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.     

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.     

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.     

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.     

Observations of hydraulic stimulations in seven enhanced geothermal system projects

Numerous stimulation tests have been performed on Enhanced Geothermal System (EGS) or Hot Dry Rock (HDR) projects during the past three decades, however, there is much room for improvement in our knowledge and understanding of the mechanisms of stimulation. This paper investigated the hydraulic stimulation tests carried out on seven EGS or HDR projects where massive volume of fluid was injected into the long open section of the well with interval of tens to hundreds of meters in the crystalline formation. The key characteristic test and performance parameters were defined and collected through extensive survey of stimulation results. Attempts were made to carry out comparative analysis on reservoir conditions, test parameters and test observations. The analysis and discussion suggest that 1) the reservoir stress regime impacts the growth of stimulated region and the reverse faulting stress regime can be favorable for the layout of multiple well system as it may lead to a horizontally or sub-horizontally oriented stimulated zone; 2) the injection pressure for activating shear slip and the associated onset of seismicity is mainly field stress controlled; 3) there is strong dependency of injectivity on injection pressure and a high pressure makes a better hydraulic injectivity during stimulation and consequently afterwards for circulation; 4) the stimulated region and number of induced seismic events are mainly injection volume controlled and the potential strategy to reduce seismic risks is either to extend stimulation in time or to separate stimulation in space; and 5) the differential stress condition is one of the necessary factors to raise a large magnitude event (LME) and the difference of maximum injection pressure achieved over that at onset of seismicity is an important additional factor to induce LMEs.     

Metal-organic frameworks for CO2 photoreduction

Metal-organic frameworks (MOFs) have attracted much attention because of their large surface areas, tunable structures, and potential applications in many areas. In recent years, MOFs have shown much promise in CO₂ photoreduction. This review summarized recent research progresses in MOF-based photocatalysts for photocatalytic reduction of CO₂. Besides, it discussed strategies in rational design of MOF-based photocatalysts (functionalized pristine MOFs, MOF-photosensitizer, MOF-semiconductor, MOF-metal, and MOF-carbon materials composites) with enhanced performance on CO₂ reduction. Moreover, it explored challenges and outlook on using MOF-based photocatalysts for CO₂ reduction.     

Influence of ambient temperatures on performance of a CO2 heat pump water heating system

In residential applications, an air-to-water CO₂ heat pump is used in combination with a domestic hot water storage tank, and the performance of this system is affected significantly not only by instantaneous ambient air and city water temperatures but also by hourly changes of domestic hot water consumption and temperature distribution in the storage tank. In this paper, the performance of a CO₂ heat pump water heating system is analyzed by numerical simulation. A simulation model is created based on thermodynamic equations, and the values of model parameters are estimated based on measured data for existing devices. The calculated performance is compared with the measured one, and the simulation model is validated. The system performance is clarified in consideration of seasonal changes of ambient air and city water temperatures.     

A comparison of electricity and hydrogen production systems with CO2 capture and storage—Part B: Chain analysis of promising CCS options

Promising electricity and hydrogen production chains with CO₂ capture, transport and storage (CCS) and energy carrier transmission, distribution and end-use are analysed to assess (avoided) CO₂ emissions, energy production costs and CO₂ mitigation costs. For electricity chains, the performance is dominated by the impact of CO₂ capture, increasing electricity production costs with 10–40% up to 4.5–6.5 €ct/kWh. CO₂ transport and storage in depleted gas fields or aquifers typically add another 0.1–1 €ct/kWh for transport distances between 0 and 200 km. The impact of CCS on hydrogen costs is small. Production and supply costs range from circa 8 €/GJ for the minimal infrastructure variant in which hydrogen is delivered to CHP units, up to 20 €/GJ for supply to households. Hydrogen costs for the transport sector are between 14 and 16 €/GJ for advanced large-scale coal gasification units and reformers, and over 20 €/GJ for decentralised membrane reformers. Although the CO₂ price required to induce CCS in hydrogen production is low in comparison to most electricity production options, electricity production with CCS generally deserves preference as CO₂ mitigation option. Replacing natural gas or gasoline for hydrogen produced with CCS results in mitigation costs over 100 €/t CO₂, whereas CO₂ in the power sector could be reduced for costs below 60 €/t CO₂ avoided.     

