Frontiers in Energy

ISSN 2095-1701

ISSN 2095-1698(Online)

CN 11-6017/TK

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Highlights of mainstream solar cell efficiencies in 2023
Wenzhong SHEN, Yixin ZHAO, Feng LIU
Front. Energy    2024, 18 (1): 8-15.   https://doi.org/10.1007/s11708-024-0937-5
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China’s policy framework for carbon capture, utilization and storage: Review, analysis, and outlook
Qiao MA, Shan WANG, Yan FU, Wenlong ZHOU, Mingwei SHI, Xueting PENG, Haodong LV, Weichen ZHAO, Xian ZHANG
Front. Energy    2023, 17 (3): 400-411.   https://doi.org/10.1007/s11708-023-0862-z
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Carbon capture, utilization, and storage (CCUS) is estimated to contribute substantial CO2 emission reduction to carbon neutrality in China. There is yet a large gap between such enormous demand and the current capacity, and thus a sound enabling environment with sufficient policy support is imperative for CCUS development. This study reviewed 59 CCUS-related policy documents issued by the Chinese government as of July 2022, and found that a supporting policy framework for CCUS is taking embryonic form in China. More than ten departments of the central government have involved CCUS in their policies, of which the State Council, the National Development and Reform Commission (NDRC), the Ministry of Science and Technology (MOST), and the Ministry of Ecological Environment (MEE) have given the greatest attention with different focuses. Specific policy terms are further analyzed following the method of content analysis and categorized into supply-, environment- and demand-type policies. The results indicate that supply-type policies are unbalanced in policy objectives, as policy terms on technology research and demonstration greatly outnumber those on other objectives, and the attention to weak links and industrial sectors is far from sufficient. Environment-type policies, especially legislations, standards, and incentives, are inadequate in pertinence and operability. Demand-type policies are absent in the current policy system but is essential to drive the demand for the CCUS technology in domestic and foreign markets. To meet the reduction demand of China’s carbon neutral goal, policies need to be tailored according to needs of each specific technology and implemented in an orderly manner with well-balanced use on multiple objectives.

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Combustion and emissions of RP-3 jet fuel and diesel fuel in a single-cylinder diesel engine
Tongbin ZHAO, Zhe REN, Kai YANG, Tao SUN, Lei SHI, Zhen HUANG, Dong HAN
Front. Energy    2023, 17 (5): 664-677.   https://doi.org/10.1007/s11708-021-0787-3
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The combustion characteristics and emission behaviors of RP-3 jet fuel were studied and compared to commercial diesel fuel in a single-cylinder compression ignition (CI) engine. Engine operational parameters, including engine load (0.6, 0.7, and 0.8 MPa indicating the mean effective pressure (IMEP)), the exhaust gas recirculation (EGR) rate (0%, 10%, 20%, and 30%), and the fuel injection timing (−20, −15, −10, and −5 ° crank angle (CA) after top dead center (ATDC)) were adjusted to evaluate the engine performances of RP-3 jet fuel under changed operation conditions. In comparison to diesel fuel, RP-3 jet fuel shows a retarded heat release and lagged combustion phase, which is more obvious under heavy EGR rate conditions. In addition, the higher premixed combustion fraction of RP-3 jet fuel leads to a higher first-stage heat release peak than diesel fuel under all testing conditions. As a result, RP-3 jet fuel features a longer ignition delay (ID) time, a shorter combustion duration (CD), and an earlier CA50 than diesel fuel. The experimental results manifest that RP-3 jet fuel has a slightly lower indicated thermal efficiency (ITE) compared to diesel fuel, but the ITE difference becomes less noticeable under large EGR rate conditions. Compared with diesel fuel, the nitrogen oxides (NOx) emissions of RP-3 jet fuel are higher while its soot emissions are lower. The NOx emissions of RP-3 can be effectively reduced with the increased EGR rate and delayed injection timing.

