Classification of Geothermal Energy Resources in Japan Applying Exergy Concept

Higher demand for energy consumption and importance of environmental issues has encouraged researchers and policy makers to consider renewable energies more seriously. Geothermal resources are a green energy source that can make a considerable contribution in some countries. Japan has the third ranking geothermal energy potential, and its geothermal electricity production is currently eighth in the world. Since the nature of geothermal resources dictates its method of utilization, it is important to categorize available resources. There is no consensus on classification of geothermal resources. Most scientists, from geologist to engineers, agree on the term temperature. However, temperature or enthalpy alone cannot describe the nature of fluids; they can have same temperature with different phases, such as saturated water or saturated steam. Using exergy for resource classification benefits their comparison, according to their ability to do work. In this paper, exergetic classification of geothermal resources was applied to 18 under‐operating geothermal power plants in Japan. Six geothermal fields have high exergy resources according to their SExI values in excess of 0.5. The remaining geothermal fields in Japan are classified in the medium resources zone. Classification results can be used by decision makers as a reference for future geothermal development. Copyright © 2012 John Wiley & Sons, Ltd.     

Classification of geothermal resources in Poland by exergy analysis—Comparative study

Geothermal resources in Poland are of growing importance for the production of renewable energy. The total installed geothermal capacity (including heat pumps) at the end of 2008 was ca. 281 MW_t, while heat sales about 1501 TJ. Poland is characterised by low-temperature geothermal resources connected mostly with the Mesozoic sedimentary formations. In the paper the estimation of thermodynamic potential of Polish geothermal fields in comparison with selected global resources was presented. Geothermal resources were classified with reference to their specific exergy and specific exergy index (SEI). These indices define the quality of the energy content of a geothermal fluid better than conventional temperature criterions.     

Research on the generation of electricity from the geothermal resources in Simav region, Turkey

One of the greatest problems in using renewable energy sources is the great variability of energy level, both in the short and long term. Geothermal energy, by nature, has high availability because the source is not dependent on weather conditions, so it is among the most stable renewable energy sources. Geothermal energy has the potential to play an important role in the future energy supply of Turkey. Although Turkey has the second-highest geothermal energy potential in Europe, electricity generation from geothermal energy is rather low.This study examines the use of geothermal energy in electricity generation and investigates the applicability of the existent geothermal energy resources to electricity generation in the Kütahya–Simav region, Turkey. The binary cycle is used in the designed power plant for electricity generation from geothermal fluid in which the percentage of liquid is high and which is at lower temperature. In this power plant, R134a is chosen as the secondary fluid, whose boiling point temperature is lower than that of water, and is used instead of geothermal fluid in a second cycle. The thermal efficiency of the designed power plant is measured to be 12.93%.     

青海省共和县地下热水水文地球化学特征研究

以青海省共和县地下热水为研究对象,对所取得的水样进行水化学以及氢氧同位素测试,并分析测试结果。结果表明：共和地区地下热水水化学类型以Cl-Na型为主;地下热水受到浅表层氧化作用的影响较大,所处环境的封闭性较差,有浅层地下水或地表水的混合;地下热水受大气降雨补给;地下热水热源和水源不是同一来源;地下热水未达到水岩平衡状态,估算出地下热水热储温度在37.59~108.22 ℃之间,循环深度为500~2000 m,属于中低温地热系统。     

Environmental and Exergetic Aspects of Geothermal Energy

Geothermal energy is already in the form of heat, and from the thermodynamic point of view, work is more useful than heat because not all heat can be converted to work. Therefore, geothermal resources should be classified according to their exergy, which is a measure of their ability to do work. In recent years there has been a remarkable growth of interest in environmental issues—sustainability and improved management of development in harmony with the environment. Environmental impact assessment is one of the most widely used tools in environmental management. In this study, the environmental and exergetic aspects of geothermal energy, namely the rapid impact assessment matrix method, and, specific exergy index, were studied first. They were then applied to the Tuzla geothermal field in Canakkale and Balcova geothermal field in Izmir, Turkey, respectively. Finally, the results obtained are given and discussed.     

