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.

This article presents computational fluid dynamics (CFD) modeling of the flow processes at a certain specimen of an external gear pump. The purpose of the developed two-dimensional (2D) CFD model is to carry out a numerical study to obtain the main characteristics of the pump flow rate, especially the flow rate as a function of the pressure and the flow rate as a function of the time. A numerical study was carried out at forty-two different operating modes that were expressed as a variation of two parameters: rotational frequency (950–1450 min⁻¹) and pressure (5–150 bar). The validation of the numerical results was carried out through an experimental study. For this purpose, a laboratory experimental setup equipped with a modern data acquisition (DAQ) system was designed and implemented. It allows the gear pump to be tested at the same operating modes as the numerical study. A validation analysis was performed by comparing the numerical and experimental results using the average relative error index (FIT). A detailed description of the 2D CFD model development (CAD model, mesh, general settings, boundary conditions, etc.) is provided. Based on the 2D CFD model, an original methodology was proposed to take into account the influence of the discharge channels on the displacement volume of the pump by adjusting the face width of the gears. Despite the limitations of the simple 2D CFD model, which are discussed in this article, a very good match between numerical and experimental results is analyzed by calculating the FIT level, which is in the range of 93–97%.

In the present paper, a numerical study of the acoustic characteristics of a new acoustic absorber construction has been conducted. The cross-section of the plastic acoustic absorber is in the form of a semi-tube. The acoustic absorber is modeled and studied with COMSOL Multyphisics ^® . Some of the simulation numerical results of the acoustic absorber model and the experimental verification are compared and presented.

Trajectory tracking control is important in order to realize autonomous driving of mobile robots. From a control standpoint, trajectory tracking can be stated as the problem of stabilizing a tracking error system that describes both position and orientation errors of the mobile robot with respect to a time-parameterized path. In this paper, we address the problem for the trajectory tracking of nonholonomic wheeled mobile robots, and an exponential trajectory tracking controller is designed. The stability analysis is concerned with studying the local exponential stability property of a cascade system, provided that two isolated subsystems are exponentially stable and under certain bound conditions for the interconnection term. A theoretical stability analysis of the dynamic behaviors of the closed-loop system is provided based on the Lyapunov stability theory, and an exponential stability result is proven. An explicit estimate of the set of feasible initial conditions for the error variables is determined. Simulation results for verification of the proposed tracking controller under different operating conditions are given. The obtained results show that the problem of trajectory tracking control of nonholonomic wheeled mobile robots is solved over a large class of reference trajectories with fast convergence and good transient performance.

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.

Novel hybrid organic/inorganic flexible devices with composite films, consisting of Ba_0.5Sr_0.5TiO₃ (BST), were prepared by inserting BST nanocoating under spray deposited Poly-vinylidene fluoride-based co-polymer PVDF-TrFE. The study validated that the crystalline structure of BST remains unaffected by the presence of polymer. The 3D atomic force microscopic image of the composite sample confirmed the improved surface roughness and contact conditions after spraying the polymer. As a result, the hybrid sample exhibited a higher polarization current with reduced impedance and parasitic inductance. The enhancement of the stability of the piezoelectric parameters at multiple bending was observed for the hybrid sample in comparison with the BST single film transducer. The drop of the root mean square (RMS) voltage was 70% after approximately 340,000 numbers of bending against less than 3% for the hybrid BST+PVDF-TrFE device. Due to the effect of the separate layers and summed net charges, the piezoelectric voltage of the hybrid device was competitive to the piezoelectric oxide films, despite the lower piezoelectric coefficient of the polymer. The proposed solution paves the path toward lead-free, wearable energy harvesting devices for low-power consuming electronic devices.