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1.
Large-scale and multipolar anisotropies of cosmic rays detected at the Pierre Auger Observatory with energies above 4 EeV
R. de Almeida, Andrej Filipčič, Jon Paul Lundquist, Samo Stanič, Serguei Vorobiov, Danilo Zavrtanik, Marko Zavrtanik, Lukas Zehrer, 2022, published scientific conference contribution

Abstract: More than half a century after the discovery of ultra-high energy cosmic rays (UHECRs), their origin is still an open question. The study of anisotropies in the arrival directions of such particles is an essential ingredient to solve this puzzle. We update our previous analysis of large-scale anisotropies observed by the Pierre Auger Observatory using the latest data collected before the AugerPrime upgrade. We select events with zenith angles up to 80 degrees, implying a sky coverage of 85%, and energies above 4 EeV, for which the surface detector of the Observatory is fully efficient. Dipolar and quadrupolar amplitudes are evaluated through a combined Fourier analysis of the event count rate in right ascension and azimuth. The analysis is performed in three energy bins with boundaries at 4, 8, 16 and 32 EeV and two additional cumulative bins with energies above 8 and 32 EeV. The most significant signal is a dipolar modulation in right ascension for energies above 8 EeV, as previously reported, with statistical significance of 6.6σ. Additionally, we report the measurements of the angular power spectrum for the same energy bins with the same dataset.
Keywords: Pierre Auger Observatory, indirect detection, surface detection, ground array, ultra-high energy, cosmic rays, anisotropy, dipole, quadropole, angular power spectrum, inclined showers
Published in RUNG: 03.10.2023; Views: 1000; Downloads: 4
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2.
Efficiency of the grid energy storage technology based on iron-chloride material cycle
Uroš Luin, doctoral dissertation

Abstract: Future high-capacity energy storage technologies are crucial for a highly renewable energy mix, and their mass deployment must rely on cheap and abundant materials, such as iron chloride. The iron chloride electrochemical cycle (ICEC), suitable for long-term grid energy storage using a redox potential change of Fe2+/Fe, involves the electrolysis of a highly concentrated aqueous FeCl2 solution yielding solid iron deposits. For the high overall energy efficiency of the cycle, it is crucial maximizing the energy efficiency of the electrolysis process. The thesis presents a study of the influence of electrolysis parameters on energy efficiency, performed in an industrial-type electrolyzer system. We studied the conductivity of the FeCl2 solution as a function of concentration and temperature and correlated it with the electrolysis energy efficiency as a function of current density. The contribution of the resistance polarization increases with the current density, causing a decrease in overall energy efficiency. The highest energy efficiency of 89 ±3 % was achieved using 2.5 mol dm-3 FeCl2 solution at 70 °C and a current density of 0.1 kA m-2. In terms of the energy input per Fe mass, this means 1.88 Wh g-1. The limiting energy input per mass of the Fe-deposit, calculated by extrapolating experimental results toward Eocell potential, was found to be 1.76 Wh g-1. For optimal long-duration electrolysis efficiency and performance, the optimal catholyte concentration range is 1-2 mol dm-3 FeCl2. We performed in situ X-ray absorption spectroscopy experimental studies to validate theoretical conclusions from literature related to the population and structure of Fe-species in the FeCl2 (aq) solution at different concentrations (1 - 4 mol dm-3) and temperatures (25 - 80 °C). This revealed that at low temperature and low FeCl2 concentration, the octahedral first coordination sphere around Fe is occupied by one Cl ion at a distance of 2.33 (±0.02) Å and five H2O at a distance of 2.095 (±0.005) Å. The structure of the ionic complex gradually changes with an increase in temperature and/or concentration. The apical H2O is substituted by a Cl ion to yield a neutral Fe[Cl2(H2O)4]0. The transition from the charged Fe[Cl(H2O)5]+ to the neutral Fe[Cl2(H2O)4]0 causes a significant drop in the solution conductivity, which well correlates with the existing state-of-the-art conductivity models. An additional steric impediment of the electrolytic cell is caused by the predominant neutral species present in the catholyte solution at high concentration. This correlates with poor electrolysis performance at a very high catholyte concentration (4 mol dm-3 FeCl2), especially at high current densities (> 1 kA m-2). The neutral Fe[Cl2(H2O)4]0 complex negatively affects the anion exchange membrane ion (Cl-) transfer and lowers the concentration of electroactive species (Fe[Cl(H2O)5]+) at the cathode surface. The kinetics of hydrogen evolution from the reaction between Fe powder and HCl acid was studied under the first-order reaction condition. The activation energy was determined to be 55.3 kJ mol-1.
Keywords: ICEC, Power-to-Solid, energy storage, hydrogen, ferrous chloride, electrolysis, Fe deposition, efficiency, XAS, structure and population, ionic species, ion association, conductivity
Published in RUNG: 18.04.2023; Views: 1782; Downloads: 26  (1 vote)
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3.
Monitoring the success of the performance of a travel agency with the Power BI tool : diploma thesis
Riste Ristov, 2022, undergraduate thesis

