## 1 Introduction  
Advancements in solar technology and solar physics are essential for the share of solar energy in the total RES mix to increase in the future. This includes improving existing solar devices and inventing new ones [1]. These innovations must enhance useful heat power and thermal efficiency while maintaining economic affordability. The traditional classification, according to which solar collectors (SCs) can be non-tracking, i.e., fixed or tracking (first classification criteria [2, 3, 4]) and non-concentrating or concentrating (second classification criteria [5, 6, 7]), is considered obsolete today. This is best illustrated by modified versions of flat-plate collectors (FPCs) and evacuated tube collectors (ETCs)--SCs that are traditionally fixed and non-concentrating. FPCs and ETCs are characterized by small dimensions [8], relatively simple manufacturing technologies [9], low production and sales costs (market availability [10]) and satisfactory thermal performance (FPCs' and ETCs' zero-loss thermal efficiencies are about 80% and 70%, respectively [11]). In other words, these are solar structures with an excellent basis for further improvements, which can be achieved with simple technological solutions and low investments. Table 1 shows some modified solutions (with an emphasis on reflectors and tracking mechanisms) implemented in the past using mathematical (math), numerical (num) and experimental (exp) scientific approaches. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**SC Type** & **Modification** & **Method** & **Description** & **Main Results** & **Year** & **Source** \\ \hline \multirow{8}{*}{Reflector} & \multirow{3}{*}{Math/Exp} & \multirow{3}{*}{Manually operated mirror in all three orthogonal directions} & \multirow{3}{*}{\(\eta\) higher by 29.55\%} & \multirow{3}{*}{2015} & \multirow{3}{*}{[12]} \\ \cline{3-3} \cline{5-6}  & & & & & \\ \  
For instance, [PERSON] et al. [31] investigated a hybrid ETC configuration, achieving the maximum outlet temperature of 87 \({}^{\circ}\)C, thermal efficiency of 56% and cooling capacity of 4.6 kW. [PERSON] et al. [32] developed an ETC with a modified design, achieving a heat transfer fluid temperature of 160.32-22 \({}^{\circ}\)C higher than standard tubes. Other studies, such as those by [PERSON] et al. [33] and [PERSON] et al. [34], explored advanced designs and working fluids, including nanofluids, to enhance performance. [PERSON] et al. reported the highest collector efficiency at 68.7%, using the 4.0 vol% Ag/EG-PW nanofluid. The measurement and simulation of the flow rate in the water-in-glass ETC were realized by [PERSON] et al. in [35]. The natural circulation flow through the water-in-glass ETC with the flat reflector behind (and parallel) was investigated numerically and experimentally in [36]. The theoretical model [37] of the double-stage direct flow in the coaxial vacuum tube collectors (VTCs) was presented and investigated by [PERSON] et al. In [38], the subject of the experimental research was the direct flow in the ETC integrated with the phase change material. [PERSON] et al. developed an interesting solar construction (parabolic SC with direct flow evacuated tube, i.e., ET) [39]. The results showed that the thermal efficiency of the presented SC can increase by 71% in some cases (maximum value). In [40], the thermal performance of the helical direct flow U-Tube ETC was analyzed. The main result of this study is that the maximum average energy and exergy efficiencies of 38.6% and 18%, respectively, were achieved in the seven-step helical tube ETC at the highest flow rate of 30 1/h. The reviewed literature reveals that when researching ETCs, the main focus is usually placed on the following parameters: reflective surfaces (and tracking mechanisms) implementation, the mutual axial distance between ETs, application in different HVAC systems and sectors (residential, public and industrial), operative temperatures (working fluid temperature and ambient temperature), mass flow, working fluid types and selecting materials.