Geothermal energy is generated from the natural Earth’s heat, predominantly resulting from the decay of naturally occurring radioactive isotopes such as potassium, thorium and uranium. The Earth’s surface heat flow, averaging 82 mW/m2 and equivalent to a total heat of approximately 42 million megawatts is the result of Earth’s internal heat. The total heat content of Earth has been estimated at around 12.6 x 1024 MJ [1].
Although the Earth possesses an immense amount of thermal energy, only a fraction of it can be exploited for practical purposes. Geothermal resources are confined to regions with favorable geological conditions for a carrier medium, such as liquid or vapor-phase water, to facilitate heat transfer from deep high-temperature regions to the surface or near-surface areas. This article will thoroughly analyze geothermal energy potential worldwide, current practices and major challenges.
Figure 1: Electricity generation from geothermal resources [2]
Geothermal Temperature Profile
The Earth temperature rises about 25–30 °C/km by going deeper into it. So, Earth’s crust temperature at 10 km would be higher than 300 °C, assuming a conductive gradient. However, most geothermal studies and utilization are executed at steeper gradients since this enables less expensive and faster drilling.
Core Earth temperatures can reach 3500–4500 °C, whereas those at the base of the continental crust are estimated to be between 200 to 1000 °C. Conduction is the major means of transferring heat from the interior to the outside. Geothermal generating wells are usually deeper than 2 km and rarely deeper than 3 km. In dry rock formations, a 1 km well will have a bottom temperature of 40 to 45 °C as compared to a 3 km well’s bottom temperature of 90 to 100°C with the typical geothermal thermal gradient.
Types of Geothermal Resources
Geothermal resources can be found anywhere in the Earth’s depth from the yearly mean temperature of around 20 to 300 °C.
Convective Hydrothermal Resources
Convective hydrothermal resources are found where the natural convection of hot water or steam carries the Earth’s heat upward. Many high-temperature convective hydrothermal resources are underlain by molten intrusions of newly cemented rocks, which reach temperatures of 500 to 1,000 °C.
Vapor Dominated Systems
These resources generate dry steam by boiling deep, salt fluids in low-permeability rocks. Larderello in Italy, Matsukawa in Japan, Geysers in northern California, etc., are some reservoirs that are utilized to generate power.
Water-dominated Systems
Wet steam is generated by water-dominated systems when significant amounts of uniformly heated groundwater rise from porous reservoirs deep. Each convection cell has an up-flow zone at its center, an outflow zone where heated water is being displaced laterally, and a downflow zone where recharging occurs. They may be visible on the surface as geysers, hot springs, natural stone deposits, chemically modified rocks, or blind resources.
Hot Dry Rocks
This category includes heat contained in rocks within 10 km of the surface that is unable to be economically retrieved using steam or hot water.
Since they contain few pores or cracks, these hot rocks store small quantity of water and have little to no interlinked permeability. New experimental methods for heat extraction involve pressurized hydraulic fracturing, followed by the recirculation of cold water down one well and the production of hot water from a second well in a closed system. Many geological settings have exploitable geothermal systems. They are classified into two groups depending on their association with magmatic activity and new volcanoes. Geothermal regions for direct geothermal energy application are discovered in both categories, while high-temperature regions normally utilized for conventional power generation are limited to the new volcanoes category.
High-temperature Fields
The temperature in these regions is usually greater than 180 °C, and volcanic activity primarily occurs near so-called plate borders. The crust of Earth is classified into a few big, solid plates that float on the mantle and move apart on average at speeds measured in centimeters per year, according to the theory of plate tectonics (the actual movements are highly erratic). Seismic activity, intense faulting, and often volcanic activity define plate borders. In plate borders, geothermal fields are especially prevalent because crust is severely fractured and permeabilized to water and other heat sources. The groundwater in these regions is heated by magmatic intrusions, which may occasionally be found several kilometers below the surface and include partially molten rock. Most plate borders are below sea level, but when volcanic activity has been intense enough to produce islands or when active plate boundaries transect continents, high-temperature geothermal fields are dispersed along the boundaries due to the decreased density of the hot water compared to the surrounding cold groundwater. A good example is the “ring of fire” of active volcanoes and geothermal activity surrounding the Pacific Ocean. Additional resources include the East African Rift Valley, Hawaii, Yellowstone, Iceland, etc.
