**1. Introduction**

The energy issue is among the top contemporary global crises [1]. Renewable and clean sources of energy need to be capitalized given the aggravated global security issues as of February 2022 and the electricity demand rising by 5.9% to 1400 terawatt hours (TWh), complemented by 14.6 gigatons of CO2 emissions from burning fossil fuels for electricity and heat production [1,2]. Photovoltaic (PV) systems, which utilize a renewable energy source in the form of sunlight, experienced increased capacity and market size. The PV module market increased to 310 gigawatts (GW) in 2022 [3], with the PV module production capacity by the end of 2021 being over 470 gigawatt-peak (GWp) [4]. PV systems covered over 5% of the global electricity generation in 2021, reducing annual CO2 emissions by as much as 1100 megatons during the year [5].

Continuous improvements in the PV system application method are required to further boost the use of PV systems in the future. A few strategies have been implemented to diversify the PV system applications, such as vertically installed PV [6], agriphotovoltaics [7], floating photovoltaics [8], building-applied photovoltaics (BAPVs) [9], and

**Citation:** Azhar, M.H.A.; Alhammadi, S.; Jang, S.; Kim, J.; Kim, J.; Kim, W.K. Long-Term Field Observation of the Power Generation and System Temperature of a Roof-Integrated Photovoltaic System in South Korea. *Sustainability* **2023**, *15*, 9493. https://doi.org/10.3390/ su15129493

Academic Editors: Prince Winston David and Praveen Kumar B

Received: 26 April 2023 Revised: 8 June 2023 Accepted: 9 June 2023 Published: 13 June 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

building-integrated photovoltaics (BIPVs) [10]. Using a PV system as an intrinsic part of a building, a BIPV system takes part in the building structure while generating power [10,11]. In BIPVs, PV systems mainly serve as façades and/or roofing systems for buildings [10–12]. In a façade system, PV modules can serve as curtains, glazing, or spandrel panels; in a BIPV roofing system, PV modules can serve as roofing tiles, shingles, standing seams, or skylights [11]. The long-term cost offset is beneficial by replacing one functional component of a building with a working PV system [13]. The BIPV roofing system is viable for use over residential houses, as there are many areas with passively functioning roofs that PV systems can replace to create hundreds of GWp [14]. Due to these advantages, BIPV implementation has been widespread in recent years [10,14]. The policies of several countries have played a major role in promoting BIPV implementation. A few examples include tax-free self-consumption of BIPV power in France [15] and Japan [16], incentives for BIPV installation in China [17], and BIPV installation subsidies in Korea [18]. Government aid has directly contributed to the rapid growth of the number of BIPV installations, with Grand View Research forecasting that the global BIPV market will increase from USD 19.82 billion in 2022 to USD 88.38 billion in 2030 [19]. More studies are required to complement this growing industry and understand the factors contributing to system performance.

Several factors affect the performance of PV modules, namely PV module manufacturing design, cell technology type, solar irradiance, wind direction and speed, mounting configuration, and temperature [20–22]. Appropriate mounting configuration can limit PV module degradation, as shown by Jordan et al., who studied the performance of aluminum back surface field (Al-BSF)–based PV modules installed in Las Vegas, USA, with different mounting configurations [23]. The authors reported reduced heat transfer into the modules with a lower amount of metal roof and increased heat transfer with a rack-mounting configuration, leading to a lower PV module degradation rate. Manufacturing design effects due to insulations behind the PV modules were studied by Gok et al., who evaluated the performance of glass/back-sheet- and glass/glass-based crystalline silicon (c-Si) PV modules installed in two different mounting configurations in Canobbio, Switzerland [24]. The authors reported that higher operating temperatures significantly impacted the glass/back-sheet module, with the performance loss rate (PLR) varying from 0.01%/year for ventilation to −0.42%/year for insulation. Conversely, the glass/glass module displayed an unexpected opposite trend, with the PLR varying from −0.10%/year for ventilation to 0.26%/year for insulation. IV measurements revealed that the reduced performance of the glass/back-sheet module originated from the deterioration of the fill factor through increased resistance, whereas the rise in the short-circuit current (Isc) was the primary driver of the insulated glass/glass module performance improvement. Kumar et al. studied different cell technology types, focusing on three different types of PV technologies: crystalline silicon (c-Si), copper indium selenide (CIS), and cadmium telluride (CdTe) modules, as BIPVs and BAPVs in Malaysia [21]. The energy-generating performance was different among the three technologies, with the c-Si, CIS, and CdTe modules generating peaks of 4240, 4280, and 4490 kWh, respectively. Singh et al. reported on the performance of high-efficiency heterojunctions with intrinsic thin-layer (HIT)-technologybased PV modules depending on different climatic conditions [25]. The authors showed that HIT technology-based PV modules were more efficient at cold partition temperatures. The impacts of urban heat islands (UHIs) and urban air pollution on BIPV energy efficiency have been extensively studied. For instance, Wang et al. reported that the UHI and solar radiation absorption caused by smog could reduce the overall PV energy generation in urban locations by more than 10% compared to that in rural locations [26]. The effects of the tilt angle and wind speed were studied by Dabaghzadeh et al., who studied the convective cooling of a PV system by modeling different tilt angles and wind speeds [27]. The authors observed the lowest temperature for a tilt angle of 45◦, aided by optimal convective cooling, irrespective of the wind speed.