Performance analysis of a CO2 heat pump water heating system under a daily change in a standardized demand

Air-to-water heat pumps using CO₂ as a natural refrigerant have been developed and commercialized. They are expected to contribute to energy saving in residential hot water supply. The objective of the research is to analyze the performance of a water heating system composed of a CO₂ heat pump and a hot water storage tank by numerical simulation. In this paper, the system performance is analyzed under a daily change in a standardized hot water demand, and some features of the temperature distribution in the storage tank and the system performance criteria such as coefficient of performance, storage and system efficiencies, and volumes of stored and unused hot water are investigated. It turns out that the daily change in the hot water demand does not significantly affect the daily averages of the COP, and storage and system efficiencies, while it significantly affects not only the daily change in the volume of hot water unused after the tapping mode, but also that in the volume of hot water stored after the charging mode. The influence of the daily change in the hot water demand on the volumes of stored and unused hot water is clarified quantitatively.     

A three-dimensional thermo-poroelastic model for fracture response to injection/extraction in enhanced geothermal systems

Water injection in enhanced geothermal systems sets in motion coupled poro-thermo-chemo-mechanical processes that impact the reservoir dynamics and productivity. The variation of injectivity with time and the phenomenon of induced seismicity can be attributed to the interactions between these processes. In this paper, a three-dimensional transient numerical model is developed and used to simulate fluid injection into geothermal reservoirs. The approach couples fracture flow and heat transport to thermo-poroelastic deformation of the rock matrix via the displacement discontinuity (DD) method. The use of the boundary integral equations, for the pressure diffusion and heat conduction in the rock matrix, eliminates the need to discretize the infinite reservoir domain. The system of linear algebraic equations for the unknown displacement discontinuities, and fluid and heat sources are used in a finite element formulation for the fluid flow and heat transport in the fracture. This yields a system of equations which are solved to obtain the temperature, pressure, and aperture distributions within the fracture at every time step. In this way, the temporal variation of the fracture aperture and fluid pressure, caused by pressurization and thermo-poroelastic stresses, are determined. Numerical experiments using the model illustrate the feed-back between matrix dilation, shrinkage, and pressure in the fracture. It is observed that whereas the poroelastic effects dominate the early stage of injection pressure profile and the fracture aperture evolution, thermoelastic effects become dominant for large injection times.     

Combination of double and single cyclic pressure alternation technique to increase CO2 sequestration with heat mining in enhanced geothermal reservoirs by thermo-hydro-mechanical coupling method

The rise in global temperature due to an unceasingly increase in non-condensable gases (NCG) prompts more development of safe and economical CCUS (Carbon Capture Utilization and Storage) technologies. Carbon dioxide (CO₂) sequestration with heat mining in deep enhanced geothermal systems (EGSs) is one of the promising methods to reduce CO₂ emitted to the atmosphere. In this study, a cyclic alternation of pressures at the injection and production wells is applied in an EGS for heat mining together with CO₂ deposit. Simultaneous alternation of the injection and production pressures can significantly increase the amount of CO₂ sequestrated compared to applying a fixed pumping or withdrawing pressures at the injector and producer respectively. At the injection well, alternation in pumping pressures at high frequency (small interval of days) increased CO₂ sequestration rate. Reducing the pumping frequency resulted in the lowering of the total amount of CO₂ sequestrated, lesser than using a fixed pumping pressure. The alternation in pumping frequency has a direct relationship to the CO₂ sequestration rate. The frequency of the injection and production pressures refers to the interval in days of the interchange in pressure between high to a low value and vice-versa. Furthermore, simultaneous alternation of pressures at the injection and production wells respectively (double cyclic method) improved geothermal heat extraction efficiency, thus higher performance for both geothermal and CO₂ sequestration projects.     

