geothermal resources for sustainable development a case study

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• Published: 18 March 2022

Geothermal energy as a means to decarbonize the energy mix of megacities

• Carlos A. Vargas   ORCID: orcid.org/0000-0002-5027-9519 1 ,
• Luca Caracciolo 2 &
• Philip J. Ball 3  

Communications Earth & Environment volume  3 , Article number:  66 ( 2022 ) Cite this article

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• Energy and society
• Sustainability

The global number of megacities is projected to increase from 33 to 43 by 2030. Megacities are critical for the world’s economy; however, their resource management is particularly challenging. The increase of energy demand, in parallel to population growth and climate change, requires urgent investment in sustainable energies. We examine the megacities of Bogotá, Los Angeles, and Jakarta and reveal that the potential geothermal resource base is enough to cover the residential electricity demand by 1.14, 4.25, 1.84 times, respectively. Geothermal energy, a clean baseload resource independent from weather conditions, could significantly contribute to energy needs, improved air quality, and the decarbonization of the world’s megacities. We conclude that it is critical that governments and public are educated about the benefits of geothermal. Moreover, those energy policies coupled with investment in research and development are needed to ensure geothermal is successfully integrated into the future energy mix.

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Introduction.

2007 was the year when, for the first time in human history, the percentage of people living in cities exceeded that of people living in the country 1 . The increasing population in urban areas determined the escalation of megacities (population of >10 million). There are 33 megacities worldwide, but it is estimated that this number will rise to 43 by 2030 2 (Fig.  1 ). Today, >50% of the world’s population lives in urban areas, which is forecast to increase to 68% by 2050. The associated greenhouse gas emissions (GHG) will grow from 70 to 80% of the world’s GHG discharges in 2050 3 .

Locations of volcanic belts (red triangles) and hydrocarbons occurrences (white circles correspond to oil fields and yellow pentagons correspond to gas fields) have been combined with distribution of megacities (green stars). The ring of fire is presented as the yellow hatched polygon. Statistics of participation of energy production technologies are shown in the lower panel (left pies), as well as costs of energy production of one kWh (right pies) for Bogotá, Los Angeles, and Jakarta cities (purple squares on the map). Coordinate system in WGS-84.

Urban centres account for 67–76% of global final energy consumption, of which an estimated 71–76% is fossil fuel derived 4 . By 2050 the energy growth for heating and cooling of buildings could increase between 7 and 40% based on 2010 statistics 5 . The Urban Heat Island (UHI), effects within mega-urban cities 6 will magnify the issues associated with global warming 7 . Importantly, megacities today generate about 20% of the world’s Gross Domestic Product (GDP). Megacities worldwide face six common challenges: transportation, electricity, water, waste, sanitation, and security. Therefore, it is clear the importance of megacities, especially in light of environmental challenges and climate change on a global scale. Dealing with these challenges requires careful planning and optimization of economic resources.

Both energy demand and consumption will increase in parallel to population growth making electricity and the decarbonization of heating and cooling needs, and the air quality are some of the most challenging issues to deal with for the future of megacities. Most megacities are distributed in less developed regions, and many of these low GDP regions are experiencing dramatic economic growth (Table  1 ). It is imperative therefore that sustainability and energy resilience is part of the development strategies for megacities. Despite the technological advancement, the use of sustainable, and greener forms of energy are underdeveloped 8 . There are comparative assessments of peak and annual electric cooling and heating electricity usage at the city-scale, including OECD (Organization for Economic Co-operation and Development) and non-OECD member cities 9 . They propose that OECD cities show a cooling electricity response of 35–90 W/°C/capita above room temperature for cooling. In tropical/subtropical cities outside the OECD suggest that current demand reaches 2–9 W/°C/capita, indicating significant growth in temperature-dependent electricity demand as air conditioning is adopted. A similar situation is observed on the heating process, with subtropical cities adopting electric heaters, increasing electricity generation and delivery concerns.

While renewable energies like wind and solar are considered potential suppliers for megacities and are highly cost-effective sources for electricity, they are also related to some technical problems. For instance, wind turbines can be noisy, occasionally impact the physical environment, are aesthetically an eyesore, and are weather dependent. Solar panels, unless installed on the roof of buildings, require considerable space. Solar panel parks not only require re-purposing of land and potential destruction of forests, but they can also change the surrounding soil’s temperature with consequences in some ecosystems. In contrast, geothermal energy is rarely considered despite several megacities worldwide are located in regions with anomalously high geothermal gradient, especially around the ring of fire, a plate boundary zone with high tectonic and volcanic activity that surrounds the Pacific Ocean (Fig.  1 ). The benefits geothermal brings over wind and solar are that it is baseload, meaning it is availably 24/day, it is not dependent on any day/night cycles and weather conditions thus, it has a high-capacity factor, bringing stability to the grid, and importantly, it requires a small land footprint 10 , 11 . Electricity derived from geothermal developments may be able not only to meet the increase energy requirements of megacities but also contribute to the future energy demands. In addition to the residential sector, there are also other industry applications that can use heat from geothermal circuits, such as agriculture in greenhouses, food preservation, textile industry, etc.

Geothermal energy for power production today can be applied using conventional, high, and low-temperature hydrothermal systems. Moreover, there are several ongoing research programs examining the possibility of supercritical geothermal systems, which significantly increase the power density 10 , 11 . Engineered concepts such as Enhanced Geothermal Systems (EGS), where the rock is fractured to increase the natural permeability extend the geothermal play potential. Furthermore, emerging Advanced (closed-loop) Geothermal Systems (AGS) as they are often called 10 , 11 , potentially open the concept of geothermal heat and power to an even wider geographic application. Geothermal energy use, particularly EGS, where fractures are introduced into the subsurface are often sighted as causing induced earthquakes. Studies past and present are designed to mitigate these concerns through detailed pre-drilling research, including stress field analysis and modelling 12 , 13 , 14 , in addition, projects require increased public relationship building and education 15 , 16 , 17 , 18 .

According to Ourworldindata 19 , Colombia is a net energy exporter with ~2.5 times its total consumption in 2019 sent outside the country (535 TWh, of which low-carbon sources represent ~68.58%). Recently, USA has become a net exporter of energy, largely due to success in its fracking industry, which supports the domestic delivery of ~100% energy of its consumption (26,291 TWh, of which low-carbon sources represent ~39.95%), and excess to other countries. In contrast, Indonesia has become a marginal importer with ~9.21% of its consumption (2475 TWh, of which low-carbon sources is ~16.95%). The three countries presented as examples in this paper are signatories of the Paris Climate Change Agreement with challenging targets to reduce greenhouse gas emissions by 2030 and promise accelerated sustainable and resilient development 20 , 21 . The treaty outlines a target reduction of emissions below the 2005 baseline, which means a decrease of 51% for Colombia: 50% for the USA, and 41% for Indonesia. These reductions are mainly based on strategies of energy efficiency, incorporation of renewable energy, waste reduction, increasing fuel efficiency in transportation and logistics, etc.

