Low-temperature Geothermal Systems Research Articles

Experimental study of a novel in-tube spray evaporative condenser for geothermal power generation

To enhance the efficiency of medium to low-temperature geothermal power systems, we designed a novel in-tube spray evaporative condenser. Experimental setups for single-tube heat exchange were devised, examining both water-water and working fluid-water configurations. The study analyzed the impact of air volume and hot water flow rate on heat transfer. Economic comparisons were made in laboratory conditions among the novel evaporative condenser, traditional evaporative condenser, water-cooled condenser, and air-cooled condenser. Results indicated: (1) The novel evaporative condenser exhibited 1.5–1.7 times higher heat flux density and 2.0–2.7 times higher mass transfer coefficient, indicating superior heat transfer performance compared to the traditional one. (2) Variations in hot water flow rate within a certain range had minimal impact on condenser performance when the condenser size was constant, providing valuable insights for condenser selection and structural design. (3) Both total power and water consumption of the novel evaporative condenser were lower than the other three types. Its daily operating cost was approximately 3/4 of the traditional evaporative condenser, 1/2 of the air-cooled condenser, and 1/4 of the water-cooled condenser, demonstrating superior economic feasibility.

Environmental impact and thermodynamic comparative optimization of CO2-based multi-energy systems powered with geothermal energy

The continuous increase in global population and energy demands has enhanced the use of fossil fuels. The evident climate change effect in recent years has brought about the necessity to reduce fossil fuels. Hence, there exists an aggregate problem of meeting energy demands and reducing carbon emissions. Renewable energy sources have been proffered as the probable solution; however, multi-energy systems are effective options/alternatives to solving this problem. In recent literature, geothermal energy has been proposed as a renewable energy source that can continuously meet energy demands. However, there exists a significant gap in literature about the most viable temperature range for geothermal energy applications in multi-energy systems. In this study, two innovative CO2-based systems namely, high-temperature geothermal multi-energy system (HTGMES) and low-temperature geothermal multi-energy system (LTGMES) are designed, modelled, analyzed, and compared using a thermodynamic approach. While the HTGMES is modelled to predominantly use CO2 as a working fluid, a novel modified absorption system is integrated with the LTGMES. The two systems are modelled to produce electricity, cooling/refrigeration effect, space heating, hydrogen, and hot water. The energy, exergy environmental, exergy destruction, and exergoeconomic methodology is used to evaluate the performance of the innovative GMESs. Also, multi-objective optimization of the HTGMES and the LTGMES is carried out in a bid to minimize the total product unit cost and maximize overall exergy efficiency. The environmental impact analysis of the proposed system is presented considering CO2 mitigation. The results from analyses showed that the overall energetic and exergetic efficiencies in steady state are 44. 22 % and 33.5 % for the HTGMES and 45.40% and 32.9 % for the LTGMES. The optimized LCOE, LCOC, and LCOH based on the total unit cost are 0.0573 $/kWh, 0.1833 $/kWh, and 11.59 $/kg for the HTGMES; 0.0323 $/kWh, 0.0032 $/kWh, and 11.9$/kg for the LTGMES. Furthermore, the optimized exergetic performance showed that the LTGMES can achieve as high as 64.54 % and 40.96 % energy and exergy efficiency.

Evaluating the Impact of Redox Potential on the Corrosion of Q125, 316L, and C276 Steel in Low-Temperature Geothermal Systems

Time series experiments were used to explore the fluid redox impact on the corrosion of Q125, 316L, and C276 steels in low-ionic-strength and neutral water at temperature and pressure conditions associated with low-temperature geothermal systems. After exposing polished samples of each steel grade to an oxidizing (H2O2) and a reducing (Zn-doped) fluid for intervals of 24 h, 1 week, and 6 weeks, the atomic force microscopy results revealed general corrosion for Q125, while 316L and C276 exhibited pitting, crevice expansion, and edge attack corrosion. Secondary depositional features are frequently found as topographic highs, adjacent to pitting corrosion. These features may be identified as there is a very strong spatial correlation between the height retrace and phase retrace surface maps. All steels became progressively rougher over time after exposure to both fluids, while the corrosion rates were more complex. Samples exposed to the reducing fluid experienced an increase in the corrosion rate over time, while C276 and 316L experienced a decrease in the corrosion rate. Finally, a novel data validation technique was developed to address the intrinsic scalability of corrosion. The results indicate that the AFM scan area does not affect the measured surface roughness over nearly three orders of magnitude.

