Fluid chemistry and temperatures prior to exploitation at the Las Tres Vírgenes geothermal field,Mexico

Generation of electricity at the Las Tres Vírgenes (LTV) geothermal field, Mexico, began in 2001. There are currently nine geothermal wells in the field, which has an installed electricity generating capacity of 10 MW_e. The chemical and temperature conditions prevailing in the field prior to its exploitation have been estimated, including their central tendency and dispersion parameters. These conditions were computed on the basis of: (i) geochemical data on waters from springs and domestic wells, and on geothermal well fluids (waters and gases); most of the sampling took place between 1995 and 1999; (ii) fluid inclusion studies; (iii) geothermometric data; and (iv) static formation temperatures computed using a modified quadratic regression Horner method.Fluid inclusion homogenization temperatures (in the 100–290 °C range) suggest that there is a high-temperature fluid upflow zone near wells LV3 and LV4 in the southern part of the field. Computed average chemical equilibrium temperatures for the geothermal fluids are ∼260 °C, based on the Na/K and SiO₂ geothermometers, and ∼265 °C, based on the H₂/Ar, and CO₂/Ar geothermometers. In general, the fluid inclusion homogenization temperatures are consistent with geothermometric data, as well as with static formation temperatures. Some of the observed differences could be related to well interference effects and different fluid production/sampling depths. The deeper geothermal waters show higher concentrations of Cl, Na, K, B, Ba, but lower concentrations of SO₄, Ca, and Mg than the shallower waters. Fluid inclusion salinities are also higher in the deeper rocks. The measured Na/Cl ratios of the geothermal well waters are more or less uniform throughout the field and are very similar to that of seawater, strongly suggesting a seawater component in the fluid of the LTV system.The heat stored in the LTV geothermal system was estimated to be at least 9 × 10¹² MJ, of which some 4 × 10¹¹ MJ (equivalent to about 148 MW_e for 30 years of operation, assuming a conversion efficiency of ∼35%) might be extracted using wells. These results indicate that the installed capacity at LTV could be safely increased from the current 10 MW_e.     

Integrated geophysical investigations in the Hisar geothermal field,Demirci, western Turkey

The shallow, low-temperature geothermal field of Hisar, in western Turkey, was studied by means of vertical electrical sounding (VES), Wenner, self-potential (SP) and very low-frequency electromagnetic (VLF-EM) profiling surveys. The VES survey, conducted along the valley in an E–W direction, provided lithological and structural information that is in good agreement with the well data, and suggests that the field is characterized by low-resistivity values (≤30 Ω m). Because the resistivity structure in a N–S direction could not be investigated due to the rough topography towards the north, SP, Wenner and VLF-EM measurements were made to identify and characterize a possible hot-fluid carrying fault/fracture zone that forms one of the boundaries of the valley. There is a good correlation between the results of the SP, VLF-EM and Wenner surveys, which confirm the existence of the NE–SW striking fault zone that had been inferred from geologic information. Tensional fractures that developed perpendicular to the fault zone were also identified.Consistent with the results of the SP and Wenner profilings, the two-dimensional model derived from VLF-EM data and a Karous–Hjelt current density pseudo-section detected the conductive fault zones bearing the geothermal fluids.     

Geochemical study of fluid inclusions from the western upflow zone of the Matsukawa geothermal system,Japan

Prior to development, the Matsukawa geothermal field was partially vapor-dominated. The youngest mineral assemblage consists of early pyrophyllite, diaspore and pyrite, and later anhydrite and quartz, implying deposition from an acidic, high-temperature fluid. Fluid inclusions in anhydrite and quartz from core and cutting samples collected in wells drilled in the western upflow zone of the field were studied to characterize the temperatures and compositions of these late fluids.The results of fluid inclusion studies indicate that the temperatures during the deposition of anhydrite and quartz were up to several tens of degrees lower than the reservoir temperatures at the time of exploitation. Fluids trapped in anhydrite had temperatures of up to 257 °C, CO₂ concentrations in the 0.4–2.6 mol% range and salinities of 1.9–11.3 wt.% NaCl. This compositional variation is related to vapor loss occurring during boiling. The data suggest that the geothermal reservoir is currently being reheated by subvolcanic intrusions.     

