Building Structure | Current Status of Metal Roof Photovoltaic Integration under the Dual Carbon Background

Source: Building Structure (ID: buildingstructure)

Current Status and Technological Advancement of Metal Roof Photovoltaic Integration

By Wu Yaohua

Abstract: Based on the goals of achieving "carbon peak" and "carbon neutrality" and the needs of building energy saving and green buildings, this paper conducts research and analysis on the current status of photovoltaic technology, photovoltaic components, and the combination of metal roofs and photovoltaics (BIPV) technology at home and abroad, as well as the business model of BAPV (attached photovoltaic) operation. It proposes key technical points for the combination of metal roofs and photovoltaics, including structural safety, durability, drainage, and efficient utilization. It also sorts out relevant domestic and international standards for metal roof photovoltaics and proposes areas that need improvement and development, such as wind load values, connection technology between photovoltaic components and metal roofs, and the lack of engineering implementation and acceptance management standards for the combination of photovoltaic and metal roof products.

 

0

Introduction

The application of photovoltaic power generation technology in buildings is an important means for the building sector to achieve the goals of "carbon peak" and "carbon neutrality." Carbon emissions from building operations in China account for one-fifth of the country's total carbon emissions, and this proportion is still increasing with urbanization, continued economic development, and improvements in people's living standards. In 2019, the existing building area in China was approximately 64.4 billion m²，and the area under construction was approximately 10 billion m^2[1].If the installable area for photovoltaic power generation is calculated as 15% of the building area, the area available for photovoltaic installation is approximately 11 billion m². Based on an average installed capacity of 130W per square meter, the installed capacity of photovoltaic power generation can reach 1,430 GW, and the annual power generation can reach 1.43 trillion kWh. Therefore, the potential for building photovoltaics in China is enormous, and the application prospects are broad.

A building photovoltaic power generation system consists of three parts: solar cells and photovoltaic components, charge-discharge control inverter components and grid-connected systems, and building roofs and walls where photovoltaic components are placed. Roofs are the most commonly used and more efficient installation carriers for building photovoltaics. Compared to exterior walls, photovoltaic components on roofs are less obstructed and are easier to construct and maintain.

This article focuses on the current status of building photovoltaics. Based on the requirements of building energy saving and green buildings, market business models, and development opportunities, it conducts research and analysis on the current status and key points of building integrated photovoltaic (BIPV) technology that combines photovoltaic cells and photovoltaic component technology with metal roofs at home and abroad. Combining the technical requirements of metal roofs, it proposes existing problems and development directions in the development of building integrated photovoltaics.

1

Development Opportunities for Building Integrated Photovoltaics

1.1 Building carbon emissions exceed half of the national total

Industry, transportation, and construction are the three major areas of energy consumption. Therefore, the construction sector is one of the main areas responsible for direct and indirect carbon emissions. According to statistics from the China Building Energy Efficiency Association^[2]，in 2018, the total energy consumption of the entire building process in China accounted for 46.5% of the national total energy consumption, of which energy consumption during the building operation stage accounted for 21.7% of the national total energy consumption. In building construction, the carbon dioxide emissions caused by the production, transportation, and construction of building materials in China have reached 2.82 billion tons, accounting for 29.4% of the national carbon emissions; carbon dioxide emissions from building operations are 2.11 billion tons, accounting for 21.9% of national carbon emissions; the sum of the two reaches 51.3% of the national total carbon emissions, making the construction sector the largest contributor to carbon dioxide emissions in the whole society. The task of carbon reduction in the construction sector is arduous and urgent.

1.2 Favorable national and local policies

In 2019, the National Development and Reform Commission issued the "Overall Plan for the Creation of a Green Life Action," which proposed strengthening technological innovation and integrated application, promoting the application of renewable energy in buildings, promoting new green construction methods, improving the application ratio of green building materials, and actively guiding the construction of ultra-low energy consumption buildings.