Experimental study on medium and low temperature geothermal ammonia power generation system based on vertical tube falling film

China’s geothermal energy resources are mainly low and medium temperature, the process of ammonia solution generation and absorption is introduced to simplify the condensation heat transfer of Karina cycle, and the vertical tube falling film structure is adopted for the generator and absorber in the system, which makes the system more suitable for medium and low temperature geothermal resources. A 5 kW ammonia water power cycle power generation experimental platform based on vertical tube falling film was built. The experimental results show that under typical working conditions, the maximum heat transfer coefficient of generator and absorber can reach 1034 w/(m2 K) and 563 w/(m2 K), respectively, and the maximum output power of the system is 3 kW.

Research on medium–low temperature geothermal power generation system

In order to make full use of the medium and low temperature geothermal resources and industrial waste heat resources, a power cycle system using ammonia–water mixed solution as the working fluid is proposed. The system can use medium and low temperature geothermal resources with a temperature of 350–450 K, and the basic circulating working fluid is ammonia aqueous solutions. Simulation calculations show that the Carnot cycle efficiency of the circulation system is 14.01%, and the theoretical cycle thermal efficiency of the circulation system is 10.28%. The article further analyzes the influence of steam turbine outlet pressure and heat source temperature on the thermal efficiency of the circulation system. This research provides a reference for the effective utilization of medium and low temperature waste heat resources.

Modelling and control strategy of a distributed small‐scale low‐temperature geothermal power generation system

At present, commercial geothermal power stations are mainly high-temperature and medium-temperature geothermal energy, while the large number of low-temperature geothermal energy resources are rarely used for power generation applications. This paper proposes a new model for energy conversion in low-temperature geothermal power systems. The basic structure and working principle of this new model are analysed, in detail, a low-temperature geothermal model and a Stirling engine model for geothermal energy conversion are established, and the control strategies on the rotor and stator sides of the Stirling-driven doubly-fed generator are discussed. Finally, the simulation analysis is carried out, and its results show that the thermal conversion efficiency of Stirling’s engine can reach 38%, the output AC voltage of the power generation system reaches the set value 50Hz, 190V, and the DC bus voltage is constant at 300V, which verified the validity of the proposed power generation model and related control strategies. This system expands the range of available heat sources in geothermal power generation, makes geothermal power generation more flexible in terms of site selection, power generation capacity and control, and provides a new idea for geothermal power generation structure.

Thermal regime of the northwestern sector of the Norte Basin (Uruguay): geothermal gradient and heat flow

The infilling of Norte Basin comprises thick Paleozoic and Mesozoic sedimentary units and Mesozoic basaltic flows mainly related to the evolution of Western Gondwana. In its northwestern sector, the Guaraní Aquifer System is located at increasing depths toward the west under thick volcanic rock coverage (up to 1,000 m), constituting a low-temperature geothermal system, which is currently used only for recreational and touristic purposes. To understand the thermal regime of the northwestern sector of the Norte Basin, the surface and subsurface temperatures were estimated from a diverse database that includes all the deep wells located in the study area. Geothermal gradients ranged between 14.4 and 29.6 °C/km, showing the highest values in the western region of the Salto Department. The heat flow maintained a mean value of 60 mW/m2, which is characteristic of resemblant continental tectonic settings. Additionally, the heat loss for every well used in this study was estimated while accounting for an outflow from the bottom hole; it was largely negligible because it remained less than the sensible heat. Therefore, the outflow water temperature can be considered as being similar to the reservoir temperature measured using adiabatic measurement equipment. The western region of the Salto Department, south of the Arapey fault, represents the area with the highest potential for future geothermal projects.

