Geothermal Resources Research Articles

Quantifying vertical borehole heat exchanger sustainability from a geothermal resource perspective in the UK

Low-temperature geothermal energy is viewed as a renewable resource that could contribute to the decarbonization of residential heating in the UK. Here, we constrain the geothermal potential of the shallow subsurface to provide the heating requirements of a typical single-family house via a vertical borehole heat exchanger (BHE), to provide a preliminary framework for the heat resource licensing of such systems. Through analytical and numerical models, we calculate the heat balance for a typical house with a garden located in a Carboniferous sedimentary basin, which represents a large proportion of the UK land mass and the regions where energy poverty is more common. Neglecting the heat recharge via groundwater flow, our results indicate that geothermal, solar heat flux and radiogenic heat production can only replenish up to 5% of the heat extracted annually by a 100 m-long BHE in heat extraction-only mode. The lack of natural recharge causes the continuous expansion of the thermal footprint at an average rate of 18 m 2 a −1 over 30 years, considering a ground thermal conductivity of 2.2 W m −1 °C −1 , with the area cooled by 1°C extending beyond the current minimum BHE spacing guidelines of 6 m. In Scotland, reinjecting c . 35% of the yearly heat requirements, which could be accomplished through borehole thermal energy storage, is shown to safeguard against resource over-exploitation and interference risks between neighbouring users in the majority of the high-demand areas. In view of a large deployment of BHEs, it is crucial to incorporate such reinjection strategies into a regulatory framework to ensure a resilient and sustainable utilization of shallow geothermal resources.

Research on the Genesis Model of Geothermal Resources in Thermal Storage

Research on the Genesis Model of Geothermal Resources in Thermal Storage

High-pressure thermal conductivity, speed of ultrasonic measurements and derived elastic modulus of sandstones with different porosity

The effect of pressure on thermal conductivity is an important for understanding the heat transfer processes in modeling applications in geothermal reservoirs and for optimization of the technology of geothermal energy extraction processes. The primary factors affecting the economics of any geothermal energy recovery process are the amount of heat present in the geothermal reservoir and rate of heat extraction, which strongly depend on the thermal properties of the reservoir rocks, which are functions of both temperature and pressure. The present paper aims to study the variation of thermal conductivity of five sandstone samples from the Germany Geothermal Field with various total (open and close) porosities of (6.8, 13.0, 15.44, 21.3, and 21.5%) with pressure up to 203 MPa, to overcome the existing lack of thermal conductivity data for the geothermal reservoir modeling. The improved steady-state heat-flow technique (contact method) was used to precisely (with an uncertainty of 4%) measure the thermal conductivity of the sandstone samples. Unlike previous studies, in the present work the effect of the contact thermal resistance on the measured values of thermal conductivity has been taken into account using a calibration procedure to increase the accuracy and reliability of the measured data. The results show that with the increase of the pressure at a fixed temperature of 293.15 K, the thermal conductivity of sandstone is linearly increasing with pressure from 0.52 to 1.73 GPa−1 depending on sandstone structural and mineralogical composition, porosity, and other characteristics, which falls in the same range reported by other authors for rock samples from different geothermal fields. The derived values of the thermal conductivity pressure coefficient are crucial for geothermal studies in order to effectively use the geothermal resources of the region, and are useful for scientific applications such as the development and testing of the accuracy, reliability, and predictive capability of existing thermal models of geothermal reservoirs. Based on the present experimental results, statistical analysis (correlation analysis) was revealed between the measured thermal conductivity of sandstone samples and open and close porosities. The role of closed and open porosities on the pressure dependence of thermal conductivity and elastic moduli is discussed. For the first time, we experimentally observed the difference of the influence of the open and closed porosities on the thermal conductivity and elastic properties of sandstones. It was experimentally confirmed that the effect of closed porosity on the thermal conductivity of sandstones is significantly greater than that of open porosity by 15 to 20%. The obtained results show that it is of great importance to study the changes in the thermal properties of the sandstones under pressure at realistic reservoir conditions for geothermal studies.