Geothermal‐based hydrogen production using thermochemical and hybrid cycles: A review and analysis

Geothermal‐based hydrogen production, which basically uses geothermal energy for hydrogen production, appears to be an environmentally conscious and sustainable option for the countries with abundant geothermal energy resources. In this study, four potential methods are identified and proposed for geothermal‐based hydrogen production, namely: (i) direct production of hydrogen from the geothermal steam, (ii) through conventional water electrolysis using the electricity generated through geothermal power plant, (iii) by using both geothermal heat and electricity for high temperature steam electrolysis and/or hybrid processes, and (iv) by using the heat available from geothermal resource in thermochemical processes. Nowadays, most researches are focused on high‐temperature electrolysis and thermochemical processes. Here we essentially discuss some potential low‐temperature thermochemical and hybrid cycles for geothermal‐based hydrogen production, due to their wider practicality, and examine them as a sustainable option for hydrogen production using geothermal heat. We also assess their thermodynamic performance through energy and exergy efficiencies. The results show that these cycles have good potential and attractive overall system efficiencies over 50% based on a complete reaction approach. The copper‐chlorine cycle is identified as a highly promising cycle for geothermal‐hydrogen production. Copyright © 2009 John Wiley & Sons, Ltd.     

Simulation of propagating fronts in geothermal reservoirs with the implicit Leonard total variation diminishing scheme

Geothermal reservoir engineering requires accurate numerical solution of the advective–diffusive transport equations for strong advective flows of multiphase nonisothermal fluids. Conventional interface weighting schemes such as upstream weighting cause numerical dispersion. Numerical dispersion can be reduced by grid refinement, but this increases execution times and computer memory requirements. As an alternative, higher-order differencing schemes can be used to reduce numerical dispersion, but they often lead to spurious oscillations. These limitations have led to the development of higher-order schemes called total variation diminishing (TVD) schemes. For geothermal reservoir engineering, these schemes must be capable of handling flows that may not be physically total variation diminishing. We have implemented TVD schemes into the implicit geothermal reservoir simulator TOUGH2. We verify the Leonard TVD (LTVD) scheme by comparison to an analytical solution for two-dimensional flow and transport. The LTVD scheme reduces numerical dispersion for tracer transport in a two-phase geothermal reinjection problem. One-dimensional simulations show that the LTVD scheme works well even if the saturation variation increases with time. Because the location of the phase front is strongly coupled to temperature, phase front propagation is sensitive to grid resolution insofar as it affects the temperature field. Phase front propagation in a composite porous medium Buckley–Leverett flow problem, where phase saturations increase upon encountering a second medium, are slightly more accurate for the LTVD scheme as compared to upstream weighting. We find that the LTVD scheme only performs well if the weighting and limiter are applied to saturation rather than to relative permeability. While there is some increased computational cost with the LTVD scheme due to increased linear equation solution time and smaller time-step size, the LTVD scheme is a practical and robust method for reducing numerical dispersion in complex flow problems relevant to geothermal reservoir engineering.     

Influence of water–rock interactions on fracture permeability of the deep reservoir at Soultz-sous-Forêts, France

Circulation of geothermal fluids through granitic fractured reservoirs leads to chemical reactions, modifying the porosity and permeability of the rock mass. FRACHEM, a thermo-hydraulic-chemical coupled computer code, was developed specifically to predict changes in the geothermal reservoir of the Soultz-sous-Forêts Enhanced Geothermal System (EGS) located in Alsace, France. This code can simulate fluid–rock interactions and determine the dissolution/precipitation reactions of eight minerals in the Soultz granite (i.e. carbonates, pyrite, silicates and aluminosilicates). Numerical simulation results of long-term fluid circulation through the 5000-m deep Soultz reservoir are comparable to those determined for the shallow reservoir and confirm the role played by carbonates in the evolution of reservoir porosity and permeability. Moreover, experiments with FRACHEM in simulating short-term fluid flow during hydraulic and/or chemical stimulations have demonstrated that the code could prove an efficient tool in reservoir engineering and management.     

