## 1 Introduction
Moreover, better temperature control can be achieved since the thermal contact resistance between PV cell and dissipater is notably reduced or even eliminated [17; 18; 19]. It is worth mentioning that by carefully selecting dielectric liquids with the right optical properties, these liquids can act as selective absorptive filters, providing effective spectral selection. Specifically, these liquids absorb solar frequencies outside the spectral response bandwidth of the selected PV-cell technology before they reach the solar cell, thereby preventing potential overheating while allowing the useful part of the solar spectrum to pass through. Consequently, dielectric liquids could serve as both optical filters and heat transfer fluids, aiming towards efficient CPVT systems [20; 21]. [PERSON] et al. [22] investigated liquid-immersed PV cells (dimethyl silicone oil) with high concentration ratios. The proposed system was compared with other configurations without liquid immersion, and promising experimental results were obtained, but it was noted that more experimental studies are needed. The electrical characteristics of GaInP/GaInAs/Ge triple-junction PV cells immersed in liquid (dimethyl silicone oil) were investigated (500\(\times\); 25 \({}^{\circ}\)C). [PERSON] et al. [23] studied silicon-based solar cells immersed in liquids. The option \"bare solar cells immersed in non-polar silicone oil\" presented the best performance, showing good stability. [PERSON] et al. [24] proposed a liquid-immersion cooling method for densely packed PV cells and high concentration ratios. PV-panel cooling to below 45 \({}^{\circ}\)C was experimentally achieved. Dimethyl silicone oil was selected, taking into account factors such as high-temperature resistance and exposure to UV light. Moreover, [PERSON] et al. [25] investigated PV cells with water-immersion cooling. The proposed system included a dish concentrator (250\(\times\)), tracking and deionised water for cooling. Degradation in module performance appeared after three days and was attributed to either the low ion concentration or the electrolytic reaction at the solar cell connections. On the other hand, the temperature distribution of the panel was rather uniform. A limited number of investigations where the fluids act as both thermal absorbers (CPVT) and optical filters have been conducted. [PERSON] et al. (2016) developed a CPVT collector (14\(\times\)) with an electrical efficiency of 4% and a peak thermal efficiency of 61%, where the nanoparticle-based receiver used polydimethylsiloxane-based oil because of its optical properties. [PERSON] et al. (2017) studied five commercial fluids, concluding that Therminol(r)VP1 achieved the highest PV efficiency of 16.39% (10\(\times\)), and fluid viscosity had the most significant impact on increasing the bulk fluid temperature. The results of the works cited above are promising, but none of these studies addressed the direct immersion of solar cells, and the proposed systems based on this cooling technique were not optimised for building integration ([PERSON] et al., 2017; [PERSON] et al., 2018; [PERSON] et al., 2019). Recently published review articles ([PERSON] et al., 2018; [PERSON] et al., 2019; [PERSON] et al., 2020) also show that there is a dearth of studies on CPV/CPVT liquid-immersion-cooling systems optimised for building-integrated applications. Considering the above-mentioned research gaps, an innovative system was developed and tested at the University of Lleida in Spain, proposing directly immersed PVTs, especially CPVTs, with optical efficiencies above 76% and configurations suitable for building-integrated applications ([PERSON] et al., 2019). With the proposed system, we aimed to substitute ordinary sun-tracking window blinds that control the amount of light entering the building with a photovoltaic-thermal collector, which, instead of blocking the direct light, converts it into electricity and heat.