Comparison of circulation times of thermal waters discharging from the Idaho batholith based on geothermometer temperatures,helium concentrations,and 14C measurements

Circulation times of waters in geothermal systems are poorly known. In this study, we examine the thermal waters of the Idaho batholith to verify whether maximum system temperatures, helium concentrations, and ¹⁴C values are related to water age in these low-to-moderate temperature geothermal systems. He/N₂ values of gas collected from thermal waters that circulate solely through distinct units of the Idaho batholith correlate linearly with Na–K–(4/3)Ca geothermometer temperatures, showing that both variables are excellent indicators of relative water age. Thermal waters that circulate in early Tertiary (45–50 Ma) granite of the Sawtooth batholith have 3.5 times more helium than thermal waters of the same aquifer temperature that circulate through the main Cretaceous granite (average 91 Ma). Hot spring waters circulating in hydrothermally altered parts of the batholith have very little dissolved helium and no correlation between He/N₂ values and geothermometer temperatures. Thermal waters discharging from the Idaho batholith are more depleted in deuterium than modern precipitation in the area. Recharge to these geothermal systems occurred from at least 10,000 BP for the cooler systems up to about 33,000 BP for the hotter systems.     

The use of soil Hg to delineate zones of upwelling in low-to-moderate temperature geothermal systems

A soil mercury survey was conducted near the town of Calistoga, California to identify and delineate a buried fault system that is thought to control the upwelling of low-to-moderate temperature geothermal fluids in the upper Napa Valley. Soil samples were collected at 100 m intervals along traverses that crossed hot springs and existing geothermal well sites. Strong mercury anomalies occur along a broadly-defined zone and in close proximity to surface thermal activity including active hot springs and silica sinter formations. In contrast, background mercury concentrations are present in locations with little or no indication of subsurface thermal activity, such as along the margins of the valley or near groundwater wells producing non-thermal water. Analysis of smoothed Hg values reveals a N65W-trending lineament of high Hg concentrations. These results suggest that soil mercury surveys can be a useful and cost-effective method for the identification and mapping of structures controlling subsurface fluid flow in low-to-moderate temperature geothermal systems.     

Laminar natural convection of non-Newtonian power-law fluids between concentric circular cylinders

The two-dimensional steady-state natural convection of power-law fluids is studied numerically between two concentric horizontal cylinders with different constant temperatures. The governing equations are discretized using finite volume technique based on second order upwind and are solved using the SIMPLE algorithm. The effects of Rayleigh number (10³ ≤ Ra ≤ 10⁵) and Prandtl number (10 ≤ Pr ≤ 10³) on the dimensionless velocity and temperature are investigated for both pseudoplastic and dilatant fluids. Also the mean Nusselt number for various values of governing parameters is obtained and discussed. The results indicate that with increasing the power-law index from 0.6 to 1.4, the mean Nusselt number decreases. In the best case among the range of parameters considered here the heat transfer rate for pseudo-plastic fluid (n = 0.6) is 170% higher than the Newtonian one and for dilatant fluid (n = 1.4) the heat transfer rate is 43% lower than the Newtonian fluid. So the pseudoplastic and dilatant fluids are more efficient than Newtonian fluids for cooling and insulating purposes, respectively. It is shown that as the Rayleigh number increases the cooling effect of pseudoplastic fluid and the insulating effect of dilatant fluid become more pronounced.     

Hydrogeochemical exploration of geothermal prospects in the Tecuamburro Volcano region, Guatemala