In July 2020, the Ministry of Housing and Urban-Rural Development, the National Development and Reform Commission, and six other ministries jointly issued the "Green Building Creation Action Plan," which identified improving building energy efficiency as one of the key tasks of the green building creation action, pointing out: promoting the development of ultra-low energy consumption buildings and near-zero energy consumption buildings, and promoting the application of renewable energy and the utilization of reclaimed water.

In November 2020, the Beijing Municipal Development and Reform Commission, the Beijing Municipal Finance Bureau, and the Beijing Municipal Housing and Urban-Rural Development Commission jointly issued the "Notice on the Promotion and Application of Photovoltaic Power Generation Systems," providing subsidies of 0.3 to 0.4 yuan per kilowatt-hour for projects utilizing photovoltaic power generation in industrial, agricultural, or school facilities. Shanghai, Guangzhou, and other places have also successively introduced subsidy standards.

In June 2021, the National Energy Administration officially issued the "Notice on Submitting Pilot Plans for County-Level (City, District) Rooftop Distributed Photovoltaic Development," intending to organize and carry out pilot work on county-level (city, district) promotion of rooftop distributed photovoltaic development nationwide. The Notice clarifies that the proportion of photovoltaic power generation that can be installed on the total roof area of party and government organs buildings should be no less than 50%; the proportion of photovoltaic power generation that can be installed on the total roof area of public buildings such as schools, hospitals, and village committees should be no less than 40%; the proportion of photovoltaic power generation that can be installed on the total roof area of industrial and commercial buildings should be no less than 30%; and the proportion of photovoltaic power generation that can be installed on the total roof area of rural residents' houses should be no less than 20%.

Currently, the cost of photovoltaic components is decreasing, and the cost of photovoltaic power generation is lower than that of coal-fired power generation. The development of photovoltaics will not have a negative impact on buildings, and it should be promoted as early as possible in new buildings and added to existing buildings^[3]. Therefore, the development of photovoltaic power generation on the building envelope is a future trend.

1.3 Huge development space for building photovoltaics

The existing building area in China is currently approximately 64.4 billion m²，and if the roof area that can be installed with photovoltaic power generation is calculated as 1/6 of the building area, the roof area available for photovoltaic installation is approximately 11 billion m²，and if the photovoltaic installation ratio is calculated as 30%, the roof photovoltaic area can reach 3.3 billion m²，and based on the current comprehensive cost of 4.6 yuan per watt, the existing buildings nationwide can provide a building photovoltaic market scale of approximately 2 trillion yuan. At the same time, approximately 2 billion m of new buildings are added annually²，and based on a roof area of 340 million m2, if the BIPV installation ratio reaches 50%, then approximately 170 million m of new roof BIPV area will be added annually²The market space could reach 100 billion yuan annually. In addition, if photovoltaic facades and awnings are considered, the market capacity will be even larger. Overall, with the impetus of policies and the maturity of building-integrated photovoltaic technology, the future market development space in this field is enormous.

On the other hand, it is relatively difficult to implement the renovation of existing buildings. Urban residential buildings are mainly composed of medium-to-high-rise modern buildings. The design service life of the building structure is long, and the load surplus that can be borne by buildings under normal circumstances is not large. Moreover, the newly promulgated reliability standards for building structural engineering have raised the requirements for safety and reliability, so the number of existing buildings that can be renovated with BIPV is limited. In comparison, industrial buildings, public buildings, towns, and rural areas are expected to become the main battlefields, especially industrial plants with metal roofs. Metal roof plants have characteristics such as large single-unit area, light weight, good seismic performance, and a design service life similar to that of photovoltaic products. However, property rights issues will become a stumbling block to the renovation of industrial plants.