Experimental study on horizontal tube spray falling film evaporator in a low temperature geothermal binary cycle power system

The horizontal tube spray falling film evaporator is an efficient evaporation equipment, which has the advantages of good heat exchange effect, small equipment volume and low cost. In this paper, R245fa is used as the working fluid to build a heat transfer test platform for the horizontal pipe spray falling film evaporator of the Organic Rankine Cycle(ORC) power generation system. The effect of factors such as the spray density of the organic working fluid, the initial temperature of the geothermal water and the flow on the heat transfer coefficient outside the tube is studied. The experimental results show that with the increase of the spray density of the organic working fluid, the initial temperature of the geothermal water, and the flow of the geothermal water, the heat transfer coefficient first increases and then decreases.

Optimization of low temperature geothermal organic Rankine power generation system

In this paper, a low-temperature geothermal organic Rankine Cycle (ORC) power generation system model is established. R245fa is used as the circulating working fluid. The calculation program is compiled using the engineering calculation software EES, and the thermodynamic parameters and thermodynamic properties of the main state points of the ORC are theoretically calculated. The effects of narrow point temperature difference, evaporation temperature, geothermal water temperature and condenser end temperature difference on system performance are analyzed. The results show that the narrow point temperature difference has a significant impact on the ORC performance. Considering system performance and economic cost comprehensively, the best narrow point temperature difference and the best condenser end temperature difference are both within the range of 5–7 K.

Mineralogy of travertine deposit in Cisolok geothermal field, Sukabumi, West Java, Indonesia

Travertine is a geothermal surface feature that forms by the deposition of carbonate from discharging supersaturated thermal bicarbonate fluid. This study was conducted to determine travertine rocks mineralogy found in Cisolok geothermal area, Sukabumi, West Java. Petrographic, diffraction, and elemental analysis were performed on several recent and paleo-travertine samples. The petrographic and diffraction method confirms the presence of calcite and microcrystalline. The paleo-travertine deposit showed a more diverse texture through chemical elemental analysis. The analysis showed that the abundance of calcite minerals and the absence of aragonite indicate the intermediate low temperature geothermal system in Cisolok.

Potential of geothermal energy in the onshore sedimentary basins of Uruguay

In Uruguay, current geothermal energy applications are restricted to recreational and balneology uses of the low-temperature Guaraní Aquifer System (Norte Basin). By integrating geological and geophysical data, we characterized three potential geothermal systems, one in each onshore sedimentary basin. In the Norte Basin, an intermediate-temperature geothermal system (> 95°C) is associated with deep aquifers (~3,500 m) in undrilled structural lows. The thermal waters could be used for a variety of indirect and direct applications. In the Santa Lucía and Laguna Merín basins, low-temperature geothermal systems (~50°C) are proposed, associated with reservoirs at depths of more than 1,000 m.

Integrated seismic ambient noise, magnetotellurics and gravity data for the 2D interpretation of the Vallès basin structure in the geothermal system of La Garriga-Samalús (NE Spain)

The integration of geophysical methods, together with the previous information of the Vallès basin area, has resulted in the creation of a new conceptual model that explains La Garriga-Samalús geothermal system. The integration of complementary geophysical methods seems to be a good option for the preliminary stages of a geothermal system exploration, especially in urban areas.An integrated seismic ambient noise, magnetotellurics, and gravity methods were used to determine the geological units and structures which control the La Garriga-Samalús geothermal system. The 2D resistivity and density models have allowed the identification of the four main units which regulate the geothermal system: the Miocene basin, the Prelitoral Range unit, the Vallès Faut Zone, and the Paleozoic basement. The interpretation of our models set the Vallès Fault Zone, which is characterized by an anomalous low resistivity and low density, as the main path for the hot fluids. Moreover, the geophysical characterization established a new geometry for the Miocene basin. The Miocene basin presents a stepwise morphology, with the minor thickness towards the fault and an increasing thickness towards the center of the basin. This geometry seems to be related to synthetic normal faults.These results have evidenced that, although, in some geothermal systems, the warm water may create an insufficient physical contrast; the appropriate use of some techniques can still be useful for the exploration of medium and low-temperature geothermal systems.

