Enhanced geothermal systems (EGS) using CO2 as working fluid—A novel approach for generating renewable energy with simultaneous sequestration of carbon

Responding to the need to reduce atmospheric emissions of carbon dioxide, Brown [Brown, D., 2000. A Hot Dry Rock geothermal energy concept utilizing supercritical CO₂ instead of water. In: Proceedings of the Twenty-Fifth Workshop on Geothermal Reservoir Engineering, Stanford University, pp. 233–238] proposed a novel enhanced geothermal systems (EGS) concept that would use carbon dioxide (CO₂) instead of water as heat transmission fluid, and would achieve geologic sequestration of CO₂ as an ancillary benefit. Following up on his suggestion, we have evaluated thermophysical properties and performed numerical simulations to explore the fluid dynamics and heat transfer issues in an engineered geothermal reservoir that would be operated with CO₂. We find that CO₂ is superior to water in its ability to mine heat from hot fractured rock. Carbon dioxide also offers certain advantages with respect to wellbore hydraulics, in that its larger compressibility and expansivity as compared to water would increase buoyancy forces and would reduce the parasitic power consumption of the fluid circulation system. While the thermal and hydraulic aspects of a CO₂-EGS system look promising, major uncertainties remain with regard to chemical interactions between fluids and rocks. An EGS system running on CO₂ has sufficiently attractive features to warrant further investigation.     

深井CO2爆破致裂干热岩系统综合效应预评价

CO₂爆破致裂工艺为建造商业化EGS储层提供了新方案,有望突破深层地热能行业发展的技术瓶颈,但目前未见CO₂致裂技术用于干热岩系统的综合性能评价研究。基于此研究不足,借助数学建模定量科学评价其综合性能,解决此技术效果的不确定性和模糊性,填补深井地热系统综合评价体系的空白。考虑到深井CO₂爆破干热岩系统是一个多变、模糊、复杂的非线性系统,该文选取AHP-FCE优化数学模型,AHP对深井爆破干热岩系统复杂问题分层,构建递阶层次结构,通过Matlab将专家的定性判断定量化,进而确定FCE中各层评价指标权重大小,再通过FCE模糊关系合成原理,对多因素影响下的深井CO₂爆破致裂干热岩系统综合评分为83.36,预评价结果为较好,可考虑将CO₂爆破致裂器用于干热岩开采。评定结果可作为深井CO₂爆破致裂干热岩建设过程中的数据支撑,为地热开采工程提供一定的理论依据。     

CO2 emissions from geothermal power plants and natural geothermal activity in Iceland

The ratio of CO₂ emissions from power plants to natural emissions is a measure of the environmental impact associated with geothermal power production. Emissions from Icelandic geothermal power plants amounted to 1.6 × 10⁸ kg year⁻¹ in 2002. Two independent estimates of natural CO₂ emissions range between 1 × 10⁸ and 2 × 10⁹ kg year⁻¹. Thus, power plant emissions are significant compared to estimated total emissions (i.e., not less than 8–16%). However, direct CO₂ flux measurements from four of the approximately 40 geothermal/volcanic systems in the country amounted to 3 × 10⁸ kg year⁻¹, indicating that these estimates of the total natural flux may be too low.     

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.     

增强型地热发电技术及发展前景

介绍了增强型地热系统及其井下换热结构,分析了各种增强型地热系统的优缺点,指出了增强型地热系统在我国的发展前景,并给出了相关建议.     

Low temperature heat source for power generation: Exhaustive analysis of a carbon dioxide transcritical power cycle

The main results of a theoretical work on the use of a low temperature heat source for power generation through a carbon dioxide transcritical power cycle are reported in this paper. The procedure for analyzing the behaviour of the proposed cycle consisted in modifying the input pressure to the turbine from 66 bar, maintained constant each evaluated temperature (60 °C, 90 °C, 120 °C and 150 °C) until the net work was approximately zero. As a result, the maximum exergy efficiency was 50%, while the energy efficiencies obtained were 9.8%, 7.3%, 4.9% and 2.4% and the net specific work was 18.2 kJ/kg, 12.8 kJ/kg, 7.8 kJ/kg and 3.5 kJ/kg, respectively. Furthermore, the effect of the addition of an internal heat exchanger, which obviously supposed an increase in the efficiency, was analyzed. The analysis of the proposed system shows the viability of implementing this type of process as an energy alternative and/or strengthener of non-conventional energy sources in non-provided zones, or for increasing the energy efficiency in the industry.     

On production behavior of enhanced geothermal systems with CO2 as working fluid

Numerical simulation is used to evaluate the mass flow and heat extraction rates from enhanced geothermal injection–production systems that are operated using either CO₂ or water as heat transmission fluid. For a model system patterned after the European hot dry rock experiment at Soultz, we find significantly greater heat extraction rates for CO₂ as compared to water. The strong dependence of CO₂ mobility (=density/viscosity) upon temperature and pressure may lead to unusual production behavior, where heat extraction rates can actually increase for a time, even as the reservoir is subject to thermal depletion. We present the first ever, three-dimensional simulations of CO₂ injection–production systems. These show strong effects of gravity on the mass flow and heat extraction due to the large contrast of CO₂ density between cold injection and hot production conditions. The tendency for preferential flow of cold, dense CO₂ along the reservoir bottom can lead to premature thermal breakthrough. The problem can be avoided by producing from only a limited depth interval at the top of the reservoir.     