Energy demand from megacities varies according to the level of development. Indicators used to evaluate the relationship between the energy use by a country and its level of development are controversial 22 ; however, a suitable variable is the energy consumed worldwide to produce the goods and services demanded by that country, i.e., its energy footprint. Thereby, megacities as strategic places where goods are produced or delivered, become intimately related with the level of development of countries that host them. Megacities in Northern America, Europe, and Japan use on average, per capita, more than 60 GJ (16.6 MWh)—and up to 100 GJ (27.7 MWh) e.g., Los Angeles and New York 23 . In some megacities, new initiatives to improve air-quality through the increased use of electric vehicles may also challenge demand on electrification.

This study was inspired by initiatives, such as the Green Deal Agreement, which highlight the need for climate neutrality by 2050 24 . We investigate the geothermal potential in Bogotá (Colombia) and follow the same workflow with Los Angeles (California) and Jakarta (Indonesia) as case studies. From the geothermal resource potential, we calculate a geothermal resource base (GRB)/energy consumption ratio. This concept could, however, be easily applied to any other megacity in the world. In addition, based on current and future energy statistics, we consider the decarbonization potential of geothermal for the three cities studied. Finally, we argue that the adoption of governmental incentives, regulation and a planned framework for decarbonizing power systems through renewable energy technologies, in particular the use of geothermal energy, could provide an important contribution to the energy demand, decarbonization, and air-quality improvement of megacities today and in the future, improving the quality of life for many people and helping to reduce energy poverty.

Energy demand of megacities

Analysis reveals that the energy demand in many megacities around the world is often below 20 GJ (5.5 MWh), largely because two-thirds of the megacities are in regions with warm climate (37% in the subtropics, 37% in the tropics, and 26% in the temperate zone; Table  2 ). In temperate climates, there is an increase in energy demand in cold months for heating. Most cities require cooling, with the energy for air conditioning being largely derived from electricity, hence depending on the available capacity of power generation and the existing grid. The implications of global warming combined with seasonal variations and the UHI effect may lead to power shortages if infrastructure/planning is not updated. The impact of the UHI alone reveals that temperatures today in cities can be an estimated 0.5–4 °C above unpopulated areas 6 . Simple analysis, presented in supplementary tables, suggests that the impact of an average 1.5 and 3.0 °C temperature increase, is a gross increase in power use principally required for cooling, resulting in an estimated 14 and 22% increase in energy. Megacities in a global warming scenario could see a dramatic increase in cooling needs, which poses a problem regarding the planning of new construction and retrofitting. Future buildings may need to be constructed to keep the heat out, rather than in.

In addition to energy demands, climate change may be putting strain on the water supply. Megacities located in the subtropics might experience lower than average precipitation as weather patterns change. Water resources could be impacted requiring access to future water supplies from desalination or recycling of wastewater, all of which increases the future energy demand. At least 19 of the 30 megacities included in Table  1 obtain more than one-third of the water supply from both surrounding areas and surface waters 25 .

When we look at the selected cities for this study, Bogotá, Jakarta, and Los Angeles, it is seen that the challenge of decarbonizing the energy for each city is strikingly different, as is the present-day per-capita energy consumption for each city (Table  3 ). The issue of decarbonization is biggest for Los Angeles, whereas the projected energy needs for Jakarta and Bogota, as these societies modernize their economies, presents a challenge of balancing improved lifestyle expectations and future decarbonization of the energy infrastructure. One common aspect that all these megacities exhibit is that they each face a decarbonization challenge, yet they all sit on a potential geothermal resource that is not currently utilized.

With respect to the geological setting for Bogotá, Jakarta, and Los Angeles, we find that all megacities are located close to subduction-related plate boundaries (Fig.  1 ). While this is not a unique to geothermal potential, it is a common theme of the megacities presented in this study. Furthermore, it is observed that geologically speaking, each region has experienced recent volcanic activity and crustal magmatic intrusions which enhance the potential of present-day geothermal anomalies for these megacities. The megacities Bogotá and Los Angeles are both located along Pacific Ring of Fire, where the Pacific plate is subducting under the Americas; however, locally they exhibit distinct geological characteristics (Figs.  1 – 3 ). Jakarta, by comparison, is located on the island of Java, which is identified as an active volcanic island arc in the Indonesian archipelago at the southern margin of the Eurasian plate (Figs.  1 , 4 ). Supplementary Note 1 contains an extended review of each megacity its current energy use, energy mix and a summary of the unique geological setting for each region.

A Spring water distribution (purple squares) in the north of Bogotá. B Calculation of the Curie Point Depth (CPD) in the area north of Bogotá 54 . Maps display normal faults (blue lines), reversal faults (red lines), right-lateral faults (cyan lines), left-lateral faults (yellow lines), active volcanoes (red triangles), inactive volcanoes (cyan triangles), and O&G fields (yellow dots). Topography from Amante and Eakins 59 . Black-dashed ellipse corresponds to the hypothetical envelope that encompasses the main geothermal lead. C Study area in a regional context; D rose diagram showing regional fault trending and the maximum principal stress direction (red arrows showing compressive regime); Rose diagrams are expressed in eight intervals of azimuth to the respective length of faulting. E Comparison between the potential geothermal power expressed as GRB (estimated in this work) and the current energy consumption in the metropolitan area of Bogotà. Coordinate system in WGS-84.

A Spring water distribution (purple squares) in the area of Los Angeles. B Calculation of the Curie Point Depth (CPD) in the area north of Bogotá 54 . Black-dashed ellipse represents a hypothetical envelope encompassing the main geothermal leads. Maps display normal faults (blue lines), reversal faults (red lines), right-lateral faults (cyan lines), left-lateral faults (yellow lines), active volcanoes (red triangles), and O&G fields (yellow dots). Topography from Amante and Eakins 59 . Black-dashed ellipse corresponds to the hypothetical envelope that encompasses the main geothermal lead. C Study area in a regional context; D rose diagram showing regional fault trending and the maximum principal stress direction (red arrows showing compressive regime); Rose diagrams are expressed in eight intervals of azimuth to the respective length of faulting. E Comparison between the potential geothermal power expressed as GRB (estimated in this work) and the current energy consumption in the metropolitan area of Los Angeles. Coordinate system in WGS-84.

A Spring water distribution (purple squares) in the area of Jakarta. B Calculation of the Curie Point Depth (CPD) in the area north of Bogotá 54 . Black-dashed ellipse represents a hypothetical envelope encompassing the main geothermal leads. Maps display normal faults (blue lines), reversal faults (red lines), right-lateral faults (cyan lines), left-lateral faults (yellow lines), active volcanoes (red triangles), and O&G fields (yellow dots). Topography from Amante and Eakins 59 . Black-dashed ellipse corresponds to the hypothetical envelope that encompasses the main geothermal lead. C Study area in a regional context; D Rose diagram showing regional fault trending and the maximum principal stress direction (red arrows showing compressive regime); Rose diagrams are expressed in eight intervals of azimuth to the respective length of faulting. E Comparison between the potential geothermal power expressed as GRB (estimated in this work) and the current energy consumption in the metropolitan area of Jakarta. Coordinate system in WGS-84.

