## 3 Results and Discussions
### Energy Efficiency  
Two different kinds of efficiency have been defined in order to assess the performance of the double-skin facade in terms of energy savings [1]:  
* Pre-heating efficiency, \(\eta\), and
* Dynamic insulation efficiency, \(\varepsilon\). The pre-heating efficiency, \(\eta\), assesses the capacity of the facade to pre-heat the ventilation air flow rate during the cold season (heating periods) and is defined as:  
\[\eta=\frac{T_{exh}-T_{i}}{T_{i}-T_{e}}. \tag{1}\]  
where:  
\(T_{exh}\)--temperature of the air exhaust (exiting from the facade), \({}^{\circ}\)C.  
\(T_{i}\)--air temperature inside the room, \({}^{\circ}\)C.  
\(T_{e}\)--exterior air temperature, \({}^{\circ}\)C.  
Figure 21: Visualization of airflow patterns in the median section of the double-skin facade channel compared to the numerical ones [37]. The dynamic insulation efficiency, \(\varepsilon\), is defined as the quota of the heat flux that enters through the outer surface of the facade in the hot season during the cooling period:  
\[\varepsilon=\frac{\dot{Q}_{r}}{\dot{Q}_{inc}} \tag{2}\]  
where \(\dot{Q}_{r}\) is the heat flux removed by the air flowing through the facade, W, Equation (3):  
\[\dot{Q}_{r}=\dot{m}_{v}\cdot c_{p}\cdot(T_{exh}-T_{e}) \tag{3}\]  
where: \(\dot{m}_{v}=v\cdot S\cdot\rho\) ventilation air mass flow rate inside the DSF channel (kg/s); \(c_{p}\), \(\rho\), specific heat of the air (1005 J/kg K) and the air density (1.165 kg/m\({}^{3}\)). The heat flux entering through the outside facing surface of the facade, \(\dot{Q}_{inc}\) (W) is defined by Equation (4):  
\[\dot{Q}_{inc}=\dot{Q}_{S}+\dot{Q}_{i}+\dot{Q}_{r}=I\cdot\tau_{i}\cdot A+U\cdot A \cdot(T_{exh}-T_{i})+\dot{Q}_{r} \tag{4}\]  
where: \(\tau_{i}\) solar transmission coefficient through exterior and interior glazings (0.42); \(A\) area of the facade module (5.2 m\({}^{2}\)); \(U\) overall heat transfer coefficient through the double glazing (1.92 W/m\({}^{2}\)K, Table 1). Finally, the equation of the dynamic insulation efficiency can be written as [1; 40]:  
\[\varepsilon=\frac{\dot{m}_{v}\cdot c_{p}\cdot(T_{exh}-T_{e})}{I\cdot\tau_{i} \cdot A+U\cdot A\cdot(T_{exh}-T_{i})+\dot{m}_{v}\cdot c_{p}\cdot(T_{exh}-T_{ e})} \tag{5}\]  
Figure 22 presents the variation of the pre-heating efficiency, \(\eta\), during the cold season (SET 4) when the channel is closed, and the double skin-facade behaves like a thermal buffer. The negative values of the pre-heating efficiency mean that the air temperature that exits from the ventilated facade, \(T_{exh}\), is lower than the temperature of the indoor air, \(T_{i}\), and there is no energy recovery. In this condition, the only benefit obtained from the ventilated facade is to increase the surface temperature of the inner glass pane inside the channel (Figure 19) and reduce the heat loss from the interior room. When \(0<\eta<1\) during the last two days of higher solar radiation at noon (29 th and 30 th November, Figure 22), the air that exits from the facade is preheated at a temperature which is higher than the indoor one but the ventilation air still requires extra heating before being introduced into the room. A maximum value of 1.83 of the pre-heating efficiency is reached at 11:00 a.m. on the seventh day, for a maximum air temperature inside the DSF channel of 28.6 \({}^{\circ}\)C, which is higher than the interior one of 19.7 \({}^{\circ}\)C, solar radiation intensity of 101 W/m\({}^{2}\) and an ambient air temperature of 14.8 \({}^{\circ}\)C. At this moment, the facade heats the ventilation air to a higher temperature than the indoor temperature. Therefore, it allows the complete compensation of ventilation losses and the partial compensation of other thermal losses through the building elements. Figure 22: Pre-heating efficiency of DSF in the fourth period (24 th–30 th November) of cold season (thermal buffer, closed channel).