## 2 Composite Solar Wall and Experimental Set-Up  
Figure 1 shows the diagram of the composite Trombe wall studied herein and designed to heat living areas. The operating mode of this solar wall is passive and conforms to a low-cost configuration. From the outside, in the presence of solar radiation, glazing creates a greenhouse effect in a non-ventilated air gap that increases the PCM wall temperature. The main characteristics of the outer storage wall surface are the solar absorption coefficient \(\alpha\), which depends on the wall surface color, and the emissivity \(\varepsilon\).  
The energy absorbed by the opaque wall is stored and then transferred with a time delay to the ventilated air gap between the storage wall and the insulating wall. The ventilated air layer plays a significant role in the ability to recover thermal energy transmitted to the room. An insulating panel serves to avoid both energy losses during periods when solar gains are insufficient and overheating in summer. However, during summertime, it is advisable to add a sunscreen in front of the glazing. The insulating panel is perforated to allow natural air thermocirculation through the lower and upper vents. In some cases, a system designed to avoid reverse thermocirculation in the ventilated air layer can be installed. A flexible plastic film placed over the lower vent was studied and proven to be effective in preventing a reverse thermosiphon with minimal air flow disruption [24; 57]. This experimental set-up does not feature a system to block reverse thermocirculation because the primary aim here is to study exchanges in the ventilated layer.  
The experimental set-up, assembled on a reduced scale 83 cm high, 73 cm wide and 1 m deep, was placed on the south side of the laboratory. The dimensions of the storage wall were: 71 \(\times\) 59 \(\times\) 4 cm\({}^{3}\).  
Figure 2 shows the solar wall set-up. It is equipped with thermocouples (T-type-diameter: 0.1 mm) to measure air and surface temperatures, plus two heat fluxmeters (tangential gradient type) to measure heat flux evolution on both sides of the storage wall. The calibration procedure, described in [58; 59], provides these sensors with an accuracy of \(\pm\)3%. The pyranometer is located in the vertical plane of the facade receiving incident solar flux. Since our laboratory window is set above the first floor, which has a white roof, the solar wall is able to benefit from the influence of solar reflection and thus receives higher solar radiation. The two heat fluxmeters with an integrated thermocouple (\(\phi_{\text{ext}}\), T\({}_{\text{ext}}\), \(\phi_{\text{int}}\) and T\({}_{\text{int}}\)) are placed on both sides of the storage wall in the central position (Figure 1). The fluxmeter (\(\phi_{\text{ext}}\)) measures the flux of heat entering or leaving the outer storage wall surface (solar radiation and exchanges between the storage wall and glazing). On the ventilated air layer side, the fluxmeter (\(\phi_{\text{int}}\)) simultaneously measures the convective exchanges between storage wall and air as well as the radiative exchanges between this same wall and the insulated wall located opposite. Two thermocouples measure air temperature at both the lower vent (T\({}_{\text{IV}}\)) and the air layer outlet at the upper vent (T\({}_{\text{IV}}\)).  
Figure 1: Vertical cross-section of the experimental set-up.