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Enhanced performance of oxygen vacancies on CO2 adsorption and activation over different phases of ZrO2
Juntian NIU, Cunxin ZHANG, Haiyu LIU, Yan JIN, Riguang ZHANG
Front. Energy    2023, 17 (4): 545-554.   https://doi.org/10.1007/s11708-023-0867-7
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The effect of oxygen vacancies on the adsorption and activation of CO2 on the surface of different phases of ZrO2 is investigated by density functional theory (DFT) calculations. The calculations show that the oxygen vacancies contribute greatly to both the adsorption and activation of CO2. The adsorption energy of CO2 on the c-ZrO2, t-ZrO2 and, m-ZrO2 surfaces is enhanced to 5, 4, and 3 folds with the help of oxygen vacancies, respectively. Moreover, the energy barrier of CO2 dissociation on the defective surfaces of c-ZrO2, t-ZrO2, and m-ZrO2 is reduced to 1/2, 1/4, and 1/5 of the perfect surface with the assistance of oxygen vacancies. Furthermore, the activation of CO2 on the ZrO2 surface where oxygen vacancies are present, and changes from an endothermic reaction to an exothermic reaction. This finding demonstrates that the presence of oxygen vacancies promotes the activation of CO2 both kinetically and thermodynamically. These results could provide guidance for the high-efficient utilization of CO2 at an atomic scale.

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CO2, N2, and CO2/N2 mixed gas injection for enhanced shale gas recovery and CO2 geological storage
Jianfa WU, Haoran HU, Cheng CHANG, Deliang ZHANG, Jian ZHANG, Shengxian ZHAO, Bo WANG, Qiushi ZHANG, Yiming CHEN, Fanhua ZENG
Front. Energy    2023, 17 (3): 428-445.   https://doi.org/10.1007/s11708-023-0865-9
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In this work, using fractured shale cores, isothermal adsorption experiments and core flooding tests were conducted to investigate the performance of injecting different gases to enhance shale gas recovery and CO2 geological storage efficiency under real reservoir conditions. The adsorption process of shale to different gases was in agreement with the extended-Langmuir model, and the adsorption capacity of CO2 was the largest, followed by CH4, and that of N2 was the smallest of the three pure gases. In addition, when the CO2 concentration in the mixed gas exceeded 50%, the adsorption capacity of the mixed gas was greater than that of CH4, and had a strong competitive adsorption effect. For the core flooding tests, pure gas injection showed that the breakthrough time of CO2 was longer than that of N2, and the CH4 recovery factor at the breakthrough time () was also higher than that of N2. The of CO2 gas injection was approximately 44.09%, while the of N2 was only 31.63%. For CO2/N2 mixed gas injection, with the increase of CO2 concentration, the increased, and the for mixed gas CO2/N2 = 8:2 was close to that of pure CO2, about 40.24%. Moreover, the breakthrough time of N2 in mixed gas was not much different from that when pure N2 was injected, while the breakthrough time of CO2 was prolonged, which indicated that with the increase of N2 concentration in the mixed gas, the breakthrough time of CO2 could be extended. Furthermore, an abnormal surge of N2 concentration in the produced gas was observed after N2 breakthrough. In regards to CO2 storage efficiency (), as the CO2 concentration increased, also increased. The of the pure CO2 gas injection was about 35.96%, while for mixed gas CO2/N2 = 8:2, was about 32.28%.

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Flow and thermal modeling of liquid metal in expanded microchannel heat sink
Mingkuan ZHANG, Xudong ZHANG, Luna GUO, Xuan LI, Wei RAO
Front. Energy    2023, 17 (6): 796-810.   https://doi.org/10.1007/s11708-023-0877-5
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Liquid metal-based microchannel heat sinks (MCHSs) suffer from the low heat capacity of coolant, resulting in an excessive temperature rise of coolant and heat sink when dealing with high-power heat dissipation. In this paper, it was found that expanded space at the top of fins could distribute the heat inside microchannels, reducing the temperature rise of coolant and heat sink. The orthogonal experiments revealed that expanding the top space of channels yielded similar temperature reductions to changing the channel width. The flow and thermal modeling of expanded microchannel heat sink (E-MCHS) were analyzed by both using the 3-dimensional (3D) numerical simulation and the 1-dimensional (1D) thermal resistance model. The fin efficiency of E-MCHS was derived to improve the accuracy of the 1D thermal resistance model. The heat conduction of liquid metal in Z direction and the heat convection between the top surface of fins and the liquid metal could reduce the total thermal resistance (Rt). The above process was effective for microchannels with low channel aspect ratio, low mean velocity (Um) or long heat sink length. The maximum thermal resistance reduction in the example of this paper reached 36.0%. The expanded space endowed the heat sink with lower pressure, which might further reduce the pumping power (P). This rule was feasible both when fins were truncated (h2 < 0, h2 is the height of expanded channel for E-MCHS) and when over plate was raised (h2 > 0).