The GRB potential is computed for each region following recognized methodologies 26 , 27 . We calculated for each region the maximum value of recoverable resource within a delineated area. Then, we simplified the model and hypothesize, firstly, that the tectonic features control the location of hot spring-waters and secondly, that the area encompassing the springs can be used to define the area and volume of potential reservoir that is storing the thermal energy.

Bogotá is in the Eastern Cordillera Basin (Fig.  2 ). Here a maximum value of recoverable resource is calculated within an area of ca. 7500 km 2 , at an average depth of 3 km, from a 30 m thick sandstone reservoir (the Eastern Cordillera Basin is approximately 11 km deep, and hosts at least one regional reservoir unit, the Une Formation 28 ). Our estimations suggest a rough value of resource of approx. 16,603.1 GWh, (Fig.  2 ), corresponding to ~1.14 times the energy per capital consumed by all people located in the metropolitan area. That means geothermal power could potentially cover the total residential power demand for Bogotá. This amount of energy may guarantee enough resources for long-term geothermal plants that take advantage of its location for supply electricity, and hot waters for industrial and recreational processes.

Los Angeles

Using the same assumptions for Los Angeles, which is located in the Los Angeles Basin (LAB) it is observed that spring-water occurrences cover an area of approx. 9.400 km 2 . This area coincides with relatively surficial Curie Point Depth (CPDs), ranging between 15 and 25 km. Having into account this figure, we have followed same approach to estimate the maximum recoverable resource, using formula (1) and assuming an average depth of 3 km, one 450 m thick reservoir related to the main grauwacke or McLaughlin’s Unit 2 29 . In this case, it is used half of the thickness than in NW Geysers Geothermal Field area 30 . Our estimations suggest a rough value of resource of approx. 312,138.8 GWh, corresponding to approx. 4.25 times the energy per capita consumed by all inhabitants that live in this megacity (Fig.  3 ).

Although the CPDs in the area are <25 km, the distribution of volcanic cones and spring-water occurrences were taken as criteria for defining the main area for focusing a detailed geothermal exploration near to Jakarta city. The ellipse envelope that encompasses them defines an area of approx. 9.100 km 2 . Using the same approach as in previous cases, we assume an average depth of 1.0 km, one 30 m thick reservoir related to the Loka andesitic pyroclastic unit (using similar thickness from the Wayang Windu geothermal field, West Java 31 ). The estimations indicate a rough value of resource of approx. 20,145.1 GWh, corresponding to 1.84 times the energy per capita consumed by all people that are currently living in this megacity (Fig.  4 ). Table  3 summarizes the main volumetric parameters used for estimating the energy and information about the total energy per capita consumed by all inhabitants for these megacities.

Discussion geothermal and megacities: How can geothermal help megacities?

Advances in Conventional, EGS and AGS, facilitate the concept of producing, low-carbon, sustainable, electricity, and heating adjacent to large population centres. In addition, low-grade heat may be used directly in district heating/cooling and Ground Source Heat Pumps, can be retrofitted, and developed in new building developments further decarbonizing heating and cooling needs for office buildings and homes 10 , 11 . Medium- and low-enthalpy geothermal energy can be used to decrease both operation costs and environmental impact from a broad range of industries including desalination of water, green houses, fish farming, brewing/fermenting, chemical products, automotive, and large-scale production industry 10 . The combined approach of geothermal for heating and cooling and for electricity production, therefore, enables geothermal to decarbonize significant amounts of the energy needs especially if it is developed in conjunction with solar photovoltaics (Solar-PV), wind power, and improved insulation technologies for existing and new office building and homes.

Geothermal and megacities: Prospective, economic, operational risks, and environmental impacts

Early studies have shown that conventional geothermal developments may be associated with the release of gasses such as hydrogen sulfide, carbon dioxide, methane, and ammonia, the amounts of gas released is significantly lower than in the case of fossil fuels. Furthermore, there are ways to mitigate this with the addition of air scrubbers, although it can add to the cost of development. In any subsurface geothermal drilling operation earthquake risks might be associated depending on the state of stress in the earth’s crust. This is increased in the case of enhanced geothermal power plants, which force water into the Earth’s crust to open fissures to increase the resource potential. However, this activity may be controlled by careful pre-drilling research including natural seismicity monitoring and by adjustments in the rate of water injection. It should be noted, however, that the above environmental concerns can be significantly reduced through AGS, and closed-loop/binary geothermal completions for both heat and power developments 10 .

There are relevant risks related to uncertainties of the properties of the geothermal field derived from the prospecting stage, but also in the management of the permeability for liquid extraction or reinjection during the lifetime of the geothermal fields. These issues relate to open-loop developments for example conventional, EGS and low-temperature geothermal completions which utilize the natural brine and host-rock interactions. In managing these power-plants the proper control of pressure and the induced seismicity are frequent concerns related to the management of the permeability that inhibit the development of this sort of renewable resource, particularly in EGS developments. For the appropriate management of a conventional geothermal field, it is essential that the entire liquid produced from geothermal wells is reinjected 32 . But this issue triggers a major operational challenge because mineral precipitation owing to boiling or cooling in the proximity of a production or reinjection well can result in reducing levels of liquid extraction or fluid injectivity gradually.

There are also significant risks linked with drilling geothermal wells, and their ultimate capacity is often challenging to assess due to large variability of the permeability on the scale of tens to hundreds of metres. Early work done to study the local stress conditions, in order to understand the present-day principal stress directions (Figs.  2 – 4 ) and fluid flow, can be used to reduce the economic risk of drilling a dry geothermal well, and significantly the understanding of open fracture orientations can help improve the location of horizontal wells, thus improving flowrates and well productivity. Other risks associated with geothermal wells are related to the influence of drilling in the reservoir, variations in well discharge pressure and temperature following long-term utilization of a geothermal structure, the presence and formation of a non-compressible gas cap, scaling of amorphous minerals, and corrosion. Nevertheless, all those identified risks are frequently taken in account in operational protocols with reduction of their impacts in the surrounding areas. For instance, the impact of the induced seismicity due to reinjection of fluids is becoming controlled by using traffic light protocols 33 .

Geothermal resource administration and regulations

Present external costs for each energy resource in each G20 country suggesting that geothermal energy becomes higher price than other sources 34 . Thus, in addition to previously identified risks and the current costs of installation and operation, the poor communication and education within local communities regarding the value of regional and local energy resources, potential hazards, and their mitigation or management, may lead to backlash and even social rejection, as well as cause low competitively in terms of price due to its low commercial penetration. If the social approach is managed properly, the geothermal resource could provide significant decarbonization of the power needs for megacities, something that promotes greater attractiveness about its use. Besides, geothermal can bring local benefits (e.g., job creation) and reduce the public expense in the energy sector. GEOENVI (2020) 35 recently reported that geothermal has brought an annual saving of 2.6% of GDP to the Icelandic economy.

