The Chinese Academy of Engineering (CAE) released the
Engineering Fronts 2022 on December 15, 2022 [
1]. Over 90 engineering research fronts and 90 engineering development fronts, including 3 engineering research fronts and 3 engineering development fronts in the fields of Energy and Electrical Science and Technologies were selected by the
Engineering Fronts 2022, which summarized the new trends and characteristics of engineering innovation in this discipline worldwide, and was an important guide for the future development of Energy and Electrical Science and Technologies.
Engineering Fronts 2022 also provided a reference for the topic selection of
Frontiers in Energy, one of the engineering journals of the CAE (Transactions of CAE).
1 Method and procedures for engineering research fronts
Under the guidance of the Energy and Electrical Science and Technologies Section of the CAE led by Prof. Shilie WENG, Prof. Guangxi YUE, and Prof. Zhen HUANG, and supported by the Office of the Ministry of Energy and Mining Engineering in Second Bureau of CAE, the engineering research fronts was selected step by step in three stages: data preparation and mining, data analysis, and expert research and judgement. In the data preparation and mining stage, the experts in this field and those in Library and Information Science determined 96 journals and 579 conferences as the retrieval data source of the research fronts. In the data analysis stage, Clarivate Analytics searched and mined the literature list, screened out the most influential papers cited in the top 10% in 2016–2021, and conducted co-citation clustering analysis on the highly influential papers to obtain the literature clustering theme. To compensate for the lack of fronts caused by the limitations of data mining algorithms or data lag, experts nominated the fronts to fill the gaps in quantitative analysis results. In the expert research and judgment stage, the experts selected ten candidate research fronts through the analysis of literature clustering theme and nomination fronts, and finally selected three engineering research fronts in the fields of Energy and Electrical Science and Technologies after extensive questionnaire survey and multiple rounds of expert meetings, and interpreted as follows.
(1) Core materials of high safety and high energy-density batteries
The emergence of battery technology has changed our lives. It is a core component of electronic devices, electric vehicles, and smart grids. The energy density of existing lithium-ion batteries (LIBs) is close to its theoretical value, which is not adapted to the electric vehicle market. Safety issues have also become a significant obstacle hindering the development of battery technology in large-scale applications. Batteries are mainly composed of cathodes, anodes, and electrolytes. The energy density of batteries depends on the electrodes, whereas electrolytes determine their safety. The energy density of an electrode depends on voltage and capacity. To realize high-energy density cathodes, high-voltage cathode materials and high-capacity sulfur materials are being explored; concerning anodes, lithium metal anodes and silicon materials are being researched; finally, regarding electrolytes, solid-state electrolytes are being investigated. Solid-state electrolytes fundamentally solve safety problem of batteries, and they enable the use of high energy-density electrodes. Solid-state electrolytes can be divided into solid polymer electrolytes, solid inorganic electrolytes, and solid composite electrolytes. Solid composite electrolytes, which are actively researched for next-generation batteries, combine the advantages of good processability of polymer electrolytes and high ionic conductivity of inorganic electrolytes.
(2) Flexibility improvement of renewable energy power generation and power grid support theory
The development of renewable energy generation, such as wind and solar power, is a promising approach for China to facilitate a clean and low-carbon transition of the energy and power industry and achieve the “” carbon neutrality goal. Different from conventional electricity generators with continuous and controllable power output, the features of renewable generation are strong volatility and weak controllability. This fundamentally changes the physical form and operation mode of power systems. With the rapid development of centralized and distributed renewable generation, as well as the gradual decommissioning of thermal power units, the uncertainty continues to increase in both the generation and consumption sides of power systems, and flexible regulation resources are becoming increasingly scarce. The power supply-demand balance and secure economic operation of power systems are facing unprecedented challenges. It is of utmost importance to investigate the theory and key technologies for flexibility improvement of renewable generation and power grid support, and for improvement of wide-area and efficient integration of a high proportion of renewable generation. Related research topics include mechanisms, operation strategies, and market mechanisms of flexibility balance in power systems; techno-economic analysis of the flexible transformation of thermal generation units and their conversion to regulating units; transient voltage support technology for large-scale wind and solar power generation systems; stability control technology of large-scale wind and solar power generation system; coordinated planning and operation of multi-period and multi-type energy storage technologies; and hierarchical aggregation, coordinated control, and distributed transaction of massive heterogeneous demand-side resources.
