Thermodynamic analysis of models used in hydrogen production by geothermal energy

Four models are developed for the use of geothermal energy for hydrogen production. These include using geothermal work output as the work input for an electrolysis process (Case 1); using part of geothermal heat to produce work for electrolysis process and part of geothermal heat in an electrolysis process to preheat the water (Case 2), using geothermal heat to preheat water in a high-temperature electrolysis process (Case 3), and using part of geothermal work for electrolysis and the remaining part for liquefaction (Case 4). These models are studied thermodynamically, and both reversible and actual (irreversible) operation of the models are considered. The effect of geothermal water temperature on the amount of hydrogen production per unit mass of geothermal water is investigated for all four models, and the results are compared. The results show that as the temperature of geothermal water increases the amount of hydrogen production increases. Also, 1.34 g of hydrogen may be produced by one kg of geothermal water at 200 °C in the reversible operation for Case 1. The corresponding values are 1.42, 1.91, and 1.22 in Case 2, Case 3, and Case 4, respectively. Greater amounts of hydrogen may be produced in Case 3 compared to other cases. Case 2 performs better than Case 1 because of the enhanced use of geothermal resource in the process. Case 4 allows both hydrogen production and liquefaction using the same geothermal resource, and provides a good solution for the remote geothermal resources. A comparison of hydrogen production values in the reversible and irreversible conditions reveal that the second-law efficiencies of the models are 28.5%, 29.9%, 37.2%, and 16.1% in Case 1, Case 2, Case 3, and Case 4, respectively.     

Geothermal energy use in hydrogen liquefaction

We propose the use of geothermal energy for hydrogen liquefaction, and investigate three possible cases for accomplishing such a task including (1) using geothermal output work as the input for a liquefaction cycle; (2) using geothermal heat in an absorption refrigeration process to precool the gas before the gas is liquefied in a liquefaction cycle; and (3) using part of the geothermal heat for absorption refrigeration to precool the gas and part of the geothermal heat to produce work and use it in a liquefaction cycle (i.e., cogeneration). A binary geothermal power plant is considered for power production while the precooled Linde–Hampson cycle is considered for hydrogen liquefaction. A liquid geothermal resource is considered and both ideal (i.e., reversible) and non-ideal (e.g., irreversible) system operations are analyzed. A procedure for such an investigation is developed and appropriate performance parameters are defined. Also, the effects of geothermal water temperature and gas precooling temperature on system performance parameters are studied. The results show that there is a significant amount of energy savings potential in the liquefaction work requirement as a result of precooling the gas in a geothermal absorption cooling system. Using geothermal energy in a cogeneration scheme (power production and absorption cooling) also provides significant advantages over the use of geothermal energy for power production only.     

Heat flow map of South America

The results of geothermal investigations carried out in South America have been compiled with the purpose of preparing regional maps of terrestrial heat flow. The compilation revealed that 655 heat flow values had been determined, giving an overall data density of 37/10⁶ km² and a representative mean heat flow of 63+-36 mW/m². The quality of the data set is variable, depending on the nature of the primary geothermal data, and the geographic distribution of the data set is also non-uniform. In spite of such difficulties a careful analysis of the data set, following suitable priority schemes, has allowed not only the determination of reliable mean heat flow values for a large number of major geological structures in South America, but also the preparation of mosaics of regional heat flow variations. Heat flow is extremely variable in the Cordilleran regions, with the eastern and southern parts having relatively high values compared to the western and northern parts. The general trend of increasing heat flow from the western coastal regions towards inland areas is interrupted by a N-S trending low heat flow belt in the Pre-Cordilleran basins. In the eastern part of the continent heat flow is low to normal (<75 mW/m²) but there are indications that in the Patagonian Platform it is higher than in the Brazilian Platform. There are, however, several isolated localities of high heat flow in the northeastern and south-central parts of Brazil. The Mesozoic rift basins (Potiguar, Recôncavo and Taubaté) are also characterized by relatively high values.In order to examine the tectonic significance of variations in the regional geothermal regime, heat flow maps have been prepared using manual and automatic contouring methods. The comparative study of automatic contour maps generated by means of a variety of data interpolation and gridding schemes has led to the identification of some geothermal features that are believed to be related to tectonic processes affecting the South American continent. Prominent among these are E-W trending belts of low heat flow in northern Peru and in central Chile (extending into the Sierras Pampeans in Argentina), as well as high heat flow belts in northern Chile (extending into the Altiplano in Bolivia) and southern Chile (extending into western Argentina). The low heat flow belts coincide approximately with zones of sub-horizontal subduction, while the high heat flow belts are situated in regions of high-angle subduction. Some of these features correlate well with the results of studies on anelastic attenuation, electrical resistivity distribution and some patterns of global seismic tomography. On the other hand, many of these features are not evident in the recent spherical harmonic analysis of global heat flow, which suggests that the use of empirical predictors based on a heat flow-age relation in devising global heat flow maps should be restricted to tectonically stable areas.     

