Modeling the CO2-based enhanced geothermal system (EGS) paired with integrated gasification combined cycle (IGCC) for symbiotic integration of carbon dioxide sequestration with geothermal heat utilization

The permeability evolution of natural fractures during shearing is one of the critical issues in enhanced geothermal system (EGS). In this study, a series of numerical simulations of shear-flow tests under different normal stresses were carried out to investigate the permeability evolution and shear behavior of a natural fracture during shearing. All numerical simulations in this study were implemented based on a coupled hydro-mechanical pore network model (PNM) of fluid flow in discrete element method (DEM). Fluid grids in the simulations are continuously refreshed as the shear progresses. The results indicate that the fracture permeability positively correlates with the shear displacement when the normal stress is relatively large. The essence of permeability evolution is the variation of the local aperture distribution. Greater fracture roughness makes it easier to form high-permeability pathways. Besides, high normal stress amplifies the effect of fracture roughness on permeability evolution. These findings can contribute to a better understanding of the hydraulic-mechanical coupling effects in the fracture shearing process of EGS. INTRODUCTION Enhanced geothermal system (EGS), as a key technology for the extraction and utilization of geothermal energy from high-temperature rock masses in deep formations, has become a research and development hotspot in the field of earth energy and renewable energy (Huang et al., 2018). Many studies have shown that fluid injection in EGS may lead to induced seismicity (Hanano, 2004; Zang et al., 2014; Gaucher et al., 2015; Kim et al., 2018). EGS utilizes fluid to create a new fracture network in hot dry rock or to further expand the original natural fracture network, thereby creating a large number of fluid flow channels. This process involves a series of complex fluid-solid coupling effects and may cause shear slips of fractures or faults (Ishibashi et al., 2018). Fracture shear slip is often accompanied by the evolution of fracture permeability, and the evolution process of fracture permeability can reflect the frictional stability of fractures to a certain extent (Wu et al., 2017). It can be seen that the study of the permeability evolution process and its influence factors in the shearing process of natural fractures is of great significance for evaluating the safety and stability of EGS.

The Organic Rankine Cycle (ORC) is commonly accepted as a viable technology to convert from low to medium temperature geothermal energy into electrical energy. In practice, the reference technology for converting geothermal energy to electricity is the subcritical simple ORC system. Over time, geothermal ORC plants with more complex configurations (architectures) have been developed. In the open literature, a large number of advanced architectures or configurations have been introduced. An analysis of the scientific literature indicates that there is some confusion regarding the terminology of certain advanced ORC system architectures. A new categorization of advanced configurations has been proposed, with a special emphasis on the application of geothermal energy. The basic division of advanced plant configurations is into dual-pressure and dual-stage ORC systems. In this study, the real potential of advanced ORC architectures or configurations to improve performance as compared with the simple ORC configuration was explored. The research was conducted for a wide range of geothermal heat source temperatures (from 120 °C to 180 °C) and working fluids. Net power output improvements as compared with the basic subcritical simple ORC (SORC) configuration were examined. The ability to produce net power with different ORC configurations depends on the magnitude of the geothermal fluid temperature and the type of working fluid. At a lower value of geothermal fluid temperature (120 °C), the most net power of 18.71 (kW/(kg/s)) was realized by the dual-pressure ORC (DP ORC configuration) with working fluid R1234yf, while the double stage serial-parallel ORC configuration with a low-temperature preheater in a high-temperature stage ORC (DS parHTS LTPH ORC) generated 18.51 (kW/(kg/s)) with the working fluid combination R1234yf/R1234yf. At 140 °C, three ORC configurations achieved similar net power values, namely the simple ORC configuration (SORC), the DP ORC configuration, and the DS parHTS LTPH ORC configuration, which generated 31.03 (kW/(kg/s)) with R1234yf, 31.07 (kW/(kg/s)) with R1234ze(E), and 30.96 (kW/(kg/s)) with R1234ze(E)/R1234yf, respectively. At higher values of geothermal fluid temperatures (160 °C and 180 °C) both the SORC and DP ORC configurations produced the highest net power values, namely 48.58 (kW/(kg/s)) with R1234ze(E), 67.23 (kW/(kg/s)) with isobutene for the SORC configuration, and 50.0 (kW/(kg/s)) with isobutane and 69.67 (kW/(kg/s)) with n-butane for the the DP ORC configuration.