Low-temperature Fields
There are four different types of these geothermal resources, all of which are independent of volcanoes and have temperatures below 180 °C:
- a) Regions related to the deep movement of meteoric water via fractures;
- b) Regions in hydrostatic pressure-affected deep high-permeability rocks;
- c) Regions in high-porous rocks at significantly higher pressure than hydrostatic; and
- d) resources found in low-porous (hot, dry) rock formations.
All of them are related to volcanic activity except (c). Types (c) and (d) are not currently used commercially.
Type (a) spring is most likely to be found in warm springs worldwide. While they can be found in various rock types and ages, warm springs that emerge along faults in valleys are more common in mountainous areas. Warm springs of this kind can be found anywhere, although they are more prevalent in regions with strong regional conductive heat flow. The ability of surface water to go deep down the Earth and back up again is crucial in this situation. Young tectonic zones are frequently rich in this type.
The most significant geothermal resource unrelated to new volcanic activity is type (b). The regional heat flow warms the water in the sediments if these are sufficiently segregated from surface groundwater by impermeable layers. As long as the sediments are permeable, it does not matter how old they are. Various aquifer depths and the geothermal inclination in the regions determine the thermal water’s temperature. However, in areas that have been researched, it is frequently in the 50–100°C range in up to 3 km depth. Although these geothermal resources are typically found during deep oil and gas drilling, they are rarely found on the surface.
Enhanced Geothermal Systems (EGS)
The idea behind Enhanced Geothermal Systems (EGS) is simple: where temperatures (150–200 °C) are high enough to make power, an extended fracture network can be formed or scaled up to develop new paths [3]. Using injection and production wells, cold surface water is conveyed to a deep reservoir and recovered as hot water or steam. The circulation system ends with additional surface installations, injection, and wells production. District heating and/or electricity production are two possible uses for the extracted heat. To effectively deploy EGS systems, several basic issues must be resolved, primarily the necessity for methods for designing, profiling, and operating deep fracture systems that may be customized to local subsurface conditions.
Geothermal Energy Utilization
Electricity Generation
Electric power is produced by turning a turbine generator designed to produce electricity using steam or hydrocarbon vapor. A vapor-dominated (dry steam) resource can be used immediately to generate electricity compared to a hot-water resource, which is needed to generate steam by lowering the pressure (flashing), typically in the 15-20% range. Some facilities employ double and triple-flash to increase efficiency. However, a bottoming cycle could be more effective in the triple-flash approach. The vapor is often condensed in a wet or dry cooling tower after it exits the turbine to optimize the temperature and pressure difference between the entering and exiting vapor and boost operating efficiency.
Nonetheless, arid regions frequently employ dry cooling. The development of binary plants is crucial for the geothermal power industry of today. Drilling expenditures and resource development impact the economics of electricity production. Thermodynamic properties of the reservoir fluid (phase and temperature) affect the electricity production per well. The number of wells needed decreases as the energy content of the reservoir fluid increases, which in turn lowers the requirement for reservoir capital expenditure. One geothermal well can produce between 1 and 5 MWe. However, some have been claimed to produce up to 30 MWe. Binary plants on the reinjection stream can provide inexpensive energy without pumping expenditures.
Direct Utilization
The main benefit of using geothermal energy for direct-use projects with low to medium temperatures is that the resources are easier to get to and can be drilled in at least 80 countries. Moreover, projects may employ standard water well drilling techniques and readily available heating and cooling equipment because there are no conversion efficiency losses. In less than a year, most projects can be fully functional. Projects might be small-scale, like those for a single house, greenhouse, or aquaculture pond, or they can be large-scale commercial operations like district heating/cooling systems, food and timber drying operations, or greenhouses.
