Operational Instability Research Articles

The Impact of Stability Factor of Train Operations to Railway Line Capacity (Kroya-Kutoarjo-Yogyakarta) Indonesia

Transportation is the backbone of development. It determines the economic growth of a country. Among any modes of transportation, railway is in need to be developed especially in the increasing of railway lines capacity in addition to the development of railway infrastructures, facilities, and the enhancement of rail operational. The determinant in rail operational is the stability in its operation. This stability can be seen by the punctuality of train arrival and departure, which is in accordance with the Graph Train Journey (Gapeka) and “delay” as the indicator. The study was conducted to determine the stability of railway operation and the delay of train travel as well as to identify the effect of the stability of railway operations to the capacity value of railway line. The study was conducted by collecting secondary data and conducting primary survey followed by data analysis. The results showed that the stability of rail operation for Kroya – Kutoarjo - Yogyakarta line was still low with an average delay of 15.60 minutes for double track and 21.68 minutes for single track. There was a decline in capacity when it was compared to the planned capacity or factual practical capacity at 22% on single track and 17% on double track. Further research on how to determine the cause of the instability of railway operation as input for the stability improvement would be needed in addition to the model or formulation to calculate line capacity in corresponding to the conditions in Indonesia.

Numerical Study on Particulate Fouling Characteristics of Flue with a Particulate Fouling Model Considering Deposition and Removal Mechanisms

Due to a large amount of particulate matter in industrial flue gas, the formation of particulate deposits on the flue wall will increase the instability of equipment operation, which needs to be solved urgently. In this paper, a numerical investigation on the characteristics of particulate deposition and removal in the furnace flue was carried out for waste heat and energy recovery. This research adopted a comprehensive fouling model combined with the discrete phase model (DPM) which was performed by the CFD framework and extended by user-defined functions (UDFs). Firstly, the particulate deposition and removal algorithms were proposed to develop the judgment criterion of particle fouling based on the Grant and Tabakoff particle–wall rebound model and the Johnson–Kendall–Roberts (JKR) theory. This model not only considered the particles transport, sticking, rebound, and removal behaviors, but also analyzed the deposition occurring through the multiple impactions of particles with the flue wall. Then, the influence of furnace gas velocity, particle concentration, and inflection angle α of the tee section on the particulate fouling were predicted. The results show that the furnace gas velocity, particle concentration, and flue structure have significant effects on particle fouling and distribution, and the particle fouling mainly occurs in the blind elbow section and the tee sections of the flue. In addition, the fouling mass of particles decreases with an increasing furnace gas velocity and the decrease in particle concentration. Lastly, the fouling mass of particles decreases with the increase in the inflection angle α of the tee section, and the location of particle fouling gradually transfers from the blind elbow section to the tee section.

Observation of Controllable Negative Differential Resistance Behaviors through Morphology‐Dependent Zinc Oxide/p‐Si Heterointerface Structures

AbstractNegative differential resistance (NDR) has received considerable attention owing to the unique switching property, which exhibits an n‐shaped current–voltage curve without a monolithic increase in current as of the applied voltage bias increases. Nevertheless, various issues in previously reported NDR devices need to be addressed, such as complex manufacturing processes, operational instability, and difficulties in large‐area processing. In addition, methods for generating and controlling NDR have not yet been developed. This paper presents the findings of morphology‐dependent NDR in which three types of zinc oxide (ZnO) build a junction with p‐Si in the form of small nanoparticles, large nanoparticles, and films. The principle of NDR behaviors is elucidated through a comprehensive study, including an analysis of the energy band structure, morphological investigation, and electrical characterization, offering control of the peak‐to‐valley current ratio (PVCR). The highest PVCR = 4.96 A A−1 is obtained in the quantum dot ZnO/p‐Si structure and the NDR characteristics occur without external stimuli, such as irradiating light, cryogenic temperatures, or controlled atmospheres.