It is important to note the innovation occurring within the geothermal industry, and new geothermal resources could be utilized by taking advantage of recent advances in geothermal completions utilizing binary, closed-loop, or AGS, which fully mitigate environmental issues common with flash-geothermal power plants 10 , 11 , 36 , 37 . While binary, closed-loop developments and AGS can increase the cost level of geothermal development, these newer concepts allow a wider deployment of geothermal energy, for power production and provide long-term cost-efficiencies to regions that prioritize GHG reduction targets 10 , 11 . These technological developments coupled with positive policy and low-carbon recognition of geothermal technologies, such as in California with the designation that binary/closed-loop technologies are of zero carbon 37 could encourage the development of more geothermal developments 10 , 11 . AGS developments are particularly interesting because they decouple the need for the co-location of permeability, heat, and fluids in the sub-surface also offering additional flexibility and a possibility to scale (i.e., larger), future, geothermal power plant developments 10 .

Penetration of geothermal has been limited due to the low-cost of fossil fuels. However, with the right incentives and forthcoming carbon tax schemes geothermal could appear increasingly attractive. Geothermal provides a low-carbon baseload power, which importantly offers grid stability, meaning it could provide a realistic pathway to the closure of coal and gas power plants 10 , 11 . To date, the preference of the, apparently, cheaper wind and solar-PV technologies, unfortunately, lock economies into a carbon-based gas or coal infrastructure 38 , precisely because wind and Solar-PV lack the baseload component. Geothermal power is also flexible, allowing power plant operators to ramp up and down the resource, therefore making it the perfect low-carbon partner with wind and solar-PV and hydroelectric resources. Given the geotectonic position of several world’s megacities, we see a huge potential for the use of geothermal power which would also reduce the environmental impact and help contrasting the energy challenge in overpopulated areas.

Regarding regulation, there are a series of legal issues which are a good guide for the proper regulation of this resource (e.g., royalty payments, use fees, environmental protection, freedom of Information, rights-of-way under the land policy and management, minerals management, leasing, operations, rights of local communities, etc.) that are permanently updated by countries and states that have a developed geothermal industry (see, for example, the USA code of the state and federal regulations 39 ), in the process of development (see, for example, the Law of the Republic of Indonesia No. 21 of 2014 about Geothermal 40 ), or that foresee the potential use of this resource in the medium or long term (see, for example, the proposal to modify the Decree 1073 of 2015 of the Ministry of Mines and Energy of Colombia 41 ).

Geothermal resource decarbonization potential for megacities

Based on the statistics and the per-capita estimates for the power use within the three studied megacities a decarbonization potential for geothermal was calculated for the three different megacities (see Supplementary Table  3 ). The calculation considered the amount of carbon emitted by the fossil fuel vs that emitted by the assumed geothermal technology, flash, or closed loop. The difference is the net decarbonization that geothermal can achieve. Supplementary Table  3 highlights the assumed full life emissions assume for the different energy technologies replaced. It was assumed that there was no need to replace existing low-carbon power sources such as wind, solar, hydrothermal, and nuclear. The decarbonization calculation was completed assuming the replacement of biomass, biogas, gas, coal, and oil sources (Supplementary Table  4 ). In total eight different scenarios were investigated assuming either 100% replacement using flash or closed-loop geothermal technologies (Scenario 1 and 5), assuming a global warming scenario with increased (120%) of present-day energy demand (Scenarios 2 and 6). Each of these scenarios was compared with only a 20% of fossil fuel demand removed by geothermal technologies (Scenarios 3, 4, 7, and 8).

Computing a decarbonization estimate introduces significant assumptions, particularly the penetration that geothermal can realistically achieve. A key uncertainty in the calculation is the true full-life cycle carbon footprint for different energy technologies. This uncertainty was important in the calculation of emissions in Bogotá where the electric system relies heavily on biomass. Biomass is controversial with respect to its carbon footprint 42 , 43 , 44 , thus the numbers presented in Supplementary Table  3 and used here could therefore be considered an upper estimate for the city. Assumptions are also derived from the estimates of the power use per capita. Firstly, this only represents the residential sector not the full transport, commercial, or industrial energy use of the megacity. The future energy demand is likely to change significantly for Jakarta and similarly Bogotá which as of today are considerably lower than Los Angeles. In future, changes in life standards and the introduction of electric vehicles will likely increase the residential electric demand on the grid. The values of per-capita and approximate population statistics used in Supplementary Table  3 , are derived from the literature and represent some errors inherent in capturing this information accurately. Regardless, the calculation gives us an idea of the current need and scope for decarbonization. In this study we also considered what the impact of climate change could be on the overall power needs whether that be heating for cooling because of 1.5 °C or 3.0 °C temperature changes (Supplementary Table  1 , 2 ), our simple calculation estimated 14–24% change in these scenarios respectively. To simplify the analysis, we assume a scenario of 20% increased power usage, this might be high for the former scenario and low for the latter scenario, but such a calculation if fraught with error due to population growth forecasts, changes in life quality and expectation and post-covid population changes. Finally, it is probably incorrect to consider that decarbonization could be achieved alone by geothermal, despite our geothermal resource results revealing there is enough geothermal potential to achieve this. It is hard to predict what is a reasonable estimate. Therefore, we begin with a modest estimate of geothermal penetration accounting for 20% of decarbonization for residential power use. By assuming only limited development of the geothermal potential the assumption is that only the best, most cost-effective geothermal projects are completed. One factor that might change the successful penetration of geothermal is the successful demonstration and cost reduction of AGS and supercritical AGS geothermal power plant developments. Today, however, it is clear with the current rate of uptake, geothermal penetration will not occur without the relevant incentives and governmental support.

The results of this simple calculation suggest that if geothermal replaced all fossil fuels in Bogotá geothermal could have a decarbonization potential of 15.4–17.0 MtCO 2 e per year. If we assume only 20% substitution of fossil-fuel by geothermal power when decarbonizing in Bogota (for example only the best geothermal projects) geothermal could only achieve a decarbonization potential of 3.1–3.4 MtCO 2 e per year. If future demand is based on global warming this could be as high as 3.7–4.1 MtCO 2 e per year based on our calculations and assumptions. For LA the results suggest ranges of 20.9–25.1, MtCO 2 e, 4.2–5.0 MtCO 2 e, and 5.0–6.0 MtCO 2 e per year. In Jakarta, we estimate 6.9– 7.9 MtCO 2 e, 1.4–1.6 MtCO 2 e, and 1.7–1.9 MtCO 2 e per year.

The estimates presented here only represent that for conventional and EGS-types of geothermal exploitation, involving heat, porosity, and high-water flow rates. These estimates already highlight that geothermal derived power can be sufficient to decarbonize the present energy needs. The reality is however geothermal might not make the level of penetration without changes in governmental regulation. In some cases, it may also help support and decarbonize the future energy needs resulting from climate change and global warming (Supplementary Tables  1 , 2 ). Importantly, geothermal derived power is not as vulnerable to environmental changes, and, importantly, it provides a low-carbon baseload power. The global share that geothermal power will contribute is 5% of the electricity by 2050 45 . In contrast to wind and solar energy, geothermal energy does not present intermittency, which would be an advantage in megacities. In fact, expanding massively its use may reduce costs and help to meet the Paris Agreement goals early. The expansion would require governments to set up policies and incentives that encouraged research and development into geothermal technologies and sub-surface exploration to encourage the development and expansion of geothermal technologies.

