38517 research outputs found
Sort by
Control and stabilization of grid-connected converters operating as constant power load in a smart transformer grid scenario
Get PDFThe development of power electronics and control technology provides improved performance of the electrical appliances, making them robust to changes in the supply voltage or frequency. However, power electronic converters behave as constant power loads (CPLs), which have negative impedance characteristics, and this non-linear behavior is challenging the stability of the grid. This paper investigates the input impedance of a three-phase low-voltage grid connected ac-dc converter that operating as a CPL when interfaced to a single DC bus and a changeable load. The possibility of adopting a Smart Transformer (ST), a power electronics converter which feeds a low-voltage grid, for the CPL stabilization is also addressed. To analyze the behavior of CPLs, a strategy of impedance modelling is employed. Gain Margin Phase Margin (GMPM) Criterion is referred to judge the stability of the system and verify the stabilization operation for CPLs. Simulation results are presented to be as a support
Sustainable conversion of carbon dioxide into diverse hydrocarbon fuels via molten salt electrolysis
Get PDFIn recent decades, the unlimited use of fossil fuels mostly for power generation has emitted a huge amount of carbon dioxide in the atmosphere which in return has led to global warming. Here we use green technology, the molten salt electrochemical system comprising of titanium and mild steel as a cathode with graphite anode whereas molten carbonate (Li2CO3-Na2CO3-K2CO3; 43.5:31.5:25 mol%), hydroxide (LiOH-NaOH; 27; 73 and KOH-NaOH; 50:50 mol %) and chlorides (KCl-LiCl; 41-59 mol%) salts as electrolytes This study investigates the effect of temperature, feed gas ratio CO2/H2Oand use of different cathode materials on hydrocarbon product along with current efficiencies. Gas chromatography and mass spectroscopy have been applied to analyze the gas products. According to GC results, more specific results in terms of high molecular weight and long chain hydrocarbons were obtained by using titanium cathodic material rather than mild steel. The results revealed that among all the electrolytes, molten carbonates at 1.5V and 425˚C produced higher hydrocarbons as C7H16 while all other produced CH4. The optimum conditions for hydrocarbon formation and higher current efficiencies in case of molten carbonates were found to be 500oC under a molar ratio of CO2/H2O of 15.6. However, the current efficiencies do not change on increasing the temperature from 425 to 500oCand is maintained at 99% under a molar ratio of CO2/H2O of 15.6. The total current efficiency of the entire cathodic product reduced clearly from 95 to 79% by increasing the temperature under a CO2/H2O ratio of 9.2 due to the reduction of hydrocarbon generation in this case, despite the formation of C7H16. Therefore, due to its fast electrolytic conversion rate and low cost (no use of catalyst) the practice of molten salts could be an encouraging and promising technology for future investigation for hydrocarbon fuel formation
The analysis of global warming patterns from 1970s to 2010s
Get PDFWhile global warming is only one part of climate change effects, it poses the highest risk to our habitats and ecologies. It is alarming that global warming has heightened in multiple locations and is intensified since the early 1970s. Since then, there are certain global warming patterns that could guide us with an overview of what mitigation and adaptation strategies should be developed in the future decades. There are certain regions affected more than another, and there are certain patterns with adverse effects on regions, sub-regions, and even continents. This study provides an insightful analysis of recent global warming patterns, those that are affecting us the most with regional climate change of different types, upsurge in frequency and intensity of natural disasters, and drastic impacts on our ecosystems around the world. By analysing the global warming patterns of these last four decades, this research study sheds light on where these patterns are coming from, how they are developing, and what are their impacts. This study is conducted through grey literature and analysis of the recorded global warming data publicly available by the NASA-GISS data centre for global temperature. This brief—but comprehensive—analysis helps us to have a better understanding of what comes next for global warming impacts, and how we should ultimately react. The study contributes to the field by discovering three key points analysed based on available data and literature on recorded global temperature, including: differences between north and south hemispheres, specific patterns due to ocean surface temperature increase, and recent impacts on particular regions. The study concludes with the importance of global scale analysis to have a more realistic understanding of the global warming patterns and their impacts on all living habitats