Geothermal reinjection experience

Reinjection into geothermal reservoirs is discussed and the experience gained by reinjection experiments in 44 geothermal fields is reviewed. Reinjection started purely as a disposal method, but has more recently been recognised as an essential and important part of reservoir management. Only a small part of the thermal energy in place in geothermal reservoirs can be recovered if reinjection is not applied. Thermal breakthrough has been observed in few geothermal reservoirs but has in all cases been found to be a manageable part of field operation. Silica scaling in surface equipment and injection wells is a delicate aspect of the reinjection process in most high-temperature geothermal fields, but silica scaling in the reservoir has not been considered a problem. Reinjection of low-enthalpy geothermal fluid into sandstone has not been successful, for reasons that are poorly understood. The location of injection wells in relation to production wells influences the ratio of injected fluid recovered in production wells. For peripheral injection, about one third of the injected fluid is commonly recovered, whereas injection within the production area results in a higher ratio of recovered fluid. Subsidence is in general of small concern in geothermal operations and micro-gravity has proved a valuable tool to estimate the recharge to geothermal reservoirs.     

World geothermal power generation in the period 2001–2005

A review has been made of all the country update papers submitted to the World Geothermal Congress 2005 (WGC2005) from countries in which geothermal electricity is currently being generated. The most significant data to emerge from these papers, and from follow-up contacts with representatives of these countries, are: (1) a total of 24 countries now generate electricity from geothermal resources; (2) the total installed capacity worldwide is approximately 8930 MW_e, corresponding to about 8030 MW_e running capacity and electric energy production is nearly 57,000 GWh (early 2005 data); (3) Costa Rica, France (Guadeloupe), Iceland, Indonesia, Italy¹, Kenya, Mexico, Nicaragua, Russia, and the USA have increased the capacity of their power plant installations by more than 10% with respect to the year 2000; (4) the new members of the geothermal electricity generating community comprise Austria, Germany and Papua New Guinea; (5) the installed capacity in Argentina and Greece is now null since their geothermal power plants have been dismantled; (6) nineteen countries have carried out significant geothermal drilling operations since 2000, with 307 new wells drilled.     

Geothermal resources in Algeria

The geothermal resources in Algeria are of low-enthalpy type. Most of these geothermal resources are located in the northeastern of the country. There are more than 240 thermal springs in Algeria. Three geothermal zones have been delineated according to some geological and thermal considerations: (1) The Tlemcenian dolomites in the northwestern part of Algeria, (2) carbonate formations in the northeastern part of Algeria and (3) the sandstone Albian reservoir in the Sahara (south of Algeria). The northeastern part of Algeria is geothermally very interesting. Two conceptual geothermal models are presented, concerning the northern and southern part of Algeria. Application of gas geothermometry to northeastern Algerian gases suggests that the reservoir temperature is around 198 °C. The quartz geothermometer when applied to thermal springs gave reservoir temperature estimates of about 120 °C. The thermal waters are currently used in balneology and in a few experimental direct uses (greenhouses and space heating). The total heat discharge from the main springs and existing wells is approximately 642 MW. The total installed capacity from producing wells and thermal springs is around 900 MW.     

地热单井回灌渗流场理论研究

在地热开发过程中经常遇到如下问题：热储压力随开采量和时间的增加而降低，地热尾水排放造成热污染和资源浪费等。回灌是解决上述问题的有效方法。针对回灌过程中渗流场的动态变化，建立地热回灌渗流场数学模型，推导了渗透系数恒定与变化时不同条件下的单井回灌公式。实例分析表明：所得公式能够表征回灌渗流场中各变量之间的相互关系，可以解释和预测回灌过程中水头与流量的变化趋势。     

Influence of late cenozoic glaciations on the geothermal field in Northern and Central Europe

On the basis of the numerical solution of Stefan-like boundary-value problems the simplest palaeoclimatic and geothermal models are analyzed. Typical histories of freezing isotherms, temperature and heat flow changes are calculated taking into account the effects of accumulation and denudation. The results show that the geothermal field demonstrates a better ability to recollect palaeocryologic environment when the phase transitions of subsurface water are involved. However, the geothermal traces of past cryogenic events in Northern and Central Europe before the middle of the Pleistocene could hardly remain up to the present day.     

