A Review of Building-Integrated Photovoltaics in Singapore: Status, Barriers, and Prospects

Abstract

Energy consumption enhancement has resulted in a rise in carbon dioxide emissions, followed by a notable greenhouse effect contributing to global warming. Globally, buildings consume one-third of the total energy due to the continued expansion of building areas caused by population growth. Building-integrated photovoltaics (BIPVs) represent an effective technology to attain zero energy buildings (ZEBs) via solar energy use. This research begins with the tropical green building concept in Singapore associated with renewable energy and gives an overview of the potential of solar photovoltaic energy. Strategies for BIPV spread in Singapore are also provided. Considering both BIPV system life cycle assessment (LCA) and BIPV industry standards and recent developments, this research determines whether Singapore should adopt this technology. Although the BIPV product market has expanded regarding BIPV products, systems and projects, there remain certain barriers to BIPV adoption in Singapore. Additionally, future research directions for tropical BIPV applications are outlined. The Singapore BIPV system serves as an example for a number of other tropical countries facing comparable challenges.

1. Introduction

Energy is considered an essential resource for economic and technological development [1]. Currently, fossil-based fuel remains the main energy source for providing power including electricity and heat. The accelerated emission of CO₂ has resulted in global warming caused by a severe greenhouse effect. In order to reach the Paris Agreement’s long-term temperature objectives, the IPCC Spatial Report on Global Warming of 1.5 °C emphasizes the need of carbon neutrality, according to ref. [2]’s research, carbon emissions must decrease by 7.6% each year. It is undeniable that the increasing population and pursuit of a modern lifestyle have dramatically boosted the energy demands and energy consumption per capita, whereas fossil fuel resources are decreasing.

The building sector accounting for one-third of the overall electricity consumption, it is estimated that by 2035, world energy consumption will increase by 50% over the 1990 level [3]. Although building energy consumption sector growth in European countries has decelerated and efforts have been made to renovate buildings to enhance energy efficiency (2% annually) [4], the rest of the world, especially the emerging economies in Asia, still faces housing pressure because of population growth [5]. In addition, the employment of various household appliances, such as computers, monitors, refrigerators, air conditioners, and ovens, has increased the consumption of household electricity because these appliances are ever more readily available today [6]. Therefore, there is an urgent need to determine alternative sustainable energy sources.

With the rapid economic development occurring over the last two decades, Singapore has evolved into a high-density city-state, and Singapore is now the second most densely populated country worldwide. Singapore, is located at 1 latitude north of the equator, at the southern end of the Malay Peninsula. The Straits of Malacca border Singapore to the west, while Indonesia and the South China Sea lie to the south and east, respectively. Due to substantial land reclamation operations, the aggregate land area has grown by 25% since the independence of Singapore. Singapore, similar to the majority of Southeast Asian countries (including Malaysia, the Philippines, Indonesia, and Brunei, which form maritime Southeast Asia, and Cambodia, Laos, Myanmar, peninsular Thailand and Vietnam), features a tropical rainforest climate, with no obvious seasonal changes, an overly high temperature, high humidity, and plentiful annual seasonal rainfall. The temperature usually varies between 23 °C and 32 °C. Southeast Asia is one of the most vulnerable places to climate change globally. Climate change poses major challenges to various industries, such as agriculture and fishery, in regard to rainfall and runoff, water quality and water supply. Climate change and the rising sea level will significantly influence the low-lying shoreline of Singapore over the next several decades.

Tropical countries mentioned above, such as Singapore, experience overly hot and high-humidity climates, and 30–50% of all electricity is consumed for cooling and ventilation purposes because heating, ventilation, and air conditioning (HVAC) systems in urban life consume much electricity [7]. Therefore, the formulation of strategies to reduce emissions, especially carbon dioxide emissions, constitutes an indispensable part of the overall emission reduction task for these countries. Additionally, Singapore is located at a low sea level and will be the first to bear the brunt of the global warming impact under the greenhouse effect. A rise in the sea level and overheated climate conditions may yield a serious negative impact on human health and ecosystems. Although Singapore contributes only 0.1% to global CO₂ emissions, it should cooperate with other countries to actively respond to the threat of global climate change and realize emission reduction commitments, which is also an overall challenge for this island country that is lacking land and natural resources [8].

Recently, photovoltaic (PV) technology has gained notable attention as a viable means of supplying energy to buildings due to the promotion of various actions in tropical countries [9]. Studies have indicated that at the end of 2017, the global installed PV capacity exceeded 400 GW [10]. Although building-integrated photovoltaic (BIPV) systems remain a niche technology, various types of PV products are increasingly available on the market. In recent years, Singaporean architects and urban planners have considered BIPVs necessary systems to achieve renewable energy production and reduce greenhouse gas (GHG) emissions [11].

A BIPV system can seamlessly integrate PV modules into external building surfaces, such as walls, roofs, shading devices, and decorative components. Moreover, it can generate clean energy. From an environmental and economic perspective, PV energy generation provides more advantages than fossil fuel-based energy generation. First, in contrast to the limited storage of fossil fuels, the solar radiation reaching the Earth’s surface every day contains 10,000 times the energy requirements of humans on a daily basis [12]. Second, the manufacturing process of PV modules produces only a small amount of carbon dioxide (20–30 g carbon dioxide equivalent (CO₂e/kWh)) [13].