1.4 Returns and Business Models of Building-Integrated Photovoltaics

The combination of photovoltaics and buildings generates electricity while also contributing to reducing the building's energy consumption. For example, the shading of photovoltaic components by sunlight is conducive to indoor temperature regulation, reducing the use of air conditioning and the building's power load. According to calculations, after playing a role in shading or controlling indoor sunlight, the reduction in air conditioning load and energy consumption achieved by BIPV may be higher than the electricity generated.^[3]。

In the past 10 years, the conversion efficiency of solar cells has been continuously improved, the power generation efficiency of photovoltaic systems has been continuously improved, and the cost of power generation has been significantly reduced. The cost of photovoltaic power generation has been reduced by about 90% in the past 10 years. In November 2019, the average levelized cost of electricity for global photovoltaic power generation released by Lazard Investment Bank in the United States has dropped to $0.04/kWh. ^[4] The cost of building-integrated photovoltaic systems has dropped from 40 yuan/W in 2015 to 5 yuan/W in 2020, and is expected to drop further to 2.5 yuan/W by 2025.^[5]According to a report by the European Solar Energy Association^[6]solar photovoltaics have become the lowest-cost electricity compared to coal-fired power, wind power, and nuclear power, with photovoltaic contract bidding prices as low as 1.85-3.20 cents/kWh. Therefore, obtaining returns on investment in building-integrated photovoltaics has become a reasonable business model. However, under the existing system, the builders, investors, and beneficiaries may be different companies, and there are still problems such as difficulties in grid connection, charging, and financing. In terms of grid connection, the power grid is not very motivated to provide free services while losing market share; for users, not connecting to the grid means that all generated electricity must be fully consumed by themselves, which may lead to waste and cannot guarantee stable power supply; in terms of financing, distributed building-integrated photovoltaic systems are attached to other people's fixed assets, and asset ownership is unclear.

2

Development Status of Building-Integrated Photovoltaics

2.1 Development of Photovoltaic Technology

Photovoltaic modules are composed of solar cells connected and tightly sealed, forming photovoltaic strings and photovoltaic arrays through series and parallel connections. Solar cells are the core components of photovoltaic modules.^[7]In recent years, solar cell technology has developed rapidly, leading to continuous improvement in its photoelectric conversion efficiency, and the cost of photovoltaic power generation systems is also rapidly decreasing. As of 2020, the industrialized photoelectric conversion efficiency of various solar cells^[4]and the current national standard "Technical Standard for Application of Building-Integrated Photovoltaic Systems" (GB/T 51368—2019)^[8](referred to as GB/T 51368) regulations on the efficiency of building-integrated photovoltaics are shown in Table 1.

In the document "Normative Conditions for the Photovoltaic Manufacturing Industry (2021 Edition)" (hereinafter referred to as "Conditions") issued by the Ministry of Industry and Information Technology, specific requirements are put forward for photovoltaic products (Table 1): the average photoelectric conversion efficiency of monocrystalline silicon cells and polycrystalline silicon cells is not less than 22.5% and 19%, respectively; the average photoelectric conversion efficiency of monocrystalline silicon modules and polycrystalline silicon modules is not less than 19.6% and 17%, respectively; the average photoelectric conversion efficiency of copper indium gallium selenide (CIGS) thin-film photovoltaic modules, silicon-based photovoltaic modules, cadmium telluride (CdTe) thin-film photovoltaic modules, and other thin-film photovoltaic modules is not less than 15%, 12%, 14%, and 14%, respectively. GB/T 51368 stipulates that the maximum attenuation rate of various battery components used in building-integrated photovoltaics in the first year of project operation is 2.5% to 5%, and the annual attenuation rate thereafter should not exceed 0.7%. The implementation of "Conditions" and GB/T 51368 will guide the development of solar cell technology towards higher photoelectric conversion efficiency.

Table 1 Conversion Efficiency of Various Solar Cells

Crystalline silicon solar cells were developed earlier, with high technological maturity and high photoelectric conversion rate; second-generation solar cells include silicon-based, cadmium telluride, and copper indium gallium selenide thin-film solar cells, which have good weak light performance, low temperature coefficient, and flexibility, and have advantages such as easy integration with buildings in building-integrated photovoltaics, but their photoelectric conversion rate is low and their stability is poor.^[7]The selection of solar cell types should comprehensively consider factors such as installation scenarios, light resources, grid conditions, and operating modes. Currently, crystalline silicon solar cells still account for a larger market share in China.

