Optimization of the distribution network operation by integration of distributed energy resources and participation of active elements : doctoral thesisNevena Srećković
, 2020, doctoral dissertation
Abstract: Distribution Networks (DNs) are evolving from a once passive to an active part of the electricity network. This evolution is driven by the current political and environmental decisions, Directives and Incentives, as well as the technological development, observed in the everincreasing integration of renewable energy resources, advanced network control and measurement devices, the upcoming energy exibility market, etc. This Doctoral Thesis deals with the problem of optimization of the technical aspects of a DN operation, enabled by the proliferated integration of the photovoltaic systems (PV) and other active devices. The main objective of the Thesis is the optimization of a DN operation in terms of minimization of electrical energy losses while ensuring the proper voltage profiles and preventing thermal overloadingof lines. Therefore, three Differential Evolution-based optimization procedures were developed and tested on real medium and low voltage DNs. The first methodology determines the optimal rooftop surfaces for the installation of PV systems, yielding minimum annual energy losses. It is based on the simultaneous consideration of high-resolution spatio-temporal solar and PV potential data, as well as long-term measured profiles of consumption and generation of electrical energy within the network of a given configuration. The second algorithm minimizes network losses in a time-discrete operation point, by determining the optimal operation of the following active elements: PV systems capable of cooperation in reactive power provision, On-Load Tap Changer equipped transformer substations and remotely controlled switches for network reconfiguration. The final algorithm was developed by a proper consolidation of the first two approaches, yielding the synergistic effects expressed as the increase of loss reduction and network exibility. The results of the performed case studies show that the locations of the highest suitability for PV installation with respect to the solar energy availability, are not necessarily the best choice from the network operation standpoint. Therefore, both standpoints should be considered simultaneously when choosing the rooftop surfaces for PV installation. Furthermore, by determining optimal hourly operation of the considered active elements, not only the additional reduction of annual network losses was achieved, but also increased accommodation of the PV systems that doesn't violate operation constraints.
Keywords: distribution network, optimization of operation, active network elements, PV system placement, minimization of losses
Published in DKUM: 11.06.2020; Views: 1155; Downloads: 249
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Analytical modelling of achieving energy efficiency with green wallsDejan Tasić
, 2019, master's thesis
Abstract: In the last years, energy efficiency has become an important issue for the EU and Slovenia, who both strive for more sustainable use of energy in buildings. As an innovative approach to such matter, green walls are vertical structures that can serve both as a façade or an insulation layer as well as an aesthetic design for building application. This thesis contains calculated annual heat (transmission) losses through different wall assemblies with and without a constructed green wall in front of them. The goal is to compare values of heat losses between those wall assemblies and the explanation of such results which will determine if green walls present a viable energy-efficient structure for buildings.
Keywords: energy efficiency, green wall, heat losses, modelling
Published in DKUM: 18.11.2019; Views: 1291; Downloads: 105
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A new somatic cell count index to more accurately predict milk yield lossesJanez Jeretina
, Dejan Škorjanc
, Drago Babnik
, 2017, original scientific article
Abstract: Intramammary infection and clinical mastitis in dairy cows leads to considerable economic losses for farmers. The somatic cell concentration in cow's milk has been shown to be an excellent indicator for the prevalence of subclinical mastitis. In this study, a new somatic cell count index (SCCI) was proposed for the accurate prediction of milk yield losses caused by elevated somatic cell count (SCC). In all, 97238 lactations (55207 Holstein cows) from 2328 herds were recorded between 2010 and 2014 under different scenarios (high and low levels of SCC, four lactation stages, different milk yield intensities, and parities (1, 2, and _>3). The standard shape of the curve for SCC was determined using completed standard lactations of healthy cows. The SCCI was defined as the sum of the differences between the measured interpolated values of the natural logarithm of SCC (ln(SCC)) and the values for the standard shape of the curve for SCC for a particular period, divided by the total area enclosed by the standard curve and upper limit of ln(SCC)=10 for SCC. The phenotypic potential of milk yield (305-day milk yield - MY305) was calculated using regression coefficients estimated from the linear regression model for parity and breeding values of cows for milk yield. The extent of daily milk yield loss caused by increased SCC was found to be mainly related to the early stage of lactation. Depending on the possible scenarios, the estimated milk yield loss from MY305 for primiparous cows was at least 0.8 to 0.9 kg day -1 and for multiparous cows it ranged from 1.3 to 4.3 kg day-1. Thus, the SCCI was a suitable indicator for estimating daily milk yield losses associated with increased SCC and might provide farmers reliable information to take appropriate measures for ensuring good health of cows and reducing milk yield losses at the herd level.
Keywords: dairy cows, milk production, lactation intervals, milk yield losses, somatic cell count index
Published in DKUM: 24.10.2017; Views: 1125; Downloads: 448
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Flow-pressure analysis of loop gas networksJurij Krope
, Peter Trop
, Darko Goričanec
, 2011, original scientific article
Abstract: This paper proposes a mathematical model underlying a computer program for flow-pressure analysis of loop gas pipe networks. The method is used on a test case with four nodes. The HAPN application for flow-pressure analyses of low pressure gas pipe networks is completely designed in object-oriented programming technology. The equations, which describe the physical flow-pressure conditions through every cross point are assumed to be continuous and the energy of every closed loop of analyzed network conserved. The system of non-linear equations was linearized by LTM (Linear Theory Method). The algorithm for numerical module LTM and the method for solution of sparse matrix are developed at the Faculty of Chemistry and Chemical Engineering, University of Maribor, Slovenia.
Keywords: fluid mechanics, gas pipe networks, linear method, non-linear programming, pressure losses
Published in DKUM: 01.06.2012; Views: 1872; Downloads: 29
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Laminar flow of a shear-thickening fluid in a 90° pipe bendJure Marn
, Primož Ternik
, 2006, original scientific article
Abstract: The non-Newtonian fluid flow in a sharp 90 curved pipe is studied numerically to obtain the pressure loss coefficient prompted by disagreement between the existing empirical correlations and results obtained by computer codes. This disagreement results from presumption of fully developed flow throughout the curvature (correlations) while the actual flow is partially developed for the Newtonian and sharp 90 curved bend non-Newtonian flows, and fully developed for slightly bent 90 curvature non-Newtonian flow. The Quadratic model is employed to accommodate the shear-thickening behavior of an electrostatic ash and water mixture. Numerical results are obtained for different values of Reynolds number. Finally, results for local pressure loss coefficient are compared with values obtained for the Power law rheological model.
Keywords: shear-thickening fluid, fluid dynamics, rheological model, pipe bend flow, numerical modelling, pressure losses
Published in DKUM: 30.05.2012; Views: 1627; Downloads: 80
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