Since the price of silicon has steeply fallen by 90%, providing a good opportunity to economically implement large-scale grid-connected PV systems [14]. Currently, connected grid PV systems are common, and smart networked energy distribution systems have gradually been established [15]. Compared to nonintegrated PV systems, BIPV installation is more convenient, as it does not use additional building space and other assembly components, such as brackets and guide rails [16]. Therefore, the use of an integrated BIPV design can reduce the total construction and material costs of a project [17]. BIPV systems represent one of the most rapidly growing market segments in the PV field. The BIPV market is expected to grow at a rate of 30% per year after 2020 [18]. This enormous growth potential allows these systems to satisfy building energy consumption requirements while lessening the dependence partially or completely on fossil fuels, thereby reducing carbon emissions and mitigating global warming. Global warming is exerting a growing and notable impact on the world. Singapore, as a small island nation on the equator, is particularly vulnerable to the effects of global warming due to its low-lying coast and high temperatures. Hence, comprehensive action is needed to mitigate this issue. Singapore has revealed its long-term emission reduction plan. Ref. [19] showed that, compared to 2005 levels, Singapore will reduce its emissions by 36% by 2030 under the Paris Agreement to 65 Mt CO₂e and by 50% to 33 Mt CO₂e by 2050 and attain net-zero emissions by the latter period of the century [20]. The cooperation between different industry sectors is necessary due to the high challenging targets.

While countries in South Asia, such as Singapore, benefit from ample solar irradiance, solar energy only supports 5% of the total energy requirements of Singapore [21]. The current research challenges include the effective application of BIPVs, whether this technology can effectively reduce carbon dioxide emissions in Singapore and its future research directions. This study illustrates Singapore’s current carbon emission scenarios and the country’s goals and progress toward GHG emission (GHGE) reduction to achieve its commitments under the Paris Agreement. Starting from the concept of green buildings in Singapore, this study examines the emission reduction role of BIPV technology and its indispensable significance. In addition, this study compares the PV energy payback time (EPBT) and greenhouse emission (GHGE) levels mentioned in other studies to those under the conditions in Singapore, explores the feasibility of BIPV technology implementation in Singapore and reviews the latest developments of BIPV technology. In addition, the current barriers in Singapore to BIPV implementation are identified. It is necessary to contribute a framework of PV manufacturers and consumers to promote finite resource efficiency in PV modules and its life cycle economy [22], including South Asia countries like Singapore. The opportunities for future BIPV research in Singapore and other tropical countries are described in the manuscript.

2. Green Building Concepts in Singapore

2.1. Singapore Building Energy Consumption Landscape

The major GHG contributor in Singapore is CO₂, primarily produced by the electricity generation sector due to the use of fossil fuels [23]. 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]. 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]:

(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]. 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].

2.2. Definition and Indicators of Green Buildings in Singapore and Singapore Green Building Masterplan (SGBMP)

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.

Table 1. Comparison of the four green building certification systems [25,26,27].

	Green Mark	DGNB	LEED	BREEAM	China Three Star
Nation	Singapore	Germany	US	UK	China
Foundation agent	Building and Construction Authority (BCA)	German Sustainable Building Council	US Green Building Council (USGBC)	Building Research Establishment (BRE)	Ministry of Housing and Urban-Rural Development of the People’s Republic of China (MOHURD)
Foundation time	2005	2007	1998	1990	2006
Focus phases	Planning, design, operation, maintenance, occupant engagement and empowerment	Planning, operation	Design, construction	Planning, operation	Planning, design, construction, operation
Evaluation sectors	(1) Sustainable design and management (2) Energy and resource management (3) Office environment (4) Workplace health and well-being (5) Advanced green and health features	(1) Ecological quality (2) Economical quality (3) Sociocultural and functional quality (4) Technical quality (5) Process quality (6) Location quality	(1) Sustainable sites (2) Water efficiency energy and atmosphere (3) Materials and resources (4) Indoor environmental quality (5) Innovation & design (6) Regional credits	(1) Management (2) Health and wellbeing (3) Energy (4) Transport (5) Water (6) Materials (7) Waste (8) Land use & ecology (9) Pollution	(1) Land savings and outdoor environment (2) Energy savings (3) Water savings (4) Material savings (5) Indoor environmental quality (6) Operations and management
Features	Suitable for tropical climates, focusing on energy efficiency and health.	Life cycle analysis of environmental, economic, and social aspects	Energy and resource consumption efficiency	Oldest method	Mainly evaluate residential and public buildings in huge quantities with large energy consumption
Reference to standards	ASHRAE 55	DIN EN ISO 14,040, 14,044, 14,025, RT 2020	ASHRAE 90.1	DIN EN ISO 14,040, 14,044, ISO 21,930	GB 50,176, 501,89, 50,736, 50,785

compares the Green Mark certification system to other major green building grading and certification systems [25,26,27].

Furthermore, the Green Mark and Singapore Green Building Masterplan (SGBMP) was launched by the BCA in 2005 and considers the 2005 building consumption level as the baseline. Three key long-term development targets were set. First, Singapore will continue to green 80% of buildings by 2030. Currently, 43% of buildings in Singapore have been assigned Green Mark certifications. Moreover, the minimum energy performance standards have been raised, requiring both new buildings and current buildings with retrofitting to achieve 50% and 40% higher energy efficiency levels, respectively, over the 2005 levels. Second, starting in 2030, 80% of the gross floor area of new development should comprise super low energy buildings (SLEBs), which are 65% more energy efficient over 2005 levels. Finally, best-in-class buildings should aim to realize an 80% higher efficiency over 2005 levels by 2030 [25]. The above future aggressive scheme of the BCA regarding energy efficiency improvement indicates that it is essential to define green buildings and the technology that can facilitate goal realization.