Many domestic photovoltaic product manufacturers have also launched photovoltaic products combined with buildings, namely building photovoltaic modules. Companies producing crystalline silicon photovoltaic modules for buildings include LONGi Green Energy Technology Co., Ltd. ("LONGi"), Hangzhou Foster Applied Materials Co., Ltd. ("Hangzhou Foster"), Shanghai Mai New Energy Technology Co., Ltd. ("Shanghai Mai"), Yingli Green Energy Holding Co., Ltd. ("Yingli"), and GCL-SI, etc. These manufacturers have developed roof photovoltaic components based on crystalline silicon photovoltaic module technology, realizing the combination of photovoltaic modules with building roofs and exterior walls; CdTe thin-film photovoltaic module manufacturers include Zhongshan Ruike New Energy Co., Ltd. ("Zhongshan Ruike"), Longyan Energy Technology (Hangzhou) Co., Ltd. ("Longyan"), etc.; perovskite thin-film photovoltaic module manufacturers include Hangzhou Finar Optoelectronics Technology Co., Ltd. ("Finar Optoelectronics"), etc.; CIGS thin-film photovoltaic module manufacturers include Hanergy Holding Group Co., Ltd. ("Hanergy") and Kaiseng Photovoltaic Materials Co., Ltd. ("Kaiseng"), etc. The photovoltaic modules for building roofs and exterior walls launched by these manufacturers can basically meet the lighting needs of building facades and interiors.

2.2 Development of Building-Integrated Photovoltaic Technology

The combination of buildings and photovoltaics (Figure 1) can be divided into two forms, namely BAPV and BIPV: (1) BAPV (building attached photovoltaic, BAPV), i.e., attached photovoltaics^[7]This involves attaching photovoltaic power generation equipment to buildings. Using existing buildings as a base, photovoltaic power generation equipment is added to the building's surface. This was a more common form in the early stages of building-integrated photovoltaic technology. Its advantages are easy modification, smaller investment, and convenient construction, while its disadvantages are that the appearance of the building after modification is not coordinated with the original architectural design style, and there may be a large gap between the visual effect and the ideal building effect. (2) BIPV, or Building-Integrated Photovoltaics, combines existing buildings with solar power generation devices to achieve the function of solar photovoltaic power generation. In this form, the photovoltaic components serve as part of the building. During the entire construction process, the photovoltaic components are designed, constructed, and installed simultaneously with the building, achieving an organic combination of photovoltaic power generation and the building, and utilizing the surface texture of the photovoltaic components. BIPV is not a simple addition of a photovoltaic power generation system to a building, but an organic combination of the two.

▲Figure 1 Metal Roof Photovoltaic Integration Components

Building photovoltaic systems can operate in two modes: stand-alone and grid-connected. Stand-alone building photovoltaics are typically used in remote areas where grid access is difficult. Furthermore, by utilizing specialized energy storage devices, surplus electricity produced by building photovoltaics during periods of good sunlight can be stored for use when there is no sunlight or when sunlight is poor, achieving self-sufficiency. When connected to the grid, excess electricity can be sent to the grid when generation exceeds local load needs; conversely, when photovoltaic power generation is insufficient, electricity from the grid can be used to ensure stable power supply.

Based on the combination of building metal roof technology and photovoltaic technology, a number of joint ventures or cooperative entities formed by leading companies in the two industries have emerged, joining forces to enter the building-integrated photovoltaic (BIPV) market, as shown in Table 2.

Table 2 Metal Roof Photovoltaic Integration Cooperative Entities

3

Technical Key Points and Issues

The combination of building photovoltaics needs to simultaneously meet the technical and functional requirements of both the building envelope system and the photovoltaic power generation system, including load-bearing capacity (wind resistance, earthquake resistance, snow resistance), building functions (fire prevention, waterproofing, insulation, lightning protection), and durability (corrosion resistance, reliable connection). It also needs to address new problems with building-integrated photovoltaic components (overheating of photovoltaic component backplanes, constraints of clamps on metal roofs, etc.).