Chemical and isotopic analyses of thermal and nonthermal waters and of gases from springs and fumaroles are used to evaluate the geothermal potential of the Tecuamburro Volcano region, Guatemala. Chemically distinct geothermal surface manifestations generally occur in separate hydrogeologic areas within this 400 km² region: low-pressure fumaroles with temperatures near local boiling occur at 1470 m elevation in a sulfur mine near the summit of Tecuamburro Volcano; non-boiling acid-sulfate hot springs and mud pots are restricted to the Laguna Ixpaco area, about 5 km NNW of the sulfur mine and 350–400 m lower in elevation; steam-heated and thermal-meteoric waters are found on the flanks of Tecuamburro Volcano and several kilometers to the north in the andesitic highland, where the Infernitos fumarole (97°C at 1180 m) is the primary feature; neutral-chloride hot springs discharge along Rio Los Esclavos, principally near Colmenares at 490 m elevation, about 8–10 km SE of Infernitos. Maximum geothermometer temperatures calculated from Colmenares neutral-chloride spring compositions are 180°C, whereas maximum subsurface temperatures based on Laguna Ixpaco gas compositions are 310°C. An exploration core hole drilled to a depth of 808 m about 0.3 km south of Laguna Ixpaco had a bottom-hole temperature of 238°C but did not produce sufficient fluids to confirm or chemically characterize a geothermal reservoir. Hydrogeochemical data combined with regional geologic interpretations indicate that there are probably two hydrothermal-convection systems, which are separated by a major NW-trending structural boundary, the Ixpaco fault. One system with reservoir temperatures near 300°C lies beneath Tecuamburro Volcano and consists of a large vapor zone that feeds steam to the Laguna Ixpaco area, with underlying hot water that flows laterally to feed a small group of warm, chloriderich springs SE of Tecuamburro Volcano. The other system is located beneath the Infernitos area in the andesitic highland and consists of a lower-temperature (150–190°C) reservoir with a large natural discharge that feeds the Colmenares hot springs.     

Geothermal energy potentials in the province of Vojvodina from the aspect of the direct energy utilization

The interest in increasing the participation of renewable energy sources (RES) in energy production arises with increasing population and growing demands for energy production and consumption, as well as the fact of the limited fossil fuels reserves. RES in the energy balance of any country has their share of energy, socio-economic and environmental benefits. Investment in energy sector in the RES domain enables Vojvodina Province to reduce energy dependence on the fossil fuel market.From the total RES potential in Vojvodina Province that is 1293 ktoe/year, around 1.7% is located in existing geothermal sources. There are 73 drills with a total capacity of 72.6 MW from which 65 drills are tested positive. Currently, 15 wells are in production, with a total power of 17.7 MW. There are 27 drills that have never been in production and which are perspective, with a total power of 42.8 MW.The aim of this paper is to perform data analysis of direct geothermal energy utilization according to the water temperature and geothermal fluid flow. According to the results of the analysis recommendations for geothermal energy utilization are given within certain sectors: agriculture (aquaculture and greenhouses), heating of the facilities and pools, industrial applications and balneology.     

Geothermal resource assessment in Korea

To estimate available geothermal energy and to construct temperature at depth maps in Korea, various geothermal data have been used. Those include 1560 thermal property data such as thermal conductivity, specific heat and density, 353 heat flow data, 54 surface temperature data, and 180 heat production data. In Korea, subsurface temperature ranges from 23.9 °C to 47.9 °C at a depth of 1 km, from 34.2 °C to 79.7 °C at 2 km, from 44.2 °C to 110.9 °C at 3 km, from 53.8 °C to 141.5 °C at 4 km, and from 63.1 °C to 171.6 °C at 5 km. The total available subsurface geothermal energy in Korea is 4.25 × 10²¹ J from surface to a depth of 1 km, 1.67 × 10²² J to 2 km, 3.72 × 10²² J to 3 km, 6.52 × 10²² J to 4 km, and 1.01 × 10²³ J to 5 km. In particular, the southeastern part of Korea shows high temperatures at depths and so does high geothermal energy. If only 2% of geothermal resource from surface to a depth of 5 km is developed in Korea, energy from geothermal resources would be equivalent to about 200 times annual consumption of primary energy (～2.33 × 10⁸ TOE) in Korea in 2006.     

Geothermal energy potential for power generation in Turkey: A case study in Simav,Kutahya

Geothermal energy and the other renewable energy sources are becoming attractive solutions for clean and sustainable energy needs of Turkey. Geothermal energy is being used for electricity production and it has direct usage in Turkey, which is among the first five countries in the world for the geothermal direct usage applications. Although, Turkey is the second country to have the highest geothermal energy potential in Europe, the electricity production from geothermal energy is quite low. The main purpose of this study is to investigate the status of the geothermal energy for the electricity generation in Turkey. Currently, there is one geothermal power plant with an installed capacity of 20.4 MWe already operating in the Denizli–Kizildere geothermal field and another is under the construction in the Aydin–Germencik field.This study examines the potential and utilization of the existing geothermal energy resources in Kutahya–Simav region. The temperature of the geothermal fluid in the Simav–Eynal field is too high for the district heating system. Therefore, the possibility of electrical energy generation by a binary-cycle has been researched and the preliminary feasibility studies have been conducted in the field. For the environmental reasons, the working fluid used in this binary power plant has been chosen as HCFC-124. It has been concluded that the Kutahya–Simav geothermal power plant has the potential to produce an installed capacity of 2.9 MWe energy, and a minimum of 17,020 MWh/year electrical energy can be produced from this plant. As a conclusion, the pre-feasibility study indicates that the project is economically feasible and applicable.     