油区地热能综合利用方案分析——以福山油田为例*

油田区域地热资源十分丰富,并且存在大量的用热需求。在油田地区综合开发地热能,不仅可以解决油田伴热、社区供暖/制冷等问题,还可以降低环境污染,减少二氧化碳的排放,有利于油田的持续稳定发展。基于海南福山油田的地热资源条件及用能需求,将油田生产伴热、热泵尾水升温和地热驱动吸收式制冷技术相结合,设计了多种油田地热能综合利用方案,并计算比较了各个方案的经济收益,进一步分析了地热水温度和流量对各个方案适用性的影响,研究结果可为其他油田区域地热资源的综合开发利用提供参考。     

从欧洲地热发展看我国地热开发利用问题

近年来,欧洲地热资源的开发利用取得了迅速的发展。我国地热能的开发利用与欧洲相比具有诸多的相似性。因此,欧洲地热利用的发展经验对我国地热开发利用具有重要的借鉴意义。本文介绍了欧洲地热开发利用的现状和趋势,分析总结了欧洲地热利用的发展模式,并针对我国地热的开发现状和存在的问题,提出了促进我国地热利用发展的对策和建议。     

The Podhale geothermal reservoir simulation for long-term sustainable production

The Podhale geothermal system, located in the southern, mountainous part of Poland, is the most valuable reservoir of geothermal waters discovered in the country to date and the one with the highest capacities in Central and Eastern Europe. Over 20 years of continuous operation has proved its stable operating parameters – a small drop in pressure and an unnoticeable temperature change. Production of over 500 m³/h of geothermal water with an 86 °C wellhead temperature is current practise, while drilling a new production well and reconstruction of an injection well allows for production that may significantly exceed 600 m³/h. To utilize these vast resources, a binary power cycle for electricity and heat production is considered by group of researchers. The results of numerical modelling of heat extraction from the Podhale reservoir are presented in the article as a preliminary step to the detailed analysis of combined heat and power production through a binary power cycle.     

Application of the computer code TOUGH2 to the simulation of supercritical conditions in geothermal systems

At the high pressures and temperatures found in deep geothermal systems, supercritical conditions can occur. Current numerical geothermal simulators are either not capable of modelling these conditions, or can do so only at significantly reduced computation speed. This paper describes modifications to the TOUGH2 simulator to extend its applicability. It employs the updated IAPWS-97 thermodynamic formulation, and uses density and temperature as primary thermodynamic variables under supercritical conditions. Results from test problems are in agreement with results produced by other simulators, giving confidence that the simulator can be used for modelling deep geothermal reservoirs.     

Power generation from geothermal resources in Turkey

This study provides information on power generation via geothermal resources and sector development. The first instance of power generation from geothermal resources was performed by a state-owned power plant at Kızıldere-Denizli, whereas the first private sector investment was the Dora-I power plant, commissioned in 2006. Legislation regulating rights ownership and certification laws was issued in 2007. The installed capacity of the geothermal resources is 311.871 MW for 16 power plants, and power generation licenses were issued for 713.541 MW at the end of 2012. The total potential geothermal power that can be generated in Turkey is estimated to be approximately 2000 MW. The geothermal fields in Turkey produce high levels of greenhouse gases, which have been deemed highly responsible for global warming. Due to high CO₂ emissions, the geothermal energy sector risks a carbon tax in the near future. For certain geothermal resources, multiple investors produce electricity from the same resource. The sector will inevitably experience severe damage unless permanent solutions are devised for problems related to sustainably managing geothermal resources and environmental problems.     

Geothermal exploration using geoelectric methods in Kestanbol, Turkey

A self-potential survey was carried out in the Kestanbol area in order to investigate the fault zones that might be associated with geothermal activity. Two fault zones, which could be interpreted as planes electrically polarized by geothermal activity, have been detected by compilation of a self-potential map. A resistivity survey, using the Wenner configuration, was able to identify the geoelectrical structures that are caused by different rocks with variable degrees of fracturation and also affected by geothermal activity. The resistivity maps and two-dimensional geoelectric models indicated a good conductive region (<10 ohm-m) in the southwest part of Kestanbol. On the basis of the results of both methods, a new location is proposed for drilling in this conductive zone.     