ABSTRACT: In Enhanced Geothermal Systems (EGS), geothermal energy extraction relies on the creation and maintenance of fractures to facilitate fluid circulation and heat transfer. Cold water injection induces significant thermal stress, causing fractures to open, propagate, and close, which in turn impacts fluid flow distribution and long-term energy recovery. This study employs a coupled fluid-solid-heat reservoir-fracture-well simulator to analyze these effects under varying gravity, stress gradients, and geothermal conditions. Simulation results indicate that thermal contraction from cold water injection leads to fracture growth, influencing both artificial and natural fracture networks. Thermo-poro-elastic effects, gravity, and stress gradients play key roles in fracture dynamics, affecting fracture conductivity, and overall system efficiency. Changes in fracture geometry over time alter flow distribution, potentially leading to uneven heat extraction and localized cooling. Understanding these complex interactions allows for the optimization of fluid injection and circulation strategies to enhance geothermal energy recovery. Properly managing thermally induced fracture behavior can improve heat extraction efficiency, maintain fracture connectivity, and extend the operational lifespan of EGS projects. By focusing on artificial fracture behavior under different mechanisms, this study provides insights into optimizing fracture design and operational strategies to achieve more efficient and sustainable geothermal energy extraction.

Performance evaluation of enhanced geothermal systems with intermittent thermal extraction for sustainable energy production

Geothermal energy extraction, which is similar to hydrocarbon well developments, requires drilling and completion to access the heat from high-temperature subsurface zones. Even though geothermal energy has its environmental benefits, the economic viability of geothermal well construction may face challenges. These challenges may also include relatively higher complexities than oil and gas wells, because of the complex drilling mechanics and higher axial rating requirements for the casing strings. This study proposes a method to reduce geothermal well construction costs to improve the financial viability of enhanced geothermal systems (EGS) through a cost-effective well design. It is well acknowledged that geothermal systems typically require a producer and an injector well to harness the subsurface heat. While doublet’s philosophy has been refined, and there are fourth to sixth generations of doublets concepts, this paper proposes a new model that includes heat loss effects into the well cost model. The paper first shows the three main cases used for the analysis: classic triplets using three identical wells and the new concept using three smaller wells. Next, a well cost model will be created for each one of the concepts, highlighting the differences and advantages. Last, given the new proposed concept, the geothermal well farm concept will be discussed. The results indicate that the total deterministic cost for constructing a pair of slimhole injector wells could be comparable to that of a single standard or a big hole injector well. The novelty of this research lies in its examination of multiple injector-producer triplet EGS wells, emphasizing cost-effective drilling and completion techniques.

Depleting fossil fuel resources and the horrible environmental impacts due to burning fossil fuels emphasize the importance of using renewable energy resources such as geothermal and solar energies. This paper compares performance of CO2 transcritical cycle, organic Rankine cycle, and trilateral Rankine cycle using a low-temperature geothermal heat source. Thermodynamic analysis, exergetic analysis, economic analysis, and exergoeconomic analysis are applied for each of the aforementioned cycles. In addition, a sensitivity analysis is performed on the system, and the effects of geothermal heat source temperature, evaporator pinch point temperature, and turbine inlet pressure on the cycle's performance are evaluated. Finally, the systems are optimized in order to minimize product cost ratio and maximize exergetic efficiency by using the genetic algorithm. Results indicate that the maximum thermal efficiency is approximately 13.03% which belongs to organic Rankine cycle with R123 as working fluid. CO2 cycle has the maximum exergetic efficiency, equals to 46.13%. The minimum product cost ratio refers to the organic Rankine cycle with R245fa as working fluid. Moreover, sensitivity analysis shows that increasing geothermal heat source temperature results in higher output power, product cost ratio, and exergy destruction ratio in all cycles.

