《1 Engineering research hotspots and engineering research focus》

1 Engineering research hotspots and engineering research focus

《1.1 Development trends of engineering research hotspots》

1.1 Development trends of engineering research hotspots

The Top 10 engineering research hotspots (Table 1.1.1) involve a number of research directions in the field of energy and mining engineering1 , among which “High-efficiency solar cells, including thin-film solar cells, perovskite solar cells, silicon heterojunction solar cells, back-contact solar cells, and back-contact silicon heterojunction solar cells,” “Research and optimization of hybrid renewable energy system,” “CO₂ fixation, storage, and utilization,” “Solar thermochemical processes,” “Wave energy and its potential assessment,” and “High-efficiency battery thermal management system” are emerging hotspots, whereas “Flow, heat/mass transfer, and combustion under extreme conditions,” “Regional industrial energy efficiency and CO₂ emission reduction,” “Stirling engine,” and “Structure, materials, and performance of the vertical-axis wind turbine” are developments in traditional research hotspots. The annual numbers of papers of each hotspot published in core journals from 2011 to 2016 are listed in Table 1.1.2.

(1) High-efficiency solar cells, including thin-film solar cells, perovskite solar cells, silicon heterojunction solar cells, back-contact solar cells, and back-contact silicon heterojunction solar cells

The photovoltaic (PV) industry mainly employs PV materials, solar cells and modules, PV inverters, and PV system integration; the key element is the solar cell. At present, commercial solar cells can largely be divided into crystalline silicon solar cells and thin-film solar cells. Scientists, technicians, and even the human kind face the pressing task of developing more efficient, economical,

《Table 1.1.1 》

Table 1.1.1 Top 10 engineering research hotspots in energy and mining engineering

¹ The Standing Committee of the Department of Energy and Mining Engineering agreed, on January 15, 2017, that at the first stage of the project, the disciplines of energy and mining engineering should only involve the energy and electrical science, technology, and engineering (thermal power engineering, electrical engineering, hydropower engineering, energy new technology), the nuclear science, technology, and engineering (nuclear energy engineering, nuclear materials and nuclear fuels, nuclear safety, protection, and environment, and applications of nuclear science and technology), the geology resources science, technology, and engineering (oil and gas resources and exploration), and the mining science, technology, and engineering (coal exploration, oil and gas engineering).

《Table 1.1.2》

Table 1.1.2 Annual number of core papers belonging to each of the top 10 engineering research hotspots in energy and mining engineering

and environmental-friendly solar cells. Up to present, owing to the flourishing development of the science and the technology of semiconductor materials—predominantly silicon—and because of the low cost and wide application of silicon, commercial solar cells based on silicon p-n junction still dominate the PV market (with a market share of over 90%). Nevertheless, silicon-based structures now have to face increasing competition from other materials, such as various semiconductor thin films and perovskite. The current trends in high-efficiency solar cells involve both the distinctive crystalline silicon (c-Si) solar cells, with an efficiency of higher than 22%, and the development of advanced thin-film solar cells. The high-efficiency crystalline silicon solar cells mainly include amorphous silicon/crystalline silicon heterojunction (a-Si/c-Si HJT) solar cells, back-contact solar cells, back-contact silicon heterojunction (BC-HJT) solar cells, bifacial solar cells, and broad-spectrum solar cells. The current research hotspots in advanced thin-film solar cells include the new perovskite solar cell, the copper–indium–gallium diselenide (CIGS) solar cell, and its substitute, the copper–zinc– tin sulfide (CZTS) solar cell. Focusing on these new PV technologies, scientists in universities and research institutions in America, Germany, Japan, and Switzerland are working together, and have already achieved remarkable breakthroughs.

(2) Research and optimization of hybrid renewable energy systems

Intermittence and fluctuation are intrinsic to renewable energies, such as solar and wind energies, and their outputs can be stabilized through a hybrid renewable energy system. Systems such as solar-wind hybrid power systems and renewable energy-gas turbine (or internal combustion engine) hybrid power systems can take full advantage of renewable energy sources as well as meet the needs of user loads. At present, there are two main categories of hybrid renewable energy systems. One category comprises the combination of different renewable energy sources—such as wind, solar, and water power— in a complementary manner to overcome the discontinuity and instability of a single energy source. The other category comprises hybrid power systems, in which a renewable energy source is combined with existing fossil fuels. Power generation in a hybrid manner can substantially improve energy efficiency, mitigate environmental pollution, and help promote a low-carbon economy and the optimization of the energy mix.

From the engineering perspective, the development of hybrid renewable energy systems calls for knowledge in disciplines such as power machinery, fluid machinery, material, power electronics, and control technologies. The current research hotspots mainly include the topological structures integration, the thermodynamic cycle analysis of hybrid energy systems, the establishment of a dynamic model of hybrid energy systems, the maximum utilization of renewable energy sources, the minimization of the utilization of natural gas, and the overall balance between the supply and demand of different energies.

(3) Flow, heat/mass transfer, and combustion under extreme conditions

Flow, heat/mass transfer, and combustion are closely related to numerous industrial applications in energy utilization systems. These applications entail various extreme conditions, such as high temperature, high pressure, and high heat flux. This engineering research focus contains mainly four branches, including evaporation and condensation, boiling heat transfer, heat transfer optimization, and combustion. Boiling, evaporation, and condensation are the main phenomena that take place in gas-liquid phase change processes, which are widely employed in the thermal management of electronic devices, the desalination of sea water, and the combustion process. Constrained by the fixed heat-transfer area of heat exchangers (heat sinks), heat transfer optimization is the maximization of heat transfer under conditions of high temperature, high pressure, ash and dust-laden settings, and high heat flux by changing the type of heat transfer (heat sink), structures, surfaces, and fluids. Combustion is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant. Combustion devices— such as gas turbines, internal combustion engines, and industrial furnaces—are currently employed as the main method to generate power. The key technologies that are the research focus of contemporary engineering involve multi-phase, multi-component turbulent flows, chemical reactions, and multi-objective optimization; these require extensive study through theoretical analysis, high-precision numerical simulation, and advanced optical diagnostics. While developing basic theories, the theoretical guides and technologies for practical industrial production should not be neglected.

(4) Solar thermochemical processes

In solar thermochemical processes, the solar energy is converted into chemical energy that is stored in hydrocarbon fuels; therefore, the efficient conversion and high-density storage of solar energy becomes possible. This technology enables regions that are rich in solar resources to convert their resources into secondary fuels, which are then transported to different regions. This technology also serves as a solution to the instability and discontinuity of power systems that solely dependent on solar energy.

