## 1 Introduction  
Solar technology is constantly developing and is currently expanding. This scientific field includes a large number of different solar constructions, such as solar collectors (SCs), photovoltaic panels (PVs), and hybrid (photovoltaic-thermal) collectors (PVTCs). Of all SCs, flat-plate collectors (FPCs) and evacuated tube collectors (ETCs) have the greatest practical and commercial applications [1; 2] because they are distinguished by a relatively simple manufacturing process, reduced dimensions, satisfactory thermal performance, and relatively low price. In addition, ETCs also offer higher thermal efficiency [3] and working fluid temperature [4] than FPCs, so they can be found almost everywhere: in the residential sector [5], in the commercial (public) sector [6], in the industrial sector [7], etc. In the mentioned sectors, ETCs are used in heating systems [8], cooling systems [9], ventilation systems [10], air conditioning systems [11], hot water systems [12], etc. Some ETC constructions are modified by concentrators [13], reflectors [14], mirrors [15; 16], and tracking systems [17; 18; 19] accompaniedby a large number of experimental [20], numerical [21], and theoretical (analytical or mathematical) [22] investigations. The hybrid configuration with ETCs is presented in [23]. The experimental investigation revealed that the solar thermal collector's maximum outlet temperature, efficiency, and maximum cooling capacity were 87 \({}^{\circ}\)C, 56%, and 4.6 kW, respectively. In [24], a comparative study was carried out on the performance of an individual SC under laboratory conditions and field conditions (experimental study). A compound parabolic concentrator (CPC) with an ETC has been used to apply boiler feed-water preheating. The performance monitoring and evaluation of the actual installation were carried out over 10 months. The system was designed to supply an average of 448,500 kWh-thermal/year, saving the equivalent amount of fossil fuel. The ETC is developed in [25] to achieve high heating-medium temperature. They used 10 mol% of ethylene glycol in water as the heat transfer fluid. The results showed that the average temperature of the heat transfer fluid in a modified tube increased to 160.32 \({}^{\circ}\)C, with an efficiency increase of 34.96%. [PERSON] et al. numerically investigated an ETC with a U-tube [26]. The thermal performance of the analyzed SC was simulated for different operating conditions. They used higher thermal conductivity working fluids: Ag, ZnO, and MgO nanoparticles in 30%:70% (by volume) ethylene glycol/pure water mixture and different nanoparticle volumetric concentrations. For example, a new ETC construction with inner concentrating, which has 10% higher thermal efficiency than classic ETCs, was proposed, fabricated, and presented in [27]. Additionally, the mathematical model of the mentioned solar collector was developed. [PERSON] et al. [28] mathematically considered a double-glazed ETC with a U-tube, where the absorber film is deposited on the outer surface of the absorber tube. In the review paper presented in [29], [PERSON] et al. analyzed ETCs with direct flow and heat pipes, providing comprehensive theoretical analysis. The review paper [30] focused on air as a working fluid in ETCs and summarized design configurations and simulation works, using nanofluids and phase change materials to understand their influence on the thermal performance of these ETCs. The state of the market, along with the available literature, shows that many ETC constructions never progress beyond the case-study phase to the commercial phase. The reasons include complex technological manufacturing procedures, costly experimental investigations, use of scarcely available materials, high manufacturing costs, high final product prices, and difficult transportation (including assembly and maintenance). Some solutions are much larger in terms of surface occupancy and volume occupancy [31] compared to the basic versions of solar collectors--indicators that authors often ignore in their works.