The major GHG contributor in Singapore is CO₂, primarily produced by the electricity generation sector due to the use of fossil fuels [23]^[1]. Although oil-fired energy plants have largely been replaced by gas-fired energy plants since 2005, 95% of all electricity is generated by natural gas in Singapore [20]^[2]. It is necessary to develop a fuel mix-based electricity generation strategy, especially including the application of renewable energy. However, Singapore is a resource-constrained city-state and has limited renewable energy options [20]^[2]:

(1)

The average wind speed in Singapore reaches approximately 2 m/s, which is lower than the 4.5 m/s criterion of commercial wind turbines.

(2)

There is no potential to implement tidal power generation due to the narrow tidal range and calm seas.

(3)

Hydroelectric power cannot be employed because there are no year-round river systems with fast-flowing water.

(4)

There are no geothermal energy sources available.

(5)

Biomass-based energy generation is not appropriate in Singapore due to the high population density and land scarcity constraints.

(6)

Nuclear power cannot be safely implemented in cities with high population densities.

Given the above reasons, solar energy is the only renewable energy source with the potential to impact the energy grid. As previously stated, BIPV systems may represent a viable solution given the limited land resources and dense metropolitan regions in Singapore. Moreover, suitable acreage for PV plants is lacking. Although rooftop surfaces can receive ample sunlight, the usable space in high-rise buildings is constrained owing to the placement of mechanical, electrical, and plumbing (MEP) infrastructures. The taller a given building is, the higher the ratio of the façade area to the roof area, and the more areas suitable for BIPV deployment occur on the façade [24]^[3]. The Singapore Building and Construction Authority (BCA) has established stringent building standards to achieve zero energy (ZEBs) and positive energy buildings (PEBs). Hence, BIPV systems comprise a critical GHGE mitigation strategy while also achieving tropical green buildings [25]^[4].

Globally, the green building concept varies because local economic and technical environment conditions should be considered. In Singapore, a certain building can receive Green Mark certification, thereby designating it as a green building. The latest Green Mark certification program revised in 2018 addresses the following 5 key sections:

(1)

Sustainable design and management, which includes Base Building Selection, integrative design and management commitment & employee engagement;

(2)

Energy and resource management, which includes air conditioning, lighting, and plug loads, water and waste;

(3)

Office environment which includes occupant evaluation, spatial quality (lighting, acoustics, office design) and indoor air quality;

(4)

Workplace health and wellbeing, which includes healthier eating & physical activity, smoking cessation and mental well-being;

(5)

Advanced green and health features which includes smart office, renewable energy and health promotion.

The Green Mark, as a certification tool, can evaluate building energy performance in the tropics and guide building stakeholders to achieve energy efficiency enhancement through the processes of site selection, design, operation, maintenance, occupant engagement, and empowerment. In addition to Singapore’s Green Mark certification system, other green building ratings and certification systems include Building Research Establishment Environmental Assessment (BREEAM) in England, Leadership in Energy and Environmental Design (LEED) in the United States, the German Sustainable Building Council (Deutsche Gesellschaft für Nachhaltiges Bauen or DGNB), and Green Building Evaluation and Labeling (GBEL) in China.

1.3. Technologies to Achieve Super Low Energy Buildings (SLEBs) in the Tropics

In 2018, the BCA announced the launch of a new program, the Green Mark for Super Low Energy Building Program (GM SLE program), as the next wave of Singapore’s green building movement, which aims to improve best-in-class building energy efficiency, the application of renewable energy either onsite or offsite, and intelligent energy management tools. The SLE program encompasses the following three types of buildings: super low energy buildings (SLEBs), zero energy buildings (ZEBs), and positive energy buildings (PEBs). These three building categories all require energy savings of at least 60% over 2005 levels, and the accounting system includes heating, cooling, ventilation, domestic hot water, indoor and outdoor lighting systems, plug load, and transportation within the building [25]^[4]. SLEB realization is a prerequisite to achieve both ZEBs and PEBs. ZEBs require all energy consumption, including the plug load, to be supplied from renewable sources onsite or offsite, while PEBs must realize an energy surplus of 10%.

