From hot rock to useful energy: A global estimate of enhanced geothermal systems potential
Graphical abstract
This study determines the theoretical, technical, optimal economic and sustainable potential of enhanced geothermal systems (EGS) on a global scale. The global potential ranges from 256 GWe to 108 TWe by 2050, depending on the selected constraints.
Introduction
Geothermal energy as a renewable energy source is commercially available today and has great potential to contribute to the growing share of renewables to meet the global future energy demand [1], [2], [3], [4]. Geothermal resources can supply energy throughout the year due to the constant flow of heat from the Earth. The use of geothermal energy for heat production is not new and has been practiced for thousands of years. However, for electricity generation, higher temperature resources, around 100-150°C or higher, are needed. Thus, the availability of geothermal energy for electricity generation is limited because such high temperature resources are mostly found near volcanically active regions, abnormally high geothermal gradients, or impermeable rock around a hydrothermal system [5]. It is expected that the contribution of geothermal power to the total global electricity generation will increase due to its high potential and cost-competitiveness. Since the depletion of natural resources, such as oil and gas, and subsequent increase in price would not affect geothermal energy, it is envisaged that geothermal power gains momentum in the years to come. However, the full potential of geothermal energy has not yet been assessed on a global scale. It should be noted that although an immense quantity of heat is stored and available within the Earth, excessive production of heat resources will result in reservoir depletion or even deterioration [6].
As of 2019, thirty countries have added geothermal capacity to their total energy mix with the total cumulative installed capacity of approximately 14.6 GW globally [7]. The US continues to be the global leader, followed by Indonesia, Philippines and Turkey. In 2018, geothermal capacities were mainly installed in Turkey and Indonesia by 294 MW and 139 MW, respectively, accounting for around two-thirds of the new capacity installed collectively [8]. Turkey experienced the highest increase in geothermal power capacity, increasing from 30 MW in 2008 to 1300 MW by the end of 2018. According to the International Energy Agency (IEA) [9], global geothermal power capacity is expected to grow to more than 17 GW by 2023. Indonesia, Kenya, Philippines and Turkey are expected to have the largest capacity additions.
Geothermal heat or direct-use of geothermal energy is one of the most versatile and oldest forms of heat production. China, the US, Sweden, Turkey and Germany accounted for roughly 66% of global geothermal heat capacity installed in 2015 [10]. The major utilisation of geothermal heat is for space heating, bathing and swimming pools, which together contribute up to 80% of direct-use of geothermal energy [10]. The remaining consumption of geothermal heat is for domestic hot water, agriculture (greenhouse heating, agricultural pond heating and agricultural drying), industrial process heat, cooling and snow melting applications [11]. Geothermal (ground-source) heat pumps are one of the fast growing applications for direct-use of geothermal energy, which allow for reduction in fossil fuel consumption and GHG emissions, as well as economic benefits [12], [13]. The heat can be collected from different depths depending on the geothermal heat system. Horizontal ground heat exchangers extract the heat at depths of 1 to 2 m. Ground water wells work at depths of 4 to less than 50 m. Energy pipes collect heat at depths of 5 to 45 m, and heat collection for borehole heat exchangers is between 10 and 250 m depths [14]. As of 2015, the leading countries in terms of heat pumps installed capacity are the US, China, Sweden, Germany and France, with a total installed capacity of 38.8 GWth [10].
