In Serbia, for heating of domestic hot water (DHW) it is customary to use electricity. As around 70% of electricity is produced by using low quality coal with high greenhouse emission, it is beneficial to environment to use solar energy by flat-plate solar collectors for heating of DHW in a solar DHW system (SDHWS). The SDHWS with variable tilt flat-plate solar collectors placed in north-south direction at roofs of houses are investigated for their optimal operation in Belgrade, Serbia. The investigated variable-tilt collectors annually take 2 tilts, 4 tilts, and 12 tilts. The used weather data are from the meteorological station. These investigations use three computer codes: EnergyPlus, GenOpt, and Hooke-Jeeves search algorithm. For different solar collectors, the investigations revile their optimum tilts that maximize the solar fraction by the SDHWS. Then, the solar fraction and avoided fossil energy by the SDHWS are maximized. In addition, the deficit in the solar fraction is estimated when the solar collectors are not at their optimum tilt.

The assessment of thermal power system reliability represents a key step towards maintaining the stability and efficiency of the system. Through the application of mathematical models, risk analysis, and maintenance strategies, it is possible to optimise system performance and reduce the risk of failures. This paper presents an assessment of the reliability of the thermal energy system in fossil fuel power plant ‘Nikola Tesla, Block B1’, based on a twelve-year failure database. By implementing the theory of reliability, based on statistics and theory of possibility, and using the simple and complex two-parameter Weibull distribution, the theoretical functions of the failure rate are determined. A significant advantage of such a study is the opportunity of an early and in-depth understanding of the logic and mechanisms of system risk behaviour, and a more precise assessment of its functioning throughout future exploitation.

Understanding the tribological behavior of additively manufactured polymers is essential for their reliable use in sliding components. Tribological tests were performed on a linear reciprocating tribometer pin-on-plate configuration using a polycarbonate sample (PC–PC). To assess the influence of additive-manufacturing-induced anisotropy, three build orientations (0°, 45°, 90°) were examined. Two normal loads of 39.24 N and 58.86 N, and two sliding velocities of 15 and 20 mm/s were selected to represent typical low-load operating conditions of polymeric components. Tests were conducted in dry contact and with two commercial lubricants exhibiting distinct rheological characteristics. Surface topography was characterized before and after testing to evaluate orientation-dependent roughness evolution, while rheological measurements provided effective viscosities at shear rates corresponding to imposed velocities. Frictional behavior was analyzed through the Stribeck parameter, showing that all configurations operated within boundary or early mixed lubrication regimes. Longitudinal specimen layer orientation (90°) was expected to give the lowest friction. In fact, dominant lowest friction in most of the examination regimes gave the 45° build orientation, whereas the 0° orientation hindered lubricant entrainment and produced the highest boundary interaction. Differences in lubricant viscosity influenced Stribeck positioning and the magnitude of friction reduction, demonstrating strong coupling between layer orientation, roughness evolution, and lubrication performance.

Accurate reciprocating tribological testing depends on the mechanical design, kinematic stability, and load application strategy of the testing apparatus. This paper presents the design, construction, and validation of a custom-built linear reciprocating tribometer developed for controlled laboratory testing. The tribometer provides a programmable stroke of up to 500 mm and a sliding-velocity range of 0.1–50 mm/s. The tribometer was validated through finite element analysis, repeated friction tests, and noise measurements. FEM results showed a maximum displacement of 0.1664 mm and a maximum von Mises stress of 7.47 MPa for the moving plate assembly under a 100 N load, while the loading portal exhibited a maximum localized displacement of 0.017 mm and a maximum stress of approximately 13 MPa under combined loading. These conditions correspond to the maximum design load of the developed tribometer. Repeated tests at 10, 15, and 20 mm/s yielded coefficients of variation of 2.84%, 5.90%, and 14.62%, respectively. Acoustic measurements showed standard deviation values in the range 1.868–3.119 dB. The obtained results confirm stable operation, limited structural deformation, and satisfactory repeatability within the analyzed operating range, indicating that the developed tribometer is suitable for controlled reciprocating tribological experiments under representative laboratory conditions.

In the modern world, where a uninterrupted energy supply is required for proper functioning, thermal energy systems are key components of the infrastructure. Considering the complexity of these systems, it is essential to develop procedures that enable specific determination of the reliability function of thermal energy systems. Analysis of the reliability of the thermal energy system is crucial for maintaining the stability and efficiency of the system. In this paper, by applying the mathematical theory of reliability to the exploitation research data and using simple and complex two-parameter Weibull distribution, the theoretical reliability functions of the thermal power plant Nikola Tesla, Block B2 were determined. A significant advantage of such a study is the possibility of an early and thorough understanding of the logic and mechanisms of risky behavior in the system, as well as a more precise assessment of its functioning throughout the future exploitation.