Geothermal water is often oxygen-free; thus, taking precautions to keep oxygen out of the system is important. Moreover, dissolved gases and minerals like arsenic, boron, etc., must be removed or separated since they are toxic to both animals and plants. Nevertheless, geothermal water frequently contains carbon dioxide (CO2), which may be captured and utilized to make carbonated drinks or to speed up plant development in greenhouses. Downhole and circulation pumps, transmission and distribution lines, heat extraction equipment, heat exchangers, peaking or backup plants (typically burning fossil fuels), and fluid disposal systems are among the typical components of a direct-use system (injection wells). Although geothermal energy is only designed for 50% of the peak load, it can often supply 80–90% of the yearly heating or cooling requirement.
Geothermal Heat Pumps
Geothermal heat pumps (GHPs) use the Earth’s crust constant temperature to supply hot and cool water for homes, offices, public buildings and schools. A compressor requires a small quantity of power input, but the energy output is four times the input. GHPs have been well-known on the commercial scene since the 1960s. In 1970, this technique was first used in Europe; now, it is widely used in the United States, Canada, France, Germany, Switzerland, Sweden, and other western European nations. GHPs are typically installed in one of two basic schematics: ground-coupled (closed loop), which may be put vertically or horizontally, and groundwater (open loop), operational in lakes and wells. The schematic selection relies on the rock and soil types at the installation, the amount of land available, and/or whether it is economically feasible to dig a water well or whether one is already there (Figure 2).
Figure 2: Heating pumps working mechanisms
Ground-coupled systems use a closed loop of HDPE pipe submerged either horizontally (between 1-2 m) or vertically (between 50-70 m) to collect and expel heat from the ground in the winter and summer, respectively [4]. The open-loop system uses groundwater or lake water directly in the heat exchanger and then discharges it into another stream, well or on the land, depending on local rules (for irrigation).
Geothermal Energy Advantages
Eco Friendly
Geothermal energy cause less harm to the environment than traditional fuels like coal and other fossil fuels with a small carbon footprint.
Renewable
The Earth will continue to receive energy from geothermal sources until they are destroyed by the sun in around 5 billion years. Since the Earth’s heated reserves are refilled naturally, it is sustainable and renewable.
Vast Possibilities
Global energy consumption has increased to about 15 terawatts, which is only a small portion of the total energy that is generated from geothermal sources. There is hope that as industrial development and research continue, more geothermal resources will become available, even if most reservoirs are now inaccessible. It is estimated that geothermal power plants can generate between 0.0035 and 2 terawatts of electricity.
Steady and Sustainable
Geothermal energy, in contrast to other renewable energy sources such as solar and wind energy, provides a consistent energy supply. This is accomplished so that the resource is always available, unlike solar or wind power.
Heating and Cooling
Water temperatures of more than 150°C are required to drive turbines in geothermal power plants. Instead, the temperature difference between a ground source and the surface might be used. A geothermal heat pump may function as a heat sink/source just two meters below the surface since the Earth is more resistant to seasonal heat changes than the air.
Reliable
Geothermal energy is a reliable source of power generation since it does not fluctuate like other energy sources like the sun and wind. We can accurately anticipate the electricity production from a geothermal plant.
No Fuel Needed
Geothermal energy is a naturally occurring resource, unlike fossil fuels, which are mined or somehow extracted from the Earth. Hence there is no requirement for fuel.
Fast Evolution
Geothermal energy is undergoing extensive research, which means that new technologies are being developed to enhance the energy process. Projects to advance and expand this business are multiplying. Several of the present drawbacks of geothermal energy will be lessened due to this rapid progress.
Geothermal Energy Technical Potential
The primary benefit of geothermal energy is that it is always available throughout the year and only needs to be turned off for maintenance. Although heating is not required throughout the year, direct-use systems typically have the capacity needed between 25 and 30%, but capacity factors for power-generating systems are often 95%. In the heating mode, heat pump systems have an operational capacity of 10–20%; this capacity is doubled if the cooling mode is also used.
Figure 3: Installed capacity of geothermal energy [5]
Geothermal heat pumps are used worldwide in direct geothermal energy usage since shallow subsurface temperatures are within their operating range everywhere. Conventional direct-use heating is restricted to locations where affordable resources exist and climate supports the demand.