There are two types of geothermal energy systems: conventional geothermal systems (hydrothermal) and enhanced (engineered) geothermal systems. The latter was previously known as Hot Dry Rock (HDR) and will be referred to as EGS hereafter in this paper. The majority of recently built and under construction hydrothermal and EGS geothermal power plants use binary-cycle technology [8]. The advantage of binary-cycle technology to other ordinary geothermal technologies is the operation with relatively low-temperature resources. However, existing geothermal plants use flash-steam and dry-steam technologies mostly, which are suitable for high-temperature resources. As mentioned earlier, conventional geothermal systems are restricted to specific geographical locations. In addition, permeable aquifers are the basis for hot water production in the standard hydrothermal technologies. In contrast, EGS can be built in larger areas, in different parts of the world, by creating a subsurface fracture system in the hot rock through hydraulic stimulation. Further, since the heat is extracted from hot basement rock at greater depths, there would be less natural permeability and fluid content. In the EGS process, high-pressure cold water travels through fractions in the rock via injection wells to capture heat from rock at great depths and returns to the surface via production wells as hot water. Then, the heat of hot water is converted into electricity using a steam turbine or a binary power plant. The cooled down water is re-injected into the ground to heat up again in a closed loop. EGS emits a very small to zero amount of GHG emissions [15]. Having said that, the average GHG emissions associated with geothermal power plants is around 120 gCO2/kWhe, which is relatively lower than that of fossil fuels power plants [16]. It is expected that the technology improvements, such as re-injection, will decrease the amount of GHG emissions significantly to 10 gCO2/kWhe [16]. EGS is comparable with other renewable energy technologies with regards to GHG emissions and environmental impacts, and in some cases have lower environmental impacts [17]. The GHG emissions from EGS range from 3.8 to 45.6 gCO2-eq/kWhe, depending on the selected applications [17].
The EGS potential estimation for the US has been proposed and evaluated by Tester et al. [18], Blackwell et al. [19], Augustine [20] and Lopez et al. [21]. Tester et al. [18] assessed the feasibility of providing 100 GWe electric capacity of EGS by 2050. They concluded that the EGS potential is even larger in the long-term, and achieving such a target is viable. Blackwell et al. [19] analysed the EGS potential for the conterminous US using temperature at depth (3–10 km) maps. It is reported that if only 2% of the EGS resource is developed, the produced energy would be 2500 times the annual primary energy consumption in the US in 2006. Augustine [20] presented the potential electric capacity of the US geothermal resources and the respective costs. The total identified hydrothermal and EGS capacities are about 36.4 GWe and 15,915 GWe, respectively. Additionally, the optimum reservoir depth has been determined using the minimum levelised cost of electricity (LCOE) at each data point. Lopez et al. [21] explored the technical potential for EGS using temperature at depth data. Similar to Augustine, the quantitative analysis method has been applied in order to find the optimum depth at the minimum LCOE. The US total technical potential for hydrothermal geothermal systems and EGS is evaluated to be around 38 GWe and 4000 GWe, respectively.
A geothermal resource estimation was carried out for Korea [22] based on constructed temperature at depth maps. The calculated subsurface geothermal energy ranges between 16.7 ZJ (4.6·105 TWhth) at 1 km depth and 101 ZJ (280·105 TWhth) at 5 km depth. Chamorro et al. have conducted studies for EGS potential estimation in Europe [23] and Iberian Peninsula [24]. Similar methods have been applied in both studies. The technical potential in terms of electrical power capacity is identified to be around 700 GWe in the Iberian Peninsula and more than 6500 GWe in Europe. Moreover, for the case of Europe, a sustainable potential term has been defined where not all the available heat content in the basement rocks can be extracted. Applying this constraint affected the technical potential estimation drastically and the final sustainable potential is found to be 35 GWe. The potential for geothermal energy in Germany has been evaluated [25]. Under the most optimistic assumptions, the available land area for constructing EGS plants is identified to be 89,000 km2, which can consist of 13,450 EGS plants with a maximum electric capacity of 474 GWe. An EGS analysis based on the subsurface temperatures data and minimum LCOE has been conducted in Europe [26]. The potential is deduced to be 19 GWe in 2020, 22 GWe in 2030 and 522 GWe in 2050. The temperature at depth calculations have also been performed to estimate the EGS potential in Great Britain [27]. The results indicate that the total technical potential is about 222.4 GWe for the depth of 6.5 km and temperature greater than 150°C. An optimal design of EGS has been assessed for the case of Switzerland considering environmental impacts [28]. The findings reveal that the shallower depths, 3500–6000 m, are more favourable for a district heating network while deeper depths are suitable for electricity production. However, the choice of appropriate technologies and applications might vary depending on the considered criteria. Hofmann et al. [29] studied the potential for EGS in the province of Alberta in Canada and concluded that Cooking Lake formation and Basal Sandstone are the most promising reservoirs, among the investigated formations, for heat extraction, even though the associated costs are higher than shallower formations. It has been claimed that China [30] has abundant EGS resources, especially in Southern parts of the country such as Yunnan, Tibet, and Southeast Coast. The evaluated EGS potential in China is about 7·109 TWhth [31]. The Chinese Academy of Geological Sciences stated that the technical extractable geothermal resources with temperature of higher than 150°C is around 8500 MWe [32]. Xia and Zhang [32] concluded that despite the availability of geothermal resources, several internal and external factors influence the development of geothermal energy in China as well as in other countries globally. These include, but not limited to, insufficient exploration of resources, lack of access to the necessary data for detailed analysis, shortage of sufficient policy support, intense competition with other energy resources, and mismatch between supply and demand.