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Reduction potential of the energy penalty for CO2 capture in CCS
Yawen ZHENG, Lin GAO, Song HE, Hongguang JIN
Front. Energy    2023, 17 (3): 390-399.   https://doi.org/10.1007/s11708-023-0864-x
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CO2 capture and storage (CCS) has been acknowledged as an essential part of a portfolio of technologies that are required to achieve cost-effective long-term CO2 mitigation. However, the development progress of CCS technologies is far behind the targets set by roadmaps, and engineering practices do not lead to commercial deployment. One of the crucial reasons for this delay lies in the unaffordable penalty caused by CO2 capture, even though the technology has been commonly recognized as achievable. From the aspects of separation and capture technology innovation, the potential and promising direction for solving this problem were analyzed, and correspondingly, the possible path for deployment of CCS in China was discussed. Under the carbon neutral target recently proposed by the Chinese government, the role of CCS and the key milestones for deployment were indicated.

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A hot future for cool materials
Xavier MOYA, Neil D. MATHUR
Front. Energy    2023, 17 (4): 447-449.   https://doi.org/10.1007/s11708-022-0854-4
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The widespread need to pump heat necessitates improvements that will increase energy efficiency and, more generally, reduce environmental impact. As discussed at the recent Calorics 2022 Conference, heat-pump devices based on caloric materials offer an intriguing alternative to gas combustion and vapor compression.

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A review on technologies with electricity generation potentials using liquified natural gas regasification cold energy
Muhammad Tauseef NASIR, Mirae KIM, Jaehwa LEE, Seungho KIM, Kyung Chun KIM
Front. Energy    2023, 17 (3): 332-379.   https://doi.org/10.1007/s11708-023-0863-y
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In modern times, worldwide requirements to curb greenhouse gas emissions, and increment in energy demand due to the progress of humanity, have become a serious concern. In such scenarios, the effective and efficient utilization of the liquified natural gas (LNG) regasification cold energy (RCE), in the economically and environmentally viable methods, could present a great opportunity in tackling the core issues related to global warming across the world. In this paper, the technologies that are widely used to harness the LNG RCE for electrical power have been reviewed. The systems incorporating, the Rankine cycles, Stirling engines, Kalina cycles, Brayton cycles, Allam cycles, and fuel cells have been considered. Additionally, the economic and environmental studies apart from the thermal studies have also been reviewed. Moreover, the discussion regarding the systems with respect to the regassification pressure of the LNG has also been provided. The aim of this paper is to provide guidelines for the prospective researchers and policy makers in their decision making.

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A methodology for regulating fuel stratification and improving fuel economy of GCI mode via double main-injection strategy
Haoqing WU, Yaoyuan ZHANG, Shijie MI, Wenbin ZHAO, Zhuoyao HE, Yong QIAN, Xingcai LU
Front. Energy    2023, 17 (5): 678-691.   https://doi.org/10.1007/s11708-022-0859-z
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Gasoline compression ignition (GCI) combustion faces problems such as high maximum pressure rise rate (MPRR) and combustion deterioration at high loads. This paper aims to improve the engine performance of the GCI mode by regulating concentration stratification and promoting fuel-gas mixing by utilizing the double main-injection (DMI) strategy. Two direct injectors simultaneously injected gasoline with an octane number of 82.7 to investigate the energy ratio between the two main-injection and exhaust gas recirculation (EGR) on combustion and emissions. High-load experiments were conducted using the DMI strategy and compared with the single main-injection (SMI) strategy and conventional diesel combustion. The results indicate that the DMI strategy have a great potential to reduce the MPRR and improve the fuel economy of the GCI mode. At a 10 bar indicated mean effective pressure, increasing the main-injection-2 ratio (Rm-2) shortens the injection duration and increases the mean mixing time. Optimized Rm-2 could moderate the trade-off between the MPRR and the indicated specific fuel consumption with both reductions. An appropriate EGR should be adopted considering combustion and emissions. The DMI strategy achieves a highly efficient and stable combustion at high loads, with an indicated thermal efficiency (ITE) greater than 48%, CO and THC emissions at low levels, and MPRR within a reasonable range. Compared with the SMI strategy, the maximum improvement of the ITE is 1.5%, and the maximum reduction of MPRR is 1.5 bar/°CA.