Prospect of solar-driven ejector-compression hybrid refrigeration system with low GWP refrigerants in summer of Guangzhou and Beijing
Get PDFIn this paper, the prospect of low-GWP refrigerants used in solar-driven ejector-compression hybrid refrigeration system was analyzed and evaluated based on the solar radiation data in summer of Beijing and Guangzhou. Prominent environmental friendly alternative refrigerants, including HCFO-1233zd(E), HFO-1336mzz(Z), HFO-1234ze(Z), HCFO-1224yd(Z) and HC-600 were chosen as the working fluids to address the environmental concerns, while HFC-245fa was used as the baseline refrigerant. The hybrid refrigeration system composed of an ejector refrigeration system as the upper stage and a vapor compression system as the bottom stage, respectively. The results indicated that the system with HFO-1234ze(Z) showed the highest COP. HFO-1234ze(Z) was followed by HCFO-1233zd(E) and HC-600, and all of them showed higher COP than that of HFC-245fa. The system with HC-600 needed more electric energy than any other systems, while HFO-1336mzz(Z) consumed the lowest electric power. Meanwhile, the performances of HFO-1234ze(Z) and HFC-245fa systems were further compared. It was found that for both working fluids, the heat load of the condenser (Qe,c), total electric energy consumption (We) and electric energy of compressor (Wcom) in August were the highest in Beijing, while in Guangzhou, the highest of them was achieved in September. It was also suggested that the solar radiation had no influence on the relative magnitude of the Qe,c, We and Wcom between HFO-1234ze(Z) and HFC-245fa systems. The Qe,c, We and Wcom of the system with HFO-1234ze(Z) were higher than those of HFC-245fa by 0.8%, 4.44%, and 3.55%, respectively. © 202
New advances in catalysts for C9 petroleum resin hydrogenation
Get PDFC9 petroleum resin is a thermoplastic polymer produced by polymerization of C9 fraction obtaining from the steam cracking unit, and could be catalytic hydrogenated to remove the ethylenic C=C bond, aromatic ring with improved physical properties. The research progress in the efficiency supported nickel or/and palladium catalysts for C9 petroleum resin hydrogenation was illustrated and reviewed, further development was discussed
A two‐stage Bayesian network model for corporate bankruptcy prediction
Get PDFWe develop a Bayesian network (LASSO-BN) model for firm bankruptcy prediction. We select fnancial ratios via the Least Absolute Shrinkage Selection Operator (LASSO), establish the BN topology, and estimate model parameters. Our empirical results, based on 32,344 US firms from 1961-2018, show that the LASSO-BN model outperforms most alternative methods except the deep neural network. Crucially, the model provides a clear interpretation of its internal functionality by describing the logic of how conditional default probabilities are obtained from selected variables. Thus our model represents a major step towards interpretable machine learning models with strong performance and is relevant to investors and policymakers
Chinese cross‐border mergers and acquisitions in the developing world: Is Africa unique?
Get PDFChina has emerged as an important partner for Africa. Not surprisingly, Chinese business and investment relations with Africa have been growing. This article contends that Africa offers a different proposition to Chinese business interests in non-African developing economies. In this optic, it takes a “comparative” institution-based view treating factors that determine Chinese multinationals' cross-border merger and acquisition (CBMA) decisions as comparatively different for Africa to the rest of the developing world. From a panel data estimation of the number of Chinese CBMAs from 2007 to 2016, we find among market size, natural resources, strategic assets, labor productivity, and institutional governance, only natural resources and market size have a distinctive effect, with Chinese investors being more attracted to African natural resources than the African market. The drive for natural resources provides impetus for Chinese MNEs to choose CBMAs over greenfield investment, and through majority ownership to exercise control. Our inference is that Africa is “significantly” different from other developing regions, in terms of CBMAs, as Chinese multinationals have a strong motive to control access to natural resources
Do national cultures matter in the containment of COVID-19?
