1. Introduction

Geothermal energy, sourced from Earth’s internal heat, is a clean, sustainable, and renewable resource that has been used one way or another for centuries. The driving force behind geothermal energy is quite simple, thermal energy from below the Earth’s core is transferred to the surface, then it is used to heat things up directly or instead converted to electric energy to be used later. Ancient Romans and Greeks used naturally occurring hot springs for bathing and for heating. In contemporary times, geothermal energy has evolved into a reasonable alternative to fossil fuels, being a significant and environmentally friendly contributor of energy for many countries.

Geothermal energy generation on a larger scale took off from the opening of the first geothermal power station in Larderello, Italy, in 1904. This plant was critical in proving that geothermal heat could be used to generate electricity on a large scale. Even though geothermal energy only makes up a small part of the global energy generation, around 0.7% as of recently, according to the U.S. Energy Information Administration [1], its usage is increasing steadily, and it’s proving itself to be a valuable, low-emission energy source that will only improve over time as it sees more technological advancements. Over the past few decades, advances in technology have made significant headway with respect to geothermal energy extraction and utilization. Geothermal energy is also typically regarded as more of a direct-use energy source, or for heat pump applications, however, since the construction of geothermal power plants, the availability and practicality of geothermal energy is much broader.

Innovations such as Enhanced Geothermal Systems (EGS) and hybrid geothermal systems that include a combination of other renewal energy sources have become potential contenders to the traditional setup. Enhanced geothermal systems involve creating artificial reservoirs in hot, dry rock formations, thereby increasing the range of geothermal resources available for usage, as shown in Figure 1, cold water is injected at very high speeds into the hot, dry rock below the surface, this creates cracks for more water to flow into, and over time it creates an artificial reservoir that can be used in places that don’t have natural ones with porous rock [2]. Advancements in drilling techniques and heat exchanger technologies have also enhanced the efficiency and effectiveness of geothermal systems, contributing to their growing role in the global energy market. A sketch is shown in Fig. 1 [3]. Hybrid geothermal systems, which attempts to integrate geothermal energy into existing renewable technology offers the potential for a more stable and dependable energy generation method. For instance, geothermal energy can serve as a base load power source while solar or wind energy can contribute to electricity generation during peak periods. This combined approach helps resolve the inconsistent and unpredictable nature of solar and wind power, enhancing the resilience of the overall energy system. By balancing supply and demand efficiently, hybrid geothermal systems can enhance the reliability and economic feasibility of renewable energy initiatives.

Figure 1. An Enhanced Geothermal System that uses hydraulic fracking to extract energy [3].

Geothermal energy is one of the most sustainable alternatives to coal as it has very low environmental impacts; in the US, geothermal power generation averages carbon emissions 35 times lower than that of coal-fueled power plants, per kWh of energy generated [4]. Unlike coal or other fossil fuels, it produces no greenhouses gases. This makes geothermal a vital component for climate change mitigation efforts. Further, much less land area is required by geothermal systems for similar amounts of energy generation as compared to alternative clean sources like solar and wind farms which often take up large areas of land. However, geothermal energy is only available in certain areas of high geothermal activity, which is often near the coast, a vital spot for the development of ports and sea access. Maintenance and expansion of applications for technologies developed for geothermal energy are important steps towards achieving global energy and environmental targets.

The recurring advancements in geothermal power removal coupled with the increasing usage highlights the capacity for additional development together with technology in this area. As innovation remains to progress, geothermal power might play a significantly crucial function in the international power mix, supplying a reliable, consistent, as well as eco-friendly choice to traditional power resources. The combination of enhanced geothermal systems, hybrid modern technologies, as well as enhanced drilling technologies guarantees to open brand-new chances for geothermal power growth adding to an extra lasting as well as resistant power future.