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A comprehensive review of wind power based power system frequency regulation
Zhang WEN, Liangzhong YAO, Fan CHENG, Jian XU, Beilin MAO, Rusi CHEN
Front. Energy    2023, 17 (5): 611-634.   https://doi.org/10.1007/s11708-023-0876-6
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Wind power (WP) is considered as one of the main renewable energy sources (RESs) for future low-carbon and high-cost-efficient power system. However, its low inertia characteristic may threaten the system frequency stability of the power system with a high penetration of WP generation. Thus, the capability of WP participating in the system frequency regulation has become a research hotspot. In this paper, the impact of WP on power system frequency stability is initially presented. In addition, various existing control strategies of WP participating in frequency regulation are reviewed from the wind turbine (WT) level to the wind farm (WF) level, and their performances are compared in terms of operating principles and practical applications. The pros and cons of each control strategy are also discussed. Moreover, the WP combing with energy storage system (ESS) for system frequency regulation is explored. Furthermore, the prospects, future challenges, and solutions of WP participating in power system frequency regulation are summarized.

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Phase transition regulation and caloric effect
Yuan LIN, Jiazheng HAO, Kaiming QIAO, Yihong GAO, Fengxia HU, Jing WANG, Tongyun ZHAO, Baogen SHEN
Front. Energy    2023, 17 (4): 463-477.   https://doi.org/10.1007/s11708-023-0860-1
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Solid state refrigeration based on caloric effect is regarded as a potential candidate for replacing vapor-compression refrigeration. Numerous methods have been proposed to optimize the refrigeration properties of caloric materials, of which single field tuning as a relatively simple way has been systemically studied. However, single field tuning with few tunable parameters usually obtains an excellent performance in one specific aspect at the cost of worsening the performance in other aspects, like attaining a large caloric effect with narrowing the transition temperature range and introducing hysteresis. Because of the shortcomings of the caloric effect driven by a single field, multifield tuning on multicaloric materials that have a coupling between different ferro-orders came into view. This review mainly focuses on recent studies that apply this method to improve the cooling performance of materials, consisting of enlarging caloric effects, reducing hysteresis losses, adjusting transition temperatures, and widening transition temperature spans, which indicate that further progress can be made in the application of this method. Furthermore, research on the sign of lattice and spin contributions to the magnetocaloric effect found new phonon evolution mechanisms, calling for more attention on multicaloric effects. Other progress including improving cyclability of FeRh alloys by introducing second phases and realizing a large reversible barocaloric effect by hybridizing carbon chains and inorganic groups is described in brief.

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Energy distribution between liquid hydrogen and liquid oxygen temperatures in a Stirling/pulse tube refrigerator
Kongkuai YING, Wang YIN, Yinong WU, Zhenhua JIANG, Jiantang SONG, Shaoshuai LIU, Haifeng ZHU
Front. Energy    2023, 17 (4): 516-526.   https://doi.org/10.1007/s11708-022-0844-6
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A two-stage gas-coupled Stirling/pulse tube refrigerator (SPR), whose first and second stages respectively involve Stirling and pulse tube refrigeration cycles, is a very promising spaceborne refrigerator. The SPR has many advantages, such as a compact structure, high reliability, and high performance, and is expected to become an essential refrigerator for space applications. In research regarding gas-coupled regenerative refrigerator, the energy flow distribution between the two stages, and optimal phase difference between the pressure wave and volume flow, are two critical parameters that could widely influence refrigerator performance. The effects of displacer displacement on the pressure wave, phase difference, acoustic power distribution, and inter-stage cooling capacity shift of the SPR have been investigated experimentally. Notably, to obtain the maximum first-stage cooling capacity, an inflection point in displacement exists. When the displacer displacement is larger than the inflection point, the cooling capacity could be distributed between the first and second stages. In the present study, an SPR was designed and manufactured to work between the liquid hydrogen and liquid oxygen temperatures, which can be used to cool small-scale zero boil-off systems and space detectors. Under appropriate displacer displacement, the SPR can reach a no-load cooling temperature of 15.4 K and obtain 2.6 W cooling capacity at 70 K plus 0.1 W cooling capacity at 20 K with 160 W compressor input electric power.

