*3.3. Real Results in Sofia, Bulgaria*

An experimental setup was placed in Sofia, Bulgaria (42◦39 1 North, 23◦23 26 East, Elevation: 590 m a.s.l.), to test the performance of isolated WFG modules throughout a year. Figure 7 shows the outdoor temperature. On the coldest winter days, the minimum temperature was below −10 ◦C, and the average daily temperature was 0 ◦C. During the hottest months, the maximum temperature reached 32 ◦C and the average temperature was 25 ◦C. The southern solar radiation reached a peak value of 400 W/m<sup>2</sup> on 21 December 2019, whereas the eastern and western were 160 W/m2 and 270 W/m2, respectively, on 21 June 2019. On summer days, the highest values were on the east and west facades (500 W/m2) because the sun angle was almost perpendicular to the vertical walls. The south facade received little radiation (200 W/m2).

**Figure 7.** (**a**) Outdoor dry bulb temperature in Sofia, Bulgaria (EnergyPlus Weather file). (**b**) Eastern, western, and southern solar radiation on facades. Sample winter day 21 December 2019 and sample summer day 21 June 2019.

Based on the outdoor simulation data, and the thermal and spectral properties of the studied WFG in Table 4, the best option for the southern facade was Case 2. It showed the highest potential for heat absorption in winter (226.6 W/m2) with the highest outlet temperature (23.17 ◦C). In summer, the maximum southern solar radiation was 200 W/m2, whereas the absorption potential was 418 W/m2, so the fluid could absorb the heat without heating the interior face of the glazing. Due to the high solar radiation in summer, the best option for eastern and western facades was Case 3. It showed the lowest absorption in summer (131.9 W/m2) with the lowest outlet temperature (18.26 ◦C). Figure 8 shows the prototype plan, with five modules facing east, five modules facing west, and five more modules on the southern facade. Unitized WFG modules were placed in three different orientations (east, south, and west) with a pyranometer measuring solar radiation on each facade. Each heat plate exchanger of the circulating device was connected to inlet and outlet water distribution systems.

**Figure 8.** (**a**) Prototype plan. Position of WFG and electronic control unit. (**b**) Pictures of the unitized module in the actual facility with the pyranometer.

The output signals were collected by one-wire probes and sent to an electronic control unit (ECU), where the developed software processed the calculations and elaborated the energy outputs. The temperature sensor network was installed in both the inlet and outlet of the plate heat exchanger. Flux meters were added to the monitoring equipment to keep a steady mass flow rate through all the modules. The temperature difference in the external WFG elements could reach 10 ◦C, depending on the exterior conditions. Glass selection for renewable production on the southern facade (Case 2) absorbed the maximum incident solar radiation and at the same time reduced indoor solar heat gains. A heat pump was used to control the inlet temperature. Figure 9 illustrates the outdoor air temperature (*T\_out*), inlet (*T\_Ei5*) and outlet (*T\_Eo5*) temperatures in two eastern WFG modules in summer conditions. The maximum temperature difference occurred from 7:00 a.m. to 10:00 a.m., when the solar radiation reached its peak value on the east facade. The southern modules' inlet and outlet temperatures (*T\_Si5*, *T\_So5*) reflected the solar radiation and outdoor temperature, and there were two peak values at 11:00 a.m. and 4:00 p.m. The maximum temperature difference between *T\_So5* and *T\_Si5* was 2 ◦C. The maximum temperature difference between the inlet (*T\_Wi3*) and outlet (*T\_Wo3*) temperatures in two western WFG modules occurred at 4:30 p.m., when the solar radiation reached its peak value on the west facade. The real measurements confirmed the simulation results because, despite the high solar radiation values on the eastern and western facades (700 W/m2), the temperature

difference between inlet and outlet was 1 ◦C. However, in the southern modules, the temperature difference was 2 ◦C when the maximum solar radiation was 470 W/m2.

**Figure 9.** Inlet and outlet temperatures of eastern WFG. Sample summer day 14 July 2020. (**a**) Module E5. (**b**) Module E1.

In winter, heat absorption does not depend directly on solar radiation due to the severity of climatic conditions. The difference between indoor and outdoor temperatures affected energy performance more than the solar radiation on the eastern and western facades. Figure 10 illustrates the outdoor air temperature (*T\_out*), the inlet (*T\_Ei5*) and outlet (*T\_Eo5*) temperatures in two eastern WFG modules. The southern WFG performance showed heat losses in the morning and in the afternoon. From 10:00 a.m. to 5:00 p.m., the outlet temperature (*T\_So5*) was higher than the inlet (*T\_Si5*), and the maximum difference reached 2.5 ◦C at 1:30 p.m. In western modules, the inlet (*T\_Wi3*) and outlet (*T\_Wo3*) temperatures showed that there were heat losses in the morning with no solar radiation and low outdoor temperature. The simulation results were validated with little energy absorption on eastern and western facades, and heat gains in the southern modules.

**Figure 10.** Inlet and outlet temperatures of eastern WFG. Sample winter day 8 January 2020. (**a**) Module E4. (**b**) Module E3.
