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Frontiers in Energy

ISSN 2095-1701 (Print)
ISSN 2095-1698 (Online)
CN 11-6017/TK
Postal Subscription Code 80-972
Formerly Known as Frontiers of Energy and Power Engineering in China
2019 Impact Factor: 2.657

, Volume 14 Issue 3

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RESEARCH ARTICLE
Key issues in development of offshore natural gas hydrate
Shouwei ZHOU, Qingping LI, Xin LV, Qiang FU, Junlong ZHU
Front. Energy. 2020, 14 (3): 433-442.  https://doi.org/10.1007/s11708-020-0684-1
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As a new clean energy resource in the 21st century, natural gas hydrate is considered as one of the most promising strategic resources in the future. This paper, based on the research progress in exploitation of natural gas hydrate (NGH) in China and the world, systematically reviewed and discussed the key issues in development of natural gas hydrate. From an exploitation point of view, it is recommended that the concepts of diagenetic hydrate and non-diagenetic hydrate be introduced. The main factors to be considered are whether diagenesis, stability of rock skeleton structure, particle size and cementation mode, thus NGHs are divided into 6 levels and used unused exploitation methods according to different types. The study of the description and quantitative characterization of abundance in hydrate enrichment zone, and looking for gas hydrate dessert areas with commercial exploitation value should be enhanced. The concept of dynamic permeability and characterization of the permeability of NGH by time-varying equations should be established. The ‘Three-gas co-production’ (natural gas hydrate, shallow gas, and conventional gas) may be an effective way to achieve early commercial exploitation. Although great progress has been made in the exploitation of natural gas hydrate, there still exist enormous challenges in basic theory research, production methods, and equipment and operation modes. Only through hard and persistent exploration and innovation can natural gas hydrate be truly commercially developed on a large scale and contribute to sustainable energy supply.

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Efficient promotion of methane hydrate formation and elimination of foam generation using fluorinated surfactants
Quan CAO, Dongyan XU, Huanfei XU, Shengjun LUO, Rongbo GUO
Front. Energy. 2020, 14 (3): 443-451.  https://doi.org/10.1007/s11708-020-0683-2
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Methane hydrate preparation is an effective method to store and transport methane. In promoters to facilitate methane hydrate formation, homogeneous surfactant solutions, sodium dodecyl sulfate (SDS) in particular, are more favorable than heterogeneous particles, thanks to their faster reaction rate, more storage capacity, and higher stability. Foaming, however, could not be avoided during hydrate dissociation with the presence of SDS. This paper investigated the ability of five fluorinated surfactants: potassium perfluorobutane sulfonate (PBS), potassium perfluorohexyl sulfonate (PHS), potassium perfluorooctane sulfonate (POS), ammonium perfluorooctane sulfonate (AOS), and tetraethylammonium perfluorooctyl sulfonate (TOS) to promote methane hydrate formation. It was found that both PBS and PHS achieve a storage capacity of 150 (V/V, the volume of methane that can be stored by one volume of water) within 30 min, more than that of SDS. Cationic ions and the carbon chain length were then discussed on their effects during the formation. It was concluded that PBS, PHS, and POS produced no foam during hydrate dissociation, making them promising promoters in large-scale application.

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Simulation of performance of intermediate fluid vaporizer under wide operation conditions
Bojie WANG, Wen WANG, Chao QI, Yiwu KUANG, Jiawei XU
Front. Energy. 2020, 14 (3): 452-462.  https://doi.org/10.1007/s11708-020-0681-4
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The intermediate fluid vaporizer (IFV) is a typical vaporizer of liquefied natural gas (LNG), which in general consists of three shell-and-tube heat exchangers (an evaporator, a condenser, and a thermolator). LNG is heated by seawater and the intermediate fluid in these heat exchangers. A one-dimensional heat transfer model for IFV is established in this paper in order to investigate the influences of structure and operation parameters on the heat transfer performance. In the rated condition, it is suggested to reduce tube diameters appropriately to get a large total heat transfer coefficient and increase the tube number to ensure the sufficient heat transfer area. According to simulation results, although the IFV capacity is much larger than the simplified-IFV (SIFV) capacity, the mode of SIFV could be recommended in some low-load cases as well. In some cases at high loads exceeding the capacity of a single IFV, it is better to add an AAV or an SCV operating to the IFV than just to increase the mass flow rate of seawater in the IFV in LNG receiving terminals.