2. Extraction methods

The methods for extracting geothermal energy have progressed rapidly over time. This has been due to a rise in technology and awareness on the importance of sustainable sources of energy. There are three categories of these methods: conventional geothermal systems, enhanced geothermal systems, and hybrid geothermal systems. Each one applies different methodologies as well as technologies to capture the Earth’s thermal energy with distinct advantages and complications.

2.1. Conventional geothermal systems

Conventional geothermal systems are based on the natural occurrence of hydrothermal resources where water or steam that is trapped in the permeable rocks is hot enough because of the Earth’s internal heat. These systems can be classified into dry steam, flash steam, and binary cycle power plants, each employing distinct mechanisms to convert geothermal heat into electricity. Dry steam power plants are the classic and most basic type of geothermal power plant. These types of power plants use only vapor from underground heated sources directly for producing electric energy through turbines. Dry steam plants are efficient but require high-temperature geothermal resources, typically exceeding 150°C. Therefore, they are limited to areas with naturally occurring steam like The Geysers in California, which is the world's largest field using dry steam method [5].

Among the different types of conventional geothermal systems, flash steam power plants must be the most common type, since can operate with lower-temperature geothermal resources at higher efficiencies. In these systems, high-pressure geothermal fluid is discharged into a tank with lower pressure, this causes the fluid to "flash" into steam, which then turns a turbine to generate electricity. Flash steam plants can handle geothermal fluids at temperatures between 150°C-370°C thus making them versatile. However, special attention must be given to management of geothermal fluids as improper handling may lead to mineral scaling and corrosion in the equipment.

Binary cycle power plants are an example of traditional geothermal energy extraction systems which are now regarded as being flexible and most modernized technology. These systems use a heat exchanger with the hot geothermal to heat a secondary fluid with lower boiling point. The secondary fluid, when vaporized runs turbines to produce electricity. Binary cycle sources typically operate within moderate geothermal areas at temperatures between 85°C and 170°C; expanding the potential for geothermal energy development in areas without high activity geothermal regions, but this also means that less energy is generated per area of geothermal plant. Additionally, binary cycle systems generate almost no emissions and have low environmental impact because they operate in closed loops.

2.2. Enhanced geothermal systems (EGS)

Enhanced Geothermal Systems (EGS) have the potential to vastly expand the availability of geothermal energy by enabling the extraction of heat from dry, impermeable rock formations that lack sufficient natural fluid or permeability. The technology behind this involves injecting highly pressurized water into hot dry rocks in geothermal regions, which enhances the permeability and absorption of the rocks through hydraulic fracturing and various other stimulation techniques. Water travels through fractures, thereby creating artificial reservoirs, returns up to the surface as it is heated by the rocks to generate electricity.

EGS technology is still in the developmental and experimental stages, but it has shown signs of being quite promising so far. One recent example is Project Red in Nevada, US, a commercial EGS system that has achieved significant geothermal flow rates and power capacity. The project demonstrated no major technical barriers for deploying similar EGS systems and showed potential to increase power capacity up to 8 MW per production well.

The main advantage of EGS is that it can accommodate a wider range of geothermal resources, meaning that it has the potential to make areas with no traditional hydrothermal activity suitable for geothermal energy production. However, drilling and reservoir stimulation costs, induced seismicity susceptibility as well as advanced monitoring and management technologies are some of the challenges that EGS has to overcome in order to maintain the life span and stability of the artificial reservoirs.

2.3. Hybrid geothermal systems

Hybrid geothermal systems integrate geothermal energy with other renewable energy sources, such as solar or wind, to enhance efficiency, reliability, and overall energy output. The combination of different energy sources is used in these systems to overcome the weaknesses and limitations associated with each different type of technology.