The applied research of thermochemical processes of solar energy includes fields such as hydrolysis reaction, methane reforming and coupling, coal gasification, and thermal decomposition of fossil fuels. Several advances have been accomplished in the design of high-temperature solar energy thermochemical reactors, the research and development of catalysts, the steam resistance array of complicated solar collector, and the material of high-temperature direct absorption vacuum pipe. In recent years, the utilization and fixation of CO₂ via solar thermochemical processes has attracted considerable attention. Metal oxides act as catalysts or redox mediators (oxygen carriers) to obtain CO or hydrocarbons by decomposing CO₂. The procedure mainly includes CO₂ decomposition and the two-step reaction of redox recycling via the thermochemical process. In that respect, researchers have focused on key technologies, such as the preparation of high-efficiency catalysts, the investigation of reaction dynamics and the mechanisms thereof, and the selectivity of target products.

(5) CO₂ fixation, storage, and utilization

CO₂ is the primary greenhouse gas. Carbon capture and storage (CCS) has been considered to be the predominant solution to the issue of reducing CO₂ emissions. The storage locations include geological structures (reservoirs of petroleum and methane, deep salt marshes, and unmineable reservoirs of coal) and oceans. CCS, deemed as a mature technology, has been put into practice in several regions around the world. Furthermore, attention has also been paid to the evaluation of the cost, the economic benefits, and the safety of CCS. In recent years, CO₂ fixation and utilization through chemical conversion has ignited the interest of the scientists who are committed to decreasing the emissions of CO₂.

CO₂ is put into both physical and chemical use. The physical properties of CO₂ are put into effect when it is used as an air-conditioner refrigerant, a curing hardener, and a food additive. Furthermore, CO₂ is applied in oil displacement technology as well; which also plays a role in CO₂ storage, owing to the fact that it can easily dissolve in crude oil, thus reducing its viscosity. The chemical use of CO₂ presents broader prospects. The activation of CO₂ molecules is a key technology, which is currently realized through biological methods, photochemical and electrochemical reduction, and heterogeneous and homogeneous catalytic reduction. Using CO₂ as a material, it may be realized by synthesizing small molecular compounds or the polymer materials as well (e.g., methane, formic acid, dimethyl carbonate, urea, salicylic acid, carbonate, carboxylic acid, and hydrocarbons). Much attention has been paid to the synthesis of gases and liquid fuels by CO2 for the purpose of energy conversion and utilization. The preparation of catalysts with superior conversion efficiency, selectivity, and stability, and both the kinetics and the mechanisms of catalytic reactions have become research hotspots.

(6) Regional industrial energy efficiency and CO₂ emission reduction

The regional industrial energy efficiency measures the benefits produced from energy consumption in the industrial production process within a given area. It is a type of input-output ratio, and it refers to the ratio of the energy cost to the benefits obtained by a company; a lower energy cost indicates higher energy efficiency. The low comprehensive efficiency of energy utilization (particularly the industrial energy utilization efficiency) is a widespread problem in developing countries. Owing to the rapidly growing economy of China, energy consumption has greatly increased, with the emission of CO₂ rising sharply. Faced with the dual challenge of economic development and CO₂ emission reduction, the regional industry should strictly control the total energy consumption, as well as adjust the industrial structure and transform the pattern of energy utilization from extensive to intensive, thus improving the energy utilization efficiency. The efficiency of coal utilization should be enhanced, and the utilization of renewable energy should be broadened based on the existing domestic energy structure. Moreover, the process of managing the pressures of energy demands while reducing the energy-supply constraints should be improved, in the process of alleviating environment pollution problems brought by energy production. The current research hotspots mainly include energy consumption analysis based on big data, real-time energy consumption measurements and statistics, high-efficiency industrial energy-conversion technology, and CCS technology.

(7) Wave energy and its potential assessment

Energy stored in wave is usually considered as a kind of ramification of solar radiation energy, and comes from wind energy. One of advantages of the wave energy is its high energy quality—mechanical energy of vibration. Furthermore, there is little power loss during wave energy transportation over a long distance. At present, the efforts of the utilization and exploitation of wave energy is mainly focused on the method of the assessing of wave energy deposit and optimization design of power generation devices.

Based on the advanced real-time monitoring system and abundant test data, the research on the assessing method is mainly concentrated on how to adopt a reasonable mathematic model and then precisely predict and estimate distribution of the wave energy deposit in a certain ocean area during a certain future period (short or long), which can provide a theoretical basis and support for feasibility analysis of wave energy exploitation programs.

In general, power generation devices for wave energy can be classified into several types according to extracting methods, such as oscillating water column type, raft-type, pendulum type, duck-like type, overtopping type, point absorber type, and so on. Among these types, oscillating water column power generation device is the earliest developed type. Nowadays, the study of the power generation device is focused on how to develop a kind of wave energy extracting equipment, which not only owns more efficient performance but also can be stably and long-term operated in oceanic environment. In addition, the effects of power generation devices on marine ecological environment and fishery industry development are also evaluated and analyzed in recent years.

(8) Stirling engine

The Stirling engine is an externally heated piston engine that operates with gas as the working medium in a closed regenerative cycle. A typical Stirling engine work cycle includes the following processes: the working medium is heated through the heat exchanger by an external heat source; then it expands and gives out effective power; after that, the working medium is cooled and compressed, and is then reused. The ideal Stirling engine consists of a cylinder with opposed pistons, a heater, a cooler, and a heat regenerator that is installed between two pistons. The pressure-swirl nozzle is typically used in Stirling engines, and it features a simple structure, reliable performance, good atomization performance, and low energy consumption. Up to now, the research on the Stirling engine has focused on the work cycle and the performance of key components.

A wide range of energy sources can be used to power Stirling engine. In addition to conventional diesel, a variety of alternative fuels, including methane, coal-bed methane, natural gas, and even straws and wood, can be used. Therefore, the performance of the Stirling engine using alternative fuels has become a research hotspot, which mainly focuses on applications in fields such as the combined cooling, heating, and power (CCHP) and standby power stations.

The Stirling engine can operate in environments that are entirely different from the atmosphere. The small vibrations and low noise levels during its operation render it ideal for power generation in submarines during their long voyages, without them leaving trails behind. At present, several countries are committed to the research and development of the Stirling engine used in submarines and submersibles. Therefore, the spray, combustion, and reliability of the Stirling engine under high back pressure are focal points of the research.

(9) High-efficiency battery thermal management system

Battery thermal management, which affects the safety, life, and performance of a battery, ensures that batteries operate in an ideal thermal environment. The following key technical problems should be solved: battery heat generation mechanism and heat generation prediction model, heat transfer medium materials and devices, highly efficient heat management system model and its design method, temperature control of the battery system, temperature difference control between battery cells. Active or passive heat management is performed on the batteries by means of external cooling, heating (or electric heating), or by maintaining a stable temperature throughout the heat transfer medium, which may be air, liquid and phase change materials, or a composite medium. By establishing a coupled simulation model and a method for the thermo-mechanical-electric multiphysics field of the battery system considering the entire working condition and the entire environment temperature condition, the research on the thermal energy management system model and the multi-objective design optimization can help to maintain the temperature of the battery system at an ideal level, and to control the temperature difference between battery cells. A highly efficiency thermal management of the battery system can be achieved via technologies such as heat pipes, heat pumps, and micro-channel heat exchangers. Intelligent control of the battery system temperature can be achieved through temperature feedback from multiple points; this feedback represents the distribution of the temperature field in the battery system. It is expected that the following studies will be carried out: thermal energy management technology based on advanced phase change materials combined with fluid heat transfer; the predictive control of battery thermal management based on battery heat generation prediction; active lean thermal management for battery cell or battery system partition; internal temperature adjustment thermal management technology of the battery cell through the external current pulse excitation or built-in heating circuit using the battery’s own energy.

