## 4 Results and Discussion
For more detailed results, it is necessary to develop mathematical models with all the specificities that characterize the solar construction. After the theoretical analysis of the thermal performance of GTCs and experimental validations, the results of the MCDM method will be presented in detail below to determine the optimal design of GTCs. Following Figure 9 and Table 3, Table 4 defines specific geometric, economic, and ecological indicators for optimal GTC type selection (\(k_{2}\) values). In this phase (for \(t_{o}\) = 20 \({}^{\circ}\)C and \(W\) = 3 m/s), the values of heat losses (\(Q_{loss}\) with \(e_{CO2}\), Equation (16), Table 3) were used when \(t_{abs}\) = 50 \({}^{\circ}\)C (SGTC with an air layer), \(t_{abs}\) = 60 \({}^{\circ}\)C (SGTC with a vacuum layer), \(t_{abs}\) = 80 \({}^{\circ}\)C (DGTC with an air layer), and \(t_{abs}\) = 90 \({}^{\circ}\)C (DGTC with a vacuum layer)--typical average values (Figure 9, Figure 10). In the same table (Table 4), the last column provides an insight into the values \(k_{3}\) (Equation (13)). Values of \(k_{4}\) can be seen in Table 2. Values of \(k_{5}\) (Equation (14)) and \(k_{6}\) (ranking of indicators) from Figure 8 are shown in Table 5. The specific indicators with the lowest values are a goal function because the aim is to minimize heat losses in GTCs. The final ranking of glass tube collectors (GTCs), derived using the multi-criteria decision-making (MCDM) method, is detailed in Table 6. The analysis reveals that simpler designs, such as the single-glazed tube collector (SGTC) with a vacuum layer, frequently emerge as superior options based on the primary geometric, economic, and ecological criteria (refer to Table 6). The double-glazed tube collector (DGTC) with a vacuum layer ranks second in the evaluation. Conversely, the single-glazed tube collector (SGTC) with an air layer occupies the lowest position according to the MCDM method results. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{ \begin{tabular}{c} **Specific** \\ **Indicator** \\ \end{tabular}  } & \multicolumn{2}{c}{**SGTC**} & \multicolumn{2}{c}{**DGTC**} & \multirow{2}{*}{**Sum**} \\ \cline{2-2} \cline{4-5}  & **Air Layer** & **Vacuum Layer** & & & \\ \hline \(Q_{loss}\) & 7.84 & 4.87 & 7.23 & 4.63 & 24.56 \\ \hline \(Q_{loss}\) & 218.98 & 136.02 & 319.92 & 204.88 & 879.8 \\ \hline \(Q_{loss}\) & 69.82 & 43.37 & 101.74 & 65.16 & 280.08 \\ \hline \(Q_{loss}\) & 2535.53 & 1575 & 3180  
Vacuum technology leads to increased embodied energy investments and, consequently, higher production costs. This chain reaction also results in higher maintenance costs. However, it significantly reduces heat losses by 37.88% compared to single-glazed tube collectors (SGTCs) with an air layer and by 35.96% compared to double-glazed tube collectors (DGTCs) with an air layer. Heat losses in solar collectors represent the portion of energy that is not utilized and is instead dissipated into the environment. Although more advanced solar collector designs often exhibit higher thermal (useful) power, this does not necessarily imply a reduction in heat losses or an increase in thermal efficiency. In many cases, as the thermal power of solar collectors increases, so do the heat losses, indicating a greater amount of unused energy. Consequently, such technical challenges must be addressed with careful, multidisciplinary approaches.