Magmatic fluid input to the Kuju-Iwoyama hydrothermal system prior to the 1995 eruption of the Kuju volcano (Kyushu, Japan)

A schematic model showing the sources of hot waters being discharged at the surface in the Kuju-Iwoyama of the Kuju volcano has been developed. Based on the isotopic characteristics of these fluids it is inferred that deep magmatic fluid mixes with thermal waters derived from rainwater in a shallow geothermal reservoir, and with local groundwaters in a deeper reservoir. These thermal waters feed hot springs that discharge waters with Cl/SO₄ ratios that differ from that of the fumaroles on Kuju-Iwoyama, due to the addition of SO₄²⁻ ions produced by the decomposition of native sulfur and mixing with magmatic fluid of high Cl content.     

Spatial distribution of temperature in the low-temperature geothermal Euganean field (NE Italy): a simulated annealing approach

The spatial distribution of groundwater temperatures in the low-temperature (60–86 °C) geothermal Euganean field of northeastern Italy has been studied using a geostatistical approach. The data set consists of 186 temperatures measured in a fractured limestone reservoir, over an area of 8 km². Investigation of the spatial continuity by means of variographic analysis revealed the presence of anisotropies that are apparently related to the particular geologic structure of the area. After inference of variogram models, a simulated annealing procedure was used to perform conditional simulations of temperature in the domain being studied. These simulations honor the data values and reproduce the spatial continuity inferred from the data. Post-processing of the simulations permits an assessment of temperature uncertainties. Maps of estimated temperatures, interquartile range, and of the probability of exceeding a prescribed 80 °C threshold were also computed. The methodology described could prove useful when siting new wells in a geothermal area.     

Buoyancy organic Rankine cycle

In the scope of renewable energy, we draw attention to a little known technique to harness solar and geothermal energy. The design here proposed and analyzed is a conceptual hybrid of several patents. By means of a modified organic Rankine cycle, energy is obtained utilizing buoyancy force of a working fluid. Based on thermodynamic properties we propose and compare the performance of Pentane and Dichloromethane as working fluids. Theoretical efficiencies up to 0.26 are estimated for a 51 m (Pentane) and 71.5 m (Dichloromethane) high column of water in a regime below 100 °C operation temperature. These findings are especially relevant in the scope of distributed energy systems, combined cycle plants, and low-temperature Rankine cycles.     

Analysis of entropy generation for distributed heating in processing of materials by thermal convection

Analysis of entropy generation has been carried out for square cavities with distributed heated sources filled with various materials involving wide range of Pr(=0.015, 0.7, 10, 1000) during the conduction and convection regime within Ra(=10³ ? 10⁵). Entropy generation terms involving thermal and velocity gradients are evaluated accurately based on elemental basis set via Galerkin finite element method. Local entropy maps are analyzed in detail for various cases and the dominance of thermal and frictional irreversibilities is studied via average Bejan number. The heat transfer irreversibility is found to dominate during conduction regime while the fluid friction irreversibility dominates the entropy generation in the convection regime, except for the low Pr fluid based on the heating configuration of the cavity. Further, the variation of total entropy generation has been observed to be similar for different heating configurations for higher Pr fluids (=10, 1000) whereas, the configuration of cavity has been found to have little effect on total entropy generation for fluids with Pr = 0.7 during both conduction and convection regimes. Thermal mixing and degree of temperature uniformity due to distributed heating in various cases are also reported and optimum cases for processing of various fluids are presented based on minimum entropy generation.     

