## 6 Conclusions  
A photovoltaic-thermal concentrator was designed, fabricated and experimentally tested at the University of Lleida, in Spain. The proposed solar system is suitable for facade-integrated applications, aiming to substitute ordinary sun-tracking window blinds with a CPVT system that converts the direct light, and its main innovation lies in the use of liquids (IPA or DIW) as part of the optical element. This study includes experiments and numerical simulations. The results demonstrate the following:  
* There is a good agreement between experimental and numerical values regarding optical performance and thermal behaviour.
* The experimental optical efficiency is around 73% for the concentrator with DIW and about 76% for the one with IPA, close to the simulation results (76% for DIW and 81% for IPA).
* Module bending is a limiting factor because it can cause important efficiency losses.
* The thermal losses of both configurations range from 14 to 20 W \({}^{\circ}\)C\({}^{-1}\)m\({}^{-2}\), indicating that the concentrator is not optimal from a thermal point of view (improvement is needed). The results also agree with the simulations, which showed losses ranging from 13 to 18 W \({}^{\circ}\)C\({}^{-1}\)m\({}^{-2}\).
* Regarding electrical performance, the fill factors for IPA and DIW are as follows: 62.8% and 61.7%, respectively. The fill-factor reduction is mainly attributed to an increased series resistance due to the interconnection of the PV cells and the micro-short circuits caused by the laser-cutting process.
* Sun-tracking is a critical factor in developing a successful concentrating solar system.
* There are striking differences between the testing unit without solar concentration and the one with solar concentration. The illuminance in the testing unit with the concentrator is about four times lower than that observed in the reference testing unit (configured with maximum shading for lighting control). Additionally, the maximum  
Figure 18: The illumination profile registered in the two testing units. The time refers to solar time. Concentrator = the testing unit with the concentrating system. Reference = the reference testing unit (without solar concentration).  
heat fluxes through the lateral walls and the ceiling and the interior temperature in the concentrator-equipped unit are reduced by approximately 50%.  
Generally speaking, the proposed liquid-based solar systems show satisfactory behaviour and offer promising possibilities for facade-integrated applications. Future research will assess efficiency improvements, multifunctional aspects to become a suitable system for facade integration and develop models to evaluate the life-cycle environmental profiles of the proposed technologies, taking into account different types of environmental indicators.  
Conceptualisation, D.C. and A.R.; methodology, D.C. and A.R.; software, A.M. and A.S.; validation, A.R., A.M. and A.S.; formal analysis, D.C. and C.L.; investigation, D.C., A.R. and A.M.; resources, D.C.; writing--original draft preparation, A.R. and C.L.; writing--review and editing, D.C.; visualisation, A.R.; supervision, D.C.; project administration, D.C.; funding acquisition, D.C. All authors have read and agreed to the published version of the manuscript.  
The authors would like to thank \"Ministerio de Ciencia e Innovacion\" of Spain, MCIN/AEI/10.13039/501100011033 for the funding (grant reference: TED2021-132843B-I00). [PERSON] thanks Institucio Catalana de Recerca i Estudis Avancats (ICREA) for the ICREA Academia award. Furthermore, this research was supported by the Generalitat de Catalunya (2021 SGR 01618). [PERSON] is an Associate Professor of the Serra Hunter programme.  
Not applicable.  
Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.  
The research data can be provided upon request. Please contact the corresponding author.  
The authors declare no conflicts of interest.