(10) Structure, materials, and performance of the vertical-axis wind turbine

The vertical-axis wind turbine is a type of small- and medium-sized wind power equipment, which can be easily installed and maintained, and has low start-up wind speed. It features prominently in the use of wind energy. It can be classified into three types, namely the drag type, the lift type, and the lift-drag type, according to the operation principle of the wheel blade. Compared with the traditional wind turbine, the vertical-axis wind turbine presents the following advantages: ① efficient use of the wind field， smaller operation area, simple structure, low weight, convenient maintenance, good response to wind from all directions, and strong wind resistance; ② longer service life and less noise pollution; ③ no need for the tail and yaw system to drive the blades; and ④ the speed-increasing gear box and generator can be installed on the ground. This type of wind turbine can be combined with solar power, small hydropower, natural gas power generation, and geothermal power to build complementary power systems.

Wind power generation technology is a comprehensive and systemic engineering that involves disciplines such as aerodynamics, computational fluid dynamics, material mechanics, automatic control, and electromechanics. The main research hotspots are the aerodynamic analysis of the blades, the high-performance blade design technology, the structural dynamics optimization, and the connection of wind turbines to the grid and control techniques.

《1.2 Understanding of engineering research focus》

1.2 Understanding of engineering research focus

1.2.1 High-efficiency solar cells, including thin-film solar cells, perovskite solar cells, silicon heterojunction solar cells, back-contact solar cells, and back-contact silicon heterojunction solar cells

(1) Concept elaboration and development status

Fossil fuels help promote the development of human society; however, the excessive consumption of the fossil fuels leads to global warming and deterioration of the environment as well, thus posing an enormous threat to the survival of mankind. Most countries have expressed their willingness to change their energy consumption strategies and develop renewable energies. Among various renewable energy sources, solar energy has attracted considerable attention because it is an inexhaustible, clean, and safe energy that can be used in several regions. In recent decades, although renewable energies represented by solar PV power generation have been growing rapidly, solar PV accounted for only 1.2% of the global electricity generation capacity by the end of 2015. In addition, the PV-generated electricity in China was 66 200 GW·h in 2016, which only accounted for 1.1% of China’s electricity consumption. It is anticipated that PV-generated electricity will account for more than 10% of the global energy supply in 2030, and will thus contribute substantially to the global energy supply and the adjustment of the global energy mix. Therefore, the solar PV industry has bright prospects both in China and in the rest of the world. The solar PV industry mainly includes PV materials, solar cells, PV modules, PV inverters, and PV system integration; the key element is the solar cell. At present, commercial solar cells can largely be divided into crystalline silicon solar cells and thin-film solar cells. It is a pressing task facing scientists, technicians and even the human kind to develop more efficient, economical and environmental-friendly solar cells.

Up to present, owing to the flourishing development of the science and the technology of semiconductor materials—which are predominantly silicon—and because of the low cost and wide application of silicon, commercial solar cells based on silicon p-n junction still dominate the PV market (with a market share of over 90%), Nevertheless, silicon-based structures now have to face increasing competition from other materials, such as various semiconductor thin films and perovskite. The current trends in high-efficiency solar cells involve both the distinctive c-Si solar cells, with an efficiency of higher than 22%, and the development of advanced thin-film solar cells. The high-efficiency crystalline silicon solar cells mainly include a-Si/c-Si HJT solar cells, back-contact solar cells, BC-HJT solar cells, bifacial solar cells, and broad-spectrum solar cells. The current research hotspots in advanced thin-film solar cells include the new perovskite solar cell, the CIGS solar cell, and its substitute, the CZTS solar cell. Crystalline silicon solar cells present great commerical potentials owing to their high efficiency, stable performance, and the abundance of silicon. Futhermore, several advanced industrial techniques are already available for c-Si solar cells. The present mainstream techniques, e.g., the passivated emitter and rear cell (PERC) and the bifacial solar cell, have both been industrialized in earnest (with efficiencies of 21.0%–21.5%). In addition, the industrial processes of a-Si/c-Si HJT solar cells, back-contact solar cells, and broad-spectrum solar cells are developing rapidly. The next-generation technology—the back-contact silicon heterojunction solar cell—is growing more and more mature. In contrast, there is room for improvement for the advanced thin-film solar cells to reach mass industrial production levels. At the present stage, the work on perovskite solar cells is mainly focused on improving the efficiency and long-term stability, increasing the cell area, and finding nontoxic substitutes, whereas the industrial process is only at the beginning. The work on CIGS solar cells technology is focused on narrowing the wide gap between the efficiency of industrial modules (~16%) and that of the small-area solar cells fabricated in laboratories, and on replacing CIGS with CZTS. It is expected that thin-film solar cells, with the flexible thin-film battery leading the way, will play an important role in the development of future PV technology as well, owing to their low material consumption, increased efficiency, and their flexibility. In fact, flexible solar cells, which use flexible materials such as plastic or metal as a substrate, have always been a subject of interest owing to their wide range of applications.

(2) Current situation in China

Since 2005, the China’s PV industry has realized striking progress. Approximaely 75% of all PV modules produced in the last two years were made in China, which is expected to remain the largest PV producer in the following years. At present, the conversion efficiency of conventional single crystalline silicon solar cells has reached 20%– 21%, and that of the multi-crystalline silicon solar cells has reached 18%–19%. Most China’s PV companies focus on conventional crystalline silicon solar cells; therefore, the China’s PV industry may obtain a leading position in this domain, establsihing the PV industry as one of the few emerging strategic industries of China with international competence. Certain important China’s PV firms, universities, and research institutions are pursuing break-throughs in the mass production of crystalline silicon solar cells that present an efficiency of over 22%. With the support of national projects—863 Program—and through international collaborations, the technologies of a-Si/c-Si HJT solar cells and back-contact solar cells have significantly progressed. However, the research on back-contact silicon heterojunction solar cells is only at its beginning, and falls behind compared to that of developed countries. In the past few years, there has been rapid progress in the research and industrialization of perovskite solar cells; China is already a competitive player in this technology. Although several universities and research institutions in China have started studying CIGS solar cells a long time ago, there is still great room for improvement to level with advanced players. In the past few years, certain key China’s companies that focus on thin-film solar cells have been importing or even purchasing CIGS from PV companies with advanced fabrication techniques from Europe and the USA to improve China’s CIGS technology. Consequently, China has become one of the leading players with its rapidly-growing CIGS technology. It should be pointed out that China still lacks a national PV laboratory similar to the Fraunhofer Institute for Solar Energy Systems (ISE) in Germany, the National Renewable Energy Laboratory (NREL) in America, the National Institute of Advanced Industrial Science and Technology (AIST) in Japan, and the Energy research Centre (ECN) in the Netherlands. The research on solar cells in China is mainly conducted in universities, research institutions, and key PV companies. Although the China’s Ministry of Science and Technology established two national key laboratories several years ago, they were both inside PV companies; this affects the application and spread of advanced PV technologies.