A Protocol for estimating the EGS potential has been proposed [33]. The Protocol set a framework, for both theoretical and technical geothermal potential estimation in different regions, using consistent methodologies and assumptions. The main goal of the Protocol is to make the results of different regions comparable for better understanding of EGS potential across the world. The current research work follows the Protocol recommendations for EGS estimation globally, where applicable, and also delved into other sources, methods and assumptions. This study is structured as follows: in section 2, the materials and methods used to evaluate the EGS potential globally are explained; in section 3, the results of modelling are presented; in section 4, a comprehensive discussion regarding the EGS potential assessment in this study, comparison of the results with other studies, and limitations and uncertainties are carried out; in section 5, a summary of the presented and discussed results is given.
Section snippets
Materials and methods
In this study, the world is clustered into a regular gridding interval of 1 degree by 1 degree, 1°×1°, to evaluate the EGS potential. This assumption is different with the 5′×5′ interval (5′ = 5 min = 0.0833°) proposed by Beardsmore et al. [33] in the Protocol, due to limited access to the data on a global level. It is challenging to find reliable and accurate data in such high spatial resolution on a large-scale. In case the data were available for the gridded cells, the actual data are
Temperature at depth
To derive temperature as a function of depth, a steady state is assumed without heat advection processes such as magmatism occurrence, intense erosion and hydrothermal convection [5]. All the above mentioned phenomena can occur only in short period of time when equilibrium thermal regime of the continental crust is considered. Therefore, the simplest form of Fourier law can be calculated based on Eq. (2). In this condition, a constant thermal conductivity, a depth-dependent temperature field,
Interpretation of the results
As shown in Table 4, Table 5, the EGS technical potential (Tz ≥ 150 °C) at various depths for the considered cost years of 2030 and 2050 are given. These values are also compared with EGS technical potential without economic and water stress constraints. The economic assessment indicates the sensitivity of the available heat content at depth intervals, which is related to the costs of drilling. The results clearly reveal the significant impact of the LCOE reduction over time on the extractable
Conclusions
With the growing demand of renewable energy and necessity for a rapid energy transition, the need to determine the maximum potential of renewable energy becomes increasingly important. Geothermal energy is a renewable energy resource that has great potential to help accelerate the shares of renewable energy in the energy mix. The conventional geothermal power system, hydrothermal, has been commercialised for a very long time. However, development of the emerging geothermal technology, EGS, has
CRediT authorship contribution statement
Arman Aghahosseini: Conceptualization, Data curation, Resources, Software, Validation, Visualization, Writing - original draft, Writing - review & editing. Christian Breyer: Conceptualization, Supervision, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors gratefully acknowledge the public financing of Tekes, the Finnish Funding Agency for Innovation, for the ‘Neo-Carbon Energy’ project under the number 40101/14. The authors would like to thank Upeksha Caldera for proofreading. Also, the authors express their gratitude to the thorough review of anonymous reviewers.
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