Search results for: Tsvetelina Merdzhanova

Authors: Uchechi Chibuko, Tsvetelina Merdzhanova, Solomon Agbo, Uwe Rau, Ursula Wurstbauer, Oleksandr Astakhov

Abstract:

Coupling of Photovoltaics (PV) to water electrolysers for production of hydrogen via water splitting holds great potential for long term energy storage and transportation. Similarly, the intermittency of PV power on short timescale from hours to days is more efficiently covered by batteries. In this work, we present two different system configurations. The first is the connection of a PV module to an alkaline electroylser/electrochemical (EC) cell stack referred to as a PV-EC system. This is carried out using a direct coupling approach for simplicity, implying the absence of any power electronics for the PV module maximum power point (MPPT) tracking. The second system involves connecting a lithium-titanium-oxide (LTO) battery (B) pack parallel to the electrolyser which is connected to a PV module and referred to as a PV-EC-B system also using a direct coupling approach. The first motivation of studying the two different systems is the possibility of improved stability and round-the-clock operation of the electrolyser in the presence of the battery pack connected to it. In PV-EC configuration the electrolyser remains naturally idle during night times/ periods of insufficient irradiance which can lead to accelerated degradation and safety concerns. For the PV-EC-B configuration, the power of the PV is split between the electrolyser and the battery during the day enabling an extended operation of the electrolyser during the night/periods of insufficient solar irradiance as the battery powers the electrolyser. We delve into two major implications of this effect using optimized experimental procedures with the use of GaAs concentrator PV modules with an efficiency of 34.5% at 17.3 suns. The first implication deals with how the solar-to-hydrogen (STH) efficiency of the systems is affected. The STH efficiency limit of the PV-EC configuration is obtained using an already established theoretical analysis of the electrolyser polarization curve. We therefore show that the experimental result of the PV-EC system yields an STH efficiency of 23.0% which is only 0.5% absolute less than its obtainable STH limit of 23.5%. However, the STH efficiency of the PV-EC-B is obtained to be 25.4% which is not only above the STH efficiency in the PV-EC configuration but also 1.9% absolute above the theoretical STH efficiency limit in the PV-EC configuration. The gain in STH efficiency is synergistic despite battery related losses as the electrolyser in the PV-EC-B configuration converts the same solar power as in the PV-EC system but at lower overpotentials. The second implication is the possibility of the electrolyser downscaling since the battery provides the possibility of running the electrolyser at lower potentials/ peak power. Our results show that the relative electrolyser size can be downscaled by a factor of 2 with the addition of a battery pack while keeping the STH efficiency without batteries. These two orthogonal investigations will be presented in well-defined steps showing determination of power requirements of the electrolyser, PV sizing, PV operation methodology using PV emulation, experimental obtained parameters and analysis of results. The results form a basis for investigation into more realistic operation, upscaling and techno-economic implications.

Keywords: photovoltaic, electrolyser, battery, solar-to-hydrogen efficiency

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Authors: Yolanda Zografova, Ekaterina Dimitrova, Tsvetelina Panchelieva, Victoria Nedeva-Atanasova

Abstract:

Aggression and violence amongst school-aged children are widely spread phenomenon, which is expanding both on a global level and in Bulgaria. The purpose of the paper is to reveal the overall range of different types and manifestations of school aggression in a specific age group (12 to 18 years old students) from the 5th to the 12th grade according to the Bulgarian education system. In addition, the research investigates the dynamics of aggressive behaviour in two parallel lines – a horizontal one (with students from the same age) and a vertical one (with students from different grade). In the current study based on the original authors’ inventory (School Aggression Questionnaire), the three main types of aggression are measured – physical, verbal and indirect. The sample consists of 300 students from schools in a big metropolitan city, a mid-sized town, and a small town. Results show that the predominant aggression type is the verbal one, but this is the predominant type for the girls in the sample, not for the boys. Another result is that the higher the school grade, the lower levels of overall aggression is shown by the students. The study of such a multi-dimensional phenomenon as the aggression will provide up-to-date scientific knowledge, important both for the development of science on these topics, and useful for public interests in relation to the balanced development of children and adolescents at school. The results provide an excellent base for the development of prevention and intervention programs in order to reduce school aggression.