3-D intrinsic attenuation tomography using ambient seismic noise applied to La Palma Island (Canary Islands)

The potential of the island of La Palma (Canary Islands) to host geothermal resources is very high, mainly due to its high volcanic activity. The primary goal of this study is to get a tridimensional image of the seismic intrinsic attenuation using ambient seismic noise and to identify anomalies that may be linked to active geothermal reservoirs on La Palma island. For this purpose, we developed a new Ambient Noise Attenuation Tomography (ANAT) technique, which uses seismic ambient noise for imaging intrinsic attenuation in 3-D at a local scale down to 5 km depth. Our research identifies two areas with high attenuation in the island’s southern region. One area could be associated with hydrothermal alteration zones beneath the Cumbre Vieja volcanic complex. Another high-attenuation zone was observed in the island’s southern part, which could be associated with extensively fractured rocks that might facilitate the circulation of heated fluids. Furthermore, we discuss the geothermal relevance of such anomalies, making a comparison with previous resistivity, S-wave velocity and density models. This study confirms that intrinsic attenuation retrieved from the coda of Rayleigh waves is more sensitive to the presence of fluids than velocity. Fluids being a key component of active geothermal reservoirs, it is reasonable to expect intrinsic attenuation anomalies in these systems. Therefore, we conclude that ANAT can be useful for geothermal exploration.

Exploring geothermal energy prospects through radioelement enrichment zones in Gabal Abu Hashim area in southeastern Aswan by geophysical and remote sensing data integration

Alteration zones in the Eastern Desert are promising for minerals and geological resources exploration. Remote sensing and geophysical techniques offer cost-effective tools for identifying new exploration sites. Additionally, their use in mapping potential alteration zones is crucial for enhancing exploration. Geophysical and remote sensing data are integrated to perform a comprehensive study of minerals alteration associated with radioactive materials under controlling geological structures. This study aims to explore the associated radiogenic heat and geothermal energy to expand the geothermal resources assessments. The study utilizes Spectral Angle Mapper classification, band algebra, principal component analysis (PCA), surface lineament density, and decorrelation stretch techniques and Multiple Criteria Decision Analysis to enhance the mapping of mineralized alterations in the study area. It uses VNIR-SWIR ASTER data to identify hydrothermal alteration minerals and dominant alteration zones, also Landsat 8 Thermal Infrared Sensor (TIRS) offers two adjacent thermal bands, benefiting land surface temperature (LST) radiation from space in the Gabal Abu Hashim area. The area contains minerals alteration indicators like chlorite, alunite, illite, kaolinite, sericite, with less abundant ferrous minerals and epidote. Airborne gamma-ray spectrometry (AGRS) was used to identify naturally occurring radioactive anomalous zones, including potassium (K), equivalent uranium (eU), and equivalent thorium (eTh), to estimate the radiogenic heat production (RHP) in selected areas of the study area. The examination of AGRS data indicated that the studied region has radioelement concentrations ranging from 2.8 to 148 ppm, 18 to 144 ppm, and 0.004 to 9% of (eU), (eTh), and (K), respectively, indicating the existence of various rock types. The Radioelements Composite Image (RCI) successfully highlighted the radioelements enriched zones associated with younger granite, older granite, and metamorphic rocks, particularly those with extensive hydrothermal alteration. The results successfully discriminated alteration zones associated with radioelements K, U, and Th potential parts in the regional shear oblique zone. The weighted overlay GIS model was used to produce the alteration zones potentiality map, and to identify five zones of significant variations in heat production across different geological formations. The northern and southeastern regions demonstrate high alterations and land surface temperature corresponding to areas of high fault density and shear zones. The results of this study reveal that the proposed methods of remote sensing and AGRS data are effective in detecting areas rich in K, eU, and eTh in alteration zones associated with high radiogenic heat production in younger granite, older granite, and metamorphic rocks.