Resources exceeding 180 oC have historically hampered power generation. There are now more possible places because of recent developments in binary cycle technology (Organic Rankine), which uses low-temperature fluids at about 100 oC. The project’s economic feasibility can be determined by the depth of the drilling, the quantity and quality of the fluid, and temperature of the resources.
Deep drilling and low-temperature resources have lately become more affordable because of the usage of combined heat and power plants. As has been done in Iceland, Germany, and Austria, district heating using waste water from a binary power plant can make a marginal project economical. Figure 4 is an example of a cascade, where the energy collected is maximized by using the geothermal fluid at progressively decreasing temperatures.
Figure 4: Cascaded geothermal energy utilization for different areas
Economic Viability of Geothermal Energy
The economic feasibility of geothermal energy system is the first critical stage in its development. The consistent heat transfer from the Earth’s surface to the system makes geothermal energy suitable for use as a base-load power plant [5]. However, this is dependent on the quality of the heat resource that is available. Nevertheless, the high price of drilling geothermal wells might make the use of low-temperature geothermal power plants unattractive in some circumstances [6].
A planned geothermal power plant may be economically assessed using different approaches. The Levelized Cost of Energy (LCOE) is an established method that takes into account the costs of running and maintaining a power plant over its lifecycle. It is defined as the cost per unit of energy over the whole project cycle that is needed to pay back the capital investment [7].
The following are the major elements for the LCOE analysis:
(1) Investment expenses
(2) Average annual production rate of electricity
(3) Lifespan
(4) Discount rate
(5) Facility availability
The geothermal power plant’s investment cost primarily affects its economic feasibility. Investment costs can be divided into surface costs and subsurface costs. Surface expenses are those related to initial and surface analysis, infrastructure design and construction, site operation, and maintenance, whereas subsurface costs are those related to well drilling [8]. Five Icelandic geothermal power facilities and their surface and subsurface costs were investigated. The surface expenses were determined to be inversely proportional to power plant size. Figure 5 shows their cost breakdown for geothermal power facilities with sizes ranging from 20 to 60 MW.
Figure 5: Cost breakdown of Iceland geothermal power plant [9]
The preliminary survey phase of developing a geothermal power plant attempts to find a geothermal resource suitable for generating power. There are four significant stages in this phase. The first stage is performing thorough research or collecting data from current studies to screen a vast region. Geological, geochemical, and geophysical investigations can be conducted if the first step’s results satisfy the geothermal project developer. To calculate the ground’s temperature gradient at the location, 3 to 5 thin boreholes are bored in the third phase. The price of well drilling significantly impacts this phase’s cost. Exploration might cost anything between one and ten million Dollars in total.
Drilling several wells is the second stage of building a geothermal power plant, which uses the energy that is stored underground to generate electricity. Depending on the scale of the power plant, this phase cost ranges from 2.5 to 50 million USD [10]. Although the methods for drilling geothermal, gas and oil wells are comparable, the cost of drilling geothermal wells may be greater than for gas and oil extraction due to the various morphology of geothermal resources [11]. Aside from lithology, other variables affect the cost of digging a geothermal well. One important aspect is the temperature of the geothermal resources. The cost of well cementing and logging will rise as the temperature of the geothermal resource rises, which will also raise the cost of well drilling. The geothermal resource quality has been demonstrated to vary with depth, and this directly affected the cost of drilling wells, which increased exponentially with depth.
The infrastructure phase involves designing and building three primary components [12]:
- Development of the production and injection systems;
- Installation of the power plants;
- and connection to the transmission grid.
Every geothermal power plant’s infrastructure design and construction costs are influenced by variables, including the geothermal resource’s chemistry and temperature [13]. The temperature of the geothermal resource impacts the size and expense of the heat exchangers used in geothermal power plants. It has been demonstrated that binary power plants are less expensive than single-flash steam power plants for geothermal resource temperatures below 175 oC, whereas single-flash steam power plants are more cost-effective for geothermal resource temperatures above 175 oC [13].