Keywords: educational psychology, School aggression, interpersonal relations, school aggression questionnaire, types of aggression

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Authors: Diana A. Dobreva, Veselina Panayotova, Albena Merdzhanova, Lubomir Makedonski, Mona Stancheva

Abstract:

Many studies suggest that marine mollusks are healthy food, characterized by low fat and high digestible proteins content. They are one of the most important dietary sources of fat soluble vitamins. The most common species of mollusks in the Bulgarian Black Sea waters are the black mussel (Mytilus galloprovincialis) and the sea snail Rapana (Rapana venosa). One of the main problems of the region is the lack of information about chemical composition of these important marine species. Due to these facts, the aim of the present work was to determine the fat soluble vitamins A, D2, D3, and E, carotenoids–β-carotene and astaxanthin, and total cholesterol contents of mollusk samples and compare them to sample of green algae (Ulva rigida). Samples were collected during autumn from north region of the Black Sea coast, and their wet tissues were used for evaluation of vitamins A, D2, D3, and E, astaxanthin, β-carotene and cholesterol compositions. All fat soluble analytes were simultaneously analyzed by RP- HPLC/UV/FL system. The results were calculated as milligrams per gram total lipid (mg.g-1TL). Alpha-tocopherol and b-carotene were most abundant in algae samples, while mussel samples presented the highest amounts of vitamin D3 (several times higher than the recommended daily intake in Bulgaria (Ordinance № 23 / 19.07.2005)). In all samples, cholesterol content was significantly low, which falls within recommendation of the same ordinance (upper daily consumption should not exceed 300 mg per day). From data, it can be concluded that all samples were characterized as beneficial sources of biologically active compounds.

Keywords: fat soluble vitamins, carotenoids, mussel, rapana, algae

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Authors: Divyansh Goel, Varun Jain

Abstract:

Classical economics works on the principle that people are rational and analytical in their decision making and their choices fall in line with the most suitable option according to the dominant strategy in a standard game theory model. This model has failed at many occasions in estimating the behavior and dealings of rational people, giving proof of some other underlying heuristics and cognitive biases at work. This paper probes into the study of these factors, which fall under the umbrella of behavioral economics and through their medium explore the solution to a problem which a lot of nations presently face. There has long been a wide disparity in the number of people holding favorable views on organ donation and the actual number of people signing up for the same. This paper, in its entirety, is an attempt to shape the public policy which leads to an increase the number of organ donations that take place and close the gap in the statistics of the people who believe in signing up for organ donation and the ones who actually do. The key assumption here is that in cases of cognitive dissonance, where people have an inconsistency due to conflicting views, people have a tendency to go with the default choice. This tendency is a well-documented cognitive bias known as the status quo bias. The research in this project involves an assay of mandated choice models of organ donation with two case studies. The first of an opt-in system of Germany (where people have to explicitly sign up for organ donation) and the second of an opt-out system of Austria (every citizen at the time of their birth is an organ donor and has to explicitly sign up for refusal). Additionally, there has also been presented a detailed analysis of the experiment performed by Eric J. Johnson and Daniel G. Goldstein. Their research as well as many other independent experiments such as that by Tsvetelina Yordanova of the University of Sofia, both of which yield similar results. The conclusion being that the general population has by and large no rigid stand on organ donation and are gullible to status quo bias, which in turn can determine whether a large majority of people will consent to organ donation or not. Thus, in our paper, we throw light on how governments can use status quo bias to drive positive social change by making policies in which everyone by default is marked an organ donor, which will, in turn, save the lives of people who succumb on organ transplantation waitlists and save the economy countless hours of economic productivity.

Keywords: behavioral economics, game theory, organ donation, status quo bias

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