Although China is a large PV manufacturer, it is still not a major PV technology innovation center. We should be aware of the severe situation faced by the PV industry. Currently, researchers in universities and research institutions in the USA, Germany, Japan, and Switzerland closely collaborate, and have made great progress in CIGS solar cells, perovskite solar cells, a-Si/c-Si HJT solar cells, back-contact solar cells, back-contact silicon heterojunction solar cells, perovskite/crystalline silicon tandem solar cells, bifacial solar cells, and broad-spectrum solar cells. When basic research on innovative PV techniques is conducted at the highest level around the world, what strategies should China’s PV specialists follow? Should China be satisfied with only being a large PV manufacturer? Only through innovation at the source and through institutional reforms can China become an advanced and innovative industrial PV power.

(3) Major countries and research institutions

In the research on distinctive high-efficiency crystalline silicon solar cells, American and Japanese companies are frontrunners. The American company SunPower and the Japanese company Panasonic have manufactured solar cells with an efficiency topping 24%, and realized the industrialization of solar cells with average efficiencies of 22.5%–23.0% by using interdigitated back contact and a-Si/c-Si HJT, respectively. Recently, the Japanese firm Kaneka has produced a crystalline silicon solar cell with a record efficiency of 26.6% by adopting the back-contact Si HJT technique. Although China has been progressing in the development of the advanced thin-film solar cells, the best techniques have been mastered in Europe, Japan, and Korea. The European thin-film solar cell alliance “Solliance” and the Ulsan National Institute of Science and Technology of Korea have both developed millimeter-sized perovskite solar cells with an efficiency that reaches 22.1%. In addition, the École polytechnique fédérale de Lausanne (EPFL) in Switzerland and the University of Oxford in England are at the forefront of the research on perovskite solar cells. The Center for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW) in Germany and the Solar Frontier of Japan have produced CIGS solar cells with an area of 0.5 cm² and efficiencies of 22.6% and 22.3%, respectively. Another recently developed field is the perovskite/silicon tandem solar cell. In 2017, Stanford University, collaborating with other universities and corporations, announced that its perovskite/crystalline silicon tandem solar cells (with an area of 1 cm² ) had reached an efficiency of 23.6%.

From Table 1.2.1, it may be seen that the major countries in which high-efficiency solar cells are studied are the USA, Germany, Japan, Switzerland, and Spain. Table 1.2.2 indicates that the research institutions that develop high-efficiency solar cells mainly include Ecole Polytech Fed Lausanne, IBM Corp., and the Fraunhofer Inst Solar Energy Syst.

Figure 1.2.1 shows the collaboration relationships between the major countries that contribute to the development of high-efficiency solar cells, which include thinfilm, perovskite, silicon heterojunction, back-contact, and

《Table 1.2.1 》

Table 1.2.1 Major producing countries or regions of core papers on the engineering research focus “High-efficiency solar cells, including thin-film solar cells, perovskite solar cells, silicon heterojunction solar cells, back-contact solar cells, and back-contact silicon heterojunction solar cells”

《Table 1.2.2 》

Table 1.2.2 Major producing institutions of core papers on the engineering research focus “High-efficiency solar cells, including thin-film solar cells, perovskite solar cells, silicon heterojunction solar cells, back-contact solar cells, and back-contact silicon heterojunction solar cells”

back-contact silicon heterojunction structures. Figure 1.2.2 shows the collaboration networks among major research institutes that focus on the aforementioned research field.

Table 1.2.3 lists the major producing countries or regions of citing core papers on the engineering research focus “High-efficiency solar cells, including thin-film solar cells, perovskite solar cells, silicon heterojunction solar cells, back-contact solar cells, and back-contact silicon heterojunction solar cells.” It may be seen that the USA, Germany, and Switzerland are the top 3 countries. Table 1.2.4 lists the major producing institutions of citing core papers on the aforementioned engineering research focus. Ecole Polytech Fed Lausanne, the IREC, and the Catalonia Inst Energy Res are the top 3 institutions.

1.2.2 Research and optimization of hybrid renewable energy systems

Developing renewable resources is an important approach to solving the problems of energy utilization and environment pollution in the world; it is an inescapable option and an effective measure to maintain the sustainable development of energy utilization. In the past ten

《Figure 1.2.1》

Figure 1.2.1 Collaboration network of the major producing countries or regions of core papers on the engineering research focus “High-efficiency solar cells, including thin-film solar cells, perovskite solar cells, silicon heterojunction solar cells, backcontact solar cells, and back-contact silicon heterojunction solar cells”¹

《Figure 1.2.2》

Figure 1.2.2 Collaboration network of the major producing institutions of core papers on the engineering research focus “High-efficiency solar cells, including thin-film solar cells, perovskite solar cells, silicon heterojunction solar cells, backcontact solar cells, and back-contact silicon heterojunction solar cells”

《Table 1.2.3》

Table 1.2.3 Major producing countries or regions of core papers that are cited by core papers on the engineering research focus “Highefficiency solar cells, including thin-film solar cells, perovskite solar cells, silicon heterojunction solar cells, back-contact solar cells, and back-contact silicon heterojunction solar cells”

years, the hybrid renewable energy system has attracted more and more attention worldwide.

From the perspective of availability of renewable energies and technology maturity, the hydropower, the wind power, and the solar power are the most realistic energy resources with broad prospects. A hybrid energy power system typically consists of two or more sets of distributed power supplies, an energy storage system, and various power electronic control devices. At present, there are two main categories of hybrid renewable energy systems. One category comprises different renewable energy sources— such as wind, solar, and water power—in a complementary manner to overcome the discontinuity and instability of a single energy source. The other category comprises hybrid power systems, in which a renewable energy source is combined with existing fossil fuels. Power generation

《Table 1.2.4》

Table 1.2.4 Major producing institutions of core papers that are cited by core papers on the engineering research focus “High-efficiency solar cells, including thin-film solar cells, perovskite solar cells, silicon heterojunction solar cells, back-contact solar cells, and back-contact silicon heterojunction solar cells”

in a hybrid manner can substantially improve energy efficiency, mitigate environmental pollution, and help promote a low-carbon economy and the optimization of energy mix.