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REVIEW ARTICLE
Progress in use of surfactant in nearly static conditions in natural gas hydrate formation
Zhen PAN, Yi WU, Liyan SHANG, Li ZHOU, Zhien ZHANG
Front. Energy. 2020, 14 (3): 463-481.  https://doi.org/10.1007/s11708-020-0675-2
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Natural gas hydrate is an alternative energy source with a great potential for development. The addition of surfactants has been found to have practical implications on the acceleration of hydrate formation in the industrial sector. In this paper, the mechanisms of different surfactants that have been reported to promote hydrate formation are summarized. Besides, the factors influencing surfactant-promoted hydrate formation, including the type, concentration, and structure of the surfactant, are also described. Moreover, the effects of surfactants on the formation of hydrate in pure water, brine, porous media, and systems containing multiple surfactants are discussed. The synergistic or inhibitory effects of the combinations of these additives are also analyzed. Furthermore, the process of establishing kinetic and thermodynamic models to simulate the factors affecting the formation of hydrate in surfactant-containing solutions is illustrated and summarized.

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Spectral emittance measurements of micro/nanostructures in energy conversion: a review
Shiquan SHAN, Chuyang CHEN, Peter G. LOUTZENHISER, Devesh RANJAN, Zhijun ZHOU, Zhuomin M. ZHANG
Front. Energy. 2020, 14 (3): 482-509.  https://doi.org/10.1007/s11708-020-0693-0
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Micro/nanostructures play a key role in tuning the radiative properties of materials and have been applied to high-temperature energy conversion systems for improved performance. Among the various radiative properties, spectral emittance is of integral importance for the design and analysis of materials that function as radiative absorbers or emitters. This paper presents an overview of the spectral emittance measurement techniques using both the direct and indirect methods. Besides, several micro/nanostructures are also introduced, and a special emphasis is placed on the emissometers developed for characterizing engineered micro/nanostructures in high-temperature applications (e.g., solar energy conversion and thermophotovoltaic devices). In addition, both experimental facilities and measured results for different materials are summarized. Furthermore, future prospects in developing instrumentation and micro/nanostructured surfaces for practical applications are also outlined. This paper provides a comprehensive source of information for the application of micro/nanostructures in high-temperature energy conversion engineering.

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A comprehensive review of renewable energy resources for electricity generation in Australia
Alireza HEIDARI, Ali ESMAEEL NEZHAD, Ahmad TAVAKOLI, Navid REZAEI, Foad H. GANDOMAN, Mohammad Reza MIVEH, Abdollah AHMADI, Majid MALEKPOUR
Front. Energy. 2020, 14 (3): 510-529.  https://doi.org/10.1007/s11708-020-0671-6
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Recently, renewable energy resources and their impacts have sparked a heated debate to resolve the Australian energy crisis. There are many projects launched throughout the country to improve network security and reliability. This paper aims to review the current status of different renewable energy resources along with their impacts on society and the environment. Besides, it provides for the first time the statistics of the documents published in the field of renewable energy in Australia. The statistics include information such as the rate of papers published, possible journals for finding relative paper, types of documents published, top authors, and the most prevalent keywords in the field of renewable energy in Australia. It will focus on solar, wind, biomass, geothermal and hydropower technologies and will investigate the social and environmental impacts of these technologies.

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Review on the design and optimization of hydrogen liquefaction processes
Liang YIN, Yonglin JU
Front. Energy. 2020, 14 (3): 530-544.  https://doi.org/10.1007/s11708-019-0657-4
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The key technologies of liquefied hydrogen have been developing rapidly due to its prospective energy exchange effectiveness, zero emissions, and long distance and economic transportation. However, hydrogen liquefaction is one of the most energy-intensive industrial processes. A small reduction in energy consumption and an improvement in efficiency may decrease the operating cost of the entire process. In this paper, the detailed progress of design and optimization for hydrogen liquefaction in recent years are summarized. Then, based on the refrigeration cycles, the hydrogen liquefaction processes are divided into two parts, namely precooled liquefaction process and cascade liquefaction process. Among the existing technologies, the SEC of most hydrogen liquefaction processes is limited in the range of 5–8 kWh/ kgLH2: liquid hydrogen). The exergy efficiencies of processes are around 40% to 60%. Finally, several future improvements for hydrogen liquefaction process design and optimization are proposed. The mixed refrigerants (MRs) as the working fluids of the process and the combination of the traditional hydrogen liquefaction process with the renewable energy technology will be the great prospects for development in near future.

