## 5 Conclusions  
The presented research aims at the development of a suitable spectrally selective surface technology that can be used in solar thermal applications in building envelopes (e.g., solar walls for passive heating, as in [36; 37]). The paper describes the reasoning behind the development of the low-cost polymer-based foil solar absorber, analysis of its spectral optical parameters, and environmental assessment of its production. These are compared with parameters of commercially available absorbers. The results indicate that the developed absorber can be applied in situations in which activation of the selective functions of its surface layers is beneficial for the thermal performance of the building envelope (regarding the heat transfer phenomena). The results of spectral optical analysis are presented in the form of spectral curves and reflectance radiative parameters. It is shown that the developed absorber has very good solar absorbance (95%). This is comparable with the best of the tested commercial products. It also has acceptably low emissivity (30%) of thermal radiation. Furthermore, the LCA of the developed absorber shows that it is considerably more environmentally friendly than the commercial products. The difference is at least 98% compared to TiNOx absorbers and 83% compared to a synthetic paint. This difference could be further increased by utilization of secondary raw materials (and renewable energy sources). The presented study suggests several future research and development prospects:  
* Feasibility. The presented study compared parameters of the developed absorber with those of several commercially available products. In conjunction with better performance, the developed absorber should also be able to compete with the commercially available products in terms of investment cost. A life cycle cost analysis should be performed to address this issue. * Durability. As mentioned before, the durability of the polymer-based foil is likely lower than that of metal-based products. Therefore, future research should include testing of environmental resistance (UV, mechanical durability, etc.) and development of suitable inbuilding options that protect the foil. * Color design. The integrated facade element will influence the overall aesthetics of the building. In terms of efficiency, the best color option is black, however \"sacrificing\" about 10-15% of absorption performance would allow a wider range of dark color options. * Environmental impacts. The drawbacks of the presented LCA are mentioned in Section 4. These indicate the need for further research of the whole life cycle of the absorber, including installation, maintenance, and disposal. This would allow identification of the most efficient means of application in buildings. * Energy efficiency. A whole life cycle perspective is also necessary from the perspective of energy efficiency. This means adding the operational energy balance to the calculations of performance and energy savings (and related environmental impacts) achievable by the installation of the absorber in various conditions (cardinal orientation of the surface, climate, etc.). Combined with further optimization of physical parameters and the production process, this would lead to an optimal balance of environmental and monetary savings for the users of buildings. Conceptualization, M.C. and D.K. ; methodology, M.C., D.K. and K.S. ; software, K.S. ; formal analysis, K.S. ; investigation, M.C., D.K. and K.S. ; resources, K.S. ; data curation, M.C., D.K. and K.S. ; writing--original draft preparation, M.C. and K.S. ; writing--review and editing, M.C. and K.S. ; visualization, K.S. ; supervision, M.C. ; project administration, M.C. ; funding acquisition, M.C. All authors have read and agreed to the published version of the manuscript. Grantova Agentura Ceske Republiky: GA 20-00630S; Vedecka grantova agentura MSVVaS SR a SAV-VEGA 1/0680/20.