One commonly employed hybrid system is the combination of geothermal and solar thermal energy. In this configuration, the temperature of geothermal fluids is augmented through solar collectors before they enter the power generation cycle [6]. Such a power plant can be made more efficient especially where there are not enough geothermal resources alone to achieve optimal performance. Hybrid geothermal-solar systems can boost the overall efficiency and output of geothermal power plants by up to 30% [7]. Another innovative hybrid approach involves combining geothermal energy with biomass. The configuration involves using geothermal heat to increase the effectiveness of bio-fuel-based combustion, or to pre-dry biomass that needs to be burned, thus reducing the energy intensity required to burn it for energy. This synergy can help raise the overall power generation output and make biomass energy systems more sustainable.

Hybrid systems offer several advantages, including increased reliability through diversified energy sources, increased efficiency and reduced environmental impact. However, they also bring about problems, since integrating these different types of energy generation significantly increases the complexity and results in higher initial capital costs. In designing an efficient hybrid system, it is important to consider local resource availability, climatic conditions and technological compatibility.

2.4. Emerging techniques and future directions

As the field of geothermal energy continues to evolve, several emerging techniques and innovative approaches are being explored to further enhance the efficiency and feasibility of geothermal energy extraction. One such approach is using advanced drilling technologies like laser assisted drilling and spallation drilling which aim to lower costs and technical difficulties associated with accessing deep geothermal resources. These advanced drilling methods could potentially lower the cost of geothermal well construction by 30% to 50%, making deep geothermal resources more economically viable [8].

Another area for investigation in this regard has been the development of systems for storing geothermal energy. These units are designed to store surplus heat during low demand periods, which can then be used during peak-load hours to increase reliability and flexibility within the geothermal power generation sector [9]. Thermal storage can be incorporated into existing geothermal plants or combined with other renewable energy sources to provide a more stable power supply. Moreover, better performing heat exchanger technology will improve efficiency in extracting geothermal energy. There have been innovative ideas towards compact heat exchangers that optimize heat transfer processes, while increasing thermal conductivity and providing resistance against corrosion on advanced materials being developed for improved efficiency in geothermal systems which could prolong their lifespan by improving operational conditions. The past century has witnessed considerable progress in methods of extracting geothermal energy as they have evolved from simple direct uses to elaborate power generating systems. The development of conventional geothermal systems enhanced geothermal systems, and hybrid geothermal systems with their inherent strengths and weaknesses have ensured that the global energy mix is complemented with a diverse range of resources. Consequently, current advancements as well as prospects can further improve the efficiency, cost efficiency and environmental friendliness of geothermic energy thereby making it an essential constituent for renewable energy portfolio in the world.

3. Utilization practices

Geothermal energy is a versatile and reliable renewable resource utilized in various applications, ranging from electricity generation to direct heating and innovative industrial processes. Geothermal energy is a highly adaptable, reliable and renewable source of electricity that has many other uses including direct heating and new industrial processes. Fig. 2 shows the usage of different types of geothermal energy application [10].

Figure 2. The usage of different types of geothermal energy application [10].

3.1. District heating and cooling systems

District heating is one of the main direct use applications of geothermal energy. To heat the buildings, residential, commercial and industrial through a network of insulated pipes steam or hot water from geothermal sources is distributed using this system. For instance, Iceland has over 85% of homes heated by geothermic energy, making it a country with abundant geothermal resources [11]. This wide acceptance demonstrates how much this renewable resource can do to reduce fossil fuel dependency for domestic heating purposes.

Less commonly used compared to district heating, cooling systems can also utilize the thermal energy from geothermal resources. District cooling systems use geothermal heat pumps to remove heat from buildings into earth thus supplying cooling effect. This means that electricity consumption for air conditioning can be significantly reduced due to efficiency in urban areas. On this basis, one can be then able to come up with an integrated approach to managing energy in terms of efficiency and sustainability whereby both aspects are considered as per se.

