5.1.2. Indoor Laboratory

The indoor characterisation of BIPV modules is performed by means of the solar simulator (Figure 3a) and electroluminescence (EL) camera (Figure 3b) provided by indoor "Solare PV Lab" of EURAC Research. The pulsed light solar simulator is in class "AAA", according to the international standard IEC 60904-9 [65]. It measures the electrical performance of PV modules, allowing the performance analysis of a PV cell or the comparison among di fferent technologies in controlled conditions. It measures the PV module's IV curve under standard conditions [65]. The measurements detect the energy performance of the module in di fferent combinations of irradiance (0–1000 <sup>W</sup>/m2) and temperature (5–75 ◦C) and its temperature coe fficients, in accordance with UNI CEI EN ISO/IEC 17025:2005 [66]. The test accredited is the Performance at STC (MQT 06.1) for PV modules according to the standard IEC 61215:2016 [67]. The electroluminescence camera is a VIS-SWIR InGaAs camera with a quantum e fficiency over 60% at 1–1.2 μm and sensor of 640 × 512 pixels. This camera enables the implementation of the test following IEC TS 600904-13:2018 [68] indications.

**Figure 3.** Indoor "Solare PV Lab": (**a**) panoramic view of solar simulator room; and (**b**) electroluminescence test execution.

#### *5.2. Indoor Testing of BIPV Technologies: First Results*

From the market analysis described in Section 4.1, two modules typologies have been selected to be tested through indoor and outdoor laboratories. The tests described hereafter constitute the first experimental campaign on coloured BIPV products. The first set of chosen modules belong to the Suncol® Tile technology, provided by Glassfer & Sunage, and consist in a sandwich PV panel of two tempered solar glass sheets within which a layer of monocrystalline cells is laminated by means of polymeric encapsulant films. The two sets of modules differ in dimensions, number of cells and customisation of the coloured front glass. The detail of the two modules typologies is given in Table 2, while Figure 4 shows the modules' samples. These modules have been selected for their high flexibility and aesthetical integration for architecturally sensitive areas, as explained in Section 4.1. Two different customisation techniques have been selected to compare their aesthetic impact and their energy performance: one has a full colour tinted front glass and one has a printed tile pattern. In both cases, an anti-reflection coating has been applied on solar cells. Furthermore, the modules are provided with an invisible mounting system and module frame that guarantees the reduction of the visual disturbance as well as their reversibility.

**Figure 4.** Tested modules samples: Suncol® Tile-Terracotta Simil RAL 8015 (**a**); and Suncol® Tile-Texturing Simil roof tile (**b**).


**Table 2.** Detail of the two tested modules typologies.

The integration of solar cells into coloured modules could results in losses in the irradiance reaching the solar cell within the module, since the colours and materials used in the glass modification have an impact on glass transmittance and light spectrum reaching the underlying cells. Some studies investigated the influence that coloured layers could have in the energy performance of c-Si PV modules, with both theoretical and experimental analyses [44,45]. One main result of these studies is that usually the theoretical colour-related power losses are lower than the actual ones, due to undesired reflections in the near infrared spectrum (NIR, wavelengths > 780 nm) that could appear. Therefore, as a first analysis for the characterisation of the two selected technologies, we decided to investigate the energy parameters of the two modules' set, with the aim of evaluating the power losses due to the front glass modification. The expected power loss due to the variations in the optical properties is analysed through the cell-to-module (*CTMx*) factor (Equation (1)), where *X* is the considered electrical parameter:

$$CTM\_{\text{X}} = \frac{X\_{\text{module}}}{\sum\_{i=1}^{n} X\_{\text{cell},i}} \tag{1}$$

This ratio is calculated using the electrical parameters of the bare cells before the assembly of the module. To calculate the CTM factor, firstly the electric performance of the two modules typologies in standard test conditions have been investigated in the indoor laboratory, with the solar simulator. The results present a normal shape of the current-voltage curves (Figure 5). The electrical parameters obtained in the standard test conditions (STC) performance test are presented in Table 3, where the power at maximum point (Pmpp), short-circuited current (Isc) and open voltage (Voc) are highlighted. Regarding the maximum power linearity respect to irradiance levels at 100–1000 <sup>W</sup>/m2, the test shows acceptable performances comparable to those in commercial transparent glass photovoltaic modules (Figure 6).

**Table 3.** Electrical parameters at standard test conditions of the tested module at maximum power point (Pmpp); current at maximum power point (Impp); voltage at maximum power point (Vmpp); open circuit voltage (Voc); and short-circuit current (Isc).


