*3.2. Transient Boundary Conditions*

Transient behavior is expected when boundary conditions such as outdoor temperature and solar irradiance vary during the day. In these following test cases, the indoor temperature was a given indoor boundary condition, and transport coefficients remained constant to avoid uncertainties in the validation process. These test cases were simulated in Sofia, and the weather file was the standard EPW file (EnergyPlus Weather). Regarding the water flow glazing, the flow rate and the inlet temperature were constant values given by Table 3. Two simulations in winter and summer were accomplished. The simulation period ran from 7 January to 11 January in winter and from 14 July to 18 July in summer. Figure 5 illustrates the thermal behavior of Case 2 and Case 3. In summer, the solar irradiance peak value was 500 W/m2, and the maximum outdoor temperature was slightly above 26 ◦C on 14 July 2020. The goal of rejecting energy was met, and the water heat gain, measured by the difference between inlet and outlet temperatures, was not above 1 ◦C on five sample summer days. The peak solar radiation in winter on the eastern facade was 180 W/m<sup>2</sup> on 10 January 2020. Due to the high infrared reflectance (above 70%) of the selected glazing and the low outdoor temperature (below 5.5 ◦C), the water heat gains were negligible. According to the steady-state analysis, Case 2 showed the best performance to heat water, as measured by the solar irradiance on the southern facade, the outdoor temperature, and the difference between inlet and outlet temperatures in southern WFG modules. In summer, the peak solar radiation was 400 W/m2, and the maximum temperature was above 26.5 ◦C on 15 July 2020. On that day, the maximum outlet water temperature was 22 ◦C when the inlet temperature was 20 ◦C, and there were water heat gains during the central hours of the day. In winter, the peak solar radiation was above 250 W/m2, and that made the water absorb heat, although the outdoor temperature was low.

**Figure 5.** Simulation results of WFG Case 2 on the southern facade and Case 3 on the eastern facade with transient boundary conditions. (**a**) Summer. (**b**) Winter.

Figure 6 shows a summary of the water heat gains on two sample days in summer and winter. Case 3 was selected for eastern and western facades because it showed the least heat absorption in summer (17 KWh), whereas Case 2 showed the highest absorption in summer (34 KWh). To reject energy, the best choice for eastern and western facades was Case 3. Case 2 had the highest heat absorption on a winter day (21 KWh) and a good value in summer (30.5 KWh). Case 2 confirmed its excellent performance on southern facades.

**Figure 6.** Accumulated energy of WFG case studies with transient boundary conditions. (**a**) Summer sample day 14 July 2020. (**b**) Winter sample day 8 January 2020.