3.1 Structural Safety of Adding Photovoltaics to Existing Metal Roofs

When adding a building photovoltaic system to an existing building roof (Figure 2), the structural safety and durability, and electrical safety of the existing building should be reviewed. The self-weight load of the added photovoltaic components (approximately 15-20 kg/m²) is added to the structure. Photovoltaic components are generally connected to existing building roofs using point-type clamps, transforming the original uniformly distributed wind load on the roof into a concentrated load transmitted by the clamps. Furthermore, a non-closed air layer is formed between the photovoltaic components and the metal roof, which has an adverse impact on the structural safety of the existing building metal roof.

▲Figure 2 Structural Connection of Photovoltaics Added to Existing Buildings

Due to the different construction times of existing buildings, previous buildings had relatively low requirements for structural safety aspects such as earthquake resistance design. Because adding photovoltaic components to the surface of the enclosure structure will increase the structural load, it is necessary to verify the structural safety of the building. Before the project implementation, it is necessary to conduct on-site surveys of the building site and environmental conditions, check the safety of the existing building structure, and conduct a feasibility assessment of the added photovoltaic system. The safety of structural components, connection nodes, and foundations, as well as the durability of structural materials, should be comprehensively considered. Based on the original design, completion documents, and on-site investigation, verification can be conducted, and a legally designated testing agency can be commissioned to conduct testing to confirm that there are no structural safety problems. Otherwise, structural reinforcement and modification should be carried out to ensure that the structural safety requirements of the building are met.

3.2 Design Wind Load

The photovoltaic components located on the outside of the roof change the local shape and surface roughness of the original roof, which will lead to changes in the roof wind load. The load effect is similar to that of decorative panels covering the metal roof.

The influence of photovoltaic components on roof wind load mainly involves three factors: 1) The shape and size of the roof and photovoltaic components. When the photovoltaic components are at an angle to the roof rather than parallel (Figure 3), a relatively large additional wind load is generated on the roof. 2) Whether there is a cavity between the roof and the photovoltaic components, and the size of the cavity. When the cavity height is large, a larger wind suction force will be generated on the roof. 3) The size of the spacing between photovoltaic components. The spacing between photovoltaic components is beneficial for uniform roof drainage, but excessive spacing will create air inlets, which is unfavorable for roof wind resistance. The influence of the fixing method and details of the photovoltaic components on roof wind load has been extensively studied in Europe^[9]However, this remains blank in China's current standards.

▲Figure 3 Fixing Method of Photovoltaic Components on the Roof

When fasteners are used to connect the photovoltaic panels to the metal roof, the negative wind pressure load on the roof changes from a uniformly distributed load to a concentrated load at the fastener locations, and the gap between the photovoltaic panel and the profiled steel sheet makes the wind resistance of the BIPV component more complex and unfavorable. Full-scale wind uplift tests should be conducted according to actual engineering practices. Dynamic wind uplift tests should also be conducted in areas with frequent strong winds to ensure wind resistance safety.

3.3 Electrical Safety

The voltage of photovoltaic components after series and parallel connection can reach more than 1000V, which is much higher than the direct contact voltage safety limit. When photovoltaic components are combined with building roofs and exterior walls, some parts of the components can be directly contacted by installers and residents, so there are risks of electric shock during installation and operation. Sufficient attention should be paid to this. Quick disconnect devices that can urgently disconnect all DC and AC circuits, as well as arc fault protection devices, should be installed in the building photovoltaic power generation system to improve the electrical safety performance of the photovoltaic power generation system.

3.4 Fire Safety

The Solar America Board for Codes and Standards conducted experimental research on the impact of roof-mounted photovoltaic arrays on roof fire rating^[10]. Combustion tests found that the air gap caused a "chimney effect," resulting in rapid flame spread that did not meet the building fire protection requirements. During overhead installation, the installation distance between the photovoltaic components and the roof, and the distance between the edges of the photovoltaic components and the roof edges, are factors that affect the spread speed and severity of fires^[11]The reason is that the direct current voltage of the photovoltaic power generation system is relatively high, and its application in buildings has fire risks such as high-voltage DC arcs. The EVA film used in photovoltaic components is a rapidly flammable material and will release hydrocarbon-containing gases at high temperatures^[12] Therefore, photovoltaic components using EVA film will release combustible gases in the event of a fire, which is not conducive to building fire prevention.