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Thoughts on strategies and paths to achieve carbon peaking and carbon neutrality in China
Xiangwan DU
Front. Energy    2023, 17 (3): 324-331.   https://doi.org/10.1007/s11708-023-0883-7
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First, a brief introduction is made to the four basic judgments and understandings of the goals of “carbon peaking and carbon neutrality.” Then, an in-depth elaboration is provided on the eight major strategies for achieving the goals of “carbon peaking and carbon neutrality,” including conservation and efficiency priority, energy security, non-fossil energy substitution, re-electrification, resource recycling, carbon sequestration, digitalization and cooperation between countries. Next, eight major implementation paths for achieving the goals of “carbon peaking and carbon neutrality” are discussed in detail, including industrial restructuring; building a clean, low-carbon, safe and efficient energy system, and renewing the understanding of China’s energy resource endowment; accelerating the construction of a new-type power system with a gradually growing proportion of new energy, and realizing the “possible triangle” of high-quality energy system development; utilizing electrification and deep decarbonization technologies to promote the orderly peaking and gradual neutralization of carbon emissions in the industrial sector; promoting the low-carbon transition of transportation vehicles to achieve carbon peaking and carbon neutrality in the transportation sector; focusing on breaking through key green building technologies to achieve zero carbon emissions from building electricity and heat; providing a strong technical support for carbon removal to achieve carbon neutrality; accelerating the construction of the integrated planning and assessment mechanism for pollution and carbon reduction, establishing a sound strategy, planning, policy and action system, and optimizing the carbon trading system. Afterwards, it is particularly pointed out that the realization of the goals of “carbon peaking and carbon neutrality” cannot be separated from the support of sci-tech innovation. Finally, it is stressed that carbon neutrality is not the end, but an important milestone. If viewed from the perspective of future energy, the significance and historical status of the goals of “carbon peaking and carbon neutrality” will be more understandable.

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Operando modeling and measurements: Powerful tools for revealing the mechanism of alkali carbonate-based sorbents for CO2 capture in real conditions
Tianyi CAI, Mengshi WANG, Xiaoping CHEN, Ye WU, Jiliang MA, Wu ZHOU
Front. Energy    2023, 17 (3): 380-389.   https://doi.org/10.1007/s11708-023-0872-x
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Alkali carbonate-based sorbents (ACSs), including Na2CO3- and K2CO3-based sorbents, are promising for CO2 capture. However, the complex sorbent components and operation conditions lead to the versatile kinetics of CO2 sorption on these sorbents. This paper proposed that operando modeling and measurements are powerful tools to understand the mechanism of sorbents in real operating conditions, facilitating the sorbent development, reactor design, and operation parameter optimization. It reviewed the theoretical simulation achievements during the development of ACSs. It elucidated the findings obtained by utilizing density functional theory (DFT) calculations, ab initio molecular dynamics (AIMD) simulations, and classical molecular dynamics (CMD) simulations as well. The hygroscopicity of sorbent and the humidity of gas flow are crucial to shifting the carbonation reaction from the gas−solid mode to the gas−liquid mode, boosting the kinetics. Moreover, it briefly introduced a machine learning (ML) approach as a promising method to aid sorbent design. Furthermore, it demonstrated a conceptual compact operando measurement system in order to understand the behavior of ACSs in the real operation process. The proposed measurement system includes a micro fluidized-bed (MFB) reactor for kinetic analysis, a multi-camera sub-system for 3D particle movement tracking, and a combined Raman and IR sub-system for solid/gas components and temperature monitoring. It is believed that this system is useful to evaluate the real-time sorbent performance, validating the theoretical prediction and promoting the industrial scale-up of ACSs for CO2 capture.

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Ga(X)N/Si nanoarchitecture: An emerging semiconductor platform for sunlight-powered water splitting toward hydrogen
Yixin LI, Sharif Md. SADAF, Baowen ZHOU
Front. Energy    2024, 18 (1): 56-79.   https://doi.org/10.1007/s11708-023-0881-9
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Sunlight-powered water splitting presents a promising strategy for converting intermittent and virtually unlimited solar energy into energy-dense and storable green hydrogen. Since the pioneering discovery by Honda and Fujishima, considerable efforts have been made in this research area. Among various materials developed, Ga(X)N/Si (X = In, Ge, Mg, etc.) nanoarchitecture has emerged as a disruptive semiconductor platform to split water toward hydrogen by sunlight. This paper introduces the characteristics, properties, and growth/synthesis/fabrication methods of Ga(X)N/Si nanoarchitecture, primarily focusing on explaining the suitability as an ideal platform for sunlight-powered water splitting toward green hydrogen fuel. In addition, it exclusively summarizes the recent progress and development of Ga(X)N/Si nanoarchitecture for photocatalytic and photoelectrochemical water splitting. Moreover, it describes the challenges and prospects of artificial photosynthesis integrated device and system using Ga(X)N/Si nanoarchitectures for solar water splitting toward hydrogen.