From the engineering perspective, the development of hybrid renewable energy systems calls for knowledge in disciplines such as power machinery, fluid machinery, material, power electronics, and control technologies.

Renewable energy sources, such as wind power and solar power, are naturally complementary, and the hydropower has the ability to adjust quickly. From the perspective of time, solar energy is abundant during daytime, whereas the wind is quite weak. However, this situation is reversed during nighttime. During sunny days, sunlight is strong, whereas wind is weak; on the other hand, this situation is reversed during rainy days. The time limitation in solar power generation and the instability of wind power generation are the inherent defects of these energy sources. Wind-solar power generation can become a reliable groundwork for the development of national economy only if they are combined with the standby powers with considerable scale and better adjustment abilities. The hydropower is a type of large-scale power supply with good adjustment performance. It can be utilized to overcome the intermittence and instability of photovoltaic and wind power generation. The wind-solar-water power system can ensure the quality of the power supply. Therefore, it is critical to ensure that the power supply of the hybrid renewable energy system is continuous and stable.

Regarding the instability of renewable energies, such as wind and photovoltaic power, China utilizes thermal power and hydropower for the peak regulation of the power system. The combined power system of renewable energies and natural gas can be used in the peak regulation of the power system; this may improve the adaptability of the power grid to the changing load, while being more environment-friendly as well. The low emission of gas turbines offers great environmental benefits; they can substitute the coal-powered engine, and can contribute to peak regulation, particularly in well-developed cities that suffer from continuously increasing pollution.

To date, domestic and foreign institutions and researchers have conducted extensive research in the field of hybrid power generation technology. The latest research trends that are related to hybrid power generation, including wind-solar systems and renewable energy-fossil fuel systems, will be introduced in the following sections.

(1) Development status of wind and solar hybrid power systems

In terms of research and development on optimization software, the US Renewable Energy Laboratory has developed the HYBRID2 and HOMER systems to simulate and optimize wind-solar hybrid power systems. Based on a genetic algorithm, the University of Zaragoza developed an optimization software package that can be applied in complementary hybrid power systems, such as wind-solar and solar-diesel engine hybrid systems. In addition, the hybrid power system of the solar-diesel engine was improved and optimized.

In terms of practical applications, a skyscraper was built in 2007 in Dubai (United Arab Emirates), the power of which is supplied by both solar photovoltaic power equipment and wind turbines. This project is the first combination of urban buildings and solar-wind hybrid power systems, which carved a new path in the application of hybrid power systems. In 2009, China built the first national wind & solar & storage demonstration project at the wind farm located in the Shangyi and Zhangbei County in Zhangjiakou City of the Hebei Province. There are 500 MW of wind power and 100 MW of photovoltaic power generation systems, and energy storage systems with a storage capacity of 75 MW.

In recent years in China, various forms and sizes of hybrid renewable energy systems have been widely applied to street lights, remote oil well equipment, highway appliances, navigation stations, oil and gas delivery and heating devices, natural reserves, residential areas, industrial parks etc.

(2) Development status of other forms of hybrid power systems

Domestic and foreign scholars have conducted studies and analyses on hybrid power generation technologies, such as wind-hydrogen-biomass-solar hybrid power systems, wind and diesel hybrid power systems, underground coal gasification and high-temperature fuel cell hybrid power systems, solar and fuel cell hybrid power systems, and solar and geothermal flash hybrid power systems.

The first wind-hydrogen-biomass-solar hybrid power plant in Germany was officially put into operation in 2010; this power plant uses wind and solar power to generate electricity. The electricity is partly connected to the grid, and the remaining part is used to electrolyze water in order to produce hydrogen power. The total installed capacity is 6 MW, and when the wind and solar power cannot meet the demand, the hydrogen and biomass energy can be utilized as a supplementary energy-generation capacity. Norway has built a new type of wind and diesel hybrid power plant with an installed capacity of 2000 kW, which conserves more than 50% of fuels. In the Sequoia National Park of the USA, Schatz Energy Research Center of Germany and the Yurok Tribe built together a solar-cell and fuel-cell hybrid power system, which provides electricity mainly for wireless communication relay stations in remote areas, with a total efficiency of up to 64% and a net efficiency of over 48%.

In Europe, to meet the objectives of the European Commission project entitled “Photovoltaic fuel-cell hybrid system for electricity and heat production for remote sites (PVFC-SYS)”, a solar-cell and fuel-cell hybrid power system was built in France, the net efficiency of which reaches up to 50%. As geothermal energy has the advantage of stability, a solar–geothermal hybrid energy pilot plant was built in Aydin, Turkey, in 2014.

(3) Future trends

Hybrid renewable systems, a systematic engineering, utilize a variety of renewable energy resources to complement each other in accordance with the conditions of different energy resources and the applications of the generated power. Through intelligent coordination control and optimal dispatch, comprehensive energy efficiency is improved, renewable energy accommodation is promoted, and the benign cycle of ecological environment becomes facilitated. At present, there are two main models.

Terminal integrated power supply system: it mainly focuses on electricity, heating, cooling, gas, and other demands of end users. As an integrated energy supply infrastructure, it is operated in line with local conditions, and uses traditional energy and new energy in a complementary manner. This type of system could meet the objectives of coordinated supply of multiple energies and comprehensive cascade utilization of energy, typically through the use of various of modes, such as the combined heating, cooling, and electricity (CHCP) system that is fueled by natural gas, the distributed renewable energy, and smart micro-grid systems.

Wind & solar & hydropower & gas & storage multienergy complementary system: this system mainly presents the advantage of utilizing a comprehensive energy base, where wind power, solar power, hydropower, coal, and natural gas are combined to generate electricity. Moreover, it utilizes the ability of regulating the peak electricity load of cascade hydropower stations and thermal power plants that have flexible adjustment abilities. The integrated operation can enhance the stability of the power output, promote the accommodation of intermittent renewable energies, including wind and solar power, and enhance the overall benefits.

According to Table 1.2.5, countries that produce the highest number of core papers in “Research and optimization of hybrid renewable energy systems” are India, China, Iran, Greece, and Malaysia; the proportion of core papers in India, China, and Iran are more than 10%. From Table 1.2.6, it may be observed that the institutions with the highest number of core papers in this research field are the Hong Kong Polytech Univ, Univ Tehran, and the Univ Malaya, where the proportion of core papers in the Hong Kong Polytech Univ and the Univ Tehran is over 10%.

In Figure 1.2.3, it may be observed that Iran, Malaysia, Indonesia, and the US have placed greater focus on the collaboration with other countries and regions in this research field. Iran has a cooperative relationship with three countries, and has co-published several papers. China has large number of published papers as well, many of which are works that have been produced in collaboration with the USA. India has the highest number of published papers; however, scarce are in collaboration with other countries and regions.

