## 5 Model Validation, Results and Discussion
During its movement towards the upper vent, the air is warmed up in contact with the storage wall before being introduced into the room; a very small portion of the energy supplied by the solar wall is transmitted to the room by means of conductive heat transfer through the insulating wall. On the other hand, during non-sunny periods or at the end of the night, when heat flux at the external surface of the storage wall is negative, the storage wall discharges its thermal energy towards the outside and becomes colder than the air in the room. As a consequence, the air circulating in the ventilated air layer during the inverse thermocirculation is affected by this convective heat transfer, in turn decreasing the air temperature. When the storage wall is cooler than the indoor environment, air enters through the upper vent, cools in contact with the walls, becomes heavier and returns to the room through the lower vent of the ventilated air layer. In this case therefore, a reverse thermocirculation effect is at work. To identify the airflow direction, it is sufficient to observe what temperature between T\({}_{\rm lv}\) and T\({}_{\rm uv}\) is closest to the room air temperature (T\({}_{\rm room}\)). A negative heat flux sign on the inner surface of  
Figure 11: Comparison of temperatures on both sides of the storage wall (model results and experimentation). T\({}_{\rm ext,(m+pcm),cal}\) and T\({}_{\rm ext,(m+pcm),exp}\) denote respectively the evolution in numerical and experimental temperatures on the exterior surface of the storage wall containing PCM; T\({}_{\rm int,(m+pcm),cal}\) and T\({}_{\rm int,(m+pcm),exp}\) denote respectively the evolution in numerical and experimental temperatures on the interior surface of the storage wall containing PCM. Figure 10: Comparison of heat fluxes on both sides of the storage wall (model results and experimentation). \(\phi_{\rm ext,(m+pcm),cal}\) and \(\phi_{\rm ext,(m+pcm),exp}\) denote respectively the evolution in numerical and experimental heat fluxes on the exterior surface of the storage wall containing PCM; \(\phi_{\rm int,(m+pcm),cal}\) and \(\phi_{\rm int,(m+pcm),exp}\) denote respectively the evolution in numerical and experimental heat fluxes on the interior surface of the storage wall containing PCM. the storage wall (\(\phi_{\rm int}\)) makes it possible to identify this phenomenon. For our model, the following conditions must be input in order to define the air flow direction: if \(\rm T_{air\,layer}>T_{room}\), air enters at the lower vent of the composite solar wall and heats the room; if \(\rm T_{air\,layer}<T_{room}\), air enters at the upper vent and exits at a colder temperature through the lower vent (Figure 13). \(\rm T_{air\,layer}\) is the mean of the temperature of the walls inside the ventilated air layer (\(\rm T_{int}\)) and the temperature of the insulated panel. It should be noted that air can exit very hot (nearly 35 \({}^{\circ}\)C ) from the air layer, and this air temperature (\(\rm T_{uv}\)) was correctly estimated. The temperature difference between measured and calculated air temperatures at the upper vent equals 0.4 \({}^{\circ}\)C on average for this period. Figure 12: Comparison of air temperatures at the upper vent of the ventilated air layer (model results and experimentation). \(\rm T_{lv}\) is the experimental air temperature at the lower vent of the ventilated air layer; \(\rm T_{uv\_cal}\) and \(\rm T_{uv\_exp}\) denote respectively the evolution in numerical and experimental air temperatures at the upper vent of the ventilated air layer; \(\rm T_{room}\) is the room-temperature fluctuation. Figure 13: Evolution in the average air temperature inside the ventilated air layer (\(\rm T_{air\,layer}\)) compared to the room-temperature fluctuation (\(\rm T_{room}\)). It can be observed that the air temperature curves at the upper vent (T\({}_{\rm uv\_cal}\),T\({}_{\rm uv\_exp}\)) show very strong similarities between experimental measurements and numerical results.