Low-enthalpy Geothermal Resources Research Articles

Techno-economic analysis of MED and RO desalination powered by low-enthalpy geothermal energy

Utilizing low enthalpy geothermal resources in various applications, including desalination, has triggered continuously growing interest in the past decade. This work offers a preliminary techno-economic evaluation of coupling a low-enthalpy geothermal resource, commonly found in regions such as the Arabian Gulf countries, and a suitable desalination technology. The desalination processes chosen, multiple effect distillation (MED) and reverse osmosis (RO), were designed as integrated energy–water systems and were compared and assessed in terms of their levelized cost of water produced. It was found that geothermal RO could potentially be a more cost-effective option for seawater geothermal desalination in the Gulf Cooperation Council (GCC) countries, based on our model results. A number of parameters, which can potentially alter the results of the analysis, were chosen to investigate their effect on the LCOW of the proposed schemes. These parameters include feed water quality, operational lifetimes of both the geothermal and desalination systems, quality of the geothermal resource, cost of well-drilling and finally, reinjection temperature of the utilized geofluid. By varying their values, the robustness of our initial model results was assessed.

Impact of a very low enthalpy plant on a costal aquifer: a case study in Southern Italy

The interest in the low-enthalpy geothermal resources is growing, owing to its wide availability, and the possibility of overcoming the problems related to energy storage. Among the low-enthalpy plants, the installation in coastal areas of the open-loop groundwater heat pump (GWHP) systems, that use groundwater as the geothermal fluid, is particularly attractive because of the presence of shallow aquifers. Nevertheless, these aquifers are often affected by seawater intrusion, so strict monitoring before GWHP installation is needed to check the feasibility of the plant. This work reports a detailed quali-quantitative characterization of a costal aquifer, in Southern Italy, for its exploitation as a very low enthalpy geothermal resource. The main groundwater parameters were monitored to assess the groundwater suitability to be used as geothermal fluid. A long-term pumping test, simulating the designed open-loop GWHP system, was performed to evaluate its potential impact on seawater intrusion. The results revealed that the open-loop GWHP plant do not affect the quali-quantitative equilibrium of the studied coastal aquifer. Furthermore, the study demonstrated that the used methodological approach is proper for the early assessment of the feasibility of the GWHP plant without affecting the seawater intrusion processes, and with minimum impact.

Study of Shallow Low-Enthalpy Geothermal Resources Using Integrated Geophysical Methods

The paper is focused on low enthalpy geothermal exploration performed in south Italy and provides an integrated presentation of geological, hydrogeological, and geophysical surveys carried out in the area of municipality of Lecce. Geological and hydrogeological models were performed using the stratigraphical data from 51 wells. A ground-water flow (direction and velocity) model was obtained. Using the same wells data, the ground-water annual temperature was modeled. Furthermore, the ground surface temperature records from ten meteorological stations were studied. This allowed us to obtain a model related to the variations of the temperature at different depths in the subsoil. Integrated geophysical surveys were carried out in order to explore the low-enthalpy geothermal fluids and to evaluate the results of the model. Electrical resistivity tomography (ERT) and self-potential (SP) methods were used. The results obtained upon integrating the geophysical data with the models show a low-enthalpy geothermal resource constituted by a shallow ground-water system.

Estimating low-enthalpy geothermal energy potential for district heating in Santiago basin–Chile (33.5 °S)

This work presents the results of a regional-scale estimation of low-enthalpy geothermal resources for district heating in the Santiago basin. The purpose of this work is to identify promising areas for the development of this type of renewable energy. The estimation was based on comparison of soil thermal properties and hydrogeological parameters, using Geographic Information System (GIS). To determine the geothermal potential, Ground Source Heat Exchanger (GSHE) coupled with heat pump was used to supply a fixed demand equivalent to the energy required to heat a Chilean standard house. The main barrier for the implementation of a GSHE coupled with heat pump is the well drilling cost, therefore the potential is presented as meters to be drilled in order to install 2 types of GSHE: 1) Borehole Heat Exchanger (BHE) and 2) Groundwater Heat Exchanger (GWHE). To assess the BHE, we used specific Heat Extraction (sHE) of sediments. To evaluate a GWHE, we used depth of groundwater table and groundwater drawdown caused by pumping water to the heat pump. The depth to be drilled ranges from 35 to 105 m in case of the BHE, while in case of the GWHE it ranges from 10 to 400 m.