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Catalytic steam reforming of tar for enhancing hydrogen production from biomass gasification: a review
Ru Shien TAN, Tuan Amran TUAN ABDULLAH, Anwar JOHARI, Khairuddin MD ISA
Front. Energy. 2020, 14 (3): 545-569.  https://doi.org/10.1007/s11708-020-0800-2
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Presently, the global search for alternative renewable energy sources is rising due to the depletion of fossil fuel and rising greenhouse gas (GHG) emissions. Among alternatives, hydrogen (H2) produced from biomass gasification is considered a green energy sector, due to its environmentally friendly, sustainable, and renewable characteristics. However, tar formation along with syngas is a severe impediment to biomass conversion efficiency, which results in process-related problems. Typically, tar consists of various hydrocarbons (HCs), which are also sources for syngas. Hence, catalytic steam reforming is an effective technique to address tar formation and improve H2 production from biomass gasification. Of the various classes in existence, supported metal catalysts are considered the most promising. This paper focuses on the current researching status, prospects, and challenges of steam reforming of gasified biomass tar. Besides, it includes recent developments in tar compositional analysis, supported metal catalysts, along with the reactions and process conditions for catalytic steam reforming. Moreover, it discusses alternatives such as dry and autothermal reforming of tar.

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RESEARCH ARTICLE
A novel cryogenic insulation system of hollow glass microspheres and self-evaporation vapor-cooled shield for liquid hydrogen storage
Jianpeng ZHENG, Liubiao CHEN, Ping WANG, Jingjie ZHANG, Junjie WANG, Yuan ZHOU
Front. Energy. 2020, 14 (3): 570-577.  https://doi.org/10.1007/s11708-019-0642-y
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Liquid hydrogen (LH2) attracts widespread attention because of its highest energy storage density. However, evaporation loss is a serious problem in LH2 storage due to the low boiling point (20 K). Efficient insulation technology is an important issue in the study of LH2 storage. Hollow glass microspheres (HGMs) is a potential promising thermal insulation material because of its low apparent thermal conductivity, fast installation (Compared with multi-layer insulation, it can be injected in a short time.), and easy maintenance. A novel cryogenic insulation system consisting of HGMs and a self-evaporating vapor-cooled shield (VCS) is proposed for storage of LH2. A thermodynamic model has been established to analyze the coupled heat transfer characteristics of HGMs and VCS in the composite insulation system. The results show that the combination of HGMs and VCS can effectively reduce heat flux into the LH2 tank. With the increase of VCS number from 1 to 3, the minimum heat flux through HGMs decreases by 57.36%, 65.29%, and 68.21%, respectively. Another significant advantage of HGMs is that their thermal insulation properties are not sensitive to ambient vacuum change. When ambient vacuum rises from 103 Pa to 1 Pa, the heat flux into the LH2 tank increases by approximately 20%. When the vacuum rises from 103 Pa to 100 Pa, the combination of VCS and HGMs reduces the heat flux into the tank by 58.08%–69.84% compared with pure HGMs.

