This paper present the results of experimental investigations of condensation of R134a refrigerant in pipe minichannels with internal diameters 0.64, 0.90, 1.40, 1.44, 1.92 and 3.30 mm subject to periodic pressure instabilities. It was established that as in conventional channels, the displacement velocity of the pressure instabilities distinctly depends on the frequency of their hydrodynamic generation. The void fraction distinctly influences the velocity of the pressure instabilities. The form of this relationship depends on the internal diameter of the minichannels and on the method of calculating the void fraction.

This paper presents the results of an experimental study and
mathematical modeling of the effect of dynamic instabilities on the condensation phase transformation of the refrigerants homogeneous R134a and its replacement in the form of isomers R1234yf and R1234ze and R404A or R507 and R448A in pipe mini-channels. In the case of homogeneous chlorofluorocarbons (CFCs), it is the 1234 isomers that are envisioned as substitutes for the withdrawn ones with high ozone depletion potential and global warming potential. For zeotropic and azeotropic mixtures, for example, these are R507 or R448A. The paper presents a dimensional analysis procedure based on the Buckingham Π theorem to develop a regression velocity model of pressure dynamic instabilities. The experimental part of the work was carried out with the use of tubular mini-channels with internal diameter 1.40–3.3 mm.

Solar energy is a unique source of renewable energy due to its availability and the unlimited quantity. It has long attracted the attention of scientists who are conducting theoretical and experimental research into its use. Solar energy plays an increasingly important role in the context of energy conservation. With the rising cost of conventional energy sources and limited access to natural resources, interest in the use of renewable energy sources is increasing. In this context, environmental protection is another factor favoring the development of technologies based on renewable energy sources. With economic development, the significance of new environmentally friendly technologies is increasing. One of the most popular ways for the average household to utilize renewable energy sources is by installing photovoltaic panels. Such an installation allows the use of solar energy to generate electricity, which contributes to reducing energy costs and protecting the environment. The article presents the results of an analysis of the exergy efficiency of prosumer photovoltaic systems found in the area of northern Poland. The analysis presented was based on actual operating parameters over a certain time interval. A key aspect is the analysis of exergy, which is not distributed in renewable energy sources (RES) systems.

The article presents an analysis of the use of Savonius wind turbines with vertical axis of rotation. The first part presents an analysis of the literature with the dentification of the properties of the basic atmospheric parameters related to the air movement referred to as wind. Used mathematical descriptions used in the analysis of air movement and enabling the identification of basic thermodynamic parameters of wind turbines with a vertical axis of rotation were presented. Then, the historical background of the development of wind turbines with a vertical axis of rotation was presented, and constructions of this type currently used were described. Proposals for modification of the configuration and design of Savonius rotors and the impact of these activities on their efficiency were analyzed. These issues were presented in relation to the experimental work carried out in the international research centers. Obvious advantages and disadvantages of using this type of equipment in the field of wind energy were indicated.