The transition towards a more affordable, reliable, and sustainable energy provision paradigm is one of the main 21st century challenges that humanity must overcome to protect the planet from the harmful effect caused by climate change. The concentration of CO2 in the atmosphere has been dramatically increasing since the pre-industrial era. If the increase of green-house gasses emissions continues unabated, this will bring dramatic consequences for planet Earth, compromising eventually the existence of many species, including the human race. To avoid a climate change catastrophe, the share of primary energy coming from renewable energy resources must increase from around 15% in 2015 to 65% in 2050. This energy transition can not rely solely on few successful technologies (i.e., solar photovoltaic, and wind energy), but it must count on a larger variety of technical solutions that are suitable for a wider range of renewable sources and diversity of circumstances. For instance, renewable thermal energy sources for power generation (i.e., geothermal reservoir, biomass fuel, and concentrated solar radiation), can provide a large portion of the world electricity demand in the future. However, the exploitation of a good portion of these sources strongly depends on the market success of technologies such as the Organic Rankine Cycle (ORC) power system. One of the key aspects to make ORC systems economically competitive, especially at the smaller sizes (⇡ 1 − 50 kW), is the realization of highly efficient turbomachinery components. The fluid-dynamic design of ORC turbomachinery significantly differs from the design of traditional machines (i.e., steam and gas turbines), and this is mainly due to the different thermo-physical properties and gas dynamic behavior of the organic working fluids. This means that design methods devised for standard steam and gas turbomachinery can not be used for turbomachinery operating in the Non-ideal compressible fluid dynamics (NICFD) region. Furthermore, no experimental campaigns have ever been carried out to create a body of empirical knowledge to support the design highly efficient ORC turbomachinery. As a consequence, the entire design process of ORC turbomachinery relies only on the use of advanced CFD software. The current trend is to couple CFD tools with numerical optimization techniques in order to automatically obtain optimal flow passage geometries. In particular, adjoint-based methods have clearly demonstrated to be the only optimization technique capable of tackling the multi-stage turbomachinery design problem, in which thousands of design variable must be concurrently optimized. Therefore, the research documented in this PhD dissertation aimed at extending the adjoint method in order to perform the fully-turbulent fluid-dynamic shape optimization of 3D multi-stage ORC turbomachinery. This document contains an extensive introduction, three main chapters, each documenting a building block towards the accomplishment of the main goal of this PhD project, and a final concluding chapter that summarizes all the research outcomes of this work and proposes future steps for research in this field. The first part of the thesis describes the extension of the RANS equations, the convective numerical schemes, and the viscous numerical schemes to the use of complex thermo-physical laws, so to simulate turbulent flows of components working in the NICFD thermodynamic region. The second part documents the derivation of the adjoint solver in order to resolve shape-optimization design problems for 2D single row of ORC turbomachinery. Finally, the last part reports the extension of the adjoint method to 3D multi-stage turbomachinery design.

The Organic Rankine Cycle (ORC) power cycle is a well-established solution for harnessing heat sources to generate energy. Presently, ORC systems predominantly employ hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) as working fluids. However, these substances possess significant greenhouse gas effects and are slated for future bans. To address this, it is imperative to establish rational selection criteria and corresponding techniques for evaluating working fluids suitable for industrial ORC applications. This chapter presents the working fluid selection criteria and screening methods for environmentally friendly working fluids. The chapter is organized as follows: (1) The fundamentals of working fluids section provide a broad introduction to the core principles of working fluids; (2) the working fluid section outlines reasonable selection criteria for identifying potential alternatives; (3) the screening of ORC working fluids section discusses possible working fluid candidates, simulation approach, and thermodynamic models in detail, which is very important to access the thermodynamic performance of ORC cycle; and (4) an example of the simulation of an ORC for working fluid selection section demonstrates the strategy for the selection of a working fluid considering a defined ORC architecture.