3.2. Geothermal heat pumps

Geothermal heat pumps (GHPs), also known as ground-source heat pumps, are another direct use application of geothermal energy. They work by capitalizing on stable temperatures existing just below the Earth’s surface, to provide heating, cooling and hot water for buildings. GHPs are highly efficient, with energy savings of up to 70% for heating and up to 50% for cooling compared to conventional systems [12]. The adoption of GHPs is essential as it will contribute considerably to reducing greenhouse gas emissions, as well as decreasing consumer energy bills.

The operation of GHPs is based on a principle where fluids pass through a series of underground pipes (known as a ground loop) to exchange heat with the ground. During cold seasons, this fluid absorbs warmth from the ground and transfers it into the building; conversely, during hotter seasons, heat from buildings is deposited back into the earth. GHPs’ ability to transfer thermal energy effectively and efficiently makes them suitable for both heating and cooling systems.

3.3. Greenhouse heating

Geothermal energy is extensively used for heating greenhouses, providing a stable and cost-effective way to maintain optimal growing conditions year-round. This is an important application in regions characterized by cold climate or where traditional sources of energy are expensive. Greenhouses can achieve significant energy savings and improve crop yields by using geothermal heat.

In the Netherlands, for instance, geothermal energy is used in warming large greenhouses which produce vegetables, flowers, and other high-value crops. Through this method, the agricultural operations emit less carbon while at the same time making greenhouse farming more economically viable. Kenya also utilizes geothermal greenhouse heating systems due to its extensive geothermal resources and heavy dependence on agribusiness.

3.4. Aquaculture

Supporting different types of aquacultures, geothermal power assists in maintaining waters which are suitable for the breeding and rearing of fish. Geothermal system-based aquaculture can increase growth rates, enhance productivity and lower operational expenses. In Iceland, they use geothermal heat to grow tilapia and arctic char among others that provides a constant environment for healthy fish growing. This use of geothermal energy does not only apply to fishery. Aquatic plants are grown as well as other marine organisms using it. Geothermal heat creates a controlled environment that optimizes growth conditions, reduces disease outbreaks and increases total production. This kind of aquaculture can provide food security and local economic benefits especially in regions with limited access to alternative energy sources as depicted in Fig. 3 [13].

Figure 3. A potential form of application of geothermal energy for fish farms [13].

3.5. Innovative uses and emerging applications

Use of geothermal energy in new ways goes beyond traditional applications towards industrial and agricultural sectors. One such innovation is the extraction of minerals through geothermal energy. Geothermal fluids often contain dissolved minerals, such as lithium, silica, and zinc, which can be extracted and processed. In this way valuable mineral resources are available in sustainable mineral rich brines, thus making it possible for economic viability of geothermal projects.

Another emerging application is the combination of geothermal energy with desalination processes. Desalination is the process of removing salt from seawater to produce fresh water, which requires a lot of energy. Such heat can be got from the earth’s crust especially when it comes to thermal desalination like multi-effect distillation (MED) or multi-stage flash distillation (MSF). This integration provides a means for addressing the problem of water shortage in dry areas that have both access to seawater and geothermally active zones. Moreover, there has been an exploration on utilization of carbon capture and storage potential within geothermal. For example, geothermal reservoirs can serve as storage sites for captured CO2 hence reducing greenhouse gas emissions. This method is called CO2 sequestration into geothermal fields that takes advantage natural capacities inherent in thermal waters [14].

4. Future improvements

The future of geothermal energy has a great deal of potential and is poised for significant advancements. Technological innovation, increased research, and a growing emphasis on sustainable energy solutions are the forces behind this insightful prediction. As the need for renewable sources of power becomes more pressing, it becomes increasingly important to have efficient, reliable, and environmentally friendly ways of tapping into geothermal energy. Key areas for future improvements include enhanced geothermal systems (EGS), hybrid geothermal systems, drilling technologies, and resource exploration techniques.

