## 2 Overview of BIPV System  
The PV modules integrated or attached to the conventional building envelopes are called BIPV. The BIPV system involves different PV technologies and their applications as a building envelope as shown in Figure 1. The PV technologies in the BIPV system can be classified as conventional and emerging solar cells in nature. In application, the integration of different PV technologies such as a crystalline, thin film, and organic technologies on the different building envelopes are covered [9]. Building envelopes are the outer part of the buildings which separate the internal environment from the outer one. The building roof, facade, and window are the primary envelopes of the system [10]. Further, it can be classified as flat/pitched PV rooftop, PV-DSF, semi-transparent PV glazing, etc. The integration of technologies mainly depends upon the building demand, size, and climatic conditions [11]. The PV modules used in the BIPV system convert between 13-20% of incident solar radiation to electricity under ideal operational conditions [12; 13]. The remaining amount of radiation absorbed by the cells is transformed into thermal energy, causing an increase in its temperature. Temperature plays an important role in PV system efficiency, as the performance of the PV module is directly related to the operating temperature of its cells. Further, the rise in temperature leads to several types of degradation such as delamination, hot spots, corrosion, glass fracture, electrical migration between module layers/interconnections, and discoloration.  
In hot environments, the BIPV system temperature can reach as high as 80 \({}^{\circ}\)C [14], which affects the system efficiency. An effective way to counteract the rate of thermal degradation of a PV module is by reducing its surface temperature. This can be achieved by using natural or forced ventilation which prevents the modules from heating during operation [15]. The performance of the modules is also affected by shading from the adjacent buildings and dust accumulation on the PV modules. In countries such as the UK, high-rise buildings mainly depend upon glazing technologies for regulating the building temperature and maintaining thermal comfort [16]. The opaque PV modules were integrated on the front face of the facade with some air gap provided to reduce the heat flux penetration in buildings as well as lowering the PV module temperature [17]. The different types of the BIPV system envelopes (Figure 1) are discussed as follows:  
(a) BIPV roof  
The rooftop envelope of a building protects the building from the outdoor environment. The PV modules are attached to flat or tilted roofs [18]. In Figure 2, the PV modules attached to a sloped roof are provided with a certain air gap from the rear side. The roof was provided with forced ventilation above the cavity opening.  
Figure 1: Categorization of BIPV system.  
2. BIPV facade This is the outer wall of a building and contains window glazing and shading devices. Generally, an air gap is provided between the outer skin of the building and the PV modules, and such an attachment of PV modules with a facade is termed a double skin facade (DSF). The cooling was provided either by natural or forced ventilation (Figure 3). Further, the ventilated hot air can be utilized in the buildings during the winter season.  
Figure 3: Schematic presentation of the DSF (**a**) forced (**b**) natural [17].  
Figure 2: Schematic presentation of the tilted BIPV roof with semi-transparent PV with forced ventilation [19].  
3. STPV glazing/windows  
STPV technology is a glass-to-glass technology, where opaque c-Si cells are equally spaced and encapsulated between the transparent glasses using ethyl vinyl acetate (EVA). STPV can replace the conventional window glazing and roof skylight (Figure 4) [18]. Apart from energy generation, the technology is also advantageous in reducing lighting loads and heat gain [19].