Thermo-economic analysis of a direct supercritical CO2 electric power generation system using geothermal heat

A comprehensive thermo-economic model combining a geothermal heat mining system and a direct supercritical CO₂ turbine expansion electric power generation system was proposed in this paper. Assisted by this integrated model, thermo-economic and optimization analyses for the key design parameters of the whole system including the geothermal well pattern and operational conditions were performed to obtain a minimal levelized cost of electricity (LCOE). Specifically, in geothermal heat extraction simulation, an integrated wellbore-reservoir system model (T2Well/ECO2N) was used to generate a database for creating a fast, predictive, and compatible geothermal heat mining model by employing a response surface methodology. A parametric study was conducted to demonstrate the impact of turbine discharge pressure, injection and production well distance, CO₂ injection flowrate, CO₂ injection temperature, and monitored production well bottom pressure on LCOE, system thermal efficiency, and capital cost. It was found that for a 100 MW_e power plant, a minimal LCOE of $0.177/kWh was achieved for a 20-year steady operation without considering CO₂ sequestration credit. In addition, when CO₂ sequestration credit is $1.00/t, an LCOE breakeven point compared to a conventional geothermal power plant is achieved and a breakpoint for generating electric power generation at no cost was achieved for a sequestration credit of $2.05/t.     

The oxygen isotope exchange between carbon dioxide and water in the Larderello geothermal field (Italy) during fluid reinjection

The oxygen isotope compositions of CO₂ and water vapor samples collected from Larderello geothermal wells after the start of the fluid reinjection program suggest that if the oxygen isotope exchange in the vapor phase does, in fact, exist, it is a very slow process when compared with the residence time of the fluids in the geothermal reservoir. This is because carbon dioxide and water vapor phases could not have equilibrated significantly in the vapor-dominated reservoir. This conclusion implies that the oxygen isotope composition of carbon dioxide may possibly be used as a tool in geothermal exploration for revealing the presence of liquid water in deep geothermal systems. Based on the interpretation of the oxygen isotope data of the CO₂, I propose that the origin of the low oxygen isotope ratios of carbon dioxide at Larderello is the high-temperature exchange with liquid water in the lower reservoir. In Larderello, the liquid water–rock interaction in the lower reservoir may have increased the ¹⁸O/¹⁶O ratio of the recharge meteoric component. By contrast, lack of high-temperature liquid water in the upper reservoir suggests that the large “δ¹⁸O shift” described for the upper-reservoir steam during the last decades reflects varying degrees of dilution of the lower-reservoir fluid by the low-¹⁸O vaporized liquid water of meteoric origin that recharges the field at shallow depth, with local contribution from still deeper high-¹⁸O water vapor of magmatic origin. The low oxygen isotope composition of the Mesozoic carbonaceous rocks that form the upper reservoir, consequently, likely represents a “fossil” record of the past hot-water geothermal stage.     

Enhanced hydrogen generation using a saline catholyte in a two chamber microbial electrolysis cell

High rates of hydrogen gas production were achieved in a two chamber microbial electrolysis cell (MEC) without a catholyte phosphate buffer by using a saline catholyte solution and a cathode constructed around a stainless steel mesh current collector. Using the non-buffered salt solution (68 mM NaCl) produced the highest current density of 131 ± 12 A/m³, hydrogen yield of 3.2 ± 0.3 mol H₂/mol acetate, and gas production rate of 1.6 ± 0.2 m³ H₂/m³·d, compared to MECs with catholytes externally sparged with CO₂ or containing a phosphate buffer. The salinity of the catholyte achieved a high solution conductivity, and therefore the electrode spacing did not appreciably affect performance. The coulombic efficiency with the cathode placed near the membrane separating the chambers was 83 ± 4%, similar to that obtained with the cathode placed more distant from the membrane (84 ± 4%). Using a carbon cloth cathode instead of the stainless steel mesh cathode did not significantly affect performance, with all reactor configurations producing similar performance in terms of total gas volume, COD removal, r_cat and overall energy recovery. These results show MEC performance can be improved by using a saline catholyte without pH control.     

A liquefied energy chain for transport and utilization of natural gas for power production with CO2 capture and storage – Part 4: Sensitivity analysis of transport pressures and benchmarking with conventional technology for gas transport

A novel energy and cost effective transport chain for stranded natural gas utilized for power production with CO₂ capture and storage is developed. It includes an offshore section, a combined gas carrier and an integrated receiving terminal. In the offshore section, natural gas (NG) is liquefied to LNG by liquid carbon dioxide (LCO₂) and liquid inert nitrogen (LIN), which are used as cold carriers. In the onshore process, the cryogenic exergy in the LNG is utilized to cool and liquefy the cold carriers, LCO₂ and LIN. The transport pressures for LNG, LIN and LCO₂ will influence the thermodynamic efficiency as well as the ship utilization; hence sensitivity analyses are performed, showing that the ship utilization for the payload will vary between 58% and 80%, and the transport chain exergy efficiency between 48% and 52%. A thermodynamically optimized process requires 319 kWh/tonne LNG. The NG lost due to power generation needed to operate the LEC processes is roughly one third of the requirement in a conventional transport chain for stranded NG gas with CO₂ capture and sequestration (CCS).     