2.3. 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]. 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%.

To adapt to the local climate, economy, and technology conditions in Singapore, relevant technology strategies were suggested to assist best-in-class buildings in SLE program realization. The following four broad areas were identified: passive strategies, active strategies, smart energy management, and renewable energy. Figure 1 lists these four areas with the corresponding technology options [25].

Through the definition of green buildings in the Singapore Green Mark program and future SLE programs, it is clearly found that the employment of renewable energy, especially BIPV technology, may be the key measure to achieve tropical green building construction in Singapore.

2.4. 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]. 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.

3. Solar Energy Potential and Its Implementation Target in Singapore

Solar Energy Potential in Singapore and Promotions

Solar energy is the largest potential renewable energy source for power generation in Singapore. Singapore receives an average solar irradiation of 1580 kWh/m²/year, which is almost 50% more solar radiation than that received by other countries in temperate areas [29]. Solar energy development can provide several benefits to Singapore, including carbon emission reduction, energy security enhancement and peak demand reduction. Due to the limited land and the rapid changes in tropical weather, in order to promote PV industry development, SERIS of Singapore has set up a short-term solar radiation prediction system with hourly observations as well as forecasts for short periods in the future. To promote PV industry development, SERIS has developed a live solar irradiance map with a 1-s resolution for Singapore based on real-time data collected from 25 irradiance stations across Singapore on a 5 km × 5 km grid, as shown in Figure 2 [21]. The total annual solar radiation value for Singapore is 1580 kWh/m²/year, which already covers the average solar radiation value for the region [30]. In addition to global horizontal irradiance and diffuse horizontal irradiance, other parameters that may impact PV production are also measured, such as ambient temperature, relative humidity, wind speed, wind direction, and air pressure.

According to [31], IEA compares solar potential of several cities such as Tokyo and Stockholm, based on geographical analysis, the results showed that comparing to other cities at higher latitudes, Singapore has a higher solar yield per square meter and a larger solar yield with flat and sloped roofs. Although its building facades have lower solar yield, it has a wider range of good yield areas. Due to Singapore’s small inland region and high population density, a holistic strategy based on existing urban contexts must be considered in its BIPV implementation. Through the use of a high-resolution 3D model, SERIS and the Singapore Land Authority conducted solar energy potential assessments. As shown in Figure 3, an area covering 36.8 km² that is suitable for PV implementation is available, comprising 35.9% roofs, 26.7% façades and 37.4% other surfaces [32]. If PV technology was implemented on all of these surfaces, Singapore’s PV capacity potential would reach 8.6 GW by 2050. Based on the [32] studies,

Table 2. An overview of the BAS and ACC deployment scenarios and their impact on PV generation and CO₂ reduction [32].

	Scenario	Estimated System Peak Demand (GW)	Installed PV Capacity (GWp)	PV Power Penetration Level	Estimated Annual Electricity Generation (TWh) and Percentage of Total Demand (%)	CO2 Emission Savings (Mt/a) *
2030	BAS	9	1.0	11%	1.28, 1.8%	0.52
	ACC		2.5	28%	3.16, 4.5%	1.29
2050	BAS	11.5	2.5	22%	3.09, 3.4%	1.26
	ACC		5	43%	6.64, 7.4%	2.71

* Singapore’s average grid emission factor (GEF) is 0.4080 kg CO₂/kWh in 2020 [33]. CO₂ emission savings equal to the multiplication of annual electricity generation and GEF.

provides the estimated installed capacity, energy yield, and CO₂ emission mitigation benefits under both a baseline scenario (BAS) and the accelerated scenario (ACC). The future targets can be achieved without using all of the potential area, it may be beneficial to utilize the building façade area for PV implementation because roofs can be not only converted to greenery roofs to offset, but also affect the heat island effect and provide additional leisure public space for the urban city are. BIPV systems technically and aesthetically compatible with the built environment.

To achieve the goal of PV implementation, Singapore started the largest PV installation project “SolarNova Program” in 2014, aiming to promote PV application in public buildings and housing estates. The SolarNova program is targeted to generate 420 GWh of solar energy every year, which is equivalent to 5% of Singapore’s total energy consumption, which is equal to powering 88,000 4-room units. Eighty percent of the population is accommodated in social housing developed by the Housing and Development Bureau (HDB), which coordinates the “solar leasing tender” with green electricity retailers and town councils in the program. The Economic Development Bureau (EDB) is responsible for driving solar ambition among government agencies, while SERIS is appointed as the technical consultant to conduct feasibility studies and site selection. HDB will offer solar developers a percentage of the initial start-up funding as an incentive, and eventually, the town council will purchase the solar system at the end of the lease.

Under this business model, the solar developer of Singapore would be responsible for the entire development cycle of the PV system, including design, financing, installation, operation, maintenance and recycling, and would focus on maximizing the efficiency of the solar system to secure the project. This encourages more solar developers to participate and reduces the cost of procuring solar systems [34].

