*3.3. Dynamic Thermal Insulation*

Because building envelope structure is ventilated and air is supplied to the indoor space, part of the heat losses can be recovered and the BIPV structure acts as dynamic thermal insulation. Efficiency of heat loss recovery is determined by comparing daily actual heat losses to theoretical ones at steady state conditions, taking into account reference thermal transmittance of the composite façade wall, which was upgraded with the BIPV structure—*Ust* 1.027 <sup>W</sup>/m2K (Figure 1). Transmission heat losses of the BIPV structure were evaluated by static thermal transmittance *Ust*, while effective thermal transmittance *Ue*ff was evaluated based on the transient thermal response of the BIPV structure. *Ust* was determined by instant indoor *Ti* and outdoor *Te* air temperatures and heat flux was measured on the internal surface of the structure .*qsi* using measured values for the period between 23:00 and 6:00+ o'clock in each day, to minimize the impact of accumulated solar energy. At that period, the heat transfer was close to the steady state, because the temperature difference (*Ti* − *Te*) was almost constant. With the *Ust* value, the impact of the double skin façade, as well as the forced ventilation of the air gap, was evaluated and compared to that of the *Ust* of the reference building envelope structure (Figure 2a). The dynamic thermal transmittance *Ue*ff was defined in the similar way, the only difference was in the evaluation period, which was in this case all day long (6:00 to 6:00+1 day). The following equations were used:

$$
\Delta L\_{\rm sf} = \frac{1}{7} \frac{1}{60} \sum\_{23 \ge 00}^{+6 \cdot 00} \frac{\dot{q}\_{\rm si}}{(T\_i - T\_c)} \cdot \Delta t\_{\rm means} \left(\frac{\rm W}{\rm m}\right) \tag{9}
$$

$$
\Delta L\_{eff} = \frac{1}{24} \frac{1}{60} \sum\_{\text{600}}^{+6.00} \frac{\dot{q}\_{si}}{(T\_i - T\_c)} \cdot \Delta t\_{\text{mens}} \left(\frac{\text{W}}{\text{m}^2 \text{K}}\right) . \tag{10}
$$

The impact of unsteady parameters (*Hglob,90*, *Te*, and *vW* ), as well as impact of ventilation air flow rate . *Va*,*in*, are considered by the effective thermal transmittance *Ue*ff. Examples of evaluation are shown in Figure 7.

**Figure 7.** An example of measured values used for the evaluation of the energy efficiency of dynamic thermal insulation of the BIPV glazed façade structure. (**a**) In the case of heavy cloudy day (1 March 2020), because the air gap was ventilated, *Ust* was equal to reference thermal transmittance of composite façade structure (*Uref* 1.027 <sup>W</sup>/m2K) and *Ue*ff was very close to the *Ust*; (**b**) even in the case of clear sky weather (21 February 2020), the *Ust* was very close to the reference value, while dynamic thermal transmittance was much lower, indicating a significant difference in diurnal transmission heat loss *Qi,sol* from the pilot BIPV glazed façade structure.

#### *3.4. Overall E*ffi*ciency of Solar Energy Utilization*

Overall efficiency of solar energy utilization by the pilot BIPV glazed façade structure includes energy gains related to production of electricity, preheating of ventilation air, and decreased transmission heat losses due to the dynamic thermal insulation. In fact, the latter is not achieved solely by utilization of solar energy but also due to lower thermal transmittance of the BIPV structure, nevertheless all indicators are normalized to diurnal received solar radiation *Hglob,90*. The overall efficiency is defined by the sum of partial efficiencies in the following way:

$$\eta\_{\text{sol},\text{RIPV}} = \eta\_{\text{PV,RIPV}} + \eta\_{\text{a,RIPV}} + \eta\_{\text{i,sol,RIPV}} = \left(\frac{E\_{PV} + Q\_{a,i} + \left| \left( Q\_{\text{loss,ref}} - Q\_{\text{loss,i}} \right) \right|}{H\_{\text{glab,90}} \cdot A\_{\text{BIPV}}}\right) (-),\tag{11}$$

where:

$$\eta\_{PV,RIPV} = \frac{E\_{PV}}{H\_{glab,90} \cdot A\_{BIPV}} = \eta\_{PV} \cdot \left(\overbrace{\frac{A\_{PV} \cdot n\_{PV}}{A\_{BIPV}}}^{0,\delta \cdot A\_{BIPV}}\right) = K\_T \cdot K\_G \cdot \left(\overbrace{\frac{A\_{PV} \cdot n\_{PV}}{A\_{BIPV}}}^{0,\delta \cdot A\_{BIPV}}\right) (-),\tag{12}$$

$$\eta\_{\mu,BIVV} = \frac{Q\_{a,in}}{H\_{glab,90} \cdot A\_{BIIVV}} = \frac{0.34 \cdot \frac{1}{60} \sum\_{8.00}^{17.00} \dot{V}\_{a,in} \cdot (T\_{a,in} - T\_e) \cdot \Delta t\_{meas}}{H\_{glab,90} \cdot A\_{BIIVV}} \tag{13}$$

$$\eta\_{\text{i,sol,BIVV}} = \left| \overbrace{\underbrace{\mathrm{LI\_{st,ref}}}\_{\mathrm{I}l\_{st,ref}} \cdot \left(T\_{i,\mathrm{avg}} - T\_{\mathrm{c,avg}}\right) \cdot 24 - \left(\frac{1}{60} \sum\_{\mathrm{elc0}}^{+600} \dot{q}\_{\mathrm{si}} \cdot \Delta t\_{\mathrm{mcons}}\right)}\_{\mathrm{J}l\_{\mathrm{sph},90} \cdot A\_{\mathrm{BIVV}}} \right| \cdot A\_{\mathrm{BIVV}} \bigg|\_{}$$

The constant 0.34 replaces the product of air density and specific heat capacity. Figure 8 shows an example of overall efficiency of solar energy utilization η*sol,BIPV* determined by measured data in two days during the experiment period.

**Figure 8.** Overall efficiency of solar energy utilization η*BIPV* of the pilot BIPV glazed façade structure (**a**) on 8th January 2020 and (**b**) on 20th February 2020; in the case (**a**) the ventilation air flow rate .*Va*,*in* was 3.5 m<sup>3</sup>/h, and consequently, the utilization of solar energy for preheating of the ventilation air decreased, while the temperature of the supply air *Ta,in* increased above indoor air temperature and efficiency εv was above 100%; the case (**b**) was the opposite because . *Va*,*in* was much higher and utilization of solar energy was higher, while preheating air efficiency ε*v* was significantly lower.