1.4. BIPV Applications in Green Buildings in Singapore

Based on the above discussion, the application of renewable energy, such as BIPV, is the key to achieving zero energy and positive energy building conditions. In addition, different types of buildings should target the realization of different levels in the SLE program. For example, low-rise and medium-sized buildings should strive to be certified as ZEBs or even PEBs because their roof areas usually provide sufficient space for PV installation. Although high-rise buildings consume much more HVAC energy and possess smaller rooftop areas than low-rise and medium-sized buildings, they have larger façade areas that can be used for PV integration, which can achieve, at a minimum, SLEB certification. Currently, as the first Southeast Asian country to do so, Singapore has implemented a carbon tax at a rate of 5 SGD/t CO₂e if any industrial entity releases GHGEs equal to or beyond 25,000 t CO₂e from 2020 to 2023 and plans to increase this carbon tax to 10–15 SGD/t CO₂e by 2030 [28]^[5]. Stakeholders in the building industry should consider these policies as a guide for decarbonization and apply the above information when setting future targets for BIPV building design and construction.

2. Recent Development of BIPV Systems

2.1. Historical Evolution of BIPV Systems

In the late 1970s, the US Department of Energy began supporting projects to enhance distributed PV systems, including supporting collaboration with the PV industry to incorporate building materials. By the 1980s, the construction industry had realized the potential of PV technology and its aesthetic acceptance, although the cost of PV technology in the 1980s impeded its development [38]^[6]. In Europe, Wohnanlage Richer was built in Munich in 1982; the residential building designed by Thomas Herzog and Bernhard Schilling, which contained polycrystalline cells on a curtain wall, became the first glass surface-integrated PV installation [39]^[7]. In 1991, Aachen’s Public Utilities building first employed PV panels as semitransparent glass in the façade [40]^[8]. The scientific literature on the subject of BIPV structures was published during that time in Europe [41]^[9]. Then, the US DOE launched a program called Building Opportunities in the United States for Photovoltaics Program to help commercialize BIPV products [42]^[10]. Meanwhile, Europe published Solar Architecture in Europe, and Japan also joined these efforts, announcing similar programs [43]^[11]. All of these plans were aimed at facilitating the commercialization of innovative BIPV projects. The International Energy Agency (IEA) established the PV Power System Initiative in 1997, which attempts to improve the architectural quality, technical feasibility, and economic viability of PV systems in the building industry [44]^[12]. Thereafter, the construction industry successfully realized projects that were developed worldwide, which were subsequently reported in a very large number of papers [26]^[13]. BIPV systems have been installed in commercial buildings since 1991, and the example usually considered the Public Utilities Building of Aachen. Throughout the world, there are more cases existing in other countries, such as the Hongqiao Railway Station building in China, which was completed in 2010 and incorporated enormous BIPV systems with a total installed capacity of 6.5 MWp; thus, the employment of solar systems integrated into buildings is one of the most important drivers of BIPV development [45]^[14].

2.2. Building-Integrated Photovoltaics (BIPVs) and Their Development

BIPV technology refers to a certain technique of PV cell employment that integrates PV cells into conventional building materials. The building skin is not only a protective layer against the elements but also a component of the structure that embodies the architectural language. Stricter building standards and regulations regarding green construction and sustainability urge architects/developers to explore high-performance façade technologies and products, such as PV materials. However, in contrast to conventional PV applications, BIPVs constitute a part of construction systems considering the context of materials, construction, jointing, manufacturing sequence and installation [39]^[7]. Because architects require a notable level of design freedom in regard to technological solutions for the customization of building skin, PV modules have greatly advanced in terms of color, form, and performance to accommodate various building skin options [46]^[15].

2.2.1. BIPV Systems

The BIPV module can replace conventional building components and function as part of the construction system. BIPV systems involve PV materials that, when combined with conventional building materials, dispense with the need for heat transfer via the building envelope [47]^[16]. Generally, there are three types of BIPV systems integrated into the building skin, as follows: roofs (BIPV tiles and skylights), façades (BIPV curtain walls and cladding walls) and accessories (BIPV shading devices and balconies). Figure 4 1 shows the general types of BIPV systems.

2.2.2. BIPV Roof Systems

Different from nonintegrated PV roof systems (such as building-attached photovoltaic (BAPV) systems), roof BIPV systems incorporate existing building roof materials, such as tiles, into the structure without the need for additional mounting structures, such as racks and rails. BIPV tiles can be similar in appearance to traditional tiles regarding color and size to meet the requirements of sensitive architectural areas. According to [48]^[17], since Singapore is located near the equator, the optimal solar radiation reception direction is 10 degrees east. Although BIPV tile products presumably achieve a high-power generation efficiency of 19.5% [49]^[18], their actual application requires further local verification. Not only do BIPV skylights generate electricity, but they also allow light into the room, thereby reducing the energy consumption of artificial lighting. According to previous studies [16]^[19], when semitransparent solar modules are employed in a sunroom, the power production decreases by 0.52% when the temperature of the PV module rises by 1 degree. When the PV module is installed directly against the building insulation material, research [50]^[20], Li et al. [51]^[21] has revealed that the temperature of the module may rise and its performance may decrease owing to the absence of circulating air. As such, an increasing number of studies [52]^[22] have focused on BIPV ventilation, which may be accomplished via natural or forced ventilation systems, and in these studies, thermal performance modeling and simulation were performed.