The precipitation of solids from potential heat exchange fluids for use in a hdr geothermal system in granite

An important consideration in the development and operation of a Hot Dry Rock geothermal system is the selection of a heat transfer fluid and the chemical composition of this fluid during circulation. The chemical reaction of the circulation fluid with the reservoir rock may lead to the undesirable corrosion or scaling of the reservoir itself, or associated engineering structures. Two potential circulation fluids for use in a high temperature (200°C) HDR system in granite in SW England, are a dilute (TDS < 120mg/1) groundwater, and a modified seawater composition. The reaction of these fluids with granite has been evaluated experimentally, with particular emphasis upon the characterisation of solid precipitates. Secondary solids associated with the reaction of groundwater with granite consist of clay and Ca-zeolite. Product fluids were alkaline (pH 9.1), of low salinity (TDS < 600mg/1) and were relatively benign for heat exchange purposes. The amount of clay precipitated may be linked to the amount of Mg in the fluid, but is less than 0.5 wt percent of the initial solid starting material. Chemical analysis of precipitated clay by analytical transmission electron microscopy reveal a range of composition between illite and smectite. Secondary solids associated with seawater-granite reaction include anhydrite, magnesium hydroxide sulphate hydrate (MHSH) and clay. The precipitation of MHSH and clay is instrumental in governing the low pH (pH 3.5) of the product fluid, which would pose problems concerning the corrosion of pumps, heat exchangers, etc in a possible HDR geothermal system. The suitability of each of the potential heat exchange fluids may be linked to their initial Mg contents which govern the acidity of the reacted fluid and the amount of precipitated clay.     

Geology and fluid chemistry of the Fushime geothermal field, Kyushu, Japan

The Fushime geothermal field is located in a depression close to the coast line. The system is characterized by very high reservoir temperature (>350°C), and a high salinity production fluid. Geological analysis shows that the main reservoir in this field occurs in a fractured zone developed around a dacite intrusion located in the center of the field. High permeability zones recognized by drilling data are found to be associated with fault zones. One of these zones is clearly associated with a NW–SE trending andesite dike swarm which was encountered in some wells.Alteration in the system can be divided into four zones, in order of increasing temperature, based on calcium–magnesium aluminosilicate mineral assemblages: i.e., the smectite, transition, chlorite and epidote zones. The feed zone is located in the chlorite and epidote zones, which can be further divided into three sub-zones according to their potassium or sodium aluminosilicate mineralogy, from the center of the discharge zone: K-feldspar–quartz, sericite–quartz, and albite–chlorite zones.Chloride concentration of the sea-water is 19,800 mg/l, and Br/Cl mole ratio is 1.55. Based on geochemical information, the reservoir chloride concentration of this field ranges from 11,600 to 22,000 mg/kg. The Clres (Cl in reservoir), Br/Cl ratios and stable isotope data indicate that the Fushime geothermal fluid originated from sea-water and is diluted by ground water during its ascent. Some fluids produced from geothermal wells show low pH (about 4). It is thought that sulfide mineral (PbS, ZnS) precipitation during production produces this acidic fluid.     

Correlations between steam saturation, fluid composition and well decline in vapor-dominated reservoirs

A large body of field data from Larderello and other vapor-dominated geothermal reservoirs shows striking temporal correlations between (1) decline of well flow rate, (2) gas/steam ratio, (3) chloride concentration, (4) degree of superheat and (5) vapor fraction. The latter is inferred from concentrations of non-condensible gases in samples of well fluid, using chemical phase equilibrium principles. Observed temporal changes in the vapor fraction can be interpreted in terms of a “multiple source” model, as suggested by D'Amore and Truesdell (1979, Models for steam chemistry at Larderello and The Geysers. Proc. 5th Workshop Geothermal Reserv. Engng, Stamford, California, pp. 283–297). This provides clues to the dynamics of reservoir depletion and to the evaluation of well productivity and longevity.     

Multiple integrated applications for low-to medium-temperature geothermal resources in Iceland