Effect of corn stover hydrochar on anaerobic digestion performance of its associated wastewater

Hydrothermal carbonation (HTC) is an effective method to enhance the fuel quality of biomass in a subcritical water environment, but generates large amounts of wastewater (HTCWW), which was converted through anaerobic digestion (AD) into methane in this study. However, the toxic and refractory substances contained in HTCWW tended to cause operation instability of the AD system. The solid product in HTC of corn stover (CS), named CS hydrochar, was modified with KOH immersion and then added to the AD reactor to improve the methanogenic performance. The results showed that the optimum dosage of modified hydrochar (MCH) was 15 g/L, and the COD removal rate was increased by 19.3% and methane yield was increased by 42.3%–301 mL/g-COD, as the pore and the oxygen-containing functional groups of MCH provided colonization points for microorganisms, and also enhanced the electron transfer efficiency among methanogenic archaea. In addition, the increased alkalinity of MCH due to alkaline modification increased the pH buffering capability, and accelerated the consumption of acetic acid and butyric acid in the early AD stage (0–8 days) and propionic acid in the late AD stage (12–18 days), which then alleviated the organic acid accumulation and reduced the lag period by 2 days. The adverse effects of toxic and refractory substances of HTCWW on the AD performance were also decreased due to the adsorption of MCH at the beginning of the AD process, and latterly the adsorbed substances could be degraded by the microorganisms colonized on the MCH surface. The finding of this study showed AD is a feasible method to recover organic energy contained in HTCWW, and the associated hydrochar can be used as an effective promoter for the AD of HTCWW.

Challenges for urban transport policy after the Covid-19 pandemic: Main findings from a survey in 20 European cities

This article explores the changes in transport and mobility patterns that were triggered by the Covid-19 pandemic, as well as citizens’ potential new expectations arising from this situation regarding public transport policy and urban planning. We conducted an extensive survey in 20 cities across 11 European countries during the second quarter of 2021, with a total of 10000 respondents. The information that we collected covered the change in the frequency of technology-based alternatives to travel -such as teleworking and online shopping-compared to the pre-pandemic period. It also included the change in trip frequency, purpose, mode and destination. Using the survey dataset as input, we applied a classification model that explains the change in mode choice based on the respondent characteristics and preferences. Our findings suggest that the changes in mobility patterns induced by the pandemic are the result of the combination of the degree of mobility restrictions imposed in each city, the adoption of new lifestyle and work patterns, and individual choices that depend on personal risk aversion. We identify three main challenges for urban transport policy: the increasing use of car that can reinforce long-term car dependency, the avoidance of public transport that may lead to financial and operational instability, and the uncertainty as regards the future of innovation in urban transport, especially as regards emerging technologies and business models.

Transient analysis of a hydropower plant with a super-long headrace tunnel during load acceptance: Instability mechanism and measurement verification

The current development trend of China's hydropower is moving toward ultra-high heads, complex geological scenarios, ultra-large capacities, and the Tibetan region. The construction of hydropower stations with super-long headrace tunnels is conducive to overcoming the constraints of complex geological conditions and making full use of hydropower resources. However, these stations have unique technical problems that limit their commercial development, including pressure fluctuations during load rejection and the operational stability of the power-frequency regulation. In this study, a potential problem in hydropower stations with super-long headrace tunnels, the instability during load acceptance, is studied by combining prototype experiments with 1D transient numerical simulations. First, according to a test case using a prototype hydropower station, the instability phenomenon occurring during a simultaneous load acceptance of two Francis turbines in a hydropower station with a 20-km headrace tunnel is described. Next, using 1D steady-state and transient simulations, the factors leading to the instability are analyzed in detail. Finally, countermeasures for the safe operation of hydropower stations are proposed based on numerical simulations and supplementary prototype experiments. Hydropower stations with super-long headrace tunnels exhibit a characteristic where the net head decreases sharply with an increase in the flow rate. Therefore, prior to load acceptance, the maximum output achievable by the multiple units under the operating net head has to be carefully determined to avoid over-adjustment of the guide vane opening and detrimental phenomena such as operational instability and extremely low pressure in the draft tube.