Get PDFPurpose
This paper empirically investigates how cultural variations in individualism and tightness affected the containment of COVID-19 using data from 54 nations during a 30-day period of government intervention.
Design/methodology/approach
The authors utilized the hierarchical regression approach to check the effects of three cultural variables – the individualism measure, taken from Hofstede’s six-dimension national culture index, and the measure of cultural tightness, based on the three tightness–looseness indexes calculated by Irem Uz (2015) and their interaction – on the changes in the prevalence rate (ΔPR) and crude mortality rate (ΔCMR) and case fatality rate (CFR) while controlling for the stringency of government responses to COVID-19, median age and population density.
Findings
Significant relationships were found between cultural variables and national performance in slowing the spread of the coronavirus, measured by ΔPR, ΔCMR and CFR. After controlling for the stringency of government responses, median age and population density, the authors found that cultural tightness and individualism as well as their interactions remain to be pivotal. Loose and individualistic cultures led to faster increases in PR and CMR and higher CFR. A four-quadrant conceptual framework is developed to categorize and discuss the national differences
COVID-19 and household energy implications: what are the main impacts on energy use?
Get PDFThis study explores the impacts of COVID-19 on household energy use. Some of these impacts are associated with longer-term energy demand changes and some could just be temporary. The study intends to present the results of a small pilot study conducted in China, by addressing household energy use. The samples are from 352 households and particularly focus on primary energy use in three periods of pre-pandemic (and pre-lockdown), start of COVID-19 outbreak and lockdown, and post lockdown. Each period is identified as a timeframe of 2.5 months, from November 2019 to late June 2020. The samples of this study highlight the primary implications of energy use, some that are understood as interim changes and some that may appear to be more prolonged. The results from the study highlight a variety of impacts on household energy use as well as prolonged impacts on transportation use. The primary household energy use are assessed in six fundamental elements of (1) transportation for commuting and leisure (for both private and public modes), (2) cooking, (3) entertainment, (4) heating and cooling, (5) lighting, and (6) the others. The results are summarized in three sections focused on major impacts on transportation use (comparison between private and public modes), cooking and entertainment, heating/cooling and lighting. The results could provide early suggestions for cities/regions that are experiencing longer lockdown. Furthermore, this study provides insights for larger-scale research in assessing household energy use/demand during times of health emergency and crises, such as the event of a pandemic
A hybrid pricing and cutting approach for the multi-shift full truckload vehicle routing problem
Get PDFFull truckload transportation (FTL) in the form of freight containers represents one of the most important transportation modes in international trade. Due to large volume and scale, in FTL, delivery time is often less critical but cost and service quality are crucial. Therefore, efficiently solving large scale multiple shift FTL problems is becoming more and more important and requires further research. In one of our earlier studies, a set covering model and a three-stage solution method were developed for a multi-shift FTL problem. This paper extends the previous work and presents a significantly more efficient approach by hybridising pricing and cutting strategies with metaheuristics (a variable neighbourhood search and a genetic algorithm). The metaheuristics were adopted to find promising columns (vehicle routes) guided by pricing and cuts are dynamically generated to eliminate infeasible flow assignments caused by incompatible commodities. Computational experiments on real-life and artificial benchmark FTL problems showed superior performance both in terms of computational time and solution quality, when compared with previous MIP based three-stage methods and two existing metaheuristics. The proposed cutting and heuristic pricing approach can efficiently solve large scale real-life FTL problems
Access Repository Dashboard
Do you manage Open Research Online? Become a CORE Member to access insider analytics, issue reports and manage access to outputs from your repository in the CORE Repository Dashboard! 👇