Industrial scale modelling of the thermochemical energy storage system based on CO2 + 2NH3 ↔ NH2COONH4 equilibrium

The thermochemical storage of energy by the system carbon dioxide, ammonia and ammonium carbamate is studied in detail. In particular, the kinetics and the thermodynamics of the reversible reaction is studied. We give two industrial models for the operation of this system. In the first, the separation of the gases NH₃ and CO₂ is achieved by compression and liquefaction of NH₃. In the second, a method of separation of the gases is proposed which is based on the solubility of NH₃ in ethanol while CO₂ is practically insoluble. The operation of this system is examined both in closed form and in the case in which CO₂ is rejected in the atmosphere, and it is taken from alcoholic fermentation or from the combustion gases of power plants burning lignite. The mass and energy balance is given, for each case, and the amount of energy losses by the use of this storage system is calculated. Finally, we give some estimates for the area of solar collectors and the amount of chemicals which are required in order to cover the energy needs of a community.     

A review of physical modelling and numerical simulation of long-term geological storage of CO2

Numerical simulations are essential to the understanding of the long-term geological storage of CO₂. Physical modelling of geological storage of CO₂ has been based on Darcy’s law, together with the equations of conservation of mass and energy. Modelling and simulations can be used to predict where CO₂ is likely to flow, to interpret the volume and spatial distribution of CO₂ under storage conditions, and to optimise injection operations. The state of the art of physical modelling and numerical simulation of CO₂ dispersion is briefly reviewed in this paper, which calls for more accurate and more efficient modelling approaches. A systematic evaluation of the numerical methods used and a comparison between the streamline based methods and the grid based methods would be valuable. Multi-scale modelling may prove to be of great value in predicting the long-term geological storage of CO₂, while highly accurate numerical methods such as high-order schemes may be employed in numerical simulations of CO₂ dispersion for local transport calculations.     

Numerical study on melting of paraffin wax with Al2O3 in a square enclosure

The melting of paraffin wax dispersed with Al₂O₃ that is heated from one side of a square enclosure with dimensions of 25 mm × 25 mm is investigated numerically. The stream function, isotherms and liquid–solid interface at different stages of the melting process are presented and discussed. The effect of orientation of the heating surfaces of a square cavity and the volumetric concentration of Al₂O₃ in paraffin wax on the melting performance of the latent heat storage system is analysed.     

Multiobjective optimisation of integrated energy systems for remote communities considering economics and CO2 emissions

The methods for designing, planning and managing integrated energy systems, while holistically considering the major economic and environmental factors, are still embryonic. However, the first phase of the design is often crucial if we want to manage resources better and reduce energy consumption and pollution. Considering integrated energy systems implies dealing with complex systems in which the synergy between the various components is best exploited (for example the thermal energy of a diesel engine produced during the night is complimented by the Rankine organic cycle of a solar thermal plant). The context of isolated communities further increases the difficulties when considering the long distance of transport required to supply fossil fuels. These sites are often located in very precarious environments, with limited or nonexistent resources except for solar energy, and with frequent additional needs for desalination (in arid zones).This paper illustrates a holistic method to rationalize the design of energy integrated systems. It is based on a superstructure (collection of models of all envisaged technologies) and a multi-objective optimisation (resources, demand, energy, emission, costs) using an evolutionary algorithm. The approach proposed allows the identification of more complete and more coherent integrated configurations characterizing the most promising designs (also taking into account the time dependency aspects). It also allows to better structure the information in view of a participative decision approach. The study shows that the economic implementation of renewable energy (solar) is even more difficult, compared to diesel based solutions, in cases of isolated communities with high load variations. New infrastructure or retrofit cases are considered.     

单位电量CO_2排放强度与单位产值CO_2排放强度关系初探

电力单位产值CO2排放强度是全国单位产值CO2排放强度的重要组成部分,其既与电力行业排放绩效即单位发电量CO2排放强度有关,也与全社会能源效率、电气化水平、资源与市场情况等因素相关。本文系统分析了有关指标的统计特征、国际对比、影响因素及趋势预测,指出"十二五"及"十三五"期间,不论是单位电量CO2排放强度,还是电力单位产值CO2排放强度,其下降幅度都是有限的,需要理性看待并科学设置规划指标。     