Kinetics of fluorescein decay and its application as a geothermal tracer

Fluorescein is a dye used to trace the path of injected fluids through geothermal reservoirs. We have measured its thermal stability at temperatures up to 300°C in hydrothermal autoclaves at various fluid compositions, pHs, and oxygen concentrations. The results of these experiments indicate that fluorescein will decay less than 10% during a one month tracer test in geothermal reservoirs with temperatures below 210°C. For tracer tests involving longer times and/or higher temperatures, the activation parameters presented in this study can be used to correct for thermal decay. These parameters were applied to a tracer test conducted at the Dixie Valley, Nevada geothermal system to correct for the thermal decay of fluorescein and to deduce the effective temperature of the injection-production flow path.     

Exploitation of geothermal energy as a priority of sustainable energetic development in Serbia

The actual global economic crisis, including all other well-known problems of sustainable development, reflects the direction of development of all countries in the world. Serbia, as a European country in its early stage of development, is trying to synchronize its progress with experience of other countries from the field of sustainable development and in accordance with rules in the field of energetic and energetic efficiency, and, as well as to promote and develop the sector of use of renewable sources of energy. On the other hand, Serbia is a country which largely depends on import of all forms of energy, which to a great extent affects its economic stability. Therefore, in Serbia the strategy for development of energetic was imposed and it considers all the aspects of development of energetic until 2015 and it also defines the priorities which can be mostly seen in the choice of forms of alternative sources of energy. These sources, based on some criteria, can be considered the most convenient for a gradual substitution of energy which is gotten from the conventional sources. Taking into account strategically defined goals and domestic potentials which are at disposal, as well as economic parameters, an alternative source of energy of basic importance for the future exploitation on the territory of Serbia geothermal energy, was chosen. The research points to the fact that Serbia will be capable to respond adequately to Kyoto protocol demands and to the European rules regarding the substitution of a certain amounts of fossil fuels by the fuel origin from the raw biological materials. The research defines the existent and non-existent capacities and the assessment of positive effects of usage of geothermal energy. At the moment, 160 long holes are being exploited whose water temperature is around 60 °C (140 °F) and their heat power reach 160 MJ/s. It was stated that adequate exploitation of existing and new geothermal sources a yearly would save about 500,000 tons of fossil fuels what is proportional to the 10% of the today's heating system. The total amount of heat accumulated at geothermal deposit sites in Serbia, up to 3 km of depth, is two times greater than the total amount of heat that may be generated by burring all available coal reserves in Serbia. Price of electrical energy produced from geothermal springs is estimated to be between 9.2 US cents/kWh and 11.5 US cents/kWh. In order to support exploitation of geothermal energy (as well as all other renewable sources of energy) the decision that all the producers of energy from renewable sources get a status of privileged producers were made.     

Potential contribution of geothermal energy to climate change adaptation: A case study of the arid and semi-arid eastern Baringo lowlands,Kenya

The impacts of recurrent droughts have increased vulnerability and reduced the adaptive capacity of the people living in arid and semi-arid lands (ASALS) of Kenya. Current interventions are short-term and curative in nature, hence unsustainable. Some of the most arid and semi-arid lands are located within the Kenyan Rift system, which has an estimated geothermal potential of about 7000 to 10,000 MWe, out of which only 200 MWe has been developed, and about 5000 MWe planned by 2030. Recent power sector reforms have built institutional structures that will accelerate development of geothermal energy. The paper analyses the potential use of geothermal energy resources in eastern Baringo lowlands between Lake Bogoria and Silali prospects, which has an estimated potential of >2700 MWe, in creating the necessary adjustments needed to adapt to the impacts of recurrent droughts by locals. Opportunities for direct and indirect uses of geothermal energy exist in climate vulnerable sectors, such as, agriculture, fisheries, water, livestock production as well as alternative income generating activities such as, tourism, micro enterprises, aloe, honey and beeswax production, fabric dyeing and others using resources sourced from within a 50 km radius. The possibility of accelerated geothermal development and proposed utilisation schemes in causing maladaptation if unsustainably implemented is also discussed. The paper draws a Lindal diagram adapted to the study area showing potential utilisation in the above sectors, and new flow diagram showing potential for cascaded use of geothermal hot water through the different processes. An estimated capacity of 100 MWt and 100 MWe can be used in the potential utilisation schemes discussed in this article to meet local adaptation and lighting needs and much less in a cascaded process. Potential barriers and possible solutions are also discussed. The study concludes that geothermal energy is a vital option for adaptation in the study area if sustainably used.     