Mathematical modeling for greenhouse heating by using thermal curtain and geothermal energy

A thermal model has been developed for the heating of a greenhouse by using inner thermal curtain and natural flow of geothermal warm water through the polyethylene tube laid on its floor. The calculations were done for a typical production greenhouse with the climatic data in the central part of Argentina during winter period. From the energy conservations point of view, the greenhouse has been divided into three zones i.e., zone I (plants under thermal blanket), zone II (space under ceiling) and zone III (space between roof and ceiling). The model has been tested with the published experimental data of air temperatures in zone I and zone II of the greenhouse. From the results, it was observed that the temperatures of air surrounding the plant mass in zone I were maintained in the range of 14–23 °C during winter night and early morning resulting in the better growth of winter growing plants against the harmful freezing effects. The predicted values of air temperature both in zone I and zone II of the greenhouse obtained from the proposed model exhibited fair agreement with the published experimental values.     

Assessment of geothermal potential of central and southern Tuscany

Central and southern Tuscany contain all the presently exploited geothermal fields of Italy, and accordingly were chosen to test the geothermal resource assessment methodology described by Muffler and Cataldi (this issue). Using new compilations at 1:200,000 of all available drill-hole, geological, gravity, and thermal-gradient data, the region was divided into 31 zones, each of reasonably homogenous geology and thermal regime. The upper 3 km of each area was then divided by horizontal surfaces into three volumes consisting of (1) the impermeable cover (Neoautochthon, Ligurids, and the upper terrigenous part of the Tuscan Series), (2) the Jurassic and Triassic rocks that form the main reservoir complex (the lower carbonate part of the Tuscan Series) and (3) the underlying Triassic and Paleozoic quartzite and phyllite of low porosity, thus allowing calculation of geothermal energy in both rock and pore water. The aggregate of these values is the “accessible geothermal resource base” of central and southern Tuscany.     

Geothermal fluid chemistry and human health

Several effects on the environment arising from the utilization of geothermal resources may affect human health. (1) Human surroundings: changes in noise level, local climate, and landscape. (2) Water pollution: hot water geothermal fields emit large quantities of saline fluids which contain fluoride, boron, arsenic, and minor amounts of other heavy metals. Heavy metal sulphide precipitates form from some geothermal waters and collect in river sediments. Processing of geothermal waters to remove heavy metals may be necessary. Sulphide, boron, ammonia, and mercury may also enter local waters from geothermal steam condensates. (3) Air pollution: emission of CO₂ and H₂S from geothermal power stations in some extreme cases may approach the outputs from similar-sized coal-fired stations. Sulphide emission can be controlled by chemical processing of condensate and waste gases. Mercury from geothermal steam is unlikely to cause local levels in air to rise by more than 1 ppb.The significant environmental problems from geothermal development are capable of technical solution but add significantly to the cost of geothermal power.     

Geothermal exploration of kos island, Greece: Magnetotelluric and microseismicity studies

This paper reports the results of magnetotelluric (MT) and microseismicity studies, conducted as part of a multi-disciplinary project to explore the geothermal potential of the island of Kos, Greece. The MT survey, comprising 18 soundings, was carried out in the bandwidth 128 Hz–40 s, in order to determine the deep conductivity structure in the geothermally prospective western part of the island. Rigorous dimensionality analysis has indicated that the geoelectric structure could adequately be approximated with 1-D interpretation tools. Two significant and seemingly communicating conductive zones of potential geothermal interest were found within the first 2 km. The first is extensive and shallow, detected at depths of 400–600 m; the second is deeper (1000–1300 m), but of considerably smaller lateral dimensions. A very deep relative conductor (<25 Ωm) was also detected at depths of 7–10 km, which is thought to comprise part of an old magma chamber with brine-saturated rocks. The microseismicity studies revealed the partial or total attenuation of shear waves in many microearthquake records. The analysis of these observations determined the vertical and lateral extent of that attenuation zone, the greatest part of which is located underneath the marine area between western Kos and Nissyros island to the south, extending approximately from near the surface to about 1.5 km depth. The nature of this zone is discussed in terms of fluid concentration due to the geothermal system of the area.     

Production engineering in geothermal technology: A review

Geothermal energy is abundant and renewable, but only a very small fraction can currently be converted commercially to electricity and heating value with today's technology. In recent years, the installed geothermal capacity worldwide has more than doubled. The increase in the use of geothermal energy is the result of a multi-disciplinary effort. Highlighted are some production engineering advances that have played a significant part in making geothermal a competitive renewable energy resource.