According to Figure 1.2.4, Institutions that actively cooperate with other institutions are the Univ Tehran, the Univ Malaya, and the Univ Tenaga Nas, all of which have a cooperative relationship with each other; their respective number of published papers rank in the top 5 positions. Two Indian institutions maintain a cooperative relationship with each other. The Hong Kong Polytech Univ in Hong Kong of China, is the institution that has published the highest number of papers; however, it has not collaborated with other organizations.

In July, 2016, in order to accelerate the multi-energy complementary integrated optimization demonstration project, the National Development and Reform Commission and the National Energy Administration issued Implementation suggestions on the promotion of the multi-energy complementary integrated optimization demonstration project. This document aims to accelerate the construction of multi-energy complementary integrated optimization demonstration projects of renewable energy, so as to improve energy system efficiency, increase effective energy supply, meet reasonable demands, drive efficient investment, and promote stable economic growth.

There are two modes of integrated demonstration projects defined in the document: one is the terminal integrated system that is based on the electricity, heating, cooling, gas, and other energy needs of users; the other one is the wind & solar & hydropower & gas & storage multi-energy complementary system that has the advantage of utilizing a comprehensive energy base, where wind power, solar power, hydropower, coal, and natural gas are combined to generate electricity. The National Energy Administration announced as many as 23 first demonstration projects, including 17 projects of the first mode, and 6 projects of the second mode.

The development of the hybrid renewable energy system is important in the construction of the intelligent energy system of “Internet Plus”, which is conducive to the improvement of the coordination ability of the renewable energy supply by promoting the utilization of renewable energies, reducing the waste of solar, wind, and hydropower owing to shortage in the accommodation ability of the power grid, and by enhancing the comprehensive

《Table 1.2.5》

Table 1.2.5 Major producing countries or regions of core papers on the engineering research focus “Research and optimization of hybrid renewable energy systems”

《Table 1.2.6》

Table 1.2.6 Major producing institutions of core papers on the engineering research focus “Research and optimization of hybrid renewable energy systems”

《Figure 1.2.3》

Figure 1.2.3 Collaboration network of the major producing countries or regions of core papers on the engineering research focus “Research and optimization of hybrid renewable energy systems”

《Figure 1.2.4 》

Figure 1.2.4 Collaboration network of the major producing institutions of core papers on the engineering research focus “Research and optimization of hybrid renewable energy systems”

efficiency of the energy system, which is of great significance in terms of facilitating the shift of the energy mix and the construction of a low-carbon and high-efficiency energy system. From the perspective of technological prowess and development trend, China is leveling with or advancing with the same pace as other players; however, its development is quicker than that of the vast majority of countries.

Certain suggestions for the development of the hybrid renewable energy system of China are the following. First, policy support should be enhanced, and efforts should be made to ensure that the implementation of the introduced policy is effective. Second, the energy pricing mechanism should be improved in a manner that reflects the technical advantages and comprehensive benefits of the multi-energy complementary integrated optimization project. Third, a corresponding power dispatch and a market trading mechanism should be established to support the operation of the multi- energy complementary system. Fourth, the standardization system should be improved, and the multi-energy complementary technology and the access to the electricity (gas) network should be standardized. Fifth, the supervision of the planning and the implementation of the major multi-energy integrated optimization demon stration projects should be strengthened. Sixth, a sector corresponding to the “Information Management System of Renewable Energy Power Project” should be set up to facilitate the project information entry, the project management, and the electricity subsidy declaration.

Table 1.2.7 shows that the countries with the largest output of citing core papers are India, Iran, and China; the proportion of citations for all three countries tops 10%. In Table 1.2.8, it may be observed that the institutions with the most output of citing core papers are the Hong Kong Polytech Univ, the Univ Tehran, and the India Inst Technol, among which the proportion of citing core papers of the Hong Kong Polytech Univ and the Univ Tehran exceeds 5%. It can be inferred that the research output and the citations of the core papers of China in this particular research field are at the forefront compared with other countries; however, most of the research output is produced from institutions located in Hong Kong, China. There is still a gap between the institutions of China’s mainland and those of Hong Kong of China and foreign institutions in terms of the number of papers.

1.2.3 Flow, heat/mass transfer, and combustion under extreme conditions

(1) Concept elaboration

This engineering research focus contains four main branches, namely the evaporation and condensation, the boiling heat transfer, the heat transfer optimization, and the combustion. Harsh extreme conditions are involved in their practical applications, such as high temperature, high pressure, and high heat flux. Boiling, evaporation, and condensation are the main phenomena that take place in gas-liquid phase change processes, which can be observed in our daily life. Evaporation and condensation play a very important role in the thermal management of electronic devices, the desalination, and the combustion process. When the fuel droplets are injected into extreme environments—such as the internal combustion engines, where the temperature is high, and wall contact takes place —the phase transition processes could directly affect the efficiency of combustion. Boiling heat transfer demonstrates great potential in the fields of heat dissipation in electronic devices and spacecraft, with very high heat flux. Under the constraints of fixed heat exchange areas of heat exchangers (or heat sinks), heat transfer optimization aims to maximize heat transfer under conditions of high temperature, high pressure, ash and dust-laden settings, and high heat flux by changing types, structures, surfaces, and fluids of the heat exchanger (or heat sink). Heat transfer optimization has been widely used in the design of heat exchangers and the system optimization of industrial processes, such as waste heat utilization. As the major means to generate power at present, combustion is widely used in several applications, such as gas turbines, internal combustion engines, and industrial furnaces. As combustion processes directly determine the power output, stability, emission, and fuel conversion efficiency of the equipment, the primary goal of the research teams in this field is to

《Table 1.2.7》

Table 1.2.7 Major producing countries or regions of core papers that are cited by core papers on the engineering research focus “Research and optimization of hybrid renewable energy systems”

《Table 1.2.8》

Table 1.2.8 Major producing institutions of core papers that are cited by core papers on the engineering research focus “Research and optimization of hybrid renewable energy systems”

find a series of techniques to achieve a highly efficient, stable, and clean combustion process for a wide range of working conditions. As the basic unit affecting the macroscopic characteristics of combustion, micro-combustion is attracting growing attention.

(2) Key technologies to be explored, and developments in different countries

Evaporation and condensation are the processes of liquid vaporization and gas liquefaction, respectively. Understanding the mechanism of evaporation and condensation has become a research hotspot because these two phenomena are closely related to the combustion process. One of the main issues in this research subject is the experimental measurement and theoretical modeling of the evaporation rate under various operating conditions. The challenges arise from multi-component fluids, droplet deformation, the geometrical shape of the droplet, droplet-to-wall interaction, and temperature differences between the liquid and the gas. The research methods include the integration of theoretical models based on physical processes with computational fluid dynamics (CFD) calculations, as well as obtaining measurements through visualization methods, such as particle image velocimetry (PIV) and high-speed cameras. Future research fields may include the study of the condensation mechanism of droplets on the nano-surface, and the determination of the evaporation coefficient through molecular simulations and statistical mechanics. International research on evaporation and condensation is concentrated in Europe and Asia. European teams, including several teams in Russia and the UK as well, have conducted some research on the evaporation of droplets. Asian researchers, such as China’s and Pakistani scholars, focus on the direct contact condensation.