There is an increasing global demand for a faster, more expansive development in the energy sector, in order to improve the standard of living of the world's population by the creation of more jobs and better living conditions. The public is, however, well aware of the damage that has been done to the environment, in the form of deforestation, despoiling of lakes and rivers and, in particular, greenhouse effects, and it is unwilling to further sacrifice its natural environment. This decision puts pressure on scientists, engineers and developers to find ways and means of attaining “sustainable energy development”. In other words, the challenge now is to achieve the sustainable development of alternative renewable energy resources. Sustainability may be achieved in a number of ways, but the one most likely to result in a rapid increase in energy output without a deleterious impact on the environment is the revamping and integration of what we already have. This paper attempts to address sustainability as it applies to geothermal energy. We describe the concept of a multiple integrated use of geothermal energy, including the tenable benefits that can be obtained from applying this concept, such as a longer reservoir lifespan, a lower specific environmental impact, and greater marketing flexibility and profitability. The paper also emphasises the importance of achieving a maximum effective temperature drop across the application, commensurate with a minimum flow rate, optimal pumping characteristics and minimal fluid extraction from the geothermal reservoir. In geothermal house heating systems this means using large and effective radiators, dual-pipe heating systems, and thermostatic controls on each radiator. Where modifications to existing house heating systems are not feasible, e.g. by conversion from a single-pipe to a dual-pipe system or installation of larger radiators, an alternative solution is to adopt a cascaded flow of the geothermal fluid through a combination of heating systems operating at different temperature levels. For economic reasons it is always better to use the geothermal water directly if its chemical quality permits us to do so, otherwise heat exchangers made of resistant materials will be needed to isolate the geothermal fluid from the heating fluid in order to avoid corrosion or scaling in the pipes and radiators. The heat exchangers should be designed in such a way as to obtain a maximum temperature drop of the geothermal fluid. The paper also describes some heating system configurations, the characteristics of geothermal heating systems and their automatic control systems, as well as recommended geothermal field management and monitoring systems. The paper also includes a few examples of existing projects to demonstrate what has already been achieved and what could be done in the future; some suggestions are also made for new developments and innovations to make geothermal energy more generally attractive and useful worldwide.     

西藏羊易EGS开发储层温度场与开采寿命影响因素数值模拟研究

西藏羊易地区具有丰富的地热能,单井开发潜力接近10 MW,对其深部热储进行EGS开采,可缓解西部能源紧缺问题。本文建立二维理想EGS开发模型,探讨深层地热开采过程中开采流量、注采方式、注入温度等参数对热储温度场分布及开采寿命的影响。基于羊易温度信息设计了12个数值模型,对比研究发现,开采流量对EGS开采的影响较大,为保证开采50年内的商业利用价值,最大开采流量应控制在0.028 kg/s以下;考虑到钻井成本,注采方式的选择以高注高采和中注高采为最佳;注入温度对热储开采影响较小,可选择40℃ ~ 80℃之间任意温度的地热尾水进行回灌,实现地热资源梯级利用。     

Chemical equilibria in icelandic geothermal systems—Implications for chemical geothermometry investigations

Chemical geothermometry represents the most important tool for estimating reservoir temperatures in the exploration of geothermal resources. Chemical equilibria between alteration minerals and solution are generally attained in geothermal systems for all major components except chloride. For the interpretation of analyses of natural waters involving geothermometry major emphasis should be placed on assessing the overall water composition with respect to mineral equilibria, rather than attempting to distinguish geothermal waters from shallow waters by a classification involving the relative abundance of major anions and major cations. Generally, cold waters may be distinguished from geothermal waters by low chloride (< 10 ppm), in conjunction with relatively low pH (6–7) and low Na/K ratios (same as the associated rock), calcite undersaturation and low √Ca²⁺ H⁺ activity ratios.     

Life cycle assessment of geothermal binary power plants using enhanced low-temperature reservoirs

Geothermal binary power plants that use low-temperature heat sources have gained increasing interest in the recent years due to political efforts to reduce greenhouse gas emissions and the consumption of finite energy resources. The construction of such plants requires large amounts of energy and material. Hence, the question arises if geothermal binary power plants are also environmentally promising from a cradle-to-grave point of view. In this context, a comprehensive Life Cycle Analysis (LCA) on geothermal power production from EGS (enhanced geothermal systems) low-temperature reservoirs is performed. The results of the analysis show that the environmental impacts are very much influenced by the geological conditions that can be obtained at a specific site. At sites with (above-) average geological conditions, geothermal binary power generation can significantly contribute to more sustainable power supply. At sites with less favorable conditions, only certain plant designs can make up for the energy and material input to lock up the geothermal reservoir by the provided energy. The main aspects of environmentally sound plants are enhancement of the reservoir productivity, reliable design of the deep wells and an efficient utilization of the geothermal fluid for net power and district heat production.     