In addition to the SolarNova Program, Singapore relies on the market to promote PV adoption instead of direct subsidizes. Chang and Li [35] studied the electricity market reform of Singapore, which has developed into a fully divested generation with competition in the retail and wholesale sectors. Supply competition and retail liberalization have brought a 9.11% price decrease in wholesale electricity [36]. Customizers could choose between various electricity retailers and their pricing arrangements. Six electricity retailers provide eco-friendly electricity plans to customers, and Geneco and Sunseap offer a 100% solar electricity option to customers. Customers can offset their carbon emission footprint and carbon tax by purchasing UN Certified Carbon Credits from retailers. Additionally, to solve the land limitation in Singapore, Singapore aims to import 100 MW solar electricity from Malaysia by 2025 and encourages collaboration between solar PV companies and investment between the two countries [37].

4. Recent Development of BIPV Systems

4.1. 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]. 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]. In 1991, Aachen’s Public Utilities building first employed PV panels as semitransparent glass in the façade [40]. The scientific literature on the subject of BIPV structures was published during that time in Europe [41]. Then, the US DOE launched a program called Building Opportunities in the United States for Photovoltaics Program to help commercialize BIPV products [42]. Meanwhile, Europe published Solar Architecture in Europe, and Japan also joined these efforts, announcing similar programs [43]. 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]. Thereafter, the construction industry successfully realized projects that were developed worldwide, which were subsequently reported in a very large number of papers [26]. 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].

4.2. 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]. 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].

4.2.1. 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]. 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 shows the general types of BIPV systems.

4.2.2. 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], 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], 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], 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], Li et al. [51] 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] 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.

4.2.3. 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] 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]. 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]. 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].

Generally, double glazing PV systems perform better in terms of heat insulation than single glazing PV systems [9]. To reduce heat transmission, an insulating layer may be applied to single glazing PV systems [55]. 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]. 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.

4.2.4. 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]. 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] 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]. 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].

4.3. Singapore BIPV Projects

BIPV roofs offer a variety of design possibilities (Figure 5a–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] is shown in (Figure 5a). 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.

The BIPV opaque roof system (Figure 5b−d) is suitable for flat, sloped, and curved roofs. To improve PV performance and reduce heat transfer to the interior, a ventilated air gap is recommended for PV integration, (Figure 5b−d) especially in tropical regions, which have hot summers. When the entire roof is a PV roof, the integration of aesthetics and structure needs to be considered, such as the visual impact as a “fifth elevation” on surrounding taller buildings such as glare and aesthetics, as well as preventing the PV panels from bending due to gravity.

An “urban living room” (Figure 5e) is formed by a PV canopy over an open space in the building complex. This shelter provides protection from solar radiation and rainwater, creating a semi outdoor space for recreation, entertainment, and sports. The BIPV skylight (Figure 5f) replaces the traditional glazing roof and shading louvers. By controlling the PV density, the BIPV skylight provides proper lighting and thermal comfort control. Due to the unique light and shadow effect cast by PV cells, it is very suitable for commercial, public, and medical buildings, creating a vivid architectural experience [60].

In Singapore, if the exterior walls are plaster and paint, the building facade needs to be repainted every five years. The paint cost is estimated at 50 SGD/m², while the cleaning fee for cladding walls is only one-fifth of the cost of paint for low- and middle-rise buildings [61]. This is an incentive for implementing BIPV cladding on buildings (Figure 5g). As the BIPV cladding walls are opaque, PV acts as the outermost skin of the building envelope, not only absorbing solar radiation to produce clean energy but also shading the wall behind, which can reduce the indoor air temperature and air conditioning energy consumption. A semitransparent BIPV curtain wall (Figure 5h) is the embodiment of a highly integrated design, which meets the requirements of the building envelope, such as weather proofing and horizontal wind load bearing. Changing the PV color, material and window-wall ratio plays an important role in energy savings of high-rise glass buildings, such as effective control of interior lighting, thermal comfort, and power generation performance [62,63].

BIPV shading devices are generally suitable for all building types, such as residential, school, office, and medical buildings. Both existing and new buildings can easily integrate BIPV shading devices on their exterior walls (Figure 5i), which reduces the thermal and visual discomfort caused by excessive solar radiation, especially for glazing curtain wall buildings and spaces such as balconies and corridors.

Table 3. Singapore BIPV projects.

No.	Project Name	Year	Application	PV Module Type	Installed Capacity (kWp/MWp)	Titled Angle	Ref
a	South Beach Tower	2016	Rooftop ventilated BIPV	CIGS	285.45	0	[64]
b	Singapore Sports Hub	2014	Rooftop ventilated BIPV	p-Si	707.46	10°	[65]
c	Cove Drive	2011	Rooftop ventilated BIPV	Monocrystalline all-back contact	44.84	10°, east	[66]
d	Zero Energy House	2008	Rooftop ventilated BIPV	a-Si	4.8	19°, northeast	[66]
e	Tanjong Pagar Center	2016	Rooftop skylight	Transparent a-Si	125	0	[67]
f	Waterfront Promenade Visitor Center	2010	Rooftop skylight	Semitransparent m-Si	32	0	[68]
g	Keppel DHCS	2013	Cladding façade	p-Si	205.58	90°, northeast	[66]
h	Tampine Grande	2007	BIPV Curtain wall	Thin film a-Si	6	90°, west	[69]
i	BCA Zero Energy Building	2009	BIPV shading device	Thin film a-Si	-	west	[70]

and Figure 5 show a collection of BIPV projects in Singapore. The summary information for the BIPV projects includes the BIPV application types, PV module types, installed capacity and titled angle.

