The optimization of machining processes is a deciding factor when increasing productivity and ensuring product quality. The response characteristics, such as surface roughness, material removal rate, tool wear, and cutting time, of the finish turning process have been simultaneously optimized. We used the Taguchi-based design of experiments L9(3⁴) in this study to test and find the best values for process parameters like cutting speed, feed rate, depth of cut, and nose radius. The Taguchi-based multi-objective grey relational approach (GRA) method was used to address the turning problem of Inconel 718 alloy to increase productivity, i.e., by simultaneously minimizing surface roughness, tool wear, and machining time. GRA and the S/N ratio derived from the Taguchi approach were utilized to combine many response characteristics into a single response. The grey relational grade (GRG) produces results such as estimations of the optimal level of input parameters and their proportional significance to specific quality characteristics. By employing ANOVA, the significance of parameters with respect to individual responsibility and the overall quality characteristics of the cutting process were ascertained. The single-objective optimization yielded the following results: minimal surface roughness of 0.167 µm, tool wear of 44.65 µm, minimum cutting time of 19.72 s, and maximum material speed of 4550 mm³/min. While simultaneously optimizing the Inconel 718 superalloy at a cutting speed of 100 m/min, depth of cut of 0.4 mm, feed rate of 0.051 mm/rev, and tool nose radius of 0.4 mm, the results of the multi-objective optimization showed that all investigated response characteristics reached their optimal values (minimum/maximum). To validate the results, confirmatory experiments with the most favorable outcomes were conducted and yielded a high degree of concurrence.

The sustainable energy transition to a low-carbon and climate-neutral economy by 2050 requires a consistent increase in the share of renewable energy sources (RESs) at the expense of the share of fossil fuels. The coal power plants in the Republic of Bulgaria have provided about one third of the annually produced electric power for decades, utilizing mainly locally available sources of lignite. The present work aimed to review the progress of the energy transition, its rejection and acceptance at the national and international scene alongside the available research for cleaner coal combustion in Bulgaria, as well as discuss a Bulgarian perspective for coal share reduction options for power generation during the energy transition. A comprehensive review was carried out, based on freely accessible data such as research and open media articles, officially published field reports, legislative and strategic acts as well as validated statistical data. Three groups of critical gaps (socioeconomic, sociotechnical and cultural and political) were indicated, claimed to be capable of guiding the just transition. Key factors influencing the process dynamics were identified and categorized in the context of the critical gaps. The peculiar policy criteria for the carbon-intensive regions are as follows: the dominant energy carriers, existing infrastructure, involved actors and choice of strategy. The observations allowed us to conclude that in addition to the efforts achieved and ambitious political will, the identification of reliable technological and socioeconomic measures is needed more than ever (accompanied by interdisciplinary research involving the technical, social and environmental and policy factors), while renewables still have long way to go towards complete substitution of the fossil fuels for power generation, transport, and manufacturing. Limited literature was found for reducing the share of coal from currently operating Bulgarian coal-fired power plants (CFPPs). Herein, short- and/or medium-term measures for carbon emission reduction were discussed, capable of promoting the limited operation of existing CFPPs, thus paving the road towards a sustainable, long-term transition. These measures concerned the typically used power units in the largest CFPPs located at the Maritsa Iztok Mining Complex (MIMC). Analyses of the biomass production, supply and cost for the same type of power units were proposed, considering the use of 100% biomass. Estimated costs, unit efficiencies and power generation were discussed along with the evaluations about the land use, ensuring a given annual productivity of wood chips from fast growing plants, e.g., Paulownia.

