*2.3. Research Methods*

The energy efficiency of functionality mode, as well as total energy efficiency of solar energy utilization, were determined on the basis of an in situ experiment. The efficiency of PV was determined on the basis of measured transmittance of solar irradiation and temperature of the inner surface of BIPV glazing TPV,si. The in situ experiment was performed between 25 December 2019 and 15 April 2020. Measured data were gathered in 1-minute intervals (Δtmeas). The solar irradiation *Gglob,90* and outdoor air temperature *Te* during the experiment are shown in Figure 3. The range of meteorological parameters appearing during the experiments are shown in Table 1.

Energy efficiency indicators were developed as diurnal values by summarizing and averaging the measured data. Although most indicators involve diurnal average values, some of them are developed using shorter time intervals. For example, the efficiency of electricity production indicators involve average meteorological data for the day-time period when *Gglob,90* > 0 <sup>W</sup>/m<sup>2</sup> or the efficiency indicators related to preheating of the ventilation air were developed taking into account the occupancy period in office buildings (8:00 to 17:00).

**Figure 3.** Solar irradiation received by BIPV *Gglob,90* and outdoor air temperature *Te,* which occurred during the experiment.


**Table 1.** Average and extreme values of meteorological variables during the duration of the experiment.

In the second evaluation step, statistical methods were used to define the influential parameters for each of the energy e fficiency indicators.

In the final evaluation step, influential parameters were involved as independent variables in the statistical evaluation of multi-parametric approximation models developed for each of energy efficiency indicators. These approximation models can be integrated into models for determining the energy performance of buildings on the basis of daily energy balance and can be used for climate conditions at least in the range of values of meteorological parameters as listed in Table 1.

## **3. Energy E** ffi**ciency Indicators**

Although experimental data was gathered in one-minute intervals, the energy e fficiency of solar energy utilization with the pilot BIPV glazed façade structure, and the indicators of each operation mode of the BIPV façade structure are presented as daily average or integral values. As such, indicators can be implemented in a monthly calculation procedure, which is still commonly used in engineering practice, and can be used for assessment of nZEB as well [27]. The indicators are schematically presented in Figure 4. In addition to those solar energy utilization indicators for which the approximation models were developed, some other are shown to emphasize the advantages of the pilot BIPV glazed façade structure.

**Figure 4.** Scheme of indicators of the e fficiency of solar energy utilization with the pilot semitransparent BIPV glazed façade structure; diurnal electricity production *EPV*; diurnal heat supplied with preheated air for space ventilation *Qa,in*; and heat gains through the opaque façade wall *Qi,sol* were the basis for developing approximation models of energy e fficiency, while others, such as static *Ust*, dynamic thermal transmittance *Ue*ff, and preheating e fficiency ε*v*, are used to emphases the advantage of the pilot semitransparent BIPV glazed façade structure.