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A comprehensive study of hydrogen production from ammonia borane via PdCoAg/AC nanoparticles and anodic current in alkaline medium: experimental design with response surface methodology
Hilal ÇELİK KAZICI, Şakir YILMAZ, Tekin ŞAHAN, Fikret YILDIZ, Ömer Faruk ER, Hilal KIVRAK
Front. Energy. 2020, 14 (3): 578-589.  https://doi.org/10.1007/s11708-020-0808-7
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In this paper, the optimization of hydrogen (H2) production by ammonia borane (NH3BH3) over PdCoAg/AC was investigated using the response surface methodology. Besides, the electro-oxidation of NH3BH3 was determined and optimized using the same method to measure its potential use in the direct ammonium boran fuel cells. Moreover, the ternary alloyed catalyst was synthesized using the chemical reduction method. The synergistic effect between Pd, Co and Ag plays an important role in enhancement of NH3BH3 hydrolysis. In addition, the support effect could also efficiently improve the catalytic performance. Furthermore, the effects of NH3BH3 concentration (0.1–50 mmol/5 mL), catalyst amount (1–30 mg) and temperature (20°C–50°C) on the rate of H2 production and the effects of temperature (20°C–50°C), NH3BH3 concentration (0.05–1 mol/L) and catalyst amount (0.5–5 µL) on the electro-oxidation reaction of NH3BH3 were investigated using the central composite design experimental design. The implementation of the response surface methodology resulted in the formulation of four models out of which the quadratic model was adjudged to efficiently appropriate the experimental data. A further statistical analysis of the quadratic model demonstrated the significance of the model with a p-value far less than 0.05 for each model and coefficient of determination (R2) of 0.85 and 0.95 for H2 production rate and NH3BH3 electrroxidation peak current, respectively.

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Thermodynamic and economic analyses of a coal and biomass indirect coupling power generation system
Buqing YE, Rui ZHANG, Jin CAO, Bingquan SHI, Xun ZHOU, Dong LIU
Front. Energy. 2020, 14 (3): 590-606.  https://doi.org/10.1007/s11708-020-0809-6
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The coal and biomass coupling power generation technology is considered as a promising technology for energy conservation and emission reduction. In this paper, a novel coal and biomass indirect coupling system is proposed based on the technology of biomass gasification and co-combustion of coal and gasification gas. For the sake of comparison, a coal and biomass direct coupling system is also introduced based on the technology of co-combustion of coal and biomass. The process of the direct and the indirect coupling system is simulated. The thermodynamic and economic performances of two systems are analyzed and compared. The simulation indicates that the thermodynamic performance of the indirect coupling system is slightly worse, but the economic performance is better than that of the direct coupling system. When the blending ratio of biomass is 20%, the energy and exergy efficiencies of the indirect coupling system are 42.70% and 41.14%, the internal rate of return (IRR) and discounted payback period (DPP) of the system are 25.68% and 8.56 years. The price fluctuation of fuels and products has a great influence on the economic performance of the indirect coupling system. The environmental impact analysis indicates that the indirect coupling system can inhibit the propagation of NOx and reduce the environmental cost.

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Energy and exergy analysis of syngas production from different biomasses through air-steam gasification
S. Rupesh, C. Muraleedharan, P. Arun
Front. Energy. 2020, 14 (3): 607-619.  https://doi.org/10.1007/s11708-016-0439-1
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Gasification is a thermo-chemical reaction which converts biomass into fuel gases in a reactor. The efficiency of conversion depends on the effective working of the gasifier. The first step in the conversion process is the selection of a suitable feedstock capable of generating more gaseous fuels. This paper analyses the performance of different biomasses during gasification through energy and exergy analysis. A quasi-equilibrium model is developed to simulate and compare the feasibility of different biomass materials as gasifier feedstock. Parametric studies are conducted to analyze the effect of temperature, steam to biomass ratio and equivalence ratio on energy and exergy efficiencies. Of the biomasses considered, sawdust has the highest energy and exergy efficiencies and lowest irreversibility. At a gasification temperature of 1000 K, the steam to biomass ratio of unity and the equivalence ratio of 0.25, the energy efficiency, exergy efficiency and irreversibility of sawdust are 35.62%, 36.98% and 10.62 MJ/kg, respectively. It is also inferred that the biomass with lower ash content and higher carbon content contributes to maximum energy and exergy efficiencies.

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Does financial development lower energy intensity?
Philip Kofi ADOM, Michael Owusu APPIAH, Mawunyo Prosper AGRADI
Front. Energy. 2020, 14 (3): 620-634.  https://doi.org/10.1007/s11708-019-0619-x
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The growth-induced effects of financial development have been well-established in the empirical literature, as well as the significance of financial development to energy demand behavior. However, the empirical evidence on the relationship between financial development and energy intensity remains sparse in the literature. Given the multifaceted nature of the effects of financial development, the proposed relationship seems a complex one and warrants an empirical investigation. Using the case of Ghana, this study provides an empirical answer to the question: does financial development lower energy intensity? To provide solid grounds for either rejection or acceptance of the null hypothesis, this study performed several robustness checks. Generally, the evidence revealed that financial development lowers energy intensity. Further, the results revealed that the price of energy, trade liberalization and industry structure play significant roles. These results have important implications for the design of macro energy efficiency policies and the creation of a ‘Green Bank’.

