## 3 Test Methodology
### Solar Heat Gain Coefficient (SHGC, G-Value) Test  
The solar heat gain coefficient (SHGC)--also named the G-value or solar factor--is the total solar energy transmittance factor through a glazing system separating two environments (generally outdoor and indoor) [22; 23; 24]. It is the fraction of external solar radiation that is admitted through a window or facade, both directly transmitted and absorbed and subsequently released inward [25]. A standardized definition of this magnitude in the case of transparent glazing systems is mainly given by ISO 9050 (2003) [26], and EN 410 (2011) [27] for a simplified calculation where the resolution of the non-linear system of equations for the heat balance is avoided. A detailed calculation is provided in ISO 15099 (2003 [28]) and EN 13363-2 (2005) [29], in which conductive-convective and radiative parts of the heat transport problem are coupled through the temperature nodes. The latter standards follow an approach with different conditions by considering either a node of temperature in each glass surface or a sole node for each layer, respectively. The G-value can be used to describe the properties of the facade as a whole, or parts of it (e.g., windows) or of components (e.g., insulating glazing units). Most related studies in the literature have focused on transparent or translucent building glazings, such as glass windows [24; 30] or transparent solar thermal collectors [31], with studies on the energy efficiency of TCPs as building envelopes being rare. In this section, the SHGC was measured with a CHB similar to that used in Figure 5. Some well-developed thermal transmittance experimental methods [32; 33; 34; 35] were applied to the two TCPs  
Figure 6: Installed specimen and thermocouples: (**a**) layout of the thermocouples; (**b**) view from outdoor side; (**c**) view from indoor side. \begin{table}
\begin{tabular}{c c} \hline \hline
**Environmental Properties** & **Temperature/Air Flow** \\ \hline Indoor side air temperature & 40 \({}^{\circ}\)C \\ Outdoor side air temperature & 20 \({}^{\circ}\)C \\ Indoor side airflow velocity and direction & 0.3 m/s, vertically upwards \\ Outdoor side airflow velocity and direction & 3.5 m/s, vertically upwards \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Simulated micro-environment conditions. with a VR of 6%. The SHGC of a specimen, G, is defined as the ratio of transmitted solar heat to the incident solar heat, and is calculated as  
\[G=\frac{Q_{\text{S,G}}}{I_{\text{S}}A_{\text{S}}} \tag{5}\]  
where \(Q_{\text{S,G}}\) is the heat flow rate transmitted through the specimen due to solar heat gain (W), \(A_{\text{S}}\) is the projected specimen surface area (m\({}^{2}\)) and \(I_{\text{S}}\) is the mean incident solar irradiance on the specimen plane (W/m\({}^{2}\)). When the steady state is attained in the metering box, \(Q_{\text{S,G}}\) can be determined from the heat balance. The CHB is a combined thermal transmittance and SHGC measurement system. Figure 7 shows the schematic cross-sectional view of the SHGC measurement mode. The metering box, guarding box and surround panel used in the thermal transmittance measurement mode were used for the indoor environmental simulation, but on the outdoor side, the climate box was replaced with a solar simulator and an external air curtain. Additionally, the indoor side boxes--i.e., the guarding and metering boxes--were mounted on an automated turning system for angular SHGC measurements. A lamp was used as the solar simulator (Figure 8). To achieve a small divergence angle and an improvement in uniformity, the lamp system was located 10 m away from the TCP (Figure 7). Figure 9 shows the normalized solar simulator spectrum in the wavelength range of 0.3 to 1.678 \(\upmu\)m, in comparison with the standard AM1.5 (air mass = 1.5) direct normal irradiance spectra recommended by ISO 9845 (1992) [36], ISO 9050 (2003) [26] and ASTM G-173 (2012) [37].