Figure 6: Geothermal energy cost and risk profile [14]
Geothermal Energy and Environment
Some greenhouse gas emissions are related to geothermal power generation, although they vary depending on the plant type (flash vs. binary) and geotherm fluid. CO2 makes up the majority of geotherm fluid emissions; in some regions, methane can also releases. Nevertheless, the emission varies by region, with Italian plants running at an average emission rate of 330 g/kWh and Icelandic geothermal power generation having low emissions in a range of 34 g/kWh [14]. Turkiya geothermal power generation plants have greater CO2 emission factors than those that use fossil fuels for generation.
Hydrogen sulfide (H2S) is also present in geothermal fluids. H2S emissions are strictly controlled and monitored. The study has demonstrated that using computer models that reinject these gases into reservoirs may be advantageous for controlling/maintaining research pressure.
Current geothermal power plant designs frequently inject generated liquids back below, preventing any net fluid discharge at the surface. Mature plants release fluid to the surface, like Wairakei in New Zealand. A bioreactor consisting of 350 km of pipe through which fluid from the power station flows before it reaches the Waikato River is one especially inventive technique utilized at the Wairakei facility to reduce the discharge of H2S. Sulfur-oxidizing bacteria have been added to this pipe, and when the plant effluent passes through the pipes, they begin to break down sulfur. As a consequence, the amount of H2S that enters the Waikato River is reduced by 80%.
Geothermal Energy and Community
In many areas, geothermal energy is a valued natural resource for domestic communities. Successful development of geothermal resources requires consideration of local interests and values and the use of locally managed ownership structures.
A study contrasts the cultural influences on geothermal development in Iceland and Japan. The study lists six cultural aspects that make up this paradigm, including individualism, long-term orientation, and the avoidance of ambiguity [15]. This study contributes to understanding the cultural backdrop for why the geothermal directory has had such distinct trajectories in Iceland and Japan. Iceland has successfully developed geothermal project clusters that integrate power, district heating, tourism, etc. and benefit the local population as a whole. According to studies, if there is more awareness of the potential effects of geothermal energy development or participation in citizen science initiatives, a confrontation with onsen owners in Japan could be averted. Good community engagement is essential. Due to inadequate interaction with communities and their stakeholders, geothermal energy projects have frequently encountered delays, challenges and termination across the world.
Challenges of Geothermal Energy
Geothermal energy is a clean and sustainable source of energy that has the potential to play a significant role in reducing our reliance on fossil fuels. However, despite its many advantages, geothermal energy also presents a number of challenges that must be overcome to fully realize its potential.
High Upfront Costs
One of the main challenges of geothermal energy is the high upfront costs associated with developing geothermal power plants. Geothermal power plants require significant investment in drilling and infrastructure, making it a capital-intensive industry. This high cost of entry can make it difficult for new players to enter the market and can limit the growth of the industry.
To address this challenge, governments can provide incentives to encourage investment in geothermal energy projects. This can include tax credits, grants, and low-interest loans to help offset the initial capital costs.
Resource Availability
Another challenge of geothermal energy is that it is not available everywhere. Geothermal resources are limited to areas with high heat flow from the Earth’s core, which is typically found near tectonic plate boundaries. This means that not all countries have access to geothermal energy, limiting its potential as a global energy source.
To overcome this challenge, governments can invest in research and development to improve exploration technologies that can help identify new geothermal resources. Additionally, the collaboration between countries can help to share knowledge and resources, allowing for more efficient development of geothermal energy projects.
Environmental Concerns
Although geothermal energy is a clean and sustainable energy source, environmental concerns are still associated with its development. Geothermal power plants require drilling deep into the Earth’s surface, which can impact local ecosystems and geothermal reservoirs. In some cases, the use of geothermal energy can also lead to land subsidence, which can cause damage to nearby infrastructure.
There is a possibility that geothermal energy will result in earthquakes. This is because excavating has altered the Earth’s structure. This issue is increasingly prevalent due to updated geothermal power plants that pump water into the Earth’s crust to enlarge fractures for enhanced resource extraction. Nonetheless, many geothermal units are placed distant from populous regions, mitigating the consequences of these earthquakes.