Existing fire performance research mainly focuses on the combustion of photovoltaic components themselves, and there is little research on fire performance after combining with buildings. The fire acceptance of BIPV still lacks regulations. Building photovoltaic components should pay special attention to the fire performance requirements such as combustion performance and fire resistance limit, especially for photovoltaic components used as building components in BIPV, double-glass photovoltaic components should be selected, and the combustion performance grade of "A" should be achieved.

3.5 Roof Drainage

Photovoltaic components are all matrix plates in the form of encapsulation added to the top of the metal profiled plates, and the rainwater on the photovoltaic plates flows along the slope and both sides. There should be a sufficiently large gap between the photovoltaic plates along the drainage slope so that all the collected rainwater in this component can flow into the profiled metal plates, preventing the water from flowing across the gap; the height of the water flow collected on both sides should not overflow over the ribs. Therefore, the generally profiled metal plates matched with the photovoltaic components are generally provided with longitudinal collection gutters (Figure 4) between the sides of the photovoltaic components and the ribs.

▲   Figure 4   Collection gutter set at the side of the photovoltaic component

In areas with heavy snowfall, snow accumulates on the photovoltaic components, and snow is more likely to cover the edges of the metal plates. It is necessary to apply sealant to the locking seams of the metal plates to prevent siphon seepage from entering the panel.

3.6   Roof Lightning Protection

Since photovoltaic components are installed on the roofs of buildings or other surrounding structures, they are easily struck by lightning. They should comply with the "Technical Specifications for Lightning Protection of Photovoltaic Building Integrated Systems" (GB/T 36963—2018)^[13] The grounding and lightning protection of the photovoltaic system, component brackets, and outer frames should be designed, constructed, inspected and maintained.

Photovoltaic components can be divided into two types: those without metal frames and those with metal frames. For photovoltaic components without metal frames, lightning protection strips need to be laid along the easily struck parts, using metal maintenance channels or metal bridges as lightning rods. If the profiled metal ribs are higher than the photovoltaic components, the metal ribs can also be used as lightning rods.

For photovoltaic components with metal frames, their metal frames can be used as lightning rods, and the metal frames are electrically connected to the photovoltaic brackets or profiled metal plates, and the electrical connection with the roof lightning protection device and metal railings should be done. The roof connection distance should not be greater than 18m, and one connection should be made every 5m on the wall.

3.7 Durability and Materials

The durability of the metal roof photovoltaic integrated system includes the durability of the metal plate, the durability of the photovoltaic component, and the durability of the connection between the two. The durability of the photovoltaic component is generally no less than 25 years, and the overall durability is required to be no less than 25 years. Therefore, when selecting coated steel sheets, it is advisable to select aluminum-zinc-based plates, aluminum-zinc-magnesium-based plates with good corrosion resistance, PVDF or HDP coatings; or stainless steel plates.

Connectors and fasteners used in building photovoltaic systems should be made of stainless steel or aluminum alloy materials. Insulation pads or other anti-corrosion measures should be installed at the contact points of different metal materials in the system to prevent electrochemical corrosion.

There are two ways to connect the profiled metal plate and the photovoltaic component: structural adhesive bonding (Figure 5) and fastener connection (Figure 6). Structural adhesives generally use silicone structural adhesives. In addition to having high strength, aging resistance, fatigue resistance, corrosion resistance, performance stability and expansion and contraction displacement capability within the expected service life, they should also meet the matching requirements with the surface coating materials of the metal plate.

▲   Figure 5   Photovoltaic component without metal frame and structural adhesive bonding

▲   Figure 6   Photovoltaic component with metal frame and fastener connection

3.8 Temperature Expansion and Contraction of Metal Roof

Due to the temperature difference between the metal roof and its underlying support structure, and the materials of the roof and support structure may also be different, there is a problem of asynchronous temperature deformation. When the length of the roof panel is large, the constrained cumulative displacement difference will lead to a large temperature stress in the roof panel. Metal roofs often use sliding rails (⊥-shaped supports, upright rolled-edge roof panels slide) or sliding grooves (sliding with grooved support connection plates) to release temperature deformation. If the connection between the metal roof and the photovoltaic component uses a fixture, the fixture fixes the rib of the roof panel to the support, causing the sliding function of the sliding rail to fail. Therefore, when using fixtures to connect photovoltaic components, sliding groove supports should be used to release temperature deformation.