Geochemical and reactive transport modeling of the injection of cooled Triassic brines into the Dogger aquifer (Paris basin, France)

As another option for exploitation of the low-enthalpy geothermal resources of the Paris basin (France), currently limited to the carbonate Dogger aquifer, brine could be extracted from deeper Triassic sandstone reservoirs and re-injected into the Dogger aquifer, which has higher injectivity. The objective of the geochemical modeling work presented here is to identify the physical–chemical processes that are likely to be induced by the re-injection of the cooled Triassic brines into the Dogger aquifer and to predict their possible consequences on the Dogger porosity. The results suggest that this re-injection process may induce precipitation/dissolution reactions. However, the impact on porosity should be limited in magnitude (a few percent) and in extent (tens of meters around the well).

A downhole heat exchanger for horizontal wells in low-enthalpy geopressured geothermal brine reservoirs

Geothermal energy is a clean, renewable energy resource that is widely available and reliable. Improved drilling and heat conversion systems make geothermal energy an increasingly attractive alternative. Downhole heat exchangers (DHEs) can accelerate the development of geothermal energy by reducing the capital cost and the risk of microquakes or subsidence. However, low-enthalpy geothermal resources are difficult to develop economically because of low heat extraction efficiency.In this study, a coaxial DHE concept is proposed to exploit forced convection driven by a downhole pump inside a horizontal wellbore. Two configurations of the proposed design are introduced, each having different flow paths for working and reservoir fluids. One system – which circulates working fluid through the inner-most tubing in the coaxial arrangement has better thermal exchange efficiency of about 29%, and is evaluated by coupling it to a simple model for a binary power generation plant and a geothermal reservoir simulator. Thermodynamic analysis evaluates the DHE performance for electricity generation. A field case study of the Camerina A reservoir (Vermillion Parish, Louisiana) demonstrates a net power of about 350kW can be generated by a turbine even after 30 years of production.

Estimating Limits for the Geothermal Energy Potential of Abandoned Underground Coal Mines: A Simple Methodology

Flooded mine workings have good potential as low-enthalpy geothermal resources, which could be used for heating and cooling purposes, thus making use of the mines long after mining activity itself ceases. It would be useful to estimate the scale of the geothermal potential represented by abandoned and flooded underground mines in Europe. From a few practical considerations, a procedure has been developed for assessing the geothermal energy potential of abandoned underground coal mines, as well as for quantifying the reduction in CO2 emissions associated with using the mines instead of conventional heating/cooling technologies. On this basis the authors have been able to estimate that the geothermal energy available from underground coal mines in Europe is on the order of several thousand megawatts thermal. Although this is a gross value, it can be considered a minimum, which in itself vindicates all efforts to investigate harnessing it.

Geophysical techniques for low enthalpy geothermal exploration in New Zealand

Shallow warm water resources associated with low enthalpy geothermal systems are often difficult to explore using geophysical techniques, mainly because the warm water creates an insufficient physical change from the host rocks to be easily detectable. In addition, often the system also has a limited or narrow size. However, appropriate use of geophysical techniques can still help the exploration and further investigation of low enthalpy geothermal resources. We present case studies on the use of geophysical techniques for shallow warm water explorations over a variety of settings in New Zealand (mostly in the North Island) with variable degrees of success.A simple and direct method for the exploration of warm water systems is shallow temperature measurements. In some New Zealand examples, measurements of near surface temperatures helped to trace the extent of deeper thermal water.The gravity method was utilised as a structural technique for the exploration of some warm water systems in New Zealand. Our case studies show the technique can be useful in identifying basement depths and tracing fault systems associated with the occurrence of hot springs.Direct current (DC) ground resistivity measurements using a variety of electrode arrays have been the most common method for the exploration of low enthalpy geothermal resources in New Zealand. The technique can be used to detect the extent of shallow warm waters that are more electrically conductive than the surrounding cold groundwater. Ground resistivity investigations using the electromagnetic (EM) techniques of audio magnetotellurics (AMT or shallow MT), controlled source audio magnetotellurics (CSAMT) and transient electromagnetic (TEM) methods have also been used. Highly conductive clays of thermal or sedimentary origin often limit the penetration depth of the resistivity techniques and can create some interpretation difficulties. Interpretation of resistivity anomalies needs to be treated in a site specific manner.