To address these concerns, it is important to conduct thorough environmental assessments before the development of geothermal energy projects. This can include monitoring of local ecosystems and geothermal reservoirs, as well as measures to minimize the impact of land subsidence.
Technological Challenges
Before a geothermal energy project can begin, extensive geological surveys must be conducted to identify suitable locations for drilling. The process can be lengthy and expensive, and there is no guarantee that the results will be successful. Exploratory drilling carries a risk of encountering dry or low-temperature wells, which can lead to project failures.
Developing geothermal power plants requires advanced technological solutions, particularly in drilling and power conversion systems. Drilling deep into the Earth’s surface requires specialized equipment and techniques, and power conversion systems must be designed to handle the high temperatures and pressures associated with geothermal energy.
Competition with other Energy Sources
Geothermal energy competes with other renewable and non-renewable energy sources in the global energy market. It can be difficult for geothermal energy to compete with traditional sources of energy, such as coal, oil, and natural gas, which have already established infrastructure and are often subsidized by governments. Additionally, other renewable energy sources such as wind and solar power can sometimes be more cost-effective in certain locations.
Awareness
There is a lack of public awareness and understanding of geothermal energy. Many people are unaware of its potential as a renewable energy source, and there is a need for increased education and outreach to raise awareness and promote its development.
The Future of Geothermal Energy
Developing enhanced geothermal systems (EGS) is one of the main priorities for future geothermal energy growth. An engineered reservoir known as EGS was developed to economically extract heat from geothermal resources with limited permeability and/or porosity [16].
Adopting EGS technology might release hundreds of megawatts of power [17]. For instance, it is thought that the technological potential of EGS in the U.S. is 100 GWe, more than 30 times the country’s current installed geothermal capacity. Although EGS technology is not widely used, a few productive plants are in use, such as Soultz-sous-Forets in France. At 200 oC, granitic foundation rocks from a 3 MWe binary plant generate energy. UK is pursuing an EGS at Redruth, Cornwall. It will generate 55 MW of thermal energy and 10 MW of electricity.
Future analyses of the generation of geothermal energy also include supercritical systems. When the fluid is at a supercritical condition, it has a lot more energy.
A high-temperature geothermal power production project is expected to cost around 40% of its overall price [18]. Various technologies are being thought about to speed up drilling (reduce drilling time) to more easily enable drilling at deeper depths and reduce drilling expenses. These technologies comprise techniques including laser, spallation, and plasma drilling. However, none of these technologies currently have commercial implementation.
The potential for using the energy in water that is “coproduced” with oil from oil wells is also expected to receive more attention in the future. Many aged oil wells generate large amounts of water as they get older (much more water than oil).
Also, efforts are being undertaken to comprehend the potential for developing offshore geothermal resources. One project under consideration is Dunquin North, off the coast of Ireland. This project is expected to use large bore horizontal wells to produce 100,000 barrels of water per day [19].
Figure 7: Installed geothermal energy capacity in different countries (2021) [20]
Summary
It is becoming increasingly common to use hot water or steam trapped in pockets beneath the Earth’s crust as an economical replacement for fossil fuels. One of the few renewable energy sources that can generate consistent electricity around-the-clock is geothermal energy. It may be cost-competitive with coal or natural gas under the appropriate circumstances, allowing countries to rely less on imported fuels and boost their energy security. Geothermal energy can significantly contribute to the decarbonization of the power sector as it is a cleaner source of electricity.
Despite geothermal energy potential, there are still obstacles to utilizing this natural resource extensively. These include significant upfront expenses for the early investigation stage and the possibility of failed exploration. Worldwide experience demonstrates that risk minimization can successfully spur investment, particularly during the preliminary exploratory stage. Geothermal energy has to be scaled up globally, which would need a significant mobilization of private sector investments assisted by risk mitigation techniques, such as employing concessional funding from public resources, climate finance, etc.