Plasmon-Induced Semiconductor-Based Photo-Thermal Catalysis: Fundamentals, Critical Aspects, Design, and Applications

Photo-thermal catalysis is among the most effective alternative pathways used to perform chemical reactions under solar irradiation. The synergistic contributions of heat and light during photo-thermal catalytic processes can effectively improve reaction efficiency and alter design selectivity, even under operational instability. The present review focuses on the recent advances in photo-thermal-driven chemical reactions, basic physics behind the localized surface plasmon resonance (LSPR) formation and enhancement, pathways of charge carrier generation and transfer between plasmonic nanostructures and photo-thermal conversion, critical aspects influencing photo-thermal catalytic performance, tailored symmetry, and morphology engineering used to design efficient photo-thermal catalytic systems. By highlighting the multifield coupling benefits of plasmonic nanomaterials and semiconductor oxides, we summarized and discussed several recently developed photo-thermal catalysts and their catalytic performance in energy production (CO2 conversion and H2 dissociation), environmental protection (VOCs and dyes degradation), and organic compound synthesis (Olefins). Finally, the difficulties and future endeavors related to the design and engineering of photo-thermal catalysts were pointed out to draw the attention of researchers to this sustainable technology used for maximum solar energy utilization.

Assessing the Drawbacks and Benefits of Ion Migration in Lead Halide Perovskites.

Since the inception of the unprecedented rise of halide perovskites for photovoltaic research, ion migration has shadowed this material class with undesirable hysteresis and degradation effects, limiting its practical implementations. Unfortunately, the localized doping and electrochemical reactions triggered by ion migration cause many more undesirable effects that are often unreported or misinterpreted because they deviate from classical semiconductor behavior. In this Perspective, we provide a concise overview of such effects in halide perovskites, such as operational instability in photovoltaics, polarization-induced abnormal external quantum efficiency in light-emitting diodes, and energy channel shift and anomalous sensitivities in hard radiation detection. Finally, we highlight a unique use case of exploiting ion migration as a boon to design emerging memory technologies such as memristors for information storage and computing.

Novel production prediction model of gasoline production processes for energy saving and economic increasing based on AM-GRU integrating the UMAP algorithm

Gasoline, as an extremely important petroleum product, is of great significance to ensure people's living standards and maintain national energy security. In the actual gasoline industrial production environment, the point information collected by industrial devices usually has the characteristics of high dimension, high noise and time series because of the instability of manual operation and equipment operation. Therefore, it is difficult to use the traditional method to predict and optimize gasoline production. In this paper, a novel production prediction model using an attention mechanism (AM) based gated recurrent unit (GRU) (AM-GRU) integrating the uniform manifold approximation and projection (UMAP) is proposed. The data collected in the industrial plant are processed by the box plot to remove the data outside the quartile. Then, the UMAP is used to remove the strong correlation between the data, which can improve the running speed and the performance of the AM-GRU. Compared with the existing time series data prediction method, the superiority of the AM-GRU is verified based on University of California Irvine (UCI) benchmark datasets. Finally, the production prediction model of actual complex gasoline production processes for energy saving and economic increasing based on the proposed method is built. The experiment results show that compared with other time series data prediction models, the proposed model has better stability and higher accuracy with reaching 0.4171, 0.9969, 0.2538 and 0.5038 in terms of the mean squared error, the average absolute accuracy, the mean squared error and the root mean square error. Moreover, according to the optimal scheme of the raw material, the inefficiency production points can be expected to increase about 0.69 tons of the gasoline yield and between about $645.1 and $925.6 of economic benefits of industrial production.

The granular sludge membrane bioreactor: A new tool to enhance Anammox performance and alleviate membrane fouling

Anammox process has be widely concerned as a novel biological nitrogen removal technology because of reduced operating costs and environmental friendliness. However, the long start-up time and operational instability of Anammox process has hindered its wide applications. A new-type hybrid granular sludge membrane bioreactor (GSMBR) combining sludge granulation with membrane filtration was developed for accelerating start-up of Anammox process and stabilizing its operation as well as alleviating the membrane fouling. Anammox start-up period was shortened to 43 days, the maximum nitrogen removing rate and efficiency reached 0.83 kg N m−3 d−1 and 97 % respectively on 115 d. Moreover, the membrane fouling is relatively light. The maximum transmembrane pressure only reached 32 kPa on 115 d, when there were still several visible micropores and channels on the fouled membrane surface. The mature brownish-red Anammox granular sludge mostly sized at 0.5–1.0 mm. The specific population shift from norank Alphaproteobacteria to Candidatus Brocadia was found. Three Anammox species were present in the mature sludge and uncultured Candidatus Brocadia sp. was the predominated bacteria. GSMBR was proven to be a robust reactor for enhancing Anammox performance. This study was helpful to promote the engineering applications of Anammox process in treating the high-strength nitrogenous wastewaters.