2.2.3. BIPV Façades

According to different integrated PV functions, façade BIPVs can be divided into two categories, i.e., BIPV cladding and curtain walls, which directly constitute the structure of the façade. Hence, it is necessary to consider the basic characteristics of the building envelope, such as weatherproofing and waterproofing. Moreover, when designing the latter wall type, in addition to the façade, indoor visibility and direct sunlight should be considered. It should be noted that previous research [3]^[23] has focused on the integration of BIPV cladding walls and phase-change materials (PCMs) to improve the efficiency and heat dissipation of PV systems. Studies have demonstrated that in other regions, BIPV systems integrated with PCMs can maintain a PV surface temperature below 29 degrees for a certain period (130 min) [53]^[24]. The BIPV curtain wall must strike a balance between visible light transmittance and power conversion efficiency while also considering the aspects of color and thermal comfort [54]^[25]. Semitransparent BIPV modules are framed within extrusions (aluminum, steel, or wood) to withstand wind loads and rainfall penetration. Curtain walls can be constructed in a variety of ways to meet many functional needs, such as thermal insulation, weather tightness, soundproofing, and waterproofing. These systems include stick curtain walls, unitized curtain walls, sealant structures, and point-fixed or suspended façades [39]^[7]. Generally, double glazing PV systems perform better in terms of heat insulation than single glazing PV systems [9]^[26]. To reduce heat transmission, an insulating layer may be applied to single glazing PV systems [55]^[27]. According to relevant research, if a PV system is directly applied to the outer skin in tropical regions, the interior temperature may increase, thereby aggravating indoor thermal comfort and humidity problems [26]^[13]. Therefore, in tropical regions, such as Singapore, the application of semitransparent BIPV windows under all building orientations offers notable potential based on indicators such as power production, artificial lighting power, and cooling energy consumption. To obtain the greatest power production advantages from different modules, multiple design methods are required to maximize the window-wall ratio under different orientations.

2.2.4. Accessories

Accessories are the external components of the building façade, such as shading devices, balustrades, and parapet walls. Both transparent and opaque BIPV modules are frequently adopted in accessories. Compared to first-generation PV cells, lightweight second-generation PV cells exhibit a higher tolerance to partial shading and high temperatures [11]^[28]. Hence, the latter cells are more suitable for use as shading devices. An adaptive solar façade, i.e., a modular dynamic shading device, should be considered. Ren et al. [56]^[29] studies indicate that the influence of shading on individual buildings vary significantly from each other. Compared to the static PV shading system, the adaptive solar façade can yield energy savings ranging from 20–80% [57]^[30]. Since this system can control both façade electricity generation and building electricity consumption monitoring, it provides a new building management method. BIPV balconies, which usually refer to BIPV balustrades and parapet walls, can highlight the architectural character of the building and its surroundings. BIPV balconies can make use of this building surface to absorb sunlight. The PV modules can be grouped together based on their orientation to form DC arrays with an exceptionally elegant appearance [58]^[31].

2.3. Singapore BIPV Projects

BIPV roofs offer a variety of design possibilities (Figure 52a–f). The application of BIPV roofs in buildings may be limited due to the challenges associated with URA and SCDF requirements. For example, adding a BIPV roof to an existing building may result in an increased gross floor area, structural issues, and unfavorable functional organization. However, BIPV roofs also offer multiple benefits, such as providing shelter from the weather (solar/rain) while producing electricity. The concept of “PV Sky Gardens” proposed by [59]^[32] is shown in (Figure 52a). By controlling the density of the grilles, a good natural ventilated environment is created underneath the canopy, which reduces the energy consumption of the cooling load and allows partial natural light to penetrate. The PV modules combined with the grilles are developed as modular components that are convenient for installation and disassembly. This solution enables the symbiotic use of three resources, i.e., natural light, wind and solar radiation.