BIPV research has mainly focused on temperature, power generation, and the correlation between the two parameters. Kumar et al. reported on different temperature and

performance losses for different PV module types, with the c-Si BIPV exhibiting 13.6% reduced performance, the CIS system exhibiting 12.8% reduced performance, and the CdTe system exhibiting 8.8% reduced power generation efficiency [21]. Poulek et al. compared the temperature of BIPV modules to that of PV modules with conventional configurations and its relation to energy production [28]. The authors used a modeling approach to compare BIPV modules with free-standing PV modules in four different climates while conducting a field study in Prague, Czech Republic. The field study indicated that the temperature of the BIPV modules was higher by more than 5 ◦C compared to that of the conventionally installed PV modules. At the same time, a difference of 3–5% in energy production due to the increased module temperature was evident. Using their model, the authors observed that climate differences from cold to hot temperatures had a negligible effect on the BIPV performance degradation; however, the PV module degradation in area with very hot temperatures was rapid. Kim et al. studied BIPV module temperatures with different insulations by conducting a field study on a miniature house with a tilted BIPV roofing system in Daejeon, South Korea [29]. The locations of the insulation were different: One system had insulation behind the PV modules (i.e., a warm roofing system), while another system had insulation behind the ceiling (i.e., a cold roofing system). The comparison of the two systems showed that the BIPV power generation of the cold roofing system was 7% higher than that of the cold roofing system. D'Orazio et al. analyzed different BIPV configurations in Ancona, Italy [30]. The authors compared rack-mounted high-ventilated, moderate-ventilated, and non-ventilated BIPV systems to see the effect of natural ventilation with the addition of air gaps. Rack-mounted PV modules exhibited the lowest PV module and air-back temperatures. In contrast, non-ventilated BIPVs exhibited the highest temperatures. The authors found that an air gap of 0.04 m between the PV modules and the roof was sufficient to create a difference of less than 3% in annual power generation prowess. Kaplanis et al. conducted a modeling study of the aging effect of BIPV and BAPV systems [31]. PV module aging was predicted to increase the PV module temperature relative to the case of the reference modules. Given the expected increase in PV module temperature when installed in a BIPV system compared to the case of a conventional installation, the aging effect might become more prominent.

Several studies have focused on decreasing the PV module temperature in BIPV systems. Mittelman et al. modeled a cooling channel in an attempt to increase the performance of PV modules in BIPVs and observed that adding 0.02–0.20 m air space between the PV modules and the roof can decrease their temperature by 10–20 ◦C and thus increase their energy-generating performance by 1–2% [32]. Other attempts to decrease the PV module temperature incorporated phase change materials (PCMs). Karthikeyan et al. studied the effect of a non-contact composite PCM on the optimal PCM thickness and observed that the optimum PCM thickness was 2.5 cm, yielding an average of 6.7 ◦C PV module temperature reduction [32]. Hasan et al. studied five different PCMs to decrease the BIPV module temperature [33]. The authors used CaCl2 as PCM and achieved an 18 ◦C lower BIPV module temperature for 30 min. In a more extended observation of 5 h, a 10 ◦C temperature reduction was maintained.

Considering the findings of previous studies on BIPV and PV modules in general, the issue of system temperature and power in BIPVs will persist with the growth of the BIPV industry, especially concerning how temperature affects the PV performance or the building [10]. Several short-term studies on the effect of BIPV module temperature on module performance—involving either modeling or field investigations—have been conducted [21,28–33]; however, only a few studies have reported on the effect of PV modules installed on top of roof tile systems and how the temperature of the roof tiles with attached PV modules compares to that under roof tiles without attached PV modules. The specific weather and climate of the regions where BIPVs are installed are also important for comparison. While temperature-reducing efforts have been generally fruitful, there are several associated drawbacks. Natural circulation might allow dust to collect in the air gap, reducing heat transfer. At the same time, several PCMs are costly and hazardous

to the environment [34]. Additionally, PCMs can achieve limited BIPV module cooling due to their short cooling duration. Because of all these issues and how long-term studies (i.e., studies longer than a year) are not available, a miniature house BIPV system was manufactured to better understand the long-term temperature conditions of the roofing system with metal tiles and attached PV modules without dedicated ventilations and PCMs for 2.5 years in South Korea. The comparison of the temperature of the roof tiles with that under the PV modules was used to assess the possibility of installing PV modules on the entire roofing system in this roofing configuration with metal tiles. The contributions of this study are as follows:


The findings of this study address questions regarding the effect of PV module installation on the roof temperature and the PV module's performance over time.