The Use of Infrared Thermography on the Measurement of Microstructural Changes of Reservoir Rocks Induced by Temperature

A variation of temperature produces a change in the microstructure of the rock due to the mineral thermal expansion and its residual strain. Depending on the temperature cycle and texture, microstresses may lead to the development of preexistent cracks or the creation of a new and irreversible cracking. The effect of temperature on reservoir rocks is an important topic since it conditions the permeability and the fluid flow. Two main questions arise from this: the first is if an irreversible cracking threshold is attained in the reservoir rocks at low temperature geothermal systems (around 100 °C); the second one is about the influence of thermal fatigue by the repetition of heating–cooling cycles on the different rock types. To answer these questions, four reservoir rocks (chalk, sandstone, fresh granite, and weathered granite) were submitted to two different thermal regimes. The first test was conceived to detect the irreversible cracking threshold, and for that, the rocks were submitted to progressive heating (90°, 100°, 110°, 120°, and 130 °C). The second test consisted of doing cycles of fast heating of the samples up to 200 °C. The microstructure variation was assessed by means of a scanning electron microscope, mercury porosimetry, and capillary water uptake combined with passive infrared thermography. Infrared thermography is an emerging tool in the field of rock study, used to detect water masses or determine thermal properties. The water transfer during the capillary tests of the rocks, before and after the tests, was monitored with this technique. In addition, the cooling rate index, a non-destructive parameter to detect cracking development, was calculated. The results made it possible to differentiate the behaviours in relation to the rock type, with a chalk and a weathered granite less susceptible to thermal stresses than a fresh granite and sandstone. In addition, infrared thermography resulted in being a very useful indirect technique to detect the changes on the surface, although they do not always correlate to the bulk microstructural changes.

Mineralogy of travertine deposit in Ciseeng geothermal field, Parung, West Java, Indonesia

Travertine deposits are carbonate rocks consists mainly of calcium carbonate minerals, supersaturated from bicarbonate-rich water. Both recent and fossil travertine deposits are found in Ciseeng geothermal field, West Java. The travertine deposits are distributed around three distinctive areas Tirta Sayaga, Mount Panjang and Mount Peyek. This study aims to determine minerals found in travertine deposits in Ciseeng geothermal field. Mineralogical analysis can determine the evolution of hydrology system and local climate change. Principal methods of analysis consists of petrographic and diffraction methods. Analysis results indicate similar mineral composition of calcite and microcrystalline carbonate (micrite) between recent and fossil travertine deposit with more diverse texture in fossil travertine due to the evolution of water composition affecting deposition and diagenesis process. The absence of aragonite indicates a low-temperature geothermal system.

A multigeneration system of combined cooling, heating, and power (CCHP) for low-temperature geothermal system by using air cooling

A new definition of the energy, exergy, economic and environmental model (3,4E-Chaiyat models) was used to evaluate cascade air-cooled multigeneration from a single-stage organic Rankine cycle of 10 kWe, a single-stage absorption chiller of 15 kW, and a drying room of 20 kW, respectively. From the study results, a first-law efficiency of 17.23% was found to be higher than a second-law efficiency of 15.13%. According to a life cycle assessment (LCA), a single score of the energy model of approximately 0.0250Pt was lower than that of the exergy model of approximately 0.1223Pt. In the 3E model, a levelized energy cost per life cycle assessment (LEnCA) was approximately 0.0017 USD⋅Pt/kWh2 for an investment cost of 159,729 USD. At the same time, in the 4E model, a levelized exergy cost per life cycle assessment (LExCA) was approximately 0.0414 USD⋅Pt/kWh2, which was lower than that of the water-cooled multigeneration system (approximately 0.0442 USD⋅Pt/kWh2). Thus, the suitable cooling type for the cascade geothermal-multigeneration system used in San Kamphaeng, Thailand, is the air-cooled type.