3.9 Metal Roof Maintenance

Dust accumulation on the surface of photovoltaic components will reduce the efficiency of photoelectric conversion, so regular cleaning is necessary, and relevant cleaning equipment and passages need to be set up. For building roof photovoltaics in snowy areas, safe passages that facilitate manual snow melting and cleaning should also be set up.

3.10 Building Insulation, Heat Insulation and Energy Saving Requirements

In the process of converting light energy into electrical energy, the conversion efficiency of photovoltaic roof solar cells ranges from 15% to 25%. In other words, solar cells can only convert 15% to 25% of light energy into electrical energy, while the remaining 85% to 75% is heat energy. Due to the increase in heat energy, the battery temperature increases, which in turn leads to a decrease in power generation efficiency and shortens the service life of solar cells.

As a building surface component, the heat generated by the photovoltaic component during power generation will have a certain adverse effect on building heat insulation and fire prevention. How to maintain a lower operating temperature of the photovoltaic cell to improve power generation efficiency is another key issue in the application of photovoltaic roof systems^[14]The gap between the metal roof profiled sheet and the back of the solar panel can be reasonably set and utilized to form a cooling channel. The heat from the photovoltaic cells is taken away by the channel, and this heat can also be utilized. This system is a photovoltaic-thermal integrated utilization system. While ensuring power output, it reduces building energy consumption due to increased domestic heating, reduces indoor HVAC load, and achieves both building energy saving and the promotion of photovoltaic applications.

3.11 Lack of Communication Between the Construction and Photovoltaic Industries

Traditional photovoltaic products focus on improving conversion efficiency and reducing costs, lacking understanding of the construction industry and the performance of building materials, and neglecting considerations for building requirements such as waterproofing, lighting, ventilation, and durability.

4

Building Photovoltaic Related Standards

4.1 International Standards

International standards related to building-integrated photovoltaics mainly include European standards EN, International Electrotechnical Commission standards IEC, and American and British standards (Table 3). These standards are divided into three categories: photovoltaic components or materials, building-integrated photovoltaic power generation systems, and building performance and safety. Among them, European standards are more systematic and comprehensive.

Table 3 International Standards Related to Building-Integrated Photovoltaics

The standards for building-integrated photovoltaic components or materials mainly include IEC 63092-1∶2020^[15]、EN 50583-1∶2016^[16]. IEC 63092-1 and EN 50583-1 provide definitions and classifications of building-integrated photovoltaic components, and also require that photovoltaic components meet the requirements of building materials, such as mechanical properties, water tightness, airtightness, and optical properties.

The standards for building-integrated photovoltaic power generation systems mainly include IEC 63092-2∶2020^[17] and EN 50583-2∶2016^[18]. Among them, IEC 63092-2 and EN 50583-2, based on photovoltaic industry standards, put forward requirements for wind and snow load resistance and energy-saving insulation performance of building-integrated photovoltaic power generation systems, and EN 50583-2∶2016 also proposes a method for determining the fire rating test according to the installation location and materials of photovoltaic components.

The implementation standards for building-integrated photovoltaics on building performance and safety include EN 61730^[19]、EN 61215^[20]、PD CEN/TR 16999∶2019^[21]etc. Among them, EN 61730 is about the requirements and test methods for lightning protection, fire protection, and personnel injury safety qualification certification of photovoltaic components. EN 61215 is about the design qualification and typing of ground-mounted crystalline silicon photovoltaic components. PD CEN/TR 16999∶2019 is a technical report document on the structural design and normal use safety regulations related to the fixing of solar photovoltaic components and photovoltaic-thermal components to roofs and walls (excluding waterproofing, insulation, and fire protection).

