In this blog post, we will examine the characteristics and utilization conditions of geothermal energy—the thermal energy found underground—as well as the principles behind geothermal power generation and geothermal heat pumps, along with the limitations of geothermal energy and the technologies designed to overcome them.
The utilization and securing of energy sources are among the critical issues that the international community is collectively focusing on today. Many countries are racing to devise solutions to energy problems and are actively investing in and developing related future industries. Energy sources not only enable technological advancement and industrial operations but are also essential for sustaining modern life. However, fossil fuels and uranium—natural resources that account for the majority of the energy sources used by humanity—have limited reserves, their supply can become unstable depending on the international economic situation, and their use generates environmental pollutants. These issues are driving research and development in renewable energy. Renewable energy is categorized into various types depending on the energy source and is characterized by its use of resources that are naturally and continuously replenished. Among these, there is a unique form of energy obtained using technology similar to the drilling and excavation processes required to extract conventional fossil fuels. This is geothermal energy, which utilizes not a specific substance found underground but the underground heat itself as an energy source.
Geothermal energy literally refers to the heat possessed by objects underground or the heat that exists underground. To understand why geothermal energy is used as an energy source, we must first understand the unique characteristics of this underground heat. The first characteristic of geothermal energy is that temperature fluctuations due to the seasons and time of day are smaller compared to those at the Earth’s surface, and temperatures remain relatively stable below a certain depth. Although underground temperatures vary depending on region, geological conditions, and depth, generally even in shallow layers close to the surface, seasonal temperature fluctuations are significantly reduced compared to the surface, and temperatures remain relatively stable year-round below a certain depth. In other words, unlike the Earth’s surface, where temperature differences vary greatly by season, temperatures underground remain relatively constant. While indoor and outdoor temperatures at the surface show significant seasonal fluctuations, underground temperatures—though they vary with depth—exhibit a relatively small annual range of variation. The second characteristic is that, generally, temperatures increase with depth. Below a certain depth, sufficient thermal energy can be harnessed for power generation or direct heat utilization. Although the rate at which underground temperatures increase with depth varies depending on the region and geological conditions, this geothermal gradient serves as a key basis for utilizing geothermal energy. The stability of temperature and the availability of sufficient thermal energy—the characteristics of geothermal energy mentioned earlier—provide the basis for its use as an energy source. Furthermore, these characteristics also represent the strengths of geothermal energy when compared to renewable energy sources such as solar and wind power, where power generation fluctuates depending on weather conditions and time of day.
However, to utilize geothermal energy with these strengths, several conditions must be met. In traditional geothermal power generation, the first requirement is the presence of sufficient heat at an appropriate depth. While temperatures generally rise with depth, sufficient heat at an appropriate depth is necessary to ensure the economic viability of geothermal energy use. Otherwise, drilling would be required to reach the deep underground heat sources; as depth increases, drilling costs rise, and in some cases, drilling or development itself may become difficult. The second condition is water. In traditional geothermal systems, water plays a crucial role in effectively transferring geothermal heat to the surface. Water is primarily found in cavities or pores within underground strata—that is, small cracks within rocks or strata, or gaps between the particles that make up the strata. The third condition is permeability, which indicates the degree to which water can pass through strata or rocks. Adequate permeability is necessary for water—the medium that transfers geothermal heat—to remain mobile. Traditional geothermal power generation can only be carried out effectively when geothermal resources meet these three conditions; in areas where water or permeability is insufficient, additional technology and costs are required. However, recent research is focused on overcoming the limitations of these conditions through the development of technologies that artificially ensure fluid flow even when sufficient natural fluid and permeability do not exist underground.