Thermodynamic performance analysis of an improved coal-fired power generation system coupled with geothermal energy based on organic Rankine cycle

Performance optimization of pinnate horizontal well in geothermal energy utilization with orthogonal test

Hybrid solar–geothermal power generation: Optimal retrofitting

Many believe that enhanced geothermal systems (EGS) can greatly increase the extraction of geothermal energy worldwide, helping to decarbonize heat and electricity production. Effective communication is key to realizing the potential of EGS, yet we currently know little about how the public perceives this emerging technology. This exploratory study contributes to the literature with a cross-national survey in the United States (n=1003) and Switzerland (n=1028), two countries with active EGS projects. Specifically, we explore how EGS support relates to beliefs about the deep underground and perceptions of EGS as tampering with nature. The results show that respondents tend to perceive the deep underground as part of nature, dangerous, and unpredictable. The majority are positive about using the deep underground as a resource, although there were variations regarding specific underground activities. In both countries, EGS support is greater for respondents who perceive the underground as something for human use, perceive more benefits than risks from EGS, and support their country's transition to renewable energy. In Switzerland, EGS support is positively related to trust in industry developers and negatively related to perceptions that EGS is tampering with nature. The results offer novel theoretical insights into perceptions of the deep underground in relation to energy development. From a practical standpoint, the results suggest that those seeking to develop EGS may want to consider how to familiarize individuals with current subsurface energy activities, including efforts to protect the underground from unwanted consequences of "tampering," alongside engaging in discussions about the risks and benefits of EGS.

How to give a full play to the advantages of zeotropic working fluids in organic Rankine cycle (ORC)

Geothermal energy is renewable, sustainable and available in large amounts. The hot dry rock (HDR) systems, in particular, have the largest potential for long-term sustainability and therefore draw a lot of attention. The combination of horizontal wells and the technology of multiple transverse fractures technology is an attractive approach of such HDRs, which is called in this paper as the specific EGS (enhanced geothermal system). The main objective of this paper is to study the heat extraction over a period of 20 years by water circulating in a deep geothermal reservoir using this specific EGS. The wellbore flow module, T2WELL/ECO2N, is implemented in the parallelized simulator TOUGH2MP as a new code TOUGH2MP-WELL/EOS3, which enables coupled wellbore–reservoir simulations. Using this newly developed code, the sensitivity of heat extraction against various parameters of the formation and fractures is assessed. The influences of the fracture geometry and the space of two neighborhood fractures on the geothermal energy performance are analyzed. According to the results in this paper, the injection rate, the wellbore radius and the fracture permeability are three main influence factors for the distribution of total flow through individual fractures. A large injection rate, a small wellbore radius and a large fracture width will cause the short-circuit effect, which reduces the performance of this specific EGS. Compared with the classic HDR system (doublet or triplet vertical wells + single fracture), this specific EGS has a much higher performance and a longer duration of the economic production. Furthermore, the horizontal wells can also be drilled in both directions of the minimum horizontal stress instead of in just one direction, so that the energy performance is doubled and the investment cost per kW energy production is decreased and the advantage of this specific EGS is significantly enlarged.

We explore the feasibility of combining the circulation of supercritical CO2 in EGS reservoirs with integrated gasification combined cycle (IGCC) power generation. The symbiotic benefits of this pairing increase net power output over the use of either system in isolation, reduces water use per MWe, and substantially reduces fugitive emissions over conventional thermal power plants. A prototypical plant set in the arid southwestern U.S. (environs of Albuquerque, NM) could sustain anticipated circulation rates of 800–1600 kg/s of CO2 in a 200 °C reservoir, resulting in a thermal output of ∼150–300 MW-thermal to augment the 550 MW from IGCC. This design would reduce annual emissions by 8,200 tons of NOx, 20,000 tons of SO2, and 4.35 million tons of CO2 over conventional thermal plants in an optimal pairing.