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Design and modeling of a free-piston engine generator
Jinlong WANG, Jin XIAO, Yingdong CHENG, Zhen HUANG
Front. Energy    2023, 17 (6): 811-821.   https://doi.org/10.1007/s11708-022-0848-2
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Free-piston engine generators (FPEGs) can be applied as decarbonized range extenders for electric vehicles because of their high thermal efficiency, low friction loss, and ultimate fuel flexibility. In this paper, a parameter-decoupling approach is proposed to model the design of an FPEG. The parameter-decoupling approach first divides the FPEG into three parts: a two-stroke engine, an integrated scavenging pump, and a linear permanent magnet synchronous machine (LPMSM). Then, each of these is designed according to predefined specifications and performance targets. Using this decoupling approach, a numerical model of the FPEG, including the three aforementioned parts, was developed. Empirical equations were adopted to design the engine and scavenging pump, while special considerations were applied for the LPMSM. A finite element model with a multi-objective genetic algorithm was adopted for its design. The finite element model results were fed back to the numerical model to update the LPMSM with increased fidelity. The designed FPEG produced 10.2 kW of electric power with an overall system efficiency of 38.5% in a stable manner. The model provides a solid foundation for the manufacturing of related FPEG prototypes.

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Promoting hydrogen industry with high-capacity Mg-based solid-state hydrogen storage materials and systems
Li REN, Yinghui LI, Xi LIN, Wenjiang DING, Jianxin ZOU
Front. Energy    2023, 17 (3): 320-323.   https://doi.org/10.1007/s11708-023-0889-1
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Three-dimensional composite Li metal anode by simple mechanical modification for high-energy batteries
Min HONG, Zhiyong WANG, Zhangqin SHI, Zheng LIANG
Front. Energy    2023, 17 (5): 569-584.   https://doi.org/10.1007/s11708-023-0875-7
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Lithium (Li) metal is believed to be the “Holy Grail” among all anode materials for next-generation Li-based batteries due to its high theoretical specific capacity (3860 mAh/g) and lowest redox potential (−3.04 V). Disappointingly, uncontrolled dendrite formation and “hostless” deposition impede its further development. It is well accepted that the construction of three-dimensional (3D) composite Li metal anode could tackle the above problems to some extent by reducing local current density and maintaining electrode volume during cycling. However, most strategies to build 3D composite Li metal anode require either electrodeposition or melt-infusion process. In spite of their effectiveness, these procedures bring multiple complex processing steps, high temperature, and harsh experimental conditions which cannot meet the actual production demand in consideration of cost and safety. Under this condition, a novel method to construct 3D composite anode via simple mechanical modification has been recently proposed which does not involve harsh conditions, fussy procedures, or fancy equipment. In this mini review, a systematic and in-depth investigation of this mechanical deformation technique to build 3D composite Li metal anode is provided. First, by summarizing a number of recent studies, different mechanical modification approaches are classified clearly according to their specific procedures. Then, the effect of each individual mechanical modification approach and its working mechanisms is reviewed. Afterwards, the merits and limits of different approaches are compared. Finally, a general summary and perspective on construction strategies for next-generation 3D composite Li anode are presented.

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Polymeric nanocomposites for electrocaloric refrigeration
Yu CAI, Qiang LI, Feihong DU, Jiawang FENG, Donglin HAN, Shanyu ZHENG, Shihao YANG, Yingjing ZHANG, Binbin YU, Junye SHI, Xiaoshi QIAN
Front. Energy    2023, 17 (4): 450-462.   https://doi.org/10.1007/s11708-022-0858-0
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Electrocaloric refrigeration represents an alternative solid-state cooling technology that has the potential to reach the ultimate goal of achieving zero-global-warming potential, highly efficient refrigeration, and heat pumps. To date, both polymeric and inorganic oxides have demonstrated giant electrocaloric effect as well as respective cooling devices. Although both polymeric and inorganic oxides have been identified as promising cooling methods that are distinguishable from the traditional ones, they still pose many challenges to more practical applications. From an electrocaloric material point of view, electrocaloric nanocomposites may provide a solution to combine the beneficial effects of both organic and inorganic electrocaloric materials. This article reviews the recent advancements in polymer-based electrocaloric composites and the state-of-the-art cooling devices operating these nanocomposites. From a device point of view, it discusses the existing challenges and potential opportunities of electrocaloric nanocomposites.