If the necessary conditions for geothermal energy utilization are met, it can be harnessed in various ways; the most representative examples include geothermal power generation and GHP (Geothermal Heat Pump). Geothermal power generation involves drilling to an appropriate depth and then bringing high-temperature water or steam from underground to the surface to generate electricity. Geothermal fluids generated in the high-temperature, high-pressure environment underground rise to the surface through boreholes. Depending on the power generation method, steam is either sent directly to a turbine, or steam separated from the high-temperature geothermal water is used to drive the turbine and generate electricity. The used steam or geothermal water can be cooled or have its heat recovered before being reinjected underground; geothermal power generation occurs through this cycle between the subsurface and the surface. Meanwhile, GHP is a technology that takes advantage of the fact that underground temperatures remain relatively constant throughout the year, making it suitable for both heating and cooling. While the temperature of the shallow subsurface varies depending on the region and depth, it is far more stable than seasonal fluctuations in outdoor air temperature. Therefore, this system can be used for heating and cooling by utilizing the relatively warm underground heat in winter and dissipating heat from buildings into the relatively cool subsurface in summer. Although it does not directly generate electricity, by using the ground—where temperatures remain relatively constant—as a heat source or heat storage medium and transferring heat via a heat pump, the energy required for heating and cooling buildings can be effectively reduced.
Due to these characteristics, geothermal energy is a viable energy source; however, it also has drawbacks and limitations. A major issue is that it is difficult to find areas that meet the necessary conditions—such as heat, fluid, and permeability—required for traditional geothermal power generation. If these conditions are not sufficiently met, various developmental and technical problems arise during the geothermal utilization process. If sufficient heat is not present among the necessary conditions, drilling must be extended deeper to secure an adequate heat source. Although temperatures tend to rise with increasing depth, drilling to such depths not only entails enormous costs but also presents technical challenges. Furthermore, there are cases where there is not enough water underground to transfer geothermal heat. If water is scarce, it is difficult to effectively transfer geothermal heat to the surface. Furthermore, even if water is present, the utilization of geothermal energy may be difficult if the permeability of the strata is low, resulting in poor water mobility through the strata’s pores. These issues often occur in combination rather than independently. Recently, to overcome these limitations, next-generation technologies—such as Enhanced Geothermal Systems (EGS), which artificially create fluid flow underground to recover heat, and closed-loop geothermal systems—have been developed in addition to existing geothermal power generation technologies. In addition to local conditions and development and technical challenges, other issues that must be considered when utilizing geothermal energy include the potential for induced earthquakes during drilling and rock stimulation, as well as environmental impacts resulting from the movement of underground fluids. Therefore, to expand the use of geothermal energy, it is necessary to advance not only resource exploration and drilling technologies but also technologies for continuously monitoring the subsurface environment and managing safety.
To address these challenges in geothermal energy utilization, the geothermal power sector is conducting research to further advance existing drilling technologies while also developing techniques to artificially create environments within the subsurface that facilitate water flow and heat exchange. Notably, technologies such as hydraulic stimulation or hydraulic fracturing—which involve injecting high-pressure fluid to create artificial fractures in the rock formation and establish pathways for fluid flow—are being utilized. Recently, active research has also been conducted to apply drilling and subsurface structure analysis technologies, which have advanced in the oil and gas industry, to geothermal applications. Furthermore, the use of geothermal energy—such as in ground-source heat pumps (GHPs), which utilize underground heat directly—is gradually expanding as a means to reduce fossil fuel consumption and improve the energy efficiency of building heating and cooling systems. Research and technological development aimed at utilizing geothermal energy not only for electricity generation but also for heating, cooling, and heat supply are also continuing. Along with these efforts, if sufficient research and technological achievements are accumulated in the future, geothermal energy—another underground energy source that can complement or replace existing fossil fuel-based energy sources—has the potential to play the role of a dark horse in the future energy transition. In particular, unlike solar or wind power, geothermal energy has the advantage of being relatively less affected by weather conditions and being able to continuously supply heat and electricity; as such, it is gaining attention as a complementary energy source that enhances the stability of energy systems. Furthermore, geothermal energy is viewed as an area of opportunity for South Korea—a country with high dependence on fossil fuels—to diversify its energy sources and enhance energy security.