5. Life Cycle Assessment (LCA)

LCA is a method used to evaluate material and energy flows and their consequences during the life cycles of items while also monitoring environmental and resource sustainability. LCA can be applied to evaluate investment and system production levels considering environmental impacts throughout the lifetime of implemented PV systems. The IEA has defined LCA guidelines for PV systems [71] that are accepted by the International Organization of Standardization (ISO) [16]. There are three major LCA phases according to these guidelines, as follows:

(1)

definition of the technical specifications and features of PV systems;

(2)

description of the modeling methodologies to perform LCA of PV systems;

(3)

reporting and dissemination of PV system LCA results.

The energy payback time (EPBT) and GHGE are the recommended and most commonly used metrics for LCA of PV systems.

5.1. EPBT

The EPBT, as a typical indicator used to evaluate energy generation systems, is the required time during which the PV system generates the same amount of energy as that utilized throughout its lifetime [72], i.e., the system creates the same amount of energy as it consumes during its lifetime. The EPBT can determine if and to what degree a PV system achieves a net energy gain throughout its lifetime as follows Equation (1) [16]:

$$\mathrm{EPBT}=\frac{{\mathrm{E}}_{\mathrm{input}}+{\mathrm{E}}_{\mathrm{BOS},\mathrm{E}}}{{\mathrm{E}}_{\mathrm{output}}}$$

(1)

where E_input is the PV module energy demand (MJ) during its lifetime, including the energy for PV module manufacturing, transportation, installation, operation, maintenance and disposal, E_BOS,E is the BOS energy demand (MJ), including cabling, inverters, batteries, other electronic and electrical components, and structural frames, and E_output denotes the primary energy savings attributed to electricity generation by the PV system [73].

5.2. GHGE

Other than fossil fuel-based power systems, PV systems convert solar energy into electricity, thus reducing the emissions of CO₂, CH₄, N₂O and chlorofluorocarbon during power generation. Therefore, the GHGE can function as a key assessment indicator for the LCA of PV systems. The GHGE rate is the emission rate of GHG per unit of electricity produced by PV systems (g CO₂e/kWh), which can be expressed as follows Equation (2) [16]:

$${\mathrm{GHGE}}_{\mathrm{rate}}=\frac{{\mathrm{GHGE}}_{\mathrm{total}}}{{\mathrm{E}}_{\mathrm{LCA}\_\mathrm{output}}}=\frac{{\mathrm{GHGE}}_{\mathrm{pv}}+{\mathrm{GHGE}}_{\mathrm{BOS}}}{{\mathrm{E}}_{\mathrm{LCA}\_\mathrm{output}}}$$

(2)

where GHGE total denotes the total GHGE during the life cycle (g CO₂e), E_{LCA_output} is the total electricity generated by the PV system during its life cycle (kWh), and GHGE_PV and GHGE_BOS are the GHGE_total of the PV modules and BOS components (g CO₂e), respectively [73].

According to [10,74],

Table 4. Summary of the CED, EPBT and GHGE of the different solar PV technologies.

Type of PV Technology	Range of CED (MJ/m2)	Range of EPBT (Years)	Range of GHG Emissions (g CO2e/kWh)
m–Si	2860–5253	2.1–12.1	30–46
p–Si	2699–5150	1.7–3.3	37
a–Si	710–1990	2.7–3.2	37.6
CdTe	790–1803	0.7–3.2	32.4
CIS	1069–1684	1.6–2.9	69

summaries the EPBT and GHGE rates of the five main PV technologies is provided in, including mono-Si, poly-Si, a-Si, CdTe and CIS PV cells.

Compared to other energy sources, wind and hydropower achieve lower EPBT (0.2–2.3 and 0.24–3.09 years, respectively) and GHGE rate (6.2–46.0 and 2.2–74.8 g CO₂e/kWh, respectively) values than those of the considered PV technologies [74,75]. Although PV technologies exhibit higher influence values due to their increased energy consumption in the manufacturing process, they are safer than nuclear energy within high-density urban contexts, such as Singapore, and yield better environmental impacts than coal plants, biomass fuels and combined-cycle gas turbines.

5.3. BIPV Standards

Because PV panels are considered building components, they are required to obey both PV and building industry standards in terms of their electrical, safety and other features. BIPV always has to deal with the following two different standardization and regulation schemes: one is the often-regulated local building codes and international ISO standards derived from the requirements of the building side, and the other is regulated from the electrical side, with international IEC standards and mandatory, not fully harmonized local regulations.

In 2016, the EN 50583 series “Photovoltaics in buildings” was published at the European level, and other suggestions for additional work items were launched globally. In October 2018, the updated ISO/TS 18178 (laminated solar PV glass) for ISO TC160 (glass in buildings) was released, which specified the appearance, durability and safety requirements, test methods and designations for laminated solar photovoltaic (PV) glass for use in buildings. Laminated solar PV glass is defined as laminated glass with integrated photovoltaic power generation. ISO 12543 (Laminated Glass and Laminated Safety Glass for Architectural Glazing) is referenced for many requirements other than electrical characteristics and permits the use of various types of glass (float glass, patterned glass, etc.), solar cells (crystalline silicon solar cells, thin-film solar cells, etc.) and interlayers (polyvinyl butyral, ethylene vinyl acetate, etc.).