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A review on different theoretical models of electrocaloric effect for refrigeration
Cancan SHAO, A. A. AMIROV, Houbing HUANG
Front. Energy    2023, 17 (4): 478-503.   https://doi.org/10.1007/s11708-023-0884-6
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The performance parameters for characterizing the electrocaloric effect are isothermal entropy change and the adiabatic temperature change, respectively. This paper reviews the electrocaloric effect of ferroelectric materials based on different theoretical models. First, it provides four different calculation scales (the first-principle-based effective Hamiltonian, the Landau-Devonshire thermodynamic theory, phase-field simulation, and finite element analysis) to explain the basic theory of calculating the electrocaloric effect. Then, it comprehensively reviews the recent progress of these methods in regulating the electrocaloric effect and the generation mechanism of the electrocaloric effect. Finally, it summarizes and anticipates the exploration of more novel electrocaloric materials based on the framework constructed by the different computational methods.

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CC@BCN@PANI core-shell nanoarrays as ultra-high cycle stability cathode for Zn-ion hybrid supercapacitors
Shixian XIONG, Hongcheng KE, Lei CAO, Yu WANG, Qian ZHU, Liqin ZHONG, Lanlan FAN, Feng GU
Front. Energy    2023, 17 (4): 555-566.   https://doi.org/10.1007/s11708-023-0882-8
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Exploring cathode materials that combine excellent cycling stability and high energy density poses a challenge to aqueous Zn-ion hybrid supercapacitors (ZHSCs). Herein, polyaniline (PANI) coated boron-carbon-nitrogen (BCN) nanoarray on carbon cloth surface is prepared as advanced cathode materials via simple high-temperature calcination and electrochemical deposition methods. Because of the excellent specific capacity and conductivity of PANI, the CC@BCN@PANI core-shell nanoarrays cathode shows an excellent ion storage capability. Moreover, the 3D nanoarray structure can provide enough space for the volume expansion and contraction of PANI in the charging/discharging cycles, which effectively avoids the collapse of the microstructure and greatly improves the electrochemical stability of PANI. Therefore, the CC@BCN@PANI-based ZHSCs exhibit superior electrochemical performances showing a specific capacity of 145.8 mAh/g, a high energy density of 116.78 Wh/kg, an excellent power density of 12 kW/kg, and a capacity retention rate of 86.2% after 8000 charge/discharge cycles at a current density of 2 A/g. In addition, the flexible ZHSCs (FZHSCs) also show a capacity retention rate of 87.7% at the current density of 2 A/g after 450 cycles.

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Automotive revolution and carbon neutrality
C. C. CHAN, Wei HAN, Hanlei TIAN, Yanbing LIU, Tianlu MA, C. Q. JIANG
Front. Energy    2023, 17 (6): 693-703.   https://doi.org/10.1007/s11708-023-0890-8
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The automotive industry is in the midst of a groundbreaking revolution, driven by the imperative to achieve intelligent driving and carbon neutrality. A crucial aspect of this transformation is the transition to electric vehicles (EVs), which necessitates widespread changes throughout the entire automotive ecosystem. This paper examines the challenges and opportunities of this transition, including automotive electrification, intelligence-connected transportation system, and the potential for new technologies such as hydrogen fuel cells. Meanwhile, it discusses the key technologies and progress of the hydrogen energy industry chain in the upstream hydrogen production, midstream hydrogen storage and transportation, downstream hydrogen station construction and hydrogen fuel cells in turn. Finally, it proposes the directions for future layout, providing guidance for future development.