Conclusions

Geothermal is a baseload technology that increases grid stability and meets significant energy resilience needs, without intermittency, additional costs for storage, which wind and solar cannot offer. This leads us to argue that in the future, geothermal energy may provide flexibility with dispatchable power to megacities as geothermal power plants are developed. Geothermal power plants are also much less space-consuming and have lesser impact on landscape compared to wind and solar.

Analysis of the geothermal potential for the megacities: Bogotá, Los Angeles, and Jakarta, reveals that the estimated RGB can meet present day residential energy consumption. Calculated RGB/Consumption ratios reveal geothermal can respectively provide 1.14, 4.25, and 1.84 times the energy demand for these megacities.

In scenarios where geothermal replaces the present-day energy produced by fossil fuels it is estimated that the decarbonizing potential of geothermal in Bogotá, Los Angeles, and Jakarta, could be 15.4–17.0, 20.9–25.1, and 6.9–7.9 MtCO 2 e. In modest scenarios where geothermal is introduced and offsets 20% of the current fossil fuel use, thus developing only the most cost-effective resources, it is estimated that geothermal can contribute to 3.1–3.4, 4.2–5.0, and 1.4–1.6 MtCO 2 e decarbonization based on present day energy use assumptions.

We argue therefore that Megacities would benefit from the sustainable and resilient form of energy to supply decarbonizing not only residential but also industry, transportation, and other needs. Critical to the uptake of geothermal energy, however, is a positive government action that may include a carbon tax, investment into research and development of geothermal resources, and the establishment of policies or tax breaks that encourage the exploration and development of geothermal resources. The execution of long-term investment policies into the research and development of geothermal energy is critical not only for improving the chances of economic exploration, but also in speeding up the rate of innovation and the safe and sustainable use of geothermal resources. Finally, if geothermal solutions are investigated, and proper promotion is presented to communities, then additional benefits of a low-carbon energy system may be found, allowing to reach near-zero emissions globally in 2050, raising the quality of life for many people and helping to reduce energy poverty.

Methodology

The concept of GRB refers to the thermal energy Q stored in volume V of rock in the subsurface. It is usually estimated using the volumetric heat in place (VHIP) approach 26 . It depends on both thermal rock properties and the distribution of the geothermal gradient. Although this approach provides a general idea of the thermal energy on large scales, significant uncertainties remain due to the variability of the medium of variables such as density ρ, porosity Φ, heat capacity C , and temperature contrast (∆ T ) in the region of interest. This method is based on Eq. 1 , where the subscript r refers to rock and w to water. In this work, we assumed typical values of ρ , Φ, and C reported in literature 46 , 47 , 48 , 49 , 50 , 51 , 52 .

However, determining the potential geothermal power is difficult due to the scarcity of available information. Furthermore, uncertainties relating to newer AGS techniques and their efficiency at mining heat vs. utilizing hydrothermal waters to extract heat are still open for debate, since commercial developments are still not commonplace. Therefore, assessing a pure-play thermal resource is not included in this analysis.

In the three areas of interest, the GRB assessment is based on estimates from O&G wells and associated spring-waters. Using the EMAG2 database 53 , there are estimations of the Curie Point depths (CPDs) for these areas ranging between 15 and 30 km 54 . Hence, we calculated geothermal gradients. Generally, the most surficial CPDs match with areas of the largest density of spring waters (LDSW). In those areas, we defined regional plays with geothermal gradients higher than 30 °C/km. In cases such as the northern Bogotá, where there are limitations in resolution of the EMAG2, we integrate it with the local magnetic databases 55 to improve the resolution and geometry of estimations of the CPDs by using the code Pycurious 56 , 57 . Details of the method and parameters used are provided in the online data repository. We assumed that geothermal gradient anomalies may be responsible for the heating of the water springs dislocated along the fault systems and their complex intersections. We simplify the model and hypothesize, firstly, that the tectonic features control the location of hot spring-waters and secondly, that the area encompassing the springs can be used to define the area and volume of potential reservoir that is storing the thermal energy. Therefore, we assume the elliptical envelope that encompasses the LDSW as the best alternative for a better constraining of the area. While the areas, depths, and thicknesses of recoverable resources have been assumed according to the regional geology of the three areas, average porosity of 5%, fluid heat capacity of 4000 J/kg °C, rock heat capacity of 850 J/kg °C, specific heat capacity of 920 J/kg °K, and efficiency as low as 0.1% have been set as general values for the three areas. Table  3 summarizes the main volumetric parameters used for estimating the potential geothermal energy 26 , 27 .

The Curie depth point, or that depth at which rocks lose their permanent magnetic properties due to temperatures near or higher than the isotherm 580 °C, is typically estimated using the radial power spectra of magnetic anomalies within overlapping rectangular windows regularly spaced in a region of interest. In this work we have used the approach proposed by Bouligand 56 to estimate the thickness of a buried magnetic source by fits an analytical solution to the entire power spectrum.

The method used and implemented in the code Pycurious 57 , fits an analytical function that depends on three terms: the depth to the top of magnetic sources z t , the thickness of magnetic sources Δ z , and a fractal parameter β . The theoretical function that represents the radial average of the logarithm of the power spectrum of magnetic anomalies randomly distributed in the subsurface is defined by the next two equations 58 :

where \({\Phi }_{B1D}({k}_{H})\) and \({\Phi }_{B2D}({k}_{x},{k}_{y})\) are the radial power spectrum and the 2D power spectrum of magnetic anomalies, respectively, \({\overrightarrow{{{{{{\boldsymbol{k}}}}}}}}_{{{{{{\boldsymbol{H}}}}}}}=(kx,ky)\) is the wave number in the horizontal plane, \({{{{{{\boldsymbol{k}}}}}}}_{{{{{{\boldsymbol{H}}}}}}}=|{\overrightarrow{{{{{{\boldsymbol{k}}}}}}}}_{{{{{{\boldsymbol{H}}}}}}}|\) is its norm, and θ is its angle with respect to k x . The orientation of the geomagnetic field appears only in the constant C , and the shape of the radial power spectrum is independent of the direction of the geomagnetic field 58 . In this work, we evaluated diverse size of windows and after an optimization process, the Pycurious code found the best parameters z t , Δ z , β , and C . Overlapping of windows was of 10 km, covering all area bounded between 1.5–8.5 N and 70–77 W, and presenting results for the window 3.5–6.5 N and 72–75 W.

Data availability

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

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Acknowledgements

The authors are grateful to the DAAD (Project 57448047: Geothermal Energy for Megacities: Phase 1, Bogotá - Colombia) and MINCIENCIAS (Projects 80740-053-2019, 110185271555-852-2019, 80740-909-2020, and 80740-157-2018) for funding the projects and allowing the academic exchange between the Friedrich-Alexander University (Erlangen-Nürnberg) and the Universidad Nacional de Colombia—UNAL (Bogotá). Geophysical monitoring infrastructure, including GHG stations in the campus of the UNAL, is being supported by TIGO—Colombia. Harald Stollhofen and two anonymous reviewers are greatly acknowledged for the constructive criticism and guidance that helped shape the final version of this paper.