Future improvements within EGS will focus on overcoming current technical challenges like induced seismicity and reservoir management. Advanced monitoring and modelling techniques are essential for predicting and mitigating seismic events associated with EGS operations. In this regard, improvements in water injection as well as heat extraction processes must be ensured for long-term sustainability of EGS reservoirs. These technologies are being advanced by projects such as Frontier Observatory for Research in Geothermal Energy (FORGE) located at Utah, USA aiming at ensuring widespread adoption and commercialization of EGS [15]. Future improvements in hybrid systems will focus on optimizing the integration of various energy sources and developing advanced control systems to manage their interactions. Research is also directed towards creating hybrid power plants that can seamlessly switch between geothermal and other renewable energy sources, ensuring a consistent energy output regardless of environmental conditions. Such advancements will enhance the overall efficiency and reliability of renewable energy systems, contributing to a more resilient and sustainable energy infrastructure.

The future of drilling technologies will also see the integration of real-time data analytics and machine learning to optimize drilling operations. These technologies can provide valuable insights into subsurface conditions, allowing for more accurate targeting of geothermal reservoirs and reducing the risk of dry wells. Additionally, advancements in drilling materials and equipment, such as high-temperature-resistant drill bits and advanced casing materials, will be crucial for improving the durability and reliability of geothermal wells [16].

Successful development of geothermal energy projects requires the efficient and precise exploration of resources. Conventional exploration methods are frequently costly and time-consuming and can be highly speculative. The next generation of exploration techniques would focus on improving the accuracy and efficiency of these procedures by employing advanced geophysical and geochemical techniques, remote sensing, and machine learning algorithms [17]. Advanced investigation methods like seismic imaging or electromagnetic surveys could help in better understanding subsurface conditions that identify potential geothermal reservoirs with more accuracy. Additionally, application of big data analytics and machine learning will enable further fine-tuning through analyzing large datasets for patterns as well as correlations that suggest presence of geothermal resources. These developments will lower risks and expenses involved in exploring for geothermal power hence making it a viable economic alternative to investors.

5. Conclusion

The future of geothermal energy appears promising, with many chances for technological breakthroughs and improvements. And these are namely advanced geothermal systems (AGS), integrated hybrid geothermal systems, improved drilling techniques and unconventional resource discovery techniques that could revolutionize the scope and efficiency of geothermal energy sources. These changes will not only enhance the economic attractiveness but also make sure that there is a more sustainable, resilient energy infrastructure [18][19]. No other source of renewable energy has such capabilities as this one in terms of being a ‘bridge fuel’ to a low-carbon future; it provides constant and reliable supply of electricity at competitive prices with minimal environmental impact. The ability to provide stable and consistent power supply together with the possibility of reducing greenhouse gas emissions makes it attractive for incorporation in the present-day energy mix as well as serving as an important alternative to traditional sources of power generation. Geothermal power becomes increasingly part of world’s global energy landscape due to continuous technological advancements that address current limitations while seizing unlocked opportunities.

The whole world’s energy demand is expected to increase significantly with time, and this can only be achieved by ensuring that all the available sources of renewable energy are fully utilized. In this context, the integration of geothermal energy with other renewable sources such as solar and wind through hybrid systems will help in boosting the efficiency and reliability of renewable energy production at large. It is obvious that such holistic approach to energy management will be critical for meeting expanding global energy requirement and reducing climate change impacts. Policy support and investment in research and development are essential for driving future growth of geothermal energy. Governments should work together with private sector stakeholders on regulatory frameworks, financial incentives, as well as innovative research projects. This would create a conducive environment for geothermal power generation thereby fast-tracking the deployment of cutting-edge technologies that maximize the potential of this sustainable source of energy. In conclusion, future enhancements in geothermal power may turn it into one of the key players worldwide energy map. With technological development, increased efficiencies and strategic mergers with other renewable sources such as solar or wind power, geothermal energy can contribute greatly towards sustainable and resilient future energies. Geothermal energy will become increasingly vital in powering the world sustainably and responsibly as one continue to push the limits further outwards [20].