4.2 Domestic Standards

Domestic standards related to building-integrated photovoltaics are divided into national standards, industry standards, local standards, and group standards in terms of hierarchy, and into building-integrated photovoltaic materials and product standards and building-integrated photovoltaic engineering standards in terms of categories, as shown in Table 4.

Table 4 Current Domestic Standards Related to Building-Integrated Photovoltaics

Building-integrated photovoltaic related product standards mainly include three aspects^[7]: photovoltaic components and materials (such as GB 29551, GB/T 29759, GB/T 37268, JG/T 465, JG/T 492, etc.); photovoltaic power generation systems (such as GB/T 16895.32, GB/T 19064, JGJ/T 365, DB 13/T 2826 and T/CECS 10094, etc.); photovoltaic component test methods (such as GB/T 37052, GB/T 38344 and GB/T 38388, etc.). These test method standards are new standards launched in the past 3 years.

Most of the engineering construction standards related to building-integrated photovoltaics have been issued and implemented in the past 3 years. The overall standards include GB/T 51368—2019, CECS 418—2015, T/CBDA 39—2020, and special standards include GB/T 36963—2018, GB/T 37655—2019, etc. Among them, the "Technical Standard for Application of Building Photovoltaic Systems" (GB/T 51368—2019) provides a comprehensive regulation on the design, construction, acceptance, and operation of building photovoltaic systems for new, renovated, and expanded buildings, including photovoltaic power generation systems and building structures, materials, equipment, power generation, energy storage, lightning protection, safety, and testing. Although the standard "Technical Regulations for Skylights and Metal Roofs" (JGJ 203—2012) related to metal roofs has some general regulations on the materials, design, and connection of photovoltaic metal roofs, it still needs to be refined for engineering practice, and some clauses are no longer applicable due to the rapid development of photovoltaic technology.

China has been carrying out standard system reform in recent years. Some regulations on design service life and structural load in the above engineering standards are not fully coordinated with newly promulgated general specifications and need to be improved; the design methods of wind load and snow load corresponding to different installation methods of photovoltaic components on roofs are still blank; the connection calculation, construction, and node requirements for different situations such as metal roofs, rolled roofs, and tile roofs need to be supplemented.

Although the photovoltaic industry is an emerging industry, its development speed is very fast, while the standard compilation of the construction industry requires a relatively long process, resulting in the relatively lagging update of engineering standards and atlases. It is necessary to strengthen the guidance of engineering and support the healthy development of building-integrated photovoltaics.

5

Conclusion

Based on the research of the development status and application market of building-integrated photovoltaics, combined with the development analysis of photovoltaic applications in metal roof projects and related standards, the following conclusions are drawn:

(1) The application of photovoltaic power generation technology in buildings is an important and effective means for the construction field to achieve the goals of "carbon peak" and "carbon neutrality." Currently, the cost of photovoltaic power generation has been significantly reduced, and power generation efficiency has continued to improve. It has become a reasonable business model to obtain benefits from investment in separated building-integrated photovoltaics. In line with the national "dual carbon" development strategy and encouraging policies, photovoltaic buildings will usher in great development opportunities. The existing building photovoltaic renovation stock market reaches 2 trillion yuan, and the annual output value of new building photovoltaics exceeds 100 billion yuan, with broad prospects.

The combination of building metal roofing and photovoltaics is still in its infancy. Further research and improvement are needed in key technical performance and node construction aspects, such as the design wind load values and wind resistance performance of BAPV (Building-Applied Photovoltaics) for retrofitting existing buildings and BIPV (Building-Integrated Photovoltaics) for new buildings, structural safety inspection and evaluation of existing metal roofs, drainage, durability, fire performance, the connection between photovoltaic components and metal roofs, and the integrated use of photovoltaic and photothermal energy.

Existing standards related to photovoltaic building roofs focus on photovoltaic components and photovoltaic power generation systems. Standards combining them with building products are lacking or impractical. Standards for structural safety, durability, fire performance, acceptance, and management are urgently needed to promote the healthy development of the building photovoltaic industry.

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