Laminar free convection from a horizontal semi-circular cylinder to power-law fluids

Extensive numerical results on the flow and thermal fields are presented for free convection from a semi-circular cylinder (flat base upward) immersed in quiescent power-law fluids for the following ranges of conditions: Grashof number, 10 ? Gr ? 10⁵, Prandtl number, 0.72 ? Pr ? 100, and power-law index, 0.2 ? n ? 1.8. The heat transfer characteristics are analyzed in terms of the isotherm patterns, local and average Nusselt number as functions of the pertinent dimensionless parameters. The flow field is visualized in terms of the streamline patterns adjacent to the surface of the cylinder for a range of values of the Grashof number, Prandtl number and power-law index. A separated flow region forms at as low values of the Prandtl number as Pr = 0.72 for n ? 1 (Newtonian and shear-thickening fluids); whereas for shear-thinning fluids (n < 1), the flow remains attached to the cylinder surface over the range of conditions encompassed here. The bubble size grows with Grashof number and it shrinks with Prandtl number. In order to quantify the deviation from the Newtonian behaviour, the normalized values of average Nusselt number are analyzed as a function of the power-law index. In addition, a correlation is proposed for average Nusselt number as a function of the Grashof number, Prandtl number and power-law index. In general terms, shear-thinning fluid behaviour enhances heat transfer whereas shear-thickening has adverse influence on it.     

Potential of geothermal heat exchangers for office building climatisation

Low depth geothermal heat exchangers can be efficiently used as a heat sink for building energy produced during summer. If annual average ambient temperatures are low enough, direct cooling of a building is possible. Alternatively the heat exchangers can replace cooling towers in combination with active cooling systems. In the current work, the performance of vertical and horizontal geothermal heat exchangers implemented in two office building climatisation projects is evaluated.A main result of the performance analysis is that the ground coupled heat exchangers have good coefficients of performance ranging from 13 to 20 as average annual ratios of cold produced to electricity used. Best performance is reached, if the ground cooling system is used to cool down high temperature ambient air. The maximum heat dissipation per meter of ground heat exchanger measured was lower than planned and varied between 8 W m^?1 for the low depth horizontal heat exchangers up to 25 W m^?1 for the vertical heat exchangers.The experimental results were used to validate a numerical simulation model, which was then used to study the influence of soil parameters and inlet temperatures to the ground heat exchangers. The power dissipation varies by ±30% depending on the soil conductivity. The heat conductivity of vertical tube filling material influences performance by another ±30% for different materials. Depending on the inlet temperature level to the ground heat exchanger, the dissipated power increases from 2 W m^?1 for direct cooling applications at 20 °C up to 52 W m^?1 for cooling tower substitutions at 40 °C. This directly influences the cooling costs, which vary between 0.12 and 2.8€ kW h^?1.As a result of the work, planning and operation recommendations for the optimal choice of ground coupled heat exchangers for office building cooling can be given.     

Effect of aspect ratio on natural convection in power-law liquids from a heated horizontal elliptic cylinder

The influence of aspect ratio and shear-dependent viscosity on free convection heat transfer from a horizontal heated elliptic cylinder in power-law fluids has been investigated. In particular, the coupled momentum and energy equations have been solved numerically over the following ranges of conditions: Grashof number, 10 ? Gr ? 10⁵; Prandtl number, 0.72 ? Pr ? 100; power-law index, 0.3 ? n ? 1.5 and aspect ratio, 0.2 ? E ? 5. The new extensive results demonstrate the influence of the Grashof number (Gr), Prandtl number (Pr), power-law index (n) and aspect ratio (E) on the macroscopic heat and momentum transfer characteristics like local and average values of Nusselt number (Nu) and drag coefficients (C_D). Further insights are developed by examining the structure of the flow and temperature fields adjacent to the cylinder. Broadly speaking, all else being equal, shear- thinning fluid behaviour promotes heat transfer whereas shear-thickening viscosity has a deleterious effect on it with reference to that in Newtonian fluids. Also, the rate of heat transfer gradually increases as the cylinder shape passes from blunt to slender with respect to the direction of gravity. Finally, the present numerical values of the Nusselt number are correlated using a simple analytical form which facilitates interpolation of the present results for the intermediate values of the governing parameters. The paper is concluded by presenting detailed comparisons with the previous numerical and experimental results available in the literature, especially in Newtonian fluids.