Geothermal energy has steadily grown in installed capacity over the past ten years, and in 2021, it will be around 15.6 gigawatts. Due to fewer emissions and renewable resource utilization, environmentally friendly technologies are in demand worldwide, and geothermal technologies are one of the emerging renewable energy trends.
Endnotes
[1] Dickson, M. H., and Fanelli, M. (2012/09/05/). What is Geothermal Energy? http://www.unionegeotermica.it/what_is_geothermal_en.html
[2] Geothermal. (2022). IRENA. https://www.irena.org/Energy-Transition/Technology/Geothermal-energy
[3] Montagud, M. E. M., and Chamorro, C. (2017). Geothermal power technologies. In Reference Module in Earth Systems and Environmental Sciences (pp. 51-61). Elsevier.
[4] Rafferty, K. (2001). An information survival kit for the prospective geothermal heat pump owner.
[5] Fernández, L. (2023). Global geothermal energy capacity 2021 Statista. Retrieved Feb 8, 2023 from https://www.statista.com/statistics/476281/global-capacity-of-geothermal-energy
[6] Van Erdeweghe, S., Van Bael, J., Laenen, B., and D’haeseleer, W. (2018). Feasibility study of a low-temperature geothermal power plant for multiple economic scenarios. Energy, 155, 1004-1012.
[7] Dowling, A. W., Zheng, T., and Zavala, V. M. (2017). Economic assessment of concentrated solar power technologies: A review. Renewable and Sustainable Energy Reviews, 72, 1019-1032.
[8] Coskun, A., Bolatturk, A., and Kanoglu, M. (2014). Thermodynamic and economic analysis and optimization of power cycles for a medium temperature geothermal resource. Energy conversion and management, 78, 39-49.
[9] Stefansson, V. (2002). Investment cost for geothermal power plants. Geothermics, 31(2), 263-272.
[10] Soltani, M., Kashkooli, F. M., Souri, M., Rafiei, B., Jabarifar, M., Gharali, K., and Nathwani, J. S. (2021). Environmental, economic, and social impacts of geothermal energy systems. Renewable and Sustainable Energy Reviews, 140, 110750.
[11] Lukawski, M. Z., Anderson, B. J., Augustine, C., Capuano Jr, L. E., Beckers, K. F., Livesay, B., and Tester, J. W. (2014). Cost analysis of oil, gas, and geothermal well drilling. Journal of Petroleum Science and Engineering, 118, 1-14.
[12] Henneberger, R. (2013). Costs and financial risks of geothermal projects. Geothermal exploration best practices. Istanbul: International Finance Corporation.
[13] Hance, C. N. (2005). Factors affecting costs of geothermal power development. Geothermal Energy Association.
[14] Archer, R. (2020). Geothermal energy. In Future Energy (pp. 431-445). Elsevier.
[15] Shortall, R., and Kharrazi, A. (2017). Cultural factors of sustainable energy development: A case study of geothermal energy in Iceland and Japan. Renewable and Sustainable Energy Reviews, 79, 101-109.
[16] Tester, J. W., Anderson, B. J., Batchelor, A., Blackwell, D., DiPippo, R., Drake, E., Garnish, J., Livesay, B., Moore, M., and Nichols, K. (2006). The future of geothermal energy. Massachusetts Institute of Technology, 358.
[17] Bronicki, L. (2016). Introduction to geothermal power generation. In Geothermal Power Generation (pp. 1-3). Elsevier.
[18] Kipsang, C. (2015). Cost model for geothermal wells. Geology.
[19] O’Sullivan, J. M. (2019). Developing Geothermal Resource Plays Offshore Ireland: A Case Study On The Lower Cretaceous Dunquin North Carbonate Build-Up, Southern Porcupine Basin. AAPG European Region, 3rd Hydrocarbon Geothermal Cross Over Technology Workshop.
[20] Richter, A. (2022). ThinkGeoEnergy’s Top 10 Geothermal Countries 2021 – installed power generation capacity (MWe). https://www.thinkgeoenergy.com/thinkgeoenergys-top-10-geothermal-countries-2021-installed-power-generation-capacity-mwe
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