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 CO
2 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 CO
2 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 (CO
2e/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 CO
2e and by 50% to 33 Mt CO
2e 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.
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]:
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 CO
2, CH
4, N
2O 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 CO
2e/kWh), which can be expressed as follows Equation (2) [
16]:
where GHGE total denotes the total GHGE during the life cycle (g CO
2e), 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 CO
2e), respectively [
73].
According to [
10,
74],
Table 4 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 CO
2e/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].
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 CO
2 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.