Low-enthalpy geothermal resources for electricity production: A demand-side management study for intelligent communities

The geological conditions in Greece contributed to the creation of important low-enthalpy geothermal energy resources (LEGERs). The resources are divided into low, medium and high enthalpy, or temperature, based on criteria that are generally based on the energy content of the fluid. LEGERs are those sources of the hot water whose temperature is between 25 and 100°C, which are used for heating residences and in the agricultural or industrial sector. The investigation for the exploitation of low-enthalpy geothermal fluids, which began around 1980, intensified in the last two decades. The low-enthalpy geothermal potential in Greece is rather significant as most of the geothermal fields have been found in regions with favourable developmental conditions, and it seems that they do not present serious environmental or technical exploitation problems. LEGER areas are abundant in Greece, mainly in the eastern and northern part of the country, as well as in many of the Aegean Islands. The aim of this work is to review the options for managing wind load by using low-enthalpy geothermal energy for electricity (through heat pump utilisation) according to the local energy demand.

An Organic Rankine Cycle off-design model for the search of the optimal control strategy

Power generation from low enthalpy geothermal resources using Organic Rankine Cycle systems is markedly influenced by the temperature level of the heat source and heat sink. During plant operation the actual temperature of the geofluid may be different from the value assumed in the design phase. In addition, the seasonal and daily variations of the ambient temperature greatly affect the power output especially when a dry condensation system is used. This paper presents a detailed off-design model of an Organic Rankine Cycle that includes the performance curves of the main plant components. Two capacitive components in the model have the key function of damping the temporary disequilibrium of mass and energy inside the system. Isobutane and R134a are considered as working fluids, mainly operating in subcritical and supercritical cycles, respectively. The off-design model is used to find the optimal operating parameters that maximize the electricity production in response to changes of the ambient temperatures between 0 and 30 °C and geofluid temperatures between 130 and 180 °C. This optimal operation strategy can be conveniently applied both to already existing plants and to the choice of new design plant configurations.

Structural and hydrogeological features of a Lias carbonate aquifer in the Triffa Plain, NE Morocco

The rising demand for water and the contamination of shallow water table aquifers has led authorities in NE Morocco to look for deeper groundwater resources in the Triffa Plain, namely in Lower Jurassic (Lias) dolomitic limestones. The liassic aquifer is of strategic importance for the development of the region, however, its hydrodynamic behaviour is poorly understood due to lack of hydrogeological data and block structure.This article presents a first effort towards understanding the structure and hydraulic behaviour of the aquifer. Exploration borehole data and results from geophysical campaigns were integrated into a GIS environment to build a preliminary model of the aquifer structure. The aquifer behaves as an unconfined aquifer in the northern part of the Béni Snassen Mountains (the recharge area), but as it dips to the north, it becomes confined by marls and shales of the Middle/Upper Jurassic. Even though piezometric level data are scarce, a tentative piezometric map was produced. Three blocks separated by NW–SE trending faults in a horst and graben structure, with distinct flow behaviours were identified: Berkane, Fezouane and Sidi Rahmoun blocks. Those blocks also show differences in hydraulic conductivity distribution. As a result of the reaction with the dolomitic limestones, the groundwater is of calcium-magnesium bicarbonate type. Groundwater temperature as measured in springs ranges from 29°C to 37°C in springs and constitutes a potential low enthalpy geothermal resource.