Feasibility Study on Geothermal Dolomite Reservoir Reinjection with Surface Water in Tianjin, China

Reinjection is thought to be the most effective way to maintain reservoir pressure and production capacity for hydrothermal resources. The use of external water injection to replenish deep geothermal reservoirs is a new approach in China to addressing the problems of declining groundwater levels and energy depletion caused by the excessive and uneven exploitation of geothermal resources. However, the key challenge and focus of the feasibility assessment of this method lies in the chemical compatibility of the external water with the native geothermal reservoir water and surrounding rocks. In this paper, we discuss the geochemical response of a dolomite reservoir to lake water injection based on experiments on water–rock interaction in the Wumishan formation in the Dongli Lake area of Tianjin. The results show that after reactions with dolomite, the TDS of the reacted water decreases, indicating the occurrence of precipitation. According to the calculation results obtained using the PHQREEC program, the precipitation amount is found to be quite limited. Geochemical analysis indicates that at the initial stage of the reactions, plagioclase dissolves and releases alkaline metals like Ca-, Na-, SiO2- and Al-bearing compositions, leading to the oversaturation and precipitation of dolomite and calcite. As the reaction progresses, a portion of the dolomite dissolves, while the calcite continues to precipitate at a later stage. Illite precipitates and its effects on reservoir structure depend on its shape. Based on the experimental data, it can be concluded that the dolomite reservoir will be slightly affected by the reinjection of lake water; however, it is still a good method for the sustainable development of geothermal resources.

Numerical simulation of mine heat hazard governance and geothermal resource exploitation using extraction-ventilation collaborative method

Numerical simulation of mine heat hazard governance and geothermal resource exploitation using extraction-ventilation collaborative method

Feasibility of coaxial deep borehole heat exchangers in southern California

This paper investigates the feasibility of coaxial deep borehole heat exchanger (CDBHE) applications to the University of California San Diego (UCSD) campus. By collecting different geophysical source data for various formations and well logs around the UCSD campus, a multilayered thermophysical model for the ground on the site is established. Water circulation within a closed coaxial loop system considers the geothermal energy extraction under uncertainty consideration of the unknown deeper layers heat flow gradient as coupled with the variation of pipe insulation properties, flow rates, outer pipe diameter, grout, and depths between 1 and 4 km. A finite-element framework models the Navier–Stokes fluid flow and heat transfer in the CDBHE system, validated with a field test on CDBHE from the literature. Results show that a 4-km CDBHE could produce a thermal power of 600 kW under the optimum geological conditions at the UCSD site: the water flow rate of 2.78 L/s and a ground thermal gradient of 60 ℃/km. Thermal power shares from different layers indicate that deeper formation layers contribute more to the thermal power than the shallower layers because increasing the CDBHE length from 1 to 4 km can lead to a maximum of 900% increase in thermal power and a 50% expansion in thermal plume for a CDBHE with an insulated inner pipe between the upper and lower bound heat flow bounds. An inner pipe with an insulated depth of 2 km produces only 1–6% less power than a fully insulated inner pipe for the 4-km CDBHE, and thus, a partially insulated vacuum-insulated tube (VIT)-plastic inner pipe is suggested as the best practice. Furthermore, the CDBHE thermal power increases by 5% when the grout thermal conductivity increases from 1 to 3.65 W/(K∙m), close to the formation thermal conductivity, and then maintains almost the same, and the 4-km CDBHE with flow rates of 2.78–6.94 L/s at the UCSD site can directly supply a low-temperature heating radiator system for room heating. This study suggests practical ranges for geothermal energy extraction for southern California. A CDBHE with a well-insulated inner pipe of 0.05 W/(m∙K), the thermal power of lower and upper-bound heat flow cases can vary by 60% from the mean. Finally, water as the working fluid is more efficient than CO2, doubling CDBHE's thermal power. The effects of the investigated factors provide guidelines for future geothermal resource exploitation in southern California.