Gold Partitioning in a Model Multiphase Mineral-Hydrothermal Fluid System: Distribution Coefficients, Speciation and Segregation

The characteristics of Au partitioning in a multiphase, multicomponent hydrothermal system at 450 °C and 1 kbar pressure were obtained using experimental and computational physicochemical modelling and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis. Sphalerite and magnetite contained 0.1–0.16 ± 0.02 µg/g Au and coexisted with galena and bornite which contained up to 73 ± 5 and 42 ± 10 µg/g Au, respectively. Bornite and chalcopyrite were the most effective Au scavengers with cocrystallization coefficients Au/Fe and Au/Cu in mineral-fluid system n–n × 10−2. Sphalerite and magnetite were the weakest Au absorbers, although Fe impurity in sphalerite facilitated Au uptake. Using the phase composition correlation principle, Au solubility in minerals was estimated (µg/g Au): low-Fe sphalerite = 0.7, high-Fe sphalerite = 5, magnetite = 1, pyrite = 3, pyrite-Mn = 7, pyrite-Cu = 10, pyrrhotite = 21, chalcopyrite = 110, bornite = 140 and galena = 240. The sequence reflected increasing metallicity of chemical bonds. Gold segregation occurred at crystal defects, and on surfaces, and influenced Au distribution due to its segregation at crystal interblock boundaries enriched in Cu-containing submicron phases. The LA-ICP-MS analysis of bulk and surficial gold admixtures revealed elevated Au content in surficial crystal layers, especially for bornite and galena, indicating the presence of a superficial nonautonomous phase (NAP) and dualism in the distribution of gold. Thermodynamic calculations showed that changes in experimental conditions, primarily in sulfur regime, increased the content of the main gold species (AuCl2− and AuHS0) and decreased the content of FeCl20, the prevailing form of iron in the fluid phase. The elevation of S2 and H2S fugacity affected Au partitioning and cocrystallization coefficients. Using Au content in pyrite, chalcopyrite, magnetite and bornite from volcanic-sedimentary, skarn-hosted and magmatic-hydrothermal sulfide deposits, the ranges of metal ratios in fluids were estimated: Au/Fe = n × 10−4−n × 10−7 and Au/Cu = n × 10−4−n × 10−6. Pyrite and magnetite were crystallized from solutions enriched in Au compared to chalcopyrite and bornite. The presence of NAP, and associated dualism in distribution coefficients, strongly influenced Au partitioning, but this effect does not fully explain the high gold fractionation into mineral precipitates in low-temperature geothermal systems.

Assessing the suitability of lithium-related geothermometers for estimating the temperature of felsic rock reservoirs

Lithium-related geothermometers are relatively rarely used because they are based on empirical relations derived from poorly known water-mineral equilibrium reactions and do not always give valid estimations. In this study, we evaluated the applicability of Na-Li and Li-Mg geothermometers based on hydrochemical data for geothermal waters from nine geothermal systems in felsic reservoirs in China. The results indicate that the Na-Li geothermometer provided by Michard (1990), which is similar to those given by Fouillac and Michard (1981) and Verma and Santoyo (1997) with Cl <0.3 mol/L, is suitable for fully equilibrated waters from felsic reservoirs and their estimates better represent the temperatures of the deep rather than the shallow reservoirs. The Na-Li ion-exchange equilibrium in thermal waters cannot be reached without a prior Na-K equilibrium, which ensures that the Na concentration remains constant. Also, variations in the Na-Li ratios may be significantly unpredictable such that the Michard (1990) Na-Li geothermometer may coincidentally give valid temperature estimates for thermal waters from the low-temperature geothermal systems. Furthermore, the Li in the thermal waters may originate from water-mineral reactions or mixing with magmatic water or both. In reservoirs consisting of basalt and felsic rocks, variations in hydrothermal alteration may be the key factor that explains the large differences in Na-Li equilibria in the thermal waters. Additionally, small measurement errors could bring about large deviations in the temperature estimations due to the large difference between Na and Li concentrations in the thermal waters from the felsic reservoirs. Unlike the Li concentration, which remains constant once the thermal waters leave the deep reservoirs, the concentration of Mg in the thermal waters is sensitive to the temperature in the shallow reservoirs. Hence, we discourage the use of the Li-Mg geothermometer for estimating felsic reservoir temperatures.