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L.C. and C.A.V. initiated the study in 2019 based on an idea to study the challenges of energy within megacities. C.A.V., L.C., and P.J.B. wrote the manuscript. C.V. developed the statistical model for geothermal resources, P.J.B. computed the forecast of heating and cooling needs for global warming scenarios and the emissions reduction scenarios.

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Vargas, C.A., Caracciolo, L. & Ball, P.J. Geothermal energy as a means to decarbonize the energy mix of megacities. Commun Earth Environ 3 , 66 (2022). https://doi.org/10.1038/s43247-022-00386-w

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Climate change and global warming have been driven by a rise in carbon dioxide (CO 2 ) concentrations in recent decades, posing a danger to environmental sustainability. Thus, this research scrutinizes the effects of two types of energy (coal and geothermal) and natural resources on CO 2 emissions in 10 newly industrialized countries (NICs). The study also considers the role of financial globalization using a data between 1990 and 2019. This research applied a fresh nonparametric econometric technique termed “method of moments quantile regression (MMQR).” This approach is resistant to outliers and produces an asymmetric connection between variables. Furthermore, the long-run estimators (AMG and CCEMG) are employed as a robustness check. The findings reveal that natural resources, coal, and economic growth contribute to the degradation of the environment in the NICs in all quantiles (0.1–0.90). However, geothermal energy aids in enhancing environmental sustainability at all quantile distributions (0.1–0.90). Our findings are robust with alternative methods (AMG and CCEMG). The research’s outcomes have the potential to help NICs nations design policies. Finally, based on the research results, a policy framework is proposed to solve the objectives of SDGs 7 and 13.

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Long-run effects of natural resources, coal, and geothermal energy on the CO 2 emissions

The study control for distributional heterogeneity using novel method of moments quantile regression (MMQR)

Natural resource and coal energy consumption increase CO 2 emissions.

Geothermal energy consumption decreases emissions.

Practical implications are reported in the lens of carbon neutrality and structural changes in NICs.

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Bashir, M.A., Dengfeng, Z., Shahzadi, I. et al. Does geothermal energy and natural resources affect environmental sustainability? Evidence in the lens of sustainable development. Environ Sci Pollut Res 30 , 21769–21780 (2023). https://doi.org/10.1007/s11356-022-23656-8

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Oil industry could help the Biden administration tap 'invisible' green energy

Kirk Siegler

BOISE, Idaho — A U.S. Department of Energy report this spring made a bold prediction.

The nascent geothermal industry, it said, has a ready workforce of 300,000 engineers, hydrologists, drillers and power plant operators ready to tap right here in this country.

All that's needed are more early adopters like Tina Riley who are willing to move over from the oil and gas sector. After two decades as a geologist with ExxonMobil in Houston, she recently moved to Idaho to help run Boise's geothermal utility.

"Boise is really well known for geothermal and I wanted to become part of it," Riley says. "I really wanted to be part of the energy transition."

Geothermal energy, which in its simplest form means tapping hot water locked in granite faults sometimes thousands of feet below the surface of the earth to create heat or electricity, is often dubbed an invisible technology. It's long been seen as underutilized. But it's also a hugely expensive renewable resource to extract compared to more conventional drilling.

Nevertheless, the White House is launching an ambitious plan to increase its development in the United States by twenty fold. And its success relies partly on a very visible industry to achieve that — oil and gas.

Boise has the oldest geothermal system in the country

Today Boise leaders see their geothermal system as a key component to meeting the city's climate neutrality goals. Since 2000, it has reduced 100,000 CO2 equivalents, or roughly the same as taking 24,000 cars off city streets. But the heating system originally came online as a cost savings measure in response to the oil crisis in the late 1970s. Geothermal had even been used to heat homes in one of the city's more affluent neighborhoods for well over a century.

Today about a hundred buildings including a Veterans Affairs campus get their heat and hot water piped in from the geothermal aquifer beneath the city. The list also includes the Idaho capitol; the only state capitol building in the country to be heated by geothermal.

A short drive away, Riley is getting out of a city owned electric car and walking to check on one of the system's well houses. It's adjacent to a popular trailhead and mountain bike park. Most residents or visitors would have no idea it's even there, more evidence it really is a mostly invisible technology.

"What we're walking along now on this road is actually an inactive fault," Riley says. Idaho's geology makes it particularly suitable for geothermal energy, she adds.

Boise has the largest and oldest municipal geothermal heating systems in the United States. Kirk Siegler/NPR hide caption

Boise has the largest and oldest municipal geothermal heating systems in the United States.

Soon, two structures built into the side of the mountain come into view. They look like bunkers. The warm water is pumped up through these well houses into a pipe system then transferred underground to nearby downtown. After it's used, it's discharged back into the aquifer near the Boise River.

Riley beams as she adds that the system is closed loop, totally renewable and emissions-free.

"It's a form of energy that just checks so many boxes and it utilizes a lot of skill sets that are transferable or applicable from oil and gas," Riley says.

There is a lot of untapped potential with geothermal

Boise's system is only used for heat because the warm water in the geothermal aquifer beneath this city of 230,000 is below the boiling point. But in other areas, geothermal can also be used to generate electricity.

In fact, scientists believe there is enough of the resource underneath the lower 48 states alone to provide power to upwards of seven million homes. But transitioning all those workers over from oil and gas is still considered a long shot. Until recently, scaling up geothermal took a backseat due to advances in fracking for natural gas.

"Where the geothermal industry is today is where the oil and gas industry was 150 years ago," says Bryant Jones, executive director of Geothermal Rising, a trade group. "You drill for oil and gas where you literally saw oil and gas bubbling up from the surface."

Tina Riley moved to Idaho recently in search of a new career working in the clean energy transition. Kirk Siegler/NPR hide caption

Tina Riley moved to Idaho recently in search of a new career working in the clean energy transition.

That's partly why Boise's system is so developed because the resource is relatively easy to access. But there are movements afoot that could change this. Bi-partisan legislation that's gaining momentum in Congress would put geothermal on the same playing field as oil and gas when it comes to permitting and new exploration on federal land.

This could bring down costs, Jones says. The geothermal industry is a fraction of the size of wind, solar and oil and gas industries in the U.S., accounting for only .4% of the total electricity generation.

"Because of that small size we just don't have enough boots on the ground in state capitols or in Washington, D.C.," he adds. "So when policies are being discussed, geothermal is often left out."

This Spring the White House did announce $60 million to scale up geothermal, funding an initial round of pilot projects including one run by Chevron. In its report this year, the Biden administration pushes to expedite new drilling in Idaho and five other states by 2030, as part of its goal to create a carbon free electricity grid by 2035.

"Geothermal is a subsurface resource just like hydrocarbons. It requires pipes. It requires drilling. These are all skills and trades that we have in the U.S.," says Amanda Kolker, who runs the geothermal program at the federal National Renewable Energy Lab. "It's a much smoother transition to geothermal than to maybe some other technologies."

There is little data available beyond anecdotes about how many workers are actually transitioning or interested in moving to geothermal from conventional fossil fuels industries . Scientists at the Colorado lab have made gains in the last three years improving efficiency and drilling techniques but they're still far behind oil and gas.