Ground temperature profiles and thermal rock properties at Wairakei, New Zealand

Here we present ground temperature data from Wairakei, Taupo, New Zealand. The surface zone in this area extends to 0.5 m, while the shallow zone extends to at least 7.39 m. It is expected that the deep zone starts close to this depth and will exhibit a constant temperature of approximately 14.3 °C, 1–2 °C higher than the mean ambient temperature at this site. Shallow ground temperatures are predominantly influenced by the ambient air temperature and to a lesser extent the amount of rainfall. Modelling of the thermal waves, taking conductive and convective heat transfer into account, enables estimates of thermal diffusivity, volumetric heat capacity and thermal conductivity, whose mean values based on data from August 2010–April 2011 were 3.48 × 10−7 m2 s−1, 2.35 × 106 J m−3 K−1 and 0.89 W m−1 K−1 respectively. These estimates are within the same ranges as previous estimates of the thermal properties at Wairakei, although the mean values are slightly higher. This finding is consistent with the increased amount of recharge that occurred during the experimental period. Increased understanding of ground temperature and thermal rock properties is valuable for the development of low enthalpy geothermal resources as well as having applications in hydrology, agronomy and climatology.

Geophysical Techniques for Low Enthalpy Geothermal Exploration in New Zealand

Shallow warm water resources associated with low enthalpy geothermal systems are often difficult to explore usinggeophysicaltechniques,mainlybecausethewarmwatercreatesaninsufficientphysicalchangefromthehostrockstobe easily detectable. In addition, often the system also has a limited or narrow size. However, appropriate use of geophysical techniques can still help the exploration and further investigation of low enthalpy geothermal resources. We present case studies on the use of geophysical techniques for shallow warm water explorations over a variety of settings in New Zealand (mostly in the North Island) with variable degrees of success. Asimpleanddirectmethodfortheexplorationofwarmwatersystemsisshallowtemperaturemeasurements.InsomeNew Zealand examples, measurements of near surface temperatures helped to trace the extent of deeper thermal water. ThegravitymethodwasutilisedasastructuraltechniquefortheexplorationofsomewarmwatersystemsinNewZealand. Ourcasestudiesshowthetechniquecanbeusefulinidentifyingbasementdepthsandtracingfaultsystemsassociatedwiththe occurrence of hot springs. Direct current (DC) ground resistivity measurements using a variety of electrode arrays have been the most common method for the exploration of low enthalpy geothermal resources in New Zealand. The technique can be used to detect the extentofshallowwarmwatersthataremoreelectricallyconductivethanthesurroundingcoldgroundwater.Groundresistivity investigationsusingtheelectromagnetic(EM)techniquesofaudiomagnetotellurics(AMTorshallowMT),controlledsource audio magnetotellurics (CSAMT) and transient electromagnetic (TEM) methods have also been used. Highly conductive clays of thermal or sedimentary origin often limit the penetration depth of the resistivity techniques and can create some interpretation difficulties. Interpretation of resistivity anomalies needs to be treated in a site specific manner.

Zeotropic mixtures as working fluids in Organic Rankine Cycles for low-enthalpy geothermal resources

This work presents detailed simulations of Organic Rankine Cycle processes for energy conversion of low-enthalpy geothermal resources. The working fluids considered in this analysis are zeotropic mixtures. Second law efficiency of subcritical cycles is calculated for isobutane/isopentane and R227ea/R245fa depending on mixture composition, heat source temperature and temperature difference of cooling water. The use of mixtures as working fluids leads to an efficiency increase compared to pure fluids, due to a glide match of temperature profiles in the condenser and evaporator. For heat source temperatures below 120 °C the raise is up to 15%. In case of pure fluids a significant efficiency increase occurs, when the maximum pressure is reached and the pinch point shifts to the inlet of the preheater. This effect appears for R227ea at 120 °C and for isobutane at 170 °C. Using mixtures this behavior can be adjusted to higher temperatures by adding a less volatile component.