Energy analysis of cyclic parameters of organic Rankine cycle system

Abstract Geothermal energy is a kind of green and non-polluting renewable energy. Medium and low-temperature geothermal energy below 150 °C is usually used directly for heating, industrial and agricultural heating, bathing, etc., and the utilization rate is low. Organic Rankine cycle (ORC) system can be used in a low-temperature geothermal power generation systems. Thermodynamic analysis of the thermal cycle system is carried out in this paper. Based on the principle of exergy analysis, the cycle parameters such as evaporation temperature, condensation temperature, superheating temperature, and supercooling temperature are analyzed. The results show that the exergy efficiency and system output power can improve by increasing evaporation temperature and the exergy loss can decrease in the evaporator. Increasing superheating temperature enhances the output power of the expander and the exergy loss in the condenser, decreases the exergy efficiency simultaneously. Increasing condensation temperature decreases the exergy efficiency of the system and the output power of the expander, and increases the exergy loss of the system. Although the output power of the expander will not be affected by supercooling temperature, the rise of supercooling temperature will reduce the exergy efficiency of the system and increase the exergy loss of the evaporator.

Feasibility of low-temperature geothermal systems: Considerations of thermal anomalies, geochemistry, and local assets

Low-temperature geothermal power plants have great potential to provide baseload electricity worldwide. However, the technology is new and untested, and the systems require a significant financial investment. Site-specific feasibility studies are required to draw together available data and inform developers of production potential and challenges. This study applies coupled thermal, hydraulic, and geochemical modeling to characterize a horizontal geothermal well system in the Williston Basin. This study targeted thermal anomalies overlying 370 m thick halite and dolomite formations from the Devonian period. The halite and dolomite produced a 6°C thermal anomaly, or 16°C km−1 increase in the thermal gradient. A >5.0 MW power plant was achieved when targeting the Dawson Bay formation. A fractured, horizontal well in the Bakken formation was re-entered for injection, providing a savings of up to 40% of drilling costs. The geochemical analysis allowed for the calculation of potential precipitates in the production casing, and consideration of mitigation methods. In addition, the co-production of lithium was identified as potentially profitable in the Dawson Bay Formation. Considering the available resources and assets provided unique data to inform the development of a geothermal system in the Williston Basin.

Thermo-mechanical behavior of a granodiorite from the Liquiñe fractured geothermal system (39°S) in the Southern Volcanic Zone of the Andes

Fractures and faults in granitic rocks play an important role in geothermal systems because they permit the circulation of hot fluids. However, the thermo-hydro-mechanical behavior of granitic rocks has predominantly been studied at temperatures exceeding 300 °C but many geothermal systems experience temperatures much lower than this. The aim of this study was to evaluate how the depth, temperature, and amount and rate of mechanical loading associated conditions, that are realistic in low temperature geothermal system, influence the physical properties of geothermal reservoir hosting rock. We carried out both room temperature and low temperature thermo-mechanical tests on a granodiorite sample from the Liquiñe area, Chile, and performed post-experimental X-ray microtomography analysis to numerically estimate the permeability of the generated fractures. The results showed that both rock strength and rock stiffness decreased with increments of temperature treatment related to the development of thermal crack damage at temperatures > 150 °C and through the development of sub-critical cracking at constant temperatures between 50–75 °C. Slowest deformed samples also exhibited lower strengths, attributed to the development of sub-critical cracking. The cyclic triaxial loading test indicated that significant mechanical fracture damage was only initiated above 80% of the peak stress regardless of the number of repeated loading cycles at lower stresses. Low-temperature treatment appears to be a conditioning factor, but not the dominant factor in controlling the physical properties of reservoir hosting rocks. Our findings indicate that thermal crack damage is likely important for developing microfracture related permeability at depths between around 2–6 km where the temperature is sufficiently high to induce thermal cracking. At shallower depths, such was previously estimated the reservoir of Liquiñe, thermal crack damage is only generated adjacent to fractures that remain open and circulate the hot fluids but sub-critical cracking over time reduces the strength of rocks in lower temperature regimes. These processes combined to produce a geothermal reservoir in Liquiñe which likely first required the presence of a highly fractured fault zone.