Immobilization of Microbes for Biodegradation of Microcystins: A Mini Review.

Harmful cyanobacterial blooms (HCBs) frequently occur in eutrophic freshwater ecosystems worldwide. Microcystins (MCs) are considered to be the most prominent and toxic metabolites during HCBs. MCs may be harmful to human and animal health through drinking water and recreational water. Biodegradation is eco-friendly, cost-effective and one of the most effective methods to remove MCs. Many novel MC-degrading bacteria and their potential for MCs degradation have been documented. However, it is a challenge to apply the free MC-degrading bacterial cells in natural environments due to the long-term operational instability and difficult recycling. Immobilization is the process of restricting the mobility of bacteria using carriers, which has several advantages as biocatalysts compared to free bacterial cells. Biological water treatment systems with microbial immobilization technology can potentially be utilized to treat MC-polluted wastewater. In this review article, various types of supporting materials and methods for microbial immobilization and the application of bacterial immobilization technology for the treatment of MCs-contaminated water are discussed. This article may further broaden the application of microbial immobilization technology to the bioremediation of MC-polluted environments.

Fluorination-Induced Charge Trapping and Operational Instability in Conjugated-Polymer Field-Effect Transistors.

Fluorination of a conjugated polymer backbone is an effective strategy to control the microstructure and electronic structure of a conjugated polymer. Although fluorination has been widely reported to increase charge carrier mobility, its effect on the operational stability of electronic devices has not been extensively investigated. Here, the effect of fluorination of a conjugated polymer backbone on charge trapping and the operational stability of organic field-effect transistors is investigated. The results show that the device based on a fluorinated conjugated polymer exhibits relatively poor operational stability despite its greater charge carrier mobility compared with that in the device based on its nonfluorinated polymer counterpart. Experimental results reveal that the low stability originates from the greater degree of shallow trapping of charge carriers within the fluorinated polymer thin film and that the shallow trapping is closely related to the presence of minority charge carriers. A mechanism of charge trapping is proposed.

Carbonic Anhydrase-Mimicking Supramolecular Nanoassemblies for Developing Carbon Capture Membranes.

As a ubiquitous family of enzymes with high performance in converting carbon dioxide (CO2) into bicarbonate, carbonic anhydrases (CAs) sparked enormous attention for carbon capture. Nevertheless, the high cost and operational instability of CAs hamper their practical relevance, and the utility of CAs is mainly limited to aqueous applications where CO2-to-bicarbonate conversion is possible. Taking advantage of the chemical motif that endows CA-like active sites (metal-coordinated histidine), here we introduce a new line of high-performance gas separation membranes with CO2-philic behavior. We first self-assembled a histidine-based bolaamphiphile (His-Bola) molecule in the aqueous phase and coordinated the resulting entities with divalent zinc. Optimizing the supramolecular synthesis conditions ensured that the resultant nanoparticles (His-NPs) exhibit high CO2 affinity and catalytic activity. We then exploited the His-NPs as nanofillers to enhance the separation performance of Pebax MH 1657. The hydrogen-bonding interactions allowed the dispersion of His-NPs within the polymer matrix uniformly, as confirmed by microscopic, spectroscopic, and thermal analyses. The imidazole and amine functionalities of His-NPs enhanced the solubility of CO2 molecules in the polymer matrix. The CA-mimic active sites of His-NPs nanozymes, on the other hand, catalyzed the reversible hydration of CO2 molecules in humid conditions, facilitating their transport across the membranes. The resulting nanocomposite membranes displayed excellent CO2 separation performance, with a high level of stability. At a filling ratio as low as 3 wt %, we achieved a CO2 permeability of >145 Barrer and a CO2/N2 selectivity of >95 with retained performance under humid continuous gas feeds. The bio-inspired approach presented in this work offers a promising platform for designing durable and highly selective CO2 capture membranes.