Over the course of more than three decades, the International Electrotechnical Commission (IEC) has created a set of standards for photovoltaic (PV) modules and systems in order to describe and evaluate their electrical performance [76]. Furthermore, several ISO (International Organization for Standardization) standards apply to BIPV modules and systems as building components. And, a new attempt was undertaken within IEC TC82 (82/1339/DC) in 2017 to form a project team, i.e., PT 63092 “Building Integrated Photovoltaics (BIPV)”, which includes specialists from ISO, IEC, and IEA PVPS Task 15 which contains IEC 63092 specifies BIPV module requirements and BIPV system requirements.

In Singapore, building professionals, such as architects and planners, are encouraged to refer to the above guidelines while designing and constructing BIPV systems. These guidelines were jointly issued by the Urban Redevelopment Authority (URA) and BCA and cover BIPV installation, safety requirements, maintenance activities, and restrictions for conservative regions. Additionally, the Green Mark program, a sustainable building certification program, grants points for performing solar potential and feasibility studies at the design stage, in addition to PV installation in buildings. Singapore adopted the major international PV standards and modified these standards to satisfy local requirements, e.g., SS IEC 61215, SS IEC 61730, and SS 601.

Moreover, due to the Singapore Statutes, the Codes of Practice for Fire Applications in Buildings published by the Singapore Civil Defense Force (SCDF) highlight roof PV arrangement requirements to ensure fire protection [77]. Additionally, a solar photovoltaic (PV) roof-mounted module is in the scope of fire safety works inspected by appropriate registered inspectors [78]. In addition, the Ministry of Sustainability and the Environment (MSE) regulations state that solar photovoltaic panels are regulated non-consumer products [79] and the solar photovoltaic panel material recovery target is 70% [80]. Moreover, MSE requests the submission of an energy use report each year from Singapore’s registered corporations, which shall cover each business activity under the operational control of the registered corporation, and the fraction of photovoltaic material manufactured that uses the fluorinated compound fraction is on the lists of data on processes and activities resulting in greenhouse gas emissions [81].

6. Barriers to BIPV Implementation in Singapore

As a densely populated city-state, Singapore notably contains vast façade areas of high-rise buildings, thus creating an ideal area for BIPV deployment. However, there are several barriers to widespread BIPV implementation in Singapore. Based on several studies [9,82,83,84] on a multistakeholder approach, it has been demonstrated that even though the driver of BIPV development accomplishes both Green Mark certification and CO₂ emission reduction, the barriers to BIPV implementation in Singapore can be classified into the following five groups: policy barriers, economic barriers, product barriers, human and social barriers, and information barriers, as summarized in

Table 5. The barriers to BIPV implementation.

Policy barriers	Difficulties in obtaining governmental approvals
	Uncertainties in BIPV policies in the long-term
	Low electricity tariff from conventional sources
	Lack of standards, codes or guidelines
Economic barriers	The high upfront capital cost of BIPV
	The long payback period of BIPV systems
Product barriers	Lack of BIPV modular products
	The low-energy conversion efficiency of BIPV systems
	Reliability problem
	Heat transfer issues
	Difficulties regarding cabling and connection
	Unstable power generation quality
	The complexity of the BIPV system
Human resources and social barriers	Lack of professionals
	Lack of public education and awareness of BIPV
Information barriers	Lack of information on BIPV products, suppliers and policies
	Lack of life cycle cost analysis knowledge
	Lack of BIPV demonstration projects
	Lack of design tools

.

7. Future Research Needs of BIPV

7.1. Prefabricated BIPV Façade Module Products

According to a previous study [85], the most cost-effective way to reduce the cost of BIPV systems is to develop prefabricated BIPV façade modules for new buildings and renovations. Both the BIPV industry and prefabrication building industry share similar features in that they both demand a highly automated manufacturing process to produce items offsite, which requires high capital and upfront expenses. In addition, onsite construction simply requires module assembly and erection. The combination of these two areas exhibits the potential to drastically reduce onsite staffing costs. Due to the high level of automation and standardization in the Japanese prefabricated housing industry, a relevant prefabricated BIPV industry business model has been established and disseminated [86].

In recent years, many studies have concentrated on prototype designs of BIPV modules (Figure 6). A façade that integrates PVs with precast concrete (PVPC) has been developed in Shanghai, China [51] and is suitable for high-rise building construction integrated with renewable energy (Figure 6a) which PVPC façades can eliminate the need for steel frames, thus lowering costs and freeing up more outside space for installation, in addition to lowering the building heating gain and cooling load. However, the air gap between the PV panel and concrete surface is a stationary cavity, which may not provide an efficient heat dissipation solution. In addition, in another study, a prefabricated BIPV wooden façade (Figure 6b) was developed comprising the following three major layers: the outer skin of the PV system, a natural ventilated air cavity in the middle and an inner framed wooden panel with thermal insulation [87]. However, this prototype might not fit within the context of Asian countries with high population densities, high-urban density settings and a scarcity of forestland resources. Furthermore, a lightweight glass block-integrated DSSC (Figure 6c) was examined [88]. The PV glass block can be installed, assembled and constructed using joints, allowing for easy dismantlement and replacement. Due to the limitations of third-generation PV cells, their power generation efficiency is limited. The European Commission Horizon 2020 project for modular façade retrofitting with renewable energy technology (MFRRn) was reviewed [89]. There are three distinct ways to retrofit renewable energy technologies, i.e., frame-based systems, layer-based systems, and the combination of these two systems (Figure 6d). Following preceding studies, it is essential to consider all aspects of the integration of a high-efficiency PV module, ventilated PV system, automatic manufacturing process, and convenient installation and erection procedures without sacrificing the range of customizations to adapt to existing and new buildings.