Table 4 provides CTM factors for both the types of PV modules and a reference monocrystalline module with clear front glass. The results show power losses in accordance with those calculated by Peharz and Ulm [45] using a numerical model for RAL colours between 8000 and 8050, which is about −21% in comparison with zero reflective devices. We obtained −20.7% for Suncol® Tile Terracotta and 31.2% for Suncol® Tile Texturing Roof tile. Thus, our test shows agreemen<sup>t</sup> with the numerical model for the PV modules of uniform terracotta colour. These performance losses are expected: they depend on the layers superposed to the c-Si cells (EVA polymeric encapsulant and glass pane) and solar cells and strings interconnection. This behaviour is enhanced by the colours and ceramic ink used to customise the modules' front glass pane which hinders the PV performance, due to the optical and physical behaviour of the coloured layers, that depends on the modules' hue and coverage percentage [65,66].

**Table 4.** CTM loss in Suncol® Tile Terracotta and Texturing Roof tile modules.


As a further analysis, we provide hereafter the results of electroluminescence control technique, which is increasingly relevant in the analysis of PV modules quality. It basically shows the path taken by the electrons along the circuitry in the module. Several issues can be detected such as diverse mechanical breaks in cells or disconnected areas. The test was realised with a VIS-SWIR InGaAs camera and focused on the effect of the non-transparent glass in the EL signal emission from the module; therefore, from normal to near 0◦ incidence shooting was carried out in the indoor lab (Figure 7). The first results in normal incidence demonstrate a good reception of the signal compared to transparent glass technology (Figure 8). For small angles of incidence, no blind spot has been detected in both types of modules and the transmission of the EL signal is still acceptable. Thus, the results of the EL test show a good electrical response of the modules regardless the incident angle of the radiation. This is a critical aspect when performing outdoor operation and maintenance (O&M) activities in real installations where the position of the panel can be diverse depending on the building and limitations for shooting can be multiple (Figure 9).

**Figure 5.** STC performance of PV modules under analysis: (**a**) Suncol® Tile Terracotta; and (**b**) Suncol® Tile Texturing Roof tile.

**Figure 6.** Power at maximum point according to different irradiance levels.

**Figure 7.** Electroluminescence test at different shooting angles.

**Figure 8.** Electroluminescence images obtained at normal incidence shooting for: (**a**) Terracotta; and (**b**) Texturing Roof Tile.

(**a**) (**b**)

**Figure 9.** Electroluminescence images obtained at small incidence shooting for: (**a**) Terracotta; and (**b**) Texturing roof Tile.

#### *5.3. Experimental Design of Outdoor Testing*

The roof testbed described in Section 5.1.1 is conceived with an interdisciplinarity approach focusing on the three integration aspects mentioned in Section 3 (technology, aesthetic and energy). This interdisciplinarity approach makes it unique with respect to other existing BIPV outdoor setups described in [70] that provides an overview of the existing international BIPV R&D testing facilities. Indeed, the proposed experimental set up is not conceived for testing stand-alone modules performance but rather a large portion of envelope BIPV systems, offering the opportunity to investigate the aesthetical and technological integration of real scale installations, involving several modules. The added value of this kind of testbed is that it will offer the opportunity to different stakeholders in the BIPV community (students, designers and the public and heritage authorities involved in the energy issues) to witness first-hand innovative BIPV technologies, better understanding the benefits of coloured PV systems.

A first qualitative study has been performed on the aesthetic integration and technical compatibility. The ensemble of PV modules are inserted to minimise their visual impact, guaranteeing: (i) 100% coverage of the roof surface; (ii) aesthetic and chromatic integration with traditional clay roof tiles (the selection terra-cotta colour, both with and without the texturing pattern); (iii) geometrical and chromatic uniformity inserting two different kinds of PV parallels in parallel rows; (iv) coplanarity with the roof line; (v) colour rendering under different exposure and tilting conditions; and (vi) pattern continuity for the tile texturing PV cells. Furthermore, the PV modules are integrated into the roof testbed through a back attached tile-type mounting system that guarantees successful integration from the aesthetical point of view since no mounting system or module frame is disturbing visual appearance (Figure 10). Considering the technology integration, technical compatibility and system reversibility are evaluated. Technical compatibility refers mainly to the reduction of moisture accumulation on the backside of the panels, while reversibility refers to the use of mounting systems to remove the panels without affecting the integrity of the roof. Figure 11 shows the roof layout (Figure 11a) and its electrical configuration (Figure 11b). From a qualitative point of view, the testbed design has been conducted with particular focus to the functional requirements for the roof outer layers, such as water resistance and moisture accumulation prevention. The first qualitative findings show no accumulation of water or moisture problems to appear. Quantitative measurements will be provided in future works to rigorously evaluate the technology performance in this respect. Moreover, the back attached tile-type mounting system guarantees the reversibility of the system without affecting the original roof, as requested for the preservation of heritage systems.

**Figure 10.** Testbed realisation on the outdoor roof testbed. To perform quantitative tests, the two module types Suncol® Terracotta and Suncol® Texturing Roof Tile are connected in two strings to a multi-string (two maximum power point trackers) grid-connected inverter. PT100 temperature sensors are applied on the backside of eight modules, as displayed in Figure 10b.

**Figure 11.** Testbed design: (**a**) roof layout; and (**b**) electrical configuration.