Complex network analysis of slaughterhouse waste anaerobic digestion: From failure to success of long-term operation

The study explored slaughterhouse waste (SHW) as prime feedstock associated with and without supplement of an external slowly degradable lignocellulosic carbon source to overcome the synergistic co-inhibitions of ammonia and fatty acids. Long-term solid-state digestion (SSD) and liquid-state digestion (LSD) were investigated using a mixture of pork liver and fat. At 2.0 g volatile solids (VS) L−1 d−1 of organic loading rate (OLR), the two reactors of SSD experienced operational instability due to ammonia inhibition and volatile fatty acid (VFA) accumulation while LSD successfully produced 0.725 CH4 L CH4 g−1VS during 197 d of working days under unfavorable condition with high total ammonia nitrogen (>4.7 g/L) and VFAs concentration (>1.9 g/L). The network analysis between complex microflora and operational parameters provided an insight for sustainable biogas production using SHW. Among all, hydrogenotrophic methanogens have shown better resistance than acetoclastic methanogens.

Donor Engineering Tuning the Analog Switching Range and Operational Stability of Organic Synaptic Transistors for Neuromorphic Systems

AbstractOrganic artificial synapses are becoming the most desirable format for neuromorphic computing due to their highly tunable resistive states. However, repressively low analog switching range, inferior memory retention, and operational instability greatly hinder the further development of organic synapses. Herein, two donor‐acceptor copolymers consisting of electron‐deficient isoindigo coupled with variable donating moieties for three‐terminal organic synaptic transistors (TOSTs) are reported. It is found that the synaptic function and device stability of TOSTs are significantly improved by enhancing the electron‐donating strength of donor units. Polymer alkylated isoindigo‐bis‐ethylenedioxythiophene exhibits high analog switching range of 170 ×, two orders of magnitude higher than that of normal organic neuromorphic devices. They also demonstrate excellent memory retention of over 5 × 103 s, low switching energy of 13 fJ, and ultrahigh operational stability with 99% of its original current after 100 000 write‐read events in air. Furthermore, the high viability of strong donor strategy is showcased by demonstrating flexible TOSTs with stable synaptic function after repeated mechanical bending as well as organic synapses capable of simulating image information processing. Overall, this work highlights the advantages of the strong donor functionalization strategy to boost the synaptic performance and device stability of TOSTs.

Predicting reverse flow characteristics in equal-height-difference passive heat removal system for ocean nuclear power

The equal-height-difference passive heat removal system (EHDPHRS) via natural circulation driving and utilizes ocean as infinite heat sink for heat dissipation, showing broad application prospect in ocean nuclear power field; but the heat removal pipe normally encounters the reverse flow of cool seawater into the pipe, leading to operation instability and even condensation-induced water hammer. In this study, the integration of volume of fluid model and condensation model based on surface renewal theory is used to simulate the reverse flow characteristics in the heat removal pipe under single-and two-phase natural circulation conditions. The interfacial heat and mass transfer are calculated by user-defined functions (UDF). The numerical results reveal that the buoyancy force (Fb) and condensation driving force (FΔP) both accelerate the reverse flow, while the inertial force (Fi) restrains the reverse flow; the reverse flow length presents monotonous decrease with increasing the flow velocity under single-phase conditions and parabolic-like trend for two-phase flow. A prediction correlation for the reverse flow length is developed using dimensionless Reynolds number, Grashof number and vapor volume fraction, covering single- and two-phase conditions. The predicted reverse flow length by the correlation agrees with 94% of the results within deviations of ±15%.

Design optimization and granularity analysis of district heating systems for distributed solar heating access