(3) Synthesis of ammonia by electrochemical nitrogen reduction in organic system
Ammonia is the second most produced commodity in the world. Motivated by the global shift toward sustainable energy, ammonia synthesis by electrochemical nitrogen reduction reaction (NRR) has attracted wide attention and is regarded as a promising alternative to traditional Haber-Bosch process. The electrochemical NRR can be performed in aqueous and organic systems and the later usually have higher Faradaic efficiency. Among the investigated modes of NRR, a lithium- mediated process in organic electrolytes distinguishes itself by outstanding ammonia yield rate and Faradaic efficiency. The lithium-mediated process takes place in three steps. Electrodeposition of metallic lithium first occurs, and then lithium-nitrogen based compound is produced by spontaneous reaction with reactive lithium. Based on this, the ammonia is formed by the protonation reaction. The performance of organic lithium-mediated systems is directly correlated with characteristics of lithium depositions. The study of lithium-mediated NRR is mainly focused on optimization of electrolytes, including the selection of proton donor, lithium salt, and additive. The highest reported current density and Faradaic efficiency are approximately 1 A/cm2 and 100%, respectively, which are even better than the targets set by the REFUEL program of the US Department of Energy (current density is 0.3 A/cm2; faradaic efficiency is 90%; energy efficiency is 60%). Before practical use, higher energy efficiency and better durability must be accomplished.
2 Method and procedures for engineering development fronts
The selection of engineering development fronts was also conducted in three stages. The experts in this field and those in Library and Information Science determined about 117 document patent retrievals. Clarivate Analytics, based on the Patent Research and Analytics Platform of Derwent Innovation (DWPI), clustered the topics of the top 10000 highly influential patent families cited in the Patent Research and Analytics Platform of DWPI in 2017–2021 according to the title and abstract of DWPI, obtained a ThemeScape patent map that could quickly, and intuitively present the development technologies in the fields of Energy and Electrical Engineering.
The experts in this field interpreted the development fronts of alternative projects from the patent map, nominated the engineering development fronts of alternative projects to supplement the data analysis results, and finally selected 12 engineering development fronts of alternative projects. After extensive questionnaire survey and several rounds of expert meetings, 3 engineering development fronts in fields of Energy and Electrical Science and Technologies were finally selected, and interpreted as follows.
(1) Large scale wind solar complementary power generation and stable grid connection technology
Wind power and solar power are naturally complementary. Wind-solar-storage complementary power generation systems formed with energy storage systems can overcome the volatility and randomness of wind and solar resources, smooth over all power fluctuations in the large-scale wind-solar bases, and ensure the stable operation of new energy high-penetration power system. In recent years, with the continuous increase of the proportion of wind and solar power generation, wind and solar hybrid power generation has evolved toward high voltage levels and multi-station coordination. In addition, the forms of energy storage used for coordination and complementarity are also changing to green and low carbon; this is the case of large-scale hydrogen storage and pumped storage stations. The key technologies involve in the large-scale wind-solar-storage hybrid power generation system including site selection and capacity determination of wind farms, photovoltaic power stations and energy storage power stations; wind-solar-storage coordinated control, dispatch and economic operation technology; the active power grid supporting technology of wind-solar-storage hybrid power generation system; unplanned and off-grid seamless switching technology of wind-solar-storage hybrid power generation system; stable control technology of wind-solar-storage hybrid power generation system.
In the future, the focus of large-scale wind-solar-storage hybrid power generation will be, among other topics, on the development of active grid-supporting technology, unplanned grid-connected and off-grid seamless switching technology, and broadband oscillation suppression to support the steady operation of high-energy-penetration-rate power systems.
(2) Fast and flexible peak shaving technology for coal-fired units
Rapid and flexible peak shaving of coal-fired units is a coal-fired power generation technology with capabilities of rapid response and safe and stable operation at low loads according to the load change command of the power grid. For countries/regions that use coal-fired power generation at large scale, this technology is the only one, and consequently key, to increase the proportion of renewable energy power generation and ultimately achieve carbon neutrality.
