## 4 Experimental Set-Up  
In order to investigate the thermal performance of the developed PCM-perlite composite integrated with thermal insulation under moderate climatic conditions (the city of Lodz, 51\({}^{\circ}\)46 N & 19\({}^{\circ}\)27 E, Poland, Central Europe), a special experimental set up was constructed. The main assumption was to build a test stand that would allow to take into account all thermal processes and disturbances occurring in a real environment for which the proposed solution was designed. For this purpose, part of the existing building external wall was reconstructed. Since the thermal performance of the external partition is strongly influenced by both internal and external heat fluxes, it was assumed that experimental set-up should include not only the considered wall, but also an adjacent room. The proposed PCM-perlite composite was designed mainly with modern, lightweight constructed office buildings in mind. Thus, the experimental wall was constructed as steel framework with a ventilated rain screen wall, and covered with BIPV panels from outside with a centrally positioned square window. The area around the window was divided into test sections with dimensions 60 cm \(\times\) 60 cm (Figure 2). Such an approach was taken to conduct simultaneous doubled tests of developed composite and reference, basic constructions (a mineral wool without PCM). It was assumed that evaluation of the proposed solution should be made by comparison to the reference case. The basic external wall was built from the following layers (pointed from inside): 1 cm of chipboard, 20 cm of mineral wool, 5 cm of ventilated air gap, and external lining made from a copper indium selenide (CIS) PV panel. The modified wall differed from the insulation layer, which was covered from the outside with an additional, thermally active composite (PCM-perlite), Figure 3.  
Figure 2: Front view of the facade with internal division into 12 sections. REF: Reference. Each tested section of the experimental wall was equipped with eight temperature sensors (Figure 3). The first one measured the temperature on the internal surface of the wall, which can be treated as an internal boundary conditions for the mineral wool. The second one was placed in the middle of the thermal insulation layer and was installed to check the influence of the PCM-perlite composite on the thermal response of the insulation. The next four sensors were concentrated around the PCM-perlite layer. Such compaction of the sensors was introduced in order to increase the accuracy of the measurements and for mutual verification. Since latent heat storage is an isothermal process, accurate readouts are crucial for robust analysis of PCM performance. The last two temperature sensors were installed in the ventilation cavity and on the back side of the PV panels. These were used for the verification of compatibility of external boundary conditions for all sections. The digital temperature sensors, which have a high precision bandgap reference with a proportional-to-absolute-temperature (PTAT) output and a low-power and high-precision analog to digital converter (ADC), were used. The temperature sensors were fully calibrated. Two types of sensors were used (Figure 4): TSc 506 (sensors 1-6) and TSc 306 (sensors 7 and 8). It was stated that due to the high thermal resistance of the insulation layer and thus small temperature fluctuations, very high accuracy was required to precisely compare the results. High accuracy of the measured values was also required to the describe performance of the PCM-perlite layer, which is very sensitive to temperature fluctuations. According to this, the most accurate sensors, with an accuracy of \(\pm\)0.1 K in a range of 40 K (from 5 to 45 \({}^{\circ}\)C), were applied (Table 1). Due to the fact that the measurement range of this sensor is \(-\)10 to 60 \({}^{\circ}\)C, it could not be used for the measurement in the places numbered 7 and 8 (Figure 4), since temperature in these locations could exceed this range.