## 4 Discussion  
Previous sections describe the development of the new solar absorber and compare its spectral parameters and environmental performance with commercially available products. The results provide several points for discussion:  
* Optical parameters. The performance data presented in Table 2 can be utilized to determine the efficiency of the compared absorbers in various solar thermal applications (e.g., solar thermal collectors in [30; 42]). P1, S3, and R2 absorbers share the best solar absorbance (\(\alpha_{\lambda}\) = 0.95). This means that they would also have identical net absorbed energy. The results also show that thermal emissivity of the developed P1 absorber is approximately 25% worse than that of commercial selective absorbers R1-R3 due to the presence of the polyethylene absorber layer. Thermal emissivity is a key parameter for the determination of the heat transfer coefficient and the heat loss of any solar thermal system (e.g., solar facades with selective absorbers as described in [12; 20; 33; 49]). When evaluating efficiency of the absorber, both parameters should be included in the selectivity parameter. In general, changes in selectivity do not adequately reflect how it affects the system efficiency. A particular solar thermal application needs to be specified for it to properly identify the theoretical efficiency of a solar absorber. Then, a figure of merit (_FM_) representing the efficiency can be calculated. In this paper a simple theoretical case of 80 \({}^{\circ}\)C absorber temperature and incidence solar radiation of 1000 W\(\cdot\)m\({}^{-2}\) was utilized for this purpose. The ratio of the net solar power absorbed by the system (i.e., absorbed power minus radiated power) to the incident power in the system simply conveys the system efficiency. Compared to the ideal absorber, due to emissivity changes of all of the evaluated absorbers, the decrease in efficiency varies: 22% (P1), 58% (S3), and 8-9% (e.g., R2 and R3). This makes the developed P1 absorber only slightly less efficient (up to 14%) compared to the R2 and R3 absorbers. However, it should be noted that the difference is related to ambient temperature: a smaller difference between the absorber temperature and ambient temperature reduces the difference in the efficiency of the absorbers. * Environmental impacts. The LCA quantified the environmental impacts related to the production of the developed P1 absorber. It identified (primary) plastics and their processing as major contributors. This means that utilization of recycled materials (in accordance with circular economy principles) and renewable energy sources could further improve the environmental performance of the absorber. Such improvement would also benefit other compared absorbers, especially TiNOx-based R2 and R3, because the LCA confirmed that their high environmental  
Figure 5: Comparison of the summed normalized environmental impacts of the developed absorber P1 with S3 paint and R2 and R3 TiNOx absorbers (per 1 m\({}^{2}\)). impacts are related to the metal substrate (as already indicated in [33]). In fact, the results show that primary aluminum and copper have approx. 49% and 99% shares of the normalized environmental impacts of the respective absorbers (due to the energy intensity of the metal processing). However, even following a 90% reduction of the embodied environmental impacts of recycled metals, as suggested in the literature (e.g., [50]), the developed absorber would still be superior. * Operating temperature. This is one of the key parameters for operation of the spectral selective coatings. Some of the existing material concepts are limited by the operating temperature, e.g., up to 200 \({}^{\circ}\)C or less. However, the intended application (in building envelopes) does not presuppose exceeding 100 \({}^{\circ}\)C. In this range, the suggested polymer-based foil absorber should perform adequately, considering its intended building integration. * Operating conditions.