*3.3. Effect of System Temperature on Power Conversion*

To determine how the system temperature affects the power-generation efficiency of this BIPV system, monthly power conversion and system temperature were compared and analyzed, and tabulated in Table 4. Figure 8 shows that the trends of system temperature and power conversion are opposite to each other. The system temperature trends followed the seasonal changes in South Korea, i.e., the system temperature increased in spring and summer and decreased in autumn and winter. Conversely, power conversion decreased in spring and summer and increased in autumn and winter.


**Table 4.** Power conversion from January 2020 to June 2022.

**Figure 8.** Power conversion and system temperature in (**a**) 2020, (**b**) 2021, and (**c**) 2022.

As mentioned previously, the system temperature is one of the factors affecting the power generation performance of a BIPV system [21,26–33,35–37]. In this BIPV roofing system, it was apparent that the system temperature affected the performance of the PV modules. The highest efficiency of 11.42% was achieved in December 2020, when the system temperature was the lowest. In contrast, the lowest efficiency of 5.24% occurred in July 2021, when the system temperature was 10 ◦C above the ambient temperature. This correlation shows how a system temperature increase negatively impacts the PV module efficiency, dropping to half its maximum value at the peak system temperature. A PV temperature increase leads to an increase in carrier concentration, thereby enhancing the rate of carrier recombination and leading to decreased open-current voltage (VOC), fill factor (FF), and thus performance [38].

Figure 9 shows that during the 2.5-year observation period, there was no significant decline in system performance regarding power conversion. The largest drop in power conversion was in July 2021, which decreased by ~1% compared to the previous year. Such a large decline in efficiency was not evident in the rest of the months when efficiency decreased on average only by 0.25% compared to the values in the previous years. These findings confirm that this particular BIPV system can maintain its performance with a negligible effect on efficiency over 2.5 years.

**Figure 9.** Comparison of monthly power conversion from January 2020 to June 2022.

### **4. Conclusions**

The long-term performance of a BIPV system of 160 Wp was evaluated using outdoor monitoring in South Korea for 2.5 years. This study revealed the intercorrelations between power generation, solar irradiance, and system temperature. While solar irradiance mainly affected power generation, system temperatures contributed to power generation fluctuations, following the seasonal changes from cold to warm and again to cold temperatures throughout the year. Results showed that half of the BIPV system's performance was lost due to temperature fluctuations. Therefore, addressing this issue is crucial, particularly in hot regions. Factors such as wind direction, ventilation, and the type of PV technology could play important roles in achieving better economic value for the installed PV system. The temperature behind the PV modules was lower than that behind the roof tiles, implying that installing such a BIPV system will not increase the temperature of the roofing system. This finding suggests that the comfort of the room underneath the PV modules may be uncompromised by using this configuration in the climate of South Korea. Therefore, expanding the use of PV modules as an entire roofing system by using this configuration has great potential. This study proves that one of the generally associated temperature concerns—the roofing temperature—is not necessarily concerned with this BIPV system configuration.

**Author Contributions:** Conceptualization, W.K.K., M.H.A.A., S.A. and S.J.; Methodology, W.K.K., M.H.A.A., S.A. and S.J.; Data curation, M.H.A.A., S.A. and S.J.; Formal analysis, M.H.A.A., S.A. and S.J.; Resources, W.K.K., J.K. (Jitaek Kim) and J.K. (Jungtaek Kim); Funding acquisition, W.K.K.; Supervision, W.K.K.; Writing—original draft, M.H.A.A., S.A. and S.J.; Writing—review and editing, W.K.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the 2021 Yeungnam University Research Grant.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