The goal of this research is to optimize the input parameters utilized in dry CNC drilling of forging steel to attain sustainable machining. Particular emphasis will be placed on achieving high productivity while minimizing the impact on surface quality. To achieve the aforementioned goal, three Taguchi-based multi-criteria decision-making (MCDM) approaches, such as traditional gray relational analysis (GRA), weighted gray relational analysis (WGRA), and data envelopment analysis ranking (DEAR), were used for simultaneous optimization of the MRR and Ra. In Taguchi’s L12 (2⁴) orthogonal array design, the cutting mode parameters—such as cutting speed, depth of cut, feed rate, and point angle—have been chosen as the input parameters for the modeling and analysis of the drilling process characteristics. The process of determining the effect of the input parameters on the output parameters was carried out with the use of analysis of variance (ANOVA). The best results from the studies were Ra = 2.19 and MRR = 375 mm³/s, which corresponded to Taguchi’s single optimization levels, S₂F₁D₁A₂ and S₂F₂D₂A₁, respectively. In the next step, the performance values obtained for each MCDM technique were reoptimized using the Taguchi method, and the optimal levels were obtained: for traditional GRA, the level S₂F₁D₂A₁ (Ra = 2.52 µm, MRR = 125 mm³/s); for WGRA, the level S₂F₁D₁A₁ (Ra = 2.31 µm, MRR = 83 mm³/s); and for DEAR, the level S₂F₂D₂A₁ (Ra = 4.42 µm, MRR = 375 mm³/s), respectively. Lastly, in order to compare the experiments’ performance, validation tests were carried out. The results of the experiments using multi-objective optimization show that traditional GRA improved the overall quality response characteristics by 29.86% compared to the initial setup parameters, while weighted GRA improved them by 34.48%, with the DEAR method providing an improvement of 96%. Based on the findings of this investigation, the DEAR optimization method outperforms the GRA method. As a result, the proposed methods are useful tools for multi-objective optimization of cutting parameters.

In this paper, analyses of Francis turbine failures for powerful Pumped Hydraulic Energy Storage (PHES) are conducted. The structure is part of PHES Chaira, Bulgaria (HA4—Hydro-Aggregate 4). The aim of the study is to assess the structure-to-concrete embedding to determine the possible causes of damage and destruction of the HA4 Francis spiral casing units. The embedding methods that have been applied in practice for decades are discussed and compared to those used for HA4. A virtual prototype is built based on the finite-element method to clarify the influence of workloads under the generator mode. The stages of the simulation include structural analysis of the spiral casing and concrete under load in generator mode, as well as structural analysis of the spiral casing under loads in generator mode without concrete. Both simulations are of major importance. Since the failure of the surface between the turbine, the spiral casing, and the concrete is observed, the effect of the growing contact gap (no contact) is analyzed. The stresses, strains, and displacements of the turbine units are simulated, followed by an analysis for reliability. The conclusions reveal the possible reasons for cracks and destruction in the main elements of the structure.

The present study deals with an accident analysis of the “Chaira” Bulgaria high-pressure Pumped Hydroelectric Energy Storage (PHES), especially the failures of the Francis large-scale Hydraulic Unit No. 4 (HU4). The causes of overloading and cracking of the stay vanes and their functionality are investigated. Despite many studies on the behavior of the runner and its safe operation, the complete destruction of the system due to the stay vanes failure has not been sufficiently studied and published in the scientific literature. The aim is to assess the reasons that have led to this unprecedented accident of the “Chaira” PHES of the total destruction of the stay vanes. Based on the historical data and the operating modes, as well as on the identification of the critical elements, an objective assessment of the risks of damage of the stay vanes and of the entire unit is proposed. A virtual prototype is built for the simulation of the system behavior in working and emergency conditions. The loads, stresses, and deformations are precisely calculated. The critical elements of the highly-loaded blocks are evaluated. Low-cycle material fatigue is with a high degree of confidence the dominant reason for the stay vanes complete destruction. The results of the research are used to propose measures for the possible rehabilitation and/or replacement of the unit.

This paper discusses the dynamics of a novel energy harvester that converts heat into mechanical vibrations of two polyvinylidene fluoride (PVDF) piezoelectric cantilevers that generate electrical energy using a shape memory alloy (SMA) filament. The vibrations are generated by a symmetrical system of two masses placed on the SMA filament, which moves transversely due to its own longitudinal temperature contractions and extensions. Temperature differences over a heat source of constant temperature are the cause of the periodic changes in length of the SMA filament. An experimental setup was created to study the harvester by measuring the mass displacements and electrical voltages generated by the piezoelectric cantilevers. Data were obtained on the dependence of the output voltage and power on the load resistance of the consumer. The experimental results are validated by a multiphysics dynamical model, taking into account the relationships between the mechanical, thermal and electrical domains. The vibrational modeling of the SMA filament takes into account the hysteresis properties and their characteristics when the time gradient changes, leading to the appearance of minor and sub-minor hysteresis. Research has shown that from a heater with a constant temperature of 70 °C, the maximum power obtained is 3.6 μW at a load resistance of 4 MΩ, and a maximum voltage of 5.8 V was generated at a load resistance of 13 MΩ. An important feature of the proposed design is the possibility of miniaturizing the mechanical system.