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Experimental investigation of multiphase flow behavior in drilling annuli using high speed visualization technique
Alap Ali ZAHID, Syed Raza ur REHMAN, S. RUSHD, Anwarul HASAN, Mohammad Azizur RAHMAN
Front. Energy. 2020, 14 (3): 635-643.  https://doi.org/10.1007/s11708-018-0582-y
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Imaging with high definition video camera is an important technique to visualize the drilling conditions and to study the physics of complex multiphase flow associated with the hole cleaning process. The main advantage of visualizing multiphase flow in a drilling annulus is that the viewer can easily distinguish fluid phases, flow patterns and thicknesses of cutting beds. In this paper the hole cleaning process which involves the transportation of cuttings through a horizontal annulus was studied. The two-phase (solid-liquid) and the three-phase (solid-liquid-gas) flow conditions involved in this kind of annular transportation were experimentally simulated and images were taken using a high definition camera. Analyzing the captured images, a number of important parameters like velocities of different phases, heights of solid beds and sizes of gas bubbles were determined. Two different techniques based on an image analysis software and MATLAB coding were used for the determinations. The results were compared to validate the image analyzing methodology. The visualization technique developed in this paper has a direct application in investigating the critical conditions required for efficient hole cleaning as well as in optimizing the mud program during both planning and operational phases of drilling. Particularly, it would be useful in predicting the cuttings transport performance, estimating solid bed height, gas bubble size, and mean velocities of bubbles/particles.

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Performance of rolling piston type rotary compressor using fullerenes (C70) and NiFe2O4 nanocomposites as lubricants additives
Ruixiang WANG, Yihao ZHANG, Yi LIAO
Front. Energy. 2020, 14 (3): 644-648.  https://doi.org/10.1007/s11708-017-0453-y
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A novel way for a compressor to improve its coefficient of performance was proposed in this paper. Fullerenes (C70) and NiFe2O4 nanocomposites were modified by span 80 and dispersed in refrigeration oil by solid grinding (SG). Besides, the tribological properties of the nanocomposites were investigated using a four ball friction tester. The results show that when the mass concentration of fullerenes nanocomposite is higher than 60 ppm and the concentration of nano-oil is 2 g/L, the friction coefficient decreases from 0.13 to 0.06 which means the wear is reduced. The coefficient of performance of the compressor under the air conditioning test condition can be raised by 1.23%.

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Multi-objective optimization in a finite time thermodynamic method for dish-Stirling by branch and bound method and MOPSO algorithm
Mohammad Reza NAZEMZADEGAN, Alibakhsh KASAEIAN, Somayeh TOGHYANI, Mohammad Hossein AHMADI, R. SAIDUR, Tingzhen MING
Front. Energy. 2020, 14 (3): 649-665.  https://doi.org/10.1007/s11708-018-0548-0
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There are various analyses for a solar system with the dish-Stirling technology. One of those analyses is the finite time thermodynamic analysis by which the total power of the system can be obtained by calculating the process time. In this study, the convection and radiation heat transfer losses from collector surface, the conduction heat transfer between hot and cold cylinders, and cold side heat exchanger have been considered. During this investigation, four objective functions have been optimized simultaneously, including power, efficiency, entropy, and economic factors. In addition to the four-objective optimization, three-objective, two-objective, and single-objective optimizations have been done on the dish-Stirling model. The algorithm of multi-objective particle swarm optimization (MOPSO) with post-expression of preferences is used for multi-objective optimizations while the branch and bound algorithm with pre-expression of preferences is used for single-objective and multi-objective optimizations. In the case of multi-objective optimizations with post-expression of preferences, Pareto optimal front are obtained, afterward by implementing the fuzzy, LINMAP, and TOPSIS decision making algorithms, the single optimum results can be achieved. The comparison of the results shows the benefits of MOPSO in optimizing dish Stirling finite time thermodynamic equations.

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