Advanced Electrical Power Generation Technology Using Renewable & Clean Low-Enthalpy Geothermal Energy Sources

Advanced nonconventional renewable & clean energy technologies which are used for generation of electricity have shown real promise and received renewed interest in recent years due to an increasing concern of environmental issues of greenhouse gas (GHG) emissions responsible for global warming effects, environmental pollution, and the limitations and conservation of natural energy resources. One of these innovative emerging technologies includes renewable low-enthalpy geothermal energy sources for clean electrical power generation. This promising technology offers potential applications in generation of electric power which can be produced using renewable low-enthalpy geothermal energy resources available in many parts of the world. In this paper, the basic concept of Organic Rankine Cycle (ORC) binary power technology using low-enthalpy geothermal heat sources is introduced and its potential applications and limitations for small-scale geothermal power generation and its relevant environmental and economic considerations are presented and discussed. Also, some recent developments and patents related to the energy conversion of low-enthalpy geothermal energy into electrical power with their important and relevant applications are reviewed. The list of patents reviewed here is by no means supposed to be thorough or complete but demonstrates, however, evolution trends related to low-enthalpy power generation.

Hyper-permeable granite: lessons from test-pumping in the Eastgate Geothermal Borehole, Weardale, UK

Abstract Extremely high permeability has been found at 410 m depth in the Weardale Granite, UK, during exploratory drilling for low-enthalpy geothermal resources. The Eastgate Borehole was sunk down the axis of an ancient, sub-vertical hydrothermal vein structure (the Slitt Vein), proving a transmissivity in excess of 4000 darcy m (= 3 × 10 −9 m 2 m), which is one of the highest values ever measured deep within a granite intrusion. Sustained hydraulic testing over 24 h showed that this transmissivity was not local to the borehole: sustained flows and steadying of drawdown suggest that this transmissivity persists along the strike of the Slitt Vein structure. Although this finding is encouraging in relation to the prospects for geothermal energy development in this and similar granites, it raises disquieting issues in relation to assumptions commonly made during planning for radioactive waste isolation concerning the feasible maximum permeability likely to be encountered in granite.

Distilled water production using geothermally heated seawater

The basic technical and economic issues related to the production of distilled water from geothermally heated waters in coastal areas of Baja California, Mexico, are compiled and reviewed. A mathematical model that takes into consideration water temperature, production flow rate and plant design is presented. The use of colder waters in the condenser that results in increased production rates and reduced initial investments is studied, as well as different possible plant designs. It is shown that in coastal areas fresh water produced from this type of low-enthalpy geothermal resources can be cost effective.

04/01544 Optimization study of combined refrigeration cycles driven by an engine: Zhao, Y. et al. Applied Energy, 2004, 76, (4), 379–389

This work presents the results of a regional-scale estimation of low-enthalpy geothermal resources for district heating in the Santiago basin. The purpose of this work is to identify promising areas for the development of this type of renewable energy. The estimation was based on comparison of soil thermal properties and hydrogeological parameters, using Geographic Information System (GIS). To determine the geothermal potential, Ground Source Heat Exchanger (GSHE) coupled with heat pump was used to supply a fixed demand equivalent to the energy required to heat a Chilean standard house. The main barrier for the implementation of a GSHE coupled with heat pump is the well drilling cost, therefore the potential is presented as meters to be drilled in order to install 2 types of GSHE: 1) Borehole Heat Exchanger (BHE) and 2) Groundwater Heat Exchanger (GWHE). To assess the BHE, we used specific Heat Extraction (sHE) of sediments. To evaluate a GWHE, we used depth of groundwater table and groundwater drawdown caused by pumping water to the heat pump. The depth to be drilled ranges from 35 to 105 m in case of the BHE, while in case of the GWHE it ranges from 10 to 400 m.