Renewable energy has the characteristics of fluctuation, intermittence, and uncertainty. Besides, there is a significant space-time difference between the supply of renewable energy and energy consumption of terminal users. Therefore, to ensure the safety of the power grid and stable supply of energy, it is necessary to rely on coal-fired units for rapid and flexible peak shaving in an extensive period of time as long as the energy storage technology remains immature and expensive. The main technical development directions include: ① ultra-low load pulverized coal boiler combustion technology and circulating fluidized bed combustion technology under various coal qualities; ② hydrodynamic safety and reliability, and economy of coal-fired units, under ultra-low load and varying load operation; ③ rapid start and stop technology of coal-fired units; ④ rapid response mechanism of unit load and coupling technology of coal-fired units and various energy storage systems; ⑤ influence of rapid load change of coal-fired units on heating surface materials; ⑥ full-load ultra-low emission technologies and carbon dioxide emission reduction assessment of coal-fired units; ⑦ economic and efficient thermal power decoupling technology and wide load peak shaving technology for thermal power coal-fired units. The core indicators of fast and flexible peak shaving technology are the response speed to the load change command of the power grid. The development trend is a progressively faster peak shaving speed, in which the supercritical unit reaches above 5% PE/min (rated load/min), and the subcritical unit reaches above 8% PE/min (rated load/min).
(3) Ammonia-fueled engine technology
Fossil fuel-based engines, which are commonly used in power generation, land transportation, ship power, aerospace industry, and other sectors, are one of the major contributors to carbon dioxide emissions. Ammonia, a carbon-free fuel, is viewed as a viable approach to minimize carbon emissions with respect to traditional carbon-based fuels. However, the combustion of ammonia may encounter some issues, including poor stability and misfire due to high self-ignition temperature and slow flame propagation speed. In addition, the emissions of nitrogen oxides (NOx) from ammonia engines is generally high because the fuel molecule itself contains nitrogen, which also hinders the development of ammonia to substitute engine fuels.
Dual-fuel technology, which combines ammonia with high-reactivity additives to improve combustion stability, is a potential path to the realization of ammonia-fueled engines. Compared with other high-reactivity fuels, hydrogen has the advantage of being another carbon-free fuel that can be produced online through ammonia cracking. High-efficiency catalytic techniques that convert ammonia into hydrogen is gaining attention. However, owing to the strong diffusivity of hydrogen and high latent vaporization heat of ammonia, the concentration and temperature distribution of ammonia-hydrogen fuel in the engine combustion chamber is often highly inhomogeneous. Therefore, a proper fuel injection strategy to achieve a reasonable distribution of ammonia fuel in the engine and effectively control the combustion phase is a key technology to be accomplished. Although the addition of high-reactivity fuels such as hydrogen improves the combustion stability of ammonia, it also increases the combustion temperature and therefore increases NOx emissions. Under fuel-rich conditions, NOx emissions may reduce to a certain extent, but it simultaneously leads to other problems such as poor combustion economy and increased unburned ammonia emissions. Therefore, it is imperative to couple after-treatment methods such as selective catalytic reduction (SCR) and ammonia slip catalyst (ASC) in ammonia-fueled engines to reduce NOx and unburned ammonia emissions.
In general, single ammonia-fuelled engine technology is still facing many challenges. To achieve the efficiency and emission optimization of ammonia fueled engines, several advanced technologies should be realized, including: ① fast and efficient ammonia online cracking to produce high-reactivity hydrogen; ② proper fuel injection strategies, such as high-temperature rich-fuel jet through pre-chamber and multi-stage injection in the main chamber considering the existence of strong turbulences; ③ moderate or intense low oxygen dilution (MILD) combustion, which allows high temperature and low-oxygen concentration to simultaneously reduce NOx emission and enhance combustion stability; ④ a strategy to prevent pre-ignition and knocking of ammonia-hydrogen fuel in large space of marine engines; ⑤ after-treatment system, such as SCR and ARC (Ammonia Slip Catalysts), with wide operation window and high conversion efficiency for NOx and unburned ammonia emissions.
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