Kolker calls geothermal exploration a very uncertain art: "Because if you can imagine, you're trying to understand what's going on underground. It's invisible, you can't see it, and your best data points are deep wells and we don't have lots of those."

Boise is looking to expand, cautiously

But geothermal is increasingly attractive because it's a stable renewable energy source. And the race has been on to find a suitable baseload fuel to supplement wind and solar.

In Boise, the former oil geologist turned geothermal manager, Tina Riley, says demand to join the city's system has grown by 25% just since 2020.

"It works around the clock, you don't have to worry about the wind or the sun shining," she says, talking over the loud hum of the pumps.

Riley says they're planning to expand slowly. They hope advancements in technologies will soon give them a better picture of exactly how much of the resource is available. But they are currently looking to add more than a dozen new buildings to the system soon.

For her part, Riley has no regrets about leaving the Texas oil patch.

"The really cool thing in my mind is that, oil and gas, as you use it, it's depleted. With a geothermal aquifer, you don't. It's a sustainable form of energy that's going to be around for many generations to come," Riley says.

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New revenue streams: Using Africa’s vast renewable energy and natural resources for premium carbon credits

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African countries could leverage their vast renewable energy resources, tropical forests, peatlands, and marine ecosystems to export premium carbon credits, providing a new revenue stream, according to the 2024 Economic Report on Africa by the United Nations Economic Commission for Africa (UNECA).

The report says, carbon markets could support Africa’s goals of resilience and prosperity, in line with Agenda 2063. They also present a potential path for achieving the Paris Agreement’s climate goals.

“A failure, however, to ensure credit additionality, appropriate governance, and high enough prices could lead to perverse market incentives that increase carbon emissions and slow the climate transition on the continent,” says the report, launched at the recently concluded 10 th Africa Regional Forum on Sustainable Development (ARFSD-10) in Addis Ababa, Ethiopia.

Nassim Oulmane, Acting Director of ECA's Technology Climate Change, and Natural Resource Management Division explains that there are two types of carbon markets that Africa could invest in: the regulatory compliance market and the voluntary carbon market (VCM).

But so far, credits from the VCM, where many African countries participate, have been only a small fraction of those supplied by the overall regulatory compliance market.

“In the VCM, the trade of carbon credits is voluntary while the compliance market, is used by companies and governments required by law to account for their greenhouse gas emissions. It is regulated by mandatory national, regional, or international carbon reduction regimes,” he explained.

“Most of the credits in the VCM have come from nature-based solutions, including forest conservation, improved agricultural cultivation, and reforestation. Energy savings from fuel efficiency and fuel switching were additional sources.”

The report shows that in 2022, while the VCM value was approaching a mere $2 billion, the value of traded carbon permits in global markets reached a record $909 billion.

On a more positive note, estimates point to the VCM reaching $10–$40 billion by 2030. A third of the VCM’s traded volume were retiring credits (that is, buying credits to count towards a commitment), but Africa contributed only 11% of this type of VCM credits in 2016.

“Africa currently realizes only around 2% of its annual potential of carbon credits,” says the report.

Explaining Africa’s potential in carbon-credits, Mr. Oulmane said the continent should invest in its untapped renewable energy potential; youthful, rapidly growing workforce; available land and other natural assets; and low emissions.

“Proceeds from sales of carbon credits can provide additional revenue for climate-smart interventions,” said Mr. Oulmane.

He added: “In addition to improving the climate, many of these interventions improve livelihoods, create jobs, spur new economic and sustainable industrial activity.”

The African Carbon Market Initiative (ACMI) estimates that 110–190 million African jobs can be created by 2050 if the carbon price per tonne reaches $80 and direct and indirect jobs are added beyond nature-based solutions.

Evolving carbon markets also present challenges for African economies.

Investing in sustainability transition

While the report shows that Cameroon, DRC, Ethiopia, Ghana, Kenya, Nigeria, Tanzania, Uganda, Zambia, and many other African countries have participated in the VCM, five countries were selected for case studies on investing in a sustainability transition: Gabon, Kenya, South Africa, Senegal, and Morocco.

Each case study reviews the national policy and strategic context and looks at the investment intervention in a sector important for promoting the transition.

GABON: Investing in sustainable forest management

Through investment in sustainable forest management, Gabon has been able to increase employment opportunities, driven by the increase in wood-processing industries in the Gabon Special Economic Zone in Nkok, from 80 in 2009 to 155 in 2018; minimized its deforestation rate; improved its social welfare with poverty incidence estimated at 33.4% and unemployment estimated at 28.8%.

KENYA: Investing in Geothermal energy

Geothermal energy development helped lift Kenya’s GDP from $70.0 billion in 2015 to $113.4 billion in 2023 and reduced its carbon footprint from power generation by displacing some traditional fossil fuel–based power generation.

SOUTH AFRICA: Investing in Renewal Energy

A pioneering model for sustainable financing, the Renewable Energy Independent Power Producer Procurement Programme (REIPPPP) with a competitive bidding process employed ensures market-driven prices, fostering cost reductions in energy production. This not only contributes to the country’s energy security but also enhances its economic competitiveness on the global stage.

SENEGAL: Investing for Green Growth

The investment in renewable energy model expanding renewable energy infrastructure has given access to millions of Senegalese to affordable electricity, fostering growth, sustainability, and resilience in the beneficiary communities. The low tariff of less than EUR 0.04 per kilowatt-hour has improved the purchasing power of the population and stimulated job creation.

MOROCCO: Sustainable finance for an ecological transition 

Morocco’s development and implementation of the different financing mechanisms under the Low Carbon Strategy - Green innovation of Agadir municipality and - Positive impact bond -has created new job opportunities and stimulated economic growth in related sectors. Climate investments in energy-efficiency measures are leading to cost savings for businesses and households, contributing to economic productivity and competitiveness.

Also in this issue

Protecting Africa's wetlands is key to combating biodiversity loss

Angola: Soyo entrepreneurs switching from oil to sustainable agriculture

The duality of the education challenge in Africa: Historical imperatives and 21st-century necessities

Unregulated Autonomous Weapons Systems pose risk to Africa

In Southern Africa, El Niño drought leaves a trail of scorched harvests and hunger

Reforming global financial structures: Paving the way for financing for sustainable development in Africa

Broaden national tax base for more resources, African countries told

More from africa renewal.