Energy, Exergy and Exergo-economic analyses of supercritical CO2 cycles for the exploitation of a geothermal resource in the Italian water dominant Amiata site

Alternative powerplant layouts to commonly applied flash technology is investigated for the water dominant geothermal location of Amiata Mountain, Italy. The proposed solution avoids flashing the brine stream by a borehole pump capable of maintaining pressurized conditions, thus reducing the amount of non-condensable gases released when reaching the ground level. Therefore, the need of a gas treatment section is completely avoided. The heat recovered from the hot brine at ground level is transferred to different supercritical CO2 cycle configurations, equipped with high efficiency Printed Circuit Heat Exchangers (PCHE). The optimal 45.5% calculated exergy efficiency was achieved for the recuperative cycle with intercooling and reheat, which however didn’t show the optimal electricity costs (8.092 c€/kWh) because of the more complex heat exchangers network. The lowest cost of electricity (7.4 c€/kWh) was found for the recuperative cycle configuration. These costs are in line with the current levels of geothermal binary cycles, but almost doubled compared to the current single flash +ORC solution, due to a 40-50% reduction of power output with sCO2 cycles. On the other hand, sCO2 binary cycles could guarantee an almost zero environmental impact, which is the main reason for social concerns related to the current flash powerplants.

The Geysers Is a Unique Geothermal Resource Presenting Equally Unique Challenges to Drillers

_ For drillers, The Geysers is a unique sort of hell. It is one of the only places on Earth where a shallow well can reach a highly fractured reservoir so hot that it only produces steam. That steam powers 18 power plants generating 835 MW of electricity, making this 45-square-mile area the world’s most prolific geothermal power producer. High drilling costs there also stand out. Drilling a well requires 60 days or so and generates a lot of damaged drill bits and drillpipe. The drilling rigs have the size and power of modern onshore rigs, but a close look reveals there’s no topdrive. Below the rig floor there is a rotary table, known as a Kelly drive because it can spin drilling pipe without being in the line of fire of the steam flowing out when running slotted casing into the steam-production zone. When they get to that zone, they switch from drilling fluid and mud to injecting a high-pressure stream of air because of the enormous fluid losses anticipated when they reach that highly fractured level. A drill bit that lasts 500 ft in this harsh, highly fractured rock has had a long run. And the rugged terrain—mountains to a Texan but hills to a Californian—makes this a hard place to move a rig. “The Geysers is its own animal, for sure,” said Sam Noynaert, a Texas A&M professor who was a key member of a team of drilling experts and scientists that drilled a well in The Geysers to test if faster drilling methods from the oil business could work there. The two upper sections went pretty well, considering. As they went deeper, they repeatedly had to deal with severe fluid losses. Still, they had shaved nearly 6 days off the average drilling time. But when they reached the highly fractured steam-producing payzone and their drilling method changed to air drilling, things got worse with drill bits failing after runs as short as 21 ft. As a Geysers drilling paper presented at the Stanford Geothermal Workshop earlier this year delicately put it: “The air section proved to be more challenging.” The Blob The Geysers has a long history. The heat source is “a large blob of silica-rich magma (which) forced its way through Earth’s crust beneath the Coast Range,” according to the description of an image from NASA. That flow about 1.3 million years ago from deep within the earth heated the overburden, making the rock brittle and prone to fracturing. Over time, water seeped into those hot fissures, creating hot springs that were awed by Native Americans and later attracted tourists to a nearby resort hotel. By the 1960s drillers were tapping the steam in those payzones for commercial power generation, which peaked at 2,000 MW in 1987. It declined from there as steam production depleted the reservoir. Eventually, the output was stabilized at its current level by injecting water from sewage-treatment plants into the production zone.