CO2单井增强地热系统取热性能研究

为提高闭式单井系统取热性能,提出一种CO2单井增强地热系统（CO2-SEGS）。建立井筒流动换热和储层热-流-固耦合的数学模型,考虑CO2可压缩性以及井纵向压力传递特性,对比分析了水和CO2在SEGS中的取热性能,研究系统取热性能与封隔间距、井筒保温的关系。结果表明：（1）额定循环流量下,井口生产温度从134.09℃降低至116.06℃;CO2在采出过程中降压膨胀做功,产生明显的温降效应,中心管井口温度比底部低约57℃。（2）井筒不同位置处CO2的密度、热容差异很大,当循环流量小于50 kg/s时,依靠浮升力作用,SEGS可实现自主循环运行,无需额外泵功。（3）当水和CO2的流量分别为15 kg/s和40 kg/s时,两者年均取热速率近似相等,CO2的采出温度比水低约40℃,而压力损耗远小于水。（4）SEGS取热性能与封隔间距以及中心管保温性能呈正相关。研究结果可为SEGS系统的开发提供参考。     

Evaluation of potential cost reductions from improved amine-based CO2 capture systems

Technological innovations in CO₂ capture and storage technologies are being pursued worldwide under a variety of private and government-sponsored R&D programs. While much of this R&D is directed at novel concepts and potential breakthrough technologies, there are also substantial efforts to improve CO₂ capture technologies already in use. In this paper, we focus on amine-based CO₂ capture systems for power plants and other combustion-based applications. The current performance and cost of such systems have been documented in several recent studies. In this paper we examine the potential for future cost reductions that may result from continued process development. We used the formal methods of expert elicitation to understand what experts in this field believe about possible improvements in some of the key underlying parameters that govern the performance and cost of this technology. A dozen leading experts from North America, Europe and Asia participated in this study, providing their probabilistic judgments via a detailed questionnaire coupled with individual interviews. Judgments about detailed technical parameters were then used in an integrated power plant modeling framework (IECM-CS) developed for USDOE to evaluate the performance and costs of alternative carbon capture and sequestration technologies for fossil-fueled power plants. The experts’ responses have allowed us to build a picture of how the overall performance and cost of amine-based systems might improve over the next decade or two. Results show how much the cost of CO₂ capture could be reduced via targeted R&D in key areas.     

Sorbent enhanced hydrogen production from steam gasification of coal integrated with CO2 capture

In this work, CO₂ capture and H₂ production during the steam gasification of coal integrated with CO₂ capture sorbent were investigated using a horizontal fixed bed reactor at atmospheric pressure. Four different temperatures (650, 675, 700, and 750 °C) and three sorbent-to-carbon ratios ([Ca]/[C] = 0, 1, 2) were studied. In the absence of sorbent, the maximum molar fraction of H₂ (64.6%) and conversion of coal (71.3%) were exhibited at the highest temperature (750 °C). The experimental results verified that the presence of sorbent in the steam gasification of coal enhanced the molar fraction of H₂ to more than 80%, with almost all CO₂ was fixed into the sorbent structure, and carbon monoxide (CO) was converted to H₂ and CO₂ through the water gas shift reaction. The steam gasification of coal integrated with CO₂ capture largely depended on the reaction temperature and exhibited optimal conditions at 675 °C. The maximum molar fraction of H₂ (81.7%) and minimum CO₂ concentration (almost 0%) were obtained at 675 °C and a sorbent-to-carbon ratio of 2.     

Pilot scale autothermal gasification of coconut shell with CO2-O2 mixture

This paper explored the feasibility and benefit of CO₂ utilization as gasifying agent in the autothermal gasification process. The effects of CO₂ injection on reaction temperature and producer gas composition were examined in a pilot scale downdraft gasifier by varying the CO₂/C ratio from 0.6 to 1.6. O₂ was injected at an equivalence ratio of approximately 0.33–0.38 for supplying heat through partial combustion. The results were also compared with those of air gasification. In general, the increase in CO₂ injection resulted in the shift of combustion zone to the downstream of the gasifier. However, compared with that of air gasification, the long and distributed high temperature zones were obtained in CO₂-O₂ gasification with a CO₂/C ratio of 0.6–1.2. The progress of the expected CO₂ to CO conversion can be implied from the relatively insignificant decrease in CO fraction as the CO₂/C ratio increased. The producer gas heating value of CO₂-O₂ gasification was consistently higher than that of air gasification. These results show the potential of CO₂-O₂ gasification for producing high quality producer gas in an efficient manner, and the necessity for more work to deeply imply the observation.