With leadership and determination, Africa can achieve the SDGs

Crabshack floating restaurant thriving above mangrove trees

How Kenyan coastal villagers are cashing in on carbon credits

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Enhanced geothermal systems (EGS) for UN sustainable development goals

D. chandrasekharam.

Department of Civil Engineering, Izmir Institute of Technology, 35430 Izmir, Turkey

Energy-water-food nexus, the three interdependent primary requirements are essential for life and the need of the hour by the world. The Covid-19 pandemic has proved that countries that are food independent can survive any stressed conditions. For food security, water is essential. Countries that are self-sufficient with water can overcome situations like what the world experienced in the last 2 years. Energy independent countries can be water secured at a cost that will disturb the environment. But when this energy is green and free from carbon footprint, it will promote sustainable development of a country, providing water and food. This technology, which is in its development stage now, will be the future “energy road” to the countries. High radiogenic granites, omnipresent, with low carbon, and low land footprint, can power the millions. In enhanced geothermal systems or engineered geothermal systems (EGS), as the name implies, a reservoir is created in a hot rock, that is devoid of water, and heat from the rock is mined by circulating water or supercritical carbon dioxide (ScCO 2 ) [ 12 ]. Three dimensional simulations of EGS systems operated with CO 2 as injection fluid (heat extraction fluid) instead of water was used by Pruess [ 13 ] indicate that gravity effects were found to be strong and capable of inducing preferential flow in the bottom of the thermal reservoir. The advantage of ScCO 2 is to avoid water based reservoir that inhibits water rock-interaction, resulting in the precipitation of secondary minerals and clogging the fracture openings. This circulating ScCO 2 is a future promising heat extraction medium with minimum liability on the thermal reservoir.

This heat from the granites is utilized for generating electricity, providing heat to support agriculture, space heating, dehydration and finally providing low-cost freshwater from the sea through desalination technology.

Unlike hydrothermal resources, that are site-specific, EGS has no such restrictions. Starting with traditional hydrofracturing technology, which was established in Soultz-sous-Forets, France and Cooper basin, Australia [ 9 , 11 ], now EGS is bouncing into a new development of a “closed-loop” system of technology, extracting heat from the granites. This method, unlike the conventional hydrofracturing method, is free from induced seismicity. The closed-loop EGS system is also known as the advanced geothermal systems (AGS) uses a co-axial U-loop in which the working fluid (water or ScCO2) will not enter the rock or flow into the rock fractures [ 2 ]. The loop acts as heat exchanger, transferring heat from the rock to the circulating fluid that is collected or flows into a production well. Simulations studies were conducted using varying injection temperatures and varying reservoir temperatures and flow rate [ 2 ]. The results show that annual power generation from this system could vary from 2 to 15 GWh with a levelized unit cost of power around US$ 20 to 110 /MWh. Over the years, with technological development this cost could be brought down. The advantage of AGS is, electricity can be generated anywhere on the earth and with minimum or zero induced seismicity that is a cause of concern with hydrofraturing based EGS systems.

I will discuss two cases: one where EGS is sustainable for water-food security and the second case, where EGS is cost-effective and helps the country to support UN SDG. The Former case is Saudi Arabia and the latter one is Turkey. Both the countries have huge EGS resources and how this energy source will help the countries to mitigate climate-related issues and secure food and water are highlighted here.

The Western Arabian shield hosts numerous granite intrusives (Fig.  1 ), spread over a cumulative area of 161,467 sq. km, with high concentration of radioactive elements (U, Th, and K) and the radiogenic heat production (RHP) of these granites varies from 5 to 134 µW/m 3 [ 6 , 7 ]). The temperatures measured in certain bore-holes along the western margin of the shield gave geothermal gradients exceeding 80 °C/km. Temperature estimates at 2 and 3 km depth gave values of 230–300 °C [ 6 , 7 ]).

Radiogenic granites with high heat generation capacity, Western Arabian Shield.

(adapted from Ref. [ 6 , 7 ])

Earlier EGS projects established in Soultz-sous-Forets, France and Cooper basin, Australia [ 9 , 11 ], estimated that 1 km 3 of such high radiogenic granites can generate 79 × 10 6 kWh of electricity. Assuming a 2% recovery of heat from such granites, the power generation capacity of the Midyan granite alone (Fig.  1 ), located at the NW corner of the shield, exceeds 160 × 10 12 kWh. Fluid flow and heat transfer models using ANSYS/CFX demonstrate that this amount can be enhanced further to support industrial applications [ 14 ]. Saudi Arabia, though energy independent, is a water-stressed country and depends on groundwater for agriculture and domestic needs. The annual groundwater recharge is only 2.4 billion m 3 while the groundwater extraction is 20 billion m 3 . This additional water is withdrawn from the non-renewable (fossil) aquifer- the Saq-Ram sandstone aquifer. This is not a sustainable solution to meet water demand. Hence the country depends on food imports. Saudi Arabia’s food imports have exceeded 70 million tons and the country has banned wheat production due to a lack of water for irrigation [ 8 ]. The country is unable to support per-capita “pita” (bread) production of 88 kg/y. Thus, wheat imports have surged from 1.9 to 3.03 million tonnes. To support other agricultural and domestic water supply, the country depends on 128 desalination plants, supported by fossil fuel-based energy, supplying freshwater at an energy input of 5 kWh/m 3 (MSF: Multistage Fractionation, [ 10 ]). The source of energy being fossil fuel the emissions are around 5.43 kg CO 2 /m 3 of freshwater generated. EGS (with ScCO 2 as extraction fluid) has three advantages for Saudi Arabia. It can provide base-load uninterrupted energy, and freshwater (food security) and reduce CO 2 emissions.

Turkey’s EGS potential is also huge and similar to Saudi Arabia. High radiogenic Eocene and Miocene granites are spread over a cumulative area of 6910 sq. km along western Anatolia (Fig.  2 ). Although the RHP of the granites of western Anatolia is lower than that of Saudi Arabian granites, the advantage Turkey has is the disposition of high-temperature isotherm (Curie point temperature-depth) at a very shallow depth (6 to 12 km [ 1 , 3 ] and a foundered continental and thinned crust due to the Alpine-Himalayan orogeny.

High radiogenic granites of western Anatolia, Turkey.

(adapted from Ref. [ 4 ])

However, the heat flow values of western Anatolian region are very high (~ 154 W/m 2 ), which is higher than the heat flow values of the Arabian shield. The reservoir temperatures estimated based on the bottom-hole temperature of drill holes is about 180 °C at 3 km [ 3 ]. These granites can generate a minimum amount of 546 × 10 9 kWh [ 4 ]. Turkey has established a strong geothermal culture with its hydrothermal resource, generating 1658 MWe and providing district heating and greenhouse cultivation support. Turkey ranks the third position in the world with its geothermal energy generation. With the development of EGS, Turkey will emerge as a strong country establishing United Nations’ Sustainable development Goals SDG) in another few years. With a high-temperature regime at shallow depth, the cost of drilling will drastically be come down allowing a lower levelised cost of electricity (LOCE). The entire western Anatolian region will be an experimental ground to perform innovative technology related to EGS, like developing loop technology to harness heat, carbon dioxide sequestration, supercritical carbon dioxide fluid circulation to extract heat and develop innovative methods to create fracture networks in the granites at 3 km depth. This provide an opportunity to refine the EGS technology. While the unit cost of energy projected based on earlier EGS projects (Soultz-sous-Forets, France and Cooper basin, Australia) was around 6 to 7 Eurocents, EGS projects in Turkey will provide a realistic unit cost of power in future. Like Solar PV [ 5 ], EGS has no carbon footprint and the land footprint is very low. EGS will provide sound energy-water-food security to all the countries and remove the fiscal disparity between the countries and people and lead the world towards a net-zero emissions scenario.

Acknowledgements

DC thanks TUBITAK (project No:120C079) for its support in this work.

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