In the low-carbon transition process of district heating systems (DHS), the need for widespread access to renewable energy and lower carbon emissions conflict with the existing structure of large centralized DHS. This paper proposes an optimized design of the DHS based on granularity analysis methodology to harness distributed solar energy. This study applies a hierarchical clustering method to get the possible scheme set based on the given geometrical information of an area with distributed solar heating system (SHS). This study further carries out pipe network routings and the pipeline sizing optimization to implement the scheme with the best economic performance. To disclose the relationship between granularity and SHS access ratio, we propose a quantitative granularity estimation method considering the spatial distribution of heat in the sense of Manhattan distance. This study calculates optimal schemes' total annual cost (TAC) and potential operation instability (PRI) under a series of SHS access ratios. A real DHS covering a heating area of 164300 m2, and 21 buildings is selected for a case study. At a granularity of 4.50%, TAC is reduced by 16.04%, and PRI by 58.06% compared to the existing design where granularity is not considered and one substation covering the entire area. As the SHS access ratio increases from 0 to 12%, the optimal granularity value increases from 4.50% to 16.84%, indicating a higher granularity is necessary to utilize solar energy. Our work shows that a proper selection of granularity in the design stage is essential to the economic and stable operation of DHS with renewable energy access. The result from this study is also meaningful to the design and operation control of district energy systems and integrated energy systems.

Lead‐Free Halide Double Perovskites: Fundamentals, Challenges, and Photovoltaics Applications

AbstractMetal halide perovskites (MHPs) have gained tremendous interest in photovoltaics applications reaching the power conversion efficiency (PCE) of 25.8%. Despite the rapid development of MHP‐based solar cells, the existence of toxic lead (Pb) and their operational instability are seen as key roadblocks for commercialization. A viable strategy for developing lead‐free optoelectronic devices nests in addressing the toxicity issues in these compounds by carefully and strategically replacing lead while maintaining comparable stability and PCE. In this regard, halide double perovskite with structural flexibility, as well as lower toxicity and higher stability have caught the attention of the scientific community, surging the research of lead‐free halide double perovskites with A2B(I)B(III)X6 composition. A critical analysis of such compounds is presented from a fundamental point of view considering their orbital characteristics, bonding interaction, and optical transitions and relate these properties with their solar cell performance. This review focuses on the efficiency limiting factors and critically discusses recent advancements and strategies which have been demonstrated to boost their photovoltaic performances. Finally, starting from existing double perovskite materials, prospective research directions are identified toward enhanced optoelectronic properties and device performances.

Utilization of the CW magnetrons as coherent RF sources for superconducting RF accelerators

CW magnetrons designed and optimized for industrial heaters, driven by an injection-locking signal, were suggested to power Superconducting RF (SRF) cavities. However, the CW magnetrons, which are regenerative generators, are intended to operate in self-excitation mode, so, for reliable starting they use a large regenerative gain, causing a large regenerative instability in operation. Thus, power supply ripple, even shot noise, etc., lead to multiple sidebands forming a quasi-continuous noise spectrum at the carrier frequency. Therefore, in the traditionally used mode of operation and control, such magnetrons are inapplicable for powering high Q-factor SRF cavities. A new approach to operation and control of CW magnetrons considering non-stationary processes during start-up and operation makes it possible to find a mode of almost coherent RF generation of tubes with a significant reduction in regenerative instability and noise and increased efficiency. In our work is presented substantiation for the newly developed mode with an analysis of the new approach based on experimental results.

Axial thrust instability analysis and estimation theory of high speed centrifugal pump

High-speed centrifugal pumps are widely used in the aerospace and chemical industries. However, the large axial thrust of a rotor due to high rotating speeds exacerbates the device operation instability. Herein, the axial thrust characteristics of a high-speed centrifugal pump under variable speed conditions are analyzed via numerical simulations and experiments. Results show that at high rotating speeds, although the dynamic reaction of the impeller and axial force on the inducer blade only account for a small proportion of the axial thrust, they are the main sources of axial thrust instability. Moreover, axial thrust exhibits a linear relation with the axial force acting on the front and back surfaces of the impeller, and the values of these parameters are very close. However, huge errors are afforded when these two parameters are interchanged, and many axial thrust estimation models ignore this. Therefore, considering the influence of the impeller dynamic reaction and the axial force of the inducer blade on axial thrust, this study proposes an estimation theory for predicting axial thrust. Comparison of the prediction results of the estimation theory and the numerical simulation results reveals that the maximum and minimum relative errors of these two parameters are 11.1% and 2.6%, respectively, under different rotational speeds, thereby suggesting that the prediction results of the estimation theory are sufficiently accurate. The estimation theory can greatly reduce the calculation cost, shorten the design cycle of the centrifugal pump, and provide certain reference for engineering practice.