Singapore has unique advantages in PV technology integration with prefabricated construction technologies. First, the majority of the existing housing buildings were built by the Housing Development Board (HDB) in the 1980s, which suggests that the PV system can be integrated with similar dimensions and economically deployed in large numbers [24]. In addition, design for manufacturing and assembly (DfMA) is identified as a game-changing construction method in the construction industry of Singapore, which involves structural steel, advanced precast concrete systems (APCSs) and prefabricated prefinished volumetric construction (PPVC). DfMA provides guidelines to increase the manufacturing efficiency and streamline the assembly process to reduce costs and improve the overall system performance [90]. The Solar Energy Research Institute of Singapore (SERIS) and the National University of Singapore (NUS) Department of Architecture have created a modular pod based on these design principles to assist architects and developers in simply integrating BIPV technology into building façades (Figure 7). A test building was constructed with multiple prefabricated BIPV elements (panelized walls, monsoon windows, unitized walls and fixture walls) to simulate different construction methods. The structure manufacture relied on a highly automated process of light gauge steel roll-forming machines. The multifunctionality of these prototypes enables them to meet the requirements of connection and cabling, dry assembly, and noise- and weatherproofing. The interior thermal performance of the building and the energy generation performance can be monitored and evaluated through various embedded sensors.

7.2. Productive BIPV Façade

Singapore heavily depends on energy and food imports to meet the growing domestic demand. For security considerations, it is essential to gradually increase local food and energy production levels in the future. Due to the pressure of the growing population and limited land, innovative integration of PV shading devices and vertical farming (VF) planters into building façades (Figure 8), referred to as productive façades (PFs), could be an alternative method to achieve self-sufficiency in the field of power and food for Singapore [91].

PF-related investigations have recently been proposed and conducted [91] to examine the potential of PFs in Southeast Asia and Singapore with 57 cases considering the four key aspects of building typology and morphology, plot ratio, site coverage and building height. Their study revealed that PFs could be suitable under all construction orientations in low-latitude regions. In the cases with the lowest plot ratios (PR < 1.9) and smallest building heights (<42 m), food and energy self-sufficiency, respectively, were achieved. Ref. [92] proposed two types of PF-integrated systems, namely, window façades and balconies, with 8 prototypes under four orientations (Figure 8a). The optimal design was selected via the multiple attribute decision-making (Vise Kriterijumska Optimizacija I Kompromisno Resenje or VIKOR) optimization method and installed at the Tropical Technologies Laboratory (T2 Lab) of the NUS, and five critical functions were compared (interior daylight autonomy, power generation, irradiance, vegetable productivity level and viewing angle). Kosorić et al. [20] conducted a door-to-door survey among Singapore social housing residents to collect data on social acceptance, aesthetic requirements, and maintenance of PF designs. A web survey was also conducted of local Singapore building experts regarding certain key design aspects, such as façade aesthetics, material use, views from the inside, operation, functionality, and architectural quality [93]. These two survey studies revealed positive responses among both end users and building professionals. However, the application scalability from the building scale to the city scale requires further investigation.

7.3. BIPV Recycling

Upon BIPV installation enhancement, it is essential to consider the issue of BIPV recycling. Globally, a PV waste amount totaling approximately 250 tons occurred in 2016, and the waste amount is expected to range from 1.7–8 million tons by 2030 and will reach approximately 78 million tons by 2050, with the possible value of the recovered material exceeding 15 billion US dollars by 2050 [32,94]. Therefore, PV panel recycling is an urgent environmental issue but may also yield notable economic benefits in subsequent decades.

The development of BIPV recycling can be divided into the following three fields: policy making, recycling technology and design for disassembly (DfD). In terms of policy making, Europe is the pioneer, as it has established a stringent regulatory framework, while other countries have initiated the implementation of comparable restrictions. Currently, the Waste Electrical and Electronic Equipment (WEEE) Directive 2012/19/EU [95] governs the recycling obligations of PV producers within the European Union (EU). Additionally, the Japan Photovoltaic Energy Association (JPEA) has suggested a voluntary standard for end-of-life (EoL) PV panels [10]. However, other countries with large PV deployment markets, such as the United States, India and China, have not formulated such EoL PV panel regulations. PV waste is categorized in these countries as general electronic waste or as hazardous and nonhazardous solid waste materials in the absence of specific PV regulations. Currently, approximately 10% of all EoL PV panels worldwide are recycled [32].