Boiling heat transfer refers to the heat transfer between the liquid and the heating surface, when the temperature of solid wall exceeds the liquid saturation temperature; it is highly efficient because the generated bubbles remove a significant amount of heat through vaporization. By changing the morphologies and wettability of the solid heat transfer surface, and by changing the liquid properties via the addition of nanoparticles and surfactant, the heat transfer coefficient and critical heat flux can be increased, thus meeting the heat dissipation needs of high heat flux electronic devices and spacecraft, which has become a trend in the research on boiling heat transfer. In China, the studies on boiling heat transfer are mainly conducted experimentally, and numerical simulations are conducted as well. Experimental studies in China focus on changing fluid properties (adding nanoparticles and surfactant) and the morphology of the heat transfer surface, whereas numerical simulations place emphasis on the simulation of bubble growth and spatial distribution by means of lattice Boltzmann method (LBM). In China, Tsinghua University, Shanghai Jiao Tong University, Xi’an Jiao Tong University, the North China Electric Power University, and the Institute of Engineering Thermophysics (Chinese Academy of Sciences) are the major institutions where studies on boiling heat transfer have been conducted. In South Korea, the studies on boiling heat transfer focus on understanding the mechanism of boiling heat transfer through experimental methods. In Japan, Kyoto University and Tokyo University have conducted extensive studies on boiling heat transfer. In the USA, the studies on boiling heat transfer focus on pool boiling and boiling in narrow gaps. Purdue University, the Massachusetts Institute of Technology (MIT), and Stanford University have made significant contributions.

In terms of heat transfer optimization, several basic theories have been proposed in recent decades: finite-time thermodynamics, entropy generation minimization theory, constructal theory, entransy theory, and the field synergy principle. Because there are numerous fin structures on the surface of heat exchangers, and because convective heat transfer is involved, the development of related basic theories to guide the heat transfer optimization under extreme conditions is the focus of contemporary research. The aforementioned heat transfer optimization methods have been widely used in applications in the iron and steel industry, in waste heat recovery, in heat-work conversion, and in the optimization of heat exchangers. In China, studies are focused on the development of the entransy dissipation extremum theory and the field synergy principle, as well as the combination of the entransy dissipation theory and the constructal theory. In USA and Europe, the main focus is on the entropy generation minimization theory and the constructal theory.

Combustion is a complex process, with a series of chain reactions combined on a series of microscale levels. Even slight changes in the physical or chemical process would accumulate and influence the macro characteristics of combustion. The study of the differences in combustion characteristics under various conditions, and revealing the mechanism of micro-combustion have become one of the focal interests in engineering studies both at home and abroad. In recent decades, the studies on the characteristics and mechanisms of microscale combustion have been enhanced owing to the rapid development of non-contact optical methods and to advances in CFD simulation. Meanwhile, the theoretical system of microscale combustion has been improved based on experimental and simulation studies. At present, research subjects include micro-droplets (representing the liquid fuel), coal particles (representing the solid fuel), and the coal water slurry (representing the liquid-solid fuel). The research process includes droplet breakup of secondary atomization of liquid fuel, evaporation of suspended or surface droplets, the mixture with surrounding air, the ignition, and the combustion process. The experimental methods include highspeed imaging, PIV, and laser induced fluorescence (LIF), whereas simulation studies establish accurate models of different processes based on self-developed software. Up to present, papers related to micro-combustion mainly originate from Asia and Europe. China and Russia have significantly contributed to experimental research, whereas the UK, Finland, and Iran focus on simulation studies.

(3) Current development status and future trends

Form the viewpoint of evaporation and condensation, different research teams have studied specific problems due to a number of factors involved in these phenomena, including gas-liquid temperature difference, gas flow, droplet deformation, and droplet-to-wall interaction. At present, the main research methods include visual experiments and numerical calculations. In the visual experiments, high-speed cameras, which directly observes the phase change processes, is widely used; this satisfies the need to study the mass transfer process from the macroscopic angle. Furthermore, the fluid motion inside the particles can be measured using PIV techniques. The thermogravity and thermocapillary convection during the process of droplet settling to the surface was investigated through a laser shade technique. The thermal phenomenon of the droplets was measured using a noncontact far-infrared thermal imaging technique. In the numerical calculations, one challenge is to consider a two-phase flow. The Euler two-phase, Euler-Lagrangian, and other methods are usually employed. For multicomponent fluids, the discrete multicomponent model is used when the number of components is small, while the distillation curve model and continuous thermodynamics method are used for cases in which a large number of components are involved. Future research directions include the precise determination of the evaporation/condensation rate, which directly affects the basic combustion mechanisms. In terms of theoretical modeling, possible approaches include molecular dynamics calculations and statistical mechanics modeling. In the measurement, change in the mass flux could be obtained by measuring the movement of the contact line through PIV. With all this information, the local evaporation/condensation rate can be obtained. Moreover, the local evaporation coefficient of the local heat flux can be measured using an infrared camera. In addition, it is desirable to allow the prompt removal of the droplets from the wall to enhance heat transfer in droplet condensation. Researchers have prepared nanostructured super-hydrophobic surfaces, where the adhesion is low and droplets are more likely to bounce. Such surfaces could enhance heat transfer to some extent. However, the further optimization of the wall performance still remains a long-term topic for discussion.

In the field of boiling heat transfer, the main focus is on bubble dynamics in liquid and porous media, and the effects of wettability and morphology on bubble departure and heat transfer coefficient. Experiments in this field usually employ infrared thermal imaging instruments and a high-speed camera to reveal the temperature and flowfield information, while LBM is commonly used in numerical studies. Most of the current research is experimental rather than theoretical. With the development of aerospace and electronics industries, the heat flux values of spacecraft and electronic devices are increasing, and there has been a shift from the study of macro heat/mass transfer to that of microscale boiling heat transfer mechanisms, which are attracting increasing attention. Furthermore, the application of boiling heat transfer to industrial practices and development of highly efficient heat exchanger products are considered as an important trend.

In terms of heat transfer optimization, several theories have been developed to satisfy the requirements of practical engineering applications. Finite time thermodynamics combines thermodynamics, heat transfer, and fluid dynamics to optimize the practical processes and devices in limited time and size. Constructal theory, regarded as the thermodynamics for nonequilibrium constructal problems in thermal science, provides the theoretical basis of the explanation of the generation and design of flow structures. The entropy-generation minimization theory is applied in the analysis and optimization of heat exchanger performances. However, an entropy production paradox indicates that results obtained from the entropy-generation minimization method are not optimal. In this regard, various modified objectives were defined to address the entropy production paradox. To reveal the nature of heat transfer, Guo et al. proposed a new physical quantity called entransy, which describes the total heat transfer ability. Based on the entransy concept, the principle of entransy dissipation extremum has been established, which not only provides a new direction for heat transfer optimization, but also successfully addresses the limitations and uncertainty of thermal resistance analysis and entropy-generation minimization theory. The combination of the various heat-transfer optimization methods is a trend for future research.

