In recent years, a growing global effort has been underway to reduce the Earth’s carbon footprint. One of the main strategies to achieve this goal is the utilization of available renewable energy resources. Among the largest and most inexhaustible is hydro-power. This paper presents an experimental study of three hydrokinetic turbines tested under real river conditions, aiming to evaluate their effectiveness in harnessing the kinetic energy of flowing water. The experiment is described in detail, including velocity field measurements conducted within the river section used for testing. Based on the experimental data, the main performance characteristics of the three turbines are presented, specifically their power output and efficiency. The importance of selecting an optimal riverbed site and customizing turbine runners to local flow conditions is highlighted, as even slight velocity fluctuations can significantly impact performance. Among the tested designs, the K1–6 turbine runner showed the highest power and efficiency, while the K2–4 runner provided superior rotational stability, making it promising for consistent energy output in variable flow environments

Very few experimental studies of full pinion engagement planetary gear trains have been published; therefore, their behavior under load is little known. In this paper, the results from the experimental studies of the above-mentioned gear trains are presented, whereby the bending stresses in the planet pins are displayed both in time and frequency domains by means of fast Fourier transform (FFT). The experiments are conducted on a mechanical closed-loop test rig, which was designed especially for the experiments. The bending stresses in the pins are measured by strain gauges, which are mounted in a double half-bridge configuration, thus showing the stresses in two perpendicular planes. The torque applied is 200 Nm. The radial run-out errors of the planets are measured and their relation to the pin loads are analyzed.

This article investigates the influence of planet mounting position on axle loads in double planet planetary gear trains. The study includes basic information about the type of geartrain used, theoretical data, finite element analysis simulations, and empirical experiments to provide more detailed data on the impact of planet pin position on bearing loads in different scenarios. A test model with six different setups is designed for the studies. Finite element simulations were then conducted to examine the model and to give more comprehensive information on the effect of pin location. Empirical experiments were conducted on a test rig to verify the theoretical and simulation results. The results indicate that the position of the planet pins has an impact on the distribution of load between the bearings of the outer and inner planets of the gear train. This study aims to provide more insights into the design and optimization of planetary gear trains when evaluating the loads on the bearings of the planet gears.

The study presents an analysis and guide for the synthesis of a spherical shaped gear mesh in kinematic sculptures. The specific conditions for generating such arrangements are investigated, like the mesh condition for constrained gear systems, the geometry of forming gears, possible patterns, choice of dimensions, and number of teeth. The starting geometry is also partially described, which is based on convex polyhedrons. It is a solid three-dimensional figure with flat polygonal faces that are closed, where all the angles between adjacent faces are less than or equal to 180 degrees, and there are no concave regions or indentations in its structure. Two different examples are presented based on two different polyhedrons with different gear arrangements.

Working toward replacing conventional fossil fuels with alternative renewable energy, offshore renewable energy technologies give promising solutions, even though each of the types (wind, wave, tidal and thermal) is at a different stage of development and with its own unique challenges and opportunities. This paper is focused on sea wind wave energy harvesting through a hybrid system that combines both a water turbine with oscillating blades and “Wells”-type air turbine. A test bench was designed and tested at different flow rates to study the performance of the hybrid system and more precisely the “Wells”-type turbine. Experimental studies of the main parameters of a turbine were conducted, which demonstrated the efficiency of the turbine and its good practical application. The experimental results show good agreement with the preliminary performed numerical simulations, as well as with the published data.

This study presents a comparative analysis of the potential for combined use of wind and solar energy in nine key coastal settlements on the Bulgarian Black Sea coast—Shabla, Balchik, Varna, Byala, Obzor, Nesebar, Burgas, Primorsko, and Tsarevo—selected for their diverse geographical and meteorological characteristics. The study evaluates the feasibility of implementing hybrid renewable energy systems by analyzing the average annual solar radiation and wind velocity for each location. A methodology based on physical and technical parameters is applied to determine the required installed capacity of photovoltaic systems to meet the average annual household electricity consumption of 6000 kWh. Concurrently, wind energy potential is assessed through theoretical and practical models using two turbine sizes (3 m and 6 m in diameter), which represent small-scale residential wind applications.