Study on improving heat extraction capacity of borehole heat exchanger by forced convection of groundwater

Borehole Heat Exchangers (BHEs) has the advantage of "extracting heat without extracting groundwater". Its main drawback is low heat extraction power. Aiming at developing medium-deep hydrothermal geothermal resources, a Branch Well Reinforced Coaxial Borehole Heat Exchanger (BWR-CBHE) is proposed firstly. An assisted circulation loop formed by the wellbore and geothermal reservoir is created through drilling branch well and installing electric submersible pump (ESP). By driving the forced convection of groundwater within the assisted circulation loop, heat extraction capacity of the CBHE is enhanced. Then, a numerical model of the BWR-CBHE is established. The main circulation of the working fluid within the CBHE is coupled with the assisted circulation of groundwater, and the heat extraction performance of the BWR-CBHE is calculated. Under the conditions studied in this paper, the net heat extraction power (after deducting the energy consumption of the ESP) of the BWR-CBHE is 6.09 times higher than that of conventional CBHEs. Finally, impacts of parameters, including working fluid, branch well and ESP, on the heat extraction capacity of the BWR-CBHE are discussed. The heat extraction power of the BWR-CBHE is increased significantly without extracting groundwater, which provides a new approach for the development of BHEs.

Orthogonal experimental investigation on permeability evolution of unconsolidated sandstones during geothermal fluid reinjection: A case study in the Minghuazhen Formation, Tianjin, China

Geothermal fluid reinjection (GFR) is vital for sustainable sandstone hydrothermal geothermal resource extraction. However, the permeability damage mechanisms of sandstone geothermal reservoirs due to reinjection remain unclear. This study aimed to investigate the permeability evolution of unconsolidated sandstone during GFR through an orthogonal experiment. The influence mechanism of the factors, including temperature, injection rate, confining pressure, geothermal fluid concentration (GFC), and depth, on sandstone permeability is also discussed. The results showed the sandstone samples suffered permeability damage in 34.62–72.73 %. Particle clogging emerged as the primary mechanism responsible for the permeability damage, serving as a crucial bridge in understanding the influences of the factors on sandstone permeability. Efficient GFR requires the optimization of reinjection conditions tailored to the characteristics of geothermal reservoirs. Sensitivity analysis showed that the order-sensitive characteristic of permeability damage to the factors was injection rate > GFC > temperature > depth > confining pressure. Through the optimization of reinjection parameters, the heat exploitation performance under the worst permeability damage case can be improved by 1.77–1.93 times. This study could offer valuable insights into the challenges and considerations associated with GFR in sandstone reservoirs.

Integration of airborne geophysical data for the characterization of the geothermal system in Valle del Cura, high central Andes

The Valle del Cura region, located in the High Central Andes, over the Pampean flat-slab segment, exhibits several geothermal manifestations suggesting structural rather than magmatic controls. However, the structural and favorable conditions of this geothermal system have not been studied in detail yet. By integrating high-resolution airborne geophysical data with existing geological information using GIS tools, we can better characterize the structural controls of the thermal areas. The application of the upward continuation filter and tilt-angle derivative to the reduced to pole magnetic anomaly map allowed us to delineate structural lineaments that correlate well with known faults in the area. Euler deconvolution successfully identified local structures controlling the subsurface water upwelling and provided reliable depth estimations. Results indicate that the Despoblados area is characterized by NW-oriented deep structural controls reaching depths of up to 5 km. In contrast, the Bañitos-Gollete and the northern thermal areas are controlled by the intersection of N-S with E-W shallow structures at less than 3 km deep. Aeroradiometric data provided additional information for the characterization of several lithological units based on the dominance of radioelements (K, Th and U) in near-surface rocks by using individual and ternary maps. In addition, estimations of radiogenic heat production were crucial for assessing the potential energy of geothermal resources in the region. In particular, higher average values were obtained for igneous and metamorphic basement which may contribute to the geothermal gradient and temperature distribution at depth. Further studies, including new geophysical data acquisition and field verification, are still needed to fully describe the regional and local structures involved in these geothermal areas.