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RETRACTED ARTICLE: Distributed energy management for networked microgrids in a three-phase unbalanced distribution network
Guodong LIU, Thomas B. OLLIS, Maximiliano F. FERRARIL, Aditya SUNDARARAJAN, Yang CHEN, Mohammed M. OLAMA, Kevin TOMSOVIC
Front. Energy    2023, 17 (3): 446-446.   https://doi.org/10.1007/s11708-022-0851-7
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Formic acid dehydrogenation reaction on high-performance PdxAu1−x alloy nanoparticles prepared by the eco-friendly slow synthesis methodology
Yibo GAO, Erjiang HU, Bo HUANG, Zuohua HUANG
Front. Energy    2023, 17 (6): 751-762.   https://doi.org/10.1007/s11708-023-0895-3
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Dehydrogenation of formic acid (FA) is considered to be an effective solution for efficient storage and transport of hydrogen. For decades, highly effective catalysts for this purpose have been widely investigated, but numerous challenges remain. Herein, the PdxAu1−x (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1) alloys over the whole composition range were successfully prepared and used to catalyze FA hydrogen production efficiently near room temperature. Small PdAu nanoparticles (5–10 nm) were well-dispersed and supported on the activated carbon to form PdAu solid solution alloys via the eco-friendly slow synthesis methodology. The physicochemical properties of the PdAu alloys were comprehensively studied by utilizing various measurement methods, such as X-ray diffraction (XRD), N2 adsorption–desorption, high angle annular dark field-scanning transmission electron microscope (HAADF-STEM), X-ray photoelectrons spectroscopy (XPS). Notably, owing to the strong metal-support interaction (SMSI) and electron transfer between active metal Au and Pd, the Pd0.5Au0.5 obtained exhibits a turnover frequency (TOF) value of up to 1648 h−1 (313 K, nPd+Au/nFA = 0.01, nHCOOH/nHCOONa = 1:3) with a high activity, selectivity, and reusability in the FA dehydrogenation.

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Lifecycle carbon footprint and cost assessment for coal-to-liquid coupled with carbon capture, storage, and utilization technology in China
Jingjing XIE, Kai LI, Jingli FAN, Xueting PENG, Jia LI, Yujiao XIAN
Front. Energy    2023, 17 (3): 412-427.   https://doi.org/10.1007/s11708-023-0879-3
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The coal-to-liquid coupled with carbon capture, utilization, and storage technology has the potential to reduce CO2 emissions, but its carbon footprint and cost assessment are still insufficient. In this paper, coal mining to oil production is taken as a life cycle to evaluate the carbon footprint and levelized costs of direct-coal-to-liquid and indirect-coal-to-liquid coupled with the carbon capture utilization and storage technology under three scenarios: non capture, process capture, process and public capture throughout the life cycle. The results show that, first, the coupling carbon capture utilization and storage technology can reduce CO2 footprint by 28%–57% from 5.91 t CO2/t·oil of direct-coal-to-liquid and 24%–49% from 7.10 t CO2/t·oil of indirect-coal-to-liquid. Next, the levelized cost of direct-coal-to-liquid is 648–1027 $/t of oil, whereas that of indirect-coal-to-liquid is 653–1065 $/t of oil. When coupled with the carbon capture utilization and storage technology, the levelized cost of direct-coal-to-liquid is 285–1364 $/t of oil, compared to 1101–9793 $/t of oil for indirect-coal-to-liquid. Finally, sensitivity analysis shows that CO2 transportation distance has the greatest impact on carbon footprint, while coal price and initial investment cost significantly affect the levelized cost of coal-to-liquid.

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Electrochemical CO2 reduction to C2+ products over Cu/Zn intermetallic catalysts synthesized by electrodeposition
Ting DENG, Shuaiqiang JIA, Shitao HAN, Jianxin ZHAI, Jiapeng JIAO, Xiao CHEN, Cheng XUE, Xueqing XING, Wei XIA, Haihong WU, Mingyuan HE, Buxing HAN
Front. Energy    2024, 18 (1): 80-88.   https://doi.org/10.1007/s11708-023-0898-0
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Electrocatalytic CO2 reduction (ECR) offers an attractive approach to realizing carbon neutrality and producing valuable chemicals and fuels using CO2 as the feedstock. However, the lack of cost-effective electrocatalysts with better performances has seriously hindered its application. Herein, a one-step co-electrodeposition method was used to introduce Zn, a metal with weak *CO binding energy, into Cu to form Cu/Zn intermetallic catalysts (Cu/Zn IMCs). It was shown that, using an H-cell, the high Faradaic efficiency of C2+ hydrocarbons/alcohols (FEC 2+) could be achieved in ECR by adjusting the surface metal components and the applied potential. In suitable conditions, FEC2+ and current density could be as high as 75% and 40 mA/cm2, respectively. Compared with the Cu catalyst, the Cu/Zn IMCs have a lower interfacial charge transfer resistance and a larger electrochemically active surface area (ECSA), which accelerate the reaction. Moreover, the *CO formed on Zn sites can move to Cu sites due to its weak binding with *CO, and thus enhance the C–C coupling on the Cu surface to form C2+ products.

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