temperature just below the frost line. This resource is sustainable because it will never run out. Fourth, we will save money over time because the system is energy efficient, with annual cooling and heating costs dropping by as much as 60 to 70% [18,104,105].

The lack of information on possible geothermal energy resources is one of the major challenges in the creation of geothermal projects. Places with hot springs that are naturally occurring or contain a large temperature variation may be suitable for use. In order to meet the heating demands of isolated communities in the coldest parts of the nation, which have a tough time supplying fuel for heating and cooking throughout the winter, the country can implement geothermal heat pumps. Among the top geothermal mapping facilities in the United States is Southern Methodist University’s (SMU) Geothermal Laboratory, which largely describes heat movement and temperature at depth, as well as the potential for geothermal sources. The National Renewable Energy Laboratory (NREL) of the U.S. Department of Energy has already created a geothermal energy forecast that identifies those places of hydro-thermal activity as well as the suitability of improved shallow geothermal system applications using information supplied by the Southern Methodist University Geothermal Laboratory, as well as NREL analyses for regions (EGS). One hundred fifty-four research and advancement programs are now supported by the Geothermal Technologies Office of the U.S. Department of Energy (DOE), leveraging a combined investment of approximately $500 million [106]. Thus, every project exemplifies a developing technological sector in technical and non-technical research and analysis, along with traditional hydrothermal and temperature conditions, as well as co-produced or Enhanced Geothermal Systems (EGS), technology, and techniques.

Even once geothermal energy resources have been identified, there are still multiple steps between resource discovery and power plant building. Typically, geothermal power plants’ anticipated development costs comprise not only operations, but also their source discovery, source appraisal, and digging of testing wells, which together make up around 13% of the total cost. Despite their small size, these upfront expenditures are important, since these are hazardous activities (i.e., they might result in dry holes), which have a high cost of capital. The remaining portion of the capital investment (87%) is made during the drilling and building stages. Marketing initiatives and complete shareholders’ as well as end-users’ participation have been recognized as crucial tactics to raise awareness and successfully persuade less knowledgeable areas to participate in such green economies [107]. A study conducted by Zhou et al. [108] demonstrates the huge potential and significant cost advantages of a hybrid solar–geothermal system.

Geothermal systems are important for cooling [109], based mostly on groundwater aquifers or lake cooling. This use is estimated to conserve about 90–95% of the cooling energy needed for district cooling systems [110]. One of the largest groundwater reservoirs in Norway is used to serve the Gardermoen Airport as a complementary heat sink and source for district cooling and heating systems [111]. During the cooling period, the chilled water is pre-cooled by the groundwater, with a cooling capacity of 3 MW. It is then post-cooled by a combined heat pump/refrigeration plant, with a cooling capacity of 6 MW [43,112]. Besides that, geothermal energy is more reliable, based on its availability factor compared to other renewable energy sources. Geothermal energy, despite having a reputation as an alternative energy source that is friendly to the environment, nevertheless raises a few minor environmental issues. Greenhouse gases, like hydrogen sulfide, carbon dioxide, methane, and ammonia, are released as a result of the extraction of geothermal energy from the Earth. In contrast to the use of fossil fuels, however, much less gas is released.

Table 2. Availability factor of Renewable Energy Resources.

Source	Availability Factor (%)
Solar PV	63
Wind	38
Biomass	30
Geothermal	80

shows geothermal energy’s availability in relation to other renewable energy sources [113].

6. Conclusions

The worldwide developments in geothermal forms of energy and technology are examined in this review article. Then, it covered the advantages and difficulties of using geothermal cooling systems. The paper provided a thorough analysis of geothermal energy resources. Geothermal heating and cooling systems are both efficient power generators and environmentally friendly. Unlike any conventional heating and cooling systems that rely exclusively on fuels connected to carbon, geothermal energy includes organic thermal processes that arise naturally from the Earth’s core and is both cost-effective and environmentally benign. Modern geothermal heating and cooling systems are regarded as one method of providing year-round temperatures that may be given at a particular depth.

Geothermal energy is a virtually universal foundation for sustainable, ecologically beneficial, renewable energy. Consequently, combining a number of renewable energy resources to create a hybrid system could be the answer. This best hybrid, particularly in terms of the environment, combines geothermal energy with many different renewable resources. Approximately 15 terawatts of energy are now consumed globally, which is a small proportion of the total energy that may be obtained through geothermal sources. Facilities that use geothermal energy are thought to be able to make anywhere from 0.0035 to 2 TW of energy. This is the fastest and most reliable source of renewable energy due to the lifespan of these systems, which is relatively high, as they only have a small number of movable components, while the geothermal system can typically last for at least 20 years.