In terms of microcombustion, there are plenty of influencing factors, including the physical properties of fuel, fuel components, ambient pressure/temperature, air flow, droplet structure, and droplet-wall interaction. Here, visualization experiments and numerical simulations are the primary research methods. High-speed photomicrography is widely applied to observe and analyze the variation in droplets on a microscale process. The influence of flow field and the concentration and mixing characteristics on microscale combustion could be investigated based on PIV and LIF techniques, respectively. For the viewpoint of simulating actual working conditions, various self-developed simulation methods have been employed and detailed simulation models have been used to predict the characteristics of microcombustion, including evaporation and combustion. Moreover, the comparisons between experimental and numerical results have been conducted to improve the accuracy of simulation models. Researchers from China and Russia have made outstanding contributions in experimental research, among which China’s Xi’an Jiao Tong Univ and Russia’s Tomsk Polytech Univ played a leading role. In addition, Europe and Asia achieved a remarkable progress in simulation studies, among which the England’s University Brighton, Finland’s Lappeenranta Univ Technol, and Iran Univ Sci & Technol played a vital role; until now, they have preliminarily established the evaporation and combustion model of droplets on the microscale level.

(4) Comparisons of major research countries or regions and institutions and their collaborations

Table 1.2.9 shows that China, Iran, and USA rank as the top 3 in terms of the number and proportion of core papers, citation frequency, proportion of citation frequency, and number of consistently cited papers. The statistical data indicate that these three countries have greater research prowess and input; the demand in this research field is also very strong. Note that the average citation frequency for the papers from China is only 16.08, pushing it behind the top 10 countries, among which Japan, USA, and Singapore are the top 3. This indicates that although China has conducted a large amount of research, the quality should be improved further. Figure 1.2.5 shows that USA has collaborated with many countries, including China, Iran, Malaysia, Korea, England, Japan, and India. China also shows strong collaborations with other countries, especially with Singapore. Strong connections also exist between Iran and Malaysia.

Table 1.2.10 shows that Univ Malaya, Nanyang Technol Univ in Singapore, Ferdowsi Univ Mashhad in Iran, PLA Naval Univ Engn in China, and Huazhong Univ Sci & Technol in China are very close in terms of the number of core papers. Nanyang Technol Univ in Singapore shows better performance in the number, proportion, and average citation frequency. In terms of the number of consistently cited papers, the PLA Naval Univ Engn and Huazhong Univ Sci & Technol in Wuhan, China stand out. These two universities and Xi’an Jiao Tong Univ in China account for 67% of the number of consistently cited papers; this shows their strong ability in the research field. Figure 1.2.6 shows that intensive collaborations have been established among Ferdowsi Univ Mashhad of Iran, Univ Malaya, and Islamic Azad Univ. Strong connections can also be observed between Shanghai Jiao Tong Univ and Nanyang Technol Univ, as well as between Tomsk Polytech Univ and Natl Res Tomsk Polytech Univ.

The main researchers for condensation and evaporation hail from Russia, UK, China, and some other countries. In addition, intensive collaborations exist between Russian

《Table 1.2.9》

Table 1.2.9 Major producing countries or regions of core papers on the engineering research focus “Flow, heat/mass transfer, and combustion under extreme conditions”

《Table 1.2.10》

Table 1.2.10 Major producing institutions of core papers on the engineering research focus “Flow, heat/mass transfer and combustion under extreme conditions”

《Figure 1.2.5》

Figure 1.2.5 Collaboration network of the major producing countries or regions of core papers on the engineering research focus “Flow, heat/mass transfer and combustion under extreme conditions”

《Figure 1.2.6》

Figure 1.2.6 Collaboration network of the major producing institutions of core papers on the engineering research focus “Flow, heat/mass transfer and combustion under extreme conditions”

scholars and those from other European countries. However, researchers from USA are not active in this branch. In the field of boiling heat transfer, China, Japan, Korea, and USA comprise many scholars, while Malaysia and India do not have as many. Moreover, extensive collaborations exist between Korea and USA. In the field of heat transfer optimization, constructal theory was proposed by Professor Bejan from Duke University. This theory has been widely employed in USA, Europe, and Iran. Entransy dissipation theory was proposed by Professor Guo from Tsinghua University; it is mostly studied in China and has recently gained attention in other counties. In terms of combustion, Europe still leads in the number of studies on liquid fuel at the microscale level; nevertheless, several research teams in China have also conducted in-depth study in this field in recent years. In addition, China, Iran, and Russia are the major countries studying liquid–solid fuels, such as coal–water fuel, at the microscale level. Most of the international collaborations are in Asia, where China has a strong cooperative relationship with Singapore.

In the study of evaporation and condensation, China’s scholars, such as those from Xi’an Jiao Tong University and Beijing University of Chemical Technology, have conducted several experimental and theoretical research on direct condensation, which is at the leading level. However, research on droplet evaporation is rare and requires more attention. Study on the branch of boiling heat transfer is mainly focused upon at the experimental level in China, seeking the enhancement of the coefficient of heat transfer. In recent years, some scholars in Tsinghua University, Shanghai Jiao Tong University, Xi’an Jiao Tong University, and the Institute of Engineering Thermophysics (Chinese Academy of Sciences) have made great progresses in this field. They adopted the LBM method to build liquid-vapor phase change models and explored the mechanism of phase-change heat transfer from the theoretical perspective. China’s scholars play the leading role in this field, and have suggested further theoretical studies. In the field of heat transfer optimization, some optimization concepts, such as entropy, were introduced from overseas. By contrast, entransy dissipation theory proposed by Professor Guo is at the cutting edge of this field. The entransy dissipation and construtal theories form the basis of the new optimization theory for heat transfer. In terms of combustion, China has many years of an intellectual convention of solid and liquid-solid fuels at the microscale level; and the extensive research results prove that China belongs in the leading team in this area. From the viewpoint of liquid fuels, China lags the leading countries; however, it is catching up and seeking to become a leader in this field as well.

The statistical data in Table 1.2.11 shows that China, Iran, and USA rank the top 3 in citing the core papers on the engineering research focus “Flow, heat/mass transfer and combustion under extreme conditions.” Furthermore, as shown in Table 1.2.12, the top 3 institutions citing the core papers on the engineering research focus “Flow, heat/mass transfer and combustion under extreme condi-

《Table 1.2.11》

Table 1.2.11 Major producing countries or regions of core papers that are cited by core papers on the engineering research focus “Flow, heat/mass transfer and combustion under extreme conditions”

《Table 1.2.12》

Table 1.2.12 Major producing institutions of core papers that are cited by core papers on the engineering research focus “Flow, heat/mass transfer and combustion under extreme conditions”