Research on Geothermal Geology and Heat-Forming Conditions in the Tangyuan Fault Depression

In order to gain an in-depth understanding of the geothermal geological characteristics and heat-forming conditions in the Tangyuan fault-depression area, this paper has carried out a series of detailed geological works, including geological surveys and geophysical explorations. Through these works, combined with the results of systematic drilling, sampling tests, and dynamic monitoring, we have studied, analyzed, and compared the relevant data to achieve a comprehensive understanding of the geological characteristics in the Tangyuan fault-depression area. During the research process, we preliminarily determined the stratigraphic structure and geological structure in this area and clarified the characteristics of the main geothermal reservoir and its burial conditions. These findings provide basic data support for our understanding of the formation and distribution of geothermal resources. At the same time, we have carried out a systematic analysis of the basic geothermal geological parameters, laying the foundation for the future calculation and evaluation of geothermal resource reserves. Through the collation and analysis of these research results, this paper not only provides a scientific basis for the development of geothermal resources in the Tangyuan fault-depression area but also provides specific references for future development prospect plans. This series of work will lay a solid foundation for further development and utilization of geothermal resources and promote the sustainable development of the regional economy.

Characteristics and Formation Mechanism of Geothermal Resources in Tectonically Active Regions: A Case Study of the Red River Fault in Western Yunnan Province, China

Characteristics and Formation Mechanism of Geothermal Resources in Tectonically Active Regions: A Case Study of the Red River Fault in Western Yunnan Province, China

3D Surface Velocity Field Inferred from SAR Interferometry: Cerro Prieto Step-Over, Mexico, Case Study

The Cerro Prieto basin, a tectonically active pull-apart basin, hosts significant geothermal resources currently being exploited in the Cerro Prieto Geothermal Field (CPGF). Consequently, natural tectonic processes and anthropogenic activities contribute to three-dimensional surface displacements in this pull-apart basin. Here, we obtained the Cerro Prieto Step-Over 3D surface velocity field (3DSVF) by accomplishing a weighted least square algorithm inversion from geometrically quasi-orthogonal airborne UAVSAR and RADARSAT-2, Sentinel 1A satellite Synthetic Aperture-Radar (SAR) imagery collected from 2012 to 2016. The 3DSVF results show a vertical rate of 150 mm/yr and 40 mm/yr for the horizontal rate, where for the first time, the north component displacement is achieved by using only the Interferometric SAR time series in the CPGF. Data integration and validation between the 3DSVF and ground-based measurements such as continuous GPS time series and precise leveling data were achieved. Correlating the findings with recent geothermal energy production revealed a subsidence rate slowdown that aligns with the CPGF’s annual vapor production.

Investigation of geochemically induced permeability alteration in geothermal reservoirs and its implications for sustainable geothermal energy production

Geothermal resources constitute a significant portion of the world's low-carbon, renewable energy potential, with about 75% classified as low-temperature. One such potential resource exists in Precambrian basement rocks underlying the Williston Basin in southern Saskatchewan, Canada, with a reservoir temperature of 120 °C. However, geochemically induced permeability alteration in these highly reactive low-temperature granitoid resources poses a significant risk to long-term heat production. To assess and potentially mitigate this risk, we conducted a geochemical and mineralogical study of both altered and unaltered samples. Our findings facilitated the parameterization of geochemical simulations of water-rock interactions to predict mineral volume changes and, by extension, draw inferences on porosity and permeability changes resulting from these interactions. The simulations indicate an increased mineral volume in both samples, yet geothermal alteration of the unaltered, and thus more reactive, rocks induced relative mineral volume changes about 30% greater than those in the altered rocks. The resulting absolute change in porosity is 0.5 vol% for the unaltered rocks and 0.35 vol% for the altered rocks. Utilizing an empirical porosity-permeability relationship, the computed change in permeability indicates that the unaltered basement rock experienced a greater change in total permeability than the altered basement rocks. Additional calculations demonstrate the sensitivity of the porosity-permeability equation to critical porosity and power exponent, forecasting various scenarios with permeability changes ranging from 1.0 × 10−13 to 1 × 10−20 m2. Consequently, we infer that altered, permeable zones of the examined Precambrian basement rocks are likely to offer favourable conditions for sustained, multi-decade heat production, and thus should be targeted over less altered zones to justify initial capital expenditures. Globally, geothermal heat extraction from these rocks remains an underexplored yet promising resource for generating reliable, low-carbon renewable energy, crucial in our efforts to decarbonize the global economy.