Abstract: In today’s times, when digitalisation is advancing in all areas, travel agencies need to follow trends to optimise their work as much as possible and stay competitive. Business information systems provide a more detailed insight into business processes and their operation, which helps monitor progress and adapt to possible changes to achieve the best results in the business world. Travel agencies need a quality business information system that helps employees shift from using basic office tools to more modern and effective systems for reporting and monitoring work performance. Data is the basis on which agencies adjust, optimise and build their business. Data is available in huge quantities and a variety of formats. Raw data that is not organised into a meaningful whole cannot bring added value to the company. The thesis aims to use Power BI as a tool for editing and integrating databases in travel agencies, as well as their visualisation and display, which is understandable for the user. With the help of the tool, we will create a report which will help the travel agency improve its business and make critical decisions that are critical for success.
Keywords: Travel agency, business information system, data, database, Power BI, visualisation
Published in RUNG: 30.09.2022; Views: 2029; Downloads: 67
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4.
Correlation between FeCl2 electrolyte conductivity and electrolysis efficiency
Uroš Luin, Matjaž Valant, Iztok Arčon, 2022, published scientific conference contribution abstract

Abstract: The electrolysis efficiency is an important aspect of the Power-to-Solid energy storage technology (EST) based on the iron chloride electrochemical cycle [1]. This cycle employs an aqueous FeCl2 catholyte solution for the electro-reduction of iron. The metal iron deposits on the cathode. The energy is stored as a difference in the redox potential of iron species. Hydrogen, as an energy carrier, is released on demand over a fully controlled hydrogen evolution reaction between metallic Fe0 and HCl (aq) [1]. Due to these characteristics, the cycle is suitable for long-term high-capacity and high-power energy storage. In a previous work [2] we revealed that the electrolyte conductivity linearly increases with temperature. Contrary, the correlation between the electrolyte concentration and efficiency is not so straightforward. Unexpectedly small efficiency variations were found between 1 and 2.5 mol dm-3 FeCl2 (aq) followed by an abrupt efficiency drop at higher concentrations. To explain the behavior of the observed trends and elucidate the role of FeCl2 (aq) complex ionic species we performed in situ X-ray absorption studies. We made a dedicated experimental setup, consisting of a tubular oven and PMMA liquid absorption cell, and performed the measurements at the DESY synchrotron P65 beamline. The XAS investigation covered XANES and EXAFS analyses of FeCl2 (aq) at different concentrations (1 - 4 molL-1) and temperatures (25 - 80 °C). We found that at low temperature and low FeCl2 concentration the octahedral first coordination sphere around Fe is occupied by one Cl ion at a distance of 2.33 (±0.02) Å and five water molecules at a distance of 2.095 (±0.005) Å [3]. The structure of the ionic complex gradually changes with an increase in temperature and/or concentration. The apical water molecule is substituted by a chlorine ion to yield a neutral Fe[Cl2(H2O)4]0. The transition from the single charged Fe[Cl(H2O)5]+ to the neutral Fe[Cl2(H2O)4]0 causes a significant drop in the solution conductivity, which well correlates with the existing conductivity models [3]. [1] M. Valant, “Procedure for electric energy storage in solid matter. United States Patent and Trademark Office. Patent No. US20200308715,” Patent No. US20200308715, 2021. [2] U. Luin and M. Valant, “Electrolysis energy efficiency of highly concentrated FeCl2 solutions for power-to-solid energy storage technology,” J. Solid State Electrochem., vol. 26, no. 4, pp. 929–938, Apr. 2022, doi: 10.1007/S10008-022-05132-Y. [3] U. Luin, I. Arčon, and M. Valant, “Structure and Population of Complex Ionic Species in FeCl2 Aqueous Solution by X-ray Absorption Spectroscopy,” Molecules, vol. 27, no. 3, 2022, doi: 10.3390/molecules27030642.
Keywords: Iron chloride electrochemical cycle, Power-to-Solid energy storage, XANES, EXAFS, electrical conductivity, electrolyte complex ionic species structure and population
Published in RUNG: 26.09.2022; Views: 2040; Downloads: (1 vote)
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1/x distribution in a close proximity of the Bak-Tang-Wiesenfeld sandpile
Alexander Shapoval, other performed works

Keywords: sandpile, scale-free distribution, power-law
Published in RUNG: 09.07.2021; Views: 2272; Downloads: 0
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8.
Links between HX-groups and hypergroups
Irina Elena Cristea, Michal Novak, Babatunde Oluwaseun Onasanya, 2021, original scientific article

Keywords: HX-group, hyperstructure theory, Chinese hypergroupoid, EL-hyperstructure, power sets
Published in RUNG: 03.06.2021; Views: 2209; Downloads: 0
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9.
Sand density as sandpile descriptor
Alexander Shapoval, M. Shnirman, 2008, original scientific article

Keywords: sandpile, critical density, power law exponent
Published in RUNG: 19.04.2021; Views: 2374; Downloads: 0
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10.
Crossover phenomenon and universality: from random walk to deterministic sand-piles through random sand-piles
Alexander Shapoval, M. Shnirman, 2005, original scientific article

Keywords: sandpile, universality class, power-law exponent
Published in RUNG: 19.04.2021; Views: 2336; Downloads: 0
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