地热资源的开发利用及可持续发展

地热资源作为一种新型能源矿产,具有分布广泛、易于开发等特点,其利用方式主要有地热发电和地热直接利用两种.我国具有良好的地热资源条件,主要为中低温地热资源.据计算,我国12个主要沉积盆地的地热可开采资源量为7500×1018J,相当于2560×108t标煤.当前,我国地热资源利用方式主要以供暖、洗浴、种植等直接利用为主;地热发电发展缓慢,主要分布在西藏;利用热泵技术开发地热资源得到了快速发展;油区地热资源的开发利用也取得了良好的经济和社会效益.但同时我国地热资源产业也面临着一些问题,包括大部分地区尚未开展地热资源勘查评价,影响了地热资源规划的制订及地热产业的发展;防腐、防垢技术还需要进一步加强研究;地热回灌率普遍过低;增强型地热系统研究有待加强等.为了促进地热资源的可持续发展,建议在加大地热资源勘查力度的同时,应以浅层地温能和热水型地热资源为主,发挥热泵技术的优势,开展地热资源的综合利用及梯级利用;重视和加快油气区地热资源的利用;在西藏等适宜地区加大高温地热能发电利用;集中全国优势技术力量,在一两个有利区域开展增强型地热系统技术探索;此外,走回灌开发道路是地热资源开发利用的必然选择.     

四川地热流体水文地球化学及同位素特征简析

四川省范围内有沉积盆地型和隆起山地型两类地热资源,共划分五个地热区。从五个区采取的205组水样的水文地球化学特征及184组2H、18O和61组14C特征分析显示,四川省各地热区地热流体基本来自于大气降水补给,地热流体的水文地球化学和同位素特征与其所属的热储类型和热储开放性有关。盆地型热储主要为岩溶层状热储,山地型热储主要是变质岩为主的裂隙带状热储和层状带状复合型热储。盆地型热储开放性较山地型弱,地热流体矿化度和理疗元素含量均比山地型高,易形成深埋藏的卤水,地下水平均径流时间较山地型长。本研究可为四川省地热资源未来的开发利用规划提供参考。     

The feasibility of using geothermal energy in hydrogen production

A feasibility study exploring the use of geothermal energy in hydrogen production is presented. It is possible to use a thermal energy to supply heat for high temperature electrolysis and thereby substitute a part of the relatively expensive electricity needed. A newly developed HOT ELLY high temperature steam electrolysis process operates at 800 – 1000°C. Geothermal fluid is used to heat fresh water up to 200°C steam. The steam is further heated to 900°C by utilising heat produced within the electrolyser. The electrical power of this process is reduced from 4.6 kWh per normalised cubic meter of hydrogen (kWh/Nm³ H₂) for conventional process to 3.2 kWh/Nm³ H₂ for the HOT ELLY process implying electrical energy reduction of 29.5%. The geothermal energy needed in the process is 0.5 kWh/Nm³ H₂. Price of geothermal energy is approximately 8–10% of electrical energy and therefore a substantial reduction of production cost of hydrogen can be achieved this way. It will be shown that using HOT ELLY process with geothermal steam at 200°C reduces the production cost by approximately 19%.     

A comparison of economic evaluation models as applied to geothermal energy technology

Several cost estimation and financial cash flow models have been applied to a series of geothermal case studies. In order to draw conclusions about relative performance and applicability of these models to geothermal projects, the consistency of results was assessed. The model outputs of principal interest in this study were net present value, internal rate of return, or levelized breakeven price. The models used were VENVAL, a DuPont, Inc. venture analysis model; the Geothermal Probabilistic Cost Model (GPC Model) and the Alternative Power Systems Economic Analysis Model (APSEAM), which were developed at the Jet Propulsion Laboratory (JPL); the MITRE Corporation's Geothermal Loan Guarantee Cash Flow Model (GCFM); and the GEOCOST and GEOCITY geothermal models developed by Battelle Pacific Northwest Laboratories. The case studies to which the models were applied include a geothermal reservoir at Heber, CA; a geothermal electric power plant to be located at the Heber site; an alcohol fuels production facility to be built at Raft River, ID; and a direct-use, district heating system in Susanville, CA.