Modeling of calculations on the performance optimization of a double-flash geothermal renewable energy-based combined heat/power plant coupled by transcritical carbon dioxide rankine cycle

This study investigates the integration of a double-flash geothermal system with a transcritical CO2 Rankine cycle, presenting a groundbreaking approach for enhancing efficient and sustainable energy generation. By harnessing renewable geothermal resources, this combined heat and power system aims to minimize greenhouse gas emissions, contributing to a more sustainable future. The research emphasizes multi-objective optimization to maximize net power output, net heating capacity, and overall system efficiency. The expanded stream pressures from the expansion valves, denoted as P2 and P6, are systematically varied to evaluate their impact on system performance. Significant findings reveal that as P6 increases from 260 kPa to 640 kPa, net power output rises dramatically from 2688 kW to 3163 kW. The optimal conditions for peak performance are identified at P2 of 820 kPa and P6 of 640 kPa, resulting in impressive outcomes: a net power output of 3392 kW, a net heating capacity of 14,586 kW, and an overall efficiency of 52.65 %. By delineating these relationships and establishing benchmarks for performance, this research offers valuable insights for the scientific community, policymakers, and industry stakeholders aiming to advance geothermal energy technologies.

Energy, exergy and exergoeconomic analyses and ANN-based three-objective optimization of a supercritical CO2 recompression Brayton cycle driven by a high-temperature geothermal reservoir

The use of CO2-based power cycles to harness high-temperature geothermal resources is an interesting application that has been little explored. Therefore, to obtain a more holistic understanding of these systems, in this work, a supercritical CO2 recompression Brayton cycle is assessed. For comparison, the performances of a recuperative cycle and a recompression cycle with intercooling were also computed. Detailed mathematical models were developed and then, artificial neural network-based surrogate models were adopted to conduct multi-objective optimization. Results of the baseline simulations show that, the high temperature recuperator and the turbine are the most important components from both exergy and exergoeconomic approaches. The optimizations reveal that the intercooled cycle is superior to the other layouts in all the cases while the recuperative cycle is not feasible with reinjection temperatures greater or equal to 150 °C. When the reinjection temperature is constrained to 150 °C, the intercooled cycle attains 4472.70 kW, 62.10 % and 14.42 $/GJ for net power, exergy efficiency and product unit cost, respectively, whereas without this constraint, it achieves 5394.09 kW, 61.83 %, 11.79 $/GJ. In conclusion, the adoption of advanced layouts of the CO2 cycle is beneficial for this application from both technical and economic points of view.

Hydrochemical Characteristics and Formation Mechanism of Geothermal Fluids in Zuogong County, Southeastern Tibet

Zuogong County is located in the southeast of Tibet, which is rich in hot spring geothermal resources, but its development and utilization degree are low, and the genetic mechanism of the geothermal system is not clear. Hydrogeochemical characteristics of geothermal water are of great significance in elucidating the genesis and evolution of geothermal systems, as well as the sustainable development and utilization of geothermal resources. The hydrogeochemical characteristics and genesis of the geothermal water in Zuogong County were investigated using hydrogeochemical analysis, a stable isotope (δD, δ18O) approach, and an inverse simulation model for water–rock reactions using the PHREEQC. The results indicated that the Zuogong geothermal system is a deep circulation heating type without a magmatic heat source. The chemical types present in the geothermal water from the Zuogong area are HCO3 and HCO3·SO4, and the main cations are Na+ and Ca2+. The groundwater is replenished by atmospheric precipitation and glacier meltwater. The salt content of geothermal water mainly comes from the interaction between water and surrounding rocks during the deep circulation process. The reservoir temperature of geothermal water in Zuogong is 120–176 °C before mixing with non-geothermal water and drops to 62–98 °C after mixing with 58 to 79% of non-geothermal water. According to the proposed conceptual model, geothermal water mainly produces water–rock interaction with aluminosilicate minerals in the deep formation, while in shallow areas it interacts mainly with sulfate minerals. These findings contribute to a better understanding of the geothermal system in Zuogong County, Tibet.