04/01681 Geothermal development in Hungary — country update report 2000–2002: Árpàsi, M. Geothermics, 2003, 32, (4–6), 371–377

Exploration for geothermal resources began in Romania in the early 1960s, based on a detailed geological exploration program for hydrocarbon resources that had a capacious budget and enabled the identification of eight geothermal areas. Over 200 wells drilled to depths between 800 and 3500 m have indicated the presence of low-enthalpy geothermal resources (40–120 °C). Completion and experimental production from over 100 wells during the past 25 years has led to the evaluation of the exploitable heat resources of the geothermal reservoirs. The proven reserves, with the wells that have already been drilled, amount to about 200,000 TJ for 20 years. The main geothermal systems discovered on Romanian territory are in porous permeable formations such as sandstones and siltstones (Western Plain and the Olt Valley) or in fractured carbonate formations (Oradea, Bors, and north of Bucharest). The total thermal capacity of the existing wells is about 480 MWt (for a reference temperature of 25 °C). Only 152 MWt of this potential is currently being exploited, from 96 wells (35 of which are used for health and recreational bathing), producing hot water in the temperature range 45–115 °C. In 2002 the annual energy utilisation from these wells was about 2900 TJ, with a capacity factor of 0.6. More than 80% of the wells are artesian producers, 18 wells require anti-scaling chemical treatment and six are reinjection wells. During the period 1995–2002, 15 exploration-production geothermal wells were drilled and completed, two of which were dry holes. Drilling was financed by the geological exploration fund of the State Budget, to depths varying between 1500 and 3500 m. Progress in the direct utilisation sector of geothermal resources has been extremely slow because of the difficulties encountered during the transition period from a centrally planned to a free-market economy; geothermal production is at present far below the level that could be expected from its assessed potential, with geothermal operations lagging behind in technology. The main obstacle to geothermal development in Romania is the lack of domestic investment capital. In order to stimulate the interest of potential investors from developed countries and to comply with the requirements of the large international banks, an adequate legal and institutional framework has been created, adapted to a market-oriented economy.

04/01680 Geothermal applications in Turkey: Mertoglu, O. et al. Geothermics, 2003, 32, (4–6), 419–428

Exploration for geothermal resources began in Romania in the early 1960s, based on a detailed geological exploration program for hydrocarbon resources that had a capacious budget and enabled the identification of eight geothermal areas. Over 200 wells drilled to depths between 800 and 3500 m have indicated the presence of low-enthalpy geothermal resources (40–120 °C). Completion and experimental production from over 100 wells during the past 25 years has led to the evaluation of the exploitable heat resources of the geothermal reservoirs. The proven reserves, with the wells that have already been drilled, amount to about 200,000 TJ for 20 years. The main geothermal systems discovered on Romanian territory are in porous permeable formations such as sandstones and siltstones (Western Plain and the Olt Valley) or in fractured carbonate formations (Oradea, Bors, and north of Bucharest). The total thermal capacity of the existing wells is about 480 MWt (for a reference temperature of 25 °C). Only 152 MWt of this potential is currently being exploited, from 96 wells (35 of which are used for health and recreational bathing), producing hot water in the temperature range 45–115 °C. In 2002 the annual energy utilisation from these wells was about 2900 TJ, with a capacity factor of 0.6. More than 80% of the wells are artesian producers, 18 wells require anti-scaling chemical treatment and six are reinjection wells. During the period 1995–2002, 15 exploration-production geothermal wells were drilled and completed, two of which were dry holes. Drilling was financed by the geological exploration fund of the State Budget, to depths varying between 1500 and 3500 m. Progress in the direct utilisation sector of geothermal resources has been extremely slow because of the difficulties encountered during the transition period from a centrally planned to a free-market economy; geothermal production is at present far below the level that could be expected from its assessed potential, with geothermal operations lagging behind in technology. The main obstacle to geothermal development in Romania is the lack of domestic investment capital. In order to stimulate the interest of potential investors from developed countries and to comply with the requirements of the large international banks, an adequate legal and institutional framework has been created, adapted to a market-oriented economy.