Regarding PV recycling technology, the principle of the circular economy has gained increasing acceptance as a means of transforming the current PV industry into an ideal closed-loop circular recycling system capable of efficiency improvement and energy consumption reduction during the recycling process, in addition to increasing the recycling and recovery rates. Given that crystalline silicon (c-Si)-based PV panels have accounted for 80–90% of the market over the last two decades, these c-Si PV panels are the main types of PV panels for recycling in the future, including their components, such as glass, aluminum frames, solar cells, ethylene-vinyl acetate (EVA) encapsulants, and composite Tedlar back sheets manufactured via lamination as one panel. EVA removal and PV panel dismantling are regarded as the most challenging aspects of the recycling process [46]. Nevertheless, the market for PV recycling is expected to be enormous in the near future, and there are currently only a few specific PV recycling companies in the industry.

Since PV panels on building façades function as both a power generator and building skin, the disassembly of PV panels could greatly affect building functions, involving many factors, such as cabling and connection, water- and weatherproofing features, construction risks for high-rise buildings and numerous other issues. Therefore, in contrast to BAPV and PV farms, the DfD of BIPVs should be considered at the early design stage. Due to the high energy consumption and low recycling rate in the construction industry, the DfD concept was developed in the 1990s and has only recently gained attention in mainstream practice, such as in the London Plan. Although the EU Building as Material Banks (BAMB) project [96] and the United States Environmental Protection Agency (EPA) have published standards [97] for the abovementioned design process, certain sustainability certifications also grant points for DfD, such as LEED and Green Globes. DfD targeting BIPV technology requires a comprehensive and detailed disassembly plan that includes deconstruction instructions and methods for reuse, recycling and reclamation of building components and PV panels, all of which require recording and tracking the entire life cycle. In addition, it is essential to design appropriate PV joints to facilitate disassembly and minimize the employment of heavy equipment. PV joint design should prioritize dry assembly, such as the adoption of bolts, screws or nails in connections, instead of applying chemical methods, such as sealers, glues or welding.

Although the need for PV recycling will be met within the next 5–10 years in Singapore, related regulations have been established under the concept of extended producer responsibility (EPR), which requires PV producers, including importers and manufacturers, to offer free take-back services for EoL PV panels starting in 2022.

7.4. Urban Heat Island (UHI) Effect

Urban heat island (UHI) effects exert a notable impact on the local weather conditions in cities, resulting in temperatures that are higher in urban areas than in rural areas. This phenomenon may increase energy consumption related to air conditioning by altering building thermal conditions [98] and may result in a health crisis as a result of climate warming [99]. This mainly occurs because building surface materials such as concrete and steel absorb solar radiation during the daytime, after which heat is radiated into air at night, thus increasing the ambient temperature.

The implementation of BIPV systems on building roofs and façades modifies the nature of rooftops and façades, respectively, and may affect energy transfer between the atmosphere and buildings. Several studies have demonstrated that BIPV surfaces can mitigate UHIs and reduce cooling loads in the summer. At the urban scale, Taha, Genchi et al. and Masson et al. [100,101,102] reported that PV systems on buildings exert no negative impacts on the air temperature and UHI phenomenon in high-density cities, such as Los Angeles, Tokyo and Paris, if opaque PV panels are installed in large city areas. In addition, although a considerable temperature change occurs on the building surface at the building scale, there is no effect on the local microclimate, such as in Tianjin city, China [103]. This is mostly attributed to the shading effect of PV panels, which may help reduce cooling load-related energy usage. Additionally, partial substitution of fossil fuels with solar technology may help reduce CO₂ emissions and mitigate the effects of UHIs [104].

However, there are only a few studies focusing on the effects of UHIs on BIPV performance. Studies have revealed that BIPV power generation may decline slightly due to the increased ambient temperature attributed to the UHI phenomenon, e.g., refs. [104,105,106] compared 27 available PV-related software programs and found that there is a lack of software programs that consider the UHI impact on PV panels integrated into buildings. Further investigation of the relationship between BIPV systems and local urban microclimate conditions is necessary.

Research on the UHI effect on BIPV systems in cities involves collaboration among different professionals, such as urban planners, architects, engineers and policy makers. It is necessary to compile a comprehensive city-scale database of solar potential to evaluate solar technology implementation in building envelopes in the future. An integrated platform should be developed in the future based on existing infrastructure information among various city administrative agencies. This platform can support researchers in the investigation of BIPV implementation and facilitate decision making during policy development [32].

7.5. Conclusions

Although BIPV systems have gained attention recently, the application of BIPV is still in a niche market. Taking Singapore as an example, the establishment of BIPV systems offers insights for other tropical countries facing comparable challenges. Based on the literature review, the following conclusions are highlighted:

To meet the target of the Paris Agreement, Singapore must make use of solar energy due to the limitation of energy resources and land. BIPV is applicable for high-density urban scenarios.

BIPV is an essential factor to help tropical buildings become green buildings, such as super low energy buildings, zero energy buildings and positive energy buildings.

Through a review of the state of PV technologies and BIPV applications in Singapore, the efficiency of PV cells should be improved, and demonstrative BIPV buildings should be encouraged.

Developing holistic BIPV regulation is important, as many still believe that BIPV is a technology material and not a building construction material. The lack of related local BIPV regulations hinders BIPV façade implementation.

Based on the barriers to BIPV implementation in Singapore, an information and knowledge sharing platform should be established among stakeholders and technology developers.

Future BIPV implementation research can focus on prefabricated construction, food and energy, material recycling, and mitigation of the urban heat island effect.