二氧化碳的等离子体化学固定化

二氧化碳排放控制和化学利用是全世界科学家面临的难题。人们已提出利用物理和生物方法来解决二氧化碳问题,但对二氧化碳这样一个巨大的碳资源,还没有特别有效的化学利用方法。在这方面,前人曾考虑过非均相催化。但非均相催化能耗大,而氢源紧缺;以及二氧化碳本身非常稳定,需高温激活,而高温又导致催化剂寿命很有限,很难达到工业化要求。而冷等离子体,包括各种气体放电现象,其某些作用方式类似于常规催化,但在低温高效激活二氧化碳方面更有效率。本文详细讨论了等离子体二氧化碳化学固定化的各种方法,包括直接法和间接法,并比较了等离子体法与常规催化方法的区别。讨论的物系包括：Ｈ２／ＣＯ２、ＣＨ４／ＣＯ２、Ｈ２Ｏ／ＣＯ２和Ｈ２Ｓ／ＣＯ２     

Emission components characteristics of a bi-fuel vehicle at idling condition

Natural gas (NG) represents today a promising alternative to conventional fuels for road vehicles propulsion, since it is characterized by a relatively low cost, better geopolitical distribution than oil, and lower environmental impact. This explains the current spreading of compressed natural gas (CNG) fuelled spark ignition (SI) engine, above all in the bi-fuel version, which is able to run either with gasoline or with NG. However, the aim of the present investigation is to evaluate the emission characteristics at idling condition. The vehicle engine was converted to bi-fueling system from a gasoline engine, and operated separately either with gasoline or CNG. Two different fuel injection systems (i.e., multi-point injection (MPI)-sequential and closed-loop venturi-continuous) are used, and their influences on the formation of emissions at different operating conditions are examined. A detailed comparative analysis of the engine exhaust emissions using gasoline and CNG is made. The results indicate that the CNG shows low air index and lower emissions of carbon monoxide (CO), carbon dioxide (CO₂), and total hydrocarbon (THC) compared to gasoline.     

A liquefied energy chain for transport and utilization of natural gas for power production with CO2 capture and storage – Part 2: The offshore and the onshore processes

A novel energy and cost effective transport chain for stranded natural gas utilized for power production with CO₂ capture and storage is developed. It includes an offshore section, a combined gas carrier, and an integrated receiving terminal. In the offshore process, natural gas (NG) is liquefied to LNG by liquid carbon dioxide (LCO₂) and liquid inert nitrogen (LIN), which are used as cold carriers. The offshore process is self-supported with power, hot and cold utilities and can operate with little rotating equipment and without flammable refrigerants. In the onshore process, the cryogenic exergy in LNG is used to cool and liquefy the cold carriers, which reduces the power requirement to 319 kWh/tonne LNG. Pinch and exergy analyses are used to determine thermodynamically optimized offshore and onshore processes with exergy efficiencies of 87% and 71%, respectively. There are very low emissions from the processes. The estimated specific costs for the offshore and onshore process are 8.0 and 14.6 EUR per tonne LNG, respectively, excluding energy costs. With an electricity price of 100 EUR per MWh, the specific cost of energy in the onshore process is 31.9 EUR per tonne LNG.     

Time-dependent optimization of a large size hydrogen generation plant using “spilled” water at Itaipu 14 GW hydraulic plant

In this paper hydrogen generation and storage systems optimization, related to a very large size hydraulic plant (Itaipu, 14 GW) in South America, is investigated using an original multilevel thermo-economic optimization approach developed by the Authors. Hydrogen is produced by water electrolysis employing time-dependent hydraulic energy related to the water which is not normally used by the plant, named “spilled water”.     

Opimal design and 4E evaluation of a CCHP system using an improved version of weighted mean of vectors algorithm

Using CCHP systems (combined cooling, heating, and power) as an environmentally friendly and economical procedure of energy generation has taken a step towards cost reduction and proper management of energy sources in the contemporary era, due to the fact that the air pollution, energy crisis as well as its expensive supply has become an overarching problem. In this study, a novel method for the design optimization of a CCHP system is represented. The gas turbine used as the study's primary engine is intended for a rural setting in Guilin, China. Energetic, exergetic, environmental, and economic characteristics are the four key metrics used to evaluate the method's success. For higher accuracy and convergence, a new improved version of Weighted mean of vectors algorithm (IINFO) is used. In order to demonstrate the method's increased efficiency, the accomplishments of the recommended IINFO-based methodology are compared with those of the genetic and modified owl optimizers.